Proton chemical shifts in NMR. Part 16.1 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.