The prediction of spectroscopic properties from the quartic force field of NOBr

Ab initio molecular electronic structure methods are used to determine the constants describing the infrared spectroscopy of NOBr. Second-order Mّller-Plesset perturbation theory (MP2) with a 6311G extended basis set on bromine is used to obtain an equilibrium geometry, harmonic vibrational frequencies and a quartic expansion of the potential energy surface. The cubic and quartic force constants were evaluated in mass-weighted normal coordinates using a finite-difference approach. Standard perturbation theory is then used to determine effective vibrational and rotational Hamiltonians for the fundamental vibrations and selected overtone and combination bands. The quadratic and most of the cubic potential constants are also determined experimentally. The calculated anharmonic constants are found to be in excellent agreement with experiment if a MP2 potential is used.