A systematic theoretical investigation of the relationship between heats of detonation and NBO charges and 15N NMR chemical shifts of nitro groups in nitramines and nitro paraffins

Abstract

A new quantitative method for predicting and calculating the heat of detonation for a series of nitro paraffins and nitramines employing the natural bond orbital (NBO) charge analysis and ¹⁵N NMR chemical shifts of the nitro group is established. All calculations, including optimizations, charge analysis and ¹⁵N_Nitro NMR chemical shifts, were performed using density functional (DFT) methods with 6-311+G(d,p) basis set. The results show a linear correlation between the nitro group charges and C/N-nitro bond lengths. The latter reflect the strength of the corresponding bond and thus the stability of the nitro compounds. A strong correlation was observed between the heat of detonation with the charge and ¹⁵N NMR chemical shift on the nitro group in nitramines and nitro paraffins. Nitro compounds with a higher heat of detonation have less negative nitro group charges, (Q_Nitro) and a lower value for the ¹⁵N_Nitro chemical shift in analogous compounds. From the quantitative models, the heat of detonation increase when the Q_Nitro values (positive coefficient) are larger and decrease when the ¹⁵N_Nitro NMR chemical shift (negative coefficient) is higher. The present work provides consistent models (mean square error prediction values below 0.14 MJ kg⁻¹) in a systematic way for quick estimation of heats of detonation – with or without experimental data – for a wide range of energetic materials. This practical approach is particularly useful as a tool for the design of high-energy density materials.