A DYNAMIC NMR INVESTIGATION OF THE ADAMANTYLIDENEADAMANTANE BR2 SYSTEM - KINETIC AND THERMODYNAMIC EVIDENCE FOR REVERSIBLE FORMATION OF THEBROMONIUM ION/BR(N)(-) PAIRS/

A dynamic H-1 NMR study of the adamantylideneadamantane (Ad=Ad)/Br2 sy stem has been undertaken in d4-1,2-dichloroethane as a function of exc ess [Br2] and temperature. Bromine site exchange rate constants (k(NMR )) were determined by line shape analysis of the spectral region betwe en delta 2.1 and 2.6. The temperature range between the fast and slow exchange limits for a given [Br2] is unusually narrow, being approxima tely 15-degrees-C. Increasing [Br2] at a given temperature dramaticall y reduces the k(NMR) due to the equilibrium formation of Br3-, Br5- an d Br7-/bromonium ion pairs. It is shown that the dynamic phenomena pro ceed via the rate-limiting dissociation of a 1:1 charge-transfer compl ex to free Ad=Ad + Br2. General implications for the bromination of mo re reactive olefins are discussed.