Nuclear magnetic resonance parameters in water dimer

Abstract

The changes in the computed nuclear magnetic resonance (NMR) parameters of the water dimer with respect to their monomer values were monitored as the geometry of the dimer was systematically varied. Nuclear magnetic shielding constants, shielding tensor anisotropies, nuclear quadrupole coupling constants and spin–spin coupling constants for the hydrogen bond donor and acceptor molecules were calculated at hybrid density-functional theory level. The dimer geometry was specified through the intermolecular oxygen–oxygen distance ROO and the hydrogen bond angle α. A grid of 120 geometries was selected by systematically varying these two parameters. The other geometrical parameters of the dimer were allowed to relax, keeping the two parameters fixed. As the dimer geometry was varied, all NMR parameters were observed to be smoothly behaving. Characteristic changes as a function of the intermolecular geometry were observed. These include, besides the well-known deshielding of the donor hydrogen shielding constant, also influences on the donor deuterium quadrupole coupling constant, as well as the shielding anisotropy of the donor and acceptor oxygens. We discuss the contributions to the total dimerisation effect from, on the one hand, the dominant direct interaction effect at a fixed geometry and, on the other hand, from the quantitatively relevant indirect, geometric effect. A fundamental ambiguity of this partitioning is demonstrated. By forging the general, smooth trends in all the studied NMR parameters into a specific geometric definition, we find our data to be in agreement with the widely used distance criterion for hydrogen bonding in water, ROO ≤ 3.5 A.