Hydrogen bonding versus hyperconjugation in condensed-phase carbocations.
Hydrogen bonding versus hyperconjugation in condensed-phase carbocations.
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凝聚相碳阳离子中的氢键与超共轭。
DOI：
10.1039/c3ob40737c
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发表时间：
2013
影响因子：
3.2
通讯作者：
Tham,FookS
中科院分区：
文献类型：
--
作者：
Reed,ChristopherA;Stoyanov,EvgeniiS;Tham,FookS
叔碳阳离子正电荷的超共轭稳定是其稳定性的教科书解释，其红外光谱中的低频ch波段长期以来被视为证实证据。在气相中，氩标记的t-丁基阳离子的红外光谱与在Cs对称下计算的红外光谱非常接近，这证实了这一点，但在凝聚相中的情况就不太清楚了。t-Bu+在超强酸介质（2830 cm−1）中的νCHmax与气相（2834 cm−1）中的νCHmax的一致性最近被证明是偶然的。相反，νCHmax作为反离子的函数变化很大，以一种揭示存在显著的C-H⋯阴离子氢键的方式。本文讨论了超共轭与氢键的相对重要性。通过将νCH区域的红外光谱分配给t-丁基阳离子中特定的C-H键，我们表明，t-丁基阳离子红外光谱中的低频νCHmax波段，长期以来被认为是超共轭的直接证据，似乎主要是由于h键。在i-Pr（CHB11Cl11）等仲烷基碳硼烷的红外光谱中出现类似的低频νCH波段，它们具有主要的sp3中心而不是sp2中心（因此不太支持超共轭），这也表明氢键比超共轭更重要。
Hyperconjugative stabilization of positive charge in tertiary carbocations is the textbook explanation for their stability and low frequency νCH bands in their IR spectra have long been taken as confirming evidence. While this is substantiated in the gas phase by the very close match of the IR spectrum of argon-tagged t-butyl cation with that calculated under Cs symmetry, the situation in condensed phases is much less clear. The congruence of νCHmax of t-Bu+ in superacid media (2830 cm−1) with that in the gas phase (2834 cm−1) has recently been shown to be accidental. Rather, νCHmax varies considerably as a function of counterion in a manner that reveals the presence of significant C–H⋯anion hydrogen bonding. This paper addresses the question of the relative importance of hyperconjugation versus H-bonding. We show by assigning IR spectra in the νCH region to specific C–H bonds in t-butyl cation that the low frequency νCHmax band in the IR spectrum of t-butyl cation, long taken as direct evidence for hyperconjugation, appears to be due mostly to H-bonding. The appearance of similar low frequency νCH bands in the IR spectra of secondary alkyl carboranes such as i-Pr(CHB11Cl11), which have predominant sp3 centres rather than sp2 centres (and are therefore less supportive of hyperconjugation), also suggests the dominance of H-bonding over hyperconjugation.