Cis and trans methyl nitrite: Effusive molecular beam i.r. matrix spectra; thermodynamics and ab initio calculation of the cis-trans interconversion

Abstract

The i.r. spectra of cis and trans methyl nitrite have been studied by trapping a thermal effusive molecular beam in an argon matrix. Variation of the beam source temperature allowed to distinguish between absorption bands of the thermodynamically more stable cis and of the less stable trans conformer. From the intensity behavior of selected trans/cis band pairs the enthalpy of conformer interconvesion was determined to be Δ H°(298 K) = 623(11) cal/mol. The experimental data are consistent with a statistical thermodynamic model thus supporting the assumption of sudden trapping of the conformer mixture in the matrix deposition process. In addition, the electronic energies and the optimum geometries of cis and trans methyl nitrite have been determined by ab initio gradient calculations. The difference of electronic energies Δ E T is significantly larger than the value derived from the experimental enthalpy difference Δ H°(298 K).