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 polari zation effect, is modeled by ab initio molecular orbital calculations in gr aded uniform electric fields of molecules that form chains of H-bonds. The stabilization energy, dipole moments, and relaxed geometrical structural pa rameters of these molecules are tabulated as a function of applied field. T he H-bonding interactions of a molecule are judged to be due to electrostat ic and polarization effects if all of the tabulated parameters agree with t hose calculated for an infinite chain of H-bonding molecules at the same va lue of the applied electric field. If these parameters individually agree w ith those of the infinite chain of H-bonding dimers at substantially differ ent values of the applied field, effects other than electrostatic and polar ization, such as covalent or resonance effects, must be important. Of the t wo molecules studied, urea appears to be reasonably well-described by elect rostatic plus polarization effects, whereas the other molecule, the enol of 1,3-cyclohexanedione, clearly is not.