A quantum-chemically supported classical approach to assessing the vibrational contribution to the thermal expansion of molecules. Example of LaI3

Abstract

А dynamic vibrational task has been solved with taking into account the interaction of vibrational coordinates on the base of the calculated PES. The vibrational level energies and corresponding wave functions of LaI3 molecule have been determined by the solution of three-dimensional Schrödinger equations. Theoretical calculations were performed by the DFT/PBE0, DFT/B3LYP and MP2 methods using relativistic effective pseudopotentials and a good quality basis sets for inner and outer atomic shells. Six-dimensional PES was numerically calculated by the DFT/PBE0 approach. Available gas-phase electron diffraction (GED) data were simulated by the original methodology based on the full dimensional PES. A good agreement between experimental and theoretical thermal average structural parameters supports the reliability of the theoretical data and the efficiency of the methodology implemented. It is shown that the standard approaches based on the use of the harmonic approximation, to calculate of the vibrational corrections necessary for the interpretation of GED results, at high temperatures lead to either significantly overestimated or underestimated values of equilibrium geometrical parameters. An analysis of the anharmonic vibrational levels obtained indicates the absence of Fermi resonances and the suitability of the harmonic approximation for describing fundamental vibrational transitions.