A solid state NMR and molecular orbital study of hydroxylammonium chloride

Abstract

Deuterium and nitrogen-15 NMR spectroscopy has been used to measure the2H quadrupolar coupling and15N chemical shift tensors in solid hydroxylammonium chloride, NH3OH+Cl-, (HAC). In addition, the NH3and OH dynamics have been investigated by variable temperature2H line shapes and T1measurements. The Arrhenius activation energy for NH3rotation is 22.5 ± 1.8 kJ/mol with a pre-exponential factor of 8 ± 3 × 1012s-1from line shapes and 21.3 ± 2 kJ/mol with an infinite temperature correlation time, τinf,, of 5.0 ± 0.4 × 10-14s from the T1analysis. The latter value corresponds to a pre-exponential factor of 6.7 ± 0.5 × 1012s-1, if a three-site exchange is assumed. There was no evidence for OH reorientation up to 405 K, indicating a rather strong OH···Cl hydrogen bond. Previously reported inconsistencies between crystal structure and molecular orbital derived N-O bond lengths are cleared up by performing geometry optimizations with large basis sets and taking electron correlation into account. The internal rotational potential for the isolated HA cation is calculated to be 5.8 kJ/mol at the MP2/6-31G** level, with the trans geometry preferred. Calculations that employ the neutron diffraction geometry and include the Cl-anions that surround the HA+cation yield an upper limit for the activation energy for NH3group rotation of 62 kJ/mol. Analysis of the deuterium spectrum and T1data yield nuclear quadrupolar coupling constants of 160 ± 5 kHz and 194 ± 5 kHz (η = 0.50 ± 0.05) for the ND3and OD deuterons, respectively. Density functional calculations of the deuterium and nitrogen-14 nuclear quadrupolar coupling constants at the B3LYP level show that it is necessary to include the influence of the surrounding chloride anions. We have also shown that it is possible to obtain accurate proton chemical shifts from the deuterium MAS spectrum of solid HAC-d4.Key words: solid state NMR, molecular dynamics, nitrogen 15 chemical shift anisotropy.