Spectroscopic and computational studies on a number of conformationally flexible molecules

• Main Author: Richardson, Patricia
• Published: University of Edinburgh 2001
• Subjects: 543.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.661109

The laser-desorption jet-cooling (LD-SJ) method has been applied successfully to the acquisition of LIF excitation spectra for a number of conformationally flexible molecules. The molecules 4-cyanobiphenyl, 4-hydroxybipenyl, tyramine and 4-t-butyl phenol were studied and well-resolved vibronic spectra obtained for each molecule. Modification to the experimental set-up have allowed the collection of dispersed fluorescence spectra for the first time. The dispersed emission spectra from both conformer bands of 4-t-butyl phenol, and several of the tyramine conformer bands were recorded. In the dispersed fluorescence spectrum of 4-hydoxylbiphenyl, a high enough resolution to observe individual torsional levels in the ground state could not be obtained. <i>Ab initio</i> methods have been used in tandem with the LD-SJ work to study the structural and electronic behaviour of each molecule studied experimentally. Several similarities were observed in the electronic properties of each molecule. Interactions in the HOMO were influential in determining the structure of each compound. In addition, substituent effects on the electron density in the HOMO were shown to influence the energetic stability of the various different molecular conformers. Calculations show that conjugative interactions in the HOMO also play an important role in the stability of the twisted molecular structure of biphenyl and derivatives. Substituent induced perturbations to the electron density in the lowest unoccupied molecular orbital (LUMO) can be directly linked to the curvature of the S₁ torsional potential. Substituents which are either strongly electron donating or withdrawing introduce a large asymmetry in the electron density across the inter-ring bond resulting in a reduced S₁ torsional frequency.