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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry