A multireference configuration interaction study with singles and doubles of some mesoionic rings: reaction and activation free energies for the ring‐opening reaction

For the first time, multireference configuration interaction with singles and doubles (MR‐CISD) calculations (including extensivity corrections, MR‐CISD+Q) have been performed to study the ring‐opening reactions of the following mesoionic rings and their 2, 3, and 4 methyl‐substituted derivatives: ( a ) 1,3‐oxazol‐5‐one, ( c ) 1,3‐oxazol‐5‐thiolate, and ( d ) 1,3‐thiazol‐5‐thiolate. The ring‐opening reaction of the parent 1,3‐thiazol‐5‐one mesoionic ring ( b ) has also been studied. The effect of methyl and S replacement on the reaction and activation free energies (ΔG and ΔG † , respectively) is studied. For a , the effect of methyl replacement on C2 is almost negligible, while on N3 and C4, it is significantly larger, especially in the latter position. The open structure (ketene form) of a is considerably more stable, and the replacement of the exocyclic O by S stabilizes the ketene tautomer even more and increases ΔG † considerably. On the other hand, replacement of the endocyclic O by S (yielding b ) completely prevents the formation of the open structure, while replacement of both endo and exo atoms strongly stabilizes the cyclic structure and leads to a remarkably high value of ΔG † . In some cases, the relative polarity of the stationary points is used to estimate how ΔG and ΔG † are expected to change as one goes from the gas phase to aprotic polar solvents. A linear correlation between the multiconfigurational character of the mesoionic rings and ΔG † has been observed, and such correlation is used to estimate the ΔG † values for the methyl‐substituted c and d . The largest barrier (13.78 kcal/mol) has been obtained for the 4‐methyl‐1,3‐thiazol‐5‐thiolate (4‐m‐ d ).