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Accepted Article
A Journal of
Title: Quantifying the effects of halogen bonding by haloaromatic
donors on the acceptor pyrimidine
Authors: Thomas L Ellington, Peyton L Reves, Briana L Simms, Jamey
L Wilson, Davita L Watkins, Gregory S Tschumper, and
Nathan I Hammer
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To be cited as: ChemPhysChem 10.1002/cphc.201700114
Link to VoR: http://dx.doi.org/10.1002/cphc.201700114
ARTICLE
Quantifying the effects of halogen bonding by haloaromatic
donors on the acceptor pyrimidine
Thomas L. Ellington, Peyton L. Reves, Briana L. Simms, Jamey L. Wilson, Davita L. Watkins,* Gregory
S. Tschumper,* and Nathan I. Hammer*[a]
Abstract: The effects of intermolecular interactions by a series of
haloaromatic halogen bond donors on the normal modes and
chemical shifts of the acceptor pyrimidine are investigated by
Raman and NMR spectroscopies and electronic structure
computations. Halogen bond interactions with pyrimidine’s nitrogen
atoms shift normal modes to higher energy and upfield shift 1H and
13C NMR peaks in adjacent nuclei. This perturbation of vibrational
normal modes is reminiscent of the effects of hydrogen bonded
networks of water, methanol, or silver on pyrimidine. The
unexpected observation of vibrational red shifts and downfield 13C
NMR shifts in some complexes suggests that other intermolecular
forces such as interactions are competing with halogen bonding.
Natural bond orbital analyses indicate a wide range of charge
transfer is possible from pyrimidine to different haloaromatic donors
and computed halogen bond binding energies can be larger than a
typical hydrogen bond. These results emphasize the importance in
strategic selection of substituents and electron withdrawing groups
in developing supramolecular structures based on halogen bonding.
1. Introduction
The inclusion of halogen-bonded interactions in self-assembled
material building blocks has recently garnered a great deal of
attention for enhanced and selective morphological control.[1-11]
Halogen bonding is defined by IUPAC as a noncovalent
interaction involving the net attraction between an electrophilic
region of a halogen atom of a molecular entity and the
nucleophilic region of another.[12] Here, a halogen bond acceptor
is a molecule with an electron rich region, such as the nitrogen
atoms on pyrimidine, which interacts with a positive region of
electrostatic potential (ESP) on a halogen atom on an adjacent
molecule. When covalently bound to another atom, the electron
density of halogen atoms undergo an anisotropic redistribution in
which an area of positive ESP is localized on the halogen atom
and is highly directional, aligned with the R-X covalent bond
(referred to a -hole, blue region in Figure 1), leading to near
collinear intermolecular interactions.[13-18] This linearity has been
touted as a basis for the competition of halogen bonding with the
much more recognizable hydrogen bonding in complex chemical
environments.[7, 19-23] Due to the formation of a -hole, a belt-like
region of negative ESP also emerges that is perpendicular to the
R-X covalent bond, as shown in Figure 1. This nucleophilic
region can interact with Lewis acids, allowing for amphoteric
behaviour by halogen bond donors acting as both Lewis acids
and bases.[24]
The interaction strength of these highly-directional
noncovalent interactions is tuneable through either the
modification of the substituent groups, with stronger interactions
when these groups are sufficiently electron withdrawing, or the
halogen atom which results in stronger interactions with less
electronegative and more polarizable halogen atoms (I > Br > Cl >
F).[17-18] For example, Resnati and co-workers recently
demonstrated a nice correlation between the redshift in the C-I
stretching vibrational frequency in halogen bond donor molecules
of co-crystals with the careful selection of electron withdrawing
groups on pyridine-based acceptors.[25] In that case, changing the
electron withdrawing groups of the halogen bond acceptors led to
the tunability in interaction strength. Due to the strength and the
tunability of the interaction, halogen bonding is indeed becoming
more useful as a tool to direct and control molecular assembly.[1, 7,
23, 26-29] We have recently taken advantage of these interactions for
the creation of co-crystals for optoelectronic device applications.[8,
11] For example, we demonstrated that the crystal structures that
result from using pyridyl thiophene-based donors exhibit strong
halogen bond interactions, ranging from −7.5 to −8.7 kcal mol-1,
supplemented by other secondary interactions including
stacking interactions. In fact, the stacking interactions were
slightly larger in magnitude than the halogen bond interactions for
co-crystals containing (iodoethynyl)difluorobenzene.[11]
Pyrimidine (Pm, left-most structure in Figure 2) is a two-
nitrogen analogue of pyridine whose incorporation into building
blocks offers two sites for halogen bond interactions. We and
others have previously elucidated the effects of hydrogen
bonding by water, methanol, and other co-solvent molecules on
pyridine’s and Pm’s vibrational normal modes.[30-34] In particular,
[a] T. L. Ellington, P. L. Reves, B. L. Simms, J. L. Wilson, Prof. D. L.
Watkins, Prof. G. S. Tschumper, Prof. N. I. Hammer
Department of Chemistry and Biochemistry
University of Mississippi
P.O. Box 1848, University, Mississippi 38677 (United States)
E-mail: dwatkins@olemiss.edu
tschumpr@olemiss.edu
nhammer@olemiss.edu
Supporting information for this article is given via a link at the end of
the document.
Figure 1. Iodopentafluorobenzene includes a region of positive ESP indicated
by blue at the terminus of the molecule and a nucleophilic region of negative
ESP perpendicular to the R-X covalent bond (orange belt). A second
perspective with a solid surface (right) is included to clearly show both positive
and negative ESP regions. See Experimental Section for computational details.
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ARTICLE
we showed that electron density is transferred from Pm’s lone
pairs out into the extended hydrogen bonded network and that
this charge redistribution leads to blue-shifting in select normal
modes with the two properties being directly correlated. We also
recently demonstrated that this effect is present in surfaced
enhanced Raman spectroscopic (SERS) interactions and that
the magnitude of charge transfer from Pm to adjacent silver
atoms is only about twice that for water alone.[35]
As revealed in an extensive review, many previous
spectroscopic studies have concentrated on analysis of the
halogen bond donor and in particular, shifting in the donor’s C-X
stretching frequencies when participating in a halogen bond.[18]
Here, in contrast, we experimentally and computationally
investigate the effects of halogen bond interactions by a series
of haloaromatic donors on the acceptor Pm and explore the
possible presence of other forces present in solution such as
stacking interactions. We also compare the strength of the
interactions and magnitude of the spectral shifts to that
previously observed for hydrogen bonded systems. The
haloaromatic series and Pm is shown in Figure 2 and includes
bromobenzene (BrB), iodobenzene (IB), bromopentafluorobenzene
(BrPFB), and iodopentafluorobenzene (IPFB). Bromo- and iodo-
based halogen bond donors are selected as the electron
acceptors based on the polarizability of the halogen atoms.
Donors, BrPFB and IPFB, were employed as the inductive effect
provided by the fluoro substituents increases the magnitude of
the -hole on halogen atomin turn, increasing the strength of
the halogen bonding interaction. In addition to shifts in Pm’s
vibrational spectrum, we also show that Pm’s 1H and 13C NMR
are very sensitive to halogen bonded interactions.
Experimental Section
Raman spectroscopy
Either a LabView-controlled Jobin-Yvon Ramanor HG2-S double
grating Raman spectrometer with photomultiplier tube detection
or a Horiba Scientific LabRAM HR Evolution Raman
Spectroscopy system with CCD camera detection was used for
the acquisition of solution phase Raman spectra. Resolution of
these instruments is much less than 1 cm-1 and their use allows
for the detection of small vibrational energy shifts due to
noncovalent interactions.[32, 34-38] Solutions only contained Pm
and the corresponding haloaromatic donors.
NMR spectroscopy
1H and 13C NMR spectra were recorded on a Bruker Avance
DRX-500 (500 MHz) spectrometer and are reported in ppm
using the solvent as an internal standard (perdeuterated
toluene) at 2.08 ppm and 20.43 ppm (CH3).
Computational Methods
Full geometry optimizations and corresponding harmonic
vibrational frequency computations with Raman activities were
performed on each monomer and each halogen bonded
complex with the hybrid M06-2X[39] density functional in
conjunction with a correlation consistent basis sets augmented
with diffuse functions (aug-cc-pVTZ).[40-41] Additionally, the basis
set for bromine[42] and iodine[43] atoms contain a small-core
energy-consistent relativistic pseudo-potential (aug-cc-pVTZ-PP).
This prescription was selected based on the extensive
calibration recently performed by Kozuch and Martin.[44] For
brevity, the previously mentioned basis sets will be referred to as
aVTZ throughout this work. Further, a full natural bond orbital
(NBO) analysis was performed on all clusters in order to assess
the magnitude of charge transfer (|q|) between pyrimidine and
its halogen bond donor. A pruned numerical integration grid
composed of 99 radial shells and 590 angular points per shell
was employed along with a threshold of < 10-9 for the RMS
change in the density matrix during the self-consistent field
procedure. The threshold for removing linear dependent basis
functions (magnitude of the eigenvalues of the overlap matrix)
was tightened from 10-6 to 10-7. All electronic energies have
been converged to at least 1 × 10-9 Eh while the Cartesian forces
of the gradient did not exceed 1 × 10-5 Eh/a0. Furthermore, pure
angular momentum (i.e., 5d, 7f, etc.) basis functions were used
instead of their Cartesian counterparts (i.e., 6d, 10f, etc.). All
computations were performed using the analytic gradients and
Hessians available in the Gaussian09 software package.[45]
Electrostatic potential maps were constructed using a total
electron density isosurface value of 0.004 electrons Bohr-3.
Optimized Cartesian coordinates of each monomer and each
halogen bonded complex can be found in the Supporting
Information.
2. Results
2.1 Raman spectroscopic results
Figure 3 shows six spectroscopic regions of Raman vibrational
spectra of solutions of Pm and IPFB. Raman spectra of Pm
interacting with BrB, IB, and BrPFB are included in the
Supporting Information (Figure S1 - S3). Four of Pm’s normal
modes,6b, 1, 9a, and 8b, unambiguously exhibit blue shifts
(shifts to higher energy) of +4, +5, +2, and +5 cm-1, respectively.
Raman spectra of solutions with the other three haloaromatic
donors do not exhibit blue shifts. In fact, solutions of Pm with IB
and BrB actually yield a slight red shift for 1. The maximum
observed blue shifts in IPFB solutions are smaller than we
reported previously for Pm hydrogen bonding to water[32] (13, 14,
5, 12 cm-1), ethylene glycol[34] (9, 14, 3, 9 cm-1), or methanol[34] (6,
12, 3, 8 cm-1). These modes involve motions of Pm’s carbon
and nitrogen atoms and their blue shifts have been previously
associated with changes in the C-N bond lengths as a result of
charge transfer. The result that blue shifts are only observed
with IPFB immediately suggests that other competing
interactions are likely present in solution that are larger in
Figure 2. From left to right: pyrimidine (Pm), bromo- (BrB) or iodobenzene
(IB), and bromo- (BrPFB) or iodopentafluorobenzene (IPFB).
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ARTICLE
magnitude than the halogen bond and are playing more
dominate roles.
2.2 NMR spectroscopic results
NMR spectroscopy is commonly employed to identify the
signatures of noncovalent interactions and afford insight into the
strength of those contacts.[46-48] We complexed each halogen
bond donor with Pm at a 1:2 ratio in halogen bond promoting
solvent (i.e., 250 mM; toluene-d8). As the electron density at the
nitrogen atom changes due to halogen bond formation, the
nuclei bewteen the two nitrogen atoms (i.e., the C-H group)
becomes slightly more shielded. This shielding is observed as a
difference in chemical shift of a few hundreths of a ppm when
comparing the spectrum of neat Pm to that of a 1:2 complex of
the halogen bond acceptor and donor. Figure 4 shows the 1H
and 13C NMR spectra for the resonance signals that correspond
to the hydrogen and carbon (1H = 9.163 and 13C = 159.580 ppm)
residing between Pm’s nitrogen atoms. In the case of the 1H
NMR, essentially no perturbation is observed when Pm is
interacting with BrB or IB. While for halogen bonded complexes
of Pm with either BrPFB or IPFB, an upfield shift is observed
with IPFB showing the largest chemical shift difference (9.163 to
9.136 ppm). Conventionally, attention is given to the 13C NMR
spectra as carbon atoms are less susceptible to media (i.e.,
solvent, pH, etc.) effects.[46, 49] The carbon spectra for the
halogen bonding complexes show a similar trend to that of the
proton; however, an unexpected downfield shift is observed with
BrPFB (159.580 to 159.592 ppm). Such deviations are not
unprecedented. For example, Goroff and co-workers reported an
unusual effect in the 13C NMR spectra of iodoalkynes.[49] In this
case, the authors noted - experimentally and computationally -
that combinations of solvent and bonding environment promote
dramatic changes in the 13C NMR spectrum making it
significantly different than anticipated. Our results were
confirmed by changing the solvent from toluene-d8 to benzene-
d6 and also observing a similar trend with pyridine as the
halogen bond donor (Figure S4 - S11 of Supporting Information).
Nonetheless, the largest change in the 13C chemical shifts are
for IPFB from 159.580 to 159.509 ppm.
2.3 Computational results
Figure 5 shows optimized molecular geometries for Pm
interacting with one or two molecules of BrB, IB, BrPFB or IPFB
at the M06-2X/aVTZ level of theory. The addition of a second
halobenzene or halopentafluorobenzene is made possible due
the two nitrogen atoms of Pm. Through a vibrational analysis,
each optimized structure reported was verified as a minimum (ni
= 0) on the M06-2X/aVTZ potential energy surface. Of the
structures characterized in this work, the global minimum of
Pm/BrB and of Pm/IB are C1 structures that deviate only slightly
from a perfectly perpendicular Cs orientation of the two rings and
have halogen bond intermolecular separations of 3.11 and 3.14
Å, respectively, which correspond to reductions of the sum of the
van der Waals radii (rsvdW) of 8.5% and 11.0% relative to the
sum of nitrogen (1.55 Å) and the halogen (Br: 1.85 Å, I: 1.98 Å)
van der Waals radii.[50] A second low-energy co-planar Cs isomer
of Pm/BrB (+0.12 kcal mol-1) and of Pm/IB (+0.09 kcal mol-1) was
also found and with similar halogen bond intermolecular
separations of 3.10 Å (8.8%) and 3.11 Å (11.9%). In contrast,
the global minimum of each pentafluorohalobenzene (Pm/BrPFB
and Pm/IPFB) prefers Cs symmetry and has slightly shorter
halogen bond intermolecular separations of 2.93 Å (13.8%) and
2.95 Å (16.4%), respectively. For the case of Pm/BrPFB, a C1
symmetric analogue (+0.15 kcal mol-1) was found with the same
halogen bond intermolecular separation of 2.93 Å (13.8%). This
C1 symmetric analogue of Pm/IPFB collapses to the Cs
symmetric structure described above. The addition of a second
halo- or halopentafluorobenzene, leads to various low-energy
isomers whose relative energies (E) doesn’t exceed 0.25 kcal
Figure 3. Raman spectra of Pm/IPFB solutions with decreasing mole fraction of pyrimidine going from bottom to top.
Figure 4. 1H (left) and 13C (right) NMR spectra of the proton and carbon atom
of pyrimidine indicated by the circle.
10.1002/cphc.201700114
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ARTICLE
mol-1. Furthermore, changes in intermolecular separations and
binding energies per haloaromatic donor are similar to those
seen in complexes with a single donor, suggesting that
cooperative effects are minimal. The key feature differentiating
the 1:1 and 1:2 Pm:haloaromatic complexes is whether the rings
are co-planar or perpendicular.
For Pm/(IB)2 a non-planar Cs symmetric structure was
found to be the global minimum at the M06-2X/aVTZ level of
theory with a halogen bond intermolecular separation of 3.15 Å
(10.8%). A second energetically competitive structure of C2
symmetry was also found, though it was determined to be a
transition state (ni = 1) on the M06-2X/aVTZ potential energy
surface. Further, a third low-energy planar C2v local minimum
(+0.17 kcal mol-1) was found and has a halogen bond
intermolecular separation of 3.14 Å (11.0%). Upon fluorination,
the global minimum was identified as a planar C2v symmetric
structure with a halogen bond intermolecular separation of 2.96
Å (16.2%). Additionally, a slightly higher energy C2 isomer
(+0.22 kcal mol-1) was found to be a local minimum at this level
of theory with a halogen bond intermolecular separation of 2.98
Å (15.6%). A summary of these results along with computed
binding energies and results of the NBO calculations can be
found in Table 1. Table 2 compares the maximum experimental
frequency shifts with those computed for the molecular
complexes.
3. Discussion
The result here that four vibrational modes of Pm are observed
to blue shift when complexed with IPFB is reminiscent of the
effects of hydrogen bonding by water or methanol with Pm.
Earlier, we demonstrated that these shifts are directly
proportional to the magnitude of partial charge transfer from Pm
to the hydrogen bond donors.[34] Here, we also see perturbations
in the NMR spectra of Pm when complexed with the halogen
bond donors.
Traditionally, techniques such as 15N[51] and 19F NMR[52] as
well as solid state nuclear magnetic resonance (SSNMR)[53] are
employed to investigate displacements in chemical shifts due to
halogen bonding. However, these techniques are less practical
when compared to the standard 1H and 13C NMR where
specialized chemicals (e.g., isotopic labelling) and large sample
quantities are not necessary to achieve quality data. Studies
employing solution-phase NMR demonstrate that not only can
the presence of a halogen bond be determined but the relative
strength of the intermolecular interaction of various halogen
bond donors and their corresponding acceptors can also be
elucidated.[26, 54-59] For example, the measure of efficiency for a
halogen bond donor (Cl < Br < I) was originally established by
using 19F NMR spectroscopy.[60-61] Fluorinated halogen bond
Figure 5. Optimized molecular geometries of pyrimidine interacting with one or two molecules of BrB, IB, BrPFB, or IPFB at the M06-2X/aVTZ level of theory.
Table 1.
Point group symmetries, binding energies (Ebind), relative energies
(E), halogen bond intermolecular separations (Å
)
, van der Waals radii
reduction upon complexation (rsvdW, %), and magnitude of charge transfer
(|q|, me─) from pyrimidine upon complexation. Energies are reported in kcal
mol-1.
Complex
Symmetry
Ebind
E
RX···N
rsvdW
|q|
Pm/BrB
C1
−2.27
0.00
3.11
8.5
6.2
Cs
−2.16
+0.12
3.10
8.8
Pm/IB
C1
−3.38
0.00
3.14
11.0
13.2
Cs
−3.29
+0.09
3.11
11.9
Pm/BrPFB
Cs
−4.04
0.00
2.93
13.8
15.7
C1
−3.89
+0.15
2.93
13.8
Pm/IPFB
Cs
−5.83
0.00
2.95
16.4
30.0
Pm/(BrB)2-A
C2
−4.56
0.00
3.11
8.5
12.6
Pm/(BrB)2-B
C2
−4.32
+0.24
3.10
8.8
Pm/(IB)2
Cs
−6.69
0.00
3.15
10.8
24.5
C2v
−6.52
+0.17
3.14
11.0
Pm/(BrPFB)2
C2v
−7.88
0.00
2.95
13.2
27.0
Pm/(IPFB)2
C2v
−11.03
0.00
2.96
16.2
50.5
C2
−10.81
+0.22
2.98
15.6
Ebind = E(Complex) – E(Pm) – E(halo- or haloperfluorobenzene)
ΔE = Ebind(local minimum) – Ebind(global minimum)
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ARTICLE
acceptors result in definitively stronger interactions via an
inductive effect and result in large high-field shifts in chemical
signals upon halogen bonding formation.
Although solution-based NMR spectroscopy has shown to
be a convenient method towards detecting the occurrence of
halogen bonding, these studies are modest and place particular
emphasis on the halogen bond donor.[26] Nitrogen-containing
molecules like Pm are ubiquitous in self-assembling complexes,
yet their characterization within halogen bond assemblies has
been limited.[48, 62] This lack of data on nitrogen-containing
halogen bonded acceptors is mostly due to the complexity of the
molecules and strengths of interaction, affording less distinctive
differences when compared to that of non-interacting building
blocks.[49] Here, we employ our knowledge of the Pm structure to
note significant perturbations in the NMR spectra of Pm when
complexed with IPFB. The substantial difference in chemical
shift is most likely due to the polarizability of iodine and the
inductive effect of the fluoro groups increasing the localized area
of electron density on the atom. As the lone pairs on Pm’s
nitrogen atoms interact with the region of positive electrostatic
potential on the iodine atom in IPFB, the electron density of the
nitrogen atoms decrease as some of this density is transferred.
The NMR peaks corresponding to the proton and carbon nuclei
between the two nitrogen atoms thus becomes more shielded
(i.e., spatial shielding)[63] with complexation and are the most
affected. Although the deviations in NMR chemical shifts are
small (<1 ppm), they can be accounted for by weak
intermolecular interactions between the solvent and solute that
conceivably screen the effects of halogen bonding. Nonetheless,
the trend is consistent with the blue shifts observed in the
experimental Raman spectra.
Here, |q| for Pm/(IPFB)2 is computed to be significantly
larger than that for any of the other complexes. This
corresponds to the largest shifts in the NMR spectra and the
only observed blue shifts in the experimental Raman spectra.
Furthermore, as seen in Tables 1 and 2, the |q| and rsvdW
results of halogen bonded complexes display a clear trend and
track the increasing interaction strength. These results might
appear somewhat inconsistent with the computed vibrational
results in Table 2, which suggest that blue shifts should also be
observed for the other haloaromatic donors as well, especially
BrPFB. Other intermolecular forces, such as -type interactions,
are known to compete with halogen bonding in the condensed
phases[8, 11] and are likely competing here with halogen bonding
in these solutions. The numerical agreement between the
experimentally observed blue shifts for IPFB solutions and the
computed shifts for the 1:1 Pm:IPFB complex suggests that the
halogen bond interaction begins to dominate this interaction as
the halogen bond strength increases. The unexpected red shift
in Pm’s 1 mode with BrB and IB and the downfield shift in the
13C NMR spectrum of BrPFB, however, also suggest that other
interactions such as -stacking are competing with halogen
bonding.
In the cases of similar 1:2 structures of Pm interacting with
water or methanol, |q| at the B3LYP/6-311++(2df,2pd) level of
theory was previously reported to be 38 me─ in both cases.[32, 34]
At the M06-2X/aVTZ level of theory, |q| for these 1:2
complexes is 31 me─ for water and is 34 me─ for methanol. This
compares to 51 me─ for the Pm/(IPFB)2 structure computed here
at the M06-2X/aVTZ level of theory. The magnitudes of |q| for
Pm/(IB)2, Pm/IPFB, Pm/(BrPFB)2 are computed to be much
smaller, lying between 24 and 30 me─ with binding energies
between approximately 3 and 4 kcal mol-1. The binding energy
for Pm/(IPFB)2, on the other hand, is computed to be over 5 kcal
mol-1, similar in magnitude to the strength of hydrogen bonding.
Together, these results suggest that, in solution, an appreciable
interaction strength may be required for the halogen bond
interaction to dominate over competing molecular forces. For
example, Zarić and co-workers demonstrate the significance of
perturbations involving -stacking interactions and other
competing noncovalent interactions in pyridine and hydrated
pyridine.[64] We also recently showed that although hydrogen
bond interactions are possible in similar systems, only -
stacking interactions are energetically competitive with halogen
bonding bonding. For example, in that study we found that C-
H∙∙∙F interactions are only about 1 kcal mol-1, roughly 1/8th the
magnitude of halogen interaction energies.[11] Herrebout and co-
workers previously reported vibrational red shifts of benzene's
ring breathing mode when CF3X (where X=Cl,Br,I) is the
halogen bond donor and benzene is the acceptor. Such C-X∙∙∙
interactions would explain the red shifts observed here in Pm’s
ring breathing mode when complexed with BrPFB and Pm.
Wang and co-workers also previously showed that as the
halogen bond strength between pyridine and different
haloaromatic halogen bond donors increases, the - interaction
also strengthens.[65] Here, of all the Br containing complexes, the
lowest energy 1:2 Pm/(BrPFB)2 complex is the only one that
exhibits coplanarity, which could allow it to more easily form -
interactions and could explain the downfield shift in the 13C NMR.
Although the lowest energy 1:2 Pm/(IPFB)2 complex is also
coplanar, its halogen bond interaction strength is much greater.
Table 2. Select experimental and computed Raman vibrational frequencies (in cm-1) and their subsequent perturbations upon complexation. The original locations
are the experimental Raman vibrational frequencies of pyrimidine before complexation.
Mode
Original
Location
Bromobenzene
Iodobenzene
Bromopentafluorobenzene
Iodopentafluorobenzene
BrB
(BrB)2
Expt
IB
(IB)2
Expt
BrPFB
(BrPFB)2
Expt
IPFB
(IPFB)2
Expt
ν6b
626
+2
+4
−1
+2
+5
−1
+4
+8
0
+5
+11
+4
ν6a
681
+1
+1
−1
+1
+2
0
+1
+2
0
+1
+2
+1
ν1
990
+2
+3
−2
+4
+7
−2
+4
+9
0
+6
+10
+5
ν9a
1139
+2
+4
0
+1
+2
−1
+1
+3
+1
+1
+2
+2
ν8a
1564
+1
+3
−1
0
+1
0
0
+6
0
−1
+1
+2
ν8b
1565
+2
+4
0
+3
+4
0
+4
+8
0
+7
+8
+6
10.1002/cphc.201700114
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Conclusions
The effects of intermolecular interactions by a series of
haloaromatic halogen bond donors on the vibrational normal
modes and NMR chemical shifts of the acceptor Pm have been
investigated by both Raman and NMR spectroscopies, as well
as electronic structure computations. Strong halogen bond
interactions were observed to induce upfield shifts in NMR
spectra and blue shifts in certain vibrational normal modes of
Pm. The consistency between the NMR and Raman results
suggest that these methods may be very sensitive probes for
detecting such weak interactions in the condensed phases.
These results also suggest that there is a delicate balance of
intermolecular interactions competing in solution that could
affect solid state crystal structures and that a relatively strong
interaction strength on the order of a hydrogen bond is needed
for halogen bonding to dominate and govern observed
spectroscopic shifts.
Acknowledgements
This material (experimental part) is based upon work supported
by the National Science Foundation under Grant Numbers CHE-
0955550 and CHE-1532079 (N.I.H). This material
(computational part) is based on work supported by the
Mississippi Center of Supercomputing Research and the
National Science Foundation under Grant Numbers CHE-
1338056 and IIA-1430364 (G.S.T). D.L.W. appreciates financial
support of this work from Oak Ridge Associated Universities
through the Ralph E. Powe Award. We would like to also thank
John T. Kelly for contributions to this work.
Keywords: halogen bonding • ab initio calculations • vibrational
spectroscopy • Raman spectroscopy • NMR spectroscopy
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Entry for the Table of Contents
ARTICLE
Affecting the acceptor: The effects
of halogen bond interactions on the
acceptor pyrimidine are elucidated
using Raman and NMR
spectroscopies. Changes in
vibrational normal modes and
chemical shifts stem from charge
transfer to haloaromatic donors and
interactions. Computations reveal
some halogen bond binding energies
larger than a typical hydrogen bond.
Thomas L. Ellington, Peyton L. Reves,
Briana L. Simms, Jamey L. Wilson,
Davita L. Watkins,* Gregory S.
Tschumper,* and Nathan I. Hammer*
Quantifying the effects of halogen
bonding by haloaromatic donors on
the acceptor pyrimidine
((Insert TOC Graphic here))
10.1002/cphc.201700114
ChemPhysChem
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