Evidence for C-H hydrogen bonding in salts of tert-butyl cation.
Evidence for C-H hydrogen bonding in salts of tert-butyl cation.
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叔丁基阳离子盐中 C-H 氢键的证据。
DOI：
10.1002/anie.201203958
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发表时间：
2012
期刊：
影响因子：
--
通讯作者：
Reed,ChristopherA
中科院分区：
文献类型：
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作者：
Stoyanov,EvgeniiS;Stoyanova,IrinaV;Tham,FookS;Reed,ChristopherA
叔丁基阳离子 [1] 是有机化学中的标志性中间体，超共轭是教科书对其稳定性的解释。通过将对齐的 C-H 键的电子密度提供给阳离子 C 原子上形式上空的 2pz 轨道，使正电荷离域，从而赋予 C-C 键部分双键特征，增加 H 原子上的 δ+ 电荷，并延长 C-H 键（方案 1）。超共轭在孤立（即气相）叔丁基阳离子中的重要性已由理论 [2] 彻底确立，并由通过光解获得的氩标记 tBu+ 离子的实验气相红外光谱与 Cs 对称结构的计算结果之间具有显着的一致性。 [3] Olah 等人引用了超共轭。 1964 年，解释了 SbF5 超强酸基质中 tBu+ 离子的 CÀH 拉伸 (νmax 2830 cmÀ1) 异常低的红外频率。 [4]与气相中的 νmax 值 (2834 cm-1)[3] 几乎一致，给人留下的印象是，相同的解释仅合理化了凝聚相中 tBu+ 离子的稳定性。我们现在证明，气相和凝聚相光谱数据的这种收敛是偶然的。固态 tBu+ 离子的红外光谱不仅需要通过超共轭来解释，还需要通过氢键来解释，这一想法首先由 Hollenstein 和 Laube [5] 在描述叔丁基阳离子盐的 X 射线结构中甲基和 Sb2F11 反离子之间的紧密接触时提出。
The tert-butyl cation [1] is an iconic intermediate in organic chemistry, with hyperconjugation being the textbook explanation for its stability. Positive charge is delocalized by donation of electron density from aligned CÀH bonds into the formally empty 2pz orbital on the cationic C atom, thereby giving partial double bond character to the CÀC bonds, increasing the δ+ charge on the H atoms, and lengthening the CÀH bonds (Scheme 1).The importance of hyperconjugation in isolated (ie gasphase) tert-butyl cations has been thoroughly established by theory [2] and is substantiated by the outstanding agreement between the experimental gas-phase IR spectrum of argontagged tBu+ ions obtained by photodissociation and that calculated for the Cs symmetrical structure.[3] Hyperconjugation was invoked by Olah et al. in 1964 to explain the unusually low IR frequency of the CÀH stretch (νmax 2830 cmÀ1) of tBu+ ions in an SbF5 superacid matrix.[4] Near coincidence with the νmax value in the gas phase (2834 cmÀ1)[3] has left the impression that the same explanation solely rationalizes the stability of tBu+ ions in condensed phases. We now show that this convergence of gas-and condensed-phase spectral data is fortuitous. The IR spectrum of tBu+ ions in the solid state requires explanation not only in terms of hyperconjugation but also through hydrogen bonding, an idea first put forward by Hollenstein and Laube [5] when describing close contacts between methyl groups and the Sb2F11 À counterion in the X-ray structure of a salt of a tertbutyl cation.