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The basicity of unsaturated hydrocarbons as probed by hydrogen-bond- acceptor ability: Bifurcated N-H+⋯μ hydrogen bonding

  • Author(s): Stoyanov, ES
  • Stoyanova, IV
  • Reed, CA
  • et al.

The competitive substitution of the anion (A-) in contact ion pairs of the type [Oct3NH+]B(C6F5V by unsaturated hydrocarbons (L) in accordance with the equilibrium Oct 3NH+ ⋯A- + nL ⇄ [Oct 3NH+-Ln]A- has been studied in CCl4. On the basis of equilibrium constants, K, and shifts of vNH to low frequency, it has been established that complexed Oct3NH +-L., cations with n = 1 and 2 are formed and have unidentate and bifurcated N-H+⋯μ 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, AvNH, a new scale for ranking the It basicity of unsaturated hydrocarbons is proposed: fused-ring aromatics ≤ benzene < toluene < xylene < mesitylene < durene < conjugated dienes ≈ 1 -alkynes < pentamethylbenzene < hexamethylbenzene < internal alkynes ≈ cycloalkenes < 1 -methylcycloalkenes. This scale is relevant to the discussion of It complexes for incipient protonation reactions and to understanding N-H+⋯ μ hydrogen bonding in proteins and molecular crystals. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

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