The change of enthalpies, energy differences, activation energies and equilibrium constants (at 298 K), for valence tautomerizations between 2,7-di(X)benzene sulfides (BX-X) and 2,7-di(X)thiepins (TX-X), also between 2-tert-butyl-7-(X)benzene sulfide (BX-Y) and 2-tert-butyl-7-(X)thiepin (TX-Y), are estimated at ab initio (MP2/6-311++G**, HF/6-311++G**) and DFT (B3LYP/6-311++G**) levels, where X = H, Me, Et, i-Pr, and Y = t-Bu. For X = H, benzene sulfide (BH-H) is more stable than thiepin (TH-H) (ΔH = 6.37 kcal mol-1), in the above BX-X/TX-X series. Likewise, BH-t-Bu is more stable than TH-t-Bu (ΔH = 5.62 kcal mol-1), in BX-Y/TX-Y series. In contrast, for X = Me, Et, and i-Pr, steric effects shift the equilibrium in favor of TX-Y in such a way that Keq for Bi-Pr-t-Bu/Ti-Pr-t-Bu is about 109 times greater than that for BH-t-Bu/TH-t-Bu. The B3LYP/6-311++G** calculated activation energies for inversions of thiepins to their mirror images, show higher energy barriers for all the TX-Y thiepins, compared to their corresponding TX-X analogues. Magnetic (NICS) criterion indicates virtual non-aromaticity for all TX-X and TX-Y thiepins, but extreme antiaromaticity for all their corresponding inversion transition states.
Copyright 2008 Elsevier B.V., All rights reserved.
- Ab initio
- Benzene sulfide
- Steric effects