Effects of Intramolecular Group and Solvent on Vibrational Coupling Modes and Strengths of Fermi Resonances in Aryl Azides: A DFT Study of 4-Azidotolune and 4-Azido-N-Phenylmaleimide

The high transition dipole strength of the azide asymmetric stretch makes aryl azides good candidates as vibrational probes (VPs). However, aryl azides have complex absorption profiles due to Fermi resonances (FRs). Understanding the origin and the vibrational modes involved in FRs of aryl azides is critically important towards developing them as VPs for studies of protein structures and structural changes in response to the surroundings. As such, we studied vibrational couplings in 4-azidotolune and 4-azido-N-phenylmaleimide in two solvents, N,N-dimentylacetamide and tetrahydrofuran, to explore the origin and the effects of intramolecular group and solvent on the FRs of aryl azides using density functional theory (DFT) calculations with B3LYP functional and seven basis sets, 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). Two combination bands consisting of the azide symmetric stretch and another mode form strong FRs with the azide asymmetric stretch for both molecules. The FR profile has been altered by replacing the methyl group with maleimide. Solvent changes the relative peak position and intensity more significantly for 4-azido-N-phenylmaleimide, which makes it a more sensitive VP. Furthermore, the DFT results from different basis sets can be used to predict more reliable vibrational spectra.