ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Using the ‘permutation of indices’ method proposed by Kaplan and Fraenkel, we could formulate the density-matrix equations required to fit the temperature-dependent 13C-NMR spectra observed with the title compounds. For 6Li13CHBr2 (1) and 6Li13CH2SC6H5 (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 ΔH≠ = 10.2 kcal/mol, ΔS≠ = 13.7 cal/mol·K for 1 and ΔH≠ = 11.1 kcal/mol, ΔS≠ = 20.6 cal/mol·K for 2 ((D8)THF as solvent). In the case of (6Li)butyllithium (3), the observed low-temperature spectra indicate that dimeric (3b) and tetrameric (3a) species are in dynamic equilibrium interchanging the C3H713CH2 groups (and THF molecules) bonded to the 6Li-atoms. 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 K′eq = [3b]2/[3a], was found to be 2.6·10-2 mol/1 (at -88°) and corresponds to ΔGR (-88°) = 2 ΔG°f(3b) - ΔG°f(3a) = 1.34 kcal/mol. The activation parameters of the dynamic process responsible for the exchange were estimated as ΔH≠ = 3.78 kcal/mol and ΔS≠ = -31.3 cal/mol·K. Tentative interpretation of the thermodynamic and kinetic parameters is given.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19850680707
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