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.
Rate constants for the gas-phase reactions of OH radicals with ethylene and halogenated ethylene compoundsThe usually quoted expression for the second order rate constant, for a unimolecular reaction at the low pressure limit, is valid only for strictly irreversible processes. Its application to isomerization reactions (which are to some extent reversible) is demonstrably in error; corrected expressions have been published. Attention is directed to intramolecular conversions over low barriers, for which the inappropriateness of the unidirectional expression becomes obvious. For such isomerizations we propose a model which incorporates only operationally observable states, so that an essential conceptual ambiguity is avoided. Use of this model is illustrated for the syn ~anti conversions of methyl nitrite, derived from a gas phase NMR coalescence curve (Me :Te)' The present data suggest that during isomerization the alkyl nitrites may not be completely ergodic on a time scale of 10-9 s. A regional phase-space model is proposed which has the appropriate formalism to account for this behavior.
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