Klaus Eichele: Publication Abstracts 1994 [ Go Home ]

33.4. Summary and Conclusion:
MAS frequency-dependent NMR spectra arising from dipolar-coupled crystallographically equivalent spin pairs result from the instantaneous difference in their respective chemical shifts. If the spin pair is also spin-spin coupled via the intervening electrons, then J can be extracted from MAS frequency-dependent spectra. This coupling information is not available from solution NMR studies of such an isolated chemically equivalent spin pair. In the high-speed limit, the rate of MAS is much greater than the dipolar interaction and the difference in the CS tensors of the crystallographically equivalent spins, and only a single peak at d(iso) is observed.
The J-recoupling phenomenon involving crystallographically equivalent spins was first predicted by Maricq and Waugh in 1979. An experimental example was first reported by Challoner, Nakai, and McDowell; it involved the P-31 spin pair of TBPDP. Average Hamiltonian theory predicts three distinctive types of J-recoupling pattern. The theory successfully accounts for the type I behavior observed for the P-31 spin pair of TBPDP and the unusual, type II, J-recoupling patterns observed for Cd(PPh₃)₂(NO₃)₂ and Hg(PPh₃)(NO₃)₂. The theory also accounts for spinning frequency-dependent spectra resulting from crystallographically equivalent spin pairs that are not J coupled.
Splittings in P-31 MAS NMR spectra that arise from a pair of phosphorus nuclei which are chemically equivalent in solution NMR studies are often attributed to crystallographic nonequivalence in the solid state. However, the studies outlined here clearly demonstrate that two or more peaks can arise from crystallographically equivalent nuclei that have identical isotropic chemical shifts. To interpret correctly the origin of peak multiplicities in high-resolution P-31 MAS NMR studies, it is essential to examine carefully spectra as a function of the MAS frequency.
The results reported here indicate that MAS frequency-dependent spectra involving crystallographically equivalent nuclei will be a more common phenomenon at high magnetic fields. Also, it is obvious that one can potentially extract interesting information from such MAS spectra.

A number of cadmium thiocyanate complexes of varying stoichiometry (1:2, 1:3, 1:4) have been prepared and investigated by solid-state ¹¹³Cd MAS NMR spectroscopy. The application of this technique in characterizing the various modes of linkage isomerism exhibited by the ambidentate thiocyanate ligand is discussed in detail. The ¹¹³Cd isotropic chemical shifts and chemical shift anisotropies provide information concerning the coordination number of cadmium in these complexes. For the octahedral complexes, the ¹¹³Cd isotropic chemical shifts span a range of 213 ppm with the CdN₆ complex being most shielded (d_iso = 130.0 ppm) and the CdS₆ complex being least shielded (d_iso = 342.9 ppm). Each distinct cadmium site in the cadmium thiocyanates exhibits multiplets that result from ¹¹³Cd,¹⁴N spin-spin coupling. Analysis of these multiplets reveals the number of nitrogen atoms attached to cadmium. The magnitudes of the indirect ¹¹³Cd,¹⁴N spin-spin coupling constants in these complexes vary from 37 Hz to 178 Hz and appear to be related to the cadmium-nitrogen separation.

A ³¹P NMR study of powder and single-crystal samples of ammonium dihydrogen phosphate (ADP), NH₄H₂PO₄, was undertaken in order to resolve the apparent discrepancies in the literature concerning the local symmetry at the phosphorus nucleus of the H₂PO₄ moiety. In two independent ³¹P NMR studies of solid dihydrogen phosphates evidence has been presented which indicates that the phosphorus chemical shift tensor of ADP is non-axially symmetric, in contrast to expectations based on the accepted crystal structure from X-ray and neutron diffraction experiments. In analyzing the single-crystal ³¹P NMR results presented here, it is essential to consider the influence of homonuclear ³¹P-³¹P dipolar interactions. The analysis indicates that the phosphorus chemical shift tensor is axially symmetric, d₁₁ = d₂₂ = 12 ppm and d₃₃ = -20 ppm, consistent with the expectations based on the known crystal structure. The unique component of the phosphorus chemical shift tensor is oriented along the c axis of the tetragonal crystal. The present study also demonstrates the dangers of analyzing magic-angle spinning (MAS) NMR spectra of powder samples if the nuclei under consideration are strongly dipolar coupled to neighboring spins.

The NMR line shape of an isolated pair of spin-1/2 nuclei in a static powder sample depends on the three principal components of the chemical-shift (CS) tensor, the direct dipolar spin-spin coupling tensor, and their relative orientations. Provided that the CS anisotropy is much greater than the effective dipolar coupling constant, R(eff), splittings appear at frequencies near those corresponding to each of the principal components of the CS tensor. The magnitude of the splittings provides important information concerning the orientation of the principal components of the CS tensor. Unfortunately, the equations that are generally solved to obtain this information are not independent. As a result, the values obtained for the two angles which describe the orientation of the CS tensor may not be unique. Here, we present a convenient approach, the dipolar-splitting-ratio method, which immediately indicates all possible solutions. Examples are presented to demonstrate the utility of our method in obtaining orientation information from powder samples.

In this context, refer also to the DSR program in the software list.

A solid-state P-31 CP/MAS NMR study of the 1:1 adduct of indium tribromide and a triarylphosphine permits the first unambiguous measurement of ¹J(In-115, P-31). The large quadrupole moment of In-115 (spin 9/2) generally makes such measurements impossible in solution NMR studies. Analysis of a P-31 NMR spectrum of a nonspinning powder sample indicates that the ¹J(In,P) tensor is anisotropic (J(parallel) = 1894 Hz, J(perpendicular) = 716 Hz), implicating contributions from mechanisms other than the commonly accepted Fermi-contact mechanism.

Both the phosphorus chemical shift (CS) tensor and the ¹⁹⁹Hg-³¹P spin- spin coupling tensor have been determined for the 1:1 mercury(II) phosphine complex HgPCy₃(NO₃)₂ (Cy = cyclohexyl), 1, using single-crystal ³¹P NMR spectroscopy. The phosphorus CS is found to be anisotropic with a span of approximately 90 ppm. The orientation of the principal axis system of the phosphorus CS tensor has been assigned in the molecular reference frame and is found to conform with the local site symmetry at the phosphorus atom. These results represent the first determination of the complete phosphorus CS tensor in a metal-phosphine complex. The orientation dependence of the ¹⁹⁹Hg-³¹P spin-spin coupling has been monitored as a function of crystal orientation in the external magnetic field from which the traceless part of the ¹⁹⁹Hg-³¹P J tensor has been determined. A significant anisotropy, on the order of 5400 Hz, is found to exist, indicating the participation of non-Fermi contact mechanisms in the electron mediated transmission of nuclear spin information between ¹⁹⁹Hg and ³¹P in 1. An X-ray crystallographic structure determination has also been performed for 1, revealing the presence of a polymeric chain structure in contrast to the dimeric species found previously. Crystals of 1 were found to belong to the monoclinic space group P2₁/c with Z = 4 and unit cell dimensions a = 10.360(2) A, b = 10.247(1) A, c = 21.005(4) A, b = 97.73(2) deg. The structure was refined using least-squares techniques to a final R- factor of 0.0365 based on 3070 independent reflections.

Both carbon-13 and nitrogen-15 solid-state NMR spectroscopy have been employed to characterize the carbonyl carbon and nitrogen chemical shift (CS) tensors of the amide fragment of (Z)-acetanilide (I) and (E)-N-methylacetanilide (II). These two related compounds exhibit very different structural features in the solid state, as shown by previous X-ray diffraction studies. The orientation of the principal axis system (PAS) of both the carbon and nitrogen CS tensors have been determined using dipolar-chemical shift NMR spectroscopy in conjunction with IGLO chemical shielding calculations. For I and II, the carbon CS tensors are found to be very similar. In each, the most shielded direction is perpendicular to the amide plane while the intermediate component lies approximately along the carbonyl bond. Unlike the carbon results, the three principal components of the nitrogen CS tensor reveal striking variations which can be attributed predominantly to differences in the orientation of the N-phenyl substituent with respect to the amide plane. The orientation of the PAS of the nitrogen CS tensor has been unambigously determined for I and II from the results of two separate dipolar-chemical shift NMR experiments for each compound. The intermediate component of the nitrogen CS tensor lies perpendicular to the amide plane in both compounds while the most shielded direction lies close to the C-N bond, toward C_a. Although the shielding calculations adequately reproduce the experimental shielding trends, they are less successful in determining the absolute magnitudes of the principal components of the experimental shielding tensors, which, in the case of I, can be partly attributed to the neglect of intermolecular N-H - - - O=C hydrogen bonding in the calculations.

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