MICHAEL KASHA 35 



groui) is coplanar with ihe ring, the /-orbital is fully conjugated and 

 only TT ^ 77-* transitions are expected. If the amino group is twisted 

 90° about the C-N bond, no conjugation of the /-orbital electrons 

 and the Ti-orbital electrons would be possible, because in this molecu- 

 lar conformation the orbitals differ in symmetry. In the 90° form, an 

 n -^ a* transition (see next section) might be found at higher ener- 

 gies, but an ;/ -^ tt* transition in aniline would not be expected. At 

 intermediate angles of twist, an intramolecular charge-transfer transi- 

 tion (23) from the "lone-pair" /-orbital to an empty antibonding 

 phenyl vr-orbital could occur: it may be desirable to designate this 

 as / -^ Gtt, since the "lone-pair" orbital is not non-bonding in the 

 twisted conformation, nor is Tr-antisymmetry preserved for the mole- 

 cule as a whole. To label such a transition in aniline (or diphenyl- 

 amine and triphenylamine) as n — > 77* would be inaccurate and con- 

 fusing. It is however Avorth pointing out that in one sense, viz., the 

 addition of a proton to the hetero-"lone-pair" causes spectral changes 

 in the / -^ a-n- bands of such molecules, which resemble the behavior 

 of bona-fide n -^ tt* transitions. This is clearly demonstrated in the 

 recent study of absorption bands in triarylarsines by Hochstrasser and 

 Cullen (14) . The / — > a-ir designation seems appropriate, as far as 

 orbital labelling is concerned, for describing such intramolecular 

 charge transfer transitions of high intensity. 



The aryl Group V-compounds discussed above have some electronic 

 orbital similarities to the triarylmethyl radicals. Murrell (28) has 

 assigned the long-wavelength absorptions in these molecules as intra- 

 molecular charge-transfer transitions, which could again be designated 

 as / -> a-K. Burawoy's interpretation (4, 4a) of the origin of 'R'-bands 

 was based on the fact that the solvent and substitution effects in 

 triarylmethyl radicals were similar to corresponding effects found 

 in heteroatom molecules. He came to the conclusion that the com- 

 mon ingredient in all these cases was the presence of "radical-like 

 character." Actually, we now can recognize that the common in- 

 gredient is an ??-orbital or /-orbital "lone-pair" electron, promotion of 

 which shows some similarities with respect to solvation and conjuga- 

 tive substitution effects. However, the polarizations and intensities 

 of n -^ TT* and / -^ On transitions contrast sharply. The theoretical 

 bases for the empirical classifications wdll be outlined in a later section. 



C. Interpretation of the Carhonyl Absorption Bands 



A theoretical study, based on the molecular orbital theory, by 

 Mulliken and McMurry in 1941 (27, 22) suggested a quite novel 



