Klaus Eichele: Publication Abstracts 1995 [ Go Home ]

The magnitude and orientation of the principal components of the carbonyl carbon chemical shift tensor of a typical aryl aldehyde, 3,4-dibenzyloxybenzaldehyde-¹³C_a,²H_a, 1, have been determined from the analysis of ¹³C NMR spectra of static powder samples obtained at 4.7 and 9.4 T. The principal components of the carbon shielding tensor of formaldehyde, 2, have been calculated from the experimental ¹³C nuclear spin-rotation tensor which is available in the literature. The experimental results are compared with those of ab initio chemical shielding calculations using the GIAO (Gauge-Including Atomic Orbitals) method. For 1, the most shielded component of the carbonyl carbon chemical shift tensor, d₃₃, is perpendicular to the ²H-¹³C=O plane, whereas the least shielded component, d₁₁, is within this plane approximately perpendicular to the C=O bond. The orientation of the carbon shielding tensor of formaldehyde is analogous to that of 1; however, the precise orientation of the three principal components is dictated by the higher molecular symmetry (e.g., d₂₂ is coincident with the C=O bond). The deshielding associated with d₁₁ is attributed to a large local magnetic dipole transition moment of the s <-> p* type. Analysis of ¹³C NMR spectra of solid 3,4- dibenzyloxybenzaldehyde-¹³C_a,²H_a obtained with magic-angle spinning (MAS) yields a value for the indirect ¹³C-²H spin-spin coupling constant, ¹J(¹³C,²H)_iso = 26.4(5) Hz. From ²H NMR spectra of a static powder sample of 1, the ²H nuclear quadrupolar coupling constant was estimated to be 151.5 kHz.

Carbon-13 CP/MAS NMR spectra of two closely related solid chloroketosulfones have been examined. Peaks arising from carbon nuclei which are spatially proximate to chlorine nuclei exhibit extensive broadening and in some cases fine structure, particularly at the lower applied magnetic fields. Residual ^35/37Cl-¹³C spin-spin interactions are responsible for the broadening and fine structure. By consideration of the appropriate Zeeman-quadrupolar Hamiltonian, the observed ¹³C CP/MAS NMR spectra can be reproduced by exact numerical computer calculations. The excellent agreement between the observed and calculated spectra allows one to determine both the sign and magnitude of the ^35/37Cl nuclear quadrupolar coupling constant, c, and the ^35/37Cl-¹³C indirect spin-spin coupling constant, ¹J(^35/37Cl,¹³C). The values determined for the ³⁵Cl-¹³C spin pair are: c(³⁵Cl) = -73(2) MHz and ¹J(³⁵Cl,¹³C) = -20(5) Hz. This is the first direct measurement of ¹J(^35/37Cl,¹³C); the relative magnitude and negative sign of this coupling constant is consistent with known periodic trends.

Phosphorus chemical shift tensors of the phosphinidene moiety, formally RP^2-, in several solid ruthenium carbonyl clusters have been characterized using slow magic-angle spinning ³¹P NMR techniques. Typically, the phosphorus nucleus in these systems is highly deshielded relative to the standard reference, 85% H₃PO₄(aq), with isotropic chemical shifts in the range of 200 to 550 ppm. For one representative cluster, nido-Ru₄(CO)₁₃(m₃-PPh), 2, ³¹P NMR studies of a single crystal yielded the principal components of the phosphorus chemical shift (CS) tensor (d₁₁ = 889 ppm, d₂₂ = 294 ppm, and d₃₃ = 60 ppm) as well as the orientation of the CS tensor. The deshielding is most pronounced when the applied magnetic field, B₀, is oriented approximately parallel to the P-C bond of the PPh moiety, while the greatest shielding occurs when B₀ is approximately perpendicular to the P-C bond and the open face of the nido-Ru₄ cluster. The orientation of the phosphorus chemical shift tensor is discussed in the context of Ramsey's theory of nuclear magnetic shielding and the results of extended Hueckel molecular orbital calculations. With the P-C bond axis defined as the x-axis, the large deshielding in these clusters is thought to be associated with p_y- and p_z-type orbitals localized on the phosphorus atom of the PPh moiety.

Set-up samples for ¹⁹⁹Hg CP/MAS NMR spectroscopy have been recommended which are superior in their relaxation and chemical shift anisotropy properties to the commonly employed mercuric acetate.

A ³¹P NMR study of powder samples of tetraethyldiphosphine disulfide (TEPS) has been undertaken in order to compare the results of variable-field and 2D spin-echo experiments with those of an earlier single-crystal ³¹P NMR study of TEPS (Tutunjian and Waugh, 1982). A significant discrepancy in the value of the effective dipolar coupling constant, R_eff, which is the difference between the direct dipole-dipole coupling constant and one third of the anisotropy in the indirect spin-spin coupling interaction, prompted a reinvestigation of the single-crystal ³¹P NMR study of TEPS. We also redetermined the crystal structure of TEPS by X-ray diffraction to obtain more accurate geometrical parameters. The results of our single-crystal ³¹P NMR experiment confirm the data determined from our ³¹P NMR studies of powder samples. We obtained a value of R_eff of 1650(10) Hz, compared to the previously reported value of 1100 Hz; the direct dipolar coupling constant calculated from the P,P bond length is 1805 Hz. The principal components of the phosphorus chemical shift tensor are: d₁₁ = 108 ppm, d₂₂ = 98 ppm, d₃₃ = -54 ppm. The most shielded direction is located within the SPPS plane and makes an angle of 2 deg with the direction of the PS bond.

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