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31P MAS NMR of P4S3. Crystalline-to-plastic phase transition induced by MAS in a double air-bearing stator

1989; Elsevier BV; Volume: 84; Issue: 1 Linguagem: Inglês

10.1016/0022-2364(89)90020-6

1557-8968

Thomas Bjorholm, Hans J. Jakobsen,

NMR spectroscopy and applications

Chemical-shift anisotropies (CSA) for 31P nuclei in most phosphorus compounds are usually large. Therefore, one of the main obstacles in obtaining simple and interpretable 3’P magic-angle spinning (MAS) NMR spectra at high magnetic fields, of otherwise simple polyphosphorus compounds, is a severe overlap of the large number of spinning sidebands for the individual 31P nuclei. Our ability to run very high-speed MAS spectra ( I ), and thereby reduce the complexity or suppress spinning sidebands in such spectra, has prompted 31P MAS investigations in our laboratories on phosphorus sulfides of the type P4 S, ( 3 G n < 10; including p-P4 S6, isolated and characterized for the first time during this study). These compounds exhibit a fascinating variety of structures with different molecular symmetries and crystal structures which make studies of their molecular behavior in the solid state a challenging task. For example, we have shown (2) that polycrystalline (Y-P& ( C,, symmetry), contrary to liquid state results, exhibits four nonequivalent 3’P resonances and a ‘Jpp = 232 Hz using 1D and 2D 31P MAS NMR. In this Communication we report observation of some interesting spectral features for P4 S3, including the crystalline-to-plastic phase transition, from a 3’P MAS study at various spinning speeds. The 31P MAS NMR spectrum of P4S3 (molecular structure shown in Fig. 1) at low field (0.475 T) and ambient temperature has been reported previously (3) to consist of two resonances (6 = 72 and 103 ppm relative to 85% H3P04) with an intensity ratio of 1:3. This matches the solution state spectrum (4) and the static solid state spectrum observed at 420 K ( 147°C) (5). However, our MAS investigations, performed at 7.1 T (31P at 12 1.42 MHz) using a spinner assembly with double air-bearing design for cylindrical rotors ( 1)) show that the 31P MAS NMR spectrum of polycrystalline P4S3 (Fig. 1) is indeed more complex. The room temperature, slow-speed spinning spectrum is depicted in Fig. la and shows a typical spinning sideband manifold arising from two narrow lines and a somewhat rolling baseline in the low-frequency region of the spectrum. The centerband resonances for the two narrow lines, 6 = 9 1 .O and 84.8 ppm, are easily identified from spectra recorded at somewhat higher spinning speeds as shown in Figs. 1 b and 1 c. Furthermore, these spectra reveal the appearance of an additional spinning sideband manifold consisting of rather broad lines in the low-frequency portion of the spectra. The centerband resonance, 6