Question

1. Which is better for identification, IR or UV/Vis spectrophotometry and why?

2. Why does a UV/Vis absorbance spectrum have discreet “bumps” or “absorbance bands”?

3. Why does the absorbance spectrum for most molecules in solution look very smoothed out, i.e. why doesn’t it have sharp bumps like those observed in the gas phase?  

Answer 1

1) IR (Infra Red) radiation is better for identification. UV/Vis spectroscopy gives an idea regarding the electronic environment in the molecule. However, IR spectroscopy reveals the presence of exact functional group present in the molecule. for example, the peak at 1710 cm^-1 responsible for carbonyl functional group in acetone

2) single electronic transition in the absorption spectrum should give a sharp peak, however it is not so. It is due to the fact that electronic excitation superimposed on rotational and vibrational sub levels. Just like electronic state rotational and vibrational states are also quantized. The difference in energy between two adjacent vibrational level is intermediate in between two adjacent electronic and rotational state.During electronic transition, the electron moves from a given vibrational & rotational mode in one electronic state to some other vibrtional and rotational mode in the next electronic state. Tis leads to a large number of possible transition that lies close to each other. Thus a large number of wavelength which are close enough will be absorbed resulting in the formation of a broad band. Hence during analysis of the spectrum the wavelength that gives maximum absorbance is always considered and is known as absorption maximum (λ_max).

3) Molecular interaction is more severe in a solution (due to solvent - solute interaction) than in gaseous phase. As a result of which the absorption peak in gaseous phase appears t obe sharp than in solution media.