15N Chemical Shifts in Energetic Materials: CP/MAS and ab Initio Studies of Aminonitropyridines, Aminonitropyrimidines, and Their N-Oxides

Karen L Anderson,Julio C Facelli,Lawrence H Merwin,William S Wilson

doi:10.3390/i3080858

Abstract

Solid state 15N NMR chemical shift measurements have been performed on a series of nitro- and amino-substituted nitrogen-containing heterocycles that are of interest as potential new insensitive explosives. Due to low solubilities, many of these compounds are not amenable to study by solution state methods. Theoretical calculations of 15N chemical shift parameters have been performed on the structures of interest and are reported herein. The calculated and experimental values are in good agreement. The use of a model that includes intermolecular effects and allows the proton positions of the nearest neighbors to be optimized leads to the best agreement between calculated and experimental values. The theoretical models accurately predict the effects of nitro and amino substituents on ringnitrogen chemical shifts, explaining a seeming reversal in trend that is noted in the pyridine and pyridine-1-oxide chemical shifts of the highly substituted compounds.

Highlights

Materials that exhibit good explosive properties tend to be sensitive and unstable, while insensitive materials tend to be somewhat lacking in explosive performance
Of great interest is the characterization by solid state NMR (SSNMR), which can provide valuable structural information when diffraction studies are not feasible due to the lack of good-quality single crystals [7]
The BLYP method was used because it produces results that are significantly better than Hartree Fock (HF) and similar to MP2 at a fraction of the computational cost

Summary

Introduction

Materials that exhibit good explosive properties tend to be sensitive and unstable, while insensitive materials tend to be somewhat lacking in explosive performance. To increase the energy content while maintaining insensitivity, one strategy is to utilize the stability of aromatic heterocyclic compounds, adding further stability through alternating nitro and amino groups and incorporating N-oxide functionalities in the ring for improved oxygen balance and additional energy. Using this approach, several highly substituted pyridines, pyrimidines, and their N-oxides have been investigated [4,5,6]. Of great interest is the characterization by solid state NMR (SSNMR), which can provide valuable structural information when diffraction studies are not feasible due to the lack of good-quality single crystals [7].

Experimental and Computational Methods WARNING

Results and Discussion

Conclusions