Chemical insights from high-resolution X-ray photoelectron spectroscopy and ab initio theory: propyne, trifluoropropyne, and ethynylsulfur pentafluoride.

Abstract

High-resolution carbon 1s photoelectron spectroscopy of propyne (HC triple bond CCH3) shows a spectrum in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne (HC triple bond CCF3) and ethynylsulfur pentafluoride (HC triple bond CSF5), the ethynyl carbons show only a broad structure and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution. Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5 and CF3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.

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US Department of Energy
US Department of Energy Publications
University of Nebraska - LincolnYear 
Chemical Insights from High-Resolution
X-ray Photoelectron Spectroscopy and
ab Initio Theory: Propyne,
Trifluoropropyne, and Ethynylsulfur
Pentafluoride
Leif J. S∗
Knut J. Borve∗∗
Gary L. Gard§
Nora Berrah†
Thomas X. Carroll††
Rolf Winter¶
John D. Bozek‡
Edwin Kukk‡‡
T. Darrah Thomas?
∗University of Bergen
†Western Michigan University
‡University of California - Berkeley
∗∗University of Bergen
††Keuka College
‡‡Western Michigan University
§Portland State University
¶Portland State University
?Oregon State University
This paper is posted at DigitalCommons@University of Nebraska - Lincoln.
http://digitalcommons.unl.edu/usdoepub/42

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Chemical Insights from High-Resolution X-ray Photoelectron
Spectroscopy and ab Initio Theory: Propyne, Trifluoropropyne, and
Ethynylsulfur Pentafluoride
Leif J. Sæthre,*,‡Nora Berrah,§John D. Bozek,†Knut J. Børve,*,‡Thomas X. Carroll,|
Edwin Kukk,§,†Gary L. Gard,⊥Rolf Winter,⊥and T. Darrah Thomas*,∇
Contribution from the Department of Chemistry, UniVersity of Bergen, N-5007 Bergen, Norway,
Physics Department, Western Michigan UniVersity, Kalamazoo, Michigan 49008, AdVanced Light Source,
Lawrence Berkeley Laboratory, UniVersity of California, Berkeley, California 94720, Keuka College,
Keuka Park, New York 14478, Department of Chemistry, Portland State UniVersity,
Portland, Oregon 97207-0751, and Department of Chemistry, Oregon State UniVersity,
CorVallis, Oregon 97331-4003
ReceiVed June 12, 2001. ReVised Manuscript ReceiVed August 16, 2001
Abstract: High-resolution carbon 1s photoelectron spectroscopy of propyne (HCtCCH3) shows a spectrum
in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by
distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne
(HCtCCF3) and ethynylsulfur pentafluoride (HCtCSF5), the ethynyl carbons show only a broad structure
and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively
understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated
compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution.
Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights
into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies
and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5
and CF3have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.
Introduction
Over 30 years ago Siegbahn and co-workers1showed that
inner-shell ionization energies depend on the chemical state of
the atom from which the electron is ionized. The ionization
energy increases with the oxidation state of the atom or with
the electronegativity of the ligands attached to the atom. In
addition, it is a local probe of the ability of the molecule to
accept charge at a particular site. This close relationship between
inner-shell ionization energies and fundamental chemical prop-
erties has led to numerous measurements of these energies.
Correlations between core-ionization energies and other proper-
ties have been a rich source of insights into chemical phenom-
ena.
Despite the successes that have been achieved, progress in
the investigation of hydrocarbons has been slow. The differences
in carbon 1s ionization energies in such molecules, even for
chemically inequivalent atoms, are small compared to the
experimental resolution that has been available until recently.
A case in point is propyne, HCtCCH3, which is a prototype
for the aliphatic alkynes. These reactive molecules have a rich
and varied chemistry2and play an important role in organic
synthesis. In propyne the three carbon atoms have quite different
chemistry. The HCt unit is the site for electrophilic attack and
is the most basic and possibly the most acidic end of the
molecule. The central carbon, while susceptible to attack, is not
as reactive as the HCt carbon. The CH3group is not highly
reactive, but its acidity is only slightly different from that of
the HCt group. The inner-shell photoelectron spectroscopy of
this molecule provides a basis for a better understanding of its
chemistry and, by extension, that of more complex molecules.
Measurements of the carbon 1s photoelectron spectrum of
propyne by Cavell3showed only a single asymmetric peak.
Although he analyzed this in terms of three components, each
representing a contribution from one of the carbon atoms, Cavell
expressed reservations about both the positions and the assign-
ments of each component. It was, therefore, difficult to draw
firm conclusions about the chemistry of the molecule from these
measurements.
The development of high-brightness radiation sources at third-
generation synchrotrons coupled with high-resolution spectrom-
eters has allowed a striking change in the resolution of such
experiments. Whereas Cavell worked with a resolution of about
1.1 eV, it is now possible to achieve an experimental resolution
for carbon 1s photoelectron spectroscopy of less than 0.05 eV,
or about half the natural line width (proportional to the inverse
lifetime of the 1s hole). We present here the carbon 1s spectra
‡University of Bergen.
§Western Michigan University.
†University of California.
|Keuka College.
⊥Portland State University.
∇Oregon State University.
(1) Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.;
Hedman, J.; Johansson, G.; Bergmark, T.; Karlsson, S.-E.; Lindgren, I.;
Lindberg, B. ESCA, Atomic, Molecular and Solid State Structure Studied
by Means of Electron Spectroscopy; Almqvist and Wiksell: Uppsala, 1967.
(2) Patai, S. The Chemistry of the Carbon-Carbon Triple Bond; Wiley:
New York, 1978.
(3) Cavell, R. G. J. Electron. Spectrosc. Relat. Phenom. 1975, 6, 281-
296.
10729
J. Am. Chem. Soc. 2001, 123, 10729-10737
10.1021/ja016395j CCC: $20.00© 2001 American Chemical Society
Published on Web 10/03/2001
This article is a U.S. government work, and is not subject to copyright in the United States.

Page 3

of propyne, 3,3,3-trifluoropropyne, and ethynylsulfur pentafluo-
ride (HCtCSF5) to illustrate the chemical information contained
in such high-resolution spectra. In the propyne spectrum the
contributions from the chemically inequivalent carbons are
clearly resolved. Moreover, each peak in the spectrum has a
distinctive pattern of vibrational structure associated with it.
Comparison of these structures with those in model compounds
and with predictions of ab initio electronic structure theory
allows us to assign the peaks to the appropriate carbon atoms.
With this information at hand, we are able to investigate the
relationships between the ionization energies and the distinctive
chemical properties of the different carbon atoms. The spectrum
of trifluoropropyne shows not only the expected large shift in
ionization energy of the carbon to which the fluorine atoms are
attached but also significant modification of the positions and
vibrational structure for the peaks associated with the ionization
of the other two carbons. The spectrum of ethynylsulfur
pentafluoride reflects the similarity of CF3 and SF5 as elec-
tronegative substituents. Also included are spectra of ethyne4,5
and ethane,6-8which provide models for understanding the
spectra of interest.
Methods
Experimental. Propyne and 3,3,3-trifluoropropyne were obtained
from commercial sources. Ethynylsulfur pentafluoride was synthesized
following the procedure described by Terjeson et al.9
The procedures for measuring the carbon 1s photoelectron spectra
are the same as those we have used in other studies.4,5,10,11Photons of
320 and 330 eV were obtained from beamline 9.0.1 of the Advanced
Light Source of the Lawrence Berkeley National Laboratory. The gas-
phase photoelectron spectra were measured using a Scienta SES-200
electron spectrometer12(propyne and trifluoropropyne) or in a spherical-
sector electrostatic analyzer (ethynylsulfur pentafluoride). The combined
experimental resolution in the photoelectron spectra ranged from 45
meV for propyne and ethyne to 70 meV for the other molecules. The
spectrum of CF4 was measured simultaneously with each of the
compounds of interest to provide an internal calibration; this allows us
to align the spectra with a relative uncertainty of a few hundredths of
an electronvolt. The carbon 1s ionization energy in CF4(301.898 meV13)
provides an absolute calibration of the ionization-energy scale.
Computational. The intensity of vibrational lines in the photoelec-
tron spectra were computed according to the Franck-Condon principle
as described by Thomas et al.6For each site of ionization, this procedure
uses the change in equilibrium geometry upon ionization, as well as
the vibrational frequencies and normal modes for both initial and final
states. These properties were computed from ab initio theory using the
Gaussian-9814and MolCas-415suites of programs. The procedure is
implemented by a program, g2fc,16which uses the output of Gaussian-
98 to calculate the changes in normal modes and the corresponding
Franck-Condon factors for vibrational excitation.
In all cases, vibrational frequencies and normal modes were
computed from second-order many-body perturbation theory (MP2),17
using Dunning’s correlation-consistent basis sets of triple-? quality,
cc-pVTZ.18The core-ionized carbon atom was described by the carbon
cc-pCVTZ set,19but without the core-correlating d function. The ionized
core was represented by an effective core potential (ECP)20scaled to
account for only one electron in the 1s shell.21The harmonic frequencies
calculated in this way are slightly higher than the experimentally
observed fundamental frequencies. In the case of ethyne, propyne and
trifluoropropyne, we correct the frequencies by forming scaling factors
from the ratios of observed22to computed ground-state frequencies and
apply these to the computed frequencies of the core-ionized molecules.
All scaling factors are in the range 0.95-1.00, with 0.98 being a typical
value. For ethynylsulfur pentafluoride, experimental information on the
fundamental frequencies is lacking, and all computed vibrational
frequencies for this molecule were therefore scaled by a common factor
of 0.98. Finally, in the case of propyne, the symmetric carbon-hydrogen
stretching mode at the core-ionized methyl carbon atom was described
using an anharmonic Morse potential, with parameters determined in
our previous studies of methane.21
When computing structural changes upon ionization, we first applied
the MP2/cc-pVTZ approach to the case of propyne. Comparison of
results for methane and ethane obtained at this level with results from
very accurate calculations21,7indicates that the MP2/cc-pVTZ method
gives CH bond lengths for the core-ionized molecule that are too short
by 0.2 pm. When the lengths of the methyl CH bonds in core-ionized
propyne are increased by this amount, then this level of theory gives
accurate vibrational profiles of the carbon 1s spectrum. For trifluoro-
propyne, however, the results obtained using the MP2/cc-pVTZ method
are less than satisfactory. No correction factors were available for this
molecule, and to resolve this situation, and also to identify where our
theory fails, we have reoptimized the relevant structures at a higher
level of theory. In this the core hole is explicitly included and
variationally optimized,21and electron correlation is included in terms
of configuration interaction (modified coupled pair functional, MCPF).23
Atomic natural orbitals (ANO) were used as follows: H[3s2p], C,
F[4s3p2d], and for core-ionized carbon, C*[7s5p3d].24The computed
geometric changes provide an accurate description of the CF3part of
the carbon 1s spectrum of trifluoroproyne; the acetylenic part of the
spectrum has too little structure to serve as a test. The main flaw in
(4) Thomas, T. D.; Berrah, N.; Bozek, J.; Carroll, T. X.; Hahne, J.;
Karlsen, T.; Kukk, E.; Sæthre, L. J. Phys. ReV. Lett. 1999, 82, 1120-1123.
(5) Børve, K. J.; Sæthre, L. J.; Thomas, T. D.; Carroll, T. X.; Berrah,
N.; Bozek, J. D.; Kukk, E. Phys. ReV. A 2001, 63, 012506, 1-14.
(6) Thomas, T. D.; Sæthre, L. J.; Sorensen, S. L.; Svensson, S. J. Chem.
Phys. 1998, 109, 1041-1051.
(7) Karlsen, T.; Sæthre, L. J.; Børve, K. J.; Berrah, N.; Kukk, E.; Bozek,
J. D.; Carroll, T. X.; Thomas, T. D. J. Phys. Chem. A 2001, 105, 7700-
7706.
(8) Rennie, E. E.; Ko ¨ppe, H. M.; Kempgens. B.; Hergenhahn, U.;
Kivima ¨ki, A.; Maier, K.; Neeb, M.; Ru ¨del, A.; Bradshaw, A. M. J. Phys.
B: At. Mol. Opt. Phys. 1999, 32, 2691-2706.
(9) Terjeson, R. J.; Canich, J. M.; Gard, G. L. Inorg. Synth. 1990, 27,
329-332.
(10) Carroll, T. X.; Berrah, N.; Bozek, J.; Hahne, J.; Kukk, E.; Sæthre,
L. J.; Thomas, T. D. Phys. ReV. A 1999, 59, 3386-3393.
(11) Carroll, T. X.; Hahne, J.; Thomas, T. D.; Sæthre, L. J.; Berrah, N.;
Bozek, J.; Kukk, E. Phys. ReV. A, 2000, 61, 42503, 1-7.
(12) Berrah, N.; Langer, B.; Wills, A. A.; Kukk, E.; Bozek, J. D.; Farhat,
A.; Gorczyca, T. W. J. Electron Spectrosc. Relat. Phenom. 1999, 101-
103, 1-11.
(13) Myrseth, V.; Bozek, J. D.; Kukk, E.; Sæthre, L. J.; Thomas, T. D.
J. Electron Spectrosc. Relat. Phenom. 2001. In press.
(14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh,
PA, 1998.
(15) Andersson, K.; Blomberg, M. R. A.; Fu ¨lscher, M. P.; Karlstro ¨m,
G.; Lindh, R.; Malmqvist, P.-Å.; Neogra ´dy, P.; Olsen, J.; Roos, B. O.; Sadlej,
A. J.; Schu ¨tz, M.; Seijo, L.; Serrano-Andre ´s, L.; Siegbahn, P. E. M.;
Widmark, P.-O. MOLCAS 4.1; Lund University: Lund, Sweden, 1997.
(16) Børve, K. J. g2fc; University of Bergen: Bergen, 1999.
(17) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618-622.
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10730 J. Am. Chem. Soc., Vol. 123, No. 43, 2001Sæthre et al.

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the MP2 geometries turns out to be an exaggerated contraction of the
carbon-carbon single bond, but only by 0.8 pm out of a net change of
7.2 pm. Still, an error of 11% in the bond contraction is enough to
distort the spectrum significantly. Hence, our best geometries (MCPF/
ANO) were used together with normal modes and frequencies from
the MP2 calculation.
Shifts in vertical ionization energies as well as decomposition of
ionization energies and acidities into the separate contributions from
initial- and final-state effects were computed at the same level of theory
as was used for the normal-mode analyses. It turns out that the computed
shifts are quite sensitive to the choice of ground-state geometry, and
the reported values were therefore obtained in the experimental ground-
state geometries of propyne,25trifluropropyne,26and ethynylsulfur
pentafluoride.27
Data Analysis. To model the spectra, we have assumed that each
vibronic transition that appears in the spectra has a shape that results
from combining the effects of the lifetime of the core hole, the resolution
of the experiment, and the distortion of the spectrum that results from
interaction of the photoelectron with the carbon KVV Auger electron.
The instrumental resolution function is taken to be Gaussian with a
full width at half-maximum of between 45 and 70 meV, as appropriate
for the particular measurement. On the basis of our measurements on
methane,10ethyne,5and tetrafluoromethane,28we have used an intrinsic
width of 100 meV to account for the lifetime broadening for carbon
atoms not attached to fluorine and 80 meV for carbon in the CF3group.
The effects on the spectrum of interaction of the photoelectron and
Auger electrons are modeled using the approach of van der Straten et
al.29
Results
The spectra for the three hydrocarbons, propyne, ethyne, and
ethane, are shown in Figure 1, with that of propyne in the center.
Those of ethyne and ethane are above and below, to facilitate
comparison. The spectra for the ethynyl groups of the fluorine-
containing compounds are shown in Figure 2, with the spectrum
of propyne repeated in Figure 2A for reference. Figure 3 shows
the portion of the trifluoropropyne spectrum that is attributable
to the CF3group. In Figures 2 and 3 the points represent the
data, and the lines represent ab initio theoretical calculations
of the vibrational structure. The calculated curves are discussed
in more detail below.
Propyne. The spectrum of propyne shows three major peaks
of about equal intensity, each from one of the three inequivalent
carbon atoms. Each has associated with it a characteristic
vibrational structure, and this structure provides an unambiguous
means of assigning the peaks. For comparison, Cavell’s
spectrum3is indicated in Figure 1B by the dotted line; the
additional structure that is revealed at higher resolution is
striking.
Looking at the spectrum for ethane we see a pattern that arises
primarily from excitation of the carbon-hydrogen stretching
vibration accompanying core ionization.6-8The excitation of
this vibration leads to the pronounced peak at an ionization
energy about 400 meV greater than that of the main peak. The
shoulder on the high-ionization energy side of the main peak is
due to HCH bending.6-8Other features of this spectrum are
due to additional excitations of these modes. Comparison with
the spectrum of propyne shows a strong resemblance between
the peak labeled “c” in propyne and that of ethane. We are
therefore confident in assigning this peak to the CH3group in
propyne. This assignment is also in accord with our theoretical
calculations of both its shape (discussed below) and position
(Table 1).
In ethyne, the 105-meV difference in energy between the
ungerade and gerade carbon 1s orbitals5,30is noticeable in the
spectrum, producing a shoulder on the low-ionization-energy
side of the main peak and a double peak for the first vibrational
satellite. This effect does not exist in propyne, where the carbons
are inequivalent. On the other hand, there are contributions to
the propyne spectrum from CC single-bond stretching as a
shoulder on the high-ionization-energy side of the main peak;
these are not present in ethyne. Allowing for these differences
between ethyne and propyne, we can see that the vibrational
pattern of peaks “a” and “b” are similar to what is observed for
ethyne, and therefore we assign these two peaks to the acetylenic
part of the molecule.
To assign the two acetylenic peaks to their respective carbon
atoms we can rely on chemical intuition, theoretical calculations
of the ionization energies, or theoretical calculations of the
vibrational structure. We choose the last of these, since one of
our goals has been to develop theoretical models that can be
used to understand the vibrational structure that is apparent in
X-ray photoelectron spectra of polyatomic molecules. The
procedure, described in detail elsewhere,6provides intensities
and energies for all significant vibrational excitations. Com-
bining these with our knowledge of the instrumental resolution
and intrinsic line shape, we construct the vibrational profile for
each core-ionization. The three profiles for propyne are com-
bined using least-squares fitting where the only adjustable
parameters are a constant background and the overall height
and position of each profile.
Figure 2A shows a comparison of the theoretical calculations
(solid line) with the experimental data (circles). The calculation
reproduces the essential features of the spectrum in a satisfactory
(25) Lide, D. R., Ed. Handbook of Chemistry and Physics, 76th ed.; CRC
Press: Boca Raton, 1995.
(26) Cox, A. P.; Ellis, M. C.; Legon, A. C.; Wallwork, A. J. Chem. Soc.,
Faraday Trans. 1993, 89, 2937-2944.
(27) Zylka, P.; Christen, D.; Oberhammer, H.; Gard, G. L.; Terjeson, R.
J. J. Mol. Struct. 1991, 249, 285-295.
(28) Carroll, T. X.; Børve, K. J.; Sæthre, L. J.; Bozek, J. D.; Kukk, E.;
Hahne, J. A.; Thomas, T. D. Unpublished work.
(29) Van der Straten, P.; Morgenstern, R.; Niehaus, A. Z. Phys. D 1988,
8, 35-45.
(30) Kempgens, B.; Ko ¨ppel, H.; Kivima ¨ki, A.; Neeb, M.; Cederbaum,
L. S.; Bradshaw, A. M. Phys. ReV. Lett. 1997, 79, 3617-3620.
Figure 1. Experimental carbon 1s photoelectron spectra of ethyne,
propyne, and ethane. The dotted curve in B shows the previously
available data for propyne (ref 3).
Carbon 1s Photoelectron SpectroscopyJ. Am. Chem. Soc., Vol. 123, No. 43, 2001 10731

Page 5

way. For this fit, we have assigned peak “a” to the terminal
HCt carbon, “b” to the central tC- carbon, and “c” to the
-CH3carbon; the alternate choice for “a” and “b” produces a
distinctly inferior fit. We can, therefore, conclude with confi-
dence that the peaks should be assigned as indicated. This result
shows the power of using good ab initio calculations in
conjunction with a high-resolution spectrum to provide under-
standing that is not readily apparent from the data alone. It also
indicates that similar calculations can be used to aid in the
analysis of more complex, less resolved spectra.
3,3,3-Trifluoropropyne and Ethynylsulfur Pentafluoride.
The spectrum for trifluoropropyne shows two major peaks. One,
at high ionization energy, shown in Figure 3, is due to the CF3
carbon. In keeping with the effects that are typically observed
for fluorination, this peak is shifted by about 8 eV from the
CH3 peak in propyne. The solid line in Figure 3 shows the
results of a theoretical calculation of the vibrational spectrum,
adjusted in overall height and overall position by a least-squares
fit to the data. The vertical lines show the relative intensities
and positions of the main lines contributing to the spectrum.
The theoretical result indicates that the major vibrational
excitation involves the CC single-bond stretching mode and that
there are weaker but significant contributions from CF-stretching
and CCF-bending modes.
For the ethynyl carbons (Figure 2, B and C) both molecules
show a single, broad, featureless peak. The two ethynyl peaks
that are apparent in the propyne spectrum have both been shifted
to higher ionization energies by the electronegative CF3 and
SF5groups. There is a greater shift for the terminal CH carbon
than for the central carbon, with the result that the two peaks,
which are well separated in propyne, are superimposed in the
molecules with the electronegative substituents. The sharp
vibrational structure seen in propyne is not apparent for these
two compounds. The solid lines show the results of ab initio
calculations of the vibrational structure, with the overall area
and the positions for each contributing carbon adjusted by least-
squares to give a best fit to the data. In these fits, the two
contributing peaks are constrained to have the same area. In
Figure 2, B and C, the peak with the lower ionization energy is
that for ionization at the HCt carbon, and the peak with the
higher ionization energy is that for ionization at the tC-
carbon; the reverse ordering leads to an inferior fit for
trifluoropropyne but a comparable fit for ethynylsulfur pen-
tafluoride. Both the order shown in the figure and the magnitude
of the splittings for the two molecules are in agreement with
our theoretically calculated ionization energies. For ethynylsulfur
pentafluoride, we were unable to fit the spectrum using only
the calculated spectra. Other portions of the experimental carbon
1s spectrum as well as the sulfur 2p spectrum indicate the
presence of a small impurity in the sample. As a result, we have
included a third peak in this fit to represent the contribution
from an impurity.
There is considerably more vibrational excitation in these
molecules than in propyne. In Figure 2B each group contains
more than 150 lines, whose relative intensities and positions
are given by the theoretical calculations. In Figure 2C, more
than 500 lines contribute to each group. For both molecules,
the agreement between the observed and calculated spectra is
quite good.
Ionization Energies. The fits shown in Figures 2 and 3,
together with the calibration data, allow us to assign both
adiabatic and vertical ionization energies to these molecules.
The adiabatic ionization energy is the energy to produce the
ion in its vibrational ground state. The vertical ionization energy
corresponds to the ionization energy averaged over the vibra-
tional manifold. These ionization energies, together with several
others included for reference are given in Table 1. Also included
in this table are the experimental and theoretical values of the
vertical ionization energies relative to that of ethyne (average
of2Σg+and2Σu+).
Figure 2. Carbon 1s photoelectron spectra for propyne and the ethynyl
carbons in 3,3,3-trifluoropropyne and ethynylsulfur pentafluoride. The
points show the experimental data, and the solid lines show the results
of ab initio calculations of the vibrational profiles associated with the
ionization of each carbon. In C, the line marked with an asterisk
indicates the contribution of a possible impurity.
Figure 3. Carbon 1s photoelectron spectrum for the CF3group in 3,3,3-
trifluoropropyne. The points show the experimental data, and the line
shows the results of ab initio calculations of the vibrational profile.
The vertical lines indicate the positions and relative intensities of the
significant vibrational transitions. Each line is labeled with four
vibrational quantum numbers to identify the particular modes that are
excited.
10732 J. Am. Chem. Soc., Vol. 123, No. 43, 2001Sæthre et al.

Page 6

For propyne, the vertical ionization energies are in close
agreement with those reported by Cavell,3whereas those for
trifluoropropyne are about 0.2 eV lower than those he reported.
The close agreement for propyne is striking, considering the
resolution of the earlier experiments.
Discussion
Once the peaks in the spectrum have been assigned, we turn
to the chemical information that is contained in their positions
and structures. We look first at the vibrational structure and
then consider the vertical ionization energies.
Vibrational Structure. There is a striking contrast between
the vibrational structure seen in propyne, ethane, and ethyne
and that seen in trifluoropropyne and ethynylsulfur pentafluoride.
In the hydrocarbons, the structure is characterized by features
that are sharp within the limits set by the natural line width
and the resolution, whereas in the fluoro compounds the
vibrational structure is broad and featureless. This same contrast
is seen in the differences between the adiabatic and vertical
ionization energies, which reflect the average vibrational excita-
tion that accompanies core ionization. For the first set of
compounds these are typically about 150 meV, whereas for the
second set the differences for the ethynyl carbons are about 400
meV. The immediate reason for this difference can be found in
the calculated changes in equilibrium bond lengths and bond
angles between the neutral and core-ionized molecules. For
propyne, the typical change upon core ionization is a shrinkage
of the bonds attached to the core-ionized atom, with a magnitude
of 2-5 pm and a change in the CCH bond angle of 2° or less.
For trifluoropropyne, on the other hand, the CC single bond
increases in length by 6-10 pm upon core ionization of either
of the ethynyl carbons; for ethynylsulfur pentafluoride, the
corresponding increase is 13-15 pm. For both of these
compounds, the change in CCF or CSF bond angle is 4-5°.
These results suggest that core-ionization of either of the ethynyl
carbons in these molecules leads to a considerable weakening
of the CC single bond. Accompanying this is a softening of the
potential along this coordinate and a lowering of the vibrational
frequency in this mode. As a consequence, there is a large
vibrational excitation spread over many levels and, correspond-
ingly, a broad and featureless vibrational excitation pattern.
These differences can be understood in terms of the effects
of the fluorines on the molecular charge distribution. Upon core
ionization the valence orbitals on the core-ionized atom shrink
toward the core hole, and this shrinkage is, in general,
accompanied by a shortening of the bonds attached to this atom.
In addition, there is a redistribution of the valence electrons to
delocalize the positive charge. Thus each part of the ion can be
considered to carry part of the net charge. For trifluoropropyne
and ethynylsulfur pentafluoride the CF3 and SF5 groups are
highly polar and have the positive end of their dipoles pointed
toward the positively charged ethynyl group. Coulombic repul-
sion thus weakens the CC or CS single bonds, leading to the
observed bond lengthening and pronounced vibrational structure.
A similar effect is seen in the oxygen 1s ionization of CO2,
where the CO bond also lengthens upon core ionization31
presumably for the same reason. In propyne, however, the
negative end of the dipole of the CH3group points toward the
ethynyl group, with the result that the CC single bond is
strengthened by this interaction.
Ionization Energies. The agreement between the theoretical
and experimental relative ionization energies for propyne is
excellent, providing further confirmation of the quality of the
calculations and the correctness of the peak assignments. For
the hydrocarbons, the calculated shifts in ionization energy are
within 0.03 eV of the experimental values. For the fluorinated
compounds the discrepancies between experiment and theory
are somewhat larger, as much as 0.15 eV. Despite this problem,
theory and experiment agree as to the ordering of the ionization
energies in trifluoropropyne.
The data for propyne show two striking results. The carbon
1s vertical ionization energy for the CH3group in propyne is
1.04 eV higher than it is in ethane, and for the CH group it is
0.88 eV lower than in ethyne. The simplest picture of core-
ionization energy shifts is that they reflect the charge in the
vicinity of the core-ionized atom. In this view, these results
imply that the CH group in propyne is negative and that the
CH3group is positive. This charge distribution can be under-
stood in terms of two resonance structures:
In II there has been transfer of electrons from the CH3group
to the CH group. Corresponding to this charge distribution is a
measured dipole moment of 0.784 D;25our theoretical calcula-
tions are in agreement with this and show that the direction is
as indicated in II. Quantitatively, the transfer of about -0.05e
from the CH3carbon to the CH carbon can account for both
the dipole moment and the shifts in ionization energy. Thus, a
small contribution from II produces a significant effect on the
molecular properties.
Also contributing to the positive shift of the carbon 1s
ionization energy of the CH3carbon in propyne relative to that
in ethane is that in propyne this group is attached to an sp
hybridized carbon, which is more electronegative than the sp3
hybridized carbon in ethane. Transfer of electrons from the CH3
group to the -Ct carbon should be reflected in a decrease in
the ionization energy for this carbon, and such a shift is seen,
as indicated in Table 1. This shift is, however, only about one-
third of the shift seen for the HCt carbon, suggesting that this
effect is less important than the resonance effect. Moreover,
we will see below that most of the shift for the -Ct carbon is
not due to the ground-state charge distribution but to charge
rearrangement accompanying the core ionization.
For trifluoropropyne and ethynylsulfur pentafluoride, the two
carbon peaks are shifted to higher ionization energy relative to
(31) (a) Clark, D. T.; Mu ¨ller, J. Chem. Phys. 1977, 23, 429-436. (b)
Domcke, W.; Cederbaum, L. S. Chem. Phys. 1977, 25, 189-196. (c)
Kivima ¨ki, A.; Kempgens, B.; Maier, K.; Ko ¨ppe, H. M.; Piancastelli, M.
N.; Neeb, M.; Bradshaw, A. M. Phys. ReV. Lett. 1997, 79, 998-1001. (d)
Hahne, J. A.; Carroll, T. X.; Thomas, T. D. Phys. ReV. A 1998, 57, 4971-
4973.
Table 1.
Carbon 1s Ionization Energies
molecule atom
I, adiab.
eV
I, vert.
eV
∆I, vert.a
eV
∆I, calc.a
eV
ethyne
ethane
methane
propyne
HCt
-CH3 290.545b290.714b
CH4
290.689b290.844b
HCt
290.226
tC-
290.778
-CH3 291.610
HCt
291.689
tC-
291.789
-CF3
299.454
291.621
tC-
291.731
291.128b291.249b
00
-0.54
-0.41
-0.88
-0.32
0.51
0.78
0.89
8.30
0.85
0.97
-0.52
-0.41
-0.90
-0.34
0.52
0.71
0.79
8.43
0.73
0.79
290.374
290.929
291.755
292.025
292.144
299.548
292.101
292.222
trifluoropropyne
ethynylsulfur pentafluoride HCt
aRelative to ethyne (average of2Σg+and2Σu+).bReference 13.
Carbon 1s Photoelectron Spectroscopy J. Am. Chem. Soc., Vol. 123, No. 43, 2001 10733

Page 7

propyne, with the HCt carbon ionization energy being shifted
much more than the -Ct ionization energy. As a result, the
two peaks overlap in the photoelectron spectra and can be
disentangled only with the help of theory. The overall shift is
expected and arises from the high electronegativity of the CF3
and SF5groups. The differential shift can be understood in terms
of resonance structures similar to II, but working in the opposite
direction:
The ionization energies can be related to the basicity (reflected
in the proton affinity) and the acidity (or deprotonation energy).
Because both proton addition and core-electron removal produce
a positive charge at a particular site, proton affinities and core-
ionization energies are strongly correlated with one another. For
protonation of oxygen and nitrogen in molecules where the sites
of protonation and the geometric changes on protonation are
similar, there are linear correlations with a slope of about -1.32
These correlations have been valuable in assigning sites of
protonation and in assessing the geometric changes that occur
upon protonation. Until now, similar correlations have not been
developed for protonation of hydrocarbons because of the
difficulties of resolving closely spaced peaks in the carbon 1s
photoelectron spectra. With high-resolution spectra we see the
correlation in comparing propyne with ethyne. The proton
affinity of propyne is 0.89 eV higher than that of ethyne,33and
the ionization energy at the expected site of protonation, the
CH carbon, is 0.88 eV less than in ethyne. (In this case, the
geometric changes during protonation are not the same since
the vinyl cation formed from protonating ethyne is thought to
have a bridged structure.33However, the difference in energy
between the bridged and classical structures is only about 0.2
eV.) On the basis of these numbers and the carbon 1s ionization
energy of the HCt carbon in trifluoropropyne and ethynylsulfur
pentafluoride, we can predict that the proton affinity for
protonation at this carbon will be about 1 eV lower than in
ethyne.
Looking at acidity, or deprotonation, we note that factors that
tend to make an acidic site more negative will increase the
deprotonation energy, and conversely, those that make a site
more positive will decrease this energy. In propyne, as noted
above, the ionization energy for the CH carbon is 0.88 eV less
than in ethyne, suggesting that this carbon is more negatively
charged in propyne than in ethyne. Nevertheless, the deproto-
nation energies of the two molecules are about the same. At
the CH3carbon, the core-ionization energy is 1.04 eV greater
than in ethane, and the gas-phase acidity is, in keeping with
this, 1.6 eV less for propyne than for ethane.22There is an
apparent contradiction in these observations. In both cases there
is a shift of about 1 eV in ionization energy relative to that in
an appropriate reference compound. For the CH end of the
molecule, there is, however, almost no change in acidity,
whereas for the CH3end there is a change much larger than 1
eV. Reasons for this difference are explored below.
In discussing such phenomena as core ionization, protonation,
deprotonation, and electrophilic addition it has been useful to
consider separately the effects of the initial-state charge distribu-
tion and the final-state redistribution of charge that occurs in
response to the removal or addition of a proton or electron.
Traditionally, acidities and proton affinities (basicities) have
been explained in terms of the latter, often with emphasis on
resonance effects.34In the last 15 years, however, it has become
apparent that the initial-state charge distributions have a more
important influence on relative acidities and basicities than has
charge redistribution in the ion.35-37It is, therefore, of interest
to assess the relative importance of these quantities in core
ionization.
To a reasonable approximation, these effects can be expressed
in terms of simple expressions:35,38
where ∆I is the difference in core-ionization energy for a given
atom between one compound and another, ∆A is the difference
in Brønsted acidity (deprotonation enthalpy39), and ∆P is the
difference in proton affinity. The Vs are the potential energies
of a unit positive charge at either the nucleus from which the
core electron is removed (VI), the acidic proton (VA), or the
site of protonation (VP). The Rs represent the contribution of
charge redistribution in the ion to the energy change for the
process. A simplifying set of approximations is that
and
With these approximations, eqs 1 and 3 provide the basis for
the negative correlation between core-ionization energies and
proton affinities that has been mentioned above. Similarly, eqs
1 and 2 have provided a means for experimentally estimating
∆V and ∆R, since ∆I + ∆A ) -2∆R and ∆I - ∆A ) 2∆V.35,40
These quantities can also be obtained from theoretical calcula-
tions. The accuracy of our theoretically calculated values of the
ionization-energy shifts indicates that this level of theory should
also give reasonably accurate values of ∆V and ∆R.
For acidities the calculation of VA and RA presents no
problems. The potential energy of the acidic proton, VA, is easily
calculated. Then RA can be obtained from eq 2 and the
(32) Brown, R. S.; Tse, A. J. Am. Chem. Soc. 1980, 102, 5222-5226.
See also other references given in Sæthre, L. J.; Thomas. T. D.; Svensson,
S. J. Chem. Soc, Perkin Trans. 2 1997, 749-755.
(33) Aue, D. H. In Dicoordinated Carbocations; Rappoport, Z., Stang,
P. J., Eds.; John Wiley and Sons: Chichester, 1997; pp 105-156.
(34) (a) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 3rd ed.; Allyn
and Bacon: Boston, 1973; p 597. (b) Loudon, G. M. Organic Chemistry,
2nd ed.; Benjamin/Cummings: Menlo Park, 1988; p 824. (c) Streitwieser,
A.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry,
4th ed.; Maxwell: New York, 1992; p 486.
(35) Siggel, M. R.; Thomas, T. D. J. Am. Chem. Soc. 1986, 108, 4360-
4363.
(36) Bo ¨kman, F. J. Am. Chem. Soc. 1999, 121, 11217-11222 and
references therein.
(37) (a) Solomons, T. W. G. Organic Chemistry, 5th ed.; John Wiley:
New York, 1992; p 100. (b) Vollhardt, K. P. C.; Schore, N. E. Organic
Chemistry, Structure and Function, 3rd ed.; Freeman: New York, 1999; p
832. (c) Jones, M. Organic Chemistry, 2nd ed.; Norton: New York, 2000;
p 892.
(38) Davis, D. W.; Shirley, D. A. J. Am. Chem. Soc. 1976, 98, 7898-
7903.
(39) Gas-phase acidity is more conventionally defined as the Gibbs free
energy of deprotonation. We have chosen to use enthalpy here, since the
quantities that we are concerned with are more closely related to enthalpies
than to free energies.
(40) Smith, S. R.; Thomas, T. D. J. Am. Chem. Soc. 1978, 100, 5459-
5466.
∆I ) ∆VI- ∆RI
(1)
∆A ) -∆VA- ∆RA
(2)
∆P ) -∆VP+ ∆RP
(3)
∆VI) ∆VA) ∆VP) ∆V
(4)
∆RI) ∆RA) ∆RP) ∆R
(5)
10734 J. Am. Chem. Soc., Vol. 123, No. 43, 2001Sæthre et al.

Page 8

experimental acidity.22,33Results obtained in this way are
summarized in Table 2. (For ethynylsulfur pentafluoride no
experimental value of the acidity is known. This we have
calculated using the G3 method,41which reproduces the acidity
of the other molecules within 0.07 eV (1.6 kcal/mol)).
For the core-ionization energies, the electric potential at the
nucleus is only an approximation to the effect of the initial-
state charge distribution on the core-ionization energy, as both
the finite extent of the core orbital and valence-electron
correlation need to be taken into account. Toward that end, we
have developed an “extended Koopmans’ theorem,”42which has
been used to calculate the values of ∆VIgiven in Table 3. From
comparing results obtained at this level of theory with those
calculated by a more rigorous approach42we estimate the
uncertainty in these values to be about 0.1 eV. Values of ∆RI,
also given in this table, are obtained from the expression
∆I,vert ) ∆VI - ∆RI, where ∆I,vert is the experimentally
measured shift in vertical ionization energy. We have chosen
to use experimental values of ∆I rather than theoretical ones,
because whereas the values of ∆VI can be calculated fairly
accurately from ground-state wave functions, theoretical values
of ∆RI depend on hole-state calculations of the ionization
energies, which are less certain.
A comparison of values of ∆V and ∆R obtained from
experiment and from theory is given in Table 4. Although the
results from these different approaches differ in detail, they agree
in the overall picture. For the HCt carbon a negative shift in
potential is produced when the hydrogen in ethyne is replaced
by a methyl group. A positive shift is produced by replacing
the hydrogen with a CF3 or SF5 group, with a larger shift
associated with the SF5group. The values of ∆R for this carbon
are all positive by a few tenths of an eV relative to that for
ethyne, reflecting the higher polarizability of the substituents,
relative to that for hydrogen. A major disagreement is indicated
in the last line of Table 4, which considers the methyl group in
propyne relative to the methyl group in ethane. Here we see
that the values of ∆RIand ∆RAdiffer significantly. About half
of this difference arises from geometric relaxation. For most of
the molecules considered here, the effect of geometric relaxation
on the acidity is small and about the same for each molecule.
However, the anion formed from deprotonation of the methyl
group in propyne is the same as that formed by the deprotonation
of allene, and there is a large change in structure from one
having a single bond and a triple bond, as in I, to one having
two double bonds, as in II. Despite this problem, the three
methods agree that the potential at the methyl group in propyne
is much more positive than in ethane.
To see an overall picture of the results of these calculations,
we have plotted ∆VAversus ∆VIand ∆RAversus ∆RIin Figure
4. In both 4A and 4B, we have plotted the values for the HCt
carbons relative to that for ethyne; in Figure 4A we have also
included the values for the CH3carbon relative to that of ethane.
The uncertainties shown for the points reflect the 0.1 eV
uncertainty in the theoretical results, mentioned above. Also
shown in Figure 4A is a least-squares fit of a straight line to
the HCt points; this has a slope of 0.8. It is apparent that the
values of ∆VAand ∆VIcorrelate well with one another and that
eq 4 is a reasonable approximation. For ∆R, Figure 4B, the
range of values is smaller than for ∆V, and the scatter in the
results is more apparent. The straight line in this figure has unit
slope and passes through the point for ethyne. The other data
for the HCt group scatter around this line, indicating that eq
5 may provide a useful approximation but should be viewed
with some caution for differences in ∆R of less than 0.1 eV.
For propyne, the values of ∆V indicate that the potential at
the HCt end of the molecule is negative relative to what it is
in ethyne. This effect can be attributed to resonance structure
II. Similarly, the positive values of ∆R indicate that charge
redistribution lowers the energy of the processes relative to that
for ethyne. This effect can also be attributed to II, either
enhancement of it for core ionization, or reversal of it for
deprotonation. Because of the different sign of the ∆V term in
eqs 1 and 2 the effects of potential and relaxation reinforce one
another for core ionization and tend to cancel one another for
deprotonation, leading to a large shift in ionization energy but
to a small shift in acidity, as is observed. A similar effect is
seen for carboxylic acids.43For the methyl end of propyne, the
values of ∆V indicate that the principal difference in both
ionization energy and acidity arises from the more positive
potential at the CH3group in propyne relative to that in ethane.
(41) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople,
J. A. J. Chem. Phys. 1998, 109, 7764-7776.
(42) Børve, K. J.; Thomas, T. D. J. Electron Spectrosc. Relat. Phenom.
2000, 107, 155-161.
(43) Siggel, M. R. F.; Thomas, T. D. J. Am. Chem. Soc. 1992, 114, 5795-
5800.
Table 2.
Acidities from Theory
Acidities and Initial- and Final-state Contributions to
acidity, Aa
eV
∆A
eV
∆VA
eV
∆RA
eV
Shifts Relative to Ethyne
ethyne, HCt
propyne, HCt
trifluoropropyne, HCt
ethynylsulfide pentafluoride, HCt
16.38b
16.53c
15.40b
15.07d
0
0.15
-0.98
-1.31d
00
-0.39 0.24
0.73 0.25
0.91 0.40
Shifts Relative to Ethane
ethane, CH3-
propyne, CH3-
18.22b
16.58c
00
0.99 0.65
0
-1.64
a∆H298for the deprotonation reaction, RH f R-+ H+
mental value from ref 22.cExperimental value from ref 33.dCalculated
using G3 methodology, ref 41.
bExperi-
Table 3.
Ionization Energies
Initial- and Final-State Contributions to Carbon 1s
moleculeatom
∆I, vert
eV
∆VI
eV
∆RIa
eV
Relative to Ethyne
HCt
tC-
-CH3
HCt
tC-
-CF3
HCt
tC-
Relative to Ethane
-CH3
propyne
-0.88
-0.32
0.51
0.78
0.89
8.30
0.85
0.97
-0.51
-0.07
0.62
0.99
0.91
7.92
1.18
1.16
0.37
0.25
0.11
0.21
0.02
-0.38
0.33
0.19
trifluoropropyne
ethynylsulfur pentafluoride
propyne
1.041.02
-0.02
a∆RI) -∆I,vert + ∆VI
Table 4.
Experimental Energies and from Theory
∆Vexpta
eV
Comparison of Initial- and Final-State Contributions from
∆VI
eV
Relative to Ethyne
-0.51
0.99
1.18
Relative to Ethane
1.02
∆VA
eV
∆Rexptb
eV
∆RI
eV
∆RA
eV
HCtCCH3
HCtCCF3
HCtCSF5
-0.51
0.88
1.08
-0.39
0.73
0.91
0.36
0.10
0.23
0.37
0.21
0.33
0.24
0.25
0.40
HCtCCH3
1.340.990.30
-0.020.65
a∆Vexpt) (∆I,vert - ∆A)/2
b∆Rexpt) -(∆I,vert + ∆A)/2
Carbon 1s Photoelectron SpectroscopyJ. Am. Chem. Soc., Vol. 123, No. 43, 2001 10735

Page 9

For the -Ct in propyne, ∆V is small, suggesting only small
electron transfer between the methyl group and this carbon; most
of the shift in ionization energy relative to that for ethyne is
due to relaxation.
For trifluoropropyne and ethynylsulfur pentafluoride, the
major influence on the core-ionization energies and the acidities
is the initial-state charge distribution. However, for both of these
molecules, the final-state charge redistribution plays an impor-
tant role in determining the effects of the substituents. In both
cases, ∆VIand ∆RIare positive; they tend to cancel each other
in their effect on the ionization energy (eq 1) and to reinforce
each other in their affect on acidity (eq 2). Both of these
quantities are larger for SF5than for CF3. For ionization energy,
the larger ∆VIfor SF5is effectively canceled by the larger ∆RI,
with the result that ∆I is about the same for trifluoropropyne
as it is for ethynylsulfur pentafluoride. On the other hand, the
effect of SF5on acidity is much greater than that of CF3, because
of the larger value of both ∆V and ∆R.
Electronegativities. The pronounced shift in ionization
energy of the CH3group in propyne relative to that for ethane
reflects the electronegativity of the HCtC group. On the basis
of this shift, together with ionization energies of other CH3X
compounds, the electronegativity of this group falls between
those of bromine (2.96) and iodine (2.66).44This is considerably
smaller than the estimate (based on vibrational energies) of 3.3
for HCtC given by Wells45or a value of 3.03 calculated by
Bergmann and Hinze.46On the other hand, the average of the
ionization energy and the electron affinity for this group,22which
is the Mulliken definition of electronegativity,46gives an
electronegativity of 2.24, and a correlation between methyl
proton NMR shifts and electronegativities gives 2.5.45
For CF3and SF5, the methods proposed by Bergmann and
Hinze46give electronegativities of 3.36 and 3.65, respectively,
both between chlorine and fluorine. This high electronegativity
is reflected in the effects of both of these substituents on the
carbon 1s ionization energies and the acidities of trifluoropro-
pyne and ethynylsulfur pentafluoride. However, if we look at
shifts in ionization energy for the carbon attached to these
electronegative groups (0.89 eV for trifluoropropyne and 0.97
eV for ethynylsulfur pentafluoride, relative to ethyne), we find
that they are much smaller than is found for a carbon atom
attached to a single halogen (2.6 and 1.5 eV for fluorine and
chlorine in fluoro- and chloroethene).47On the other hand, if
we compare the acidities of trifluoropropyne and ethynylsulfur
pentafluoride with those of fluoro- and chloroethyne (calculated
using the G3 method41), we find that the effect of the single
halogens (-0.2 and -0.4 eV, respectively, relative to that for
ethyne) is much smaller than is seen for trifluoropropyne (-1.0
eV) and ethynylsulfur pentafluoride (-1.3 eV). Thus, one
approach indicates electronegativities less than that of chlorine
for these substituents and the other values greater than that of
fluorine.
Measurements of the photoelectron spectra of CF3CtCCF3
and CF3CtCSF5by Brant et al.48show a lower average carbon
1s ionization energy for the second compound than for the first.
From this they concluded that the trifluoromethyl group is more
electron-withdrawing than is the pentafluorosulfur group. This
ordering of the carbon ionization energies is the reverse of what
we have seen for trifluoropropyne and ethynylsulfur pentafluo-
ride. However, in both cases, the differences in ionization
energies are small and are influenced by relaxation energies as
well as by the electron-withdrawing abilities of the substituents.
The preceding discussion illustrates the problems of relying
on a single parameter such as electronegativity to describe
electrical effects. We have already noted the differential
influence of charge redistribution in the ion, which tends to
lower the effect of an electronegative group on raising the
ionization energy and to enhance its effect on decreasing the
deprotonation energy. This influence is recognized by including
the hardness, η, as well as the electronegativity in describing
the electrical effect of a substituent,46or by three-49and four-
parameter50descriptions of electrical effects. Qualitatively,
hardness reflects the charge flow in response to a change in
charge and is, therefore, closely related to the relaxation energy
R. Specifically, the relaxation energy should be proportional to
the polarizibility of the substituent, which, in turn, is expected
to be proportional to the reciprocal of the hardness.51Thus, a
small value of hardness implies a large relaxation energy. The
hardness is often calculated using the principle of electronega-
tivity equalization, and from the approach of Bergmann and
Hinze,46we find that the values of η for CF3 and SF5 are
considerably lower than those for fluorine and chlorine.
Consequently, the effects of these substituents should be strongly
modified by relaxation, as we have seen to be the case. Similar
conclusions can be reached from a consideration of Charton’s
three-parameter treatment of electrical effects.49
(44) Electronegativities on the Pauling scale from Atkins, P. Physical
Chemistry, 6th ed.; Freeman: New York, 1998; p 941.
(45) Wells, P. R. Prog. Phys. Org. Chem. 1968, 6, 111-145.
(46) Bergmann, D.; Hinze, J. In Structure and Bonding; Sen, K. D.,
Jørgensen, C. K., Eds.; Springer-Verlag: Berlin, 1987; Vol. 66, pp 145-
190.
(47) Sæthre, L. J.; Siggel, M. R. F.; Thomas, T. D. J. Electron Spectrosc.
Relat. Phenom. 1989, 49, 119-137.
(48) Brant, P.; Berry, A. D.; DeMarco, R. A.; Carter, F. L.; Fox, W. B.;
Hashmall, J. A. J. Electron Spectrosc. Relat. Phenom. 1981, 22, 119-129.
(49) Charton, M. Prog. Phys. Org. Chem. 1987, 16, 287-315.
(50) Taft, R. W.; Topsom, R. D. Prog. Phys. Org. Chem. 1987, 16, 1-83.
(51) Politzer, P. J. Phys. Chem. 1987, 86, 1072-1073.
Figure 4. Correlations between theoretical values of ∆VA and ∆VI
and between ∆RAand ∆RI. In A, results are shown for HCt relative
to ethyne and for the CH3carbon in propyne relative to ethane. The
straight line in A represents a linear fit to the HCt results. In B, only
the HCt results are shown. The line has unit slope and passes through
the point for ethyne.
10736 J. Am. Chem. Soc., Vol. 123, No. 43, 2001Sæthre et al.

Page 10

Conclusions and Summary
From an experimental point of view, we have seen that high-
resolution inner-shell photoelectron spectroscopy provides a new
level of detail in the carbon 1s photoelectron spectra of
hydrocarbons and related compounds. For propyne, where only
a single poorly resolved structure was seen previously, the
spectrum shows clear resolution of the contributions from the
three carbon atoms as well as the vibrational structure associated
with each type of carbon 1s ionization. For trifluoropropyne
and ethynylsulfur pentafluoride, the vibrational structure and
the contributions from the chemically inequivalent carbons in
the ethynyl group are not resolved. However, since the
experimental resolution is quite good, we can be confident that
this lack of resolution in the observed spectra is chemically
significant. Ab initio calculations provide a basis for analyzing
and interpreting these spectra.
Taking the core-ionization energies determined in these
measurements together with experimental gas-phase acidities
and theoretical calculations of the factors that influence these
quantities, we are able to gain insight into the substituent effects
of the methyl, ethynyl, trifluoromethyl, and pentafluorosulfur
groups. In particular, the analysis shows that in the neutral
molecule the ethynyl, trifluoromethyl, and pentafluorosulfur
groups are strongly electron-withdrawing relative to hydrogen
or methyl groups. This electron-withdrawing ability can be
understood in terms of conventional resonance structures and
the high electronegativity of fluorine. However, in assessing
the effect of these groups upon a process such as electron or
proton removal, it is also necessary to take into account both
the initial-state charge distribution and the final-state charge
redistribution. Moreover, the magnitude of the overall effect
depends on the sign of the particle removed. For example, for
core ionization, the initial-state and final-state effects may work
in opposite directions, so that a substituent such as SF5has the
initial-state influence strongly reduced by the final-state relax-
ation. By contrast, for acidity, the initial- and final-state effects
for SF5are in the same direction, with the result that SF5has a
very strong effect on acidity.
The striking difference between the sharp vibrational structure
seen in propyne and the rather broad, featureless peaks seen in
trifluoropropyne and ethynylsulfur pentafluoride also reflects
the charge distribution in these molecules. For propyne, the
intrinsic charge distribution of the methyl group leads to a
relatively stable core-ionized molecule, whereas for the other
two molecules, the highly polar nature of the CF3and SF5leads
to considerable weakening and lengthening of the CC and CS
single bonds upon core ionization, with a corresponding effect
on the degree of vibrational excitation.
With high-resolution inner-shell spectroscopy and ab initio
theoretical calculations, we have the possibility of probing the
effect of substituents at all of the carbon atoms in a molecule.
This ability holds promise for providing new insights into
substituent effects.
Acknowledgment. T.X.C. and T.D.T. acknowledge support
by the National Science Foundation under Grant No. CHE-
9727471. G.L.G. acknowledges support by the National Science
Foundation under Grant No. CHE-9904316 and the Petroleum
Research Fund (ACS-PRF No. 34624-AC7). E.K., N.B., and
J.D.B. acknowledge support from the Divisions of Chemical
and Material Sciences, Office of Energy Research, of the U.S.
Department of Energy. L.J.S. and K.J.B. thank the Research
Council of Norway (NFR) for support.
JA016395J
Carbon 1s Photoelectron SpectroscopyJ. Am. Chem. Soc., Vol. 123, No. 43, 2001 10737