Anharmonic force field of cis- and trans-formic acid from high-level ab initio calculations, and analysis of resonance polyads.

The quadratic, cubic, and semidiagonal quartic force fields of cis- and trans-formic acid have been calculated using three different levels of theory. They all give satisfactory results, including the one at the lowest level of theory which is the MP2 method employing a basis set of triple-zeta quality. The results are used to theoretically analyze resonance polyads, including the one involving the 4(1), 5(1), 6(1), 8(1), 7(1)9(1), and 9(2) vibrational states. A semiexperimental equilibrium structure is derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. The ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-zeta quality is in excellent agreement with the semiexperimental structure.