Deuteron quadrupole coupling in hydrogen bonded molecules. V. Relationship of experiment to theory

Abstract

The purpose of this article is to examine experimental data for deuterium quadrupole coupling in a number of hydrogen bonded systems in the context of approximate, but moderately accurate, ab initio calculations of the field gradient, and to suggest a quantitative interpretation of experimental data in terms of the modification of the local electronic structure on hydrogen bonding. Systems discussed include HF, dimers of HF in the solid configuration and in the geometry of the gas phase complex, HCl, dimers and clusters of HCl in the geometry of the orthorhombic solid, formic acid, maleic acid, and several other carboxylic acids, and salts of pyridinium ion. For the case of DF, for which a plethora of calculations is available, computations of the deuterium field gradient at the STO-3G, −4/31G, −6/31G*, and −6/31G** are presented in the context of previous work. Hydrogen bonding has, according to the calculations, a substantial effect upon the magnitude of the field gradient and a much smaller effect upon its principal axis directions. According to these calculations, the deuterium coupling constant in the gas phase dimer in the interior position should be depressed by ∼ 5% from the monomer value, while the depression of the coupling constant at the exterior position should be less than 1%. In a model dimer at the solid geometry, the former is depressed by ∼ 10%. In solid DCl, these effects are somewhat smaller, of the order of 5%, but observable. For the other species examined in this investigation, effects of hydrogen bond formation on coupling constant are also substantial and always in the direction of decreasing field gradient. It is found that although the absolute values of the field gradient are strongly basis dependent, differences are remarkably constant. Calculations at STO-4/31G seem to predict the changes observed on hydrogen bonding to good precision. The partition of these changes into variation with bond length and with charge distribution is discussed. Cross-sectional charge density difference maps are presented for a variety of cases and discussed in terms of the changes which occur at constant distance. A normal feature which results from hydrogen bonding is depletion of charge at the hydrogen, accompanied by a strong accretion in the bond region. A very small buildup of charge is also found in the hydrogen bond region and other manifestations of polarization are evident in the density difference maps as well.