Motions of Protons in CH‐, CH2‐, CH3‐ and C2H5‐Halides by Cold Neutron Transmission Experiments

Abstract

AbstractTotal cross sections of CH3I, CH2DI, CH3CH2I, CD3CH2I, CH3CD2I, CH2I2, CH2Br2and CHBr3have been measured with cold neutrons between 4 Å and 17 Å. The slopesSof the scattering cross section per proton vs. the neutron wavelength indicate the different types of protonic motions in the investigated liquids.The high slopeS= 12.6 ± 0.4 barn/Å for protons in CH3I is explained by relative free internal rotation of the methyl group. This rotation is not influenced by the substitution of a proton by a deuteron as in CH2DI (S= 12.6 ± 0.4 barn/Å). But the motion of the protons becomes strongly hindered by increasing the rotational mass when substituting a proton by a halide nucleus as in CH2I2(S= 5.6 ± 0.5 barn/Å) and in CH2Br2(S= 6.2 ± 0.5 barn/Å). In CHBr3no internal molecular motion of the proton is possible and the low cross section slopeS= 2.0 ± 0.8 barn/Å shows, that the rotations of the CHBr3molecules are much more hindered than rotations of CH2Br2.In CH3CH2I the neutron‐proton scattering is weaker than in CH3I. Comparison of the CD3CH2I and CH3CD2I cross sections with the CH3CH2I result indicates within the experimental errors no difference between the scattering power of methyl and methylene protons in ethyl iodide. This leads to the conclusion that all protons in ethyl iodide participate at the same microdynamic process which exchanges momentum and energy with the cold neutron. Therefore an internal rotation of the ethyl group around the C–I bond is assumed.