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(1 1) (a) T. Ito, N. Morimoto, and R. Sadanaga, Acta Crystallogr., 5 , 775 (1952); (b) P. Goldstein and A. Paton, Acta Crystallogr., Sect. B, 30, 915 (1974). (12) I. Chen, Phys. Rev. B, 11, 3976 (1975). (13) W. R. Saianeck, K. S. Liang, A. Paton, and N. 0. Lipari, Phys. Rev. B, 12, 725 (1975). (14) A. Tachibana, T. Yamabe, M. Miyake, K. Tanaka, H. Kato, and K. Fukui, J. Phys. Chem., 82, 272 (1978). (15) T. Yonezawa, H. Konishi, and H. Kato, Bull. Chem. SOC.Jpn., 42, 933 (1969). (16) Pa-0.Lowdin, Phys. Rev., 97, 1509 (1955). (17) A. T. Amos and L. C. Snyder, J. Chem. Phys., 41, 1773 (1964). (18) T. Yamabe, K. Tanaka, K. Fukui, and H, Kato, J. Phys. Chem., 81, 727 (1977). (19) .E. Clement; and D.L. Raimondi, J. Chem. Phys., 38, 2686 (1963). (20) H. Basch, A. Viste, and H. B. Gray, Theor. Chim. Acta, 3, 458 (1965). (21) H. L. Hase and A. Schweig, Theor. Chim. Acta, 31, 215 (1973). (22) (a) J. Hinze and H. H. Jaffe, J . Chem. Phys., 38, 1834 (1963); (b)
W. F. Hwang and H. A. Kuska C. E. Moore, Natl. Bur. Stand. (US.), Ckc.,No. 467, Vol. 1-111 (1956). (23) K. H. Johnson in “Advances in Quantum Chemistry”, Vol. 7, P.-0. Lowdin, Ed., Academic Press, New York, N.Y., 1973, p 143. (24) V. BonaEie-Kouteckg and J. I. Musher, Theor. Chim. Acta, 33, 227 (1974). (25) (a) J. A. Pople, D. P. Saniry, and 0. A. Segai, J . Chem. Phys., 43, S129 (1965); (b) M. S. Gordon, J. Am. Chem. Soc., 91,3122 (1969); (c) S.E. Eherenson and S. Seker, Theor. Chim. Acta, 20, 17 (1971); (d) S. Sakaki, ibid., 30, 159 (1973); (e) S. Sakaki, H. Kato, H. Kanal, and K. Tarama, Bull. Chem. SOC.Jpn., 47, 377 (1974). (26) See, for example, H. 0. Pritchard and H. A. Skinner, Chem. Rev., 55, 745 (1955), (27) See, for example, J. N. Murrell, S. F. A. Kettle, and J. M. Tedder, “Valence Theory”, Wiley, London, 1965, p 52. (28) J. A. Popie and D. L. Beveridge, “Approximate Molecular Orbital Theory”, McGraw-Hill, New York, N.Y., 1970, p 126. (29) P. W. Atkins and M. C. R. Symons, “The Structure of Inorganic Radicals”, Eisevier, Amsterdam, 1967.
An Ab Initio Molecular Fragment Investigation of the Inversion Barrier and Syn-Anti Isomerization Mechanisms of Formaldoxime W.
F. Hwang and H. A. Kuska”
Depatfment of Chemjstty, The Universjty of Akron, Akron, Ohio 44305 (Received November 9, 1977: Revised Manuscript Received July 11, 1978)
The ab initio molecular fragment method is used to calculate two possible intermediate states in the syn-anti isomerization of formaldoxime. The results suggest that the intermediate is stabilized by a hybridization change which results in the formation of a linear C-N-0 a system.
Introduction The syn-anti isomerization of formaldoxime has been the subject of a number of experimental and theoretical p a p e r ~ . l - ~From J ~ these studies it appears that the interconversion takes place by an inplane lateral shift r
Chart I: Resonance I n t e r a c t i o n in t h e Intermediate State of F o r m a l d o x i m e
1
Case I:no resonance i n t e r a c t i o n
rather than a 180° rotation around the C-N bond. The previous theorectical papers1J2 suggest that the intermediate undergoes a nitrogen hybridization change; however, the possibility of a hybridization change on the X group was not investigated. In this paper the ab initio molecular fragment method of Chri~toffersenl~ is used to study this possibility. Procedure For formaldoxime, the initial state (syn form) was constructed the same way as trans-forma1do~ime.l~ Two possible intermediate states were constructed as described in Chart I along with the molecular fragment date summarized in Table I. The two possible intermediate states utilize different oxygen lone pair hybridizations. The complete optimized geometry calculated by Liotard12 is adopted as our input nuclear coordinates for the intermediate state of formaldoxime. Results and Discussion The calculated total energies and the energy components are compared with that of the initial state in Table 11. 0022-365417812082-2126$0 t .OO/O
resonance i n t e r a c t i o n Case 11: “resonance interaction” b e t w e e n t h e f i l l e d n - t y p e 2p, o r b i t a l of o x y g e n a n d t h e C = N ?T system
As shown in Table 111, we find two significant results. First, without a hybridization change and resultant resonance interaction (as in case I) the inversion barrier, 74.9 kcal/mol, is too high to be acceptable. Second, with the resonance interaction scheme (case 11)the absolute E-N potential increases profoundly and stabilizes the system. An examination of the individual molecular orbitals reveals that a case I predominately oxygen lone pair orbital with an energy of -0.3752 au becomes a more stable C-N-0 a bonding orbital in case I1 (the energy is -0.4036 au and the carbon, nitrogen, and oxygen pr atomic orbital coefficients are 0.16, 0.47, and 0.77, respectively). The original case I C-0 a orbital has an energy of -0.1244 au and coefficients of 0.69 and 0.35 for the carbon and nitrogen. In case I1 it is slightly stabilized, -0.1287 au, and has carbon, nitrogen, and oxygen pr coefficients of 0.70, 0.38, and -0.52. 0 I978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 19, 1978 2127
Syn-Anti Isomerization of Formaldoxime
TABLE I: Molecular Fragment Data for Intermediate State of Formaldoximea7b FSGO distance from fragment type FSGO type “heavy” atom .. C-H C-7T C-inner shell N-H N-2py (lone pair) N-7T N-inner shell
.CH, (planar R(C,H) = 1.78562447) *NH, (sp at N) lone pair e-in max 2Py
0-H
Case I: H,O (oxy u ) , bent type
0-lone pair 0-inner shell 0-H 0-lone pair 0-lone pair ( 7 ~ ) 0-inner shell
Case 11: H,O (oxy 7 ~ )sp, hybrid. at 0, one lone pair in max 2Pz All distances are reported in Hartree atomic units.
TABLE 111: Inversion Barriers of Syn-Anti Isomerization of Formaldoxime (kcal/mol)
35.8 a
Reference 12.
ab initio H-F typea
CNDOa
Huckelb
50.6
37.5
23.0
0.0 0.89803124 0.51282051
io.l
0.0 0.63354170 0.1 0.00067298 0.79678221 0.23835937 i0.1 0.00083398
Reference 13.
TABLE 11: Total Energy and Its Components of Formaldoxime intermediate state energya ground state case I case I1 kinetic energy 138.3040 137.4913 137.9003 E-N potential -477.2894 -475.6030 -476.7941 E-E potential 124.8504 124.6007 125.3204 N-N potential 70.6601 70.1556 70.1556 total energy -143.4748 -143.3554 -143.4178 inversion 74.9235 35.7675 barrier, kcal/mol a Except for inversion barriers, all energies are in Hartree atomic units
FSGO
1.130931 39 i 0.1
FSGO radii ( p ) 1.51399487 1.80394801 0.32682735 1.43795016 1.35006653 1.35873044 0.27806030 1.34008728 1.35875976 0.24041778 1.37684374 1.36888573 1.13643749 0.24089502
Reference 1.
Case I1 differs from case I in two important ways. They are the change in oxygen hybridization and the formation of a coplanar C-N-0 T system. In order to obtain an estimate of the relative importance of these two stabilizing effects, case I1 was rerun with a 45’ rotation of the OH fragments (C-N-0 rotation axis). The total energy -143.3941 au is 24.28 kcal/mol lower than the case I energy but is 14.87 kcal/mol higher than the resonance case I1 energy. Thus, both contributions are significant. Experimentally,2J0J1it is found that the inversion energy barrier in the isomerization of the imine system has a linear dependence upon the electronegativity of the substituent group X (X = NH2, NHR, OH, OR, or halogen), Le., the higher the electronegativity of X, the higher the interconversion barrier. Although the major effect of increased electronegativitymay simply be a more stable ground state,
contributions from an increased hybridization promotion energy and a decrease of C-N-X T delocalization in the excited state are suggested. The calculated inversion barriers are compared with the barriers obtained from other methods in Table 111. Except for the Huckel result, the energy barrier values obtained seem to be too high in comparison with the general behavior of imine systems (see ref 2). It is to be noted that from the pure theoretical sense the FSGO results are far more acceptable than the results from the ab initio H-F type method. However, it is not appropriate to conclude that the FSGO method is basically much better than the more extensive ab initio method. Rather, it is because of the flexibility of the FSGO method in dealing with systems within the scope of significant chemical interest in general and because of the utilization of the knowledge concerning the expected resonance interaction for the intermediate state of formaldoxime in this particular case, that the FSGO method gives more acceptable results.
References and Notes G. Ostrogovlc, 2. Simon, and F. Kerek, Rev. Roum. Chim., 15, 1453 (1970). G. Ostrogovic and F. Kerek, J . Chem. Soc., 13, 541 (1971). H. A. Staab and F. Vogtle, Tetrahedron Lett., 51 (1965). H. A. Staab, F. Viigtie, and A. Mannschreck, Tetrahedron Lett., 697 (1965). H. A. Staab, F. Vogtle, A. Mannschreck, and D. Warmb-Gerlich, Annalen, 36, 708 (1967). D. Y. Curtin and J. W. Hausser, J. Am. Chem. Soc., 83,3474 (1961). D. Y. Curtin and C. G. McCarty, Tetrahedron Lett., 1269 (1962). D. Y. Curtin, E. J. Grubbs, and C. G. McCarty, J. Am. Chem. Soc., 86, 2775 (1966). E. Huckel, Z. Phys., 423 (1930). G. Wettermark, J. Weinstein, J. Sousa, and L. Dogliotti, J. fhys. Chem., 69, 1584 (1965). H. Kessler, Tetrahedron Leff., 2041 (1968). D. Liotard, A. Dargelos, and M. Chailet, Theor. Chim. Acta, 31, 325 (1973). R. E. Chrlstoffersen, L. L. Shipman, and G. M. Magglora, Int. J. Quantum Chem. Symp.. 5, 143 (1971). R. E. Christoffersen, Adv. Quantum Chem., 6, 333 (1972). W. F. Hwang and H. A. Kuska, J . Mol. Struct., 48, 239 (1978).
