W- and X-band pulsed electron nuclear double-resonance study of a sodium-nitric oxide adsorption complex in NaA zeolites

Pulsed electron nuclear double-resonance (ENDOR) spectroscopy at W- and X-b and frequencies has been employed to characterize the structure of NO adsor ption sites involving sodium cations in zeolite NaA. The principal values o f the sodium hyperfine and nuclear quadrupole coupling tensors as well as t he orientation of their principal axes system with respect to the g tensor coordinate frame of the Na+-NO adsorption complex could be determined by or ientation-selective ENDOR spectroscopy. Such orientation-selective experime nts benefit especially from the high spectral resolution at W-band frequenc ies. Furthermore, the sodium ENDOR spectrum is drastically simplified at hi gh frequencies where the limit of weak hyperfine couplings is fulfilled. Th e dipolar sodium hyperfine coupling tensor reveals a bent structure of the formed adsorption complexes and gives access to the bond distance between t he NO molecule and the cations. The Na-23 nuclear quadrupole data indicate that the adsorption complexes are preferentially formed with the sodium ion s at the six-membered rings of the NaA zeolite structure. An analysis of th e sodium and nitrogen hyperfine coupling data shows that 96% of the unpaire d electron spin density in the Na+-NO adsorption complex is localized in th e nitrogen and oxygen 2p pi orbitals of the NO ligand molecule.