Quantitative Line‐Shape Analysis of Temperature‐ and Concentration‐Dependent ¹³C‐NMR Spectra of ⁶Li‐ and ¹³C‐Labelled Organolithium Compounds. Kinetic and Thermodynamic Data for Exchange Processes in Dibromomethyllithium, (Phenylthio)methyllithium, and Butyllithium

Using the ‘permutation of indices’ method proposed by Kaplan and Fraenkel , we could formulate the density‐matrix equations required to fit the temperature‐dependent 13 C‐NMR spectra observed with the title compounds. For 6 Li 13 CHBr 2 ( 1 ) and 6 Li 13 CH 2 SC 6 H 5 ( 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 Δ H ≠ = 10.2 kcal/mol, Δ S ≠ = 13.7 cal/mol·K for 1 and Δ H ≠ = 11.1 kcal/mol, Δ S ≠ = 20.6 cal/mol·K for 2 ((D 8 )THF as solvent). In the case of ( 6 Li)butyllithium ( 3 ), the observed low‐temperature spectra indicate that dimeric ( 3b ) and tetrameric ( 3a ) species are in dynamic equilibrium interchanging the C 3 H CH 2 groups (and THF molecules) bonded to the 6 Li‐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 Δ G R (−88°) = 2 Δ G ° f ( 3b ) – Δ G ° f ( 3a ) = 1.34 kcal/mol. The activation parameters of the dynamic process responsible for the exchange were estimated as Δ H ≠ = 3.78 kcal/mol and Δ S ≠ = −31.3 cal/mol·K. Tentative interpretation of the thermodynamic and kinetic parameters is given.