Knowledge Bank

Energies of interaction, vibrational frequency shifts, intermolecular vibration frequencies, and infrared intensity changes, all of them resulting from bringing two molecules together to form a one-to-one complex, have been calculated using the Pople-Beveridge-Dobosh INDO approximate quantum-mechanical $treatment,1$ for a number of different ``complexes” in some different geometrical configurations. Calculations are restricted to the treatment of molecules composed of atoms from the first row of the periodic table, but the calculations for $H3N-F2$, for example, provide a good model for the interactions between amines and $I2$. The extent of charge transfer from $NH3$ to $F2$, for example, has been calculated and the calculated charge distribution compared with that expected from simpler charge transfer theory and from experimental results in $I2$ complexes. The relationship between the extent of charge transfer and the frequency shift of the halogen-halogen stretching vibration, suggested empirically by Friedrich and $Person,2$ has been tested for these model systems. Calculations have been made for model systems including: $NH3$ with $F2$, $NH3$ with $H2$ (two different orientations), $NH3$ with $N2$ (two different orientations), and the water dimer. These results provide a rather complete theoretical model for understanding changes, both in frequency and in intensity, of vibrational spectra of molecules as they interact to form complexes.