Theoretical evaluation of NMR shifts in polycyclic aromatic hydrocarbons
Using computational chemistry methodology, we evaluate the proton magnetic shieldings and the corresponding chemical shifts of 12 polycyclic aromatic hydrocarbons that derive from chrysene, perylene and picene. Due to the large size of the studied compounds, we resort to density functional theory (DFT) and use it together with the B3LYP and the KT1 functionals. After a systematic method and basis set selection study carried out on methane, benzene and anthracene, the DFT(B3LYP) method and the
... 31G*, 6-31G** and 6-311++G** bases are selected to carry out the calculations, because of the efficiency in providing shifts close to the experimental data available. Additionally, we select the DFT(KT1) method together with the aug-pcS-1 basis set, and HF/6-31G* shifts are also calculated. In order to estimate the error in the theoretical results, we take as reference accurate experimental chemical shifts obtained for the molecules under investigation. Extra measurements are needed for this purpose and are included in the present work. The best combination of method and basis set is DFT(B3LYP)/6-31G**, proving to be very efficient in getting shifts close to experiment at a relatively low computational cost, and therefore we recommend it for the evaluation of proton shifts in larger polycyclic aromatic hydrocarbons.