Positional Isotope Effect for the Water Tetramer

Ab initio studies are carried out for the water tetramer with a single H replaced by D, (H2O)3(HDO). The isomer with D in a free position, H7DfO4, has ZPE 0.17 kcal/mol higher than the isomer with D in an H-bond, H7DbO4. The same ZPE difference is found for HbD7O4 vs HfD7O4. The difference may be large enough for a preference for H rather than D in a free position to be experimentally measurable. Exploration of the components of ZPE shows that the stretch component alone favors H7DfO4, but this is more than compensated by the bend and torsion components.