Knowledge Bank

Charge rearrangement during cluster formation may be determined from its effects upon the dipole moments u and the electric field gradients $\chi 0$ of the monomeric units,provided that corrections can be made for their vibrational averaging. An isotopic substitution method has enabled us to obtain vibrational amplitudes in the linear $dimer1$ and $trimer.2$ A dipole moment of 6.552(35) D has been reported for the dimer3 and we have found 10.6(2) D for the $trimer.2$ These lead to induced dipole moments Au of 0.703(3) D for the dimer and 1.75(10) D for the trimer. A similar analysis was made of $\Delta xo(14N)$. A mutual polarization model was used to predict the induced dipole moments from the four known electrical multipole moments of HCN and its bond polarizabilities. The results are 0.703 D for the dimer and 1.68 D for the trimer. The calculations give ratios of the dipoles induced in the C-N bonds which are essentially identical with the ratios of the experimental . Equations are derived for calculation of Au of an infinite H-bonded chain and are applied to HCN. The convergence of multipale expansions for the electric field due to a molecular charge distribution will be discussed. The mutual polarization model works so well on HCN dimer and trimer that any charge transfer between the HCN monomers seems likely to be very small or negligible.