G. WILSE ROBINSON 25 



or dipole-dipole depending upon whether the solvent molecules are 

 themselves polar. A striking confirmation of the role played by the 

 dipole moment in solvent interactions is afforded in the case of 

 trapped NH^ in crystalline rare gases (18) . The electronic properties 

 of this free radical in its ground state are almost identical to those 

 of HoO except that one of the lone-pair electrons is missing in NH2. 

 The dipole moment in the ground state is probably of the order of 

 1.0 Debye. In the lowest excited state the molecule, unlike HoO, is 

 linear and the dipole moment vanishes. Consequently the ground 

 state is depressed not only because of the attractive dispersion forces 

 but also because of dipole — induced-dipole interactions. The excited 

 state of NHo is depressed only by virtue of the dispersion forces. Blue 

 shifts are expected unless the dispersion forces in the excited state 

 are sufficiently large to cancel the sum of the two ground state inter- 

 actions. The electronic shifts of NHo in argon, krypton, and xenon 

 are 27.4 cm-i to the blue, 7.1 cm-i to the red, and 41.6 cm-i to 

 the red, respectively. This shows that the dipole — induced-dipole con- 

 tribution to the shift is dominant in argon, approximately equal but 

 a little smaller than the dispersion contribution in krypton, and much 

 smaller than the dispersion contribution in xenon. 



The effect of the dipole term is apparently somewhat larger in 

 71-TT* transitions. The blue shift of these transitions in polar and 

 non-polar solvents has been known for some time^^ and is an im- 

 portant characteristic of 7i-7r* transitions. The major part of this 

 shift is very probably caused by an energy change of the states them- 

 selves (the 0-0 band) , rather than by the Franck-Condon effect on the 

 local environment. This is borne out by the fact that individual vi- 

 brational bands in the formaldehyde n-n* transition in solid argon 

 (a non-polar "solvent") can be measured to within ± 50 cm-^ and 

 these all show relatively large shifts to the blue of from 220 to 320 

 rm-M This shift is of similar magnitude as the u-tt* shift in other 

 non-polar solvents. The behavior of ti-tt* transitions in polar solvents 

 is very analogous to that in these simple non-polar solvents. The 

 energy shifts are expected to be much greater because of dipole-dipole 

 interactions, and strong "charge transfer" or "hydrogen-bonded" 

 complex formation. 



5. Energy Transfer 



Energy transfer may take place only if total energy is conserved. 

 In an isolated molecule in a radiation field, transfer between states of 

 identical total energy may take place, but transfer between states of 



" See, for example, ref. 33, pp. 30. 



