Electronic energy levels and transitions in transition-metal complexes

The fullest analysis of the electronic spectrum of a gas-phase molecule can only be achieved if rotational structure is resolved. In this case, rotation constants in upper and lower states can be calculated, as well as vibration frequencies, and a detailed picture of the structure of each state can be obtained. Rotational structure is, however, rarely resolved except for molecules with only two or three atoms. This is partly because excitation often leads to unstable upper states, with short lifetimes. 

The selection rules that govern the intensities of valence electronic transitions are derived from the quantum-mechanical equations that describe the processes involved. They depend on many factors, and it is not possible to give a full account of them here you will find details in [8] and [9]. It is important, however, to understand some aspects of the rules to make sense of the electronic spectra of transition-metal complexes. This is covered in Section 9.6.2. 

6 Electronic energy levels and transitions in transition-metal complexes 

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