Are hydrogen bonds covalent or electrostatic? A molecular orbital comparison of molecules in electric fields and H-bonding environments

The extent to which hydrogen-bonding can be considered to be a covalent or resonance effect, rather than an electrostatic or electrostatic plus polarization effect, is modeled by ab initio molecular orbital calculations in graded uniform electric fields of molecules that form chains of H-bonds. The stabilization energy, dipole moments, and relaxed geometrical structural parameters of these molecules are tabulated as a function of applied field. The H-bonding interactions of a molecule are judged to be due to electrostatic and polarization effects if all of the tabulated parameters agree with those calculated for an infinite chain of H-bonding molecules at the same value of the applied electric field. If these parameters individually agree with those of the infinite chain of H-bonding dimers at substantially different values of the applied field, effects other than electrostatic and polarization, such as covalent or resonance effects, must be important. Of the two molecules studied, urea appears to be reasonably well-described by electrostatic plus polarization effects, whereas the other molecule, the enol of 1,3-cyclohexanedione, clearly is not.