Allers, Suzanne Julia Rachel; (1996) Molecular excited states geometries from resonance Raman spectroscopy. Doctoral thesis (Ph.D.), University College London.

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Abstract

This thesis is concerned with the application of two theoretical methods for finding structural changes in molecules in excited electronic states. Both methods, the Sum-over-States (SOS) method and the Overtone Transform (OT) method, utilise information from resonance Raman and electronic absorption spectroscopies. The purpose of the work is twofold: first, to test the applicability of both methods in the case of a complex inorganic molecule, and second, to use the methods to find new structural information about the molecule in an excited electronic state. A series of quadruply bonded metal-metal dimers serve as a vehicle for this dual aim. The partial characterisation of the 1δδ* excited state by previous studies allows comparisons with the SOS and OT results to be made. Furthermore, the structural changes along the metal-metal co-ordinate in this state are of great interest. Application of the SOS method to the metal-metal co-ordinate of the [M2X8]n- ions ([Re2Br8]2-, [Re2I8]2- and [Mo2Cl8]4- in the 1δδ* state has proved to be successful, and has allowed important conclusions to be drawn regarding the conversion between structural conformations in this state. It has been found that observation of a particular structure is dependent on the timescale of the experimental technique used. A comparative study of the 1δδ* state of the compounds Mo2X4(PMe3)4 (X = C1, Br or I) has provided insights into the effect of the ligands on metal-metal bonding in these systems. An attempted application of the OT method to the same systems has been found to be unsuccessful. Investigations of this failure point to problems with the method of finding values for structural parameters. The unusual behaviour of the ion [Mo2Br8]4- in the resonance Raman spectrum recorded in resonance with the 1(δ ← δ) transition is reported and explanations for the observations are proposed.

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