Sulfur-containing mesoionic compounds: Theoretical study on structure and properties

摘要

The structure of the mesoionic 1,3-dithiolylium-4-olate (2) and thiopyrylium-3-olate (3) and of their derivatives were calculated by first-principles methods. The compounds are considered as cylic derivatives of open-chain thione S-ylides. The extent of cyclic electron delocalization was discussed in terms of the molecular charge distribution and nucleus-independent chemical shift values (NICs). The calculated CS bond of S-C=O of 2 was found to be exceedingly long (>1.80 Angstrom). This result suggests a facile CS ring opening of 2 to S-ketenyl dithiomethylester. The transition structure of this reaction is nearly isoenergetic with the product of the reaction. The calculated reaction barrier amounts to about 12 kcal/mol and the ring-to-chain valence isomerization should therefore occur under thermal conditions. This result is in line with the chemical reactivity of diphenyl-2. According to time-dependent density functional theory calculations (TD-DFT) the sulfur-containing mesoionic compounds 2 and 3 should be colored. The intense long wavelength absorption of diphenyl-2 at about 550 nm is well reproduced. (C) 2002 Wiley Periodicals, Inc. [References: 37]