The basicity of unsaturated hydrocarbons as probed by hydrogen-bond-acceptor ability: Bifurcated N-H+ center dot center dot center dot pi hydrogen bonding

The competitive substitution of the anion (A(-)) in contact ion pairs of the type [Oct(3)NH(+)]B(C6F5)(4)(-) by unsaturated hydrocarbons (L) in accordance with the equilibrium Oct(3)NH(+) center dot center dot center dot A(-) + nL = [Oct(3)NH(+) center dot center dot center dot L-n]A(-) has been studied in CCl4. On the basis of equilibrium constants, K, and shifts of nu NH to low frequency. it has been established that complexed Oct(3)NH(+) center dot center dot center dot L-n cations with n = 1 and 2 are formed and have unidentate and bifurcated N-H+ center dot center dot center dot pi hydrogen bonds, respectively. Bifurcated hydrogen bonds to unsaturated hydrocarbons have not been observed previously. The unsaturated hydrocarbons studied include benzene and methylbenzenes, fused-ring aromatics, alkenes, conjugated dienes, and alkynes. From the magnitude of the redshifts in the N-H stretching frequencies, Delta nu NH a new scale for ranking the pi basicity of unsaturated hydrocarbons is proposed: fused-ring aromatics <= benzene < toluene < xylene < mesitylene < durene < conjugated dienes approximate to 1- alkynes < pentamethylbenzene < hexamethylbenzene < internal alkynes approximate to cycloalkenes < 1-methylcycloalkenes. This scale is relevant to the discussion of T complexes for incipient protonation reactions and to understanding N-H+ center dot center dot center dot pi hydrogen bonding in proteins and molecular crystals.