Vibrational state, infrared energy absorption

The techniques considered in this chapter are infrared spectroscopy (or vibrational spectroscopy), nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy (or electronic spectroscopy) and mass spectrometry. Absorption of infrared radiation is associated with the energy differences between vibrational states of molecules nuclear magnetic resonance absorption is associated with changes in the orientation of atomic nuclei in an applied magnetic field absorption of ultraviolet and visible radiation is associated with changes in the energy states of the valence electrons of molecules and mass spectrometry is concerned... 

Vibrational absorption-A low energy-level change in the vibrational state of a molecule characterized by absorption in the near infrared. 

Other detection methods. Besides XPS, other chemically sensitive techniques are available to probe the reaction. Surface reflection absorption infrared spectroscopy [130] and electron-energy loss spectroscopy [131] give detailed information on the vibrational states and thus the bonds of surface species. Gas-phase mass spectroscopic techniques provide information about the desorbing species. 

The absorption bands in the ultraviolet and visible part of the spectrum correspond to changes in the energy of the electrons but simultaneously in the vibrational and rotational energy of the molecule. In this way a system of bands is produced in the gaseous state. In the liquid state there is nothing of the rotational fine structure to be seen, and usually little or nothing of the vibrational structure, as a result of the interaction with the molecules of the solvent. With aromatic compounds in non-polar solvents such as hexane and carbon tetrachloride the vibrational structure is, however, still clearly visible in the ultraviolet absorption spectrum. This vibrational structure is mainly determined by the vibrations of the excited state, which therefore do not occur in the infrared and Raman spectrum of the normal molecule. 

Recall from Chapter 13 that the absorption of infrared energy can promote a molecule from a lower vibrational state to a higher one. In a similar fashion, the absorption of ultraviolet (UV) light can promote an electron from a lower electronic state to a higher one. Ultraviolet light has... 

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