碳𬭩离子

碳鎓离子（英語：Carbonium ion）是非经典类型的五配位或四配位碳正离子，比如五配位的离子R₅C⁺。^[1]

甲鎓离子

母体化合物甲鎓离子或写作CH₅⁺是甲烷被超强酸质子化的产物。这种离子是存在于星际介质中的活性中间体，实验室里也可以在低温低浓度的气相中制备。这种离子被认为是CH₃⁺碳正离子与一分子的氢的空轨道相互作用，并形成一个三中心二电子键。来自氢分子的两个氢原子能够与来自CH₃⁺的三个氢原子不断交换位置，所以甲鎓离子被认为是流变分子。交换反应的能垒很低，甚至在绝对零度下也能发生。甲鎓离子的构象信息可通过红外光谱得到。^[2]^[3]^[4]

平面甲烷的红外光谱显示对称与不对称C－H键的伸缩振动在3000 cm⁻¹附近，对称与不对称两键的弯曲振动在1400 cm⁻¹附近。在CH₅⁺的光谱中，在2800 – 3000 cm⁻¹附近有三个不对称伸缩振动，在1300 cm⁻¹有一个摇摆振动，在1100 - 1300 cm⁻¹有一个的弯曲振动。

参见

更多的碳鎓离子被称作非经典碳正离子，并在降冰片基类物质中发现。
鎓化合物

参考资料

^ George Andrew Olah. Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions. J. Am. Chem. Soc. 1972, 94 (3): 808–820. doi:10.1021/ja00758a020.
^ Edmund T. White, Jian Tang, Takeshi Oka. CH₅⁺: The Infrared Spectrum Observed. Science. 1999, 284: 135. doi:10.1126/science.284.5411.135.
^ Oskar Asvany, Padma Kumar P, Britta Redlich, Ilka Hegemann, Stephan Schlemmer, Dominik Marx. Understanding the Infrared Spectrum of Bare CH5+. Science. 2005, 309 (5738): 1219–22. PMID 15994376. doi:10.1126/science.1113729.
^ Xinchuan Huang, Anne B. McCoy, Joel M. Bowman, Lindsay M. Johnson, Chandra Savage, Feng Dong, David J. Nesbitt. Quantum Deconstruction of the Infrared Spectrum of CH5+. Science. 2006, 311 (5757): 60–63. PMID 16400143. doi:10.1126/science.1121166.

[1] George Andrew Olah. Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions. J. Am. Chem. Soc. 1972, 94 (3): 808–820. doi:10.1021/ja00758a020.

[2] Edmund T. White, Jian Tang, Takeshi Oka. CH₅⁺: The Infrared Spectrum Observed. Science. 1999, 284: 135. doi:10.1126/science.284.5411.135.

[3] Oskar Asvany, Padma Kumar P, Britta Redlich, Ilka Hegemann, Stephan Schlemmer, Dominik Marx. Understanding the Infrared Spectrum of Bare CH5+. Science. 2005, 309 (5738): 1219–22. PMID 15994376. doi:10.1126/science.1113729.

[4] Xinchuan Huang, Anne B. McCoy, Joel M. Bowman, Lindsay M. Johnson, Chandra Savage, Feng Dong, David J. Nesbitt. Quantum Deconstruction of the Infrared Spectrum of CH5+. Science. 2006, 311 (5757): 60–63. PMID 16400143. doi:10.1126/science.1121166.

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