The compounds HC=CXMe (X = O, S, Se) were studied by the MP2(full)/6-31G* ab initio method using the method of natural bond orbitals. The parameters of the molecular geometry were obtained. The electron density distribution in these compounds is determined to a greater extent by the electronegativity of atoms X, than by conjugation of the lone electron pairs of the heteroatoms with the triple bond.Chemical transformations of acetylene and its derivatives allow preparation of new promising monomers, cross-linking agents, and intermediates for fine organic synthesis [1,2]. Understanding of the factors affecting the reactivity of the compounds and development of new synthetic routes are impossible without comprehensive study of their electronic and steric structure by physicochemical and theoretical methods. Modern ab initio quantum-chemical methods taking into account the electron correlation reproduce the experimentally observed molecular characteristics with a high accuracy, without introducing any empirical fitting parameters [3]. Analysis of the wave functions obtained by the method of natural bond orbitals (NBO) allows transformation of the canonical molecular orbitals into orbitals of the Lewis type and interpretation of the results of quantum-chemical calculations in terms of traditional chemistry [4 3 6].Our goal was to apply this approach to the compounds HC=CXMe (X = O, S, Se) and to analyze the quantitative information on the nature of the lone electron pairs of the O, S, and Se atoms, on their interaction with the triple bond, and on their effect on the electron density distribution.Our study was based on ab initio quantum-chemical calculations taking into account the correlation energy for all the electrons on the level of the secondorder Møller3Plesset perturbation theory [3,7] in the 6-31G double-split basis set with six d functions [8]: MP2(full)/6-31G*. The calculations were performed using the GAUSSIAN 98W (Rev. A.7) [9] and GAMESS [10] software. We used standard criteria of convergence of the density matrix and energy gradient. The population analysis of the wave functions was performed with the NBO method [4 3 6]. We obtained the following total energies (E t , au): 3191.2592574 (HC=COMe), 3513.8919695 (HC=CS . Me), and 32514.0062393 (HC=CSeMe). For HC b =C a . OMe, the COC bond angle (113.4o) and the C sp 3O (1.313 A) and C sp 33O (1.434 A) bond lengths have been determined experimentally [11, 12]. We obtained the following structural parameters for this molecule: 112.9o (UCOC); 1.217 [d(C=C)], 1.315 [d(C a 3X)], 1.064 [d(C b 3H)], 1.445 [d(X3CMe)], and 1.088, 1.092, 1.092 A [d(C3H Me )]. The steric structure of HC b =C a SMe was studied by electron diffraction [13] and microwave spectroscopy [14]. The C=C bond length is 1.215(3) [13] or 1.205(7) A [14]; the C sp 3S bond length is 1.683(3) [13] or 1.685(5) A [14], and the C sp 33S bond length, 1.814(3) [13] and 1.813(2) A [14]. The CSC bond angle is 99.9o [13, 14], and the SCC bond angle, 184(3)o [13] or 182(0.3)o [14]. We obtained the followin...
The potential functions of internal rotation around the C sp 23S bond in the C 6 H 5 S(O)CH 3 and C 6 H 5 S(O)CF 3 molecules were obtained by ab initio MP2(full)/6-31+G* calculations. The stationary points were identified by solving the vibrational problems. The structures in which the plane of the C sp 23S3C sp 3 bonds is approximately perpendicular to the benzene ring plane correspond to the energy minimum. The barriers to rotation around the C sp 23S bond, corrected for the zero-point vibration energy, are 21.29 [C 6 H 5 S(O)CH 3 ] and 28.98 [C 6 H 5 S(O)CF 3 ] kJ mol !1 . The bond angles (deg) are as follows: 95.7 (CSC), 107.1 (C sp 2SO), 106.3 (C sp 3SO) in C 6 H 5 S(O)CH 3 ; 93.5 (CSC), 108.2 (C sp 2SO), 105.2 (C sp 3SO) in C 6 H 5 S(O)CF 3 . The bond lengths are as follows (A): 1.520 (S=O), 1.804 (C sp 23S), 1.810 (C sp 33S) in C 6 H 5 S(O)CH 3 ; 1.507 (S=O), 1.799 (C sp 23S), 1.870 (C sp 33S) in C 6 H 5 S(O)CF 3 . According to the results of NBO calculations, the formally double S=O bond consists of a strongly polarized covalent s bond (S6O) and an almost ionic bond.An increase in the S=O bond multiplicity relative to a single bond is mainly due to hyperconjugation by the mechanism n(O)6s*(C sp 23S) and n(O)6s*(C sp 33S) and, to a lesser extent, by interaction of the oxygen lone electron pairs with the Rydberg orbitals of the S atoms, characterized by a large contribution of the d component.
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