SEMI-CLASSICAL MODELS OF INFRARED INTENSITIES

Group Dipole Derivatives as Infrared Intensity Parameters.72 

The tiieory of die interaction of electromagnetic radiation witii vibrating molecules defines a simple relation between absorption intensities in the infiared spectra and derivatives of molecular dipole mcmient with respect to normal coordinates as given by Eqs. (1.47) and (1.48). 

The success of the theoty of vilnational frequencies is determined by the discovery of phjrsically plausible approadies in describing vibrational motion and in devising efficient mathematical metiiods for evaluation of the respective molecular parameters. Vahtable information about molecular structure has been accumulated over the years [3 6,17-26]. Numerous successful predictions of spectral properties of molecules confinn the validity and solid physical foimdation of tire models developed. 

The themies developed are best applied to ei erimental data for isolated molecules. Translated into realistic experimental conditions diis refers to molecules in the gas-phase at low partial pressure so diat no substantial intermolecnlar interactions are present As mentioned, the main reastm for these restrictimis is that in contrast to vibrational transition frequencies, die intraisities of absorptimi bands are extremely sensitive to environmental variations such as change of phase, intermolecnlar interactions and solvent effects. A good illustration in diis respect is offmed by the measured experimental intensities of CH2CI2 in the gas-phase, in solution and as a pure liquid as summarized in a review by Person and Steele [53] and givrai in Table 3.1. The experimental data reveal a strong dependence of die observed intensities on phase state and medium. 

It is clear that fiilly reliable information on die relatimiship between intensities and molecular structure can be derived fi om gas-phase experimental data. As a consequence, the range of molecules that can be investigated is much limited. Theoretical models fm quantitative assessment of die influence of solvmits on vibrational absorption intensities have been developed [54-58]. Using diese iproaches, eiqierimental data determined in solution are transformed to expectation values in the gas-phase. These transformations 

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