How steric effects favor thiepins over their benzene sulfide tautomers at theoretical levels?

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.