Conversions over low barriers. VI. The c i s–t r a n s isomerization of monothioformic acid

Abstract

The dynamics of the trans⇄cis isomerization of monothioformic acid (rotation about the HC–SH bond) is a case study for an intramolecular conversion over a significant barrier [microwave spectra indicates Vmax(θ) =3500 cm−1], where the density of vibrational states at the potential maximum is very low, ≈2.5/cm−1. We recorded the NMR spectra of dilute solutions of the acid in CD2Cl2, and of the gas at low pressures, over the temperature range 300–170 K, to determine the exchange relaxation time and to deduce the corresponding kinetic parameters. In spite of difficulties with the preparation and maintainence of the pure material, (i) correct assignment of the NMR resonances were made; (ii) the magnitude of E0sol for isomerization in solution, from k∞u, was found to be compatible with the barrier derived from microwave spectra; (iii) in the gas phase this intramolecular conversion is in the second order regime, but (iv) only an upper limit could be set on the magnitude of E0gas for isomerization, which is at least 2 kcal/mol lower than E0sol.