Electronic Transitions and Their Nomenclature

Absorptions due to 3t 3t and n 3t transitions differ from one another in several important aspects as shown in Table 6.1. 

In a molecule, the singlet electronic states are denoted by So, Si, S, etc., of increasing energy and triplet electronic states as Ti, T2, etc. So indicates the ground state of singlet electron, whereas Si, S2, etc., and Tj, T2, etc., are excited singlet and triplet states, respectively. The n t transition is the lowest energy transition for most of saturated carbonyl compounds and this transition is known as So Si and Ti- These transitions occur from the promotion of an electron from n molecular orbital to the 31 molecular orbital and these transitions are referred as (n, 3t ) and (n, 31 ) states, respectively. Similarly, S2 and T2 excited states arise from the 

1 Occurs at shorter wavelengths (220-260 nm) Occurs at longer wavelengths (270-350 nm) 

2 Substitution moves the absorption to longer wavelength Substitution moves the absorption to shorter wavelengths 

3 Shows relatively strong absorption with max of 10 -10 1 mol cm Shows relatively weak absorption with fimax of 1-10 1 moP cm  

---