RELATIVE STABILITY OF HYDROGEN AND DEUTERIUM BONDS

The relative energies of H and D bonds are due to differences in zero- point vibrational energy (ZPVE). Ab initio calculations are used to as sess the changes in this quantity that accompany all possible substitu tions of protium by deuterium in a number of complexes. The ZPVE of th e D bond is lower than that of the I-I bond in the neutral dimer and t rimer of water. This difference can be traced to one particular vibrat ional mode, the one which displaces the bridging atom away from the O. ..O axis. The heavier mass of D lowers the frequency, and hence the ZP VE associated with it. The situation reverses itself in ionic H bonds. The total ZPVE of the (H2O..H..OH2)(+) complex is higher when a D occ upies the bridging position, as compared to a terminal site. This diff erence is attributed to the intramolecular modes. Although replacement of the central H by D reduces the intermolecular ZPVE, the reduction of the intramolecular ZPVE is even larger when the substitution is mad e at a peripheral atom, so a D would tend to migrate away from a bridg ing location. This effect is noted also in the larger complex in which two methanol molecules are bound by a proton. The lower energy of a H bond as compared to a D bond is observed as well in the anionic (HOH. .OH)(-) system, although the magnitude of the preference is smaller he re. In all cases, raising the temperature, and thus invoking thermal v ibrational and entropic effects, tends to preferentially stabilize H o ver D bonds.