A Computational Study of the NMR Chemical Shifts of Polynitropyrazoles

The ¹H, ¹³C, ¹⁵N, and ¹⁷O chemical shifts of 91 nitro-1H-pyrazoles, as well as 1-amino-1H-pyrazole, were calculated using the Gauge-Invariant Atomic Orbital/Becke Three-parameter Lee-Yang-Parr [GIAO/B3LYP/6-311++G(d,p)] method. The majority of these compounds are polynitropyrazoles, including substituents such as N-nitro, C-nitro, and N-trinitromethyl groups. The calculated chemical shifts were compared with available experimental data using an empirical equation that relates absolute shieldings to chemical shifts. However, a new equation was required to accurately account for the oxygen atoms in the nitro groups. Additionally, some conformational studies were performed to better understand the chemical shifts, particularly those of the OH and NO₂ substituents at position 1. Different statistical methods were used to analyze the calculated ¹⁵N chemical shifts.