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

Although being widely studied over the past 40 years, the degradation pathway(s) for 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, commonly referred to as HNIW, are still under debate. One approach to investigate the decomposition mechanisms are by measuring the kinetic-isotope effect (KIE). Tracking how certain isotopic substitutions affect measurable parameters, e.g., chemical speciation or time-to-explosion, specific functional groups can then be assigned to certain steps in the decomposition pathway. Therefore, it is useful to synthesize and research isotopically labeled explosives. To aid in the characterization of any newly synthesized isotopically labeled HNIW variants, the vibrational spectra of the D, ¹³C, ¹⁵N (all), ¹⁵N (nitro groups), and ¹⁸O isotopologues have been theoretically predicted in the gas phase. These results are compared to experimentally measured IR and Raman spectra of both unsubstituted HNIW and ¹⁵N-labeled HNIW in which the six nitro groups were synthetically tagged with ¹⁵N atoms. The experimental isotopic frequency shift for the -NO stretching frequencies compares nicely to that theoretically calculated (~35.7 cm^-1 vs. 36.5 cm^-1, respectively) which demonstrates the accuracy of the predicted vibrational spectra. Also, analysis of the isotopic shifts for all observed spectral features have improved the current spectral assignments of the HNIW vibrational bands. This theoretical method is then likely useful for other explosives or organic molecules in which isotopic analogues are of interest.