Pulsed nuclear magnetic resonance studies of the protons of hexamethyldisiloxane adsorbed on aerosil silica

Abstract

Measurements are reported of the free-induction decays, spin–lattice relaxation and solid- and dipolar-echo responses of the protons in hexamethyldisiloxane adsorbed onto calcined and deuterated aerosil-200 silica. It is shown that the free-induction decay corresponds to a super-Lorentzian lineshape arising from an inhomogeneous distribution of dipolar second moments. A model is proposed in which a hexamethyldisiloxane molecule adsorbed on the silica surface undergoes rapid but anisotropic motions about three Cartesian axes fixed in the surface as well as, possibly, a number of internal motions. As a consequence the intramolecular dipole–dipole couplings between the protons are partitioned into an average part, which is time independent and a function only of the angle between the external field B₀ and the normal to the surface at the adsorption site, and a zero-average fluctuating part which dominates the spin—lattice relaxation behaviour. It is proposed that the average part dominates the linewidth for a given site and thus leads to the super-Lorentzian shape for the powder sample. Spin–lattice relaxation measurements are shown to be consistent with this model in that when it is measured by observing the signal at the beginning of the f.i.d. following the 90° pulse in the 180-τ-90 sequence the relaxation is multicomponent, whereas if it is monitored in the tail of the f.i.d. it is single exponential in character and has the maximum value. Similarly, solid-and dipolar-echoes are shown to be formed even from the long-time part of the f.i.d., confirming that it is dominated by secular dipolar interactions. The observation that the dipolar-echoes always have their maximum intensity at t=τ following the 90_x-τ-45_y-T-45_y sequence is also explained in terms of the inhomogeneous nature of the lineshape with respect to the dipolar couplings. The consequences of these observations for the interpretation of nuclear spin relaxation measurements on adsorbed systems is emphasised.