Ab initio calculations on conventional and unconventional hydrogen bonds - Study of the hydrogen bond strength

Different measures of H-bond strength based on X-H proton donating bond properties and on parameters of (HY)-Y-... distance (Y-proton acceptor within X-(HY)-Y-... H bridges) are investigated. Correlations between such measures and H-bond energy are studied. The parameters of H-bonds are taken from geometry of simple complexes optimized within HF/6-311++G** and MP2/6-311++G** levels of theory. The Bader theory of atoms in molecules is also applied for an estimation of electronic densities at bond critical points and Laplacians of these densities, these topological parameters are also used to define H-bond strength measures. Apart from the conventional statistical analysis, the factor analysis is applied to study the properties of H bridges. The results show that the set of geometrical, energetic, and topological variables describing the H bridge may be replaced by one new variable, one factor. It is also shown that the geometrical and topological parameters of the proton donating bond better correlate with the H-bond energy and with the new factor than the parameters of (HY)-Y-... contact.