Date of Award
Master of Science
Site-specific study of the protein using time-resolved IR spectroscopy with the assistance of vibrational probes (VPs) has been the most promising research discipline. However, azide VPs that are absorbed within the protein transparent window generate a complex absorption profile due to Fermi resonance (FR). In the current study, the azide absorption profiles of three aryl-azide compounds: 4-azidotoluene, 4-azido-N-phenylmaleimide, and 4-azidoacetanilide have been explored with basis set effect, solvent effect, intramolecular effect, and the effect of rotational isomerism. Basis set effect was studied using seven basis sets namely: 6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p) and 6-311++G(df,pd) with DFT/B3LYP. Geometry optimization and anharmonic frequency calculations have been carried out using two solvents, NNDMA and THF. Peak intensities, relative peak positions to the azido asymmetric stretch, cubic force constants, and the third-order Fermi resonance constants were analyzed. 4-azidoacetanilide has a more complex azide absorption profile that cannot explain both basis set effect and solvent effect. The DFT results show that FR patterns change with para substitution, and the azide asymmetric stretch blue shifts with substitution of methyl to maleimide, or NNDMA to THF. It has been found that rotamers depict the same features in the azide absorption profile. Moreover, theoretical vibrational spectra with the 6-311+G(d,p) basis set can describe the FTIR spectra qualitatively, but it was identified that more accidental FRs would impact on azide absorption profile than that observed in FTIR spectra.
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