A secondary isotope effect attributed to the different vibrational amplitudes of isotopologues. For example, both the mean and mean-square amplitudes of vibrations associated with C–H
bonds are greater than those of C–D
bonds. The greater effective bulk of molecules containing the former may be manifested by a steric effect on a rate or equilibrium constant.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1131 (https://doi.org/10.1351/pac199466051077)