Department of Chemistry, University of California, Riverside, California 92651, USA.
Org Biomol Chem. 2013 Jun 21;11(23):3797-802. doi: 10.1039/c3ob40737c.
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 C(s) symmetry, the situation in condensed phases is much less clear. The congruence of νCH(max) 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, νCH(max) 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 νCH(max) 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 sp(3) centres rather than sp(2) centres (and are therefore less supportive of hyperconjugation), also suggests the dominance of H-bonding over hyperconjugation.
正碳离子中的超共轭稳定化是其稳定性的教科书解释，其在红外光谱中的 νCH 频带频率较低，长期以来一直被视为确认证据。虽然在气相中，氩标记的叔丁基阳离子的红外光谱与 C(s) 对称下计算的光谱非常吻合，证实了这一点，但在凝聚相中情况要清楚得多。在超酸介质中 t-Bu(+) 的 νCH(max)（2830 cm(-1))与气相中的 νCH(max)（2834 cm(-1)) 一致，这最近被证明是偶然的。相反，νCH(max) 随抗衡离子的变化而变化很大，这表明存在显著的 C-H···阴离子氢键。本文探讨了超共轭与氢键的相对重要性问题。我们通过将 νCH 区域的红外光谱分配给叔丁基阳离子中的特定 C-H 键，表明叔丁基阳离子红外光谱中低频 νCH(max) 带，长期以来一直被视为超共轭的直接证据，似乎主要归因于氢键。在仲烷基碳硼烷（如 i-Pr(CHB11Cl11)）的红外光谱中也出现了类似的低频 νCH 带，这些碳硼烷具有主要的 sp(3) 中心而不是 sp(2) 中心（因此不太支持超共轭），这也表明氢键的主导地位超过了超共轭。
