Structural features of hydrogen thioperoxide (oxadisulfane,
H–S–O–H)
and of alkanesulfenic acids (R–S–O–H; R = CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, C(CH3)3, CF3, CCl3) and the mechanisms of their
dehydrative cyclocondensation to the respective sulfinothioic acid
(H–(SO)–S–H) and alkyl alkanethiosulfinates
(R–(SO)–S–R) have been studied using
coupled cluster theory with single and double and perturbative triple
excitations [CCSD(T)] and quadratic configuration interaction with
single and double and perturbative triple excitations [QCISD(T)] with
the cc-pVDZ basis set and also using second-order Møller-Plesset
perturbation theory (MP2) and the hybrid density functionals B3LYP,
B3PW91, and PBE1PBE with the 6-311+G(d,p) basis set. The concerted
cyclodehydration mechanisms include cyclic five-center transition
states with relatively long distance sulfur–sulfur bonding
interactions. Attractive and repulsive nonbonding interactions are
predicted in the sulfenic acids, transition states, and thiosulfinates.
In the alkyl alkanethiosulfinates attractive cyclic C–H----OS
nonbonding interactions are predicted. CCSD(T) and QCISD(T) predict
similar values for the relative energies and CCSD(T) predicts the
barrier to the cyclocondensation of H–S–O–H to
sulfinothioic acid (H–(SO)–S–H) to be
41.8 kcal/mol, and barriers in the range of 37.5 to 39.6 kcal/mol
are predicted for the condensation of alkanesulfenic acids to alkyl
alkanethiosulfinates.
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