Structure and Dynamics of Cyclopropane in Solution by Nuclear Magnetic Resonance Coupled Relaxation

Article abstract of DOI:10.1021/j150657a008

The spin-lattice relaxation of selectively labeled cyclopropane-1-13C-2,2,3,3-d₄ (13CH2C2D4) dissolved in CD2Cl2 has been studied with carbon-13 NMR.Selctive and nonselective ? pulses were applied to the carbon and proton transitions of this AX₂ spin system and partially relaxed carbon-13 spectra were obtained for various evolution periods after the pulse.All experiments were performed at -81.5, -60, -20, and +20 deg C.The data were analyzed in terms of a small-step rotational diffusion model for the rotational reorientation.The reorientation at -81.5 and -60 deg C may be characterized by two diffusion coefficients.The unique axis lies parallel to the C₃ axis of cyclopropane.At -20 and +20 deg C, only one diffusion parameter is obtained.Values obtained for the proton-proton distance at the lower temperatures agree reasonably well with values in the literature but become too large at the higher temperatures.Small but negative values for the carbon autocorrelation random field term j_C are observed at the lower temperatures.These results indicate that as temperature increases, the molecule enters a motional regime where the dipolar mechanisms become less efficient, spin rotation becomes more important, and greater uncertainty exits in the small-step motional model used to interpret the data.