NMR SHIELDINGS IN BENZOYL AND 2-HYDROXYBENZOYL COMPOUNDS - EXPERIMENTAL VERSUS GIAO CALCULATED DATA

GIAO-calculated NMR chemical shifts (H-1,C-13, and O-17) as Obtained a t Various computational levels are reported for the three parent compo unds phenol, benzaldehyde, and salicylaldehyde, and for 13 different b enzoyl and the 13 corresponding 2-hydroxybenzoyl compounds. The data a re compared with experimental solution data, focusing on the agreement with spectral patterns and spectral trends. The influence of differen t optimized geometries (HF, MP2, B3LYP, BLYP), basis sets (6-31G(d,p) up to 6-311++G(2df,2dp)), and levels of theory (HF, B3LYP, BLYP) was i nvestigated systematically by exhaustive calculations on the three par ent compounds. With regard to the results obtained from this foregoing study, the GLAO calculations for the compounds of the two series were performed at two levels of theory, HF/6-311++G(d,p) and BLYP/6-311++G (d,p) for both the B3LYP/6-31G(d,p) and the HF/6-31G(d,p) optimized ge ometries. It turned out that, with the exception of the nuclei of the hydrogen-bonded OH groups, B3LYP and HF optimized geometries yield rat her similar results. For aromatic carbons and protons, because of syst ematic shortcomings, the GIAO-HF calculations are distinctly worse tha n the GIAO-BLYP calculations. In the latter case, interchanges with re spect to the experimental spectral patterns are obtained only in few i nstances and concern nuclei with rather small chemical shift differenc es (within 4 ppm for carbons, within 0.5 ppm for hydrogens). For the n uclei of the C=O and O-H groups, the experimentally observed spectral trends are reproduced in similar quality at both the HF and the BLYP l evels of theory.