Ab initio MO calculations of sulfonium, selenonium and telluronium ylides R',X+-C-R2, (X = S, Se, Te; R' = H, Me; R2 = H, CN) have been performed to estimate their thermal stabilities toward the pyramidal inversion (vertex inversion) reaction. Geometrical parameters at both the global minima and the saddlepoints are independent of the levels of theory employed. However, the activation energies for the pyramidal inversion reaction depend on the levels of theory employed; a method superior to the Hartree-Fock level of theory was necessary for the present purposes. The activation energies for the pyramidal inversion were also found to depend on the substituents. The present computations clearly indicate that the telluronium ylides require higher activation energies for the pyramidal inversion reaction than those of the sulfonium and selenonium ylides.

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Kinetic studies were carried out on the thermal epimerization by pyramidal inversion of optically active (S)‐(−)Se‐ and (R)‐(+)Se‐4‐[(‐)‐menthyloxycarbonyl]phenyl(2′,4′,6′‐triisopropylphenyl)selenonium‐N‐toluene‐4″‐sulphonimides (1). The first‐order rate constants for the pyramidal inversion of (S)‐(−)Se‐1 and (R)‐(+)Se‐1 were determined and the activation enthalpies and entropies were calculated. A fairly large difference was found in the activation parameters between optically active (S)‐(−)Se‐1 and (R)‐(+)Se‐1, i.e. a large negative activation entropy was obtained in the case of (R)‐(+)Se‐1.

The Stern-Volmer quenching equation for use of a dualistic quencher, which serves not only as the quencher for an excited starting molecule but also as the promoting agent for product formation, was obtained by applying the steadystate approximation to the photoreaction system. The equation was applied to the kinetic analysis of the Norrish type I1 photoreaction of valerophenone using triethylamine, which serves as the excited-state quencher and as the promoting agent for product formation.