Other Nitrogen Oxides as Spin Probes

Other nitrogen oxides such as nitrogen dioxide (NO2) have been employed as a spin probe to characterize the zeolite structure, chemical properties of zeolites, and motional dynamics of small molecules in them by ESR [13]. 

NO2 is a stable paramagnetic gaseous molecule at normal temperatures. The ESR parameters of NO2 trapped in a solid matrix have been well established from single-crystal ESR measurements and have been related to the electronic structure by molecular orbital studies [39]. Thus, the NO2 molecule has potential as a spin probe for the study of molecular dynamics at the gas-solid interface by ESR. More than two decade ago temperature-dependent ESR spectra of NO2 adsorbed on porous Vycor quartz glass were observed [40] Vycor is the registered trademark of Coming, Inc. and more information is available at their website. The ESR spectral line-shapes were simulated using the slow-motional ESR theory for various rotational diffusion models developed by Freed and his collaborators [41]. The results show that the NO2 adsorbed on Vycor displays predominantly an axial symmetrical rotation about the axis parallel to the O—O inter-nuclear axis below 77 K, but above this temperature the motion becomes close to an isotropic rotation probably due to a translational diffusion mechanism. 

For the characterization of acid sites on soUds such as silica-alumina, alumina, and zeolites, di-tert-butyl niffoxide (DTBN), a stable organic nitroxide radical, was employed as a spin probe molecule by Hoffman et al. [47,48] and others [49, 50]. Gutjahr et al. [50] studied the electron pair acceptor properties of the monovalent cations such as Li+, Na+, K , and Cs in a faujacite (Y) type of zeolite by means of ESR using DTBN as a probe molecule. They reported a linear relationship between the nitrogen spin density estimated from the nitrogen /i/constants of DTBN and the electro-negativity of the zeolite cations. 

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