Abstract
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 Cs 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−1 region and represent excitations of the ν24 (skeletal torsion) and the ν23 (methyl torsion) modes. The third independent variable, the OH torsion, interacts strongly with ν23. The A1/E splitting of the ground vibrational state has been evaluated to be 0.024 cm−1 as it was expected given the high of the methyl torsional barrier (338 cm−1). A very good agreement with respect to previous experimental data concerning fundamental frequencies (νCAL − νEXP ~ 1 cm−1), and rotational parameters (B0CAL − B0EXP < 5 MHz), is obtained.
Highlights
Pyruvic acid (CH3 CO-COOH, pyruvic acid (PA)) is a prevalent species in the Earth’s atmosphere [1,2]where it is a key intermediate of keto-acid reactions
The geometrical parameters and energies of the pyruvic acid conformers were computed using the explicitly correlated coupled cluster theory with singles and doubles substitutions augmented by a perturbative treatment of triple excitations (CCSD(T)-F12 [24,25]
The computed spectroscopic properties corresponding to the most stable conformer of pyruvic acid are in a surprising good agreement with previous experimental data. This behavior was not expected since molecules with low methyl torsional barriers can present sudden problems causing divergences between “ab initio” results and observations
Summary
Pyruvic acid (CH3 CO-COOH, PA) is a prevalent species in the Earth’s atmosphere [1,2]. Kleimeier et al [11] have designated pyruvic acid to be a detectable species in extraterrestrial sources They have studied its formation by barrierless recombination of hydroxycarbonyl (HOCO) and acetyl (CH3 CO) radicals in ices of acetaldehyde (CH3 CHO) and carbon dioxide (CO2 ) modeling interstellar conditions driven by cosmic rays. The far infrared region, relevant for rotational studies, is explored using a three-dimensional variational procedure [26]. This procedure has been improved and implemented for a proper classification of the energy levels in molecules with regions showing a high density of states [27]. The two low-lying torsional modes are considered to be independent coordinates These two vibrational modes are strongly coupled as was early observed [15]. A third coordinate, the OH internal rotation, is defined as an independent coordinate since it interacts strongly with the ν24 mode
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