The Method of Local Oscillating Dipoles

From Eqs. (4), (5), and (8) (Section II), the resultant dipole moment derivative for the normal coordinate Qi is 

The simplicity and power of this approach, known as the method of local oscillating dipoles, was first demonstrated by Orgel (111), who used it to assign the spectra of octahedral molybdenum carbonyl complexes. Very similar arguments were presented simultaneously, but independently, by El-Sayed and Kaesz (59) in a discussion of the spectra of pentacarbonyl halide derivatives of Group VIIA metals. Subsequently, the model has been applied to the assignment of bands in the spectra of numerous octa- 

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Considerable attention has been focused on the interpretation of the relative intensities of the CO-stretching modes observed in the infrared spectra of the compounds M2(CO)m (M = Mn, Tc, or Re). The intensity of the high-energy 62 band in the spectra of these derivatives is large, contrary to that predicted by the method of local oscillating dipoles. 

In the present paper we calculate frequencies and intensities of OH-stretching and SOH-bending vibrational transitions as well as their combinations and overtones— the dominant vibrational transitions from 1000 cm to 20000 cm . The vibrational calculation is based on the harmonically coupled anharmonic oscillator (HCAO) local mode model [42-44] combined with ab initio calculated dipole moment functions [50]. This local mode method has been successful in the calculation of OH- and CH-stretching overtone spectra [19,51,52], The local mode parameters, frequency and anharmonicity, are obtained either from the observed experimental transitions or calculated ab initio [53-56]. 

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