## Abstract

The change of enthalpies, energy differences, activation energies and equilibrium constants (at 298 K), for valence tautomerizations between 2,7-di(X)benzene sulfides (B_{X-X}) and 2,7-di(X)thiepins (T_{X-X}), also between 2-tert-butyl-7-(X)benzene sulfide (B_{X-Y}) and 2-tert-butyl-7-(X)thiepin (T_{X-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 (B_{H-H}) is more stable than thiepin (T_{H-H}) (ΔH = 6.37 kcal mol^{-1}), in the above B_{X-X}/T_{X-X} series. Likewise, B_{H-t-Bu} is more stable than T_{H-t-Bu} (ΔH = 5.62 kcal mol^{-1}), in B_{X-Y}/T_{X-Y} series. In contrast, for X = Me, Et, and i-Pr, steric effects shift the equilibrium in favor of T_{X-Y} in such a way that K_{eq} for B_{i-Pr-t-Bu}/T_{i-Pr-t-Bu} is about 10^{9} times greater than that for B_{H-t-Bu}/T_{H-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 T_{X-Y} thiepins, compared to their corresponding T_{X-X} analogues. Magnetic (NICS) criterion indicates virtual non-aromaticity for all T_{X-X} and T_{X-Y} thiepins, but extreme antiaromaticity for all their corresponding inversion transition states.

Original language | English (US) |
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Pages (from-to) | 117-121 |

Number of pages | 5 |

Journal | Journal of Molecular Structure: THEOCHEM |

Volume | 861 |

Issue number | 1-3 |

DOIs | |

State | Published - Jul 30 2008 |

## Keywords

- Ab initio
- Benzene sulfide
- DFT
- NICS
- Steric effects
- Tautomerization
- Thiepin