Protium and deuterium

Hydrogen possesses three isotopes, protium, deuterium and tritium, selected properties of which are given in Table 1.1.

The isotopes of hydrogen exhibit greater differences in physical and chemical properties than isotopes of any other element. The origin of the differences between H and D, or between pairs of compounds such as H₂O and D₂O, lies in the difference in mass, which in turn affects their fundamental vibrational wavenumbers and zero point energies . The fundamental vibrations for H₂, HD and D₂ are at 4159, 3630 and 2990cm^-1, respectively, and from these data, the zero-point energies of H₂ and D₂ are calculated to be 26.0 and 18.4 kJmol^-1, respectively.   The total electronic binding energies for these molecules (represented by the overlap of their atomic wavefunctions) are the same, and so it follows that their dissociation energies differ by 7.6 kJmol^-1, with the D^_D bond being stronger than the H^_H bond. Similarly, an X^_D bond is stronger than the corresponding X^_H bond (where X is any element), and this difference is the basis of the kinetic isotope effect.

Table 1.1 Selected properties of the isotopes of hydrogen.