Journal of the American Chemical Society, Vol.122, No.41, 10194-10200, 2000
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-band frequencies has been employed to characterize the structure of NO adsorption sites involving sodium cations in zeolite NaA. The principal values of the sodium hyperfine and nuclear quadrupole coupling tensors as well as the orientation of their principal axes system with respect to the g tensor coordinate frame of the Na+-NO adsorption complex could be determined by orientation-selective ENDOR spectroscopy. Such orientation-selective experiments benefit especially from the high spectral resolution at W-band frequencies. Furthermore, the sodium ENDOR spectrum is drastically simplified at high frequencies where the limit of weak hyperfine couplings is fulfilled. The dipolar sodium hyperfine coupling tensor reveals a bent structure of the formed adsorption complexes and gives access to the bond distance between the NO molecule and the cations. The Na-23 nuclear quadrupole data indicate that the adsorption complexes are preferentially formed with the sodium ions at the six-membered rings of the NaA zeolite structure. An analysis of the sodium and nitrogen hyperfine coupling data shows that 96% of the unpaired electron spin density in the Na+-NO adsorption complex is localized in the nitrogen and oxygen 2p pi orbitals of the NO ligand molecule.