Molecular structure of allene from its proton nmr in a nematic solvent

The proton NMR spectrum of allene dissolved in the nematic mesophase of p, p′-di-n-hexyloxyazoxyhenzene has been obtained at 75°C. Computer analysis of this spectrum gives the parameters of the spin Hamiltonian. The structure of aliène has been determined from the elements of the spin Hamiltonian assuming that the two CH₂ planes are perpendicular to each other. It is found that the ratio of the geminal H-H bond length to the C-H bond length is 1.7133 and is in good agreement with the electron diffraction value of 1.7180. Again in comparison with electron diffraction, the C-C bond length is too short relative to the C-H bond length by 4%. This shortness could not be completely explained on the basis of molecular vibrations.