J . A m . Chem. SOC.1984, 106, 2781-2787 loss of H. from 5 and 6 to give CH2SH+ (1) are calculated to be of similar magnitude to that for the intramolecular rearrangement, hydrogen scrambling may occur upon loss of H- or D- from CD3SH+*. (iv) In addition to the experimentally well-established dimethyl sulfide (10) and ethanethiol radical cations, the sulfonium ion CH2SHCH3+.(11) is likely to be an observable C2H6S++isomer. This species is found to lie 82 kJ mol-] above CH3SCH3+.,with a barrier to rearrangement to CH3SCH,+. of 120 kJ mol-'. The barrier to dissociation of 11 to give CH2SH+and CH3. is calculated to be 98 kJ mol-', with no reverse activation energy.
278 1
(v) The present results are consistent with the CH2SH+(1) ion rather than CH3S+(2) being formed by fragmentation of dimethyl sulfide at low ionizing energies. A two-step rearrangementdissociation mechanism is found to lead to formation of CH2SH+. This mechanism yields a barrier for the formation of CH2SHf from CH3SCH3+.which agrees well with that derived from the experimentally observed appearance energy for formation of CH2SH+ from dimethyl sulfide. Registry No. 1, 20879-50-9; 2, 20828-73-3; 3, 12538-93-1; 6, 8125583-6; loa, 34480-65-4; 11, 89277-97-4.
Overtone Spectral Investigation of Substituent-Induced Bond-Length Changes in Gas-Phase Fluorinated Benzenes and Their Correlation with ab Initio STO-3G and 4-21G Calculations Kathleen M. Gough and Bryan R. Henry* Contribution f r o m the Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2. Received August 31, 1983
Abstract: The gas-phase overtone spectra of eight fluorinated benzenes are measured in the CH-stretching regions corresponding to Av = 2 to 5 and are analyzed in terms of the local mode model. Peaks corresponding to inequivalent CH bonds in monofluorobenzene and the three difluorobenzenes are partially resolved. The frequency shifts are corn ared to uI, and the peaks are assigned to the various CH bonds on the basis of the substitutent effect. CH bond lengths are determined from the shift in the overtone peak frequency relative to benzene. These values are compared with bond lengths obtained from geometry-optimized ab initio molecular orbital calculations at the STO-3G and 4-21G (r:-i'G)levels. There is excellent agreement between the values of and r,$$'". In fact, the local mode analysis would appear to provide the best available technique for determining CH bond lengths in molecules of this size. The redistribution of electron population upon substitution is examined in terms of a bond-strength parameter, which is derived from the Mulliken population analysis. This parameter correlates well with the calculated and experimental bond lengths and provides a simple physical interpretation of the observed variation in rCH.
(4;)
I. Introduction The understanding of the effect of substituents on the properties of a parent molecule is of primary importance in the organization of chemical knowledge. In particular, studies on benzene and its substituted derivatives have formed a considerable area of research for many years. In an earlier paper,' we examined the higher CH-stretching overtones of 20 substituted benzenes in the liquid phase, using the local mode model2 A given C H bond behaves as an uncoupled anharmonic diatomic oscillator whose overtone transition energies are given by AEo-" (cm-') = wu + Xu2 (1) where w is the local mode frequency, X i s the diagonal local mode anharmonicity, and u is the CH-stretching vibrational quantum number. As w is dependent on bond strength3 and X i s sensitive to steric hindrance: unique C H bond types within a molecule can be spectrally resolved. In our liquid-phase study,' the lines were too broad to allow for resolution of inequivalent hydrogens, except in the case of nitrobenzene where a partially resolved doublet was observed. The assignment of the ortho hydrogens to the highfrequency peak was subsequently confirmed by partial deutera(1) Gough, K. M.; Henry, B. R. J . Phys. Chem. 1983, 87, 3433-3441. (2) Henry, B. R. "Vibrational Spectra and Structure"; Durig, J., Ed.; Elsevier: Amsterdam, 1981; Vol. 10, pp 269-319. (3) Greenlay, W. R. A.; Henry, B. R. J . Chem. Phys. 1978, 69, 82-91. (4) Henry, B. R.; Mohammadi, M. A,; Thomson, J. A. J . Chem. Phys. 1981, 75, 3165-3174.
t i ~ n . We ~ attempted to interpret the observed variation in the positions of the overtone frequencies of the substituted benzenes in terms of u,, the inductive part of the Hammett u, after the work of Katayama et aL6s7 The correlation of frequency shift with uI was only moderately successful, and the effect clearly ceased to be additive at higher levels of substitution. In the present work, we investigate the overtone spectra of a series of fluorinated benzenes in the gas phase. Preliminary results for 1,3-difluorobenzene have already been presented.* Because of the decrease in intermolecular interactions in the gas phase, the overtone bands are narrower. Partially resolved peaks are observed corresponding to absorption from inequivalent C H bonds. This resolution leads to a greater understanding of the correlation of frequency shifts with uI. McKean and his collaborators have investigated the fundamental CH-stretching transitions in molecules where all the hydrogens but one have been replaced by d e ~ t e r i u m . ~They have (5) Gough, K. M.; Henry, B. R. J . Phys. Chem. 1983, 87, 3804-3805. (6) Mizugai, Y.; Katayama, M. J . Am. Chem. Soc. 1980,102,6424-6426. (7) Mizugai, Y.; Katayama, M.; Nakagawa, N. J. Am. Chem. SOC.1981, 103, 5061-5063. (8) Henry, B. R.; Gough, K. M. "Photochemistry and Photobiology: Proceedings of the International Conference, Jan 5-10, 1983, University of Alexandria, Egypt"; Zewail, A. H., Ed.; Harwood Academic Publishers: Chur, Switzerland, 1983; Vols. I and 11. (9) McKean, D. C. Chem. SOC.Rev. 1978, 7, 399-422; private communication.
0002-7863/84/1506-2781$01.50/00 1984 American Chemical Society
2182 J . A m . Chem. Soc., Vol. 106, No. 10, 1984
Cough and Henry r
--6
~~~~
DIFLUO'IO BENZENES
c .CL I
i
1
1°K
i
1
Figure 1. Calculated and experimentally observed overtone spectra of fluorobenzene in the regions of Au = 3 (lower curve) and Av = 4 (upper
1
\
curve). The calculated band envelopes (dashed) represent the sum of Lorentzian peaks from a computer-assisted deconvolution of the experimentally observed overtone band. The experimental spectra were obtained in the gas phase at 86 "C.
observed an excellent correlation between this CH-stretching frequency, ij&, and rCH. Not surprisingly, similar correlations have been observed in the overtone In the present work we investigate the correlation between overtone frequency shifts and values of rCHdetermined from geometry-optimized ab initio molecular orbital calculations at the STO-3G13and 4-21G14 levels. We extend this correlation to changes in the electron population distribution and attempt a simple physical interpretation of the results.
Figure 2. The overtone spectra of gas-phase 1,2-difluorobenzene (bottom), 1,3-difluorobenzene (middle), and 1,4-difluorobenzene (top) at 86 OC in the region of Au = 3. Table 1. Deconvoluted Peak Positions and Local Mode Parameters molecule fluorobenzene
11. Experimental Section All of the compounds were obtained commercially at 97+ to 99+% purity and used without further purification. The spectra were measured in the gas phase with a multiple path length gas cell (Wilks Scientific Corp., South Norwalk, Conn., Model 5720) equipped with a heating jacket. The temperature of the gas cell was kept constant at 86 OC. The procedure that was used for loading sample into the gas cell has been described e1~ewhere.l~The path length was varied, dependent on the molecule and the overtone, in the range 0.75-14.25 m. The spectra were recorded on a Beckman 5270 spectrophotometer with near-infrared and visible light sources. The digitized signal was transferred to a Nicolet 1280 computer. The data were converted to a linear energy scale, with data points at 1.22-cm-' intervals. At AuCH = 4 and 5, the absorption was often very weak so several spectra were recorded and signals averaged. In all cases, background spectra were recorded from the evacuated cell for each spectral scan and were subtracted from the sample spectra. Each spectrum was deconvoluted with a Nicolet curve analysis program which fitted Lorentzian peaks to the experimental data. The experimental and calculated spectra were plotted and compared to evaluate the quality of the deconvolution fit. The quality of the fit was also evaluated from the root-mean-square deviation. A b initio molecular orbital calculations with complete geometry optimization at the STO-3G level were performed for all the fluorobenzenes studied. The GAUSSIAN 7016 program or MONSTERGAUSS," which employs the force gradient method, was used. The results of a complete geometry optimization of benzene, fluorobenzene, 1,3-difluorobenzene, and 1,3,5trifluorobenzene with the 4-21G split-vale'nce basis set have been published.'* Using these geometries, we repeated the calculations with the MONSTERGAUSS 4-21G basis set. Changes in the electron population (10) Hayward, R. J.; Henry, B. R. Chem. Phys. 1976, 12, 387-396. (11) Mizugai, Y . ;Katayama, M. Chem. Phys. Left. 1980, 73, 240-243. (12) Wong, J. S.; Moore, C. B. J . Chem. Phys. 1982, 77, 603-615. (13) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J . Chem. Phys. 1969, 51,
2657-2664. (14) Pulay, P.; Fogarasi, G.; Pang, F.; Boggs, J. E. J . Am. Chem. SOC. 1979, 101,2550-2560. Binkley, J. S.; Pople, J. A,; Hehre, W. J. Ibid. 1980, 102, 939-941. (15) Gough, K. M.; Henry, B. R. J . Phys. Chem. 1984,88, 1298. (16) Hehre, W. J.; Lathan, W. A.; Ditchfield, R.; Newton, M. D.; Pople, J. A. QCPE 1974, I O , 236. (17) Peterson, M. R.; Poirier, R. A. MONSTERGAUSS, Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, 1981, (18) Boggs, J. E.; Pang, F.; Pulay, P. J . Comput. Chem. 1982, 3, 344-353.
1,2-difluorobenzene
1,3-difluorobenzene
1,4-difluorobenzene 1,3,5-trifluorobenzene
I ,2,3,4-tetrafluorobenzene
I ,2,3,5-tetrafluorobenzene 1,2,4,5-tetrafluorobenzene
assipnment AU -v (cm") H(2)
3
4 5 H(3), 3 H(4) 4 5 H(3) 3 4 5 H(4) 3 4 5 H(2) 3 4 5 H(4) 3 4 5 H(5) 3 4 5 3 4 5 3 4 5 2 3 4 2 3 4 2 3 4
a
8892 I1617 14234 8840 1 1 535 14130 8889 11621 14242 8859 11570 14178 8958 1 1 727 14355 8920 11661 14278 8863 I 1 564 14173 8904 11650 14261 8980 1 1 761 14401 6076 8932 11681 6108 8980 I 1 743 6089, 608Ia 8964 1 1 709
w
(cm-')
X (cni")
3140
i
3
-58.6
3127
*6
-60.3
3135
i
I
-57.3
i
0.3
3129
i
4
-58.7
i
1.0
3160i 8
-57.5
i.
1.9
3150
2
-58.8
i
0.4
3133 i 8
-59.9
i
2.0
3143 i 6
-57.9
i
1.4
i
t
0.7 1.5
3164
t
8 -56.6
i
2.0
3155
i
3
-58.9
i
1.0
3172
I
3 -59.1
i
0.9
3159
i
6
i
1.8
-57.6
Line fit uses the center of the doublet at AU = 2
distribution with fluorine substitution were calculated from the Mulliken population analysis.
111. Results
Representative gas-phase overtone spectra of the eight fluorinated benzenes at AD = 2-5 are shown in Figures 1-8. In Figure
Overtone Spectra of Fluorinated Benzenes
J . Am. Chem. SOC.,Vol. 106, No. IO, 1984 2783
-_ - -.
I TETRAFLUORO- I
GEUZEYES 3z-U3R3-
II@ ,5
I'
'; 3 -
I
\ J"
C .'
d,C
7:
'
"
+
6,
U"i, < r
i
^
c
F (cm-')
DIFLUOROBENZENES
4 d r
Figure 6. The overtone spectra of gas-phase 1,2,3,4-tetrafluorobenzene (bottom), 1,2,4,5-tetrafluorobenzene(middle), and 1,2,3,5-tetrafluorobenzene (top) at 86 "C in the region of Av = 2 I
1
I
"-
Figure 4. The overtone spectra of gas-phase 1,2-difluorobenzene (bottom), 1,3-difluorobenzene (middle), and 1,4-difluorobenzene (top) at 86 "C in the region of Av = 5.
I ,3,5 i! TRIFLUOROBENZENE
'
! I ' 1
I 1
;
I
' I
I
v (ern-') Figure 5. The overtone spectra of gas-phase 1,3,5-trifluorobenzene at 86 "C in the region of A0 = 3.
1
I TETRAFLUORO - '
11
BENZENES
p
.'-..---
L - - - _ I "ZC. 3cC.1: : q 5
sy-
,
L (crn-I )
v (cm-I)
1)
n
~
--.
f
-1
i; (cm-l)
Figure 3. The overtone spectra of gas-phase 1,2-difluorobenzene (bottom), 1,3-difluorobenzene (middle), and 1,4-difluorobenzene (top) at 86 O C in the region of Aa = 4 I
' 2,3,42' I I '\-.--
ORTHO
%. 1 7 '
?'
BENZENES
I'
Figure 7. The overtone spectra of gas-phase 1,2,3,4-tetrafluorobenzene (bottom), 1,2,4,5-tetrafluorobenzene(middle), and 1,2,3,5-tetrafluorobenzene (top) at 86 "C in the region of Av = 3.
1, the calculated spectra (dashed line) from the deconvolution program are included for Av = 3 and 4 of fluorobenzene to illustrate the quality of the deconvolution fit. The positions of the partially resolved peaks, as determined by the deconvolution of each spectrum, are listed in Table I. The values were used in eq 1 to obtain the local mode parameters, w and X,from a least-squares fit. These parameters are also given in Table I. T h e peaks were assigned to the various inequivalent CH bonds on the basis of frequency shift and correlation with the molecular orbital calculations (vide infra). The full widths at half maximum of the deconvoluted peaks are given in Table 11. The errors for the frequencies and anharmonicities in Table I are determined from the least-squares fitting procedure. They arise from uncertainties in the peak positions due to spectral breadth or to uncertainties in the deconvolution procedure. A third cause for such errors could be the presence of unresolved com-
Gough and Henry
2784 J . Am. Chem. Soc.. Vol. 106, No. IO, 1984
Tablc 111. I:requcnq Shifts (cm-' ) Relative to Benzene' a t A U --. 2 to 5 for 1,'luorinated Benzenes in the Gas Phase
1 TETRAFLUOROBENZENES 1 I
molecule fluorobenzene
3
4
5
94 42 91 61
119
128
37 123 72
I ,3-difluorobenzene
160 122 65
229 163 66
48 170 106 282 207
1.4-difluoro-
106
152
101 188
182
263
329
104
134
183
135
182
245
117
166
21 1
assignment
2
1,2-difluorobenzene
benzene
I ,3,5- tr i flu or obenzene I ,2,3,4-tetra-
fluorobenzene I ,2,3,5-tetrafluorobenzene 1,2,4,5-tetrafluorobenzene
' I:rom
ref 19. l.084k
v (cm-l) Figure 8. The overtone spectra of gas-phase 1,2,3,4-tetrafluorobenzene (bottom), 1,2,4,5-tetrafluorobenzene(middle), and 1,2,3,S-tetrafluorobenzene (top) at 86 O C in the region of Av = 4. Table 11. Deconvoluted Gar-Phase Overtone Line Width5 (cm-' ) of Fluorinated Benzenes molecule
fluorobcnzene 1,2-difluoro-
assignment
H(4)
benzene I ,3-difluoro-
H(2)
benzene
H(4) Hf5)
1,4-difluorobenzene
1,3,5-trifluorobenzene I ,2,3.4-tetrafluorobenzene 1,2,3,5-tetrafluorobenzene I ,2,4,5-tetrafluorobenzene
2
H(2) H(3), H(4) H(3)
31
4
89
103
85
121
l:
70 98
78
92 46
5
3
52 54
72 91 88
45 39
73 80
101
31
65
72
31
63
81
26
35
100
l1
40
66
l3 17 23
11
Figure 9. The correlation between CH bond lengths in fluorinated benzenes, which are obtained from frequency shifts in the overtone spectra, and CH bond lengths from ab initio molecular orbital calculations at the 4-21G level.'* study of the gas-phase alkanes,I2 have noted a correlation between overtone frequencies at Au = 6 and C H bond lengths determined experimentally or from ab initio molecular orbital theories, respectively. In particular, Wong and MooreI2 obtained the relationship
(A) =
bination peaks. These peaks can contribute to the lower overtones and involve two quanta of a lower frequency normal mode. They lead to shifts in the overtone peak positions which result in deviations from the simple Birge Sponer relation (eq l ) . The uncertainties in the line widths depend primarily on three factors: the overtone (AD),the noise level, and the presence and number of inequivalent C H bonds. Thus we estimate the uncertainties in the line widths for the five molecules with a single C H bond type to be 110% (considerably
