An ab initio Quantum-Chemical Study of C6H5S(O)CH3 and C6H5S(O)CF3

Abstract

The potential functions of internal rotation around the C $$_{sp^2 }$$ -S bond in the C6H5S(O)CH3 and C6H5S(O)CF3 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 $$_{sp^2 }$$ -S-C $$_{sp^3 }$$ bonds is approximately perpendicular to the benzene ring plane correspond to the energy minimum. The barriers to rotation around the C $$_{sp^2 }$$ -S bond, corrected for the zero-point vibration energy, are 21.29 [C6H5S(O)CH3] and 28.98 [C6H5S(O)CF3] kJ mol−1. The bond angles (deg) are as follows: 95.7 (CSC), 107.1 (C $$_{sp^2 }$$ SO), 106.3 (C $$_{sp^3 }$$ SO) in C6H5S(O)CH3; 93.5 (CSC), 108.2 (C $$_{sp^2 }$$ SO), 105.2 (C $$_{sp^3 }$$ SO) in C6H5S(O)CF3. The bond lengths are as follows (A): 1.520 (S=O), 1.804 (C $$_{sp^2 }$$ -S), 1.810 (C $$_{sp^3 }$$ -S) in C6H5S(O)CH3; 1.507 (S=O), 1.799 (C $$_{sp^2 }$$ -S), 1.870 (C $$_{sp^3 }$$ -S) in C6H5S(O)CF3. According to the results of NBO calculations, the formally double S=O bond consists of a strongly polarized covalent σ bond (S→O) 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)→σ*(C $$_{sp^2 }$$ -S) and n(O)→σ*(C $$_{sp^3 }$$ -S) 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.