10096
Anisotropic Complexes
J. Phys. Chem. A 2002, 106, 10096-10107 11B
and
13C
NMR Interaction Tensors in Decamethylcyclopentadienyl Boron
Robert W. Schurko,*,† Ivan Hung,† Siri Schauff,† Charles L. B. Macdonald,‡ and Alan H. Cowley‡ Department of Chemistry and Biochemistry, UniVersity of Windsor, Windsor, Ontario, Canada N9B 3P4, and Department of Chemistry and Biochemistry, UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: March 25, 2002; In Final Form: June 25, 2002
Solid-state double-resonance and triple-resonance 13C, 11B, and 1H NMR experiments are used to investigate two main group metallocene complexes: the decamethylcyclopentadienylborinium cation, [Cp*2B]+, and bis(pentamethylcyclopentadienyl)methylborane, Cp*2BMe. The crystal structure for the latter complex is reported herein. A combination of magic-angle-spinning and static 11B{1H} NMR experiments are used to measure 11 B nuclear quadrupole coupling constants (CQ) and rare instances of anisotropic boron chemical shielding tensors. Boron-11 nuclear quadrupole coupling constants reflect the higher spherical symmetry of [Cp*2B]+ compared to Cp*2BMe, with CQ(11B) ) 1.14 MHz in the former and CQ(11B) ) 4.52 MHz in the latter. Chemical shielding tensor spans are measured for [Cp*2B]+ and Cp*2BMe as Ω ) 73.0 and 146.0 ppm, respectively. Hartree-Fock and hybrid density functional theory (B3LYP) calculations of electric field gradient and chemical shielding tensors are in quantitative agreement with experiment, and are applied to examine the relationships between the anisotropic NMR interaction tensors and the structure and symmetry of these chemically analogous but structurally dissimilar boron complexes. Variable-temperature 11B MAS NMR, 13C CPMAS NMR, and 13C/11B/1H CP TRAPDOR NMR experiments are applied to make a preliminary investigation of motion of the Cp* rings of [Cp*2B]+.
Introduction Transition metal metallocenes, such as ferrocene, have long been the focus of structure and reactivity studies in organometallic chemistry. However, more recently there have been many reports of syntheses and structural characterizations of main group metallocenes from groups 1, 2, 13, 14, and 15.1-4 Such main group metallocenes are of great interest, not only on account of their structural diversity and the discovery of structural types that are not observed for their d-block analogues, but also because of potential industrial applications as materials precursors, polymerization catalysts, and CVD sources for ultrapure metals. Some of the main group metallocenes that feature lighter first row elements, such as the decamethylborocenium cation, [Cp*2B]+,5,6 and beryllocene, Cp′2Be, represent examples of “tightly squeezed” isoelectronic metallocenes (Cp′ ) C5R5, R ) H7, R ) Me8). In the former, only one of the pentamethylcyclopentadienyl rings has η5-coordination with the boron center, while the other ring is η1-coordinated. The [(C5R5)2B]+ cations (R ) H, Me) have been the subject of a recent computational investigation, from which it was concluded that there may be fluxional degrees of hapticity between the two rings.9 For example, one ring can be η1-coordinated initially, progress through an η2-arrangement, and end up as η5coordinated. Addition of a third ligand to the boron center generates a tricoordinate borane, in which both of the Cp* rings are expected to be η1-coordinated. From the perspective of solid-state nuclear magnetic resonance, metallocenes represent a wonderful opportunity to study * To whom correspondence should be addressed. † University of Windsor. ‡ University of Texas at Austin.
the central metal quadrupolar (i.e., spin > 1/2) nuclei in a fascinating array of structurally and chemically tunable molecules. Main group metallocenes can be synthesized that possess nonbridged or bridged Cp′ rings of variable hapticity, diverse ring substituents, variable ring dynamics, and linear or bent geometries about the central nucleus depending on the steric and electronic characteristics of the substituents. Many of the main group metallocenes currently under investigation in our laboratory possess quadrupolar nuclei that are amenable to solidstate NMR analysis by virtue of their small nuclear quadrupole moments, high natural abundances, and substantial magnetogyric ratios. The manifestation of the second-order quadrupolar interaction in magic-angle-spinning (MAS) spectra of halfinteger quadrupolar nuclei should provide much insight into the electronic environment and symmetry at the metal or metalloid centers of these metallocenes. Moreover, variable-temperature double- and triple-resonance solid-state NMR experiments are useful for providing enhanced understanding of ring dynamics in such systems. The present paper is devoted to a solid-state 11B and 13C NMR spectroscopic analysis of the borocenium cation in the salt [Cp*2B][AlCl4] and the tricoordinate borane Cp*2BMe, the structures of which are illustrated in Figure 1. The X-ray structure of the former compound has been published elsewhere;5 the X-ray analysis and synthesis of the latter compound is included herein. From a combination of MAS and static 11B NMR experiments, the 11B quadrupolar parameters (i.e., nuclear quadrupole coupling constants, CQ, and asymmetry parameters, ηQ) and boron chemical shielding tensors in these complexes have been determined. Boron-11 is an excellent NMR nucleus for studies of this type, with nuclear spin I ) 3/2, high natural abundance (80.42%), a relatively small nuclear quadrupole
10.1021/jp020800r CCC: $22.00 © 2002 American Chemical Society Published on Web 09/27/2002
11B
and
13C
NMR of [Cp*2B]+ and Cp*2BMe
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10097
Figure 1. ORTEP drawings of the (A) [Cp*2B]+ cation in [Cp*2B][AlCl4] and (B) Cp*2BMe.
moment ) 0.04100 × 10-28 m2,10 and high magnetogyric ratio ) 8.5847 × 107 rad T-1 s-1 (Larmor frequency of 115.589 MHz at B0 ) 8.46 T). There is an abundance of 11B quadrupolar data in the literature for three- and four-coordinate boron complexes; however, very few definitive cases of boron chemical shielding anisotropy have been reported.11,12 The standard boron chemical shift range is approximately 230 ppm,13 including inorganic and organometallic boron complexes and boranes. In addition to the foregoing, we have examined the [Cp*2B]+ cation using variable-temperature 11B MAS NMR, 13C CPMAS NMR, and 11B/13C/1H CP TRAPDOR14 NMR experiments. The quadrupolar powder pattern, the effectiveness of cross-polarization, and efficiency of the TRAPDOR effect all indicate the existence of rapid rotation of the η5-Cp* ring, and hitherto undescribed motions of the η1-Cp* ring in the solid state. Experimental Section Synthesis of Bis(pentamethylcyclopentadienyl)methylborane, Cp*2BMe. A solution of MeLi in Et2O (4.6 mL of a 1.4 M solution, 6.4 mmol) was added to a solution of (C5Me5)2BCl (2.03 g, 6.4 mmol) in Et2O at -40 °C. The pale yellow mixture was stirred overnight. Volatiles were removed in vacuo, and the resulting white powder was extracted with consecutive portions of hexane and dried under vacuum. Colorless crystals were obtained by sublimation of the remaining white residue at 45 °C and 0.04 Torr. Yield: 75%. mp 57-58 °C. CI HRMS: calcd for C21H33B1, 296.2675; found, 296.2687. NMR (C6D6): 1H: δ 0.389 (s, BMe), δ 1.595 (s, C5Me5). 13C{1H}: δ 13.442 (s, C5Me5), δ 124.440 (s, C5Me5). 11B: δ 85.625. X-ray Crystallography of Bis(pentamethylcyclopentadienyl)methylborane, Cp*2BMe. The crystal used for the diffraction experiments was handled, selected, and coated in perfluoro(poly)ether prior to data collection. The data were collected on a Nonius Kappa CCD diffractometer using a graphite monochromator with Mo KR radiation (λ ) 0.710 73 Å). A total of 103 frames of data were collected using ω-scans with a scan range of 1° and a counting time of 20 s per frame. The data were collected at -120 °C using an Oxford Cryostream low-temperature device. Details of crystal data, data collection, and structure refinement are listed in Table 1. Data reduction was performed using DENZO-SMN.15 The structure was solved by direct methods using SIR9716 and refined by full-matrix least squares on F2 with anisotropic displacement parameters for all
TABLE 1: Summary of X-ray Crystallographic Data for Cp*2BMe empirical formula formula weight temperature (K) wavelength (Å) crystal system space group unit cell dimensions a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) volume (Å3) Z calculated density (g cm-3) absorption coefficient (mm-1) F(000) crystal size (mm) θ range for data collection (deg) limiting indices reflections collected/unique refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole (e Å-3)
C21H33B 296.28 153(2) 0.710 73 orthorhombic P212121 7.5688 (15) 17.863 (4) 27.857 (6) 90 90 90 3766.4 (13) 8 1.045 0.057 1312 0.4 × 0.4 × 0.2 2.92-27.48 -9 e h e 9 -23 e k e 23 -36 e l e 35 7702/7702 [R(int) ) 0.0600] full-matrix least squares on F2 7702/0/419 1.017 R1 ) 0.0696, wR2 ) 0.1416 R1 ) 0.1413, wR2 ) 0.1783 0.189 and -0.255
the non-H atoms using SHELXL-93.17 All hydrogen atoms were placed in calculated positions (AFIX 137). The function ∑w(|Fo|2 - |Fc|2)2 was minimized, where w ) 1/[(σ(Fo))2 + (0.0713P)2 + (1.7084P)] and P ) (|Fo|2 + 2|Fc|2)/3. No correction for secondary extinction effects was necessary. Neutral atom scattering factors and values used to calculate the linear absorption coefficient are from the International Tables for X-ray Crystallography (1992).18 All figures were generated using SHELXTL/PC.19 Tables of positional and thermal parameters and metrical parameters are provided in the Supporting Information. Solid-State NMR. Samples were prepared under an N2 environment and packed into 3.2 mm o.d. rotors. Spectra were acquired on an 8.46 T (V0(1H) ) 360 MHz) Chemagnetics CMX-360 NMR spectrometer at the State University of New
10098 J. Phys. Chem. A, Vol. 106, No. 43, 2002 York at Stony Brook (courtesy of Prof. Clare P. Grey), with V0(11B) ) 115.515 MHz and V0(13C) ) 90.536 MHz. A Chemagnetics 3.2 mm triple-resonance HXY probe was applied for all experiments. For 11B NMR experiments, solid NaBH4 was used as a secondary chemical shift reference, with δiso(11B) ) -42.06 ppm, setting the chemical shift scale against the primary reference, which is neat liquid (C2H5)2O‚BF3 with δiso(11B) ) 0.00 ppm.20 13C NMR spectra were referenced to TMS (δiso(13C) ) 0.0 ppm) by setting the high-frequency resonance of adamantane to 38.57 ppm. Proton-decoupled 11B MAS NMR spectra were acquired with spinning speeds ranging from 5 to 20 kHz, with pulse widths of 1.5 µs, V1 ) 80 kHz, and recycle delays of 1 s. Proton-decoupled 11B Hahn echo experiments were conducted with similar pulse widths and recycle delays, with interpulse delays of 50-100 µs. Protondecoupled 13C MAS, CPMAS, and VACPMAS NMR spectra were acquired at spinning speeds of 2-6 kHz with proton 90° pulse widths of 2.5 µs, recycle delays from 2 to 8 s, and contact times ranging from 0.1 to 15 ms. Rotor-synchronized 13C/1H/ 11B CPMAS TRAPDOR experiments were conducted in a manner similar to the 13C CPMAS experiments above, except with strong on-resonance irradiation of the 11B peaks with a rf field of 80 kHz over one, three, and five rotor cycle periods. Spectral Simulations. Analytical simulations of the 11B MAS and static NMR spectra were carried out on a Pentium III computer using the WSOLIDS simulation package.21 This software incorporates the space-tiling method of Alderman and co-workers for the generation of frequency domain solid-state NMR powder patterns.22 Carbon shielding tensors were extracted from 13C MAS NMR spectra using the method of Herzfeld and Berger.23 Prior to simulating 11B MAS NMR spectra, the sidebands were summed into the isotropic centerband to produce a powder pattern resembling that obtained at infinite spinning speeds. Numerical simulations of the full central transition spinning sideband manifold were performed using the SIMPSON24 software package for general simulations of solid-state NMR spectra. Simulations were accomplished by the gcompute method of powder averaging using the zcw4180 crystal file provided in the package. For the sake of simplicity, the start and detect operators were set to I1x and I1p, respectively, requiring us only to set the delay parameter to 9999 µs in the pulse sequence section. Simulated spectra were saved in free induction decay (FID) format as ASCII files without any mathematical manipulation and converted for processing to NUTS (Acorn NMR) readable files using a script written within our research group. Ab Initio Calculations. Calculations of electric field gradient and chemical shielding tensors were performed using the Gaussian 9825 computational package on a Dell Precision 420 workstation with dual 733 MHz Pentium III processors running Red Hat Linux 6.2 (Zoot). Molecular coordinates of [Cp*2B]+ and Cp*2BMe used in calculations were obtained from crystal structure data resolved by X-ray diffraction studies from ref 5 and the present work, respectively. Computations were carried out using the restricted Hartree-Fock (RHF) method and density functional theory (DFT) with the B3LYP functional26,27 in conjunction with basis sets (6-31G**, 6-311G**, 6-311+G**) provided by the Gaussian 98 package. Chemical shielding tensors were calculated using the GIAO method.28 Due to the lack of a standard 11B absolute chemical shielding scale, calculated chemical shielding data were referenced to theoretical BH4- isotropic chemical shielding as a secondary reference using δiso(BH4-) ) -42.06 ppm, which sets the chemical shift
Schurko et al. scale to that of (C2H5)2O‚BF3 (δiso ) 0.0 ppm). This was carried out by subtracting theoretical chemical shielding data of [Cp*2B]+ and Cp*2BMe from those of BH4-, which were calculated using the corresponding methods and basis sets, and then adding -42.06 ppm. Carbon chemical shielding parameters were converted to carbon chemical shifts by calculating the absolute chemical shielding of the carbon nucleus in CtO, and setting the chemical shift to the experimentally determined value of δiso(13C) ) 187.1 ppm with respect to the carbon nucleus in TMS (δiso(13C) ) 0.0 ppm).29 The rotational barrier of the η5Cp* ring in [Cp*2B]+ was determined by calculating the selfconsistent-field (SCF) energy of the molecule, keeping all atoms fixed and rotating the η5-Cp* ring by six degree intervals. Movement of the η5-Cp* ring was accomplished using the z-matrix input format. Calculations were also conducted in which the entire molecular geometry was allowed to optimize except for the angle responsible for the relative orientation of the Cp* rings. Results and Discussion In this section we discuss the experimental and theoretical measurement of two NMR interaction tensors: the electric field gradient (EFG) tensor, V h , and the chemical shielding (CS) tensor, σ j . Both V h and σ j are second-rank tensors with nine components: σ j ) {σRβ; R, β ) x, y, z} and V h ) {VRβ; R, β ) x, y, z}. The origins of NMR powder patterns affected by these anisotropic NMR interaction tensors are well discussed elsewhere.30,31 The EFG tensor, V h , is traceless and symmetric, and can be expressed in its own principal axis system (PAS) by three principal components assigned such that |V11| e |V22| e |V33|. This can be further reduced to two commonly reported parameters: the nuclear quadrupole coupling constant, CQ ) eQV33/h (in MHz), and the quadrupolar asymmetry parameter ηQ ) (V11 - V22)/V33, where 0 e ηQ e 1. The chemical shielding tensor, σ j , has a nonzero trace and is asymmetric, though the asymmetric components of the CS tensor do not make an observable contribution to the NMR spectrum. The CS tensor describes the shielding of a nucleus relative to the bare nucleus, which has chemical shielding of 0 ppm (i.e., no shielding), and has three principal components in its PAS which are assigned from least to most shielded as σ11 e σ22 e σ33. Under conditions of rapid random tumbling, only the isotropic chemical shielding is observed, and is given by σiso ) (σ11 + σ22 + σ33)/3. Chemical shielding is often described with respect to a reference compound (i.e., the chemical shift), which typically has sharp resonance(s) that is (are) arbitrarily assigned some chemical shift value. The chemical shift tensors also has three components which are assigned from least to most shielded as δ11 g δ22 g δ33, with δiso ) (δ11 + δ22 + δ33)/3. Chemical shift and chemical shielding are related to one another via δsample ) (σref - σsample)/ (1 - σref) × 106 ≈ σref - σsample. Two additional parameters, the span and the skew, are derived from the three principal components. The span of the CS tensor, Ω, is used to describe the range of chemical shielding anisotropy, with Ω ) σ33 σ11 ) δ11 - δ33. The skew, κ, describes the degree of axial symmetry of the CS tensor, with κ ) 3(σiso - σ22)/Ω ) 3(δ22 - δiso)/Ω, where -1 e κ e +1.32 In performing time-independent analytical simulations of MAS NMR spectra of quadrupolar nuclei, we neglect the effects of chemical shielding anisotropy by assuming infinite spinning speed conditions. The appearance of the MAS NMR spectrum is therefore dependent upon only three parameters: CQ, ηQ, and δiso. In the case of static NMR spectra (i.e., stationary sample), the anisotropic chemical shielding is not averaged, and the Ω and κ make contributions to the appearance of the spectrum. In
11B
and
13C
NMR of [Cp*2B]+ and Cp*2BMe
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10099
Figure 2. Experimental and simulated 11B{1H} MAS NMR spectra at 8.46 T of (A) [Cp*2B][AlCl4], υrot ) 6.0 kHz, and (B) Cp*2BMe, υrot ) 20.0 kHz.
TABLE 2: Experimental Boron-11 Chemical Shift and Quadrupolar Parameters in [Cp*2B][AlCl4] and Cp*2BMe parameter
[Cp*2B][AlCl4]
Cp*2BMe
CQ (MHz) ηQa δiso (ppm) Ωb (ppm) κc δ11d (ppm) δ22 (ppm) δ33 (ppm)
1.14 (1) 0.10 (4) -41.3 (1) 73.0 (3) 0.98 (2) -16.7 -17.5 -89.7
4.52 (2) 0.11 (1) 81.9 (1) 146.1 (3) 0.75 (4) 136.7 118.4 -9.4
Quadrupolar asymmetry parameter, ηQ ) (V11 - V22)/V33. b Span of the CS tensor, Ω ) δ11 - δ33. c Skew of the CS tensor, κ ) 3(δ22 - δiso)/Ω. d Principal components of the CS tensor from least to most shielded: δ11 g δ22 g δ33. a
addition, the relative orientation of the CS and EFG tensors plays a role in determining the shape of the static powder pattern.33,34 The relative orientations of these tensors are described by Euler angles R, β, and γ,35 where the CS tensor is described in the EFG PAS by the rotational operation R(R,β,γ) ) RZ′′(γ) RY′(β) RZ(R), where R and γ range from 0 to 2π and β ranges from 0 to π. Complete fitting of a static NMR spectrum involves a simulation dependent upon eight independent variables, though the problem can be reduced to five variables by using data obtained from concomitant MAS NMR spectra. Numerical time-dependent simulations are used (see Experimental Section for details) to examine the combined effects of the quadrupolar interaction, the chemical shielding anisotropy, and magic-angle spinning in solid-state NMR spectra. Such simulations rely on the numerical evaluation of the timedependent Liouville-von Neumann equation of motion, with high-field truncated Hamiltonians describing the quadrupolar and chemical shielding interactions in the Zeeman frame.30,31 Boron-11 MAS NMR. Boron-11 MAS NMR spectra of [Cp*2B]+ and Cp*2BMe and best fit simulations are shown in parts A and B, respectively, of Figure 2. The parameters from these simulations (Table 2) are obtained from comparison of simulations of complementary MAS and static (vide infra) NMR spectra. As the local environment of the boron changes from the “two-coordinate” η1,η5-arrangement in [Cp*2B]+ to the three-coordinate η1,η1,Me-arrangement in Cp*2BMe, there are two conspicuous differences apparent in the NMR parameters: (i) a substantial increase in the 11B nuclear quadrupole coupling constant, CQ(11B), and (ii) considerable deshielding of the boron nucleus. The CQ(11B) increases from 1.14 MHz in [Cp*2B]+ to 4.52 MHz in Cp*2BMe, whereas the asymmetry parameters, ηQ, remain similar. A small distortion is visible in the 11B MAS NMR spectra of [Cp*2B]+, which is identified as arising from impurities in the accompanying static spectra (vide infra). The
latter CQ(11B) is comparable to that measured in trimesitylborane (CQ(11B) ) 4.75 MHz),11 with which it is structurally similar. It has been demonstrated that certain special arrangements of point charges about the vertices of centrosymmetric polyhedra can generate a zero EFG tensor at the center of the polyhedra, S0.36,37 This simple model can be used to rationalize the differences in nuclear quadrupolar coupling constants in these compounds. In the case of [Cp*2B]+, we can imagine the boron nucleus at the center of an icosahedron (i.e., 12 vertices, 20 faces), with the nearest-neighbor carbons (i.e., five from the η5-Cp* ring and one from the η1-Cp* ring) approximately positioned on six of the twelve vertices. In general, the null EFG occurs at the center of a centrosymmetric polyhedron with V vertexes if like point charges are placed at V/2 of the vertices such that none of them are related by reflection through the center point, S0.37 However, this is of course not exactly the case for this molecule, since (i) the η5-Cp* ring carbons are not positioned precisely on the icosahedron vertices, and possess different charges than the η1-coordinated Cp* ring carbon, and (ii) the EFG at the boron is sensitive to the presence of secondcoordination sphere atoms, such as methyl carbons, noncoordinated η1-ring carbons, protons, etc. Nonetheless, this spherical pseudosymmetry likely accounts for the smaller value of CQ(11B) in [Cp*2B]+ compared to that measured in Cp*2BMe. In the case of Cp*2BMe, the BC3 skeletal geometry is trigonal planar; however, overall the molecular symmetry is C1 and consequently the observed value of CQ(11B) is very large. Of equal interest is the difference in nuclear shielding in the 11B NMR spectra of these two complexes. The boron chemical shift range is typically quoted as being ca. +20 to -130 ppm for four-coordinate boron compounds and ca. +100 to -10 ppm for three-coordinate boron.13 The three-coordinate Cp*2BMe has a boron chemical shift of 81.9 ppm, which is comparable to the high chemical shift values of most three-coordinate organometallic boron complexes. In [Cp*2B]+, the presence of one η5-Cp* ring and one η1-Cp* ring results in a large increase in nuclear shielding, with δiso(11B) ) -41.3 ppm. Such a change in shielding in the presence of the η5-Cp* ring is not surprising, as a rough perusal of solution NMR literature quickly shows that the majority of η5-bis(cyclopentadienyl) coordinated main group metals have isotropic chemical shifts occurring to low frequency of standard chemical shift ranges. For instance, the magnesium chemical shifts normally range from 0 to 100 ppm (from reference compound MgSO4‚7H2O to Et2Mg);13 however, Cp2Mg has a chemical shift of -85.4 ppm. Substitution of various Lewis bases at the Mg site, which cause bending of the metallocene structure, result in decreased nuclear shielding (i.e., higher chemical shifts).38 Similar results have been observed for main group metallocenes such as the aluminocenium cation,
10100 J. Phys. Chem. A, Vol. 106, No. 43, 2002
Schurko et al.
Figure 3. Experimental and simulated 11B{1H} static NMR spectra at 8.46 T of (A) [Cp*2B][AlCl4] and (B) Cp*2BMe. Simulations with and without the effects of boron chemical shielding anisotropy are included for comparison.
δiso(27Al) ) -114.5 ppm39 (standard shift range 0-300 ppm from Al(H2O)63+), stannocene, δiso(119Sn) ) -2199 ppm40 (organometallic tin complexes from -400 to 200 ppm, with respect to SnMe4 at 0 ppm), and plumbocene, δiso(207Pb) ) -6150 ppm40 (organometallic lead complexes from -400 to +400 ppm with respect to PbMe4 at 0.0 ppm). A metal with a single η5-coordinated Cp′ ring should have a chemical shift somewhere between the limits of three-coordinate planar and bis-η5-Cp′ coordinated metallocenes. Boron-11 Static NMR. To better understand the origins of nuclear magnetic shielding in these complexes, static 11B NMR spectra were acquired for both complexes, with a view to observing boron chemical shielding anisotropy. Simulation of the static spectrum using only the quadrupolar parameters obtained from the 11B MAS NMR spectra indicate that additional anisotropic NMR interactions are present. It is clear that there is an anisotropic boron chemical shielding tensor in [Cp*2B]+, with the following parameters obtained from best fit simulations: δiso ) -41.3 (1) ppm, Ω ) 73.0 (3) ppm, and κ ) 0.98(2) (Figure 3A). The relative orientation of the EFG and CS tensors is described by Euler angles R ) β ) γ ) 0°, which indicates that the largest component of the EFG tensor, V33, and the most shielded component of the CS tensor, δ33, are coincident. A small “hump” in the spectrum is visible at approximately -40 ppm, which results from an impurity with a relatively sharp resonance, and constitutes
