The CC/DFT Route towards AccurateStructures and Spectroscopic Features for Observedand Elusive Conformers of Flexible Molecules:Pyruvic Acid as Case Study

摘要

The structures, relative stabilities as well as the rotational and vibrational spectra of the three low-energy conformers of Pyruvic acid (PA) have been characterized using a state-of-the-art quantum-mechanical approach designed for flexible molecules. By making use of the available experimental rotational constants for several isotopologues of the most stable PA conformer, Tc-PA, the semi-experimental equilibrium structure has been derived. The latter provides a reference for the pure theoretical determination of the equilibrium geometries for all conformers, thus confirming for these structures an accuracy of 0.001 Å and 0.1 deg. for bond lengths and angles, respectively. Highly accurate relative energies of all conformers (Tc-,Tt- and Ct-PA)and of the transition states connecting them areprovided along with the thermodynamic propertiesat low and high temperatures, thus leading toconformational enthalpies accurate to 1 kJmol⁻¹. Concerning microwavespectroscopy, rotational constants accurate toabout 20 MHz are provided for theTt- and Ct-PAconformers, together with the computedcentrifugal-distortion constants and dipolemoments required to simulate their rotationalspectra. For Ct-PA, vibrationalfrequencies in the mid-infrared region accurate to10 cm⁻¹ are reported along withtheoretical estimates for the transitions in thenear-infrared range, and the correspondinginfrared spectrum including fundamentaltransitions, overtones and combination bands hasbeen simulated. In addition to the new datadescribed above, theoretical results for theTc- and Tt-PAconformers are compared with all availableexperimental data to further confirm the accuracyof the hybrid coupled-cluster/density functionaltheory (CC/DFT) protocol applied in the presentstudy. Finally, we discuss in detail the accuracyof computational models fully based ondouble-hybrid DFT functionals (mainly at theB2PLYP/aug-cc-pVTZ level) that avoid the use ofvery expensive CC calculations.