Electronic structure of chiral halomethanes

Article abstract of DOI:10.1021/jp0116959

The electronic structure of chiral halomethanes was studied for the analysis of intermolecular interactions. Their photoelectron spectra contain unanmbiguously assigned bands which acts as an internal probe for such interactions. The study of the electron

Full text of DOI:10.1021/jp0116959

J. Phys. Chem. A 2002, 106, 465-468
465
Electronic Structure of Chiral Halomethanes
Igor Novak* and Dong Bo Li
Department of Chemistry, National UniVersity of Singapore, Singapore 117543, Singapore
Anthony W. Potts
Department of Physics, King’s College London, London WC2R 2LS, U.K.
ReceiVed: May 3, 2001; In Final Form: August 8, 2001
We report the study of the electronic structures of CHBrFI and CHClFI using HeI/HeII photoelectron
spectroscopy. The orbital interactions are discussed on the basis of high-level quantum chemical calculations
and empirical arguments.
Introduction
SCHEME 1
Chiral halomethanes are the smallest stable chiral molecules,1
and one of them (CHBrClF) has been the subject of several
investigations dealing with the important physical and chemical
aspects of chirality. The physical aspect comprises energy
difference between enantiomers which can be expected on the
basis of parity violating weak interactions. However, no energy
difference was observed in a high-resolution vibrational spec-
troscopic2 study of CHBrClF although experimental sensitivity
was 3 orders of magnitude higher than the size of the expected
enantiomeric effect. The chemical aspect comprises enantiomer
separation, measurement of optical rotation, and the determi-
nation of molecule’s absolute configuration.3,4 Recently, a
further important aspect of chirality emerged when “chiral
photoionization” was observed.5 To interpret asymmetry in
photoelectron distributions from chiral molecules (induced by
circularly polarized VUV light) the previous knowledge of
electronic structure of such molecules is, of course, essential.5
Besides the chirality aspect, the electronic structure of chiral
halomethanes allows the accurate determination of intramo-
lecular orbital interactions between geminal halogens. Out of
the five possible chiral halomethanes (enantiomeric pairs), two
(CHBrClF and CHBrClI) have been studied by UV photoelec-
tron spectroscopy.6,7 We report in this work the improved
synthetic procedure for the preparation and the electronic
structure analysis of CHBrFI and CHClFI. The remaining chiral
halomethane (CBrClFI) is the subject of our current research
program.
SCHEME 2
conveniently, CHClI2 can even react with mercuric fluoride at
room temperature to afford CHClFI in a reasonably good yield
and of high purity (Scheme 2).
Preparation of Bromofluoroiodomethane (1). CHBrI2 (8.7
g, 25 mmol) and mercuric fluoride (3.0 g, 12.5 mmol) were
charged into a 50 mL stopcock, round-bottomed flask under
nitrogen. The mixture was continuously stirred and slowly
heated to 60 °C for 1 h. After cooling, the products were
removed under high vacuum with a dry ice condenser to give
the light-sensitive liquid, which was redistilled under water-
aspirator to afford 1 (3.61 g, 60.4%) as the light-sensitive liquid.
bp 36-38 °C/30 mmHg (Lit.1 bp 50 °C/50 mmHg), H NMR
1
(300 MHz, CDCl3) δ 7.58 (d, JH-F ) 49.1 Hz); 13C NMR (75
MHz, CDCl3) δ 36.55 (d, JC-F ) 316.1 Hz); 19F NMR (282
MHz, CDCl3) (TFA) δ -15.60 (d, JF-H ) 53.3 Hz); EIMS
239.8 (M+); EIHRMS: m/z calcd for CHBrFI: 237.8289;
found: 237.8287; IR (neat): 908.6, 735.3, 673.2 cm-1
.
Preparation of Chlorofluoroiodomethane (2). The fresh
chlorodiiodomethane (25.4 g, 84 mmol) was charged into a 250
mL stopcock, round-bottomed flask under nitrogen. With
stirring, mercuric fluoride (10.0 g, 42 mmol) was added in small
portions at 0 °C under nitrogen. After addition, the resulting
mixture was allowed to warm to room temperature and stirred
for 36 h. The products were removed under high vacuum with
dry ice condenser to give a light-sensitive liquid, which was
redistilled under nitrogen to afford 2 as a light-sensitive liquid
(3.85 g, 23.6%). bp 74-76 °C (Lit.1 bp 35 °C/150 mmHg), 1H
NMR (300 MHz, CDCl3) δ 7.64 (d, JH-F ) 50.5 Hz); 13C NMR
(75 MHz, CDCl3) δ 55.04 (d, JC-F ) 306.2 Hz); 19F NMR (282
MHz, CDCl3) (TFA) δ -11.04 (d, JF-H ) 53.3 Hz); EIMS
193.8 (M+); EIHRMS: m/z calcd for CHClFI: 193.8794;
Experimental Section
Synthesis. The reported methods for the preparation of
CHBrFI and CHClFI involve the classical Hunsdiecker reactions
of silver bromofluoroacetate with silver chlorofluoroacetate
under demanding conditions, such as high temperature, anhy-
drous environment, etc.8 Since the use of pure, dry silver salts
is often difficult due to their thermal instability, we have tried
to find more convenient and milder methods for the preparation
of CHBrFI and CHClFI by starting from CHBrI2 and CHClI2,
respectively.9,10 CHBrI2 reacts with mercuric fluoride at 60 °C
to give high purity CHBrFI in a good yield (Scheme 1). More
found: 193.8798; IR (neat): 908.5, 735.0, 651.0 cm-1
.
Spectral Measurements. The HeI/HeII photoelectron spectra
* Author to whom correspondence should be addressed.
were recorded on the Perkin-Elmer PS 16/18 spectrometer at
10.1021/jp0116959 CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/15/2001

466 J. Phys. Chem. A, Vol. 106, No. 3, 2002
Novak et al.
TABLE 1: Experimental Ionization Energies (Ei/eV), Theoretical Ionization Energies (ROVGF/eV), Band Assignments, and
Vibrational Frequencies (ν/cm-1) for CHBrFI and CHClFI
band
Ei ((0.01 eV)
ROVGF
assignment
ν ( 80 cm-1
CHBrFI
X, A
B, C
D
E
F, G
H
9.86, 10.49
11.02, 11.55
12.83
13.66
16.8, 16.8
18.0
9.82, 10.01
10.80, 11.27
12.73
13.77
16.95, 17.27
nI
nBr
σ(CH/CI)
σ(CH/CBr)
nF, nF
σ(CF)
C2s
420 (C-Br str; X band)
1090 (C-F str; B band)
I
20.6
J
22.0
I5s
CHClFI
X, A
B, C
D
E
F
10.06, 10.64
11.62, 12.41
13.46
14.18
17.0
9.94, 10.05
11.30, 12.11
13.30
14.33
17.18
nI
nCl
410 (C-Br str.), 140 (C-I str.)
σ(CH/CI)
σ(CH/CBr)
nF
G
H
18.2
20.5
17.48
σ(CF)
C2s
I
22.1
I5s
room temperature. The HeI spectra were recorded with the
electron pass energy of 2 eV and the resolution of 15 meV as
measured on Ar+ 2P3/2 peak. The HeII spectra were measured
at 40 eV pass energy and 300 meV resolution in order to
generate sufficient signal. The rising background above 20 eV
in HeII spectra is due to high pass energy.
Calculations. The ab initio calculations were performed with
Gaussian 98 set of programs.11 The calculations were performed
with full optimization at the CCD level using 6-311+G(3df,-
3pd) basis sets for carbon, hydrogen, chlorine, and bromine
atoms and SDD effective core potential set13 for iodine (which
includes relativistic effects). Subsequently, the ROVGF method14
was used to obtain the ionization energies.
cross-section for Br4p and I5p orbitals on going from HeI to
HeII excitation.15 On the other hand, the F 2p photoionization
cross-section decreases only marginally upon change from HeI
to HeII. This leads to the relative band intensity increase for
F2p ionization as is shown by the increased intensity of 16.8
eV band. The assignment of inner valence ionizations at >20
eV (Table 1) was based on the comparison with the spectrum
of CH2ClI.16 However, it is well established that the ionizations
in the inner valence region cannot always be described on the
Results and Discussion
The photoelectron spectra of CHBrFI and CHClFI are shown
in Figures 1 and 2. The unambiguous assignment of both spectra
(Table 1) can be readily obtained by comparison with the
reported spectra of CHBrClF6 and CHBrClI7 (Figure 3). The
HeII measurements confirm the nature of orbital ionizations
(halogen lone pairs or σ-bonding orbitals) via the well-known
variation of relative band intensities upon increasing photon
energy. The first four bands (X-C) show a pronounced decrease
in their relative intensity on going from the HeI to HeII photon
excitation. This is due to a large decrease of photoionization
Figure 2. HeI and HeII photoelectron spectra of CHClFI.
Figure 3. Energy level diagram for chiral halomethanes studied in
Figure 1. HeI and HeII photoelectron spectra of CHBrFI.
previous works6,7 and this work.

Electronic Structure of Chiral Halomethanes
J. Phys. Chem. A, Vol. 106, No. 3, 2002 467
TABLE 2: Lone Pair Ionization Energy Splitting (∆Ei/EV)
halogen lone pair energy can be affected by resonance type
interactions with other halogen lone pairs and with bonding
σ-orbitals (whose energies are lower than Br, Cl, or I lone pairs).
In CF3I, the increased splitting implies a destabilizing
interaction with σ-orbitals since the F2p energies are consider-
ably lower than I5p and thus make F2p-I5p interaction unlikely.
In CHBrFI the increased splitting of Br4p suggests the
existence of strong I5p-Br interactions. The interactions with
σ(CX) orbitals are less prominent, because if this were not the
case one would expect an overall reduction in the Br4p splitting.
Also, the broadening of Br4p component at higher ionization
energy provides further evidence of strong Br4p-I5p interac-
tions. The broad, symmetrical bandwidth usually indicates more
bonding character and hence more delocalized nature of the
corresponding ionized orbital and this can be rationalized
through orbital mixing/interaction. This argument is based on
the Franck-Condon principle. Br4p band at higher ionization
energy is broader and has a symmetrical profile. I5p-Br4p
interactions would also lead to a decrease in I5p splitting. The
fact that none is observed suggests the predominance of
relativistic effects (see above).
in Halomethanesa,b
molecule
∆Ei (Br)
∆Ei (Cl)
∆Ei (I)
CH3Br21
CH3I21
0.32
0.62
CF2BrCl18
CH2BrCl19
0.30
0.36
0.30
0.31
19
CHBrCl2
19
CBrCl3
CF3I17
0.73
0.59
0.63
0.55
CH2ClI16
CHBrFI
CHBrClI7
CHBrClF6
CHClFI
0.23
0.53
0.35
0.33
0.22
0.55
0.79
0.58
a Only the splittings >0.2 eV are listed. b The UPSdata are from
references given as superscripts.
basis of Koopmans approximation (i.e., configuration interaction
type processes need to be invoked). We must thus add that these
ionizations have mixed C2s and halogen np character. The
expanded scans of some bands (not shown in Figure 1) revealed
vibrational progressions which were assigned by comparison
with other halomethanes16-19 (Table 1).
In the molecules with no symmetry, all intramolecular
interactions become possible and the separations between lone
pair energies provide a measure of such interactions. Further-
more, in halomethanes the small number of well resolved,
halogen lone pair bands makes these molecules suitable for fine
probing of intramolecular interactions.
In CHClFI a very pronounced splitting of Cl3p ionizations
is observed (Figure 3); the strongest in all halomethanes (Table
2). The existence of this splitting, both gives further evidence
for and can be rationalized by the “CHF effect” described
previously.6
Conclusion
The observed splitting in the bromine, chlorine, and iodine
lone pair ionizations (Table 2) reflects both the spin-orbit
coupling (SOC) and spatial interactions between orbitals local-
ized on different halogens and/or various σ-bonding orbitals (N.
B. Cl3p lone pairs have a very small relativistic SOC effects
<0.1 eV due to its relatively small atomic number). SOC
description strictly applies only to molecules with high sym-
metries. In an attempt to unravel different contributions to the
measured splitting we have used the ROVGF method which
does not include the relativistic SOC effect, but only spatial
interactions. The difference between the calculated and measured
splitting can then be used to gauge the relative contribution of
each effect. Tables 1 and 2 and Figure 3 suggest that for I5p
orbitals the SOC effect predominates, while for Cl3p only the
spatial interactions are important. For Br4p both effects are
important as can be seen, for example, from distinctly different
bromine lone pair splittings in CHBrFI vs CHBrClF (Table 2).
The spatial orbital interactions are often described in terms
of inductive and resonance effects. The electrostatic field nature
of inductive effect causes equal shifts in lone pair orbital
energies of both lone pair components (without changing the
measured splitting), while the resonance effect influences the
two components differently (and leads to measurable change
in the splitting). A similar approach was outlined previously
for dihaloalkanes X(CH2)nX (X ) Br, I, n) 1-5), but the
presence of two identical halogens (Br or I) gives rise to four
lone pairs bands.20 The existence of four bands complicates the
analysis and prevents the direct use of measured spin-orbit
coupling as the probe for intramolecular interactions. We use
experimental lone pair splitting for methyl bromide and methyl
iodide as references and search for discrepancies from these
values in other halomethanes as evidence of intramolecular
halogen-halogen interactions. Two halomethanes exhibit un-
usually large/small splittings in Br4p and I5p: CF3I exhibits
an increase in I5p, while CHBrFI shows an increased splitting
for Br4p. To understand these observations one must recall that
Chiral halomethanes are good case studies for detailed
analysis of various intramolecular interactions. Their photo-
electron spectra contain a small number of well resolved,
unambiguously assigned bands which can act as an internal
probe for such interactions. The interactions can thus be
analyzed without recourse to various theoretical models of
population analysis, which are often subject to ambiguous
descriptions of bonds, bond orders, and partial atomic charges
(e.g., Mulliken population analysis, NBO analysis, AIM method,
etc.)
Acknowledgment. We thank the CPEC Centre at the
National University of Singapore for financial assistance.
References and Notes
(1) Quadbeck-Seeger, H.-J.; World Records in Chemistry; Wiley-
VCH: Weinheim, 1999.
(2) Daussy, Ch.; Marel, T.; Amy-Klein, A.; Nguyen, C. T.; Borde, Ch.
J.; Chardonnet, Ch. Phys. ReV. Lett. 1999, 83, 1554.
(3) Constante, J.; Hecht, L.; Polavarapu, P. L.; Collet, A.; Barron, L.
D. Angew. Chem., Int. Ed. Engl. 1997, 36, 885.
(4) Constante-Crassous, J.; Marrone, T. J.; Briggs, J. M.; McCammon,
J. A.; Collet, A. J. Am. Chem. Soc. 1997, 119, 3818.
(5) Bo¨wering, N.; Lischke, T.; Schmidtke, B.; Mu¨ller, N.; Khalil, T.;
Heinzmann, U. Phys. ReV. Lett. 2001, 86, 1187.
(6) Novak, I.; Ng, S. C.; Potts, A. W. Chem. Phys. Lett. 1993, 215,
561.
(7) Novak, I.; Ng, S. C.; Jin, S.; Potts, A. W. J. Chem. Phys. 1997,
106, 9963.
(8) Haszeldine, R. N. J. Chem. Soc. 1952, 4259.
(9) Auger, M. V. C. R. Acad. Sci. 1908, 146, 1037.
(10) Dehmlow, E. V.; Stu¨tten, J. Liebigs Ann. Chem. 1989, 187.
(11) 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.;

468 J. Phys. Chem. A, Vol. 106, No. 3, 2002
Novak et al.
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.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, ReVision A.9;
Gaussian, Inc.: Pittsburgh, PA, 1998.
(12) Curtiss, L. A.; McGrath, M. P.; Blaudeau, J.-P.; Davis, N. E.;
Binning, R. C.; Radom. L. J. Chem. Phys. 1995, 103, 6104.
(13) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol.
Phys. 1993, 80, 1431.
(16) Novak, I.; Benson, J. M.; Potts, A. W. Chem. Phys. 1986, 107,
129.
(17) Cvitasˇ, T.; Gu¨sten H.; Klasinc, L.; Novak, I.; Vane`ik, H. Z.
Naturforsch. 1977, 32a, 1528.
(18) Cvitasˇ, T.; Klasinc, L.; Novak, I.; Gu¨sten, H. Int. J. Quantum Chem.
QCS 1980, 14, 305.
(19) Novak, I.; Cvitasˇ, T.; Klasinc, L.; Gu¨sten H. J. Chem. Soc. Faraday
Trans. 2 1981, 77, 2049.
(20) Novak, I.; Klasinc, L.; Kovae`, B.; McGlynn, S. P. J. Mol. Struct.
1993, 297, 383.
(14) Ortiz, J. V. J. Chem. Phys. 1988, 89, 6348.
(15) Yeh, J. J. Atomic Calculation of Photoionization Cross-sections
and Asymmetry Parameters; Gordon and Breach: Langhorne, 1993.
(21) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S.
Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules;
Japan Scientific Societies Press: Tokyo, 1981.