J. Am. Chem. SOC.1991, 113,4768-4776
4768
Mechanistic Study of the Electrocyclic Ring-Opening Reaction of Thiirane Joseph E. Fowler, Ian L. Alberts, and Henry F. Schaefer III* Contribution from the Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602. Received September 21, 1990 Abstract: The electrocyclic ring opening of the C-C bond in thiirane is investigated in detail within the context of ab initio molecular orbital theory. The conrotatory and disrotatory reaction pathways are studied with three different basis sets ranging from double-t plus polarization (DZP) through triplet plus double polarization (TZZP) in conjunction with self-consistent-field (SCF), two-configuration SCF (TCSCF), and configuration interaction with single and double excitations (CISD) levels of theory. The conrotatory and disrotatory stationary points were located on the potential energy surface and characterized via harmonic vibrational frequency analyses. At all levels of theory considered, the conrotatory stationary point is found to be a genuine transition state. The disrotatory stationary point, located with the two configuration methods, is found to have two imaginary vibrational frequencies. The conrotatory process is shown to be the favored mechanistic route for thiirane ring opening by 13.2 4.0 kcal mol-' with the zero point vibrational energy correction, relative to the disrotatory pathway at the highest level of theory employed (TZZP TCSCF-CISD+Q). The potential energy hypersuurface for thiirane ring opening is compared with the corresponding surface for oxirane.
Introduction The stereoselective nature of the electrocyclic ring-opening reaction of cyclopropyl anion leading to allyl anion via C-C cleavage can be rationalized in terms of orbital symmetry arguments and is a direct consequence of the reaction mechanism. Rotation of the terminal methylene groups in the same direction defines a conrotatory process and in the opposite direction a disrotatory process. For various cyclic systems, whether a ring opening reaction pathway is allowed or forbidden can be determined by applying the principle of conservation of orbital symmetry. Woodward and Hoffmann thus postulated that the conrotatory mechanism should be favored for the thermolysis and the disrotatory mechanism for the photolysis of the cyclopropyl anion.IJ Extensive experimental+I2 and the~retical'~-~' inves-
(1) (a) Woodward. R. E.; Hoffmann, R. J . Am. Chem. Soc. 1965,87,395. (b) Hoffmann, R.; Woodward. R. E. J . Am. Chem. Soc. 1965, 87,2046. (2) Woodward, R. B.;Hoffmann, R. Angew. Chem., Inr. Ed. Engl. 1%9, 8,781. (3)Huisgen, R.; Scheer, W.; Huber, H. J . Am. Chem. Soc. 196'1.89.1753. (4)(a) Linn, W. J. J . Am. Chem. Soc. 1965,87,3665.(b) Linn, W. J.; Benson, R. E. J . Am. Chem. Soc. 1%5,87, 3667. (5)Do-Minh, T.;Trouolo, A. M.; Griffin, G. W. J. Am. Chem. Soc. 1970, 92, 1402. (6) Crawford. R. J.; MacDonald, H. H. J. Can. J. Chem. 1970, 50,428. (7)Huisgen, R. Angew. Chem., Inr. Ed. Engl. 1977,16, 572. (8)Wong, J. P. K.; Fahmi, A . A.; Griffin, G. W.; Bhacca, N. S. Tetrahedron 1981, 37,3345. (9)(a) Gill, H. S.;Langrebe, J. A. J . Org. Chem. 1983, 48. 1051. (b) Gisch, J. F.; Langrebe, J. A. J . Org. Chem. 1985,50. 2050. (1 0) Engel, P. S.Chem. Rev. 1980,80, 99. (11) Brauman, J. 1.; Archie, W. C. J . Am. Chem. Soc. 1972, 94,4262. (12)Steinmetz, M. G.; Srinivasan. R.; Leigh, W. J. R N . Chem. Inrermed. 1984,5, 57. (13)Schilling, E.; Snyder, J. P. J . Am. Chem. Soc. 1975,97,4422. ( I 4) Hayes, E. F. J . Chem. Phys. 1969,51, 4787. (15) (a) Yamaguchi, K.; Fueno, T. Chem. Phys. Lerr. 1973,22,471.(b) Yamaguchi, K. Chem. Phys. Lcrr. 1975,33,330. (16)(a) Bigot, E.; &in, A.; Devaquet, A. J . Am. Chem. Soc. 1979,101, 1095. (b) Bigot, E.; Sevin, A.; Devaquet, A. J. Am. Chem. Soc. 1979,101, 1101.
(17)Houk, K. N.;Rondan, N. G.; Santiago, C.; Gallo, C. J.;Gandour, R. W.; Griffin, G. W. J . Am. Chem. Soc. 1980,102,1504. (18)(a) Jean, Y.; Volatron, F. Chem. Phys. LLrr 1981,83, 91. (b) Volatron, F.; Anh, N . T.; Jean, Y. J . Am. Chem. Soc. 1983,105, 2359. (c) Volatron, F. Cun. J . Chem. 1984, 62,1502. (19)Feller, D.; Davidson. E. R.; Borden, W. T. J . Am. Chem. Soc. 1984, 106, 2513. (20)Tachibana, A.; Koizumi, M.; Okazaki, 1.; Teramae, H.; Yamabe, T. Theor. Chim. Acta 1987,71,7.
0002-7863/91/1513-4768$02.50/0
tigations that have been undertaken confirm the validity of these predictions, in particular for heterocyclic systems isoelectronic with the cyclopropyl anion, such as aziridine and oxirane (H2CXCH2,X = NH, 0). The C-C electrocyclic ring opening of these heterocycles, leading to azomethane and carbonyl ylides, respectively, was shown to occur in a concerted manner in accordance with the Woodward-Hoffmann (W-H) rules. Studies concerning azomethane and carbonyl ylides have formed an active area of chemical research since they have been used as intermediates in the synthesis of many organic Clearly, the relative activation barriers for the conrotatory (in C, symmetry) and disrotatory (in C, symmetry) ring-opening pathways determine the preferred route. The barrier for conrotatory ring opening via C-C bond cleavage for unsubstituted oxirane was found to be 47.6 kcal/mol with use of multireference configuration interaction (MR CISD) techniques and the 4-3 1G basis &.I8 The barrier height for the disrotatory reaction pathway was predicted to be 10.9 kcal mol-' above that of the conrotatory pathway; however, the corresponding disrotatory stationary point was shown to be an energy maximum with respect to two coordinates of reaction. Thus the disrotatory stationary point is not a genuine transition state at this level of theory.I8 A nonsynchronous pathway in C1symmetry was also proposed for oxirane18 in which one methylene group is initially allowed to rotate, the other being kept orthogonal to the COC plane. Energetically the stationary point for the nonsynchronous pathway in CI symmetry was predicted to lie 8.3 kcal mol-' above the conrotatory stationary point in C, symmetry. Also the open structure, carbonyl ylide, was found to be 34.1 kcal mol-' above the ring form, oxirane. In comparison, theoretical studies for cyclopropane ring opening at the two-configuration self-consistent-field (TCSCF) level with the 3-21G basis set showed that the conrotatory stationary point in C, symmetry was below the disrotatory stationary point in C, symmetry, although by less than 1 kcal m ~ l - l . ~ ~ Vibrational analysis of planar unsubstituted carbonyl ylide by Yamabe and co-workersZ0a t the 4-31G TCSCF level of theory (21) Horsley, J. A.; Jean, Y.; Moser, C.; Salem, L.; Stevens, R. M.; Wright, J. S.J . Am. Chem. Soc. 1972,94,279. (22)Hayes, E. F.; Siu, A. K. Q. J . Am. Chem. Soc. 1971,93, 2090. (23) Kato, S.;Morokuma. K. Chem. Phys. Leu. 1979,65, 19. (24)Doubleday, C.;McIver, J. W.; Page, M. J. Am. Chem. Soc. 1982, 104.6533. (25)Goldberg, A. H.; Dougherty, D. A. J. Am. Chem. Soc. 1983,105,284. (26)Yamaguchi, Y.; Osamura, Y.; Schaefer, H. F. J . Am. Chem. Soc. 1983,105, 7506. (27) Yamaguchi, Y.; Schaefer, H. F. J . Am. Chem. Soc. 1984,106, 51 IS. (28)Dewar, M. J. S.; Kirschner, S.J . Am. Chem. Soc. 1974,96,6809. (29)Breulet, J.; Schaefer, H. F. J . Am. Chem. Soc. 1984, 106, 1221. (30)Spellmeyer, D. C.; Houk, K. N. J. Am. Chem. Soc. 1988,110,3412. (31) Yoshimine, M.; Pacansky, J.; Honjou, N. J . Am. Chem. Soc. 1989, I l l , 2785.
0 1991 American Chemical Society
Electrocyclic Ring- Opening Reaction of Thiirane showed two imaginary frequencies, 4553 (a2) and 369i cm-’ (bl). Following the a2mode, a nonplanar structure was located involving pyramidalization of the two methylene groups and was found to be stabilized by less than 1 kcal mol-’ relative to the planar isomer.20 Following the bl mode yielded the ring isomer.20 A number of other distinct structures for carbonyl ylide, involving rotation of the methylene groups, have been examined with molecular orbital theory.1b27 Much research in recent years has been concerned with the comparison of systems containing first-row atoms with the analogous isovalent systems containing second- (or third) row atoms. The similarities and differences in the structures and bonding of such systems have been analyzed ~ l o s e l y . ~ The ~-~~ purpose of the current research is to investigate the effect of the second-row atom on the electrocyclic ring-opening reaction of H2CSCH2,the thiirane molecule. The electrocyclic ring opening of thiirane via C-C cleavage, leading to thione methylide, has been studied with semiempirical methods.)* The conrotatory barrier was found to be lower in energy than the disrotatory barrier in accordance with the conservation of orbital symmetry (W-H rules). The quantitative results of this work, however, should be treated cautiously due to the nonrigorous nature of the CNDO procedure employed. Contrary to the analogous oxygen case, thione methylides cannot be generated by the electrocyclic ring opening of thiiranes because of the relative weakness of C-S as compared to C-C bonds. However, experimental investigation^^^*^ have been able to examine the related cyclization of thione methylides (which had to be generated from different precursors). The geometrical parameters and harmonic vibrational frequencies of (cyclic) thiirane have previously been determined the~retically,~’ and the predicted values are in satisfactory agreement with the experimentally derived values.4143 To our knowledge there has been no high-level theoretical determination of the molecular properties of the open isomer (thione methylide) or the activation barriers for conrotatory and disrotatory motion. In this research a detailed a b initio theoretical analysis of the thiirane ring opening potential energy surface was performed involving location of stationary points corresponding to the above mentioned structures employing methods that include electron correlation effects. Harmonic vibrational frequency analyses were carried out to characterize each stationary point and to determine the zero-point vibrational energy (ZPVE). The relative energies of the ring and open isomers and the energetic competition between the conrotatory and disrotatory pathways will be discussed at various levels of theory and compared with the corresponding oxirane values. The current research is concerned with the synchronous conrotatory (in C2 symmetry) and disrotatory (in C, symmetry) ring-opening mechanisms. We will also examine whether the W-H rules are applicable in a straightforward manner for thiirane, as they are for the first-row analogue oxirane. Ring opening of oxirane is also known to occur via C 4 cleavage.I6 However, the criterion of potential reversibility of an oxirane opening via C-O (32) Luke, B. T.; Pople, J. A.; Krogh-Jespersen,M. B.; Apeloig, Y.;Kami, M.; Chandrasekhar, J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1986,108,270. (33) Apeloig, Y. The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; pp 57-225. (34) Schmidt, M. W.; Truong, P. T.; Gordon, M. S. J. Am. Chem. SOC. 1987, 109, 5217. (35) Korkin, A. A. Inr. J . Quantum Chem. 1990, 38, 245. (36) Kutzelnigg, W. Angew. Chem., Inf. Ed. Engl. 1984, 23, 272. (37) Rice, J. E.; Amos, R. D.; Handy, N. C.; Lee, T. J.; Schaefer, H. F. J . Chem. Phys. 1986.85, 963. (38) Snyder, J. P. J. Am. Chem. SOC.1974, 96, 5 0 0 5 . (39) (a) Kellog, R. M.; Wassenaar, S.; Buter, J. Tetruhedron Leu. 1970, 4689. (b) Buter. J.; Wassernaar, S.; Kellog, R. M. J . Org. Chem. 1972, 37, 4045. (40) Arduengo, A. J.; Burgess, E. M. J. Am. Chem. SOC.1976,98,5020. (41) Allen, W. D.; Bertie, J. E.;Falk, M. V.; Hess, B. A.; Mast, G. B.; Othen, D. 0.; Schaad, L. J.; Schaefer, H. F. J. Chem. Phys. 1986,84,4211. (42) Cunningham, G. L.; Boyd, A. W.; Myers, R.J.; Gwinn, W. D.; Le Van, W. I. J. Chem. Phys. 1951, 19, 616. (43) Okiye, K.; Hirose, C.; Lister, D. G.; Sheridan, J. Chem. Phys. Lett. 1974, 24, 1 1 1.
J . Am. Chem. SOC.,Vol. 113, No. 13, 1991 4169 o*(a)\
Thiirane
x*(a)
Conrotatory
Thione Methylide
(C,)
Thiirane
Disrotatory
Thione Methylide
(CS)
Figure 1. Schematic correlation diagrams for the conrotatory and disrotatory isomerization of thiirane to thione methylide.
cleavage is not appropriate for thiirane analysis. Not only is C-S more likely than is C-0 cleavage, but the corresponding diradicals easily stabilize via loss of elemental S. Electronic Structure Considerations For W-H allowed pathways of ring-opening reactions the single configuration level often represents a reasonable zeroth-order wave function for a qualitative description of the process. W-H forbidden pathways of ring-opening reactions, however, usually require MCSCF based methods for even a qualitative description. The selection of the configurational space for the MCSCF is not always obvious, and this aspect will be discussed for the ring opening of thiirane. The stereoselectivity of the electrocyclic ring opening of thiirane may be understood in terms of a correlation diagram involving C-C ub, u*, and ring n” orbitals of thiirane that transform smoothly into three C-S T orbitals of planar thione methylide, as shown schematically in Figure 1. This is analogous to the diagram for oxirane ring opening.16 The methylene motion dictates the behavior of the molecular orbitals centered on the carbons, while molecular orbitals centered mainly on sulfur tend to correlate with their sulfur counterparts since they are not significantly affected by the motion. During the conrotatory process the symmetry of the molecule is constrained to C2,while during the disrotatory process the symmetry is constrained to (Cs).The correlation diagram for the conrotatory process (Figure 1) shows a crossing between a pair of occupied orbitals and this is referred to as a symmetry-allowed mechanism.’V2 Following the disrotatory pathway the C-C ub bonding orbital (a’ in C, symmetry) of thiirane appears to correlate with the unoccupied C-S n* antibonding orbital (a’) of thione methylide and the occupied nonbonding ?rn orbital (a’) correlates with the occupied C-S n bonding orbital. The C-C uband nnorbitals are both of a’ symmetry and thus an avoided crossing results. The unoccupied C-C u* antibonding orbital (a”) of thiirane correlates with the occupied nonbonding n” orbital (a”) of thione methylide and, therefore, the disrotatory process involves the actual crossing of an occupied and unoccupied pair of orbitals and corresponds to a symmetryforbidden mechanism. From the correlation diagram it can be argued that the conrotatory mechanism can be qualitatively described with a single configuration wave function. Conversely, description of the disrotatory process evidently requires at least a two-configuration wave function which spans the occupied-virtual orbital crossing. This may also be understood by consideration of the ground-state
Fowler et al.
4110 J. Am. Chem. SOC.,Vol. 113, No. 13, 1991
valence-electron configurations of the various species involved in the ring-opening reaction. For thiirane the ground-state configuration can be described by (in C , symmetry)
...2bl21a228a123bl25b20
(1)
and the ground-state configuration for the open structure may , symmetry) be expressed by (in C
...5 b2’8aI22bl 1a223b10
(2)
The symmetry is constrained to C2 during the conrotatory process. In C2symmetry the above electronic structures correlate with the following configuration
...6b28a27b29a28b0
(3)
thus a single-configuration wave function correctly describes the conrotatory ringopening pathway. For the disrotatory mechanism C, symmetry is maintained. With this constraint thiirane and thione methylide give rise to distinct configurations thiirane in C,
-
...9a’25a’’210a’21 la’26a’’0
thione methylide in C,
-
(4)
...5a”9a’210a’26a’’21 la’O
(5)
It is evident that a single configuration model is inadequate. Configuration 4,the electronic structure for thiirane in C, symmetry, corresponds to the ( T * ) configuration ~ of the open structure
...5bz28a122bl23b121a20
-
-
(6)
-
--
(7)
resulting from the ( T * ) ~[la? 3bI2]double excitation, while configuration 5 , the electronic structure for thione methylide in C,symmetry, corresponds to the (a*)2configuration of cyclic thiirane ...2b121a223b125b228alo
-
resulting from the ( u * ) ~[8aI2 5b?] double excitation. This excitation was used rather than ( T ” ) ~ (a*)2 [3bI2 5b?] since the C-C bond is broken during ring opening. The two-configuration wave function (4 5 ) encompasses the occupied-virtual orbital crossing for the disrotatory process, as suggested by Figure 1, and smoothly correlates with both the ring and open structures in the C, symmetry point group. For the conrotatory process a single configuration for each stationary point will, in principle, suffice on the basis of orbital symmetry; however, for a more unbiased comparison of the activation energies for the conrotatory and disrotatory mechanisms, a second configuration was incorporated in the wave function for the conrotatory process described by
+
...6b28a27b28b29a0
(8)
Configuration 8 correlates smoothly with doubly excited configurations ( 6 ) and (7) of thiirane and thione methylide, respectively. The two-configuration wave function (3) (8) is also consistent with the two-configuration wave function for the disrotatory process (4) + ( 5 ) .
+
(see below) is from the same source as for carbon, with the polarization function orbital exponent a d ( 0 )= 0.85. The more flexible basis set of
triple-r plus double polarization function (TZZP) quality was utilized since energy differences, particularly involving cyclic structures, are often sensitive to the quality of the basis set.“,” This TZ2P orbital set involves McLean and chandler'^'^ 6s5p contraction of Huzinaga’s” 12s9p primitive set for sulfur and Dunning’s45 5s3p and 3s contractions of Huzinaga’s IOs6p and 4s sets for carbon and hydrogen, respectively, augmented with two sets of Cartesian d-like functions for C, S, and p-like functions for H. The polarization function orbital exponents employed were ad@) = 1.0, 0.25, q ( C ) = 1.5, 0.375, a (H) = 1.5, 0.375. This basis set is lOs6p2d/5~3pZd), H(4sZp/3s2p). designated S ( 12s9p2d/6s5pZd), The TZZP/DZP basis set consists of TZ2P on C and S and DZP on the H atoms. The hydrogen s functions have been scaled by the standard factor of 1.2 in all three basis sets. The geometries of the various structures involved in thiirane ring opening have been fully optimized within the given symmetry constraints with use of the two-configuration self-consistent-field (TCSCF) analytic first derivative method.s0 The nature of the stationary points was determined via prediction of harmonic vibrational frequencies with analytic TCSCF second derivative technique^.*^*^' The geometries of the three species involved in the conrotatory process, thiirane, thione methylide, and the conrotatory stationary point, were also optimized using single configuration SCFSoand single reference configuration interaction CISD3’ss2 analytic gradient procedures. At the S C F level of theory harmonic vibrational frequencies were determined via analytical second-derivative techniques5’J4 and at the CISD level of theory via a finite difference of analytic gradients. For the sake of comparison, the geometries of the structures involved in oxirane ring opening were optimized at the TCSCF level with the DZP basis set. The three species involved in the oxirane conrotatory process were also subject to geometry optimizations at the DZP S C F level of theory. The frozen core and frozen virtual approximation was applied at the CISD level of theory. Specifically, the seven lowest occupied molecular orbitals (S Is, 2s, 2p; C Is-like orbitals) were held doubly occupied (frozen cores) and the three highest lying virtual orbitals (S Is*, C Is*-like orbitals) were deleted (frozen virtuals) in all configurations. Otherwise all singly and doubly excited configurations with respect to the S C F (SCF-CISD or 1R CISD) and TCSCF (TCSCF-CISD or 2R CISD) references have been included. With the TZ2P basis set the TCSCF-CISD wave function involves 212 027 and 223 299 configurations for thiirane and thione methylide in C , symmetry and 419662 and 419 483 configurations for the conrotatory and disrotatory stationary points in C, and C, symmetries, respectively. These CISD wave functions were determined by using the shape driven graphical unitary group app r o a ~ h . The ~ ~ effect of unlinked quadruple excitations on the CISD relative energies is estimated by incorporating the Davidson correction,% and the subsequent energy differences are labeled CISD+Q. Refined energetic predictions were obtained from TZ2P 1R CISD and DZP, TZZP/DZP, TZ2P 2R CISD single-point energies at the appropriate S C F or TCSCF optimized geometries.
e(
Results and Discussion
In Tables I-VI we report the optimized geometries, dipole moments, harmonic vibrational frequencies, and infrared intensities for the isomeric forms of C2H,S considered in this investigation at various levels of theory. Relative energies for the thiirane isomers are reported in Tables VII. Total and relative energies for the oxirane system are shown in Table VIII. Thiirane. The symmetry of thiirane was constrained to the C , point group during the geometry optimizations and the resulting
Theoretical Procedures Three basis sets, DZP, TZZP/DZP, and TZZP, were employed in this study. The DZP basis set is constructed from the standard HuzinagaDunningdouble-f contraction of Gaussian functions augmented with a set of Cartesian d-like functions for C, S, and p-like functions for H. This complete double-r plus polarization (DZP) basis set is designated S ( 1 I s7pld/6s4pl d), C(9~5pld/4sZpld),H(4sl p/2sl p). The polariza= 0.5, ad(C) = 0.75, tion function orbital exponents employed were ap(H) = 0.75. The oxygen DZP basis set used for oxirane calculations ~
~
~~~~~~
(44) (a) Huzinaga. S.J . Chem. Phys. 1965,42, 1293. (b) Huzinaga, S. Approximate Atomic Wavefunctions 11, Department of Chemistry Report, University of Alberta, Edmonton, Alberta, Canada, 1971. (45) (a) Dunning, T. H. J . Chem. Phys. 1970,53,2823. (b) Dunning, T. H. J. Chem. Phys. 1971,55, 716. (46) Dunning, T. H.; Hay, P. J. Modern Theoreticol Chemistry; Schaefer, H. F., Ed.; Plenum: New York 1977; Vol. 3, pp 1-27.
(47) Alberts, I. L.; Grev, R. S.;Schaefer, H. F. J . Chem. Phys. In press. (48) Bernholdt, D. E.; Magers, D. H.; Bartlett, R. J. J. Chem. Phys. 1988, 89, 3612. (49) McLean, A. D.; Chandler, G. S. J . Chem. Phys. 1980, 72, 5639. (50)Goddard, J. D.; Handy, N. C.; Schaefer, H. F. J . Chem. Phys. 1979, 71, 1525. (51) Yamaguchi, Y.; Osamura, Y.; Fitzgerald, G.; Schaefer, H. F. J . Chem. Phys. 1983, 78, 1607. (52) Brooks, B. R.; Laidig, W.D.; Saxe, P.; Goddard, J. D.; Yamaguchi, Y.; Schaefer, H. F. J . Chem. Phys. 1980, 72, 4652. (53) Osamura, Y.; Yamaguchi, Y.; Saxe, P.; Vincent, M. A.; Gaw, J. F.; Schaefer, H. F. Chem. Phys. 1982, 72, 131. (54) Saxe, P.; Yamaguchi, Y.; Schaefer, H. F . J. Chem. Phys. 1982, 77, 5647. (55) Saxe, P.; Fox, D. J.; Schaefer, H. F.; Handy, N. C. J . Chem. Phys. 1982, 77. 5584. (56) Langhoff, S.R.; Davidson, E. R. Int. J. Quuntum Chem. 1974,461.
J . Am. Chem. SOC.,Vol. 113, No. 13, 1991 4771
Electrocyclic Ring- Opening Reaction of Thiirane
Table 1. Theoretical Prediction of the Total Energy, Structure, and Dipole Moment" for Thiirane level of theory energy r,(C-S) r,(CC) r,(C-H) B,(SCH) B,(CCH) B,(CSC) F DZP S C F -0.558 154 1.817 1.479 1.077 115.2 118.2 48.0 2.257 1.817 1.471 1.07 1 114.8 118.2 47.7 2.237 TZ2P S C F -0.600 901 118.0 48.2 1.979 DZP ClSD -0.954 037e 1.820 1.487 1.084 115.5 118.0 49.0 2.214 DZP TCSCF -0.573 584 1.815 1 SO4 1.077 1 15.4 TZ2P/DZP TCSCF -0,614332 1.816 1.495 1.074 115.1 118.1 48.6 2.210 TZ2P TCSCF -0.6 16 448 1.815 1.495 1.072 115.0 118.1 48.6 exDerimentd 1.820 1.492 1.078 114.9 117.8 48.4 "Energy in hartree, bondiengths in A, angles in deg, dipole moment ( M ) in D. bAdd -475 hartrees. CWithinclusion of the Davidson correction, the DZP CISD+Q energy is -476.003 747. "The experimental structure parameters are ro and Bo values from refs 42 and 43. Table 11. Theoretical Harmonic Vibrational Freuuencies DZP amrox descriDtion symmetry SCF C-H stretch a1 3308 (19.2)c CHI scissor a1 1629 (2.7) C-C stretch a1 1231 (4.3) CHI wag a1 1138 (0.1) a1 686 (46.9) C-S stretch
and Infrared Intensities" for Thiirane TZZP DZP DZP TZ2P/DZP SCF CISD TCSCF TCSCF experimentb 3297 (14.5) 3246 3304 (21.3) 3278 (16.8) 3014 (Xd.') 1630 (3.3) 1572 1614 (4.2) 1613 (5.0) 1457 (7) 1216 (2.4) 1199 1182 (1.6) 1179 (1.1) 11 10 (4) 1140 (0.3) 1099 1104 (0.2) 1097 (0.2) 1024 (1) 654 (46.3) 690 678 (45.2) 647 (45.1) 627 (75 - .Y) C-H stretch a2 3393 (0.0) 3376 (0.0) 3336 3392 (0.0) 3361 (0.0) C H 2 twist a2 1294 (0.0) 1307 (0.0) 1241 1255 (0.0) 1268 (0.0) CHI rock a2 972 (0.0) 976 (0.0) 94 1 974 (0.0) 975 (0.0) C-H stretch bl 3407 (12.6) 3392 (2.8) 3348 3405 (14.3) 3376 (3.8) 3088 (18) CHI rock bl 1041 (2.7) 1043 (3.0) 1004 1044 (2.6) 1044 (2.7) 945 (8) CHI twist bl 893 (0.7) 897 (0.6)) 872 867 (0.7) 871 (0.5) 824 ( C l ) C-H stretch b2 3301 (22.4) 3291 (11.3) 3240 3298 (24.2) 3273 (15.2) 3013 (100 - x') CHI scissor b2 1590 (0.5) 1594 (1 .O) 1535 1583 (0.1) 1584 (0.2) 1436 (5) CH2 wag b2 1183 (43.6) 1198 (28.8) 1127 1171 (42.2) 1186 (28.0) 1051 (80) C-S stretch b2 740 (0.7) 725 (0.4) 725 746 (0.5) 727 (0.2) " Harmonic frequencies in cm-' and infrared intensities in km mol-I. bExperimental frequencies from ref 41. cInfrared intensities in parentheses. dThe experimental infrared intensity is a relative value. 'Overlapping bands, x and y change value with change of isotopomer. Table 111. Theoretical Prediction of the Total Energy, Structure, and Dipole Momento for Thione Methylide level of theory energyb r.(C-S) f.(C-C) fAC-HJ r.(C-HJ B.(SCH.) BJHCH) B.(CSC) DZP SCF -0.470 102 1.619 2.749 1.074 1.073 123.3 121.1 116.3 TZ2P S C F -0.519965 1.605 2.731 1.069 1.068 123.5 121.2 116.6 1.080 DZP ClSD -0.873 13lC 1.635 2.761 1.078 123.2 115.2 121.3 DZP TCSCF -0.507 532 1.666 2.774 1.074 1.072 123.0 121.2 112.8 TZ2P/DZP TCSCF -0.551 069 1.647 2.758 1.070 1.069 123.2 121.3 113.7 -0.553 055 TZ2P TCSCF 1.647 2.758 1.069 1.068 123.2 121.3 113.7 "Energy in hartree, bond lengths in A, angles in deg, dipole moment ( M ) in D. *Add -475 hartrees. cWith inclusion of the Davidson the DZP CISD+Q energy is -475.927 632. Table IV. Theoretical Harmonic Vibrational Frequencies DZP approx description symmetry SCF C-H stretch a1 3474 (O.O)b C-H stretch a1 3346 (1.0) a1 1565 (2.7) CH2 scissor CHI rock a1 1136 (5.2) C-S stretch a1 1009 (0.6) CSC bend a, 371 (4.0)
and Infrared Intensities" for Thione Methylide TZ2P DZP DZP SCF CISD TCSCF 3452 (1 .O) 3422 3472 (0.8) 3333 (0.4) 3290 3341 (5.2) 1569 (2.4) 1504 1552 (0.0) 1133 (6.1) 1081 1095 (1.2) 1003 (0.1) 962 847 (15.0) 378 (3.5) 350 347 (0.3)
TZ2PIDZP
TCSCF 3441 (0.1) 3316 (2.7) 1554 (0.2) 1094 (1.3) 849 (8.0) 355 (0.6)
N
0.020 0.232 0.044 0.610 0.654 0.652 correction,
TZ2P TCSCF 3453 (0.2) 3328 (2.3) 1556 (0.2) 1096 (1.8) 850 (7.9) 355 (0.6) 510 (0.0) 60 (0.0) 612 (27.4) 395 (146.0)
424 (0.0) 297 474 (0.0) 510 (0.0) 702 (0.0) 584 157i (-) 18i (-) 639 (41.2) 654 595 (29.1) 611 (27.4) 730 (201.8) 528 325 (183.6) 390 (148.4) C-H stretch b2 3474 (0.0) 3453 (0.5) 3423 3472 (0.0) 3441 (0.5) 3452 (0.5) C-H stretch b2 3345 (12.4) 3331 (14.5) 3289 3342 (0.8) 3317 (0.2) 3330 (0.2) CH2 scissor b2 1531 (72.8) 1536 (56.7) 1476 1524 (2.7) 1526 (4.5) 1527 (4.5) C-S stretch b2 1171 (247.9) 1162 (234.9) 1139 1081 (2.8) 1074 (9.9) 1075 (9.6) CHI rock b2 962 (5.1) 957 (4.5) 920 948 (0.8) 944 (1.1) 945 (0.9) "Harmonic frequencies in cm-I and infrared intensities in km mol-'. bInfrared intensities in parentheses. CThea2 symmetry CHI wagging and twisting modes are heavily coupled. CHI twiste CH2 wagc CH2 twist CHI wag
a2 a2 bl bl
414 681 610 729
(0.0) (0.0) (93.8) (221.6)
structuresare shown in Table I. T h e valence electron configuration of cyclic thiirane is described by eq 1. Multiple bonding is not significantly involved in the CSC ring since t h e out-of-ring plane l a 2 and 2b, molecular orbitals correspond to C-H bonding orbitals,
and 3bl is localized predominantly on sulfur. This is exemplified by the C-C and C-S bond lengths: 1.487 and 1.820 A respectively at the DZP SCF-CISD level, which are within the range of typical single bond distances for cyclic compounds.
Fowler et al.
4112 J . Am. Chem. SOC.,Vol. 113, No. 13, 1991
Table V. Theoretical Prediction of the Total Energy, Structure, and Dipole Moment' for the Conrotatory Transition State level Of theory energ? r,(c-s) r,(C-Hb) O,(SCH.) B,(HCH) O,(CSC) a,(H,CSc) 6,(HbCSC) f,(C
