An ab initio Quantum-Chemical Study of C_6H_5S(O)CH_3 and C_6H_5S(O)CF_3

摘要

The potential functions of internal rotation around the C_(sp~2)-8 bond in the C_6H_5S(O)CH_3 and C_6H_5S(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~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 [C_6H_5S(O)CH_3] and 28.98 [C_6H_5S(O)CF_3] 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 C_6H_5S(O)CH_3;93.5(CSC),108.2(C_(sp~2)SO),105.2(C_(sp~3)SO)in C_6H_5S(O)CF_3.The bond lengths are as follows(A):1.520(S=O),1.804(C_(sp~2)-S),1.810(C_(sp~3)-S)in C_6H_5S(O)CH_3;1.507(S=O),1.799(C_(sp~2)-S),1.870(C_(sp~3)-S)in C_6H_5S(O)CF_3.According to the results of NBO calculations,the formally double S=O bond consists of a strongly polarized covalent a 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)->sigma*(C_(sp~2)-S)and n(O)->sigma*(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.