Nobeli, Irene; (1998) Characterising organic hydrogen bonds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The intermolecular geometry and relative strengths of hydrogen bonds between alkanol fragments and aromatic heterocycles, containing nitrogen and/or oxygen acceptors, have been studied using a combination of statistical studies of organic crystal structures in the Cambridge Crystallographic Database, and intermolecular perturbation theory calculations. The variation in the strength of the hydrogen bond with the nature of the acceptor, the orientation of the donor molecule, and the influence of neighbouring groups is well reflected in the electrostatic component of the intermolecular energy. To model this variation, an anisotropic electrostatic model, such as distributed multipoles centred on the atoms, is required. This anisotropic electrostatic model, combined with a simpler empirically fitted 6-exp dispersion-repulsion potential, successfully predicts the main trends in the relative strengths of the hydrogen bonds between a number of aromatic heterocycles and methanol, and it predicts intermolecular geometries for the complexes that qualitatively agree with the most commonly observed patterns in the database. Changes of the atomic charge density due to hybridisation, neighbouring atoms, or molecular conformation inevitably affect the other contributions to the intermolecular energy, and thus such changes need to be modelled to provide a more accurate potential. The overlap model is investigated as a means of deriving the exchange-repulsion from the overlap of the monomer wavefunctions for two very different N H-O hydrogen bonds, and for the oxalic acid dimer. The overlap model is shown to be a good method for splitting-up the total intermolecular repulsion into atom-atom contributions, although convergence problems of the series of anisotropic terms mean that the analytical expression of the overlap is not very useful for organic systems. Overall, a systematically derived potential, which uses distributed multipoles for the electrostatics and the overlap model for the repulsion, is successful in modelling both polymorphs of oxalic acid, and is thus more realistic than most earlier empirically fitted transferable potentials.

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