Tautomerism of pyrazoline-5-thione and triazoline-5-thione studied by variable temperature photoelectron spectroscopy

Abstract

Photoelectron spectra at variable temperatures demonstrate that the rate of thermal isomerization of pyrazoline-5-thione (1a) into methylthiopyrazole(s) in the gaseous phase is metal catalysed. Similar isomerizations of triazolethione (2a and 3a) are at best only slightly so.Furthermore, photoelectron spectra and MNDO calculations show that gaseous methylthiopyrazoles (1b and 1c) and methylthiotriazoles (2b and 2c) exist predominantly in planar form for the b isomer and in nonplanar form for the c isomer. The ΔH0 for the rotameric equilibria is 2.7–4.2 kJ•mol−1.