Observation of electron-coupled nuclear spin-spin interactions in the silicon-29 MAS NMR spectrum of low albite

Abstract

Silicon-29 MAS NMR spectra at a wide range of magnetic fields are presented for a well characterized feldspar, low albite. The splitting of silicon-29 resonances in these spectra is independent of magnetic field and is thus attributed to electron-coupled silicon-29 nuclear spin-spin coupling with aluminum-27. To our knowledge, this work represents the first time such a splitting has been observed for silicon-29 in the solid state. In a continuing effort to obtain high-resolution silicon-29 MAS NMR spectra of minerals and zeolites (I, 2), the spectrum of an extensively studied feldspar, low albite, was measured. Because the structure of this mineral is well characterized, a number of researchers (I, 3,4) have included albite in the development of correlations between structure and 29Si chemical shift. In addition, a recent neutron diffraction study (5) on this end member of the plagioclase feldspar group has demonstrated unequivocally that the aluminum present in low albite is highly ordered and resides almost exclusively in the T1O tetrahedral site. Thus, this material offers a well-defined system to investigate the effect of aluminum ordering on the silicon-29 MAS NMR spectrum. A sample of low albite from Amelia County, Virginia, was provided by Dr. Carl A. Francis of Harvard University. Silicon-29 MAS NMR spectra were measured at 39.6, 53.6, and 99.4 MHz on JEOL FX-200, JEOL FX-270 and Bruker AM-500 Fourier transform NMR spectrometers, respectively. Silicon-29 spin-lattice relaxation times were estimated to be between 100 and 150 s at 53.6 MHz by the T-null, inversion-recovery method. Spectra were typically measured by accumulating approximately 1000-2500 FIDs using a 30-90“ flip angle and a 30150 s recovery time. The spectra are referenced to tetramethylsilane (TMS) using solid hexamethylcyclotrisiloxane (HMCTS) as a secondary reference. The chemical shift of solid HMCTS with respect to a 30 ~01% solution of TMS in deuterochloroform was found to be -10.0 ppm.