Investigating hyperpolarized Xe and CPMAS for spin polarization transfer to surface nuclei: a model study

Several experimental techniques have been developed to utilize spin-polarized xenon gas for sensitivity and selectivity enhancement in surface studies using solid-state NMR. Although previously reported as a viable spin polarization transfer mechanism, the details of high-field cross-polarization (CP) have not been thoroughly investigated. We recently reported observations of CP from an adsorbed layer of hyperpolarized xenon (HP Xe) to a variety of surface nuclei at temperatures as high as 323 K [J. Am. Chem. Soc. 105 (2001) 1412]. In this paper, we investigate many of the issues associated with HP Xe surface CP studies, including polarization transfer kinetics and the effects of temperature on the dynamics. Protonated and methylated silica samples are used as model systems for comparison. A comparison of the rate analysis data from CP and SPINOE (Spin Polarization-Induced Nuclear Overhauser Effect) experiments provides information on the origin of the difference in polarization transfer efficiencies between the two techniques. Lineshape analysis of ¹H spectra for CP and SPINOE experiments demonstrates the difference in selectivity of methods due to longer SPINOE evolution times that lead to greater spin diffusion. The results of this work help to assess the viability of HP Xe CP as a surface analysis technique.