Variable temperature study of the cross-relaxation dynamics in the hyperpolarized xenon-induced enhancement of surface nuclei

The cross-relaxation rate constants for a three spin system involving hyper polarized Xe-129, C-13, and H-1 nuclei have been determined over a wide tem perature range (136-216 K) through observation of the polarization enhancem ents from spin polarization induced nuclear Overhauser effect (SPINOE) expe riments. A model system of methoxy groups adsorbed on high surface area sil ica was studied. A maximum polarization enhancement of 3.4 was observed for the methoxy carbon. Xenon-carbon and xenon-proton cross-relaxation were fo und to be dependent on both the xenon coverage and xenon diffusion across t he silica surface. The temperature dependence of the cross-relaxation rate constants was effectively fit using a hopping model. Possible improvements for the SPINOE experiment are suggested including alternative substrates wh ich would retard xenon diffusion across the surface and improve xenon-carbo n cross-relaxation leading to significantly larger SPINOE enhancements.