Benchmarking quantum mechanical calculations with experimental NMR chemical shifts of 2-HADNT

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In this study, both GIAO-DFT and GIAO-MP2 calculations of nuclear magnetic resonance (NMR) spectra were benchmarked with experimental chemical shifts. The experimental chemical shifts were determined experimentally for carbon-13 (C-13) of seven carbon atoms for the TNT degradation product 2-hydroxylamino-4,6-dinitrotoluene (2-HADNT). Quantum mechanics GIAO calculations were implemented using Becke-3-Lee–Yang–Parr (B3LYP) and other six hybrid DFT methods (Becke-1-Lee–Yang–Parr (B1LYP), Becke-half-and-half-Lee–Yang–Parr (BH and HLYP), Cohen–Handy-3-Lee–Yang–Parr (O3LYP), Coulomb-attenuating-B3LYP (CAM-B3LYP), modified-Perdew–Wang-91-Lee–Yang–Parr (mPW1LYP), and Xu-3-Lee–Yang–Parr (X3LYP)) which use the same correlation functional LYP. Calculation results showed that the GIAO-MP2 method gives the most accurate chemical shift values, and O3LYP method provides the best prediction of chemical shifts among the B3LYP and other five DFT methods. Three types of atomic partial charges, Mulliken (MK), electrostatic potential (ESP), and natural bond orbital (NBO), were also calculated using MP2/aug-cc-pVDZ method. A reasonable correlation was discovered between NBO partial charges and experimental chemical shifts of carbon-13 (C-13).