DFT Studies of Solvent Effect on the Strongly Coupled Vibrations of 4-Azidoacetanilide

DFT calculations were performed to study the solvent effect on the vibrations of 4-azidoacetanilde in the range including the azido asymmetric stretch, i.e. 2000-2300 cm-1, using N, N-dimethylacetamide (NNDMA) and tetrahydrofuran (THF) as solvents. B3LYP functional was employed in the DFT calculations with 7 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). The results show that there are 7 combination bands coupling with the azide asymmetric stretch mode. As a result, the absorption profile of 4-azidoacetanilde is very solvent dependent. Specifically, stronger couplings of vibrations of 4-azidoacetanilde are predicted in NNDMA and the intensity of the azide asymmetric stretch is substantially reduced. Interestingly, the frequency of the azide asymmetric stretch of 4-azidoacetanilde in NNDMA is essentially the same as that of 4-azidotoulene. The intensity of the azide asymmetric stretch of 4-azidoacetanilde in THF is slightly increased with respect to that of 4-azidotoulene. Furthermore, the azide asymmetric stretch of 4-azidoacetanilde in THF remains nearly the same as in NNDMA but is blue shifted with respect to that of 4-azidotoulene. This indicates that a more complex and sensitive absorption profile will be expected when 4-azidoacetanilde is used as IR probe.