Position-Specific ²H/H Equilibrium Isotopic Fractionation Factors in Alkane, Alkene, and Aromatic Molecules: A Density Functional Theory Approach

Advances in position-specific deuterium (²H) analyses provide powerful and accurate means for evaluating organic H isotopic signatures and fractionation factors. Density functional theory can aid in determining ²H-H equilibrium fractionation factors at specific positions on organic compounds. This work evaluates a DFT method that provides improved accuracy with respect to experimental data over previous methods while allowing for calculations on higher molecular weight compounds that are useful organic geochemical markers (biomarkers). We used computational quantum chemistry and applied the density functional theory method B3LYP and multiple basis sets. Based on accuracy criteria, we selected the basis set, 6-311++G(d,p), to calculate ²H-H equilibrium fractionation factors (α) on primary, secondary, tertiary, sp², and aromatic C atoms for a model set that included methane, ethane, propane, 2-methylbutane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,6-dimethyloctane, 2,6,10,14-tetramethylpentadecane (pristane), 2,6,10,14-tetramethylhexadecane (phytane), twist-boat and chair cyclohexane, axial and equatorial methylcyclohexane, 2-heptanone, (E) and (Z) 2-pentene, and benzene with water. Two conformers each of cyclohexane, 2-pentene, and pristane were used to explore how thermodynamic weighting using the Boltzmann partition function could improve the α results for models of organic compounds that occur in more than one isomeric form. The B3LYP method coupled with the 6-311++G(d,p) basis set provided the most accurate α results, when compared with results from B3LYP coupled with 17 other basis sets. The calculated equilibrium α values for ²H on primary, secondary, and tertiary C atoms follow the observed trend in the deuterium preference for carbon-hydrogen bonds relative to water: α3° > α2° > α1°. Further, ln(β) values predicted from the DFT calculations also agree with fractionation estimated within 4%, as calculated using observed FTIR vibrational frequencies. Application of B3LYP/6-311++G(d,p) will aid in the assessments of position-specific ²H-H equilibrium fractionation factors (α) for comparison with experimental studies. Estimated position-specific alpha values can be scaled with temperature and used to predict molecular-averaged delta values, such as determined using compound-specific isotope analyses (CSIA). The ability to adjust position and molecular-averaged calculations for dominant isomeric forms makes this a useful approach to study equilibrium H isotope distributions in larger compounds of organic geochemical, biogeochemical, and environmental interest.