Phenylnaphthalenes: Sublimation Equilibrium, Conjugation, and Aromatic Interactions

Article (PDF Available)inThe Journal of Physical Chemistry B 116(11):3557-70 · February 2012with86 Reads
DOI: 10.1021/jp2111378 · Source: PubMed
Abstract
In this work, the interplay between structure and energetics in some representative phenylnaphthalenes is discussed from an experimental and theoretical perspective. For the compounds studied, the standard molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were determined by the measurement of the vapor pressures as a function of T, using a Knudsen/quartz crystal effusion apparatus. The standard molar enthalpies of formation in the crystalline state were determined by static bomb combustion calorimetry. From these results, the standard molar enthalpies of formation in the gaseous phase were derived and, altogether with computational chemistry at the B3LYP/6-311++G(d,p) and MP2/cc-pVDZ levels of theory, used to deduce the relative molecular stabilities in various phenylnaphthalenes. X-ray crystallographic structures were obtained for some selected compounds in order to provide structural insights, and relate them to energetics. The thermodynamic quantities for sublimation suggest that molecular symmetry and torsional freedom are major factors affecting entropic differentiation in these molecules, and that cohesive forces are significantly influenced by molecular surface area. The global results obtained support the lack of significant conjugation between aromatic moieties in the α position of naphthalene but indicate the existence of significant electron delocalization when the aromatic groups are in the β position. Evidence for the existence of a quasi T-shaped intramolecular aromatic interaction between the two outer phenyl rings in 1,8-di([1,1'-biphenyl]-4-yl)naphthalene was found, and the enthalpy of this interaction quantified on pure experimental grounds as -(11.9 ± 4.8) kJ·mol(-1), in excellent agreement with the literature CCSD(T) theoretical results for the benzene dimer.

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Available from: Carlos F R A C Lima
Phenylnaphthalenes: Sublimation Equilibrium, Conjugation, and
Aromatic Interactions
Carlos F. R. A. C. Lima,
Marisa A. A. Rocha,
Bernd Schröder,
Lígia R. Gomes,
§
John N. Low,
and Luís M. N. B. F. Santos*
,
Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto,
P-4169-007 Porto, Portugal
§
CIAGEB-Faculdade de Ciências da Saúde, Escola Superior de Saúde da UFP, Universidade Fernando Pessoa, P-4200-150 Porto,
Portugal
CICECO, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB24 3UE, Scotland
*
SSupporting Information
ABSTRACT: In this work, the interplay between structure
and energetics in some representative phenylnaphthalenes is
discussed from an experimental and theoretical perspective.
For the compounds studied, the standard molar enthalpies,
entropies and Gibbs energies of sublimation, at T= 298.15 K,
were determined by the measurement of the vapor pressures as
a function of T, using a Knudsen/quartz crystal effusion
apparatus. The standard molar enthalpies of formation in the
crystalline state were determined by static bomb combustion
calorimetry. From these results, the standard molar enthalpies
of formation in the gaseous phase were derived and, altogether
with computational chemistry at the B3LYP/6-311++G(d,p)
and MP2/cc-pVDZ levels of theory, used to deduce the relative molecular stabilities in various phenylnaphthalenes. X-ray
crystallographic structures were obtained for some selected compounds in order to provide structural insights, and relate them to
energetics. The thermodynamic quantities for sublimation suggest that molecular symmetry and torsional freedom are major
factors affecting entropic differentiation in these molecules, and that cohesive forces are significantly influenced by molecular
surface area. The global results obtained support the lack of significant conjugation between aromatic moieties in the αposition
of naphthalene but indicate the existence of significant electron delocalization when the aromatic groups are in the βposition.
Evidence for the existence of a quasi T-shaped intramolecular aromatic interaction between the two outer phenyl rings in 1,8-
di([1,1-biphenyl]-4-yl)naphthalene was found, and the enthalpy of this interaction quantified on pure experimental grounds as
(11.9 ±4.8) kJ·mol1, in excellent agreement with the literature CCSD(T) theoretical results for the benzene dimer.
INTRODUCTION
Phenylnaphthalenes are organic compounds consisting of a
variable number of phenyl substituents bonded to a unit of
naphthalene, as depicted in Figure 1.
Phenylnaphthalenes and related compounds encounter a
wide number of applications. They are of interest for the fields
of polymer chemistry, nanotechnology, and molecular
biology.
13
Substituted phenylnaphthalenes can be used as
enzyme inhibitors and antibacterial compounds.
4,5
The relative structural simplicity of phenylnaphthalenes can
be explored in order to obtain some fundamental knowledge on
the physicalchemical properties of these systems, both in bulk
and gas phases. In this work, a systematic study of some
phenylnaphthalenes was carried out in order to isolate and
characterize the factors that lead to structural and energetic
differentiation. These factors are basic physicalchemical
phenomena, like enthalpic or entropic effects, electronic
delocalization, specific inter and intramolecular interactions,
which, although present in many molecular systems, are often
superimposed by a myriad of other factors, complicating the
task of properly isolating them from the globally intertwined
ensemble of phenomena.
Received: November 18, 2011
Revised: January 31, 2012
Published: February 22, 2012
Figure 1. Schematic molecular structure of phenylnaphthalenes,
showing the αand βpositions of naphthalene.
Article
pubs.acs.org/JPCB
© 2012 American Chemical Society 3557 dx.doi.org/10.1021/jp2111378 |J. Phys. Chem. B 2012, 116, 35573570
Figure 2 presents the structures and the abbreviations that
will be used throughout this paper for the eight phenyl-
naphthalenes studied. Three groups of structural isomers can be
distinguished as follows: A, 1-phenylnaphthalene (1-PhN) and
2-phenylnaphthalene(2-PhN);B,1-([1,1-biphenyl]-4-yl)-
naphthalene (1-bPhN), 2-([1,1-biphenyl]-4-yl)naphthalene
(2-bPhN), 1,8-diphenylnaphthalene (1,8-diPhN) and 1,4-
diphenylnaphthalene (1,4-diPhN); C, 1,8-di([1,1-biphenyl]-4-
yl)naphthalene (1,8-dibPhN) and 1,2,3,4-tetraphenylnaphtha-
lene (1,2,3,4-TPhN). This subdivision is useful because more
direct comparisons can be made between isomers, and chemical
stability can only be meaningfully compared within the same
potential energy surface (PES).
The presence of aromatic moieties can induce an extensive
conjugation along the πsystems, which is associated with
charge delocalization and electronic correlation, and may lead
to significant molecular stabilization. Additionally, the existence
of adjacent phenyl substituents (like in 1,8-diPhN), opens the
possibility for considerable intramolecular steric repulsions
and/or aromatic interactions. Aromatic interactions are non-
covalent intra- and/or intermolecular interactions involving at
least one aromatic system. The relatively large polarizability of
aromatics and the quadrupole moment created by the interplay
between the σand πframeworks of an aromatic group can lead
to the establishment of significant dispersive van der Waals and
electrostatic interactions, respectively.
611
Three types of
structures are generally observed for the interaction between
two aromatic groups: T-shaped (T), parallel-displaced (PD),
and stacked (S).
8,9,11
The T structure is in its essence a C
H···πinteraction.
8,11,12
Aromatic interactions are cohesive
forces present in the bulk phases of aromatic compounds,
andtheyhaveanirrefutableimportanceinmolecular
recognition processes and in defining the conformation of
molecules and macromolecules, like proteins or DNA.
9,13,14
Aromatic interactions are also a hot topic in supramolecular
chemistry.
9,15,16
This work is a combined structural/energetic study of this
family of compounds, comprising the following: X-ray
crystallography for the determination of crystal phase
structures; Knudsen/quartz crystal effusion for vapor pressure
measurement as a function of T, and consequent derivation of
the thermodynamic quantities of sublimation; combustion
calorimetry for evaluation of the enthalpies of formation of the
crystalline compounds, and consequent derivation of the
enthalpies of formation in the gas phase in combination with
the sublimation data; UVvis spectroscopy for exploring the
electronic properties of some relevant phenylnaphthalenes; a
computational study, using B3LYP and MP2 methods, as a
support of the experimental results and assessment of gas phase
geometries and energetics.
A series of homodesmotic reaction schemes is used in order
to detect and quantify, on pure experimental grounds, some
important electronic effects that can influence energetics in
some of the molecules under study.
EXPERIMENTAL SECTION
Synthesis, Purification, and Characterization of the
Studied Compounds. 1,2,3,4-Tetraphenylnaphthalene was
commercially obtained from Sigma-Aldrich, washed with
boiling methanol and sublimed under reduced pressure. All
the other compounds, excluding 1,4-diphenylnaphthalene, were
synthesized and purified as described elsewhere.
17,18
1,4-Diphenylnaphthalene. A 40 mL solution of K2CO3(28
mmol) in water was added to a solution of 1,4-dibromonaph-
thalene (7 mmol), phenylboronic acid (25 mmol) and
palladium acetate (2 mol %) in 40 mL of DMF. The resultant
solution was stirred at 90 °C for 9 h. The crude product was
extracted with ethyl acetate. The organic phase was washed
with water and NaOH(aq) 1 M, and evaporated yielding a
yellowish solid (yield (%) = 66). The compound was washed
with boiling methanol and sublimed under reduced pressure to
yield pure white crystals of 1,4-diPhN (mp = 134.8135.7 °C).
1H NMR (400 MHz, CDCl3, 300 K, TMS): 8.01 (2H, dd, H5
and H8,Jortho = 6.5, Jmeta = 3.3), 7.617.44 (14 H, m); 13C
NMR (100.6 MHz, CDCl3, 300 K): 141.7, 140.8, 132.8, 131.1,
129.2, 128.2, 127.4, 127.3, 126.8.
The purity of the compounds was verified by gasliquid
chromatography, using an HP 4890 apparatus equipped with a
HP-5 column, cross-linked, 5% diphenyl and 95% dimethylpo-
lysiloxane, showing a %(m/m) purity greater than 99.8% for all
the compounds.
Structural Characterization. Crystals suitable for X-ray
diffraction of 1-bPhN were grown from evaporation of a
CH2Cl2/isooctane (1:1) solution. Crystals of 1,8-dibPhN and
1,8-diPhN were grown from evaporation of a CH2Cl2solution.
Crystals of 1,4-diPhN were grown from evaporation of a
CH2Cl2/acetone (1:1) solution. The intensity data was
collected in a Bruker-Nonius CCD diffractometer. Data
collection, cell refinement and data reduction were made with
the software package of the diffractometer: APEX2 and
SAINT.
19
Absorption correction was performed with SA-
DABS.
19
The structure was solved and refined using the
following software: OSCAIL
20
and SHELXL97.
21
H atoms were
treated as riding atoms with CH(aromatic), 0.95 Å. The
crystal of 1,4-diphenylnaphthalene was a merohedral twin with
a twin component ratio of 66/34. Molecular graphics were
made with ORTEP
22
and PLATON.
23
The complete set of
Figure 2. Schematic representation of the phenylnaphthalenes studied
in this work and the adopted abbreviations: 1-phenylnaphthalene (1-
PhN); 2-phenylnaphthalene (2-PhN); 1-([1,1-biphenyl]-4-yl)-
naphthalene (1-bPhN); 2-([1,1-biphenyl]-4-yl)naphthalene (2-
bPhN); 1,8-diphenylnaphthalene (1,8-diPhN); 1,4-diphenylnaphtha-
lene (1,4-diPhN); 1,8-di([1,1-biphenyl]-4-yl)naphthalene (1,8-
dibPhN); 1,2,3,4-tetraphenylnaphthalene (1,2,3,4-TPhN).
The Journal of Physical Chemistry B Article
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structural parameters in CIF format is available as an Electronic
Supplementary Publication from the Cambridge Crystallo-
graphic Data Centre, CCDC: 698100 (1-bPhN), 668280 (1,8-
dibPhN), and 802922 (1,4-diPhN). Information about crystal
data, data acquisition conditions and refinement parameters for
these compounds is given as Supporting Information. Despite
all efforts, crystals suitable for X-ray diffraction for 2-PhN and
2-bPhN could not be obtained.
UVVis Spectroscopy. UVvis spectra for 1-PhN (1.835
×105mol·dm3), 2-PhN (1.995 ×105mol·dm3), 1-bPhN
(1.576 ×105mol·dm3), and 2-bPhN (1.379 ×105
mol·dm3), in the region of (1901090) nm, in CH2Cl2
were measured with an Agilent 8453 diode array UVvis
spectrometer, at T= 298.1 K; the molar concentrations are
given in parentheses. A quartz cell with a path length of 10.00
mm was used. Temperature control was achieved by means of a
Julabo F25 HP refrigerated circulator.
Combustion Calorimetry. 1-([1,1-Biphenyl]-4-yl)-
naphthalene. The standard molar enthalpy of combustion,
ΔcHm
o,atT= 298.15 K, was measured in an isoperibol static
bomb combustion calorimeter with a twin valve bomb of
internal volume of 0.290 dm3, formerly used at the National
Physical Laboratory, Teddington, U.K. The measurements were
performed mainly as previously described, although a few
changes in technique, due to different auxiliary equipment, were
applied.
24,25
Other Compounds. The standard molar enthalpies of
combustion, ΔcHm
0,atT= 298.15 K, were measured in an
isoperibol mini-bomb combustion calorimeter.
26
The mini-
bomb is made of stainless steel with 0.46 cm wall thickness and
18.2 cm3of internal volume. The internal fittings located on the
head of the mini-bomb (electrodes, crucible support and sheet)
are all made of platinum.
The program LABTERMO
27
was used to compute the
corrected adiabatic temperature change, ΔTad. The densities, in
g·cm3, of the compounds were taken from the crystallographic
data presented in this work and in the literature: 1-bPhN, 1.25;
1,4-diPhN, 1.26; 1,8-diPhN, 1.14;
28
1,8-dibPhN, 1.28; 1,2,3,4-
TPhN, 1.19;
29
2-PhN, 1.25 (considered the same as for 1-
bPhN); 2-bPhN, 1.25 (considered the same as for 1-bPhN).
The values of (u/p)Tat T= 298.15 K were assumed to be 0.2
J·g1·MPa1.
30,31
The corresponding energetic correction
usually leads to negligible errors in the final combustion
results. Standard state corrections were calculated for the initial
and final states by the procedures given by Hubbard et al.
32
and
by Good and Scott.
33
The relative atomic masses used were
those recommended by the IUPAC Commission in 2005.
34
Knudsen/Quartz Crystal Effusion. The vapor pressures of
the studied compounds were measured as a function of
temperature by the combined Knudsen/quartz crystal effusion
method recently developed in our laboratory.
35
This technique
is based on the simultaneous gravimetric and quartz crystal
microbalance mass loss detection, enabling the use of a
temperature-step methodology, and having the advantages of
smaller sample sizes and effusion times, and the possibility of
achieving temperatures up to 650 K. In a typical Knudsen
effusion experiment, the system is kept at high vacuum,
enabling free effusion of the vapor from the cell, while the oven
is kept at a fixed temperature, T.
Computational Details. All theoretical calculations were
performed using the Gaussian 03 software package.
36
The full
geometry optimizations were performed using the Moller
Plesset perturbation theory with a second order perturbation
(MP2) and the correlation consistent basis set cc-pVDZ, and
density functional theory (DFT) with the hybrid exchange
correlation functional (B3LYP) and the 6-311++G(d,p) basis
set. The spin-component-scaled MP2 approach (SCS-MP2)
37
was also used for the calculation of the ground-state electronic
energies. Some of the phenylnaphthalenes studied are prone to
establish intramolecular dispersive interactions. For that reason
were chosen a method that is virtually blind to these
interactions (B3LYP) and one that can describe them
satisfactorily (MP2), for then the presence of such interactions
in one molecule will probably lead to a significant discrepancy
between the B3LYP and MP2 results.
RESULTS
X-ray Crystallographic Structures. The X-ray structure
determination for 1-bPhN, 1,8-dibPhN, 1,4-diPhN, and 1,8-
diPhN confirmed the expected molecular structures. A
literature search revealed that the crystalline structures for
1,8-diPhN
28
and 1,2,3,4-TPhN
29
have been previously
determined. The geometrical parameters and supramolecular
structure obtained in this work, corresponding to low
temperature data acquisition of 1,8-diPhN, compares well
with the structural data published by Tsuji et al. at room
temperature and thus it will not be discussed here.
28
The
molecular structures for 1,4-diPhN, 1,8-dibPhN, and 1-bPhN
are presented in Figures 35.
Compound 1,4-diPhN crystallizes with two molecules on the
asymmetric unit whose carbon atoms are labeled as C1X and
C2X for molecule 1 and molecule 2, respectively. The main
bond lengths for the compounds are within the expected range
for aromatics. The distances found for CC bonds of the
phenyl rings show values within the typical range of 1.384(13)
Å, corresponding to the mean value for the CarCar bond in
aromatic compounds.
38
Nevertheless, a significant elongation of
the C9C10 bond length on the naphthalene ring in 1,8-
dibPhN (1.439(3) Å) and 1,4-diPhN (1.442(7) and 1.436(7)
Å) is observed when compared with the corresponding value
Figure 3. View of 1,4-diPhN with our numbering scheme. Displace-
ment ellipsoids are drawn at the 30% probability level. Molecule 1 with
C1X labels and molecule 2 with C2X labels.
The Journal of Physical Chemistry B Article
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for naphthalene at 135 K (1.424(2)) Å.
39
For compound 1-
bPhN, no significant differences in the geometrical parameters
of the naphthalene ring is observed when compared with
naphthalene in spite of the 1,1-biphenyl substitution. Table 1
shows the dihedral angles between the mean planes of the
phenyl rings and the naphthalene ring for the characterized
compounds, and for 1,8-diPhN
28
and 1,2,3,4-TPhN.
29
Com-
pounds with phenyl rings inserted in ortho or peri positions of
the naphthalene moiety show higher dihedral angles. This
variation lies within the 11°to 23°range reflecting the expected
higher steric hindrance of ortho/peri substitution.
Table 2 shows the dihedral angles between the mean planes
of each of the phenyl rings for 1-bPhN and 1,8-dibPhN.
The supramolecular structures of 1-bPhN and 1,8-dibPhN
are stabilized by three CH···πintermolecular interactions,
whereas in 1,4-diPhN there are no significant CH···πor π···π
interactions. The geometric parameters for the interactions are
summarized in Table 3 and depicted in Figures 68.
In 1-bPhN the center of gravity of the C1C2C3C4
C9C10 aromatic ring of the naphthalene moiety acts as
acceptor for two short CH···πinteractions from which the
donor H atoms are the ortho hydrogens attached to the outer
biphenyl ring. These link symmetry related molecules into
chains running parallel to the a-axis (Figure 6). A third and
longer C(7)H(7)···π(141) interaction involving the C7H7
donor of the naphthalene ring and the center of gravity of the
C141C146 phenyl ring links the previous chains to form
sheets lying parallel to the ab plane (Figure 6). Similarly, in 1,8-
dibPhN, it is the C1C2C3C4C9C10 aromatic ring of
the naphthalene moiety located at (x,y,z) that is the acceptor
of two CH···πinteractions: one involving the C4H4 donor
of the naphthalene ring at (0.5 x,0.5 + y, 0.5 z), linking
the molecules in antiparallel chains, and one involving the
C15H15 acceptor located at (x,1+y,z) that links the two
molecules into a chain along the b-axis by a unit translation
(Figure 7). These chains are then linked by a third C(844)
H(844)···π(141) interaction to form a sheet which lies parallel
to the bc plane (Figure 8). It is interesting to note that the D
A···A angular values (Table 3) for the shorter CH···π
interactions in 1,8-dibPhN show that they are practically
coaxial, favoring the electrostatic dipolequadrupole contribu-
tion for this type of interaction.
811
Combustion Calorimetry. The detailed results for
combustion calorimetry of the compounds studied are provided
as Supporting Information. The products of combustion in the
experiments consist of a gaseous phase and an aqueous mixture
for which the thermodynamic properties are known. The values
of ΔcUm
0refer to the combustion reactions of the studied
compounds, represented by the general equation:
++
→+
nm
nm
C H (cr) ( /4)O (g)
CO (g) ( /2)H O(l)
nm 2
22 (1)
Table 4 lists the derived standard molar energies of
combustion, ΔcUm
0(cr), the standard molar enthalpies of
combustion, ΔcHm
0(cr), and the standard molar enthalpies of
formation, ΔfHm
0(cr), of the crystalline solids. In accordance
with normal thermochemical practice, the uncertainties
assigned to ΔcHm
0(cr) and ΔfHm
0(cr) are twice the overall
standard deviation of the mean and include the uncertainties in
calibration and in the auxiliary quantities used. To derive
ΔfHm
0(cr) from ΔcHm
0(cr), the standard molar enthalpies of
formation of H2O(l) and CO2(g) at T= 298.15 K, (285.830
±0.042) kJ mol1and (393.51 ±0.13) kJ mol1, respectively,
were used.
40
Heat Capacity Measurements and Estimations. The
values of Cp,m
o(cr), at T= 298.15 K, were measured for 1,4-
diPhN and 1,2,3,4-TPhN using a high-precision heat capacity
drop calorimeter, developed by Wadsö
41,42
at the Thermo-
chemistry Laboratory of Lund, Sweden, afterward transferred to
Porto, Portugal, in where it was modernized with regards to the
temperature sensors and electronics, and enabling the use of
computer data acquisition in a user-friendly environment.
43
For
Figure 4. View of 1,8-dibPhN with our numbering scheme.
Displacement ellipsoids are drawn at the 30% probability level.
Figure 5. View of 1-bPhN with our numbering scheme. Displacement
ellipsoids are drawn at the 30% probability level.
The Journal of Physical Chemistry B Article
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the other compounds, Cp,m
o(cr), at T= 298.15 K, was estimated
by an additive method based on the equation:
=+
C
CnC
(compound, cr)
(naphthalene, cr) (Ph)
(Ph increment, cr)
p
pp
,m
o
,m
o,
m
o
(2)
where Cp,m
o(naphthalene, cr) = 165.7 J·K1·mol1, n(Ph) is the
number of phenyl substituents in the compound, and Cp,m
o(Ph
increment, cr) is the mean value for the increment in Cp,m
o(cr)
per phenyl ring, estimated by a least-squares method, using the
experimental values of Cp,m
o(cr) for: naphthalene (165.7), 1,4-
diPhN, 1,2,3,4-TPhN, biphenyl (198.4), o-terphenyl (274.8), p-
terphenyl (278.7), p-quaterphenyl (362.5), and 1,3,5-triphe-
nylbenzene (361.0); all values in parentheses are in J·K1·mol1
and were taken from Domalski.
44
The theoretical values of
Cp,m
o(g), at T= 298.15 K, for 1-bPhN, 1,4-diPhN, and 1,8-
diPhN were computed at the B3LYP/6-311++G(d,p) level of
theory using the scaling factor of 0.9688 for the fundamental
frequencies calculations.
45
Correction for hindered rotation of
the phenyl groups was not considered since it is generally of
relatively small magnitude and lies within the uncertainties
derived for the Δcr
gCp,m
ovalues. For the other compounds
Cp,m
o(g) was calculated according to an equation analogous to
Table 1. Dihedral Angles between the Mean Planes of the Phenyl Rings and the Naphthalene Ring for the Characterized
Compounds and for 1,8-diPhN
28
and 1,2,3,4-TPhN
29
dihedral angles/deg
C1
a
C2
a
C3
a
C4
a
C8
a
1-bPhN 56.52(11) −− − −
1,8-dibPhN 60.64(9) −− 73.80(9)
1,4-diPhN 53.1(2) −− 50.4(2)
52.2(2) −− 54.2(2)
1,8-diPhN 66.48(10) −− 67.10(10)
1,2,3,4-TPhN 66.50(7) 68.81(8) 65.17(8) 71.48(8)
76.04(8) 62.55(8) 62.55(8)
b
76.04(8)
b
a
Attachment atom position.
b
There are 1.5 molecules in the asymmetric unit. One of the molecules sits on a 2-fold crystallographic axis, which
bisects the naphthalene moiety lengthwise.
Table 2. Dihedral Angles between the Mean Planes of Each
of the Phenyl Rings for 1-bPhN and 1,8-dibPhN
dihedral angles/deg
C14
a
C84
a
1-bPhN 28.23(12)
1,8-dibPhN 42.34(9) 30.05(11)
a
Attachment atom position.
Table 3. Intermolecular Interactions, Distances, and Angles
(Å and deg)
a
compound DH···AH···A
/Å D···A/Å DA···A
/°
1-bPhN C(7)H(7)···π(141)i2.94 3.708(3) 139
C(142)H(142)···π(1)ii 2.79 3.599(3) 143
C(146)H(146)···π(1)iii 2.71 3.565(3) 150
1,8-dibPhN C(4)H(4)···π(1)iv 2.75 3.691(3) 172
C(15)H(15)···π(1)v2.87 3.679(3) 144
C(844)
H(844)···π(141)vi 2.64 3.587(3) 174
a
π(141) is the centroid of the C141C146 ring, π(1) is the centroid
of the C1C2C3C4C9C10 ring of naphthalene. Symmetry
codes: (i)x,1+y,z;(ii)x,1/2+y,1/2z;(iii)1x,1/2+y,1/2
z;(iv)1/2x,1/2+y,1/2z;(v)x,1+y,z;(vi)1/2x,1/2
+y,3/2z.
Figure 6. View of the supramolecular structure of 1-bPhN, showing
the CH···πintermolecular interactions. Hydrogen atoms not
involved in the motifs are not included.
Figure 7. View of the supramolecular structure of 1,8-dibPhN,
showing the CH···πintermolecular interactions. Hydrogen atoms
not involved in the motifs are not included.
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eq 2, using the following calculated values of Cp,m
o(g) (at the
same level of theory) for the estimation of Cp,m
o(Ph increment,
g) by a least-squares method: naphthalene (134.2), 1-PhN
(217.6), 2-PhN (217.8), 1-bPhN (300.8), 2-bPhN (301.2), 1,4-
diPhN (300.5), and 1,8-diPhN (300.9); all values are in
J·K1·mol1. For the least-squares methods the average errors
of 1.9 (for Cp,m
o(cr)) and of 0.1 (for Cp,m
o(g)) between the
experimental/theoretical and estimated heat capacities were
obtained, corresponding respectively to approximately 0.6%
and 0.05% deviation. The uncertainties in the estimated and
theoretically calculated heat capacities were assumed to be of
±2.0 J·K1·mol1for Cp,m
o(cr) and of ±5.0 J·K1·mol1for
Cp,m
o(g). The global heat capacities results of the compounds
studied are presented in Table 5.
Knudsen/Quartz Crystal Effusion. For some of the
compounds studied, the standard molar enthalpies, Δcr
gHm
0,
and entropies, Δcr
gSm
0, of sublimation at the mean temperature,
T, were derived using the integrated form of the Clausius
Clapeyron equation:
=−pabT
l
n/
(3)
where ais a constant and b=Δcr
gHm(T)/R. Figure 9 presents
the plots of ln (p/Pa) = f(T1/K1) for the compounds
studied, obtained in the Knudsen/quartz crystal effusion
apparatus, recently developed in our laboratory.
35
The detailed experimental results are given as Supporting
Information. The molar enthalpy of sublimation, Δcr
gHm, at the
mean temperature, T, was derived from the parameter bof
the ClausiusClapeyron equation, and the molar entropy of
sublimation at p(T)andatT,Δcr
gSm(T,p(T)),
is calculated as:
Δ
⟨⟩ ⟨⟩ =Δ ⟨⟩ ⟨⟩STpT HT T(,()) ()/
cr
gmcr
gm(4)
The standard molar enthalpy of sublimation, Δcr
gHm
0,atT=
298.15 K is determined by eq 5:
Δ
=Δ ⟨⟩+ ⟨⟩Δ
H
HT T C
(298.15 K)
( ) (298.15 ) p
cr
gm
o
cr
gmcr
g,
m
o(5)
where:
Δ
=−CC C(g) (cr)
pp pcr
g,m
o,m
o,m
o
(6)
The values for the heat capacities, Cp,m
o(cr) and Cp,m
o(g), are
those presented in Table 5. The standard molar entropy of
sublimation, Δcr
gSm
0,atT= 298.15 K, was calculated according
to eq 7:
Δ
=Δ ⟨⟩ ⟨⟩ +Δ
⟨⟩− ⟨⟩
SSTpTC
TRppT
(298.15 K) ( , ( ))
ln(298.15/ ) ln{ / ( )}
pcr
gm
0cr
gmcr
g,m
0
0
(7)
where po=10
5Pa.
DISCUSSION
Data Compilation. A compilation of all relevant
thermodynamic data, at T= 298.15 K, for the phenyl-
naphthalenes studied in this work and naphthalene is given in
Table 6. The standard molar enthalpies of formation in the gas
phase, ΔfHm
0(g), at T= 298.15 K, for the compounds studied,
were calculated according to eq 8:
Δ
=Δ +Δ
H
HH
(g, 298.15 K)
(cr, 298.15 K) (298.15 K)
fm
0
fm
0cr
gm
0(8)
Sublimation Thermodynamics. The data presented in
Table 6 clearly indicates the existence of a significant
differentiation in the enthalpies and entropies of sublimation
among the studied phenylnaphthalenes.
In these compounds the entropy of sublimation can be
approximately dissected into the following terms:
1. The increase in Δcr
gSm
0due to the presence of additional
phenyl substituents.
2. Molecular symmetry: higher symmetry leads to higher
S0(cr) (more symmetry increases the number of atomic
permutations by molecular rotation that are compatible
with a given macrostate, thus increasing the number of
ways into which the crystal lattice can be realizedthis is
valid since a unique set of coordinates within the lattice
can be attributed to each atom, and thus each atom in the
Figure 8. View of the supramolecular structure of 1,8-dibPhN,
showing the sheets that lie parallel to the bc plane. Hydrogen atoms
not involved in the motifs are not included.
Table 4. Derived standard (p0=10
5Pa) Molar Energies of
Combustion, ΔcUm
0(cr), Standard Molar Enthalpies of
Combustion, ΔcHm
0(cr), and Standard Molar Enthalpies of
Formation, ΔfHm
0(cr), in the Crystalline State, at T= 298.15
K, for the Compounds Studied
ΔcUm
0(cr)/kJ
mol1ΔcHm
0(cr)/kJ
mol1ΔfHm
0(cr)/kJ
mol1
2-PhN 8143.9 ±2.2 8151.3 ±2.2 140.2 ±3.0
1-bPhN 11145.9 ±3.0 11155.8 ±3.0 212.0 ±4.2
2-bPhN 11132.2 ±2.9 11142.2 ±2.9 198.3 ±4.1
1,4-diPhN 11156.7 ±3.2 11166.6 ±3.2 222.7 ±4.3
1,8-diPhN 11177.4 ±3.2 11187.4 ±3.2 243.5 ±4.3
1,8-dibPhN 17171.1 ±4.8 17186.0 ±4.8 376.7 ±6.5
1,2,3,4-TPhN 17204.1 ±4.6 17219.0 ±4.6 409.7 ±6.4
Table 5. Heat Capacities, Cp,m
o(cr) and Cp,m
o(g), at T= 298.15
K, for the Compounds Studied
Cp,m
o(cr)/J·K1·mol1Cp,m
o(g)/J·K1·mol1
1-bPhN 328.3 ±2.0
a
300.8 ±5.0
1,4-diPhN 324.0 ±1.4 300.5 ±5.0
1,8-diPhN 328.3 ±2.0
a
300.9 ±5.0
1,8-dibPhN 490.9 ±2.0
a
467.9 ±5.0
a
1,2,3,4-TPhN 491.6 ±0.9 467.9 ±5.0
a
a
Estimated values (see text).
The Journal of Physical Chemistry B Article
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crystal is distinguishable from all the others), and/or
lower S0(g) (the Pauli exclusion principle does not allow
microscopically indistinguishable rotational states in a
symmetric molecule), therefore decreasing Δcr
gSm
0.
4860
3. The internal rotational freedom of the phenyl
substituents in the gas phase: higher barrier heights
being associated with a decrease of S0(g), and thus a
decrease in Δcr
gSm
0.
The molecules of 1-bPhN and 2-bPhN belong to the C1
symmetry point group, having associated a symmetry number
of 1, σsym = 1. On the contrary, the X-ray structures of 1,8-
diPhN and 1,4-diPhN present a molecular geometry consistent
with the C2point group, for which σsym = 2. This nonthermal
entropic contribution should decrease Δcr
gSm
0by about Rln(2)
= 5.8 J·K1·mol1, either by increasing S0(cr), decreasing S0(g),
or a combination of both to varying extents.
6064
Because of
the presence of one adjacent phenyl neighbor, the barrier
height associated with hindered phenyl rotation in 1,8-diPhN
should be significantly higher than in the other isomers and
contribute to a decrease in S0(g), and hence in Δcr
gSm
0. For the
decrease in S0(g) to be directly reflected in Δcr
gSm
0, one must
assume that in the solid phase the respective vibrational modes
are hindered to the same extent in all isomers, having very high
rotational barriers due to the constraints imposed by the crystal
lattice. It can also be assumed that the phenyl internal rotations
in gaseous 1-bPhN and 1,4-diPhN are rather alike. Figure 10
illustrates the reasoning presented above.
Nevertheless, the 2-bPhN isomer does not seem to fit this
rationalization in Δcr
gSm
0. The molecule has no symmetry (σsym =
1), and at a first glance, the phenyl rotors in the gas phase are
not more hindered than the ones in the 1-bPhN isomer.
Notwithstanding, the lower than expected Δcr
gSm
0of 2-bPhN can
also be related with hindered rotation of the phenyl
substituents. Because of the β-phenyl relation, this isomer is
more likely to present an enhanced electron delocalization (as
it will be shown in a following section); a fact that is associated
with higher ring coplanarity and should increase the energetic
penalty associated with internal phenyl rotation, thus lowering
S0(g). This in turn suggests a more planar molecular geometry
for 2-bPhN, which can lead to some positional degeneracy in
the crystal lattice, thus increasing S0(cr).
Figure 9. Plots of ln(p/Pa) = f(T1/K1) for the compounds studied, obtained in the Knudsen/quartz crystal effusion apparatus.
Table 6. Compilation of Relevant Thermodynamic Data, at
T= 298.15 K, for the Phenylnaphthalenes Studied in This
Work and Naphthalene
compound Δcr
gHm
0/kJ mol1Δcr
gSm
0/J·K1.mol1ΔfHm
0(g)/kJ mol1
naphthalene 72.70 ±0.04
46
168.1 ±0.1
46
150.6 ±1.5
47
1-PhN −−252.3 ±3.4
a
2-PhN 107.8 ±0.6
17
211.5 ±1.8
17
248.0 ±3.1
1-bPhN 138.9 ±0.8 245.0 ±2.1 350.9 ±4.3
2-bPhN 140.2 ±1.3
17
228.0 ±3.6
17
338.5 ±4.3
1,4-diPhN 132.5 ±0.6 237.4 ±1.6 355.2 ±4.3
1,8-diPhN 126.4 ±0.5 230.9 ±1.4 369.9 ±4.3
1,8-dibPhN 178.5 ±1.1 280.2 ±2.7 555.2 ±6.6
1,2,3,4-TPhN 154.2 ±0.8 246.9 ±2.1 563.9 ±6.4
a
Estimated value (see details in the text).
Figure 10. Entropic diagram showing the possible causes for the
differentiation in Δcr
gSm
0observed among the three isomers: 1-bPhN,
1,4-diPhN, and 1,8-diPhN.
The Journal of Physical Chemistry B Article
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Relative to the 1,8-dibPhN and 1,2,3,4-TPhN isomers, the
large difference in Δcr
gSm
0can be mainly attributed to the
different degree of hindered rotation of the phenyl substituents
in the gaseous phase. The X-ray structures of both compounds
are consistent with σsym = 2, and thus there should be no
significant differentiation in Δcr
gSm
0due to molecular symmetry.
In the 1,2,3,4-TPhN isomer the two β-phenyl substituents are
surrounded by two adjacent phenyl neighbors, being therefore
highly hindered relative to internal rotation, whereas the
torsional profiles of the two outer phenyl substituents in 1,8-
dibPhN should have a considerably lower rotational barrier and
resemble the one for biphenyl. As demonstrated by the X-ray
structure of 1,8-dibPhN, the outer phenyls are significantly
further away from each other than the phenyl substituents
directly bonded to naphthalene. In this way, differentiation in
S0(g) should mainly arise due to the significantly different
internal rotations associated with the two highly hindered β-
phenyl substituents in 1,2,3,4-TPhN, and the two less hindered
outer phenyl rings in 1,8-dibPhN.
The Δcr
gSm
0data obtained in this work was fitted to a
semiempirical additive model analogous to the one proposed
recently for estimation of Δcr
gSm
0in polyphenylbenzenes.
60
It
was found that the previous model describes quite well the
presented set of data for Δcr
gSm
0(including naphthalene and
seven phenylnaphthalenes studied), by the following equation:
Δ
=+ − σ
β + ‐
α
Sabn cR
dn en
(Ph) ln( )
( / phenyl/biphenyl) (peri
phenyl)
cr
gm
0sym
(9)
where a,b,c,d, and eare fitting parameters; σsym is the external
symmetry number of the molecule, which corresponds to the
number of unique orientations of the rigid molecule that only
interchange identical atoms; n(Ph) is the total number of
phenyl substituents; n(α/β-phenyl/biphenyl) is the number of
αand βphenyl substituents with zero phenyl neighbors (e.g.,
the two phenyls in 1,4-diPhN or the phenyl in 2-PhN), plus
those that are bonded to another phenyl ring (e.g., the outer
phenyl of 1-bPhN or 2-bPhN); n(peri-α-phenyl) is the number
of αphenyl substituents with one adjacent peri phenyl neighbor
(e.g., the two phenyls in 1,8-diPhN). Symmetry numbers, σsym,
were taken from the X-ray crystallographic structures and/or
from the MP2/cc-pVDZ optimized geometries as: naphthalene
(4),
39
2-PhN (1), 1-bPhN (1), 2-bPhN (1), 1,4-diPhN (2), 1,8-
diPhN (2), 1,8-dibPhN (2), 1,2,3,4-TPhN (2).
29
The
parameters dand etake into account the respective
contributions from hindered rotation relative to the most
hindered rotor, the β-phenyl in 1,2,3,4-TPhN. Applying a least-
squares method, the values for the fitting parameters were
found to be: a= 187.1 J·K1·mol1,b= 9.7 J·K1·mol1,c= 1.5,
d= 18.3 J·K1·mol1, and e= 14.9 J·K1·mol1, giving an
average error between the experimental and estimated Δcr
gSm
0
values of 2.4 J·K1·mol1(approximately 1% deviation). The
compound 2-bPhN is clearly an outsider, and therefore was not
considered in the fitting. For 2-bPhN to be part of the
correlation, a contribution of ca. 13 J·K1·mol1for its Δcr
gSm
0
should be considered. Curiously, this value is very near R
ln(4), the contribution that is to be expected if 2-bPhN
presents a structural degeneracy in the crystal lattice consistent
with a planar linear molecule, for which σsym = 4. Unfortunately,
suitable crystals of 2-bPhN for X-ray diffraction could not be
obtained, in order to confirm this hypothesis. Interestingly, the
value of c(constant multiplying the factor R·ln(σsym)) is very
similar to the one observed in our previous work concerning
polyphenylbenzenes (c= 1.4).
60
As referred to that paper, this
result suggests that symmetry leads to entropic differentiation
both in crystal and gas phases and that its influence should be
more correctly described in terms of continuous symmetry
numbers.
50,59,60
By nature, the intermolecular interactions in these com-
pounds are mostly of the van der Waals type. Hence, the
cohesive energy of the solid is expected to depend substantially
on the available free molecular surface for interaction, with a
higher intermolecular surface increasing the likelihood of short
atomic contacts, thus contributing to stronger intermolecular
interactions. In fact, for the C22H16 isomers the enthalpy of
sublimation increases in the order: 1,8-diPhN < 1,4-diPhN < 1-
bPhN 2-bPhN, which nicely follows the increase in molecular
surface area. 1,8-diPhN has two blocked aromatic πfaces, which
leads to a decrease of the cohesive energy. The biphen-4-yl
isomers are the ones with larger surface areas, in agreement
with their greater Δcr
gHm
0in this series. The 1,4-diPhN isomer is
an intermediate case, since both phenyl substituents are bonded
to the αposition of naphthalene, which reduces slightly the
volume extension of the aromatic πclouds relative to 1-bPhN
and 2-bPhN. A similar trend is observed for the C34H24 isomers
(1,8-dibPhN and 1,2,3,4-TPhN). In 1,2,3,4-TPhN only two π
faces from a total out of eight are arising from the four phenyl
groups, which hence are free to interact intermolecularly,
whereas in 1,8-dibPhN, there are four unblocked πfaces, and
an increased extension of electron density due to the biphen-4-
yl relation of the phenyl substituents. Moreover, as more
phenyl substituents cluster around naphthalene, the less prone
the naphthyl moiety is to participate in significant van der
Waals intermolecular contacts. This explains the significantly
higher Δcr
gHm
0for the 1,8-dibPhN isomer. Linear correlations
were obtained by plotting Δcr
gHm
0as a function of the number of
phenyl substituents in naphthalene for the following series: A,
naphthalene, 2-PhN, and 2-bPhN (R2= 0.9995); B,
naphthalene, 1,8-diPhN, and 1,8-dibPhN (R2= 0.9999). This
indicates the existence of a simple additive pattern in
intermolecular interactions in this class of compounds as the
number of structurally similar phenyl rings is incremented.
Similar findings, respecting enthalpies of sublimation, were
observed in our previous work concerning the related series of
polyphenylbenzenes.
60
Gas Phase Energetics. Estimation of ΔfHm
0(g) for 1-
Phenylnaphthalene. The experimental determination of
ΔfHm
0(g) of 1-phenylnaphthalene was not possible, since the
purification of this compound to the required standards was not
successfully achieved. Considering that in α-phenylnaphtha-
lenes conjugation between the naphthyl and phenyl moieties is
very weak (support for this fact comes from UVvis and 1H
NMR spectroscopy
18,65,66
), estimation by an additive method
was applied. The X-ray structures and optimized gas phase
geometries at B3LYP/6-311++G(d,p) and MP2/cc-pVDZ
levels of theory for the α-phenylnaphthalenes studied indicate
a phenyl-naphthyl dihedral angle near 60°, far away from
coplanarity, in all the cases. Therefore, the two homodesmotic
gas phase reactions presented in Figure 11 should be nearly
athermal, in accordance with what is expected if the energetic
factors are additive. Our computational results support this
observation, giving values of ΔrEel(g, 0 K), in kJ·mol1, of 0.3
(HR1) and 0.0 (HR2) at the B3LYP/6-311++G(d,p), and of
The Journal of Physical Chemistry B Article
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0.4 (HR1) and 1.1 (HR2) at the SCS-MP2/cc-pVDZ levels
of theory.
Hence, ΔfHm
0(1-PhN, g) = (252.3 ±3.4) kJ·mol1was
derived, as the resulting mean value from HR1 (252.9 ±2.3)
and HR2 (251.8 ±4.4), assuming ΔHRHm
o(g) = 0.0 kJ·mol1
for both reactions. The experimental values, in kJ·mol1,of
ΔfHm
0(benzene, g) = (82.9 ±0.9),
47
and ΔfHm
0(biphenyl, g) =
(182.0 ±0.7)
67
were used.
General Overview. The values of ΔfHm
0(cr) for all the
compounds presented in Table 6, except for 1-PhN, were
determined by combustion calorimetry. In this way, for the
calculation of the uncertainties in ΔrHm
0(g) for all the
forthcoming reactions, the uncertainties related to the
calibration of the calorimeters and the formation of CO2(g)
and H2O(l) in the course of the corresponding combustion
reactions can be ignored since these contributions cancel out in
the homodesmotic reactions used.
Before proceeding with the analysis of gas phase energetics it
is worth noting the significant similarity that was observed
between the X-ray and optimized MP2/cc-pVDZ molecular
structures, for the cases where both of them could be obtained.
Moreover, due to the relatively low molecular flexibility of these
systems, one should not expect to observe noteworthy
deviations between the equilibrium structures adopted in the
crystal and gas phases. These observations suggest that the X-
ray structures can be used in order to give some support to gas
phase energetics in the discussion that follows.
Figure 12 presents a general homodesmotic separation
reaction scheme that can be used to compare gas phase
energetics among the phenylnaphthalenes studied, and Table 7
summarizes the experimental results for ΔsepHm
0at T= 298.15
K.
Using this approach, some direct comparisons can be made
between all the compounds studied, with positive/negative
ΔsepHm
0values indicating that the total interaction between all
phenyl groups in naphthalene is stabilizing/destabilizing
relative to separated biphenyl and naphthalene.
These results clearly indicate significant energetic differ-
entiation among the compounds studied.
Conjugation in β-Phenylnaphthalenes Leads to Signifi-
cant Energetic Stabilization. As referred before, the results
obtained in this and other works point to a lack of conjugation
between naphthalene and phenyl substituents in the αposition.
The main cause for this phenomenon may be related with the
significant steric interaction between the ortho hydrogen of the
phenyl rings and the adjacent peri hydrogen of naphthalene,
which prevents the achievement of more planar geometries
(smaller angle Φ(Phnaphthyl)), and thus a significant overlap
of electron density that would lead to enhanced electron
delocalization. Table 8 presents the values for the ringring
dihedral angles, Φ(ringring), obtained in this work by X-ray
crystallography and theoretical calculations at the MP2/cc-
pVDZ level of theory, with respect to all the ringring bonds in
the compounds studied. The Phnaphthyl dihedral angles for
the α-phenylnaphthalenes are of about 60°, while β-phenyl-
naphthalenes and biphenyl derivatives present substantially
lower ringring dihedral angles, for instance Φ(Ph-Ph) = (44.4
±1.2)°in gaseous biphenyl.
68
From Table 6 one can see that in the gas phase 1-bPhN is
slightly more stable than 1,4-diPhN. The difference in ΔfHm
0(g)
for these two isomers can be related to the different molecular
Figure 11. Homodesmotic gas phase reactions used for the estimation
of ΔfHm
0(g) for 1-PhN. These reactions should be practically athermal,
as supported by B3LYP/6-311++G(d,p) (DFT) and SCS-MP2/cc-
pVDZ (MP2) calculations. Values of ΔrEel(g, 0 K) in kJ·mol1.
Figure 12. General gas phase separation reaction for the phenyl-
naphthalenes studied.
Table 7. Experimental Results for ΔsepHm
0,atT= 298.15 K,
for the Homodesmotic Reaction Scheme Presented in Figure
12
compound nΔsepHm
0(g)/kJ mol1
1-PhN 1 2.6 ±3.9
2-PhN 1 1.7 ±2.0
1-bPhN 2 2.1 ±2.5
2-bPhN 2 10.3 ±2.5
1,4-diPhN 2 6.4 ±2.8
1,8-diPhN 2 21.1 ±2.7
1,8-dibPhN 4 8.2 ±3.2
1,2,3,4-TPhN 4 16.9 ±2.9
Table 8. RingRing Dihedral Angles, Φ(ringring), for All
the RingRing Bonds for the Compounds Considered in
This Work, Derived from X-ray Crystallography and
Theoretical Calculations at the MP2/cc-pVDZ Level of
Theory
Φ(ringring)/deg
compound X-ray MP2
1-PhN 58
2-PhN 42
1-bPhN
a
57; 28 57; 42
2-bPhN 42; 42
1,4-diPhN 53; 52 58; 58
1,8-diPhN 66; 67 56; 56
1,8-dibPhN
a
74; 61/30; 42
1,2,3,4-TPhN
b
74; 71/64; 66
a
Left values: Phnaphthyl torsions/right values: PhPh torsions.
b
Left values: α-phenyl substituents/right values: β-phenyl substituents.
The Journal of Physical Chemistry B Article
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environment of the phenyl substituents. While in 1-bPhN one
phenyl ring has a biphenyl-like relation, for which Φ(Ph-Ph) =
28°(X-ray), in 1,4-diPhN both phenyl substituents are bonded
to the αposition of naphthalene, with Φ(Ph-naphthyl) = 52°
(X-ray). This is a good indication that a main factor that
energetically distinguishes these two isomers in the gas phase is
the degree of planarity of the phenyl rings.
The β-phenylnaphthalenes studied, 2-PhN and 2-bPhN, are
significantly more stable than their corresponding αisomers,
with the stabilization being more pronounced in 2-bPhN.
Figure 13 presents the experimental gas phase isomerization
enthalpies, ΔisomHm
0(g), at T= 298.15 K, and the theoretically
calculated electronic isomerization energies, ΔisomEel(g), at T=
0 K (ZPE and thermal correction contributions were neglected
since they are usually of little importance in an isomerization
reaction), alongside with their respective UVvis spectra.
Figure 13 shows that a significant bathochromic shift in the
UVvis spectra is observed on going from the αto the β
isomers. It can also be observed that on going from 2-PhN to 2-
bPhN the λmax varies significantly, whereas from 1-PhN to 1-
bPhN it is virtually unchanged. As can be seen in Table 8, β-
phenylnaphthalenes tend to present lower values of Φ(Ph-
naphthyl), which is consistent with greater conjugation and
stability. The bathochromic shift is more pronounced for 2-
bPhN, being nicely accompanied by a more negative
ΔisomHm
0(g). These results constitute a good indication that
enthalpic stability in phenylnaphthalenes is related to the extent
of conjugation between the phenyl and naphthyl moieties in
these molecules. Nevertheless, the theoretical results are not
able to distinguish between the two isomerization reactions.
The fact that both the enthalpic stabilization and bathochromic
shift are considerably more pronounced in 2-bPhN suggests
that a significant electron delocalization, not totally accounted
for by the theoretical results, exists in this molecule and extends
through the whole πsystem. In reality, the orbital overlap
between the various rings in 2-bPhN appears to be more
extensive than considered by the computational calculations. In
this way, a geometry more closer to planarity than the one
obtained computationally may be expected for this molecule.
Intramolecular T-Shaped Aromatic Interaction in 1,8-
Di([1,1-biphenyl]-4-yl)naphthalene. Figure 14 presents two
homodesmotic reactions planned with the intention of
evaluating experimentally and computationally the intra-
molecular interaction between the adjacent phenyl substituents
in 1,8-diPhN and 1,8-dibPhN. The experimental results for the
standard gas phase enthalpies of reaction, ΔrHm
0(g), at T=
298.15 K, and electronic energies of reaction, ΔrEel(g), at T=0
K, are presented (ZPE and thermal corrections were neglected
since they are usually of little importance in a homodesmotic
reaction).
A repulsive interaction is clearly observed between the two
phenyl substituents in 1,8-diPhN. Because of their close
proximity favorable intramolecular aromatic interactions could
be established. However, the two aromatic rings are too close,
and the repulsive short-range contribution dominates the
interaction at this distance. In the crystal structure of 1,8-
diPhN
28
one can observe that the distance between the
centroids of the two Ph rings is of 3.51 Å, which matches the
theoretical calculated equilibrium distance of the parallel
displaced benzene dimer,
812
but the distance between the
two carbon atoms directly bonded to naphthalene is only 2.98
Å. The enthalpic destabilization then probably arises from this
Figure 13. Gas phase isomerization reactions for α- and β-phenylnaphthalenes, and respective UVvis spectra, recorded in CH2Cl2,atT= 298.1 K.
Enthalpic stabilization is associated with a bathochromic shift. Experimental (in bold), B3LYP/6-311++G(d,p) (DFT), and SCS-MP2/cc-pVDZ
(MP2) values in kJ·mol1are presented.
Figure 14. Homodesmotic gas phase reactions used for the evaluation
of the intramolecular aromatic interactions in 1,8-diPhN and 1,8-
dibPhN. Experimental (in bold), B3LYP/6-311++G(d,p) (DFT), and
SCS-MP2/cc-pVDZ (MP2) values in kJ·mol1are reported.
The Journal of Physical Chemistry B Article
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close proximity at the base of the rings, and from the slight
geometric deformation of the molecule, which is more
perceived in the increase of the angle between the phenyl
rings and naphthalene and in the increase of the Phnaphthyl
dihedral angles (see Table 8).
Figure 15 illustrates the parallel displaced orientation that the
two phenyl rings adopt relative to each other in the crystal
phase of 1,8-diPhN.
28
Nevertheless, the fact that the DFT result
is significantly more positive than the experimental and MP2
data indicates that attractive dispersive contributions are
significant for the total interaction energy, which is typical of
a parallel displaced aromatic interaction.
812
It is well
recognized that most DFT methods, including B3LYP, are
not able to properly describe the attractive components of van
der Waals interactions.
8
Since 1,4-diPhN can practically be viewed as a combination
of two noninteracting aromatic rings, the 1,4-diPhN 1,8-
diPhN isomerization reaction is also a good way of evaluating
the intramolecular interaction in 1,8-diPhN. The values of
ΔrHm
0(g) for this reaction, in kJ·mol1, are 14.7 ±2.3
(experimental), 29.9 (B3LYP), and 11.5 (SCS-MP2). As
expected, these results are very similar to the ones obtained
for the first homodesmotic reaction presented in Figure 14.
There is no reason for the interaction between the two
phenyl substituents directly bonded to naphthalene in 1,8-
dibPhN to be less repulsive than in 1,8-diPhN. Their respective
crystallographic structures show that the geometric parameters
relative to this pair of rings (centroidcentroid distance, bond
lengths and angles, angles and dihedral angles relative to
naphthalene) are very similar for both molecules. Hence, the
significantly lower ΔrHm
0(g) observed for the second
homodesmotic reaction presented in Figure 14 is probably
due to an attractive aromatic intramolecular interaction
between the two outer phenyl rings in 1,8-dibPhN. The X-
ray crystal structure of 1,8-dibPhN is consistent with this
finding and suggests an interaction geometry that approaches
the T-shaped structure of the benzene dimer. As shown in
Figure 16, the two outer phenyl rings in this molecule can
establish two CH···πintramolecular contacts. This explains
why the two PhPh dihedral angles, as shown in Table 8, are
significantly different in this molecule. One ring tends to
become perpendicular relative to the other, increasing Φ(Ph
Ph), in order to maximize the two CH···πcontacts. The
intramolecular distances are shown in Figure 16 and are within
the typical values for this type of interaction.
812,69
The aromatic interaction enthalpy can then be easily
calculated as the difference in ΔrHm
0(g) for the two reactions
presented in Figure 14, which gives ΔHm(CH···π)=(11.9 ±
4.8) kJ·mol1, in gaseous 1,8-dibPhN. This value is in perfect
agreement with recent high-level ab initio calculations at the
CCSD(T) level of theory at the basis set limit, for T-shaped
benzene dimers.
812
The true equilibrium geometry for this
intramolecular interaction in the crystal phase of 1,8-dibPhN is
presented in Figure 17. Strictly speaking, this is not a typical T-
shaped structure, but the two interacting CH bonds of one
ring still adopt a favorable orientation toward the πcloud of the
other aromatic ring. The molecular geometry in the gas phase
should closely resemble the one adopted in the crystal;
however, the absence of an intermolecular potential may allow
the establishment of a more perpendicular T-shaped structure
for this interaction in the gas phase. Owing to the nature of the
molecules shown in Figure 14, the existence of the aromatic
interaction between the outer phenyls in 1,8-dibPhN is the
most obvious explanation to the difference observed in
ΔrHm
0(g). The outer rings in this molecule are relatively far
away from each other, and thus it seems logical that they adopt
a structure that more resembles the T-shaped one in order to
maximize the intramolecular interaction. A geometry more
closer to parallel displaced would increase the distance of short
contacts between the rings (the X-ray structure indicates a
centroid-centroid distance of 5.2 Å) and probably dilute the
interaction to a marginal value, reducing the difference in
ΔrHm
0(g) for the two reactions. According to recent theoretical
studies, the calculated Potential Energy Surface (PES) for the
parallel displaced benzene dimer points to an interaction
energy, ΔEint, at these distances of 23kJ·mol1,
8,10
well
below 11.9 kJ·mol1as indicated by our experimental results.
1,2,3,4-Tetraphenylnaphthalene. The experimental results
indicate that the four adjacent phenyl substituents in the
molecule of 1,2,3,4-TPhN do not lead to significant repulsive
Figure 15. Schematic picture showing the parallel displaced
orientation of the phenyl substituents in the crystal phase of 1,8-
diPhN.
28
Figure 16. Intramolecular T-shaped (CH···π) aromatic interaction
between the outer phenyl rings in 1,8-dibPhN: X-ray crystallographic
structure (left) and schematic representation (right). The two C
H···πinteraction distances (in Å) and the two PhPh dihedral angles
are shown.
Figure 17. Schematic picture showing the relative orientation of the
two outer phenyl rings in the crystal phase of 1,8-dibPhN.
The Journal of Physical Chemistry B Article
dx.doi.org/10.1021/jp2111378 |J. Phys. Chem. B 2012, 116, 355735703567
steric interactions. The first homodesmotic reaction scheme in
Figure 18 indicates that intramolecular repulsions are much
more prevalent when the phenyl groups have a peri relation,
like in 1,8-diPhN. In this situation, the phenyl rings are very
close to each other and overlap considerably more electron
density than the phenyl substituents in 1,2,3,4-TPhN. This
scheme also indicates that, within experimental error, the
energetics of 1,2,3,4-TPhN can be approximated by the additive
contribution of one naphthalene and four α-phenyl sub-
stituents. This can also be observed by plotting ΔfHm
0(g) as a
function of the number of Ph substituents for the following
series: naphthalene, 1-PhN, 1,4-diPhN, and 1,2,3,4-TPhN. This
yields a straight line with R2= 0.99996, thus supporting the
validity of the group additivity assumption in these compounds.
When the two α-phenyl and two β-phenyl relations in
1,2,3,4-TPhN are considered in a homodesmotic scheme (last
reaction in Figure 18), an enthalpic destabilization is observed.
In 1,2,3,4-TPhN, each β-phenyl substituent is surrounded by
two phenyl neighbors, and thus they are forced to adopt a more
perpendicular geometry relative to the naphthalene plane, a fact
that is proved by their high Ph-naphthyl dihedral angles (see
Table 8). This departure from planarity shall disrupts the
conjugation between the β-phenyl and naphthyl moieties that
was observed in 2-PhN, and is probably the main source of
destabilization in 1,2,3,4-TPhN. Hence, with respect to
energetics, the β-phenyls in this molecule can be regarded as
α-phenyls. The combination of these findings precludes the
existence of strong repulsive interactions in the molecule of
1,2,3,4-TPhN. Moreover, a significant deformation of the
naphthalene ring is not observed in the crystal structure of this
compound.
29
Nevertheless, because ΔrHm
0(g) is slightly positive
for the homodesmotic reaction presented in Figure 18 that
includes 1,4-diPhN one cannot discard the existence of a small
repulsive contribution in 1,2,3,4-TPhN. In principle, the phenyl
rings in 1,2,3,4-TPhN can be compared with o-phenyl
substituents in benzene. As noted in a previous work, the
presence of o-phenyl rings in benzene do not impose significant
destabilization in polyphenylbenzenes.
60
Despite being rela-
tively close to each other (prone to steric hindrance), the
contribution of dispersive interactions between the rings,
typical of aromatic π···πinteractions, prevents a noteworthy
destabilization of these molecules. This partial cancellation of
effects thus allows the rings in 1,2,3,4-TPhN to be treated like
virtually noninteracting α-phenyls.
CONCLUSION
The relationships between structure and energetics in a series
of relatively simple phenylnaphthalenes has been explored,
experimentally and theoretically, in order to properly character-
ize and quantify the relevant fundamental chemical aspects that
lead to enthalpic and entropic differentiation among these
compounds. It was found that the differences in the entropy of
sublimation can be rationalized in terms of molecular symmetry
and hindered internal rotation of the phenyl substituents.
Higher molecular symmetry increases S0(cr) and/or decreases
S0(g), thus decreasing Δcr
gSm
0, while more hindered rotations
decrease S0(g), and consequently Δcr
gSm
0. The trend in the
enthalpies of sublimation can be understood by considering the
surface area of the molecules, showing that as the surface area,
and hence the likelihood to establish short atomic contacts,
increases the cohesive energy of the crystalline solid becomes
stronger.
With respect to molecular energetics, it was found that
conjugation in α-phenylnaphthalenes is practically absent,
leading to a scenario of group transferability in ΔfHm
0(g) for
these compounds. On the other hand, a significant enthalpic
stabilization, due to enhanced electron delocalization, was
observed in β-phenylnaphthalenes, a phenomenon that is
particularly pronounced in 2-bPhN. The global results suggest
that enthalpic stability in these systems is strongly related with
Figure 18. Homodesmotic gas phase reaction schemes used for the evaluation of energetics in 1,2,3,4-TPhN. Experimental values in kJ·mol1.
The Journal of Physical Chemistry B Article
dx.doi.org/10.1021/jp2111378 |J. Phys. Chem. B 2012, 116, 355735703568
the dihedral angles adopted by the various phenyl substituents,
with lower dihedrals being associated with increased molecular
stability. While the intramolecular interaction between the two
phenyl substituents in 1,8-diPhN is strongly repulsive, a
stabilizing aromatic interaction between the two outer phenyl
rings in 1,8-dibPhN was found, and quantified on pure
experimental grounds as (11.9 ±4.8) kJ·mol1. The X-ray
crystallographic structure of 1,8-dibPhN corroborates this
finding and indicates that the rings adopt a quasi T-shaped
structure, establishing two CH···πintramolecular contacts.
Intramolecular repulsion was found to be of little importance in
1,2,3,4-TPhN. However, the existence of adjacent phenyl
substituents precludes the achievement of more planar
geometries of the β-phenyl rings, canceling the enthalpic
stabilization associated with the phenylnaphthyl conjugation
at this position.
This rationale derived from the study of relatively simple
compounds can be of great assistance for a complete and robust
characterization of some fundamental aspects of chemistry that
influence the behavior of more complex chemical systems.
ASSOCIATED CONTENT
*
SSupporting Information
Detailed description of the techniques, detailed X-ray
crystallography, combustion, and sublimation results, UVvis
spectra for 1-PhN, 2-PhN, 1-bPhN, and 2-bPhN in CH2Cl2,at
T= 298.1 K, and total electronic energies obtained from the
optimized structures for benzene, biphenyl, naphthalene, 1-
PhN, 2-PhN, 1-bPhN, 2-bPhN, 1,4-diPhN, and 1,8-diPhN. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: lbsantos@fc.up.pt. Telephone: +351 220402536. Fax:
+351 220402520.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Thanks are due to Fundação para a Ciência e Tecnologia
(FCT) and Programa Operacional Ciência e Inovação 2010
(POCI 2010) supported by the European Community Fund
FEDER for the financial support to Project POCI/QUI/
61873/2004. C.F.R.A.C.L., M.A.A.R., and B.S. also thank FCT
and the European Social Fund (ESF) under the third
Community Support Framework (CSF) for the award of the
research grants: SFRH/BD/29394/2006, SFRH/BD/60513/
2009, and SFRH/BPD/38637/2007, respectively.
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    • "K) = (161.4 AE 1.4) kJ mol À1 for 1,2,3,4-tetraphenylnaphthalene reported from the ME–QCM study [31] was proved in this work with QCM and TGA measurements. The results are indistinguishable (seeTable 3) within the experimental uncertainties. "
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