Conformational characteristics of methyl nitrite: A cryospectroscopic study

Mid-infrared spectra of solutions in liquid argon, krypton, and xenon of me thyl nitrite, which occurs as a mixture of cis and trans conformers, have b een investigated. In liquid argon and in liquid krypton at temperatures bel ow 150 K, the rate of conformational equilibration is found to be negligibl e, while at temperatures above 160 K in liquid krypton, and in liquid xenon , the equilibration is near-instantaneous. The standard enthalpy difference DeltaH degrees between the conformers in liquid krypton was measured in th e 165-205 K interval to be 3.02(9) kJ mol(-1), with the cis as the more sta ble conformer. Starting from solutions containing nonequilibrium. populatio ns of the conformers, the conformational equilibration was studied as a fun ction of time at five different temperatures between 150 and 160 K in liqui d krypton. From these kinetic data, the extinction coefficient ratios for t he nu (3) and nu (8) conformational doublets have been determined to be 1.2 (2) and 4.8(8), respectively. The extinction coefficient ratio for nu (8) w as combined with the value for DeltaH degrees to yield the value of 5.1(2) J K-1 mol(-1) for the standard conformational entropy difference DeltaS deg rees. The kinetic data have also been used to calculate the enthalpy and en tropy of activation for the interconversion from trans to cis conformer, an d were found to be 44.0(14) kJ mol(-1) and 16(9) J K-1 mol(-1), respectivel y. The results are discussed in light of literature data and in light of pe rturbation Monte Carlo calculations on solvation enthalpy and entropy.