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

Abstract

The proton resonance spectra of a variety of condensed aromatic compounds including benzene, naphthalene, anthracene, phenanthrene, pyrene, acenaphthylene and triphenylene were obtained in dilute CDCl 3 solution. Comparison of the proton chemical shifts obtained with previous literature data for CCl 4 solution shows small but significant differences. A previous model (CHARGE6) for calculating the proton chemical shifts of aliphatic compounds was extended to aromatic compounds. This was achieved by including an automatic identification 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 was a function of the number of adjoining rings and b) the molecular ring current was proportional to the molecular area divided by the molecular perimeter. This, plus the inclusion of deshielding steric effects for the crowded protons in these molecules, gave a good account of the observed chemical shifts. The model was also applied successfully to the non-alternant hydrocarbons of fulvene and acenaphthylene and to the aliphatic protons near to and above the benzene ring in tricyclophane and [10]cyclophane.The Huckel calculation of the π electron densities in CHARGE6 was used to calculate the π electron densities in substituted benzenes. The π-inductive effect was used to simulate the effect of CX 3 groups (X = H, Me, F) on the benzene ring. These together with the long range effects of the substituent 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 shifts 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 error of the observed vs. calculated shifts was ca. 0.1 ppm. The model also allows the interpretation of the shifts in terms of the separate interactions calculated in the programme, i.e. π electron densities and steric, anisotropic and electric field effects. Previous correlations of the proton SCS with π electron densities and substituent parameters are shown to be over simplified. The relative proportions of these different interactions are very different for each substituent and for each ring proton.

1H chemical shifts in NMR: Part 19. Carbonyl anisotropies and steric effects in aromatic aldehydes and ketones

“…Conversely, the H-10 proton of 9-hydroxyanthracene was calculated at too low an υ value (calculated 7.57 vs observed 7.80). The calculated SCS have the correct signs (note the value for H-10 in anthracene is 8.43υ), 23 but are much too large. This difference is not due to the carbonyl anisotropy or to steric or electric field effects, as these effects decrease very rapidly with distance (see Table 5).…”

“…Anthraldehyde was also optimized at the B3LYP/6-31G** level as this was the geometry used previously for anthracene parameterization. 23 9-Acetylanthracene was too large to run at these higher levels of theory but 9-methoxy-and 9-hydroxyanthracene were run at the B3LYP/3-21G* level. All the calculations were carried out on a PC.…”

“…The assignments of the spectra of benzaldehyde (1) and acetophenone (5) are straightforward. Those of 2-chlorobenzaldehyde (2), 23 2-hydroxybenzaldehyde (3), 29 indan-1-one (7) 25 . 2-hydroxyacetophenone (8), 29 fluoren-9-one (10), 30 2,4,6-trimethylacetophenone (13), 27 9-anthraldehyde (14) 28 and 9-acetylanthracene (15) 28 have been given previously.…”

An NMR, IR and theoretical investigation of 1H Chemical Shifts and hydrogen bonding in phenols

“…This effect known as the -shift is obviously of great interest when studying alcohols in aromatic systems, such as the phenols under investigation here. The corresponding effect for the carbon atom has however been studied in detail previously 42,43 and the same approach can be applied in determining this effect on the OH atom. This effect must thus be investigated prior to including the non-bonded geometrical effects studied above.…”

Quantum vs. classical models of the nitro group for proton chemical shift calculations and conformational analysis

“…For aromatic ring systems the ring current is calculated from the equivalent dipole approximation, 21 according to eq. (6):…”