A general correlation for the 129 Xe NMR chemical shift-pore size relationship (δ versus D) in porous silica-based materials over the range 0.5-40 nm has been demonstrated: δ ) δs/(1 + D/b), with δs ) 116 ( 3 ppm and b ) 117 ( 8 Å for the 34 materials studied. The correlation may be used in the characterization of silica samples with unknown pore structure. Even within this general correlation, subsets of materials of similar origin display finer correlations that indicate an acute sensitivity to details of the pore surfaces.The NMR spectroscopy of adsorbed 129 Xe has evolved into a sophisticated scientific tool for the study of different aspects of the structure and topology of internal voids in porous substances. 1 Although some disadvantages of the technique have been noted, 2 it remains attractive and popular, as judged by the fact that in 2001 about 50 Xe NMR-related papers were published. The latest improvements include the production of hyperpolarized xenon (HP Xe), 3 which gives a dramatic increase in the sensitivity for a variety of applications. 4 Besides such practical novelties, important theoretical developments directed at a much needed understanding of chemical shielding phenomena of confined 129 Xe are in progress. 5 For example, through empirical correlations were proposed between isotropic Xe chemical shifts and the pore size (δ-D correlation) in zeolites 6a as well as clathrates and solid Xe, 6b there is still no clear quantitative appreciation of the origin of this behavior. Attempts to extend it beyond zeolites have failed, and it has been pointed out that different correlations must exist for small pores, with a diameter less than about twice the diameter of a Xe atom, and large pores, as in the latter case account must be taken of Xe not adsorbed on the pore walls. 7We have shown previously that a simple fast exchange model explains a qualitatively similar, yet distinct, δ-D correlation found for mesoporous amorphous silica gels with a wide range of mean pore diameters from 2 to 40 nm. 8 Assuming that the 129 Xe NMR chemical shift of xenon adsorbed in mesoporous silica is a dynamic average between the gas and adsorbed states, it is straightforward to derive a parabolic dependence of the 129 Xe NMR chemical shift, δ (ppm), on the mean pore diameter, D (Å), where δ s is the chemical shift characteristic of interactions of Xe with the silica surface, and the parameter b depends on the pore geometry (η), the adsorption constant (K), and the temperature (T), as 8The mean pore diameter (D) is usually given through the volume-to-surface ratio as D ) ηV/S, where the geometry factor η is dependent on the model adopted for the pores. It can vary from 2.8 in a model of randomly packed globular particles (D ) 2.8V/S), to 4 for cylindrical pores (D ) 4V/ S) or 6 for unconnected spherical pores (D ) 6V/S).In this work we show that the model is more generally applicable by extending the range of silica-based porous materials, and we illustrate the correlation with previous work 8, 9 and new results 10 on...

Bright blue light with a maximum at 450 nm is emitted by both structural isomers of the novel, stable B (O)(7-azain) Ph (7-azain=7-azaindole anion) on irradiation with UV light. The isomer shown in the picture has approximate C symmetry (the other isomer approximate C symmetry) and electroluminesces when used as the emitting layer in an electroluminescent device.

129Xe NMR spectroscopy with a continuous flow of dilute hyperpolarized xenon directed into the sample region of an NMR probe (either static or spinning) allows the rapid recording of spectra that are characteristic of the void space in micro- and mesoporous solids. The high sensitivity of the method allows the time-resolved, in-situ monitoring of processes involving sorbents such as activation, adsorption, and desorption.