MUTUAL POLARIZATION OF MONOMER CHARGE-DISTRIBUTION IN (HCN)2, (HCN)3, AND (HCN)-INFINITY

Abstract

A simple classical mutual polarization model accurately predicts the induced dipole moments of linear HCN dimer and trimer. The model employs the first four nonzero electrical molecular moments of each HCN, and both the molecular polarizability and the C-H and C-N bond dipole polarizabilities. The model is extended to linear H-bonded oligomers up to the pentamer, and also to an infinite linear H-bonded chain, appropriate for comparision with the H-bonded chains present in the HCN crystal. For the dimer and trimer, a relation is seen between the change in the electric field gradient at the N of HCN and the calculated induced C-N bond dipole moment. A semiquantitative proportionality constant, derived based on this relation, is used to enhance understanding of the underlying cause of the large increase in the N14 quadrupole coupling constant in the HCN crystal relative to the free HCN monomer. This increase results from the strong polarization of the C-N bond by the local field at each HCN caused by the rest of the HCN crystal lattice.