Determination of the Full 207Pb Chemical Shift Tensor of Anglesite, Pbso4, and Correlation of the Isotropic Shift to Lead–Oxygen Distance in Natural Minerals
Total Page：16
File Type：pdf, Size：1020Kb
crystals Article Determination of the Full 207Pb Chemical Shift Tensor of Anglesite, PbSO4, and Correlation of the Isotropic Shift to Lead–Oxygen Distance in Natural Minerals Otto E. O. Zeman 1, Jennifer Steinadler 1, Rupert Hochleitner 2 and Thomas Bräuniger 1,* 1 Department of Chemistry, University of Munich (LMU), Butenandtstr. 5-13, 81377 Munich, Germany; [email protected] (O.E.O.Z.); [email protected] (J.S.) 2 Mineralogical State Collection Munich (SNSB), Theresienstr. 4, 80333 Munich, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-89-2180-77433 Received: 23 December 2018; Accepted: 11 January 2019; Published: 15 January 2019 207 Abstract: The full Pb chemical shift (CS) tensor of lead in the mineral anglesite, PbSO4, was determined from orientation-dependent nuclear magnetic resonance (NMR) spectra of a large natural single crystal, using a global fit over two rotation patterns. The resulting tensor is characterised by the reduced anisotropy Dd = ( 327 4) ppm, asymmetry h = 0.529 0.002, − ± CS ± and d = ( 3615 3) ppm, with the isotropic chemical shift d also verified by magic-angle iso − ± iso spinning NMR on a polycrystalline sample. The initially unknown orientation of the mounted single crystal was included in the global data fit as well, thus obtaining it from NMR data only. By use of internal crystal symmetries, the amount of data acquisition and processing for determination of the CS tensor and crystal orientation was reduced. Furthermore, a linear correlation between the 207Pb isotropic chemical shift and the shortest Pb–O distance in the co-ordination sphere of Pb2+ solely surrounded by oxygen has been established for a large database of lead-bearing natural minerals. Keywords: 207Pb-NMR; single-crystal NMR; chemical shift tensor; anglesite; lead–oxygen distance 1. Introduction For the structural characterization of periodic solids, either in single or multicrystalline form, X-ray crystallography is an invaluable tool [1]. As a complementary technique for the elucidation of structure and dynamics in solids, including amorphous ones, nuclear magnetic resonance (NMR) spectroscopy has become an established method [2–4], to the extent that, since 2014, a “Commission on NMR Crystallography and Related Methods” exists in the International Union of Crystallography [5]. In the context of NMR crystallography, many questions (e.g., the determination of asymmetric units, assignment of space groups) may already be answered by considering only the isotropic chemical shift diso of the NMR-observed nuclide [2]. The deceptively simple scalar diso is, however, the result of a contraction (of the isotropic part) of the second-rank chemical shift (CS) tensor: 1 d = (d + d + d ) (1) iso 3 11 22 33 Here, d11, d22, and d33 are the components of the main diagonal of the CS tensor, which are the ’eigenvalues’ in its principal axes system. The full CS tensor d reflects the spatial distribution of electrons around the observed nucleus, which ’shield’ it from the external magnetic field to a certain extent. Therefore, a more complete picture becomes available when the full chemical shift tensor is known, which provides possible information on co-ordination, the influence of electron lone pairs, etc. The eigenvalues d11, d22, and d33 may be determined by measuring the NMR spectrum of a Crystals 2019, 9, 43; doi:10.3390/cryst9010043 www.mdpi.com/journal/crystals Crystals 2019, 9, 43 2 of 15 Version November 22, 2018 submitted to Crystals 2 of 15 multicrystalline (’powder’) sample under static conditions, as exemplified by Reference [6]. It is also possible to derive these eigenvalues from the rotational side-band pattern of magic-angle spinning (MAS) spectra [7] to even higher precision [8]. In unrivalled accuracy, however, the eigenvalues of 31 pairs etc. The eigenvalues d11, d22, and d33 may be determined by measuring the NMR spectrum of a the CS tensor may be extracted from orientation-dependent measurements of single crystals [9,10]. 32 polycrystalline (’powder’) sample under static conditions, as exemplified by Ref. [6]. It is also possible Furthermore, the orientation of the CS tensor in the crystal structure, expressed by the corresponding 33 to derive these eigenvalues from magic-angle spinning (MAS) spectra at various spinning speeds [7] eigenvectors, is unequivocally only accessible via single-crystal NMR spectroscopy. (In certain cases, 34 to even higher precision [8]. In unrivalled accuracy, however, the principal components of the CS indirect determination might be possible from the relative orientation of the CS tensor to the dipolar 35 tensor may be extracted from orientation-dependent measurements of single crystals [9]. Furthermore, coupling tensor [11].) In spite of these obvious advantages, single-crystal NMR is comparatively 36 the orientation of the CS tensor in the crystal structure, expressed by the corresponding eigenvectors, seldom employed for tensor determination. This may be traced to (at least) three problems with 37 is unequivocally only available from single-crystal NMR spectroscopy. In spite of these advantages, this method: 38 single-crystal NMR is comparatively seldom employed for tensor determination. This may be traced 39 to at(i least) Data three acquisition difficulties and of evaluation this method: is rather time-consuming when compared to static or MAS NMR of polycrystalline samples; 40 (i) Data acquisition and evaluation is quite time-consuming when compared to static or MAS NMR (ii) tensor eigenvector determination in the crystal frame requires knowledge of the exact orientation 41 of polycrystallineof the crystal on samples, the goniometer axis; 42 (ii) Tensor eigenvector determination in the crystal frame requires knowledge of the exact orientation (iii) single crystals of sufficient size must be available because of the comparatively poor 43 of the crystal on the goniometer axis, signal-to-noise ratio of NMR spectroscopy. 44 (iii) Single crystals of sufficient size must be available because of the comparatively poor 45 signal-to-noiseOne purpose ratio of this of NMR paper spectroscopy. is to show that, in many cases, these difficulties may be overcome. As for problem (i), the amount of necessary work could be greatly reduced by exploiting internal 46 One purpose of this paper is to show that these difficulties can often be overcome. As for (i), the amount crystal symmetries, i.e., considering the NMR resonances of symmetry-related atoms in the unit cell 47 of necessaryas an additional work can dataset be greatly acquired reduced for an by independent exploiting internal but virtual crystal rotation symmetries, axis [12 i.e.–14]. considering The same 48 the NMRconcept resonances can be used of to symmetry-related address problem (ii) atoms by calculating in the unit the cell exact as orientation an additional of the data rotation set acquired axis from 49 for anNMR independent, data alone but[15– virtual,17]. Regarding rotation problem axis [10 (iii),–12]. many The systemssame concept are amenable can be used to crystal to address growth (ii), by 50 by calculatingmethods available the exact in a orientation standard laboratory. of the rotation In addition, axis fromnatural NMR minerals data offer alone a great [13–15 supply]. Regarding of single 51 (iii),crystalline many systems material are already amenable prepared to crystal by nature, growth often by of methods sufficient available size for NMR in a investigations standard laboratory. [16–19]. 52 In addition,To illustrate natural how minerals to tackle offer the above a great listed supply problems of single (i)–(iii), crystalline here we material present already the determination prepared by of 207 53 nature,the fulloften ofPb sufficient chemical shiftsize fortensor NMR of anglesite, investigations PbSO4 [,14 using–17]. a As single a case crystal study from to a illustrate natural mineral points 207 54 (i)–(iii),deposit, we here as pictured present in the Figure determination1a. Anglesite of the crystallises full Pb in chemical the orthorhombic shift tensor space of anglesite,group Pbnm PbSO[204],, 207 55 usingwith a single the location crystal of from the a naturalPb in the mineral unit cell deposit, shown as in shownFigure1 inb. Fig. 1 Left. (a) (b) FigureFigure 1. Left: 1. ( Singlea) Single crystal crystal of anglesite, of anglesite, PbSO PbSO4, from4, from location, Monteponi Country Mine, (mineralogical Iglesias, state Sardinia/Italy collection inventory(Mineralogical no. XXXXX). State Right: Collection, Unit cell Munich of anglesite inventory viewed MSM along 6140). the (b standard) Unit cell orientation of anglesite of the according crystal 1 shape,to seeReference also Table. [20]. The lead lead atoms atoms (gray, (grey) located at Wycko at ffWpositionYCKOFF 4positionc, related 4c) by are a related glide plane by glides in ac and(a 4 c a), 3 1 3 glide( planea 4 c) and in bc(,4 arebc), coordinated( 4 bc), and are by ten co-ordinated nearest oxygen by the atoms ten nearest (red). oxygen The sulfur atoms atoms (red). (yellow) The sulfur are located atoms 1 3 a in the(yellow)ab plane, are with located their in thecovalent(ab 4 ) bondsand (ab to4 ) oxygenplanes, in with tetrahedral their covalent coordination bonds to also oxygen shown. in tetrahedralDrawing generatedco-ordination with the Vesta also shown.program (Drawing [26]. generated with the VESTA program [21]). 207 207 = 56 NMRNMR spectroscopy spectroscopy of ofPb (thePb (the only only stable stable isotope lead ofisotope lead with with a a nuclear nuclear spin, namelynamely, II = 1/2) is a valuable analytical tool in solid-state research. As a ‘heavy’ nuclide, lead has a very large 57 has become a valuable analytical tool in solid state research. As a ’heavy’ nuclide, it has a very large 207 58 chemical shift range, extending over approximately 10 000 ppm, which makes Pb a sensitive reporter 207 59 of the local electronic state [18–20]. Recently, there has also been intensified interest in Pb-NMR 60 to characterise organolead halide perovskites, which have been identified as promising materials 207 61 in photovoltaics [21–23].