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

Abstract

Total cross sections of CH3I, CH2DI, CH3CH2I, CD3CH2I, CH3CD2I, CH2I2, CH2Br2 and CHBr3 have been measured with cold neutrons between 4 Å and 17 Å. The slopes S of the scattering cross section per proton vs. the neutron wavelength indicate the different types of protonic motions in the investigated liquids.The high slope S = 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 CHBr3 no internal molecular motion of the proton is possible and the low cross section slope S = 2.0 ± 0.8 barn/Å shows, that the rotations of the CHBr3 molecules 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.