G. WILSE ROBINSON 23 



to produce ihe repulsive interaction and the blue shift. These blue 

 shifts can be produced artificially by "trapping" molecules in a solid 

 composed of smaller molecules (18), or by high pressure techniques 

 (1). Since bands remain sharp in certain low temperature solids, 

 precise measurements of "solvent shifts" can be made by that tech- 

 nicjue. Naphthalene in solid xenon, for example, shows blue shifts of 

 50 ± 10 cm-^ in its two lowest-lying tt-tt* singlet-singlet electronic 

 transitions.^ Much larger shifts to the blue are expected in smaller 

 atom "solvents" such as krypton and argon. ^^ 



It is important to realize that the electronic origin [ (0-0) band ] 

 of a transition is shifted a different amount than the absorption 

 maximum. This is explained by the Franck-Condon principle applied 

 to the molecule plus environment. A change in equilibrium struc- 

 ture of the local environment upon molecular excitation caused by 

 different e and cr in the excited state will cause an additional shift 

 corresponding to the extra lattice energy of the excited molecule in 

 an unstable environmental state. TJiese shifts are probably much 

 smaller than the electronic shifts themselves because of the strong 

 tendency of the solvent molecules to resist local expansion or com- 

 pression. This is indicated by the study of sufficiently simple systems 

 where individual vibrational bands of the spectrum can be resolved 

 and accurately measured. 



It should be remarked that attractive intermolecular interactions 

 decrease the electron density in the molecule and also in its inter- 

 acting partner since electronic charge is transferred to the "inter- 

 molecidar bond." Repulsive interactions increase the electron density 

 in both interacting species. Such effects can be observed upon vi- 

 brational energy levels of the molecules, decreasing the vibrational 

 stretching force constants in the first case and increasing them in the 

 second to produce vibrational red and blue shifts respectively.^^ 



'The vibrational analysis (lef. 35) of the 2900A 'Bo^ — \\,g transitoin in naph- 

 thalene is apparently in error. Vibrational bands assigned to excited vibrational 

 levels in the ground state were found at 4.2° R. If the original assignment were 

 accepted, the solid xenon perturbation would have to shift the spectrum over 1300 

 cm-' to the red. This is much more difficult to explain than a slight blue shift 

 of similar magnitude as the blue shift in the 'B,,u — '.\,g transition. The large red 

 shift (— 2250 cm-') reported for the 'B.^ — KX,,. transition in pure crystalline 

 naplithalene (ref. 20) becomes 754 cm-', more in keeping with the 462 cm"' red 

 shift of the 'B;,u — '.A,g transition in the crystal. 



'" Recent work has shown the shifts in argon to be 243 cm' and 348 cm' to the 

 blue for the 'Bau — W^g and 'B^u — '.\ig transitions respectively. 



" The situation for bending \ ibrations is more complex, owing to the fact that 

 the average dipole moment of a molecule such as H^O or NH3 decreases with 



