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

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 SCO of 2 was found to be exceedingly long (>1.80 Å). 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. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002