Gas-phase infrared spectrum of methyl nitrate

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 ν1 = 3034.1 cm−1; ν2 = 2961.5 cm−1, and ν13 = 3014.5 cm−1 and the methyl deformation ν15, 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 ν4 = 1667.0; ν5 = 1431.8; ν6 = 1291.2; ν7 = 1177.9; ν8 = 1018.5; ν9 = 855.4; ν10 = 658.3; ν11 = 571.0; ν12 = 344.2; ν14 = 1444.7; ν16 = 760.6; ν17 = 204.5; and ν18 = 133.7.