Gas-phase infrared spectrum of methyl nitrate

Abstract

The infrared spectrum of methyl nitrate has been reinvestigated. Analysis of the 0.125 cm −1 room temperature spectrum has been carried out with the assistance of high-level ab initio calculations using coupled-cluster theory. Use of second-order vibrational perturbation theory (VPT2), with allowances—when necessary—for resonances has permitted the complete assignment of all fundamental vibrational modes in this molecule. A few errors in previous assignments are corrected (notably the CH stretching modes that are now assigned as ν 1 = 3034.1 cm −1; ν 2 = 2961.5 cm −1, and ν 13 = 3014.5 cm −1 and the methyl deformation ν 15, assigned to 1155.7 cm −1), and the work also presents a highly accurate empirically adjusted force field that reproduces all fundamental frequencies to within 0.1 cm −1. This Hamiltonian should be accurate enough to predict two- and three-quantum transitions to within a few cm −1. The remaining fundamental frequencies (in cm −1) are ν 4 = 1667.0; ν 5 = 1431.8; ν 6 = 1291.2; ν 7 = 1177.9; ν 8 = 1018.5; ν 9 = 855.4; ν 10 = 658.3; ν 11 = 571.0; ν 12 = 344.2; ν 14 = 1444.7; ν 16 = 760.6; ν 17 = 204.5; and ν 18 = 133.7.