Proton chemical shifts in NMR. Part 14. Proton chemical shifts, ring currents and pi electron effects in condensed aromatic hydrocarbons and substituted benzenes

The proton resonance spectra of a variety of condensed aromatic compounds i ncluding benzene, naphthalene, anthracene, phenanthrene, pyrene, acenaphthy lene and triphenylene were obtained in dilute CDCl3 solution. Comparison of the proton chemical shifts obtained with previous literature data for CCl4 solution shows small but significant differences. A previous model (CHARGE 6) for calculating the proton chemical shifts of aliphatic compounds was ex tended to aromatic compounds. This was achieved by including an automatic i dentification of both five- and six-membered aromatic rings based on atomic connectivities plus a dipole calculation of the aromatic ring current. The ring current intensity in the molecules was calculated by two alternative methods. a) The ring current intensity in the individual benzenoid rings wa s a function of the number of adjoining rings and b) the molecular ring cur rent was proportional to the molecular area divided by the molecular perime ter. This, plus the inclusion of deshielding steric effects for the crowded protons in these molecules, gave a good account of the observed chemical s hifts. The model was also applied successfully to the non-alternant hydroca rbons of fulvene and acenaphthylene and to the aliphatic protons near to an d above the benzene ring in tricyclophane and [10]cyclophane. The Huckel calculation of the pi electron densities in CHARGE6 was used to calculate the pi electron densities in substituted benzenes. The pi-inducti ve effect was used to simulate the effect of CX3 groups (X=H, Me, F) on the benzene ring. These together with the long range effects of the substituen t groups identified previously allowed a precise calculation of the SCS of a variety of substituents on all the benzene ring protons. The model gives the first accurate calculation of the proton chemical shift s of condensed aromatic compounds and of the proton SCS in the benzene ring . For the data set of 55 proton chemical shifts spanning 3 ppm the rms erro r of the observed vs. calculated shifts was ca. 0.1 ppm. The model also all ows the interpretation of the shifts in terms of the separate interactions calculated in the programme, i.e. pi electron densities and steric, anisotr opic and electric field effects. Previous correlations of the proton SCS wi th pi electron densities and substituent parameters are shown to be over si mplified. The relative proportions of these different interactions are very different for each substituent and for each ring proton.