MOLECULAR SPECTROSCOPY AND STRUCTURE
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Research efforts are under way on a variety of projects involving the use of fluorescence spectroscopy of jet-cooled molecules and Fourier transform infrared (FT-IR) and laser Raman spectroscopies. Computer methods for quantum mechanical calculations and on-line instrument control are utilized and developed.
The dynamics of conformational energy changes in electronic ground and excited states are investigated by analyzing the far-infrared, mid-infrared, Raman and fluorescence data using quantum mechanical computational techniques. From such investigates on molecules with ring inversions, psuedorotations or internal rotations, the equilibrium structures, potential energy barriers, pathways of interconversion and intramolecular forces can be determined. Recent work has concentrated on molecules that require two vibrational coordinates to define their conformations. For example, the conformational changes of cyclopentanes and cyclohexenes containing heteroatoms can be investigated using far-infrared and Raman spectra along with two-dimensional potential energy calculations.
A tunable Nd:YAG based dye laser system is used to study the conformations of molecules in electronic excited states. A supersonic jet system cools the molecules so that vibrational excited states are thermally depopulated. Fluorescence excitation spectra and ultraviolet absorption spectra such as those shown below for 1,3-benzodioxole, then can be recorded and analyzed to determine the vibrational potential energy surfaces for the electronic excited states of interest. Dispersed fluorescence spectra aid in the analyses of the ground state.
