Computation of 1H NMR chemical shifts: structural assessment of energetic materials.

Context: Nuclear magnetic resonance (NMR) spectroscopy is a highly valuable tool that is extensively employed for the structure elucidation of organic compounds and in various domains of chemistry. The density functional theory (DFT) and gauge-independent atomic orbital (GIAO) calculation strategy was established to predict reliable NMR chemical shifts with low computational expense and assist in resolving ambiguities in complex structure assignments. Here, we present the DFT-GIAO NMR chemical shift prediction method employed for the first time on a variety of energetic materials. We have predicted ¹H NMR shifts using computationally inexpensive single-point calculations for 48 energetic compounds comprising aliphatic, aromatic and heterocyclic backbones with various explosophoric functionalities. In DFT optimization and GIAO NMR computations at the B3LYP/6-311G+(2d,p) level, we predicted the ¹H chemical shifts for various energetic compounds and validated them with corresponding experimentally measured NMR spectra. The mean absolute deviation in predicted ¹H NMR shifts in CH protons ranges from 0.01 to 2.15 ppm. We illustrate the application of B3LYP/6-311G+(2d,p) method to assign the stereochemistry or highly deshielded protons due to electronegative explosophoric groups in energetic compounds with reasonable accuracy. This study also establishes the structure-chemical shift relationships of energetic materials. We believe that the reliable structural assignments provided by the DFT-GIAO method offer a user-friendly yet underutilized tool for structural validation and assignment of ambiguous and complex energetic molecular structures.

Methods: The optimization and proton chemical shift prediction for all selected energetic compounds were carried out at the B3LYP/6-311G+(2d,p) level of theory, utilizing Gaussian 09 software package.