Knowledge Bank

The structure and dynamics of HCl dimers trapped in an argon matrix have been investigated by a new technique which permits the observation of the N.M.R. proton spectrum at low temperatures. This spectrum, for the randomly oriented crystallites of the matrix, consists of a Pake doublet arising from the proton-proton dipolar interactions of the dimer. The splitting of this doublet defines a structural parameter which is very sensitive to the average orientations of the HCl units of the dimer but relatively insensitive to even large torsional oscillations within the molecule. Calculation of this structural parameter for several theoretical conformations has shown our experiment is consistent with an equilibrium configuration in which one proton lies on the line joining the two chlorine atoms while the other makes an angle of about 120 degrees to this axis. This result will be compared with structures deduced from molecular beam studies of similar hydrogen bonded Van der Waal pairs. In addition, information obtained from the temperature dependence of the N.M.R. line shape relating to molecular motions in the matrix will be presented.