(1 9.W 1.85)Using the 'permutation of indices' method proposed by Kupian and Fruenkel, we could formulate the density-matrix equations required to lit the temperature-dependent I3C-NMR spectra observed with the title compounds. For 6Li'3CHBr2 (1) and 6Li'3CH2SC,H, (2) an exchange mechanism is proposed by which monomers interchange C-and Li-atoms via a non-observed dimeric intermediate; the activation parameters of these intermolecular dynamic processes have been found to be A H # = 10.2 kcal/mol, AS' = 13.7 cal/mol.K for 1 and A H # = 11.1 kcal/mol, A S # = 20.6 cal/mol.K for 2 ((D,)THF as solvent). In the case of ('Li)butyllithium (3), the observed low-temperature spectra indicate that dimeric (3b) and tetrameric (3a) species are in dynamic equilibrium interchanging the C3H,"CH2 groups (and THF molecules) bonded to the 6Li-atonis. The relative concentrations of the dimer and of the tetramer have been determined by peak integration or by line-shape fitting; the 'pseudo'-equilibrium constant, defined by KLq = [3bI2/[3a], was found to be 2.6 -mol/l (at -88") and corresponds to dG, (-88") = 2 AGi(3b) -dGi(3a) = 1.34 kcdl/mol. The activation parameters of the dynamic process responsible for the exchange were estimated as A H # = 3.78 kcal/mol and A S z = -31.3 cal/mol.K. Tentative interpretation of thc thermodynamic and kinetic parameters is given.
An inversion barrier between enantiomorphic saddle conformers of nickel(ii) ccccc-octaethyl-pyrrocorphinate in solution is observed by 1H n.m.r. spectroscopy.
Iterative least‐squares lineshape fitting of 1H‐decoupled 13C‐DNMR. spectra is advantageously used for the investigation of symmetrical or asymmetrical intramolecular (two‐structure) exchange processes. The least‐squares procedure adopted allows the following parameters to be either optimized by computer or kept constant (according to experimental conditions): the nuclei populations (taking into account line intensity effects resulting from differences in spin‐lattice relaxation times and/or Overhauser enhancements), the corresponding chemical shifts and natural linewidths (characterizing the sites undergoing exchange), the fractional population of one structure (in the case of asymmetrical exchange), one of the chemical rates, the base‐line position, and the base‐line tilt. The relative sensitivity of the lineshape with respect to the fitted parameters as well as the importance of correlations among these parameters have been investigated and tested with examples. The reliability of the kinetic parameters as obtained by the iterative least‐squares lineshape fitting procedure is discussed.
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