Vibrational mode analysis of isomorphous hydrogen-bonded guanidinium sulfonates with inelastic neutron scattering and density-functional theory

The inelastic neutron scattering spectra of a set of homologous lamellar mo lecular crystals containing two-dimensional hydrogen bonded sheets of guani dinium ions and sulfonate moieties of organomonosulfonates have been measur ed. The spectra were collected in the 35-105 meV range using a filter analy zer spectrometer and in the 0-35 meV energy range using a time-of-flight in strument. Parallel and perpendicular vibrational modes, associated with hyd rogen displacements with respect to the hydrogen-bonded sheets, were analyz ed between 35 and 105 meV by employing different scattering geometries. Com parison of the filter analyzer spectra of the isomorphous guanidinium salts of methanesulfonate, triflate and dithionate enabled assignment of specifi c modes while providing direct unambiguous measurement of the effect of the organic substituent on the strength of the (guanidinium)N-H .O-S(sulfonate ) hydrogen bonds. Comparison of the time-of-flight spectra of the three com pounds revealed that a mode observed at 7 meV corresponded to rotational mo tion of the methyl group. Density functional theory reproduced the experime ntal spectrum of guanidinium methanesulfonate, enabling assignment of the l attice vibrational modes. The scattering data compare well with a previousl y reported vibrational mode analysis of structurally homologous guanidinium nitrate. (C) 2001 American Institute of Physics.