J. Phys. Chem. 1989, 93, 577-588 CO, is the major pathway. Formation of the 1,4-endo-peroxide such as that reportedIsa in the excitation of the C C T pairs of 2,3-dimethylnaphthalene and 1,2,3,4-tetramethylnaphthalenewith oxygen in various solvents is not observed, since an expected absorption band of the cis olefinic C-H out-of-plane mode a t 130-1565 cm-' is not found in the product spectrum. This implies the absence of singlet oxygen, 02(lAg), in the reaction of the CCT pairs of aromatic compounds and oxygen, as was the case with the alkenes studied before.'S2 Comparisons with the CCT Photochemistry of Alkenes. In our previous study of the CCT photochemistry of alkenes in oxygen matrices, four types of reactions were observed: (i) oxygen adduct formation observed for 2,3-dimethyl-2-butene for excitation in the visible region, (ii) cis-trans isomerization observed for cisand trans-2-butene, (iii) oxidative cleavage of the double bond to form corresponding carbonyl compounds in the UV region, and (iv) oxidative fragmention cleavage of the double bond to form C 0 2 and CO, which is more important as the excitation energy increases. In the excitation of the aromatics-oxygen pair, reactions of t y p (iii) and (iv) are commonly observed for all the compounds studied here. Therefore, it can be concluded that the oxidative cleavage of the double bond, either olefinic or aromatic, is the most characteristic type of reaction in the excited charge-transfer or ion-pair state of the pairs of unsaturated organic compounds and oxygen. The cis-trans isomerization is indicative of the formation of the triplet alkene. Although there is no direct evidence of the formation of the triplet state for aromatic compounds,
577
their lower reactivity compared to that of the alkenes would be attributable to a more efficient quenching process forming the triplet state; quenching of the fluorescence of benzene by oxygen has long been thought28to proceed by producing the triplet state via the C T state. Comparison with Catalytic Photooxidation on Metal Oxide. Catalytic photooxidation of organic compounds on a meta! oxide surface such as that of T i 0 2 has been t h o ~ g h t ' to ~ ~proceed - ~ ~ via the [D+...02-] state in the presence of oxygen. In this repsect it is very interesting to note that efficient COz formation (mineralization) in the photooxidation of toluene on TiOZhas been reported by Ibusuki and TakeuchLzo Although the reaction mechanism was not discussed, nearly complete oxidation of all the carbon atoms in toluene to COz may proceed via successive oxidative cleavage of the double bonds besides the unimolecular fragmentation of the carbonyl oxide type biradical in eq 9b. Sancier et aI.lgareported that irradiation of sand adsorbing organic compounds emits C 0 2 very efficiently. Such a process might also involve the ion-pair state and would be relevant to the C C T photochemistry studied here. Registry No. 02,7782-44-7; C02, 91-20-3; CO, 630-08-0; benzene, 7 1-43-2; toluene, 108-88-3;p-xylene, 106-42-3; mesitylene, 108-67-8; durene, 95-93-2; styrene, 100-42-5; naphthalene, 9 1-20-3;formaldehyde, 50-00-0; benzaldehyde, 100-52-7. (28) Brown, R. G.; Phillips, D. J . Chem. SOC.,Faraday Trans. 2 1974, 70, 630.
A Theoretical Study of the Harmonic Vibrational Frequencies and Infrared Intensities of XCH,CH,SCH,CH,X and XCH,CH,SH (X = H, CI) Carlos Sosa,t Rodney J. Bartlett,*.t KuHalim KuBulat,? and Willis B. Person*,+ Quantum Theory Project and Department of Chemistry, University of Florida, Gainesville, Florida 3261 1 (Received: January 25, 1988; In Final Form: June 20, 1988)
The atomic polar tensors, vibrational frequencies, and infrared intensities were calculated for the XCH2CH2SH and XCH2CH2SCH2CH2X(X = H, C1) series of molecules by using ab initio molecular orbital methods, with a split-valence basis set (3-21G). All the geometries were optimized at the SCF level. Atomic polar tensors were computed by analytical differentiation of the dipole moment at the Hartree-Fock level. The effects of correlation corrections as well as the effect of anharmonicities have been approximated by scaling the vibrational frequencies using a single scaling factor. The calculated force constants with the 3-21G basis set were used to obtain the potential energy distribution for each of the normal modes and to interpret and assign the experimental vibrational spectra of these molecules. For ethanethiol the calculated spectra for gauche and trans conformers can be compared with experimental spectra. The average deviation of the calculated wavenumbers (with the single scaling factor of 0.89) from the experipental values is less than 5%, with the worst discrepancy being for the vibrations involving the S atom. The calculated intensity pattern appears to be in good agreement with the experimental spectrum, and the calculated potential energy distributions (PEDs) appear to be in relatively good agreement with the experimental assignment for these ethanethiol molecules. An intensity sum rule is stated and examined, with surprisingly good agreement for these molecules. Because of the lack of experimental data for the larger molecules, no detailed comparison is made here between experimental and calculated spectra. However, we have compared our calculated spectra with those calculated (wavenumbers and PEDs, but no intensities) for diethyl sulfide and for 2,2'-dichlorodiethyl sulfide using the empirical transferable force constants recommended by Shimanouchi et al. for these molecules. The agreement is similar to that found for ethanethiol-good except for vibrations involving S and CI atoms, but with some other differences indicating that our calculated force field is not identical with the empirical Shimanouchi force field, leaving some room for future investigations.
Introduction In recent years the implementation of analytic gradient techniques for self-consistent (SCF) and correlated energies as well as molecular properties has proven invaluable for the prediction of vibrational frequencies and infrared intensities.I-l2 At the present time calculations of correlated harmonic frequencies are 'Department of Chemistry. 'Quantum Theory Project.
0022-3654/89/2093-0577$01.50/0
generally limited to small and medium size molecules.8-'0 However, the agreement between experimental and harmonic theo(1) Pulay, P. In Modern Theoretical Chemistry; Schaefer 111, H. F., Ed.; Plenum: New York, 1977. (2) Komornicki, A.; Ishida, K.; Morokuma, K.; Ditchfield, R.; Conrad, M. Chem. Phys. Lett. 1971, 45, 595. (3) Dupuis, M.; King, H. F. J . Chem. Phys. 1978, 68, 3998. (4) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S . Int. J . Quantum Chem. 1979, S13, 225.
0 1989 American Chemical Society
578 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 retical frequencies can be improved by scaling the theoretical v a l ~ e s . l ~ -The ~ ~ scale factor approximates the effects of correlation corrections as well as the effects of anharmonicity. This allows the prediction of vibrational frequencies for large plyatomic systems such as the XCH2CH2SCH2CH2Xseries of molecules and provides a guide to the interpretation of vibrational spectra for such complicated molecules. On the other hand it is not possible to scale the predicted absolute infrared intensities to bring them into better agreement with experiment. The question that we wish to explore in this series of studies of the infrared spectra of the halogenated derivatives of thiols and thioethers is whether the theoretical calculations predict spectra that are close enough to the experimental assignment of the experimental spectra. These molecules provide an interesting challenge for the theory because of the heavy sulfur and chlorine atoms. We are particularly interested in whether the predicted frequencies obtained from the calculated frequencies using a single scaling factor are useful for assigning the experimental spectra of these molecules. Experimental results from studies of Raman spectra of 2,2'dichlorodiethyl sulfide (mustard) (ClCH2CH2SCH2CH2Cl),2chlorodiethyl sulfide (ClCH2CH2SCH2CH3),and diethyl sulfide (CH3CH2SCH2CH3)have recently been reported by Christesen.I6" These results have been analyzed and compared with theoretical Raman intensities.'6b The interpretation of the experimental spectrum is complicated by a lack of detailed knowledge of the structure of these three large molecules as well as by the lack of knowledge of characteristic frequency values for the many different possible vibrational modes. On the other hand a large number of experimental studies have been publishedL7for ethanethiol (CH3CH2SH) which is closely related to the above series of molecules. Theoretical studies have been carried out at the semiempirical level by Dewar and co-workers'* for CH3CH2SH. Ohsaku and co-workers have reported the first a b initio S C F calculations on CH3CH2SH.19
Sosa et al.
Figure 1. Optimized geometry for (A) gauche-ethanethiol; (B) transethanethiol; and ( C ) 2-chloroethanethiol.
A (5) (a) Goddard, J. D.; Handy, N. C.; Schaefer 111, H. F. J . Chem. Phys. 1979, 71, 1525. (b) Brooks, B. R.; Laidig, W. D.; Saxe, P.;Goddard, J. D.; Yamaguchi, Y.; Schaefer 111, H. F. J . Chem. Phys. 1980, 72, 4652. (6) Krishnan, R.; Schlegel, H. B.; Pople, J. A. J . Chem. Phys. 1980, 72, 9654. (7) (a) Osamura, Y.; Yamaguchi, Y.; Schaefer 111, H. F. J . Chem. Phys. 1981, 75, 2919. (b) Osamura, Y.; Yamaguchi, Y.; Schaefer 111, H. F. J. Chem. Phys. 1982, 77, 383. (8) (a) Harrison, R. J.; Fitzgerald, G . B.; Laidig, W. D.; Bartlett, R. J. Chem. Phys. Lert. 1986, 124, 291. (b) Fitzgerald, G. B.; Cole, S. J.; Bartlett, R. J. J . Chem. Phys. 1986, 85, 1701. (9) (a) Fitzgerald, G . B.; Harrison, R. J.; Laidig, W. D.; Bartlett, R. J. J. Chem. Phys. 1985,82,4379. (b) Harrison, R. J.; Fitzgerald, G. B.; Laidig, W. D.; Bartlett, R. J. Chem. Phys. Lett. 1985, 117, 433. (IO) Bartlett, R. J. In Geometrical Derivatives of Energy Surfaces and Molecular Properties; Jargensen, P., Simons, J., Eds.; NATO AS1 Series; D. Reidel: Boston, 1986; p 35. ( I 1) (a) Amos, R. D. Chem. Phys. Lett. 1984, 208,185. (b) Amos, R. D. Geometrical Derivatives of Energy Surfaces and Molecular Properties;
Jargensen, P.; Simons, J., Eds.; NATO AS1 Series; D. Reidel: Boston, 1986. (12) (a) Yamaguchi, Y.; Frisch, M. J.; Caw, J. F.; Schaefer 111, H. F.; Binkley, J. S . J . Chem. Phys. 1986, 84, 2262. (b) Frisch, M. J.; Yamaguchi, Y.; Gaw, J. F.; Schaefer 111, H. F.; Binkley, J. S. J. Chem. Phys. 1986, 84, 531. (c) Sosa, C.; Schlegel, H. B. J . Chem. Phys. 1987,86, 6937. (13) Pople, J. A.; Schlegel, H. B.;Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A,; Hout, Jr., R. F.; Hehre, W. J. Int. J . Quantum Chem. Symp. 1981, 15, 269. (14) DeFrees, D. J.; McLean, A. D. J . Chem. Phys. 1985 82, 333. (15 ) Fogarasi, G.; Pulay, P. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1985; Vol. 14, and references therein. (16) (a) Christesen, S.D., private communication. (b) KuBulat, K. Ph.D. Dissertation, University of Florida, 1989. (17) (a) Hayashi, M.; Imaishi, H.; Ohno, K.; Murata, H. Bull. Chem. SOC. Jpn. 1971, 44, 872. (b) Hayashi, M.; Imaishi, H.; Kuwada, K. Bull. Chem. Soc. Jpn. 1974,47,2382. (c) Nakagawa, J.; Kuwada, K.; Hayashi, M. Bull. Chem. SOC.Jpn. 1976,49,3420. (d) Schmidt, R. E.; Quade, C. R.J. Chem. Phys. 1975,62, 3864. (e) Manocha, A. S.; Fateley, W. G.; Shimanouchi, T. J. Phys. Chem. 1973, 77, 1977. (18) (a) Dewar, M. J. S.; Lo, D. H.; Ramsden, C. A. J. Am. Chem. SOC. 1975, 97, 1311. (b) Dewar, M. J. S.; McKee, M. L. Compur. Chem. 1983, 4 . 84. (19) Ohsaku, M.; Ichiishi,T.; Imarnura, A,; Hayashi, M. Bull. Chem. SOC. Jpn. 1984, 57, 2791.
B
C
Figure 2. Optimized geometry for (A) diethyl sulfide; (B) 2-chlorodiethyl sulfide; and (C) mustard gas (2,2'-dichlorodiethyI sulfide).
In this study we shall present results from ab initio calculations of the equilibrium geometries, harmonic vibrational frequencies, and absolute infrared intensities for the XCH2CH2SCH2CH2X and XCH2CH2SH (X H , C1) series of molecules. In order to determine partially the effect of conformational changes, we have carried out calculations for the gauche and trans conformers of CH3CH2SH. Because detailed experimental studies have been reported for e t h a n e t h i ~ l , we ' ~ shall be able here to examine some details of the comparison between the predicted spectrum and the experimental spectrum. For the larger molecules the results from the recent experimental studiesI6 have not yet been reported so that we shall consider this comparison later.'6b Shimanouchi and co-workers have studied diethyl sulfide experimentallya and have (20) (a) Ohta, M.; Ogawa, Y.; Matsuura, H.; Harada, I. Shimanouchi,
T. Bull. Chem. SOC.Jpn. 1977,50, 380. (b) Shimanouchi, T.;Matsuura, H.; Ogawa, Y.; Harada, I. J . Phys. Chem. Ref. Data 1978, 7, 1323; 1980, 9, 1149. (c) Matsuura, H.; Tasumi, M. Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1983; Vol. 12.
XCH2CH2SCH2CH2Xand XCH2CH2SH (X = H, C1)
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 579
TABLE I: Optimized Geometries for trans-Ethanethiol, gauche-Ethanethiol, and 2-Chloroethanethiol
HSCHICH, type
bond" 2- 1 3-2 4-1 5-2 6-2 7-3 8-3 9-3
length 1.901 [1.899Id(1.820: 1.829c) 1.532 [1.530](1.529,1.530) 1.353 t1.3531 (1.322,1.328) 1.078 [ 1.0781 (1.090,1.088) 1.078 [ 1.0791 1.083 [1.083](1.092,1.093) 1.083[1.083] 1.084 [ 1.0861 (1.095) HSCHZCH, angle" 4- 1-2 98.2 [97.0Id(96.2?95.2c) 1-2-3 108.3 [112.4](108.6) 1-2-5 107.8 [105.8](109.4,110.9) 1-2-6 107.8 [105.8] 3-2-5 111.3 [111.8](110.2,109.6) 3-2-6 111.3 [111.8] 5-2-6 110.0 [108.9](108.9) 2-3-7 110.7 [110.8](110.6) 2-3-8 110.7 [110.8] 2-3-9 109.7 [109.9](109.7,109.7) 7-3-8 108.7 [108.5](108.1) 7-3-9 108.5 [108.8](108.9) 8-3-9 108.5 [108.8] 4- 1-2-3 180.0 [63.2] 1-2-3-8 60.0 [59.8](58.8e) 5-2-3-9 60.0 [61.2](60.V)
c-s
c-c
H-S H-C H-C H-C H-C H-C
type H-S-C
s-c-c
S-C-H S-C-H C-C-H C-C-H H-C-H C-C-H C-C-H C-C-H H-C-H H-C-H H-C-H H-S-C-C S-C-C-H H-C-C-H
c-s c-c H-S H-C
HSCHzCHzCl bond" 2- 1 3-2 4- 1 5-2
length 1.902 1.513 1.352 1.076
H-C
7-3
1.074
type
c1-c
9-3 HSCHzCH2Cl anglea 4-1-2 1-2-3 1-2-5
type H-S-C
s-c-c S-C-H
1.895
97.3 106.9 108.6
C-C-H
3-2-5
111.0
H-C-H C-C-H
5-2-6 2-3-7
110.6 113.0
c-c-c1
2-3-9 7-3-8 7-3-9
108.2 111.7 105.1
H-C-H H-C-C1
"See Figure 1 for definitions of angles and bond lengths. Angles are in degrees and lengths in angstroms. bExperimental results from ref 17b. Experimental results from ref 17d. dOptimized parameters for the gauche-CH3CH2SHare in square brackets. TABLE 11: Ootimized Geometries for Diethvl Sulfide, 2-Chlorodiethvl Sulfide. and 2,2'-Dichlorodiethvl Sulfide at the HF/3-21G Level
type
c-s c-c
CH,CH2SCH2CH, length bond" 1.891 2- 1 4-2
1.533
H-C
6-4
1.085
H-C
8-2
1.080
H-C
12-4
1.083
type
c-s-c s-c-c S-C-H C-C-H H-C-H
s-c-c S-C-H
H-C-H C-C-H C-C-H H-C-H H-C-H
CH,CH2SCH2CH, angle" 2- 1-3 1-2-4 1-2-8 4-2-8 8-2-9 1-3-5 1-3-10 10-3-1 1 2-4-6 2-4- 1 2 6-4- 1 2 12-4-13
100.1 109.1 107.9 111.1
109.6 109.1 107.9 109.6 110.0 110.6 108.5 108.6
CH3CH2SCH2CH2CI bond' length c-s 2- 1 1.894 1.895 c-s 3- 1 c-c 4-2 1.513 c-c 5-3 1.532 1.899 6-4 c1-c H-C 7-5 1.084 H-C 8-2 1.078 10-3 1.079 H-C 12-4 H-C 1.074 14-5 1.083 H-C CH,CHZSCH~CH~C~ type angle' c-s-c 2-1-3 99.4 s-c-c 1-2-4 107.6 S-C-H 1-2-8 108.6 C-C-H 4-2-8 110.9 H-C-H 8-2-9 110.2 s-c-c 1-3-5 108.8 S-C-H 1-3-10 107.6 C-C-H 5-3-10 111.4 H-C-H 10-3-1 1 109.9 c-c-c1 2-4-6 108.5 C-C-H 2-4- 1 2 113.1 C1-C-H 6-4- 1 2 104.9 H-C-H 12-4-1 3 111.7 C-C-H 3-5-7 109.7 C-C-H 3-5-14 110.7 H-C-H 7-5-14 108.5 H-C-H 14-5-15 108.7 type
type
c-s c-c c1-c
ClCH2CH2SCH2CH2CI length bond" 2- 1 1.896 4-2
1.513
6-4
1.895
H-C H-C H-C H-C
8-2 1.077 10-3 1.077 12-4 1.074 14-5 1.074 ClCH2CH2SCH2CH2CI type angle" c-s-c 2-1-3 98.7 s-c-c 1-2-4 107.4 S-C-H 1-2-8 108.5 C-C-H 4-2-8 111.0 H-C-H 8-2-9 110.5 s-c-c 1-3-5 107.4 S-C-H 1-3-10 108.5
c-c-c1 C-C-H C1-C-H H-C-H
2-4-6 2-4-1 2 6-4-1 2 12-4-13
108.3 113.0 105.0 111.7
"See Figure 2 for definitions of angles and bond lengths. Angles are in degrees and lengths in angstroms. developed a set of empirical transferable force constants that they have used to predict spectra of chlorine- and sulfur-containing normal paraffins.20b.c Since the spectra calculated by using these empirical force constants agree quite well with experimental spectra where such comparisons have been made, we may expect that calculated spectra with their force constants20b,care a reasonable representation of the experimental spectra. For this reason
we have included values calculated from their force constants20b,c in Tables VI1 and IX for diethyl sulfide and for ClCH2CH2SCH2CH2C1,respectively, as some kind of comparison of our calculation with earlier experimental results. However, a more detailed and critical analysis of the comparison between our calculated results and the experimental spectra of the molecules larger than ethanethiol will be given elsewhere.16b
580 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
Sosa et al.
TABLE 111: Total Energies and Dipole Moments Calculated at the SCF Level with the 3-21G Basis Set
primitive functions 165 141 117 99 75 75
molecule CICH,CH,SCH,CHXl CICH;CH;SCH;CH; CH3CH2SCH2CH3 CICH2CHzSH trans-CH3CH2SH gauche-CH3CH2SH
contracted functions 91 80 69 54 43 43
E, au -1465.423 61 -1008.70761 -551.98973 -93 1.0637 1 -474.347 25 -474.347 66
IA D 1.305 3.060 1.990 1.868 2.193 2.316 (1.560 & 0.032)"
"Experimental value in parentheses, obtained from ref 17b.
Method The results for calculated geometries are collected in Tables I and 11. The equilibrium geometries for trans-ethanethiol, gauche-ethanethiol, 2-chloroethanethiol, diethyl sulfide, 2chlorodiethyl sulfide, and 2,2'-dichlorodiethyl sulfide (mustard) were optimized by using the GAUSSIAN-82 system of programs2' with a split-valence (3-21G)22 basis set. The optimization gave the local minimum all-trans, C, structures for mustard gas and diethyl sulfide shown in Figure 2. For 2-chlorodiethyl sulfide, trans-ethanethiol, and 2-chloroethanethiol (Figures 1 and 2) the all-trans geometries have C, symmetry. For gauche-ethanethiol, the symmetry is Cl. There are expected to be other local minima on the potential energy surface that should also be explored. Vibrational frequencies were computed with the ACES system of programs23using analytical second derivatives at the Hartree-Fock levelz4with the 3-21G basis set. At the S C F level, the absolute infrared intensities were computed by analytically differentiating the dipole moment with respect to the Cartesian coordinates'2c of the nuclei
where E',,, are the density matrix elements, (vlcull.~)are the dipole moment integrals, and C U are , ~ the Cartesian components (x, y, and z ) ; A designates the Cartesian displacement coordinates referred to center A. The derivatives of p,,,,are obtained by solving The atomic polar tensors were the C P H F equation^.^^*^^ transformed from Cartesian coordinates to normal modes, Qi, to obtain the absolute infrared intensities in the double harmonic approximation by26 Ai =
l2l
TABLE I V Harmonic Vibrational Frequencies, Infrared Intensities, and Potential Energy Distribution Analysis (PED) Predicted for zauche -Ethanethiol
wavenumbers, cm? normal coord scaledb exptlC 1 2966 2980 2917 2967 2 3
2912
2930
4
2897
2902
5 6 7 8 9 10 11 12
2847 2333 1485 1482 1461 1408 1273 1256
2875 2571 1462 1452 1437 1377 1269 1246
13
1083
1093
14 15 16
1041 913 846
1051 970 867
17
711
740
18 19 20 21
580 311 241 189
658 319 241 191
2
( 4 ~ tN= o ) 3 0 0 0aQi ~
where N is Avogadro's number and c is the velocity of light.
Results and Discussion Geometries. The optimized geometries for tram-ethanethiol, gauche-ethanethiol, and 2-chloroethanethiol are summarized in Table I. The total energies and dipole moments computed for the optimized geometries are collected in Table 111. The C-S bond length predicted a t the S C F level with the 3-21G basis set for trans- and gauche-CH3CH2SH overestimates the experimental value by almost 0.1 A. The C-C bond length as well as the H-S and H-C bond lengths predicted for the same molecules agree with the reported experimental parameters within f0.02 A. The largest change in geometrical parameters from trans- to (21).Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; Whiteside, R. A,; Schlegel, H. B.; Fluder, E. M.; Pople, J. A. GAUSSIAN 82; Carnegie-Mellon University: Pittsburgh, 1983. (22) Binkley, J. S.; Pople, J. A,; Hehre, W. J. J . Am. Chem. SOC.1980, 102, 939. (23) ACES (Advanced Concepts in Electronic Structure), an ab initio program system, authored by: Bartlett, R. J.; Purvis 111, G. D.; Fitzgerald, G. B.; Harrison, R. J.; Lee, Y.S.; Laidig, W. D.; Cole, S.J.; Magers, D. H.; Salter, E.A.; Trucks, G. W.; Sosa, C.; Rittby, M.; Pal, S.Quantum Theory Project, University of Plorida, Gainesville, FL, 1987. (24) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Inr. J . Quantum Chem. 1979, S13, 225. (25) Garret, J.; Mills, I. M. J. Chem. Phys. 1968, 49, 1719. (26) Wilson, E. B.; Decius, J. C.; Cross, R. C. Molecular Vibrations; McGraw-Hill: New York, 1955.
A,
km/mol PED**d 9.2 CH, asvm str (94-) 25.9 CH; sym str (55+ j CH, asym str (14+), ip CH, asym str (30-), op 16.7 CH2 sym str (32+) CH, asym str (62+), op 6.5 CHI sym str (12-) CH, asym str (80+), ip 22.2 CH, sym str (94-) 28.7 S-H str (loo-) 3.7 CH3 asym def (87+), ip 11.7 CH, asym def (86+), op 6.6 CH2 sciss (99-) 6.6 CH, sym def (loo+) 31.1 CH2 wag (84-) 0.7 CH2 twist (61+) CH3 rock (25+), op 10.6 HSC bend (21-) CH2 twist (19-) CH, rock (24+), op CHI rock (20-) 4.0 CH, rock (59+), ip 8.2 C-C str (86+) 16.1 HSC bend (52+) CH2 twist (lo-) CH, rock (24+), op 2.0 HSC bend (24-) CH, rock (lo+), op CH2 rock (60+) 14.1 C-S str (87-) 1.1 S(CH2) def (86+) 2.5 CC tors (91+) 31.5 S-H tors (94-) XAi 259.7
"Percentage contributions greater than or equal to 10% are given in parentheses. bScale factor is 0.89 for all modes. cFrom ref 30a. dDefinitions of symmetry coordinates are the same as given by Shimanouchi et a1.20b*cNote that "XCH2 def" in these definitions corresponds to the XCC bends. The + and - signs give the relative phases of each symmetry coordinate in the normal coordinate (see ref 20b,c). str = stretch; def = deformation; tors = torsion; sciss = scissors; ip = in plane; op = out of plane. gauche-ethanethiol (except for the S C C H torsional angle) is for the SCC angle which is predicted to be 108.3O for the trans isomer compared to 112.4' for the gauche structure. Comparing CH3CH2SH with ClCH2CH2SH, the change from hydrogen to chlorine appears to have little effect on the C-S, C-C, H-S, and H-C bond lengths in the HSCH2- fragment; the difference between the calculated values of these bond lengths in 2-chloroethanethiol compared to CH3CH2SH appears to be only on the order of 0.002 A. The H - C bond lengths of the CICH2- fragment are calculated to be shorter by about 0.01 8,than the H-C bond lengths in the CH3- group of the CH3CH2SH molecule. The 3-21G basis set appears to predict bond angles that agree with the experimental angles within *2O. The largest discrepancy in Table I occurs in the prediction of the H C S angle (1-2-5) of the
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 581
XCH2CH2SCH2CH2Xand XCH2CHzSH (X = H , CI) TABLE V Harmonic Vibrational Frequencies, Infrared Intensities, and Potential Energy Distribution Analysis (PED) Predicted for trans -Ethanethiol
wavenumbers, cm-’ normal coord scaledb AVC 1 2967 +1 2 2918 +1 3 +4 2916 4
2903
5 6 7 9 10 11 12
2855 2335 1488 1484 1465 1408 1274 1252
13
1072
8
14
1023
15
924
16
807
17
769
18 19 20 21
586 292 232 158
A,
km/mol PEDasd 14.7 CH, asvm str (95+) 10.3 CH; as;, str (95-), op 35.8 CHI sym str (64+) CHp asym str (36+), ip +6 0.2 CH2 sym str (36+) CHp asym str (62-), ip +8 19.4 CH, sym str (97-) +2 34.0 S-H str (loo+) +3 4.3 CH, asym def (92+), ip 9.4 CH, asym def (91+), op +2 6.5 CHI sciss (99+) +4 7.7 CHp sym def (101-) 0 53.0 CHI wag (89+) +1 0.6 CHz twist (57+) -4 CH, rock (30+), op 4.2 C-C str (lo-) -1 1 HSC bend (13+) CHI rock (59+), ip 0.1 CH2 twist (41+) -18 CHp rock (35-), op CH2 rock (19+) +11 8.1 C-C str (69+) CH, rock (14+), ip -39 6.0 C-C str (19+) HSC bend (74+) 5.6 CHI, rock (25+), op +58 CHI rock (71+) +6 7.9 C-S str (89+) -19 4.8 CHI def (86+) -9 1.1 CC tors (90-) 32.2 S-H tors (93+) -3 1 E A i 265.9
sym a” a’’ a’ a’ a’ a’ a’ a‘‘ a’ a’ a’ a’’ a’
TABLE VI: Harmonic Vibrational Frequencies, Infrared Intensities, and Potential Energy Distribution Analysis (PED) Predicted for all -trans -2-Chloroethanethiol
wavenumbers normal scaled? A, coord cm-’ km/mol 1 0.3 3026 2 0.7 2988 3 4.4 2955 4 3.6 2930 20.2 5 2343 5.8 6 1466 10.9 7 1461 8 0.6 1289 9
1280
0.0
10
1229
63.1
11
1110
0.6
12
1009
6.2
13
926
0.1
a” a’
14
816
6.1
a’
15
753
4.9
a”
16
668
2.4
a’ a‘ a” a“
in Table IV. As in Table IV. Difference between freauencv predicted for trans- and guuche-CHpCH2SH;Au = vtrnns- ugBych~.dAL in Table IV.
17
614
96.9
18
264
1.4
a As
HSCH2- group. Enlarging the minimal basis set with one set of polarization functions on carbon and sulfur does not improve the agreement with experiment. It may be pointed out that some of the good agreement with experiment for values calculated by using small basis sets such as the 3-21G basis set is fortuitous. It is well-known that d-functions and correlation corrections are required to predict accurate ge~metries.~’ Similar trends can be expected for the XCHzCH2SCH2CHzX series of molecules (Table 11). The C-S distance is predicted to increase by less than 0.03 A as hydrogens are substituted by chlorine. The C-S distance predicted a t the S C F level with the 3-21G basis set is 1.891, 1.894, and 1.896 A for CH3CH2SCH2CH3, CH3CH2SCH2CH2CI,and ClCH2CH2SCH2CH2Cl, respectively. Extensive calculations on the CH3SH molecule28 indicate that the effect of polarization functions on the calculation is to predict a CS bond length that is shorter by about 0.04 A. The C H bond distance is underestimated with the 3-21G basis set by about 0.03 A compared to experiment. The error in the CC1 bond.length with the 3-21G basis set at the S C F level can be estimated from previous theoretical where it was found to be overestimated a t the HF/3-21G level by about 0.1 8, when compared with experiment. Enlarging the basis set by including polarization functions decreases the predicted C-Cl bond length by 0.09 A. Vibrational Frequencies and Infrared Intensities. Extensive calculations indicate that harmonic vibrational frequencies calculated a t the Hartree-Fock level with a split-valence basis set (27) DeFrees, D. J.; Raghavachari, K.; Schlegel, H. B.; Pople, J. A. J . Am. Chem. SOC.1982, 104, 5576. (28) Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. A b Initio Molecular Orbital Theory; Wiley: New York, 1985.
19
193
11.4
20
106
0.7
21
82
50.6
PEDasC
CI(CH2) asym stretch (92-) S(CH2) asym stretch (92-) C1(CH2) sym stretch (98-) S(CH2) sym stretch (98-) H-S str (loo+) S(CH2) sciss (92-) C1(CH2) sciss (93-) S(CH2) wag (48+) C1(CH2) wag (47-) S(CH2) twist (45+) CI(CHI) twist (37+) S(CH2) rock (lo-) S(CH2) wag (47+) C1(CH2) wag (50+) S(CHz) twist (41+) CI(CH2) twist (54-) C-C stretch (72-) HSC bend (20+) S(CH2) twist 12-) S(CH2) rock (39-) CI(CH2) rock (43+) C-C str (24+) HSC bend (69+) S(CHz) rock (47+) CI(CH2) rock (43+) C-C1 str (IO-) C-S str (48-) S(CHz) def (14+) CI(CHI) def (21+) C-CI str (6%) C-S str (31+) C-CI str (22+) C-S str (22+) S(CH2) def (19+) C1(CH2) def (32+) S(CHz) def (57+) C1(CH2) def (42-) H-S tors (32+) CC tors (60-) H-S tors (68+) CC tors (32+)
svm a” a” a’ a‘ a’ a’ a’ a’
a’’ a’ a’’ a’ a’’ a’ a” a’ a’ a’ a’ a’ a” a”
X A i 290.9
OAs in Table IV. bAs in Table IV. CSameas footnote d given in Table IV.
are generally 10-15% too high when compared with experiment due to correlation effects and a n h a r m o n i ~ i t y . ~ As ~ . ~pointed ~ out previously, this overestimation a t the Hartree-Fock level can be partially reduced by enlarging the basis set. A set of d-functions on heavy atoms and a second set of p-functions on hydrogen lowers the error to 4-10%.12c The remaining errors arise due to neglect of electron correlation. We have developed a new tool of MBPT(2) analytical second derivatives which we have shown further reduces the error to less than 5%.8 In the current work, however, we limit ourselves to Hartree-Fock theory, and in order to overcome the known deficiencies in the predicted harmonic vibrational frequencies, they have been scaled by the suggested multiplicative factor of 0.89.13 XCHzCH2SH. The vibrational frequencies, infrared intensities, and the potential energy distributions (PEDs) for gaucheethanethiol, trans-ethanethiol, and 2-chloroethanethiol are summarized in Tables IV, V, and VI, respectively. The description of the normal modes is based on the PEDs calculated from the force constants at the HF/3-21G level; a more accurate description of the normal modes requires force constants calculated with a large basis set, including correlation corrections. Early experimental studies indicated the existence of two different rotamers (29) Yamaguchi, Y.; Schaefer 111, H. F. J . Chem. Phys. 1980, 73, 2310.
Sosa et al.
582 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 TABLE VII: Harmonic Vibrational Frequencies, Infrared Intensities, and Potential Energy MsMbution Analysis (PEb) Predicted for all-tnrw -Dietby1 Sulfide, Compared with ‘Experimental” Values Calculated from tbe Shimanoucbi Set of Empirical Transferable Force Constantsm,‘
normal coord‘
calculated intensities, km/mol sym
1
wavenumbers* (scaled). cm-’ 2948
2
2947
47
3
2913
15
4
2913
0
5
2908
56
6
2908
13
7
289 1
12
8
289 1
1
9
2853
2
10
2852
50
11
1489
7
12
1489
1
13
1485
0
14
1484
18
15
1468
1
16
1461
14
17
1406
8
18
1406
5
19
1290
16
20
1254
0
21
1251
63
22
1247
0
23
1065
0
24
1028
1
25
1027
11
0
experimental PEDc CH2 asym str (44+) CH2 asym str (44-) CH2 asym str (44-) CH2 asym str (44-) CH, deg str (42-) CH, deg str (46-) CH, deg str (46+) CH, deg str (42-) CH, deg str (45+) CH, deg str (34+) CH2 sym str (11+) CH, deg str (35+) CH, deg str (46-) CH2 sym str (lo-) CH2 sym str (40+) CH2 sym str (39+) CH2 sym str (41-) CH2 sym str (42+) CHI sym str (48+) CH, sym str (48+) CH, sym str (48+) CHI sym str (49-) CH, deg def (46+) CH, deg def (46+)
wavenumbers: cm-’ 2963
CH, deg str (92+)
2963
CH, deg str (92+)
2962
CH, deg str (83+) CH, deg str (16+) CH, deg str (83+) CH, deg str (16-) CH2 asym str (53+) CH2 asym str (42+)
2962 2943 2943
CH2 asym str (53+) CH2 asym str (42-)
2907
2880
CH2 sym str CH2 sym str CHI sym str CHz sym str CH, sym str
2880
CH, sym str (95+)
1466
CH3 deg def (32+) CH, deg def (32+) CH2 sciss (lo-) CH2 sciss (lo-) CH3 deg def (41+) CH, deg def (4G) CH, deg def (41+) CH, deg def (41+) CH, deg def (39+) CH, deg def (39-) CHI sciss (27-) CH2 sciss (27-) CH, deg def (1 1-) CH, deg def (1 1-) CH2 sciss (25-) CHI sciss (25+) CH, sym def (17-) CH, sym def (17+) CH, sym def (43+) CH, sym def (43+) CHI sym def (34+) CH, sym def (34-) CH2 sciss (17+) CH2 sciss (17-) CH2 wag (42+) CH2 wag (42+) CH2 wag (45+) CH2 wag (45-)
2905
CH, deg def (46+) CH, deg def (46-) CH, deg def (45-) CH, deg def (46+) CH, deg def (46-) CH, deg def (45-) CHI sciss (49-) CH2 sciss (49-)
1463
CHI sciss (49+) CHI sciss (49-)
1431
CHI sym def CH, sym def CH, sym def CH, sym def
1388
(49-) (51+) (51-) (49-)
PED^
1463 1462 1442
1378
CH2 wag (45-) CH2 wag (45-) CH, rock (15+) CH, rock (15+) CHI twist (29+) CH2 twist (29+) CH2 wag (43+) CH2 wag (43-)
1273
CH, rock (15+) CH, rock (15-) CH, twist (28+) CH2 twist (28-) CH, rock (33+) CH, rock (33+)
1255
CH, rock (17-) CH, rock (17-) CH, rock (11+) CH2 twist (20+) CHI rock (11+) CH2 twist (20+) CH, rock (35-) CH3 rock (35+)
1046
1270
1258
1075
1041
(49+) (48+) (49-) (49+) (95+)
CH2 twist (33-) CH2 twist (33+) CH, rock (6+) CH2 twist (32+) CH2 twist (32+) CH, rock (7-) C-C str (20-) C-C str (20-) CHI rock (16+) CH, rock (16+) C-C str (21-) C-C str (21+) CH2 rock (21+) CH, rock (21-) CH, rock CHI rock CH2 rock CHI rock
(20+) (2G) (15+) (15+)
XCH2CH2SCH2CH2Xand XCHzCHzSH (X = H , C1)
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 583
TABLE VI1 (Continued) calculated intensities, km/mol sYm 0
experimental wavenumbers," cm-I PED" 1032 CH, rock (20+) CH; rock (20+j CH2 twist (15+)
normal coorda
wavenumbersb (scaled), cm-I
26
1015
27
918
7
28
91 1
4
C3-C5 str (46-) C2-C4 str (46+)
979
29
779
12
787
30
761
0
31
620
1
32
609
9
33
324
2
34
300
1
35
227
0
36
226
0
37
127
0
38
70
0
39
47
1
CH, rock (14+) CHI rock (14+) CH2 rock (34+) CH2 rock (34+) CH, rock (12-) CH, rock (12+) CHI rock (36-) CH2 rock (36+) S-C3 str (34+) S-C2 str (34+) S-C3 str (48+) S-C2 str (48-) CH2 def (46+) CH2 def (46-) CH2 def (20-) CH2 def (20-) S-C3 str (14-) S-C2 str (14-) CSC bend (23-) C3-C5 tors (47-) C2-C, tors (47-) C3-C5 tors (49-) C2-C, tors (49+) CH2 def (1 6+) CH2 def (16+) CSC bend (66-) C3-S tors (49-) C2-S tors (49-) C3-S tors (50-) C2-S tors (50+)
PEDc CH, rock (18-) CH; rock (18+) CHI twist (21+) CHI twist (21-) C3-Cs str (42+) C2-C4 str (42+)
985
778
688 68 1 349 339
256 251 153 85 54
CH2 twist (15+) C-C str (21-) C-C str (21-) CH, rock (16-) CH, rock (16-) C-C str (25-) C-C str (25+) CH, rock (14-) CH,o rock (14+) CH2 rock (32+) CH2 rock (32+) CH, rock (19+) CH, rock (19-) CH2 rock (33-) CH2 rock (33+) CH,o rock (18+) CH rock (18+) C-S str (42-) C-S str (42-) C-S str (48+) C-S str (48-) CH2 def (52+) CH2 def (52-) CSC bend 27+) CHI def (13+) CH2 def (13+) C-S str (9+) C-C tors (48+) C C tors (48-) C-C tors (47+) C C tors (47+) CSC bend (78+) CH2 def (38-) CH2 def (38-) C-S tors (49+) C-S tors (49+) C-S tors (49-) C-S tors (49+)
E A , 383 a Numbered in order of decreasing calculated wavenumbers. bThese numbers are obtained from the calculated wavenumbers multiplied by 0.89. 'The symmetry of the normal coordinate is given by the class of C2, trans molecule. Here a, has dipole moment changes in the plane of the backbone, perpendicular to the chain; bl is in the plane parallel to the chain; b2 is out of the plane of the chain; and a2 is out of plane, but inactive. The PEDs are described in terms of symmetry coordinated designed by Shimanouchi et al.20b*E"These values were calculated by using the transferable empirical set of force constants from Shimanouchi et a1.20b,c
for ethanethiol: a trans (C,) form and two equivalent gauche ( C , ) forms.% Experimental resultsMaindicate that the gauche structure is more stable than the trans by about 1 kcal/mol. In the case of 2-chloroethanethiol additional rotamers are expected due to the possibility of rotational conformers around the C-C bond as well as the C-S bond. The structure calculated in this study corresponds to the trans,trans rotamer. Table IV lists also the experimental wavenumbers for the fundamental modes of ethanethiol reported by Smith, Devlin, and The values they reported were obtained from infrared and Raman spectra of liquid or solid CH3CH2SH, for the most part, and so may be slightly shifted from the frequencies for the isolated molecules. However, such shifts are expected to be no larger than the errors in our predicted frequencies due to the small basis set. The agreement between the experimental frequencies and assignment and the scaled calculated frequencies and PEDs is very good indeed, except for the predicted wavenumbers for the S-H stretching mode (scaled predicted value is 2333 cm-I compared (30) (a) Smith, D.; Devlin, J. P.; Scott, D. W. J . Mol. Spectrosc. 1968, 25, 174. (b) Scott, D. W.; Crowder, G . A. J. Mol. Spectrosc. 1968,26,477. ( c ) Inagaki, F.; Harada, I.; Shimanouchi, T. J. Mol. Spectrosc. 1973, 46, 381.
with 2571 cm-' observed) and for the C-S stretching mode (scaled predicted value is 580 cm-I compared with 658 cm-I observed). For these two modes, much better agreement between the observed and calculated wavenumbers is found if the scaling factor is chosen to be 1.O instead of 0.89. This difference in scaling factor for the modes involving the sulfur atom is not unexpected,Is since we do not expect the small 3-21G basis to predict accurately the frequency of motions involving second-row atoms. More extensive calculations have shown that including polarization functions on sulfur reduces the error to about 20-50 cm-1.28 This problem can also be overcome by properly scaling the force constants.'% Except for these two modes, the calculated wavenumbers scaled by a single constant factor of 0.89 agree with the experimental values within a root-mean-squared deviation of f 2 5 cm-l, which is within the error due to anharmonicity or to effects from intermolecular interactions in the solution or liquid phase. In fact, not only do the predicted wavenumbers agree very well with the experimental values but also the major contributions from our calculated PEDs agree with the experimental assignments, except that some high-frequency modes assigned by Smith, Devlin, and Scott30a to CH3 stretching modes are assigned by our treatment to CH2 stretching modes. In many cases these modes are strongly mixed in our PED analysis, and the extent of mixing
584
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
will depend strongly on the exact values of the force constants. Hence, it is possible, for example, that the calculated force constants for C H bonds in the CH3 group should be scaled by a slightly different scaling factor than is appropriate for that bond force constant in a C H 2 group. Probably the most interesting feature of Tables IV and V is the comparison between the predicted spectra for the two conformers. W e see in Table V that the frequencies for the two conformers are often predicted to be very nearly the same (within 10 cm-l) so that the spectral bands for the two conformers would overlap in the experimental spectrum. Only a few low-frequency modes are predicted to be distinguishable. In particular, the band a t 783 cm-l was observed to disappear when the sample crystallized, and thus it was assigned by Smith, Devlin, and Scott3& to the C H 2 rock of the trans form; this band is predicted here at 769 cm-'. We see that in the gauche form this motion couples with the H S C bending motion (this coupling is forbidden by symmetry for the trans form) causing a splitting of these motions, in agreement with the explanation offered by Smith, Devlin, and based on their unpublished normal-coordinate analysis. According to our calculation the S H torsion is predicted at 189 cm-] for the gauche conformer and a t 158 cm-' for the trans conformer, in excellent agreement with the observed values (191 and 158 cm-I, r e s p e c t i ~ e l y ~ ~Furthermore, ~*~). our predicted values for the methyl torsion about the C-C bond (241 and 232 cm-' for gauche and trans conformers, respectively) are in excellent agreement with the experimental assignment of absorption at 247.5 cm-' 17e to that mode in the gauche conformer. Our. calculated value for the trans conformer (232 cm-l) supports the explanation30a that the failure to observe this mode experimentally for the trans form is indeed because of the overlap between this mode with that for the gauche conformer. Finally it is worth making some comment about the predicted intensities for vibrations from the two different conformers. Examination of Tables IV and V shows that the predicted pattern for the intensities is different for each form. For example, the intensity pattern predicted in the C H stretching region for the trans conformer is 15, 10, 36, 0.2, and 19 km/mol for the first five normal modes in order of decreasing frequency, while for the gauche conformer the corresponding pattern is predicted to be 9, 26, 17, 6, and 22 km/mol. Although the pattern is predicted to change, the total intensity (80 km/mol) in this region is exactly the same for each conformer, as would be expected if the change in the intensity pattern occurs only because the normal-mode mixing and geometric configuration change. If the intensity parameters did not change a t all from one conformer to the other, then the sum of intensities in Table IV should be exactly the same as the sum in Table V (ignoring the intensity sum, Q, for the pure rotation^).^' As seen in those tables, the intensity sum for gauche ethanethiol is 259.7 and 265.9 km/mol for the trans form, suggesting that the changes in the APTs are very minor. CICH2CH2SH. The substituent effect on the vibrational spectrum when a C1 atom is substituted for an H atom in the CH3 group may be seen by comparing the spectrum predicted for the trans,trans conformer of ClCH2CH2SH in Table VI with the spectrum predicted for the trans conformer of CH3CH2SH in Table V. We note that the general pattern of absorption in the two molecules is quite similar. The C H stretching modes in CICH2CH2SHare predicted slightly, but significantly, higher in frequency and decidedly lower in intensity (intensity sum is 9.0 km/mol for the four C H stretching modes in C1CH2CH2SH, compared with an intensity sum of 80 km/mol for the five C H stretching modes of CH3CH2SH). The HS bond vibrations are predicted to be almost exactly the same in both molecules, except for the HS torsion, which strongly couples with the torsional mode about the C-C bond and then both modes shift strongly to lower frequencies in the C1CH2CH2SH compound. The most important difference between the two molecules is the predicted appearance of the strong C-CI stretching a t 614 (31) Person, W.
B.;KuBulat, K.J . Mol. Struc?. 1988, 173, 357.
Sosa et al. cm-I (scaled) in C1CH2CH2SH. This mode is responsible for the predicted increase in the intensity sum (291 km/mol for CICH2CH2SHcompared to 266 km/mol for CH3CH2CH$H) despite the decrease in intensity predicted for the C-H stretches. The C-C1 stretch is predicted to be strongly coupled with the C-S stretching mode and both couple with the low-frequency SCC and CCCl bending modes. The accuracy of this prediction, of course, depends on the accuracy of the prediction of the C-S and C-CI bond force constants. If the errors in the prediction are the same for both bond force constants (corresponding to a constant scaling factor of (0.89)2 for both calculated force constants) then the mixing will be as reported here in Table VI. However, if the error is different (for example, if the C-S force constant is predicted correctly with scale factor of 1.0, as it was for CH3CH2SH, but the force constant for the C-C1 bond must be scaled by (0.89)2), then the predicted mixing in Table VI would be wrong (in this case there would be less coupling of these modes than in Table VI). In our case, we believe that the scaling constants for both force constants cfccl and fcs) should be (1 .0)2 compared to scaling constants of (0.89)2 for all other force constants. For this case, we predict that the wavenumbers for vcs and vccl will be 750 and 689 cm-l, with PEDs expected to be close to those reported in Table VI. XCH2CH2SCH2CH&. The vibrational frequencies, infrared intensities, and the potential energy distributions for the all-trans conformers of diethyl sulfide, 2-chlorodiethyl sulfide, and mustard gas are summarized in Tables VII, VIII, and IX. These structures are shown in Figure 2. The definition of the internal and symmetry coordinates are the same as those described by Shimanouchi et al.20b,cWe note that the spectra calculated for these molecules are, to some extent, just "doubled" versions of the spectra of their model compounds. Thus, each C H stretching mode predicted for CH3CH2SHis predicted to be doubled in the CH3CH2SCH2CH3 molecule; for example, the asymmetric stretching mode of the CH2 group predicted (scaled) a t 2966 cm-I for trans-CH3CH2SH (Table V) is coupled in the diethyl sulfide to give in-phase and out-of-phase combinations of this motion (Table VII). Because of the symmetry, one combination has zero intensity; because the coupling between the two C H 2 groups is weak, both modes are predicted to have the same frequency. There are some changes predicted in the way that the C H 3 vibrations couple with the C H I modes, and PEDS and intensities change somewhat from CH3CH2SHto CH3CH2SCH2CH3,but it is easy to see the correspondence between the normal modes in these two molecules. As we go down the two tables in frequency, there are differences due to removal of the H atom from CH3CH2SH to form the diethyl sulfide, so that S-H vibrations no longer couple with vibrations of the CH3CH2- groups in the latter, causing changes in PEDs, frequencies, and intensity distributions. Also the low-frequency modes ("fingerprint region") are more sensitive to the chain and hence to the change in structure from CH3CH2SH to CH3CH2SCH2CH3.Nevertheless, it is quite easy to see the pairs of bands in the latter molecule that correspond to single modes in the former. Identical comments can be made about the relationship between the spectrum of mustard (C1CH2CH2SCH2CH2Cl)(Table IX) and its model compound (CICH2CH2SH) (Table VI). For the unsymmetrical molecule CH3CH2SCH2CH2C1,the calculated spectrum may be more correctly described as a superposition of the spectrum from the CH3CH2S group on that from the C1CH2CH2S group. Again, the relationship between the frequencies of the normal modes is easy to see in comparing spectra calculated for trans-CH3CH2SH (in Table V) and for C1CH2CH2SH(in Table VI) with that for all CH3CH2SCH2CH2C1 (Table VIII). The relationship between the intensities of the corresponding modes in each molecule is not so obvious, and we shall leave the discussion of this subject to a later paper. Although the intensities of the individual bands do not show an obvious relationship from one molecule to another, there is some reason to expect that the intensity sums are related.3* Thus we may expect that the intensity sum for CH3CH2SCH2CH3will be
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 585
XCH2CH2SCH2CH2Xand XCH2CH2SH (X = H , C1)
TABLE VIII: Harmonic Vibrational Frequencies, Infrared Intensities, and Potential Energy Distribution Analysis (PED) Predicted for 2-Chlorodiethyl Sulfide
normal coord 1
2 3 4 5 6 7
wavenumbers scaled! A, cm-’ km/mol 3025 0.5 2971 2.0 2956 17.2 2955 4.6 2917 8.2 2916 5.5 291 1 28.8
8
2897
3.9
9 10 11 12
2855 1488 1484 1467
22.6 4.3 10.1 2.6
13 14
1461 1460
8.6 15.6
15 16
1407 1294
6.8 18.2
17
1278
0.5
18
1266
26.4
19
1250
0.0
20
1218
56.4
21
1107
0.1
22 23
1050 1024
5.6 0.7
24
975
12.3
PED“vc C4(CH2)asym str (96+) C2(CHi) asym str (96+) C3(CH2) asym str (91-) C4(CH2) syln str (99-) CH, asym str (92+) C2(CH2) sym str (99-) C3(CH2) sym str (25-) CH, asym str (74-) C3(CH2) sym str (75-) CH, asym str (24+) CH, sym str (98+) CH, asym def (92-) CH, asym def (91+) C2(CH2) sciss (68+) C3(CH2) sciss (28+) C4(CH2) sciss (96-) C2(CH2) sciss (27+) C3(CH2) sciss (71-) CHI sym def (101-) C2(CH2) wag (39+) C4(CH2) wag (26-) C3(CH2) wag (30+) C2(CH2) twist (44+) C2(CH2) rock (lo-) C4(CH2) twist (37+) C4(CH2) wag (35+) C3(CH2) wag (53+) C3(CH2) twist (57-) CH, op rock (30-) C2(CH2) wag (53-) C4(CH2) wag (37-) C2(CH2) twist (41+) C4(CH2) twist (54-) CH, ip rock (65+) C3(CH2) twist (40+) C3(CH2) rock (20+) CH, op rock (35-) C2-C4 str (88-)
sYm a” a” a”
normal coord 25
wavenumbers scaled: A, cm-’ km/mol 924 0.1
26 27
913 773
8.9
a’
28
750
1.8
a’
29
697
8.0
a’ a’‘ a’
5.5
a’
a’
30
620
93.6
a” a’
31
596
19.1
32
339
1.6
a’
a’ a’
33
246
2.5
a‘
34 35
a”
226 226
0.1 5.6
a’ a”
36
105
8.4
a’
37
85
1.9
a”
38
55
2.9
a’
a”
39
a’
32
0.5
PED“,‘ C2(CH2) twist (13-) C2(CH2) rock (39-) C4(CH2) rock (42+) C3-C5 str (88+) C3(CH2) rock (62+) CH, op rock (23+) C2(CH2) rock (43+) C4(CH2) rock (37+) C2-S str (49+) C2(CH2) def (18-) C4(CH2)def (18-) C-CI str (43-) C2-S str (28+) C3-S str (25-) C-CI str (27-) C3-S str (57+) CSC bend (12+) C4(CH2) def (15-) CJ(CH2)def (50+) CSC bend (29-) C2(CH2)def (41-) C4(CH2)def (lo+) C3-C5 tors (96-) C-Cl str (12+) C2-S str (19+) C4(CH2)def (39+) C,(CH2) def (21+) C2-C4 tors (77+) C3-S tors (16-) CSC bend (51+) C2(CH2)def (27-) C4(CH2)def (12+) C2-S tors (18+) C2-C4 tors (16+) C3-S tors (64+) C2-S tors (81-) C3-S tors (19+)
SY*
a” a’
a“ a” a’ a’
a‘ a’
a’ a“ a’
a” a’ a”
a”
E A i 422
‘As in Table IV. bAs in Table IV. cSame as footnote d given in Table IV.
just twice the sum for the CH3CH2S group. We can estimate the latter sum by subtracting the intensity contributions due to the S-H group from the total intensity sum for trans CH3CH2SH (266 km/mol, from Table V). W e subtract the intensity of the SH stretch, of the H S C bend, and of the SH torsion to obtain 266 - 34 - 6 - 32 = 194 km/mol as the intensity sum for the CH3CH2Sgroup. Hence the intensity sum for CH3CH2SCH2CH3 is predicted to be 2 X 194 plus the intensity of the CSC bend (0.0 km/mol) plus that for the C S torsion (+1 km/mol) ’= 389 km/mol, in excellent agreement with the actual sum, 383 lan/mol! Similarly, we calculate the intensity sum for the ClCH2CH2Sgroup to be 291 - 20 - 6 - 51 = 214 km/mol (from Table VI). Thus the intensity sum for CH3CH2SCH2CH2C1is predicted to be 194 + 214 plus the intensity of the CSC bend (3.0 km/mol) and for the CS torsion (3.0 km/mol) = 414 km/mol, compared with 422 km/mol in Table VIII. That for CICH2CH2SCH2CH2C1 is predicted to be 2 X 214 3 0 = 431 km /mol, compared with 448 km/mol found in Table IX. (It is probable that the intensity sum for the CICH2CH2S- group is underestimated here, perhaps due to subtraction of too much intensity for the S-H torsion of C1CH2CH2SH. If we estimate the intensity sum for this group instead of dividing the total intensity of mustard by two, CH3CH2SCH2CH2C1is then estimated to be 224 km/mol, so that the intensity sum for CH3CH2SCH2CH2CIis 194 224 6 = 424 km/mol, in much better agreement with the value (422 km/mol) in Table VIII.) It is of some interest here to compare our calculated spectra for diethyl sulfide (Table VII) and 2,2’-dichlorodiethyl sulfide (Table IX) with that calculated by using force constants recom-
+ +
+
+
mended by Shimanouchi et aL2ObVcas transferable force constants for molecules of this type. Calculations of the vibrational wavenumbers and PEDs (no predictions for intensities) for diethyl sulfide had been compared with their experimental studies2& of the spectra for this molecule. Their calculated wavenumbers agreed very well (within 10 cm-l) with their experimental spectra (dominated, of course, by the spectra of the more stable gauche conformers). Their assignment of the experimental spectrum is based upon their calculated PEDs, so of course the agreement is very good. However, since their force field is chosen to be transferable among a large number of these kinds of molecules, their PEDs may be taken as the best “experimental” values available. Hence the agreement (or disagreement) between our predicted PEDs and the experimental values given in Tables VI1 and IX is a good measure of the ability of the calculation based upon the 3-21G basis set to reproduce the experimental values. Of course, one must keep in mind the possibility also that some modification of the Shimanouchi force field might improve the agreement between our calculated values and theirs, and at the same time improve the prediction of spectra for all of the compounds considered in the establishment of the transferable force constants they have used. In examining the comparison between the PEDs shown in Table VII, we see that the general agreement is actually very good. As can be expected for calculations made with this small basis set, there are, however, a number of differences. The major discrepancy between the two calculations for the C H stretching region (3000-2800 cm-’) is that our calculation predicts that the CH stretching force constants (hence stretching frequencies) for the
586
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
Sosa et al.
TABLE I X Calculated Harmonic Frequencies, Infrared Intensities, and Potential Energy Distributions (PEDs) for 2,2'-Dichlordiethyl Sulfide (Mustard), Compared with "Experimental"Values Calculated from the Shimanouchi Set of Empirical Transferable Force Constantsm*c
calculated normal coord'
intensities, km/mol
1
wavenumbers* (scaled), cm-I 3024
2
3024
1
3
2978
0
4
2977
2
5
2954
2
6
2954
6
7
2920
1
8
2920
6
9
1466
2
10
1461
10
11
1461
4
12
1459
22
13
1299
2
14
1281
1
15
1278
6
16
1276
0
17
1235
46
18
1211
53
19
1112
0
20
1108
0
21
992
2
22
964
16
23
935
0
24
919
0
0
SYm
experimental
PED' CI(CH2) asym str (48+) CI(CH,) asym str (46-) Cl(CH2) asym str (47-) CI(CH2) asym str (49-) S(CH2) asym str (47+) S(CH2) asym str (47-) S(CH2) asym str (48-) S(CH2) asym str (48-) CI(CH2) sym str (49-) C1(CH2) sym str (49+) CI(CH2) sym str (SO-) CI(CH2) sym str (49-) S(CH2) sym str (49-) S(CH2) sym str (49-) S(CH2) sym str (49-) S(CH2) sym str (SO-) S(CH2) sciss (44+) S(CH2) sciss (44+)
wavenumbers: cm-' 3005
C1(CH2) asym str (98+)
3005
C1(CH2) asym str (98+)
2956
C1(CH2) sym str (76+) C1(CH2) sym str (23+) C1(CH2) sym str (76+) CI(CH2) sym str (23-) S(CH2) asym str (57+) S(CH2) asym str (41+) S(CH2) asym str (57+) S(CH2) asym str (41-) S(CH2) sym str (50+) S(CH2) sym str (49+) S(CH2) sym str (50-) S(CH2) sym str (49+) S(CH2) sciss (34-) S(CH2) sciss (34-) CI(CH2) sciss (1 3+) CI(CH2) sciss (13+) CI(CH2) sciss (40+) C1(CH2) sciss (40-) CI(CH2) sciss (33+) C1(CH2) sciss (33+) S(CH2) sciss (1 1+) S(CH2) sciss (1 I + ) S(CH2) sciss (38-) S(CH2) sciss (38+) CI(CH2) wag (38-) C1(CH2) wag (38-) S(CH2) wag (1 1+) S(CH2) wag (11+) CI(CH2) wag (39-) C1(CH2) wag (39+) S(CH2) wag (1 1+) S(CH2) wag (I]-) S(CH2) twist (21+) S(CH2) twist (21-) C1(CH2) twist (19-) C1(CH2) twist (19+) S(CH2) twist (20-) S(CH2) twist (20-) C1(CH2) twist (20+) CI(CH2) twist (20+) S(CH2) wag (43-) S(CH2) wag (41+) CI(CH2) wag (12-) CI(CH2) wag (12+) S(CH2) wag (40+) S(CH2) wag (39+) C1(CH2) wag (13+) C1(CH2) wag (13+) C1(CH2) twist (25+) C1(CH2) twist (25-) S(CH2) twist (24+) S(CH2) twist (24-) C1(CH2) twist (25+) CI(CH2) twist (25+) S(CH2) twist (24+) S(CH2) twist (24+) C-C str (37-) C-C str (37-) C-C str (44+) C-C str (44-) CI(CH2) rock (22+) CI(CH2) rock (22+) S(CH2) rock (20+) S(CH2) rock (20+) CI(CH2) rock (23-) C1(CH2) rock (23+) S(CH2) rock (19-) S(CH2) rock (19+)
2956 2944 2944 2906 2904 1439
CI(CH2) sciss (45+) CI(CH2) sciss (44+) C1(CH2) sciss (46-) CI(CH2) sciss (46+)
1434
S(CH2) sciss (47-) S(CH2) sciss (47+) S(CH2) wag (30-) S(CH2) wag (30-) C1(CH2) wag (18+) C1(CH2) wag (18+) S(CH2) twist (23+) S(CH2) twist (23+) C1(CH2) twist (19+) C1(CH2) twist (19+) S(CH2) wag (15+) S(CH2) wag (15-) CI(CH2) wag (33-) C1(CH2) wag (33+) S(CH2) twist (21+) S(CH,) twist (21-) CI(CH2) twist (20+) CI(CH2) twist (20-) S(CH2) wag (18+) S(CH2) wag (18+) C1(CH2) wag (31+) C1(CH2) wag (31+) S(CH2) wag (34-) S(CH2) wag (34+) CI(CH2) wag (15-) C1(CH2) wag (15+) S(CH2) twist (21+) S(CH2) twist (21+) C1(CH2) twist (27-) C1(CH2) twist (27-) S(CH2) twist (21-) S(CH2) twist (21+) C1(CH2) twist (26+) C1(CH2) twist (26-) C3-C5 str (44+) C2-C4 str (44+) C3-C5 str (48-) C2-C4 str (48+) S(CH2) rock (21-) S(CH2) rock (21-) C1(CH2) rock (20+) CI(CH2) rock (20+) S(CH2) rock (18+) S(CH2) rock (I&) CI(CH2) rock (23-) C1(CH2) rock (23+)
1410
1428
1306
1305
1282
1281
1218
1218
1149
1145
1043 1014 988
980
PED^
XCH2CH2SCH2CH2Xand XCH2CHzSH (X = H , C1)
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 587
TABLE IX (Continued) calculated intensities, km/mol sYm 10
normal coord' 25
wavenumbersb (scaled), cm-l 759
26
748
0
27
708
4
28
674
1
29
625
164
30
607
44
31
305
6
32
294
1
33
195
0
34
188
11
35
104
19
36
97
0
37
54
38
39
experimental
PED' S(CH,) rock (22-) sicH;j rock (22-j C1(CH2) rock (23-) C1(CH2) rock (23-) S(CH2) rock (25+) S(CH2) rock (25-) CI(CH2) rock (20+) CI(CH2) rock (20-) S(CH2) def (12+) S(CH2) def (12+) C3-S str (20-) C2-S str (20-) C1(CH2) def (1 1-) CI(CH2) def (1 1+) C3-S str (28+) C2-S str (28-) C5-C1 str (32-) C,-Cl str (32+) C3-S str (16+) C2-S str (16-) C5-CI str (35+) C4-CI str (35+) C3-S str (15-) C2-S str (1%) S(CH2) def (17+) S(CH2) def (17-) CI(CH2) def (lo+) C1(CH2) def (lo-) C5-Cl str (14+) C4-Cl str (14-) C1(CH2) def (26-) C1(CH2) def (26-) CSC bend (39+)
wavenumbers,d cm-l 79 1
756
756
75 1
723
696
36 1
348
S(CH2) def (13-) S(CH2) def (13-) C5-CI str (lo-) C4-Cl str (lo-) C3-S str (15-) C2-S str (15-) S(CH2) def (22-) S(CH2) def (22+) CI(CH2) def (27+) C1(CH2) def (27-) C3-Cs tors (46+) C2-C, tors (46-) C3-Cs tors (40+) C2-C4 tors (40+)
218
2
S(CH2) def (18-) S(CH2) def (18-) CSC bend (49+)
75
42
0
C3-S tors (44+) C2-S tors (44+)
60
26
0
C3-S tors (49+) C2-S tors (49-)
43
198
130 117
PED^ C-S str (26-) c-s str (26-j C-CI str (lo-) C-CI str (lo-) S(CH2) rock (24+) S(CH2) rock (24+) C1(CH2) rock (23-) CI(CH2) rock (23-) C-S str (32-) C-S str (32+) C1(CH2) def (9+) S(CH2) rock (26-) S(CH2) rock (26+) CI(CH2) rock (22+) Cl(CH2) rock (22-) C-CI str (34+) C-C1 str (34-) C-S str (12-) C-S str (12+) C-C1 str (35+) C-C1 str (35+) C-S str (15-) C-S str (15-) CSC bend (49+) C1(CH2) def (25-) Cl(CH2) def (25-)
S(CH2) def (18+) S(CH2) def (18-) C-CI str (9-) C-CI str (9+) C-S str (11+) C-S str (1 1+) S(CH2) def (11+) S(CH2) def (11+) CI(CH2) def (33-) CI(CH2) def (33+) S(CH2) def (29-) S(CH2) def (29+) C-C tors (42-) C-C tors (42+) C-C tors (32-) C-C tors (32-) C-S tors (12+) C-S tors (12+) C-S tors (37+) C-S tors (37+) C-C tors (12+) C-C tors (12+) CSC bend (S8+) S(CH2) def (38-) S(CH2) def (38-) CI(CH2) def (12+) C-S tors (45+) C-S tors (45-)
C A , 449 a
Footnotes are the same as those given in Table VII.
CH2group are larger than those for the corresponding values for the CH3 group. For the CH2and CH3bending modes (1500-1200 cm-') the agreement is quite acceptable. The C C stretching force constants are predicted to be considerably smaller than the experimental values so that the predicted wavenumbers (918 and 91 1 cm-') for these modes are lower than the experimental values (at 1075 and 1046 cm-', strongly mixed with the CH3 rock). As expected, the predicted wavenumbers for the C S stretching modes (620 and 609 cm-') are considerably lower than the experimental modes a t 688 and 681 cm-I. The agreement for the PEDS for
the remaining modes of diethyl sulfide is remarkably good. In general the predicted wavenumbers are somewhat too low, suggesting that better agreement between the numerical values predicted for the wavenumbers would be in better agreement with the experimental values if the scaling factor were increased slightly from the standard value of 0.89. An examination of the comparison between our calculated spectrum and the experimental spectrum for 2,2'-dichlorodiethyl sulfide shown in Table IX indicates the same general agreement for this more complicated molecule. Once again the agreement
J. Phys. Chem. 1989, 93, 588-592
588
between the calculated and experimental frequencies varies somewhat, indicating that better agreement might be obtained by using somewhat different scaling factors for C H and CC force constants, for example. In the CCI and CS stretching frequency region (790-690 cm-I in the experimental spectrum) the calculation is not in very good agreement with experiment, as expected. Furthermore, the heavy atom (S and CI) involvement in the low-wavenumber region (in general below 790 cm-I) introduces considerable error in our calculated spectrum in this region. In spite of these problems, the general agreement between the calculated spectrum and the experimental spectrum, even for this very complicated molecule, is encouraging. W e shall not discuss further here the experimental spectra of these compoundsi6" and the use of our calculated spectra for their assignment, since a detailed analysis of this problem will be the subject of later papers. However, we should point out that the comparison (given above) of our calculated spectra with previous experimental studies leads us to expect that the predicted spectrum will be in very good agreement with the experimental spectrum for the isolated, all-trans conformers of these molecules except
for the values predicted for the C-S, S-H, and C-CI stretching frequencies, using the 0.89 scaling factor. The comparison between the spectra calculated for the two CH3CH2SH conformers indicates how the conformational changes will affect the predicted spectra, and we shall discuss this subject in more detail elsewhere.'6b Acknowledgment. C.S. thanks Dr. G. B. Fitzgerald for valuable help with the frequency calculations for mustard gas. The calculations for the vibrational spectrum of mustard were performed on the CRAY-XMP with a grant from N S F a t the Pittsburgh Supercomputing Center. This work was supported by the Chemical Research Development and Engineering Center, Aberdeen, MD, Contract DAAA15-85-C-0034. We are grateful also to Professor H. Matsuura for providing us with a copy of the MVIB program for normal-coordinate calculations. Registry No. Ethanethiol, 75-08-1; 2-chloroethanethiol,4325-97-7; diethyl sulfide, 352-93-2; 2-chlorodiethyl sulfide, 693-07-2; 2,2'-dichlorodiethyl sulfide, 505-60-2.
Computational Determination of the Structures and Some Properties of Tetrahedrane, Prismane, and Some of Their Aza Analogues Peter Politzer* and Jorge M. Seminario Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 (Received: April 26, 1988)
We have carried out an ab initio self-consistent field computational study of tetrahedrane, prismane, and nine of their aza analogues, in which C-H units have been replaced by nitrogens. Structures optimized at the 3-21G level were used to compute molecular electrostatic potentials, as guides to reactive behavior, and bond deviation indexes as quantitative indicators of bond strain. Within each set of azaprismane isomers, the most stable is the one having the fewest N-N bonds. The exceptional length of these bonds, approximately 1.59 A, may reflect a tendency to rupture. In the tetrahedranes, the bonds are quite highly strained but become less so as the number of nitrogens increases. The degrees of bond strain are not as great in the prismanes and do not necessarily diminish as more nitrogens are introduced. There are negative electrostatic potentials associated wtih the C-C bonds in tetrahedrane and prismane, indicating that these bonds can serve as initial sites for electrophilic attack. These potentials are greatly weakened or eliminated by the introduction of nitrogens. In the azatetrahedranes and azaprismanes, there are strong and extensive negative regions near the nitrogens, suggesting significant basicity; these also become weaker as the number of nitrogens increases.
Introduction Tetrahedrane, I, and prismane, 11, are highly strained hydrocarbons, with estimated strain energies of roughly 130 kcal/mol.' While molar strain energies higher than this have been attributed to certain other cage-type molecules (e.g., cubane, 111, 157 kcal/mol'), tetrahedrane and prismane are certainly among the very highest on a weight basis or relative to the number of C-C bonds.
@a"! P
I
--
>-
I1
__
(4) Scott, L. T.; Jones, M., Jr. Chem. Rev. 1972, 72, 181. (5) Baird, M. S . J . Chem. SOC.,Chem. Commun. 1974, 197. (6) Peterson, R. F., Jr.; Baker, R. T. K.; Wolfgang, R. L. Tetrahedron Lett.
I11
This intrinsic instability suggests that the syntheses and isolation of these compounds may be extremely difficult, and indeed tetrahedrane is as yet unknown. However, prismane has been prepared and found to be stable a t room temperature;2 this may reflect the fact that several of its likely potential transformations are thermally forbid den.'^^,^ Since analogous restrictions apply ~
~~~~~
to tetrahedrane,'~~.~ its eventual synthesis remains a realistic possibility; indeed it has been implicated as an intermediate in several different reaction^."^ Derivatives of both molecules are known. Tetra-tert-butyltetrahedrane has been prepared,'O and there have been a t least indications of the formation of other substituted tetrahedranes.'~ll-'~ Several alkyl derivatives of prismane have been i~olated.lJ~-~~
~~
(1) Greenberg, A.; Liebman, J. F. Strained Organic Molecules; Academic: New York, 1978. (2) Katz, T. J.; Acton, N. J . A m . Chem. SOC.1973, 95, 2738. (3) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1969, 8, 789.
0022-3654/89/2093-0588$01.50/0
1969, 4749. (7) Shevlin, P. B.; Wolf, A. P. J . Am. Chem. SOC.1970, 92, 406, 5291. (8) Ona, H.; Yamaguchi, H.; Masamune, S. J . Am. Chem. SOC.1970,92,
7495. (9) Rodewald, L.B.; Lee, H.-K. J . Am. Chem. SOC.1973,95,623,3084. (IO) Irngartinger, H.; Goldmann, A.; Jahn, R.; Nixdorf, M.; Rodeward, H.; Maier, G.; Malsch, K. D.; Emrich, R. Angew. Chem. 1984, 96, 967; Angew. Chem., Int. Ed. Engl. 1984, 23, 993. (11) Maier, G. Angew. Chem., Int. Ed. Engl. 1974, 13, 425. (12) Rauscher, G.; Clark, T.; Poppinger, D.; Schleyer, P. v. R. Angew. Chem., Int. Ed. Engl. 1978, 17, 276. (13) Liebman, J. F.; Greenberg, A. Chem. Rev. 1976, 76, 31 1. (14) Viehe, H. G.; Merenyi, R.; Oth, J. F. M.; Senders, J. R.; Valange, P.Angew. Chem., Int. Ed. Engl. 1964, 3, 755. (15) Wilzbach, K. E.; Kaplan, L. J . Am. Chem. SOC.1965, 87, 4004.
0 1989 American Chemical Society
