Highly correlated ab initio study of the low frequency modes of propane ands various monosubstituted isotopologues containing D and 13C

Abstract

With the purpose of providing some clues that could incite to their spectral recordings and also to explore the far infrared spectra at low temperatures, the energy levels corresponding to the three lowest frequency modes of propane and various monosubstituted isotopologues containing deuterium and 13C are determined variationally using a flexible model in three dimensions. Five vibrationally corrected potential energy surfaces are computed using CCSD(T) ab initio calculations. In spite of the quality of these highly correlated potentials in molecules with similar structures, it was proven that an empirical adjustment of the surfaces would enclose accurately the experimental and theoretical frequency residuals and therefore it is also used in present work. Interacting terms, energy levels and tunneling splittings are provided for CH3CH2CH3, CH313CH2CH3, 13CH3CH2CH3, CH2DCH2CH3 and CH3CHDCH3. Infrared and Raman transitions of CH3CH2CH3 are assigned. Correlation between symmetry species of the five isotopologue symmetry groups (G36, G36, G18, G6 and G18¿, respectively), are established for the classification of the levels and torsional splittings. The rotational constants are determined with CCSD(T)/CBS (A0=29263.46 MHz, B0=8454.10 MHz and C0=7466.64 MHz) with a non-relativistic procedure. Fundamental anharmonic frequencies corresponding to the high and medium amplitude modes are computed for all the isotopologues. The adjusted parameters are accurate enough to be employed in further spectral analysis. c the Owner Societies 2013