Single-Crystal ²⁷Al NMR of Andalusite and Calculated Electric Field Gradients:  the First Complete NMR Assignment for a 5-Coordinate Aluminum Site

Abstract

Andalusite, Al2SiO5, contains 5- and 6-coordinate aluminum sites, and is a preeminent model for 27Al NMR spectroscopy. We describe a combined NMR, crystallography, and theory project: single-crystal 27Al NMR spectra at 298 K, the crystal structure of andalusite at 115 K, and electric field gradient calculations. The low-symmetry 5-coordinate site is a stiff test of the computational methods. In addition, the chemical shift tensor is measured for the 5-coordinate site. The small body of 27Al NMR data, especially for rare 5-coordinate sites, inspires the calculation of NMR parameters. We explore the accuracy of two approaches for 27Al EFG calculations: first, ab initio molecular orbital calculations of small clusters embedded in an array of point charges; second, full-potential linearized augmented plane wave density functional calculations of the crystal. The agreement between the experimental EFG orientation and that from the full-crystal density functional theory is remarkably close, differing by only 0.17° for the 6-coordinate site and 1.56° for the 5-coordinate site. The calculated value of C q is in error by −0.254 MHz for the 5-coordinate site. The embedded cluster molecular orbital results are significantly less accurate, with orientation errors exceeding 45°.

Ultra‐wideline 27Al NMR Investigation of Three‐ and Five‐Coordinate Aluminum Environments

“…For instance, Bryant et al acquired a full frequency-stepped 27 Al CT spectrum of an (aminato)hexaaluminum cluster which is about 150 kHz in breadth at 9.4 T (C Q ( 27 Al) = 17.5 MHz) with a total acquisition time of 46 h. [15,18] The acquisition of an even broader 27 Al wideline NMR spectrum (breadth of about 400 kHz, C Q ( 27 Al) = 37 MHz) is impressive, though there is much interference from FM radio signals in these spectra, owing to the moderate signal-to-noise (S/N) ratio. [14] Low symmetry aluminum coordination environments occur in many important solid materials; clearly, the rapid acquisition of solid-state 27 Al NMR spectra for aluminum sites with large values of C Q ( 27 Al) would be invaluable.…”

“…Thus, similar techniques were applied to acquire ultra-wideline 27 Al NMR spectra of [ Figure S3), and, as a result, QCPMG experiments do not result in a significant signal enhancement (that is, the effectiveness of QCPMG is limited by a very short T 2 ). For instance, the 40 MG loops acquired in the QCPMG experiments on [Me 2 -Al(m-OTHF)] 2 provided a gain in integrated intensity of only 1.26 compared to the normalized Hahn-echo spectrum, and no gain was observed in analogous experiments [14,[56][57][58][59] Perhaps more intriguing are the values of h Q , which are close to 1 for both systems, indicating that V 11 is the unique component of the EFG (that is, j V 33 j % j V 22 j > j V 11 j ). This value of h Q is rather unexpected given the unusual geometry of the five-coordinate aluminum atoms in these samples.…”

“…[4,7,11] Conversely, three-and five-coordinate aluminum environments occur less often, and are sometimes extremely difficult to characterize by NMR spectroscopy. Nonetheless, some solid-state 27 Al NMR experiments have been conducted on five-coordinate aluminum species, such as luminescent aluminum coordination complexes [12,13] and andalusite (Al 2 SiO 5 ), [14] as well as three-coordinate aluminum species including (aminato)-and (propanolato)aluminum clusters and methylaluminoxanes. [15] As the spherical ground-state electronic symmetry about a quadrupolar nucleus decreases, as is the case for many threeand five-coordinate aluminum centers, augmented electric field gradients (EFGs) occur, which in turn give rise to an increase in the magnitude of the quadrupolar interaction (the interaction between the nuclear electric quadrupole moment, eQ, and the EFG within the molecular or ionic system).…”

Electric field gradient calculations forLixTiS2and comparison withLiBredow1, Heitjans2, Wilkening3 2004Phys. Rev. B936870View full textAdd to dashboardCite

“…Experimentally, the coupling of the electric field gradient and the quadrupole moment tensor, Q, of nuclei with spin quantum number I ജ 1 is accessible by nuclear magnetic resonance (NMR) techniques. [18][19][20] First-principles quantum-chemical investigations of quadrupolar interactions in solid systems are based on two kinds of models, the embedded cluster model [21][22][23] and the periodic supercell model, either based on plane-waves [24][25][26][27] or on atom-centered basis functions. [28][29][30] While periodic models take into account long-range electrostatic interactions in crystalline systems, the results obtained with model clusters strongly depend on their size and shape.…”

29Si AND 23Na MAS NMR SPECTROSCOPIC STUDY OF THE POLYTYPES OF THE TITANOSILICATE PENKVILKSITE

“…The FC LAPW model has been shown to be the most accurate method for calculating the electric density and EFG in an ordered crystal at this time (Bryant et al 1999, Zhou & Sherriff 2004 29 Si MAS NMR spectra samples 3155, 3121 and 3156. The parameters of the simulated spectra are given in Table 2.…”