Pressure-dependent gas-phase ¹H NMR studies of conformational kinetics in a homologous series of alkyl nitrites

Pressure-dependent rate constants for the syn ⇌ anti conformational exchange in gaseous alkyl nitrite (RONO, R = CH₃, C₂H₅, n-C₃H₇, n-C₄H₉)-CO₂ mixtures have been obtained from line-shape analyses of ¹H NMR spectra at 300 MHz. The observed pressure dependence of the exchange rate constants agrees with predictions based on RRKM theory, demonstrating that intramolecular vibrational energy redistribution (IVR) is occurring at, or near, the statistical limit in all four nitrites at internal energies between 11 and 13 kcal mol^-1. Thus, internal energy in the additional vibrational degrees of freedom associated with each incremental increase in alkyl chain length in this series is available for the syn-anti conformational process.