Vibrational Spectra of HNIW and its Isotopologues: A Combined Experimental and Computational Study

Incorporating isotopically labelled materials in degradation experiments could help unravel the mechanism(s) of decomposition through use of the kinetic isotope effect. Characterizing synthesized isotopologues however requires an understanding of what observable signals are affected by the isotopic substitution. As vibrational spectroscopy can distinguish between isotopologues, it is an ideal characterization technique to evaluate isotopic variants. To this end, the vibrational spectra of HNIW and its deuterated (H-2), C-13, N-15 (all), N-15 (nitro), and O-18 isotopologues have been computationally predicted in the gas phase using density functional theory. These results are compared to experimentally measured FTIR/ATR and Raman spectra of both unsubstituted HNIW and N-15-labeled HNIW in which the six nitro groups were synthetically tagged with N-15 atoms (N-15(nitro)-HNIW). The experimental isotopic frequency shift for the -NO2 asymmetric stretching frequencies agrees with that theoretically calculated (similar to 35.7 cm(-1) vs. 36.5 cm(-1), respectively). Furthermore, analysis of the theoretically predicted frequency shifts for all isotopologues suggest the -NO2 bending modes are lower in frequency than previously reported. This assignment is supported by the experimentally measured isotopic shift of similar to 10.1 cm(-1) for these features (consistent with the predicted shift of similar to 13.1 cm(-1)). This work expands our current understanding of the vibrational modes in HNIW as well as provides a method for future work on similar systems.