13C NMR chemical shift assignments of nitrated benzo[a]pyrenes based on two-dimensional techniques and DFT/GIAO calculations

Abstract

Nitrated benzo[a]pyrenes are known to be mutagenic, carcinogenic and teratogenic to bacterial and animal cells. For differentiation of isomers and environmental analytical trace studies, spectroscopic information is necessary. The present studies report the 13C NMR spectra of 1-, 3- and 6-nitrobenzo[a]pyrene measured in CDCl3. Complete 13C NMR assignments based on 2D NMR spectroscopy (HMQC and HMBC) were carried out. Further, the computation of fifty 13C chemical shifts of 1-, 3-, and 6-nitrobenzo[a]pyrenes by using GIAO B3LYP/6-311+G(d,p)//B3LYP/6-31+G(d), 6-31+G(d,p), 6-311+G(d), 6-311+G(d,p), 6-311G(d,p) levels of theory was investigated. For 1-, 3- and 6-NBaP the calculated chemical shifts δcalc versus δexpt plots of chemical shifts fall on a linear correlation line with r2 > 0.90. The GIAO B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p)level of theory was found to yield chemical shifts in good agreement with experiment with r2 > 0.90. The most expensive method (larger basis set) has provided the best agreement with the experiment. It is therefore important to continue to seek computational methods that can predict precise chemical shifts in nitrated or related compounds.