Thianthrene resonance energies

The folded nature thus demonstrated experimentally is consistent with the conclusion (63T471 64JA164) from molecular orbital (MO) calculations that, in thianthrene, there is little overlap between a sulfur lone pair and the two p orbitals adjacent to it (78PS1). Similarly, later calculations showed that the resonance energy of thianthrene is essentially that of the two benzene rings (80T2711). 

Resonance energies have been reported164 (see also ref. 154a) for a few conjugated sulfur compounds thiolacetic acid (4-5 kcalmole-1), thiourea (27 kcalmole-1), thiosemicarbazide (28 kcalmole-1), thiophene (20 kcalmole-1), and thianthrene (17 kcalmole-1 difference between the resonance energy of thianthrene and that of two benzene molecules). These values were calculated from the heats of combustion and bond energies, which are 61.5 kcal for the C—S, 115 kcal for the C=S, 87.5 kcal for the S—H, and 67 kcal for the S—S bonds.156 Calorimetry of sulfones has been studied intensively.156... 

The early evaluations of DE (Section 2.26.2.1) for 1,4-dithiin and thianthrene, which suggested that both ring systems have aromatic character, received support from an empirical measurement of the resonance energy of thianthrene using combustion methods. The compound was found to have a resonance energy of 71 kJ mol-1 in excess of that for two benzene rings (55ACS847). However, it will be recalled that more recent theoretical treatments... 

Finally, we turn to the molecule thianthrene, in which two benzene molecules are joined by sulphur atom bridges, to give a central six-membered ring. The heat of formation of gaseous thianthrene is + 63 0 kcal/moled The resonance energy of this molecule, in excess of that of two benzene nuclei, is obtained from the following formal reaction ... 

Hence the resonance energy of thianthrene, in excess of the resonance energy of two benzene nuclei, is (2 AH — AH) = 16 8 kcal/ mole. 

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