24 LIGHT AND LIFE 



Taking the force constants (which vary as the square of the fre- 

 quencies) to be roughly proportional to the overall bonding electron 

 density, one can estimate the gain or loss of such electrons because of 

 the interaction. A situation uncluttered by more complicated effects 

 is the vibrational perturbations on molecules in crystalline rare gases. 

 For the simple molecular fragment NH, the bonding electron density 

 loss'- in the bond is .9%, 1.4%, and 2.4% respectively in solid argon, 

 krypton, and xenon. Usual vibrational shifts are a bit smaller than 

 these since the "local polarizabilities" of normal solvent molecules 

 are smaller than those of the rare gases. 



Charge transfer interactions. Many solvent-solute interactions are 

 somewhat more complex than the ones discussed thus far. In certain 

 cases the affinity of the solvent molecule for electrons is anomalously 

 large if there are low-lying unfilled orbitals in the solvent. If the 

 solute is a better donor in the ground state than the excited state, 

 then the ground state of the solute is depressed in energy more than 

 the excited state and a blue shift results even though attractive forces 

 are dominant. Such "charge transfer" complexes (21) are common, 

 the iodine-pyridine 1:1 complex being a good example (30). They 

 bridge the gap between the weaker van der Waals' complexes and 

 molecules bound together by strong valence forces. Since the vibra- 

 tional force constant of the donor molecule is decreased, the vibra- 

 tional spectrum is expected to be red shifted (27), characteristic of 

 attractive interactions. In the iodine-pyridine complex, the I2 vi- 

 brational frequency is decreased from 213 cm— ^, the gas phase value, 

 to 184 cm— 1 in the complex. Ignoring any possible effective mass 

 change, about 7% of the bonding electron density is removed from 

 the lo bond. In halogen-benzene systems (where multiple interactions 

 are probably important) 2.8%, 2.6%, and 2.1% of the bonding elec- 

 tron density of CL„ Br2, and I^, respectively, appear to be donated to 

 the environment. The decrease here, instead of an expected increase, 

 may in part be accounted for by repulsive interactions which become 

 increasingly important for the larger halogen molecules. 



Interactions involving dipole moments. Another important class 

 of spectral shifts results if the dipole moment of a molecide is different 

 in the two combining electronic states. Since charge density changes 

 with electronic excitation, changes of dipole moment are to be ex- 

 jiected. In liquids or solids the jioint dipole approximation is poor, 

 but a rough description of the interaction is dipole-induced — dipole 



excitation of the bending frequency. This contributes a blue shift. Any anoma- 

 lously large "hydrogen bonding" would also contribute a blue shift. See ref. 37. 

 "On the assumption that the reduced mass for vibration is unaltered. 



