Lattice dynamics of the pyrazine crystal studied by coherent inelastic neutron scattering

Abstract

The lattice vibrational excitations at a number of points in the Brillouin zone for a deuterated pyrazine crystal at room temperature have been measured by means of coherent inelastic scattering of neutrons. These give values for all 9 adiabatic elastic constants, 25 excitations at symmetry points in the Brillouin zone, and dispersion curves in the (0 η0) direction. These have been interpreted in the quasiharmonic, rigid molecules, approximation using an atom‐atom 6‐exp potential plus electrostatic intermolecular forces. This gives 3 parameters to be fixed. Fitting to the observed excitations gives physically reasonable values for these parameters. The fit obtained is not perfect (χ = 3.4) but good enough [(ω_obs − ω_calc)/ω_calc ∼ 7%] to say that this model describes the intermolecular forces in pyrazine well. The C–H ··· N hydrogen bond at long ranges is well described by a 6‐exp potential plus an important electrostatic interaction between the N lone‐pair electrons and the C–H bond dipole. The asymmetry in the Nitrogen atom electronic distribution does not appear to have any effect on the dynamics other than its electrostatic contribution. It is suggested, from the pattern of the deviations of the dynamics and crystal structure from the model, that the errors in the model derive from neglect of three body atom‐atom electronic correlations.