Calculation of secondary β-deuterium equilibrium isotope effects on protonation and deprotonation of methanol and methanediol

Secondary β-deuterium equilibrium isotope effects are calculated for protonation and deprotonation of methanol and of methanediol using ab initio quantum-mechanical methods. Improved results are obtained by multiplying all calculated force constants by a single empirical scaling factor. Scaled 4-31G force constants enable isotope effects to be calculated reliably whereas use of the STO-3G basis may lead to incorrect predictions. Calculated geometries, force constants and isotope effects may be rationalised successfully by the theory of negative hyperconjugation.