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Proton chemical shifts in NMR: Part 17. Chemical shifts in alkenes and anisotropic and steric effects of the double bond

The 1H NMR spectra of a number of alkenes of known geometry were recorded in CDCl3 solution and assigned, namely ethylene, propene, 4‐methylcyclohexene, 1,4‐dimethylcyclohexene, methylene cyclohexane (in CFCl3–CD2Cl2 at 153 K), 5‐methylene‐2‐norbornene, camphene, bicyclopentadiene, styrene and 9‐vinylanthracene. These results together with literature data for other alkenes, i.e. 1,3‐ and 1,4‐cyclohexadiene, norbornene, norbornadiene, bicyclo[2.2.2]oct‐2‐ene and α‐ and β‐pinene, and other data allowed the determination of the olefinic shielding in these molecules. The shielding was analysed in terms of the magnetic anisotropy and steric effects of the double bond together with a model (CHARGE7) for the calculation of the two‐ and three‐bond electronic effects. For the aromatic alkenes ring current and π‐electron effects were included. This analysis showed that the double bond shielding arises from both anisotropic and steric effects. The anisotropy is due to the perpendicular term only with a value of Δχ(CC) of −12.1 × 10−6cm3mol−1. There is also a steric deshielding term of 82.5/r6 (r in Å). The shielding along the π‐axis changes sign from shielding at long range (>2.5 Å) to deshielding at short range (<2 Å). The model gives the first comprehensive calculation of the shielding of the alkene group. For the data set considered (172 proton chemical shifts) ranging from δ=0.48 to 8.39, the r.m.s. error of observed vs calculated shifts was 0.11 ppm. Copyright © 2001 John Wiley & Sons, Ltd.