Large Amplitude Motions of Pyruvic Acid (CH-CO-COOH).

Torsional and rotational spectroscopic properties of pyruvic acid are determined using highly correlated ab initio methods and combining two different theoretical approaches: Second order perturbation theory and a variational procedure in three-dimensions. Four equilibrium geometries of pyruvic acid, Tc, Tt, Ct, and CC, outcome from a search with CCSD(T)-F12. All of them can be classified in the C point group. The variational calculations are performed considering the three internal rotation modes responsible for the non-rigidity as independent coordinates. More than 50 torsional energy levels (including torsional subcomponents) are localized in the 406-986 cm region and represent excitations of the ν (skeletal torsion) and the ν (methyl torsion) modes. The third independent variable, the OH torsion, interacts strongly with ν. The A/E splitting of the ground vibrational state has been evaluated to be 0.024 cm as it was expected given the high of the methyl torsional barrier (338 cm). A very good agreement with respect to previous experimental data concerning fundamental frequencies (ν - ν ~ 1 cm), and rotational parameters (B - B < 5 MHz), is obtained.