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

Abstract

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 ( 2 H), 13 C, 15 N (all), 15 N (nitro), and 18 O 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 15 N-labeled HNIW in which the six nitro groups were synthetically tagged with 15 N atoms ( 15 N nitro À HNIW). The experimental isotopic frequency shift for the À NO 2 asymmetric stretching frequencies agrees with that theoretically calculated (~35.7 cm À 1 vs. 36.5 cm À 1 , respectively). Furthermore, analysis of the theoretically predicted frequency shifts for all isotopologues suggest the À NO 2 bending modes are lower in frequency than previously reported. This assignment is supported by the experimentally measured isotopic shift of ~10.1 cm À 1 for these features (consistent with the predicted shift of ~13.1 cm À 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.

Syntheses and characterization of isotopically labeled 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB)**

“…However, a more detailed examination including modelling shows an extremely small amount of not fully deuterated material. This has been reported elsewhere [35].…”

“…FTIR spectra were recorded and 90 + % isotopic substitution. However, in a recent study, a very small amount of another compound has been detected and showed a less-than 100 % deuterium substitution [35]. The 18 O labeling targeted the nitro groups.…”