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

Abstract

With the purpose of providing some clues that could encourage the spectral recordings of propane and various monodeuterated and ¹³C isotopologues and also to explore their far infrared spectra at low temperatures, the energy levels corresponding to their three lowest frequency modes 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 the present work. Interacting terms, energy levels and tunneling splittings are provided for CH₃CH₂CH_3, 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 , respectively) is 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) using 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.