A molecular orbital explanation for the BN bond shortening in H3BNH3 on going from the gaseous to the solid state

A simple molecular orbital model has been applied to explanation of the BN bond shortening in H3BNH3 on going from the gaseous to the solid state. In this model, the shortening is attributed to the bond order increase that is caused by the fact that each atom in the crystal experiences different external electrostatic potential to each other and thus the orbital energy level of each atom is changed. To illustrate this model, Effective Fragment Potential (EFP) method has been applied to the system consisting of a H3BNH3 molecule and 30 dipole moments whose magnitudes are determined by Lorentz's local field theory. This EFP computation has brought significant BN bond shortening (1.668 1.623 A), which is about 50% of the actual shortening. The factor of the remaining discrepancy has been analyzed by Morokuma decomposition under EFP and localized orbital analysis. These analyses have revealed that the remaining discrepancy is almost compensated by incorporating the dihydrogen bonds (BH···HN) that are formed by the orbital interaction between the bonding orbital of the BH and the antibonding orbital of the NH. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008