Hydrogen bond versus polar effects: an ab initio analysis on n --> pi* absorption spectra and N nuclear shieldings of diazines in solution.

The complex nature of the effect of H-bonding solvents on electronic and magnetic properties of diazines in dilute solution is analyzed by comparing results obtained with continuum, discrete, and mixed continuum-discrete solvation methods. For comparison, other, nonprotic solvents are also considered. The interpretation of the results shows that strong H-bonding effects, such as those exerted by water molecules on diazines nitrogens, are accompanied by comparable (or at least not negligible) long-range polar interactions. It is also shown that a continuum model not only well describes such "bulk effects" but becomes essential to get the correct description of the interactions due to explicit H-bonded molecules. This double action (direct, as an additional long-range field, and indirect, through the H-bonded molecules) significantly modifies the solute electronic and nuclear charge distribution and the related response properties. This picture is confirmed by an NBO analysis on single diazines and the corresponding H-bonded clusters with and without an external continuum solvent.