碳鎓離子

碳鎓離子（英語：Carbonium ion）是非經典類型的五配位或四配位碳正離子，比如五配位的離子R₅C⁺。^[1]

甲鎓離子

母體化合物甲鎓離子或寫作CH₅⁺是甲烷被超強酸質子化的產物。這種離子是存在於星際介質中的活性中間體，實驗室里也可以在低溫低濃度的氣相中製備。這種離子被認為是CH₃⁺碳正離子與一分子的氫的空軌道相互作用，並形成一個三中心二電子鍵。來自氫分子的兩個氫原子能夠與來自CH₃⁺的三個氫原子不斷交換位置，所以甲鎓離子被認為是流變分子。交換反應的能壘很低，甚至在絕對零度下也能發生。甲鎓離子的構象信息可通過紅外光譜得到。^[2]^[3]^[4]

平面甲烷的紅外光譜顯示對稱與不對稱C－H鍵的伸縮振動在3000 cm⁻¹附近，對稱與不對稱兩鍵的彎曲振動在1400 cm⁻¹附近。在CH₅⁺的光譜中，在2800 – 3000 cm⁻¹附近有三個不對稱伸縮振動，在1300 cm⁻¹有一個搖擺振動，在1100 - 1300 cm⁻¹有一個的彎曲振動。

參見

更多的碳鎓離子被稱作非經典碳正離子，並在降冰片基類物質中發現。
鎓化合物

參考資料

^ 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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