Spectroscopic Investigations of Selected Cyclic and Bicyclic Molecules

Abstract

Several cyclic molecules along with two bicyclic molecules were investigated using infrared (IR), Raman, ultraviolet (UV) absorption, fluorescence excitation (FES), and single vibronic level fluorescence (SVLF) spectroscopies. The laser-induced fluorescence spectra (both fluorescence excitation and single vibronic level fluorescence) of jet-cooled 1,3-benzodioxan (13BZN) along with its ultraviolet absorption spectra have been recorded and analyzed in order to determine the vibrational quantum levels in both the ground and S1(pi,pi*) electronic excited states. A detailed energy map was established and utilized to better understand the structural and conformational differences between the ground and excited electronic states. Ab initio and DFT calculations were also carried out to complement the experimental work. The data allowed one-dimensional potential energy functions in terms of the ring-twisting coordinate to be calculated. A complete study of all of the vibrations of 13BZN in both its S0 ground and S1(pi,pi*) excited state was done utilizing several types of spectral data including infrared and Raman spectra. The vibrational frequencies of 13BZN were compared to those of the very similar 1,4-benzodioxan. The Raman and infrared spectra of dipicolinic acid (DPA), a component of anthrax spores, and dinicotinic acid (DNic) and their salts (CaDPA, Na2DPA, and CaDNic) have been recorded and the spectra have been assigned. Ab initio and DFT calculations were carried out to predict the structures and vibrational spectra and were compared to the experimental results. Theoretical calculations were also carried out for DPA dimers and DPA+2H2O to better understand the intermolecular interactions. 3-Methylindole (3MI), which serves as a structural model for the tryptophan side chain in proteins, has been investigated using vapor phase Raman spectroscopy. The vapor phase spectrum of 3MI complements previously reported Raman studies of 3MI solutions and related tryptophan derivatives. The analysis of the Raman spectrum of 3MI vapor was also supported here with newly obtained vapor phase infrared data and ab initio calculation to refine previous vibrational assignments. The present results provide an improved basis for assessing the dependence on the indoyl Raman signature on the local environment of the tryptophan side chain of proteins.