Protonenresonanzspektren orientierter Moleküle: Acetylenverbindungen

Abstract

The proton magnetic resonance spectra of ethyne (acetylene, I), 3-chloro- and 3-bromo-propyne (II, III), 1,4-dichloro-2-butyne (IV), propyne (V), 1,3-pentadiyne (VI) and 2,4-hexadiyne (VII) were investigated in the nematic phase of 4,4-di-n-hexyloxy-azoxybenzene. A theoretical analysis was carried out of the 4-spin spectrum of a rotating methyl group and one coupled proton (e. g. compounds V and VI) in terms of the two direct and one scalar spin-spin couplings and the chemical shift difference between the two groups of protons. Correspondingly, the spectrum of two equivalent rotating methyl groups (e.g. compound VII) was calculated in terms of the two direct and one scalar spin couplings. The chemical shift differences of the signals of the acetylenic protons in the nematic and the isotropic phase indicated that compound I was preferentially oriented perpendicular to the magnetic field, whereas an increasing parallel orientation of the carbon triple bond with respect to the magnetic field was found for compounds II, III, V and VI. The chemical shift anisotropy of the acetylenic protons were between 8.1 × 10-6 and 13.1 × 10-6, whereas the values for the methyl protons were almost zero. The absolute signs of the scalar couplings J (H, CH3) of V and of J (CH3, CH3) of VII were found to be negative and positive, respectively, in accordance with theoretical predictions of Karplus. Values for the relative proton-proton distances were calculated for compounds V, VI and VII and were compared with microwave and electron diffraction data. The agreement was good with the microwave data for V. Discrepancies with the less accurate structural data available for VI and VII are tentatively explained by the assumption that in these diacetylenes the H—C—H-bond angles of the methylgroups are slightly increased (∼ 110°) compared to V (108°25′).