Isotope chemistry and molecular structure. Carbon and oxygen isotope chemistry

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Abstract
The relationships between force constants and the isotope chemistry of carbon and oxygen are calculated for H2O, CO2, CH2O, CH4, C2H4, C2H6, and C6H6. Significant differences are found from the general features of protium–deuterium isotope chemistry. These are shown to be associated with a structural effect. Hydrogen is always an end atom except for hydrogen bonded moleules. Carbon is generally tetrahedrally bonded and its isotope chemistry shows significant contributions from the interaction between stretching and bending modes. These interactions lead to deviations in additivity of the total isotope effect from the contributions of the individual force constants. Stretching forces dominate the isotope chemistry of carbon and oxygen as they do in hydrogen. They account for 70%–90% of the reduced partition function ratios. Correlations are made between the stretching force contributions and molecular structure. It is shown that while significant differences exist between the specific contributions calculated from different force fields for methane, ethane, and benzene, the absolute value of ln(s/s′) f is rather insensitive to the detailed structures of the F matrices studied.