Stoyanov Evgenii S, Stoyanova Irina V, Reed Christopher A
Department of Chemistry, University of California, Riverside, Riverside, California 92521, USA.
Chemistry. 2008;14(26):7880-91. doi: 10.1002/chem.200800337.
The competitive substitution of the anion (A(-)) in contact ion pairs of the type [Oct3NH+]B(C6F5)4 (-) by unsaturated hydrocarbons (L) in accordance with the equilibrium Oct3NH+...A(-) + nL right arrow over left arrow [Oct3NH+...Ln]A(-) has been studied in CCl4. On the basis of equilibrium constants, K, and shifts of nuNH to low frequency, it has been established that complexed Oct3NH...+Ln cations with n=1 and 2 are formed and have unidentate and bifurcated N--H+...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 nuNH, a new scale for ranking the pi basicity of unsaturated hydrocarbons is proposed: fused-ring aromatics<or=benzene<toluene<xylene<mesitylene<durene<conjugated dienes approximately 1-alkynes<pentamethylbenzene<hexamethylbenzene<internal alkynes approximately cycloalkenes<1-methylcycloalkenes. This scale is relevant to the discussion of pi complexes for incipient protonation reactions and to understanding N--H+...pi hydrogen bonding in proteins and molecular crystals.
在四氯化碳中研究了不饱和烃（L）按照平衡式Oct3NH⁺...A⁻ + nL⇌[Oct3NH⁺...Ln]A⁻对[Oct3NH⁺]B(C6F5)4⁻型接触离子对中阴离子（A⁻）的竞争性取代。基于平衡常数K以及νNH向低频的位移，已确定形成了n = 1和2的配合物Oct3NH...⁺Ln阳离子，它们分别具有单齿和分叉的N - H⁺...π氢键。此前尚未观察到与不饱和烃的分叉氢键。所研究的不饱和烃包括苯和甲基苯、稠环芳烃、烯烃、共轭二烯和炔烃。根据N - H伸缩频率的红移量ΔνNH，提出了一种用于对不饱和烃的π碱度进行排序的新标度：稠环芳烃≤苯＜甲苯＜二甲苯＜均三甲苯＜杜烯＜共轭二烯≈1 - 炔烃＜五甲基苯＜六甲基苯＜内炔烃≈环烯烃＜1 - 甲基环烯烃。该标度与起始质子化反应的π配合物讨论以及理解蛋白质和分子晶体中的N - H⁺...π氢键相关。
