Qualitative UV spectroscopic study

The electronic structure of SO2 has been the subject of several theoretical studies which have provided a basis for interpretations of UV and photoelectron spectroscopic measurements These will not be reviewed in detail here since they all agree on those features which are important for a qualitative understanding of transition metal complex formation. The ground state configuration obtained from the extended Hiickel approach, for example, is (Iai) (lb2) (2ai) (lbi) (3ai) (2b2) (la2) (3b2) (4ai) (2bi) (4b2) (5ai) using 2 s, 2 p oxygen orbitals and 3 s, 3 p sulfur orbitals as a basis. The salient features from the M-SO2 bonding standpoint are the HOMO (4ai) and the LUMO (2bi), depicted in Fig. 2. These two orbitals can be described qualitatively as follows ... 

The energy states associated with molecules, like those of atoms, are also quantized. There are very powerful spectroscopic methods for studying transitions between permitted states in molecules using radiation from the radiowave region to the UV region. These methods provide qualitative and quantitative information about molecules, including detailed information about molecular structure. 

If the objective is identification (qualitative analysis), it suffices to compare the spectrum of the analyte with that of a standard, both recorded in the same solvent and at an identical pH. This is not the main application of UV-Vis spectrophotometry as the best results in this context are provided by spectroscopic methods considered more effective for the study of the molecular structure of organic compounds (infrared, nuclear magnetic resonance, mass spectrometry, and X-ray diffraction). However, UV-Vis spectrophotometry is a source of relevant supplementary information that helps in the elucidation of molecular structures of drugs, impurities, metabolites, intermediate compounds of degradation, etc. 

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