Klaus Eichele: Publication Abstracts 1997 [ Go Home ]

Molybdenum-95 nuclear quadrupolar coupling constants, c(⁹⁵Mo), and quadrupolar asymmetry parameters, h, for sodium molybdate dihydrate, hexacarbonyl molybdenum(0), pentacarbonyl 5-methyl-dibenzophosphole molybdenum(0) and pentacarbonyl bis(diphenylphosphino)methane molybdenum(0) were obtained from solid-state ⁹⁵Mo NMR measurements at 26 MHz (9.4 T). The first direct measurements of ¹J(⁹⁵Mo,¹³C) and ¹J(⁹⁵Mo,³¹P) from solid-state ⁹⁵Mo NMR spectra are reported. Also, the first example of a ¹³C/¹²C isotope effect on ⁹⁵Mo shielding is reported for solid Mo(CO)₆; Ds-Mo(^13/12C) = -0.316 ppm. Direct-dipolar spin-spin interactions involving protons and molybdenum (i.e., ¹H-⁹⁵Mo) are relatively weak and do not appear to make significant contributions to ⁹⁵Mo NMR line shapes when spectra are acquired with magic-angle spinning. Hexacarbonyl molybdenum is proposed as a useful set-up sample for solid-state ⁹⁵Mo NMR studies. Our results for the pentacarbonyl phosphine molybdenum complexes indicate that solid-state ⁹⁵Mo NMR studies should be feasible for a range of molybdenum(0) octahedral complexes. These studies will be facilitated by using applied magnetic fields well above 10 T.

The molecular structure of 5-methyldibenzophosphole pentacarbonyl molybdenum(0), 1, has been determined by X-ray crystallography. The crystal is monoclinic C2/c, Z = 8, with unit cell dimensions of: a = 31.113(2) Å, b =7.7917(5) Å, c = 17.9522(12) Å, b = 122.135(4). Least-squares refinement converged to R(F) = 0.0245 for 2407 independent reflections. The molecular structure is typical of phosphine substituted metal carbonyls. It contains an approximate mirror plane which bisects the dibenzophosphole framework. Phosphorus-31 NMR spectra of powder and single-crystal samples of 1 have been obtained with cross-polarization and ¹H high-power decoupling. The ³¹P CP/MAS NMR spectra exhibit exceptionally well-resolved satellites due to spin-spin coupling interactions with ^95,97Mo (I = 5/2). Using first-order perturbation theory, the multiplets have been analyzed to yield ¹J(^95,97Mo,³¹P) = 123(2) Hz and estimates of the molybdenum nuclear quadrupolar coupling constants, c(⁹⁵Mo) = -0.87 MHz, c(⁹⁷Mo) = 10.1 MHz. Phosphorus-31 NMR spectra of a large single crystal of 1 have been investigated as a function of orientation about three orthogonal axes in the applied magnetic field. Analysis of the data yields the three principal components of the phosphorus chemical shift tensor, d₁₁ = 112 ppm, d₂₂ = -23 ppm, d₃₃ = -40 ppm; d₂₂ lies close to the Mo-P bond (8 deg), while d₁₁ lies in the approximate mirror plane perpendicular to the Mo-P bond. The phosphorus chemical shift tensor determined for 1 is compared with the limited anisotropic phosphorus shift data available in the literature.

Cobalt-59 NMR spectra of a single crystal of tris(2,4-pentanedionato-O,O') cobalt(III) were obtained as a function of crystal orientation in an applied magnetic field of 9.40 T. The analysis provides the magnitudes and orientations of the ⁵⁹Co nuclear quadrupole coupling and chemical shift tensors for each of the two magnetically distinct but crystallographically equivalent cobalt sites. The cobalt electric field gradient and chemical shift tensors are not coincident, but their unique components are close to the approximate C₃ axis of the complex. Small deviations from perfect octahedral symmetry at the cobalt nucleus result in a significant nuclear electric field gradient and highly anisotropic chemical shift. The cobalt nuclear quadrupole coupling constant is 5.53(0.10) MHz with an asymmetry of 0.219(0.005), while the chemical shift tensor has a span of 1174(25) ppm. The principal components relative to the isotropic chemical shift, d_iso = 12498(5) ppm, are: d₁₁ = 698(22) ppm, d₂₂ = -222(12) ppm, and d₃₃ = -476(5) ppm. The quadrupolar tensor was characterized by examining splittings between the satellite transitions, while the chemical shift tensor was characterized by analyzing the central transition and correcting for the second-order quadrupolar interaction. The results obtained in this study are compared with those of previous ⁵⁹Co NMR studies.

Single-crystal NMR was used to characterize the cesium-133 chemical shift and electric field gradient (EFG) tensors in CsCd(SCN)₃. The principal axes of the two interaction tensors are not coincident, a reflection of the general positioning of cesium nuclei within the unit cell. Relative orientations of the chemical shift and EFG tensors have been determined, but assignment of the two magnetically distinct sites remains elusive. The span of the chemical shift, 94.4 ppm, is moderate in comparison with other cesium salts, and the magnitude of the nuclear quadrupole coupling constant, 148 kHz, is in the midrange of those reported for cesium compounds. Excellent agreement is observed between experimental ¹³³Cs NMR spectra of a stationary powder sample and spectra calculated using NMR parameters from the single-crystal analysis. Moreover, simulations indicate that the static line shape is very sensitive to the relative orientation of the chemical shift and EFG tensors. Experimental ¹³³Cs NMR spectra obtained with magic-angle and variable-angle spinning are well reproduced by calculations utilizing single-crystal NMR data.

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