Calculations of proton chemical shifts in olefins and aromatics

A semi-empirical calculation of the partial atomic charges in organic compounds based on molecular dipole moments (CHARGE3) was developed into a model capable of predicting proton chemical shifts in a wide variety of organic compounds to a reasonable degree of accuracy. The model has been modified to include condensed aromatic hydrocarbons and substituted benzenes, alkenes, halo-monosubstituted benzenes and halo-alkenes. Within the aromatic compounds the influence of the π electron densities and the ring current have been investigated, along with the α, β and γ effects. The model gives the first accurate calculation of the proton chemical shifts of condensed aromatic compounds and the proton substituent chemical shifts (SCS) in the benzene ring. For the data set of 55 proton chemical shifts spanning 3 ppm the rms error of the observed vs. calculated shifts was ca. 0.1 ppm. The model also has been expanded in order to calculate and predict the proton chemical shifts for a series of unsaturated hydrocarbons, calculating the anisotropy of the C=C bond along with the shielding or Van der Waals effects, and quantify the γ effects. For the data set of 171 chemical shifts spanning a range of ca. 7.906 ppm, the scheme fits the experimental data to an rms error of 0.11 ppm. The model was then applied to a variety of halo-aromatic and halo-alkene compounds. The calculated halogen SCS were in reasonable agreement with the observed data. The application of lanthanide induced reagents on α,β unsaturated ketones has also been investigated in order to deduce molecular structures and to obtain the assignment of the spectra of these molecules. (author)