N-V transitions

The UV spectra of quinoxalines have been examined in several solvents. In cyclohexane, three principal absorptions are observed (Table 2). In hydroxylic solvents the vibrational fine structure disappears and in methanol or water the weak n- v transitions are obscured by the intense v - tt transition (79HC(35)l). 

A weak absorption (e = 10-100) suggests an n —v transition of an isolated carbonyl group. If this absorption is found in the region 270-350 nm an aldehyde or ketone is probable. 

The longer-wave maxima (corresponding to an N-V transition) of the 1,2-dithiolium salts obtained by protonation of the 1,2-dithiaful-valenes (184) are affected by substitution in the phenyl residue to the same extent as those of the similarly substituted simple derivatives in Table VIII (Nos. 3-10).45... 

It is commonly accepted that n — - v transitions of a base always shift toward higher frequency upon H bond formation (the n —>- ir blue shift ). 

In the spectral range, adjacent to the rr-ir band from the long wavelength side, an n-v transition of low intensity is present in pyridine (49). When an H-bond is formed between the basic nitrogen atom of pyridine and the solvent molecules, or the silanol groups of the adsorbent, this band is displaced to the shorter wavelengths, but being of low intensity, it cannot produce the enhancement effect and structure described. 

First, there are the N-> V transitions which are from the bonding orbital in the ground state of a molecule to a higher energy orbital (antibonding orbital)2. The class of N -> V transitions is large and one or more may occur for a molecule. For a transition between a orbitals as in the case of paraffins, the N V transition may be written as a 7r transitions. The <7 -> ultraviolet region. While some of the 7r r transitions are observed in the far... 

R-bands) due to n - V transitions are observed at longer wavelengths. In polar solvents the n-r-n bands are shifted to lower wavelengths with loss... 

With the exception of ethylene, all the olefins exhibit very similar absorption spectra. They show broad and intense absorption bands (e 10,000) with the maxima in the region 172-194 ma (Figure 9.1). The bands are assigned to the r (N V) transitions associated with the double bonds. For analytical purposes, the olefins may be conveniently grouped according to the number of alkyl groups attached to the carbons of the double bond. 

Fig. 4. Strength distribution (matrix-element squared) for the RO matrix element Ml connecting excited states of " T1 to the 0" ground state of ° Pb. The theoretical distribution has been folded with a Gaussian for ease of viewing and to simulate the experimental resolution which might pertain for the inverse charge-exchange reaction. The dominant n-v transition is indicated for the major structures.

---