The structural and electrochemical features of layered 0.5Li 2 MnO 3 ?0.5LiMO 2 electrodes, in which M = Mn 0.52x Ni 0.52x Co 2x (0 ¡ x ¡ 0.5), have been studied by powder X-ray diffraction, electrochemical differential-capacity measurements, 7 Li magic-angle-spinning nuclear magnetic resonance, and X-ray absorption near-edge spectroscopy. Li 2 MnO 3 -like regions in the as-prepared samples were observed for all values of x, with transition-metal cation disorder between the LiMO 2 and Li 2 MnO 3 components increasing with cobalt content (i.e., the value of x). The structural disorder and complexity of the electrochemical redox reactions increase when the Li 2 MnO 3 -like regions within the electrode are activated to 4.6 V in lithium cells; interpretations of structural and electrochemical phenomena are provided.

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ABSTRACT:The two-dimensional nuclear magnetic resonance correlation spectroscopy (2D NMR COSY) spectrum of an AX spin system of spin-1 has been calculated by numerical density matrix calculations. The mathematical expressions found are valid for an AX spin system of any spins. These expressions should be used to calculate the 2D NMR COSY spectrum of an AX spin system of high spin nuclei as their use will significantly simplify the calculations. More precisely, one needs to calculate only one set of coefficients despite the need of two steps in the phase cycling to achieve phase modulation during t 1 .

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Two-dimensional (2D) 59Co correlation spectroscopy (COSY)/double-quantum-filtered (DQF)COSY experiments are reported for three tetrahedral mixed-metal clusters HFeCo3(CO)11L with L = PPh3, P(OMe)3, and PCy3 (Cy = cyclohexyl) in which the L-substituted Co center is chemically different from the other two. The 2D 59Co COSY and DQFCOSY NMR spectra of these clusters in solution prove the existence of a scalar coupling constant between the 59Co nuclei. To determine this value for each cluster, 2D 59Co COSY and DQFCOSY NMR spectra have been simulated by numerical density-matrix calculations. The predicted spectra mimic well the features of the experimental spectra if a scalar coupling is introduced between the Co nuclei. It was initially observed that the scalar coupling constants between the Co nuclei obtained from the 2D COSY and DQFCOSY NMR spectra differed significantly. In contrast to the 2D COSY spectra, the diagonal and cross peaks are of comparable intensity in the 2D DQFCOSY spectra, which leads to a considerable increase in the accuracy of the determination of the scalar coupling constant.

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P and 59 Co NMR have been applied to study the structure and dynamics of the tetrahedral mixed-metal cluster HFeCo 3 (CO) 9 [P(OCH 3 ) 3 ] 3 both in the solid state and in solution. The 31 P chemical shift (CS) anisotropy, the direct (D) and indirect (J) dipolar 31 P-59 Co interactions, and the relative orientation of the CS, D, and J tensors have been determined by iterative fitting of the 31 P MAS NMR spectra at two magnetic field strengths (4.7 and 7.1 T). The quadrupole coupling constant as well as the isotropic part and anisotropy of the CS tensor at the 59 Co nucleus has been evaluated by a moment analysis of the solid-state central transition line shape. The 31 P and 59 Co NMR data, which are both influenced by second-order quadrupolar shifts, clearly show a departure from the C 3V molecular symmetry due to a solid-state packing effect. Using the static interaction parameters obtained by solid-state NMR, it was possible to evaluate the overall rate of molecular motion in solution from 59 Co relaxation measurements. The analysis of the solution-state 31 P NMR saddleshaped spectrum gives the same 1 J( 31 P-59 Co) coupling constant as in the solid state. Moreover, no Q-CS relaxation interference effects are detected in agreement with the very weak contribution of the CS anisotropy to the 59 Co relaxation predicted by the solid-state NMR data.

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