Deuterium fractionation factors for carbon–hydrogen bonds: Calculations using scaled quantum‐mechanical force constants

The determination of uniform scaling factors for force constants calculated at the STO‐3G and 4‐31G levels of ab initio SCF MO theory is described; scaled 4‐31G force constants provide reasonable estimates of reduced partition function ratios for deuteriated/protiated molecules. Gas‐phase deuterium fractionation factors relative to methane calculated using scaled 4‐31G force constants are, however, consistently too high, whereas those obtained by the MP2/6‐31G* and scaled STO‐3G methods are inconsistent. Scaled 4‐31G deuterium fractionation factors relative to water for a range of structural moieties correlate linearly with experimental aqueous‐phase values, but are also consistently overestimated. Substitution at a hydrogen‐bearing carbon atom affects the deuterium fractionation factor through the combined effect of changes in reduced mass and changes in force constants other than for CH stretching.