Ab Initio Calculations of Deuterium Isotope Effects on Hydrogen and Nitrogen Nuclear Magnetic Shielding in the Hydrated Ammonium Ion.

Abstract

H-1 and N-14 nuclear shieldings of the ammonium ion and of its mono- and tetrahydrated species in directional as well as non-directional coordinations are calculated ab initio using the localized orbital/local origin (LORG) method; counterpoise calculations are included to test for basis set superposition effects. The variation of the H-1 and N-14 shieldings with N-H distance is essentially linear in the bonding region, while the large-scale variations illustrate the transition from solvated ammonium to ammonated hydroxonium complexes. The deuterium isotope effects for these systems are evaluated in a static approximation, and for the directionally coordinated NH4+ (H2O)4 ion the computed two-bond deuterium isotope effect on H-1 correctly reproduces the unusual (i.e. negatively signed) experimental result, while the bare ion and non-directionally coordinated NH4+(H2O)4 yield positively signed effects. The decisive factor for the sign inversion is found to be the directional solvation of the deuterium atom, with an ammonium-water distance not exceeding ca. 3 angstrom, and the effect can be traced to a difference in the distance variation of the shielding perpendicular and parallel to the N-H bond. For the N-14 nuclear shielding again the results obtained for directionally coordinated NH4+(H2O)4 are in better agreement with experiments than the results obtained for the bare ion or for non-directional solvation.