Equilibrium carbon isotope fractionation factors of hydrocarbons: Semi-empirical force-field method

We have calculated the reduced partition function ratios for carbon isotopes (beta-factor) of 67 hydrocarbons (alkanes, alkenes, alkynes, cycloalkanes, and aromatics), including their 267 single-substituted isotopomers. The calculations were performed using the harmonic oscillator - rigid rotator model and the Urey/Bigeleisen-Mayer approach. Normal frequencies of molecular vibrations of the isotopologues were calculated from the molecular structures data and valence field force constants stored in the Light-handled Elucidation of Vibrations (LEV) database elaborated by Dr. Gribov and his colleagues in Russia. The LEV database is constructed from experimental spectroscopic and structural data by solving inverse problems of molecular vibrations. The LEV is internally consistent and the most comprehensive database available to date. The beta-factor were calculated in the temperature range of 200-800 K with a 10 K step. Our calculations predict that the beta-factors increase with an increasing number of C atoms within the same groups of hydrocarbons (e.g., alkanes). Our calculations also show a general descending order of C-13 enrichments among the different groups of hydrocarbons: cycloalkanes, aromatics, alkenes (double bonds) and isoalkanes, alkanes, alkynes (triple bonds). Position-specific, intramolecular isotope effects within hydrocarbons are determined by the beta-factors of C in the different functional groups in the order: quaternary (C), tertiary (methine, CH), secondary (methylene, CH2), primary (methyl, CH3) and double bond (C-C), saturated bond (C-C), triple bond (HC (math)). Our calculations on bulk and position-specific carbon isotope beta-factors of the hydrocarbons, which are generally consistent with very limited ab initio calculations in the literature, are internally consistent and the most comprehensive to date for future applications to position-specific isotope geochemistry of hydrocarbons.