Klaus Eichele: Publication Abstracts 1993 [ Go Home ]

Carbon-13 NMR spectra of solid samples of acetanilide-13C(CO) have been examined under conditions of cross-polarization (CP) with and without magic-angle spinning (MAS). The ¹³C NMR line shapes are perturbed by direct dipolar coupling to the adjacent quadrupolar ¹⁴N nucleus. The ¹³C NMR spectra have been fully analyzed employing first-order perturbation theory for ¹⁴N. Residual ¹³C,¹⁴N splittings observed in the ¹³C CP/MAS NMR spectra of the carbonyl and C_a region of acetanilide indicate an upper limit of the ¹⁴N nuclear quadrupolar coupling constant of -3.2 MHz. The largest component of the electric field gradient (EFG) tensor at ¹⁴N is oriented perpendicular to the amide plane, while the smallest component lies in the amide plane about 20 deg off the N-H bond axis toward C_a. This orientation of the EFG tensor at nitrogen is in excellent agreement with that calculated using MO theory. Carbon-13 NMR spectra of static samples of acetanilide-13C(CO) exhibit typical ¹⁴N dipolar coupled chemical shift powder line shapes similar to those observed for the amide fragment in peptides. Asymmetric ¹⁴N,¹³C dipolar splittings are apparent in the d₁₁ region of the powder pattern, but symmetrical splittings are observed at d₃₃, while the splittings at d₂₂ are not resolved. Analysis of the static ¹³C NMR spectrum yields the magnitudes and orientations of the principal components of the carbonyl carbon chemical shift tensor. The most shielded component, d₃₃, is perpendicular to the amide plane, while the least shielded component is oriented 30 deg off the C-N bond vector toward C_a. In contrast to the MAS spectra, the static spectrum is found to be insensitive to the magnitude and orientation of the ¹⁴N EFG tensor, contrary to previous reports. The origin of the asymmetric ¹⁴N,¹³C dipolar splittings observed in the ¹³C NMR spectra of static samples of acetanilide and the amide fragment of peptides is discussed in detail.

The synthesis of the solid solution series Ga_xZn_8-xP_xSe_(2-x)[BO₂]₁₂ (x = 0, 0.5, 1, 2) is reported and the local and average long-range structure are discussed based on X-ray diffraction data, UV/visible spectroscopic measurements, and a variety of field-dependent static and magic-angle spinning solid-state NMR experiments. Inclusion of GaP within the borate sodalite analogue results in the formation of an isolated P-31,Ga-69/71 spin pair that exhibits resolved scalar coupling in the P-31 MAS NMR spectra. The analysis reveals that there are four different intracage structures distributed in small domains throughout the powdered materials and these are identified as [Zn₄Se]⁶⁺, [GaZn₃P]⁶⁺, [Zn₄P]⁵⁺ and, by inference, [Ga₂Zn₂P]⁷⁺. P-31 spin diffusion experiments suggest that the materials are single phase solid solutions, and powder X-ray diffraction data, which do not reveal more than one phase for any composition, support this conclusion.

Values for the indirect, one-bond spin-spin coupling constants J (Ru-101,99, P-31) involving the quadrupolar ruthenium nuclei Ru-101,99 (S = 5/2) and P-31 have been measured for the first time via analysis of the satellite structure in the P-31 CP/MAS spectra of the solid complexes all-trans-Ru(PEt₃)₂(CO)₂(CC-Ph)₂ (1) and (m-H)Ru₄(CO)₁₀(m-PPh₂)[m₄-PPh(C₆H₄)] (2). The P-31 CP/MAS spectrum of 1 with satellites expanded is illustrated. Values of J(Ru-99,P-31) lie in the range 105-159 Hz for 1 and 2, and J(Ru-101,P-31) lies in the range 145-174 Hz for 2. The parameters J and d (the residual dipolar coupling) are potentially useful probes of structure and bonding in solid ruthenium complexes.

Phosphorus-31 CP-MAS spectra of Cd(NO₃)₂(PPh₃)₂ have been obtained as a function of spinning frequency. Although the two P-31 nuclei are crystallographically equivalent and have the same isotropic chemical shifts in the solid state, they exhibit spinning-rate-dependent MAS spectra which have been analyzed to obtain the value of ²J(P,P). At high spinning rates, the spectra are analogous to "A₂" spectra in isotropic solutions, while at slower spinning rates, the spectra are more characteristic of strongly coupled "AB" solution spectra. The AB spectra are unusual in that d(A) = d(B) and J(A,B) is given by the splitting between the alternate peaks in the four-peak multiplet as opposed to the splitting between the outer and adjacent inner lines. This assignment was confirmed by a 2D CP-MAS J-resolved experiment. The unusual spinning-rate-dependent MAS line shapes result from recoupling of the J interaction between the two crystallographically equivalent nuclei via anisotropic interactions, i.e., weak homonuclear dipolar coupling and differences in the orientation dependence of the chemical-shift tensors. Such spinning-rate-dependent MAS line shapes are predicted to be a more frequent observation at higher applied magnetic fields.

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