Proton chemical shifts in NMR. Part 16.Proton chemical shifts in acetylenes and the anisotropic and steric effects of the acetylene group

Abstract

The proton resonance spectra of a number of acetylenes of fixed geometry were recorded in dilute CDCl₃ solution and assigned. These were acetylene, equatorial- and axial-cyclohexylacetylene at −60 °C, 1,4-di-1-adamantylbutadiyne, 1-ethynyl-t- and -c-4-tert-butylcyclohexan-r-1-ol, 2-exo-ethynylnorbornan-2-oland 2,2′-ethyne-1,2-diyldibornan-2-ol. The aromatic acetylenes measured were phenylacetylene, o-ethynyltoluene, 2-ethynylnaphthalene and 9-ethynylanthracene. This data together with previous literature data for but-1-yne, but-2-yne, pent-1-yne, tert-butylacetylene, p-ethynyltoluene, 1-ethynylnaphthalene and 2-ethynylpropene allowed the determination of the acetylene substituent chemical shifts (SCS) in a variety of molecules. These SCS were analysed in terms of the magnetic anisotropy and steric effects of the acetylene group together with a model (CHARGE7) for the calculation of the two-bond and three-bond electronic effects. For the aromatic acetylenes ring current and π electron effects were included.

Analysis of the SCS showed that the acetylene SCS were due to anisotropic and steric effects plus electronic effects for near protons. A value of Δχ^CC of −11.1 × 10⁻⁶ cm³ mol⁻¹ was obtained together with a steric coefficient of 56.6 Å⁶. Better results were obtained with both effects operating from the carbon atoms.

The model gives the first comprehensive calculation of the SCS of the acetylene group. For the data set considered of 88 proton chemical shifts spanning ca. 8.0 ppm the rms error of observed vs. calculated shifts was 0.074 ppm.