Differences in the Hydrogen and Deuterium Bonds

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Abstract
Differences in equilibrium properties of hydrogen- and deuterium-bonded systems are considered. The potential energy surface is assumed to be independent of the isotopic species and the differences attributed to vibrational motion. A useful understanding of the problem can be gained by considering a hydrogen-bonded species as if it were a linear triatomic molecule AH … B with three significant vibrational modes—the A—H stretching mode and the doubly degenerate H-bond bend. Differences in the stabilities of H- and D-bonded species are related to differences in the zero-point energies of these vibrations. Three problems are considered in detail: (i) the relative stabilities of dimeric species such as HF … DF and DF … HF; (ii) heats of vaporization of condensed phases such as water and heavy water; and (iii) the molar volumes of H- and D-bonded species. It is concluded that there can be no short answer to the question: ‘Is the hydrogen bond stronger than the deuterium bond?’ The equilibrium properties of systems containing hydrogen or deuterium bonds are dependent on vibrational motion of the nuclei and therefore reflect the shape, as well as the depth, of the potential energy surface.

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