Beryllium-9 NMR Study of Solid Bis(2,4-pentanedionato-O,O‘)beryllium and Theoretical Studies of ⁹Be Electric Field Gradient and Chemical Shielding Tensors. First Evidence for Anisotropic Beryllium Shielding

Abstract

Despite the favorable NMR properties of 9 Be (I ) 3 / 2 ), NMR spectroscopy of this nucleus in the solid state remains comparatively unexplored, perhaps owing to the extreme toxicity of beryllium and its compounds. We present here an integrated experimental and theoretical study of the Be chemical shielding (CS) and electric field gradient (EFG) tensors in bis(2,4-pentanedionato-O,O′)beryllium [Be(acac) 2 ]. Interpretation of the 9 Be NMR data was facilitated by crystal X-ray diffraction results, which indicate two crystallographically unique sites (Onuma, S.; Shibata, S. Acta Crystallogr. 1985, C41, 1181). Beryllium-9 NMR spectra acquired at 4.7 and 9.4 T for magic-angle spinning (MAS) and stationary samples have been fitted in order to extract the nuclear quadrupole coupling constant (C Q ), asymmetry parameter (η), and isotropic chemical shift (δ iso ). The best-fit nuclear quadrupole parameters for the two sites were determined to be C Q (1) ) -294 ( 4 kHz, η(1) ) 0.11 ( 0.04; C Q (2) ) -300 ( 4 kHz, η(2) ) 0.15 ( 0.02. Our analyses of the stationary samples also reveal a definite anisotropy in the beryllium CS tensor and allow us to place upper and lower limits on the spans of 7 and 3 ppm. This is the first evidence for anisotropic shielding in beryllium. Ab initio calculations of the beryllium CS tensors in Be(acac) 2 at the RHF level indicate spans ranging from 7 to 9 ppm; this represents a substantial fraction of the total known chemical shift range for Be (<50 ppm). The calculated C Q s are also in good agreement with the experimental results. To put the Be(acac) 2 results in context, calculations of the beryllium CS tensors for a series of compounds encompassing the known range of 9 Be chemical shifts are also presented. The calculations are in outstanding accord with experimental data from the literature. On the basis of calculations for linear molecules, it is shown that the assumption that the 9 Be chemical shift is governed essentially by the diamagnetic term is erroneous. For some of these molecules, the calculated Be CS tensor spans are greater than the total known chemical shift range.

Characterization of Tricoordinate Boron Chemical Shift Tensors: Definitive High-Field Solid-State NMR Evidence for Anisotropic Boron Shielding

“…24 The IGLO ab initio method has been applied to the calculation of 13 C isotropic chemical shifts in various classical and nonclassical carbocations. 25,62 Kutzelnigg has presented an explanation for the large range of values for δ iso ( 13 C).…”

“…8 An ongoing line of research in our group has been the characterization of chemical shift tensors of half-integer quadrupolar nuclei, e.g., cesium, 9 molybdenum, 10 aluminum, 11 copper, 12 and beryllium. 13 Here, we extend this work to boron.…”

“…Prompted by Olah's 13 C NMR studies of carbocations, 23 parallels between the 13 C isotropic chemical shifts of carbocations and the 11 B isotropic chemical shifts in analogous neutral compounds have been illustrated in the literature. 24 The 13 C shifts have been rationalized 25 in terms of the paramagnetic shielding term in Ramsey's theory of nuclear magnetic shielding. 26 Here, we extend this discussion to rationalize the difference between the boron CS tensors of boranes and borates.…”

Structure and Dynamics of Homoleptic Beryllocenes: A Solid‐State 9Be and 13C NMR Study

“…[35] Furthermore, two dynamic processes are observed in accordance with the initial proposition by Wong and co-workers: [13] [41] Solid-state NMR spectroscopy of metal nuclei at the heart of metallocenes is a powerful means for studying structure and dynamics in these molecules, since the orientation-dependent NMR tensors are very sensitive to changes in the electronic environment about the nuclei. Relevant solidstate NMR examples include [42] Li NMR spectroscopy of lithocenes, [46] 23 Na NMR spectroscopy of CpNa-TMEDA, [47] 119 Sn and 207 Pb NMR spectroscopic characterization of a variety of stannocenes and plumbocenes, [48,49] single-crystal 59 Co NMR spectroscopy of [Cp 2 Co]NO 3 ·H 2 O, [50] and a…”

“…[51] Although 9 Be is a half-integer quadrupolar nucleus with a small nuclear quadrupole moment (nuclear spin I = 3/2, Q = 5.288 10 À30 m 2 ), [52,53] 100 % natural abundance and receptivity of 78.7 compared to 13 C, there are relatively few examples of solid-state 9 Be NMR spectroscopic studies in the literature, likely because of the extreme toxicity of berylliumcontaining compounds. Solid-state 9 Be NMR spectra of [Be(acac) 2 ] have been reported, [42] along with studies that focus mainly upon the characterization of beryllium sites in minerals. [42,[54][55][56][57][58][59][60][61][62][63] The 9 Be quadrupole coupling constants measured for these compounds all fall within the relatively narrow range of 30-700 kHz.…”