A molecular orbital explanation of bond distance variation caused by hydrogen bond formation

For hydrogen bond systems X–D–HA–Y, a simple molecular orbital model is proposed to understand the mechanism of the bond distance variations caused by the hydrogen bond formation. This model explains the bond distance variations for X–D and A–Y as follows. Electrostatic potential that the electrons in a molecule receive from other molecules causes the changes in atomic orbital energy differences between the bonded atoms. Then, the changes in the orbital energy differences make the bond orders larger or smaller and consequently the bond distances vary. The validity of this model has been confirmed by the effective fragment potential method, using the test systems of (HCOOH)₂, HCONH₂ (formamide) crystal and BF₃·2H₂O crystal.