RATE OF BRIDGE-TERMINAL PROTON EXCHANGE IN MU-DIMETHYLAMINO-DIBORANE

The intramolecular hydrogen exchange in μ-dimethylamino-diborane has been studied by fitting the line shape of the 11B nmr spectrum as a function of temperature. Relaxation matrix theory was used to establish a model for the line shape, and it was found necessary to consider the temperature dependence of both the exchange rate constant and the line-width parameter in order to reproduce the observed spectra over the full temperature range studied (213-373 °K). A nonlinear least-squares program was used to extract values of these parameters from the spectra. The boron-bridge hydrogen and the boron-terminal hydrogen coupling constants were found to be 33.0 and 129.3 Hz, respectively. They are of the same sign and temperature independent. The exchange process was studied in solution in 1,2-dimethoxyethane, tetrahydrofuran (four concentrations), methylcyclohexane (two concentrations), and the neat liquid. The rate constant is found to have two components and may be written k(T,C) = k1(T) + Ck2(T). where k1 is the rate constant in inert solvents and in the neat material, k2 is the rate constant for the ether-catalyzed exchange process, and C is the concentration of the ether. The results obtained were Δ,S‡ = 0.4 ± 0.3 cal/deg and ΔH‡ = 16.9 ± 0.1 kcal/(deg mole) in methylcyclohexane and the neat liquid, ΔS‡ = - 25.5 ‡ 0.1 cal/deg and ΔH‡ = 6.1 ± 0.4 kcal/(deg mole) for the catalyzed process in tetrahydrofuran, and ΔS‡ = - 28.7 ‡ 0.4 cal/deg and ΔH‡ = 6 ± 1 kcal/(deg mole) in 1,2-dimethoxyethane. The ranges are standard errors. © 1969, American Chemical Society. All rights reserved.