Valence shell transitions

Transitions in which the transition moment is at right angles to the internuclear axis are Q — N and the bands typically have low intensities. In molecular orbital terminology these are t - n transitions (9). Both V - N and Q - N transitions occur without a change in the principal quantum number and are valence shell transitions in contrast to Rydberg transitions, R - N where a change of the principal quantum number does occur. The symbol I describes a triplet state formed in a valence shell transition. 

In addition to the above valence-shell transitions it may be that within the manifold of low energy excited states also those derived from the le" - 4e excitation can be found. Inspection of Figure 4 reveals that the states ( A /, Aj, E") resulting from le"... 

The excitations considered so far are called valence shell transitions or sub-Rydberg transitions, as opposed to another type of excitations, found also in unsaturated compounds, namely the Rydberg transitions 68>. 

The spectrum of halothane (CF,—CHClBr) is shown in Fig. 11. The presence of the bromine atom gives rise to a Br -> (C—Br) type valence-shell transition. Its center is near 49000 cm 340). The band of next higher frequency... 

In the ground state both electrons are assigned to Ggls. Then an electronic transition can take place whereby one of the electrons is promoted from Ogls to a,Is. If this was the only possible electronic transition only two excited states would result one singlet and one triplet. The transition would be a valence-shell transition since both Ogls and Oyls are built from the same atomically unexcited AOs. 

Atoms as well as molecules have electronic transitions that are not of the Rydberg type. For atoms the famous D-lines of sodium (3s,3p) are an example. For molecules all the familiar (vr, n ) and (n, rr ) transitions of olefins and aromatic molecules are examples of non-Rydberg, valence-shell (or intravalency) type transitions. For typical valence-shell transitions the orbital of the excited electron is not much larger than the molecular core. Bands due to such transitions cannot be ordered into series. The orbital of the excited electron is usually antibonding in one or more bonds wliile Rydberg orbitals because of their large size are, in most cases, essentially non-bonding. 

In the ground state aju and eig are filled. The transitions from these two MOs to the empty e2u and bag MOs can be considered as valence-shell transitions (it, n ) since ail these MOs, including those which are empty in the ground state have been obtained from a basis set containing only atomically unexcited AOs. (This might seem to be simple. In actual fact only a part of the bands which could result from these transitions had been identified). 

A and term values we can locate the corresponding bands. Very high term values often indicate bands due to valence-shell transitions. The IPs are needed for all this, however. 

In fluoroalkanes the lowest excited states are Rydberg In the fluoro-methanes 3s and 3p are followed by strong valence-shell transitions. In higher per-fluoro paraffins the 3s band seems to lie lower than the lowest valence-shell transition -... 

The UV spectrum of cyclopropane is quite different from those of the alkanes we have examined so far. Whereas acyclic alkanes and cyclic alkanes other than cyclopropane and cyclobutane have only Rydberg bands in the lower frequency (and photo chemically important) part of the spectrum, for cyclopropane the lowest Rydberg bands intermingle with bands due to valence-shell transitions. Two structured bands at 63000 and 78000 cm (159 and 128 nm) have been assigned to the 3e a,3p) and (3e o,4p) transitions on the grounds of the similarity of their vibrational fine structure with that of the respective photoelectron bands and their term values. Two other bands, near 70000 cm (143 nm) and 83000 cm (120 nm)... 

Rydberg bands. According to Schwarz these have valence-shell character. Actually, the term value is quite high, 23200 cm . Just as for methane, the orbital of the excited electron in these states is probably of an intermediate type. These assignments are in basic agreement with those of Robin except that he assigns the weak band at 64100 cm to an additional valence-shell transition. In our opinion, however, the existence of this band is questionable. 

As to triplet states, for Rydberg states they are very close to the corresponding singlet and they are seldom considered separately. For valence-shell transitions they, of course, require a separate study. For this, mercury sensitization has been successfully applied in many cases In the case of ethylene the main primary process is still H2 formation, both 1,1 and 1,2. 

In conjunction with a study of bonding in the nitrate ion, Maria etal. have reported the electronic absorption spectra of all four boron halides 6X3. Vacuum-u.v. spectra of BF3, BCI3, and BBrg have been obtained, and tentative assignments made. The lowest-frequency band in BCI3 and BBrg is assigned to a valence-shell transition of the type. 

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