A molecular mechanics and ab initio prediction of the 1H chemical shifts of pinanes

Molecular mechanics calculations plus the application of a refined Karplus equation gave the conformations of 19 pinanes. These range from a Y-shaped geometry in the apopinene and α-pinene series to a pseudo chair conformation in β-pinene, nopinone and verbanone, a flattened chair in pinocarvone and the pinocarveols and a distorted Y shape for iso-verbanone. These structures were then used as input to predict the 1H chemical shifts of these compounds by semi-empirical (HSPEC) and ab initio GIAO calculations, the latter at the B3LYP hybrid DFT level using 6-31++G** basis set. The two methods gave generally good agreement with the 184 observed shifts with rms errors 0.07 ppm (HSPEC) and 0.10 ppm (GIAO), but the GIAO calculations gave several significant (>0.25 ppm) errors. One was for the H3 proton in apopinenone and other α,β unsaturated ketones, the others occurred for protons in close proximity to hydroxyl groups. To provide more information smaller analogues of known geometry and chemical shifts were subject to the same analysis. In cyclopentenone the Gaussian geometry gave good agreement with the observed shifts but the MMFF94, MMX and MM3 geometries all gave errors for different protons. These results show clearly that the molecular geometries of the α,β unsaturated ketones are responsible for the errors. The errors for the alcohols were examined using ethanol as model and were shown to be due to the different possible conformations of the OH group. Similar GIAO calculations on substituted methanes gave good agreement for the methyl compounds but poor agreement for di and tri halosubstituted methanes. The above method of molecular mechanics plus GIAO calculations is shown to be a very useful tool for the investigation of molecular geometries and conformations. However, multihalogen compounds may require different basis sets for accurate calculations.