Computational approach towards vibrational spectroscopic detection of molecular species relevant to atmospheric chemistry and climate science: The formic acid rotamers

A rigorous but still feasible computational approach is implemented that aims to provide a fundamental theoretical basis for an in-depth understanding of vibrational spectroscopic properties of molecular systems relevant to atmospheric chemistry and climate science. The mentioned properties are, on the other hand, crucial in the context of experimental detection of the title molecular species and their noncovalently bonded clusters. Rotamers of formic acid are treated as particular example. Potential energy surface of free formic acid was thoroughly explored at Möller-Plesset perturbation theory level, including corrections up to the second order with a rather flexible basis set for orbital expansion (MP2/6-311++G(3df, 3pd)), as well as employing density functional tight binding approach (DFTB). Anharmonic O-H(D) stretching vibrational frequencies were calculated using several algorithms. It was found that MP2 level of theory leads to excellent agreement between theory and experiment without using any arbitrary scaling factor when the difference between O-H(D) stretching frequencies in the case of both rotamers is in question. DFTB performs significantly inferior to MP2 with this respect, while reproducing the absolute frequencies of individual rotamers rather well. Fully relaxed HCOH torsional potential was calculated as well and the frequencies of HCOH torsion in the cis- and trans-well were calculated either by solving the torsional Schrödinger equation variationally or by second-order perturbation theory. The agreement between theory and experiment is again excellent in cases when experimental data are available.