Inelastic incoherent neutron scattering study of the methyl rotation in various methyl halides

The inelastic incoherent neutron scattering technique has been applied in an energy range from 0.4 μeV to 60 meV to measure the rotational excitations of the methyl groups in polycrystals of CH3I, CH2DI, CD3I, CH3Br, CD3Br, and CH3Cl in a temperature range 5<T[K]<190. Tunnel splittings of the librational groundstate of 2.44 and 0.9 μeV were found for CH3I and CH3Br, respectively. Excitations to the higher excited librational states are identified via the temperature dependence and the isotope effects of the modes in the meV regime. The rotor–rotor coupling between neighboring methyl groups is found to be small. Thus single particle rotational potentials were derived from the observed eigenenergies. The potential strength increases going from the iodide via the bromide to the chloride. In CH3Br the rotational potential does not change on deuteration. In CH3I a very unusual increase in the rotational potential with deuteration is found. Besides the methyl librations, large amplitude motions around axes perp...

[1]  M. Prager,et al.  Coupled pairs of quantum rotors in (CH3)2SnCl2: An inelastic neutron scattering study , 1986 .

[2]  B. M. Powell,et al.  The crystal structures and phase transition of methyl bromide , 1986 .

[3]  M. Punkkinen,et al.  Proton spin-lattice relaxation and ammonium tunnelling in (NH4)2ZnCl4 , 1985 .

[4]  W. Häusler,et al.  Tunneling of coupled methyl groups , 1985 .

[5]  A. Anderson,et al.  Raman and far infrared spectra of the two solid phases of methyl bromide: CH3Br and CD3Br , 1985 .

[6]  A. Anderson,et al.  Raman and infrared studies of the lattice vibrations of some halogen derivatives of methane , 1985 .

[7]  H. Zimmermann,et al.  Determination of the lattice vibrations of imidazole by neutron scattering , 1985 .

[8]  A. Anderson,et al.  Raman and infrared spectra of crystalline methyl iodide , 1984 .

[9]  M. Paley,et al.  Coupled tunnelling motion of a pair of methyl groups in lithium acetate studied by inelastic neutron scattering , 1984 .

[10]  W. Müller-Warmuth,et al.  Rotational tunnelling and methyl group reorientation in Sn(CH3)4 by inelastic neutron scattering and nuclear magnetic resonance , 1983 .

[11]  A. Hewson.  The temperature dependence of inelastic neutron scattering in rotational tunnelling systems. I. Formulation and perturbation theory , 1982 .

[12]  P. Brüesch,et al.  Phonons: Theory and Experiments III , 1982 .

[13]  Z. Gamba,et al.  Lattice dynamical calculations on azabenzene crystals: The distributed dipole model , 1981 .

[14]  S. Clough,et al.  The correlation of methyl tunnelling and thermally activated reorientation , 1981 .

[15]  M. Paley,et al.  The temperature dependence of methyl tunnelling motion in three acetates , 1981 .

[16]  A. Hüller.  The temperature dependence of rotational tunneling , 1980 .

[17]  W. Müller-Warmuth,et al.  Rotational tunnelling in methylpyridines as studied by NMR relaxation and inelastic neutron scattering , 1978 .

[18]  D. F. Eggers,et al.  The infrared and Raman spectra of solid CH3F and CD3F: Crystal structure inferences , 1977 .

[19]  T. Shimanouchi,et al.  Crystal Vibrations and Intermodular Interactions of CH3X and CD3X (X=Cl, Br, and I) , 1976 .

[20]  T. Shimanouchi,et al.  Raman spectra and crystal structures of methyl halides , 1976 .

[21]  Y. Hayafuji,et al.  The Crystal Structures of Methyl Bromide and Methyl Iodide , 1973 .

[22]  Chris W. Brown,et al.  Raman Spectra of the Crystalline Methyl Halides , 1970 .

[23]  Mitsuo. Ito.  Raman Spectrum of Polycrystalline Methyl Bromide , 1964 .

[24]  W. J. Lafferty,et al.  Far Infrared Spectra of Solid Methyl Halides , 1962 .

[25]  M. Jacox,et al.  Solid‐State Vibrational Spectra of the Methyl and Methyl‐d3 Halides , 1961 .

[26]  D. Dows.  Infrared Spectra of Crystalline CD3Cl and CD3Br , 1960 .

[27]  R. D. Burbank.  The Crystal Structure of Methyl Chloride at -125°1 , 1953 .