Molecular Structure of Cyclopropane from Its Proton NMR in a Nematic Solvent

The proton nuclear magnetic resonance spectrum of cyclopropane has been measured in nematic p,p′-di-n-hexyloxyazoxybenzene. The spectrum has been analyzed by determining the parameters of a spin Hamiltonian which permit its computer simulation. Several ways to determine the structure of cyclopropane from the elements of the spin Hamiltonian are employed, with all giving very similar geometric parameters. With the assumption of D3h symmetry for cyclopropane, and a C–C bond length of 1.510 Å determined by electron diffraction, we are able to fit the NMR spectrum with a structure having a C–H bond length of 1.123 Å and an HCH angle of 114.4°, which are to be compared with the corresponding gas phase electron diffraction values of 1.089±0.003 Å and 115.1±1°. The anisotropic motion corresponds to the molecular plane tending to be parallel to the applied magnetic field. For indirect spin—spin couplings we find JHH(cis)=+9.5±1 Hz, JHH(trans)=+5.5±1 Hz, and a positive sign for JCH, the indirect coupling between 13C and a directly bonded proton. The effects on the observed spectrum of JHH(gem) and JCH′, the indirect coupling of carbon 13 to a proton on another carbon, are too small to permit their determination. The shortness we find of the C–C bond relative to the C–H bond length is interpreted as a result of molecular vibrations. It appears that the analysis of NMR in nematic media provides an important method of precise molecular structure determination.