on the occasion of his 80th birthday A normal coordinate analysis of the vibration spectrum of crystalline V20, is carried out in the assumption of a Urey-Bradley force field. The calculated frequencies are adjusted to 29 experimental infrared and Raman frequencies by an automatic force constant refinemeat program. Analysis of the potential energy distribution and the atomic displacements allows the classification of the modes into nine types of oxygen vibrations and three types of chain modes. A discussion is given of effective charges and infrared intensities. An assignment of the infrared spectrum of polycrystalline V,O, is added. The inErared (IR) spectrum of V,O,, and to a lesser extent the Raman spectrum, have been the subject of several papers [l to 111. Due to the complicated lattice structure and the high number of fundamentals, the assignment of experimental frequencies without normal-mode calculations has necessarily been restricted t o a few bands in the higher-frequency range. A thorough understanding of the vibration spectrum is however of both academic and practical importance. The latter is illustrated by the frequent use of the TR spectrum of V,O, catalysts as a monitor for bond changes which occur a t different stages of the reaction.Some years ago, Gilson et al. [8] have published an extended list of IR and Raman frequencies of V,O,, together with calculated frequencies and bond assignments. As we have already shown in a previous paper [lo], the experimental frequencies as determined by Gilson et al. deviate substantially from the correct values. We have therefore carried out new normal-mode calculations starting from our own IR and Raman data. The calculated frequencies are fitted to the experimental ones by adjusting the force constants in an automatic least-squares fitting program. We found that a simple valence force field is inadequate for a reasonable fit to some of the important bands. The fit was markedly improved by allowance for repulsive action between non-bonded atoms (UB force field).
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