The Remarkably Stabilized Trilithiocyclopropenium Ion, C3Li3+, and Its Relatives

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• 作者：Eluvathingal D. Jemmis ; Govindan Subramanian ; Alexander J. Kos ; and Paul v. R. Schleyer
• 刊名：Journal of the American Chemical Society
• 出版年：1997
• 出版时间：October 8, 1997
• 年：1997
• 卷：119
• 期：40
• 页码：9504 - 9512
• 全文大小：294K
• 年卷期：v.119,no.40(October 8, 1997)
• ISSN：1520-5126

文摘

The structures and energies of lithiated cyclopropenyl cations andtheir acyclic isomers(C₃H_3-nLi_n⁺,n =0-3) have been calculated employing ab initio MO (HF/6-31G*) anddensity functional theory (DFT, Becke3LYP/6-311+G*) methods. The cyclic isomers (4,6, 10, and 14) are always favored, butwhen lithium is substitutedsequentially along the C₃H₃⁺,C₃H₂Li⁺,C₃HLi₂⁺, andC₃Li₃⁺ series, the acyclicforms (5, 7, 11, 16)becomeprogressively less competitive energetically. A triply bridgedc-C₃(-Li)₃⁺ geometry,14, was preferred over theclassical form 3 by 8.7 kcal/mol. A single lithiumsubstituent results in a very large (67 kcal/mol) stabilizationofthe cyclopropenyl cation. The favorable effects of further lithiumsubstitution are attenuated but are still large: 48.2and 40.5 kcal/mol for the second and third replacements, respectively.Comparison with polyamino-substitutedcyclopropenyl cations suggestc-C₃Li₃⁺ (3 and14) to be a good candidate for the thermodynamically moststablecarbenium ion. The stabilization of the cyclopropenyl cationafforded by the excellent -donor substituentNH₂(42.8, 33.4, and 23.7 kcal/mol for the first, second and thirdNH₂ groups, respectively) is uniformly lower thanthecorresponding values for Li substitution. The total stabilizationdue to two NH₂ groups, and a Li (128.2kcal/mol)is higher than that due to three NH₂ groups (99.8kcal/mol). All the lithiated cyclopropyl radicals are computedtohave exceptionally low adiabatic ionization energies (3.2-4.3 eV) andeven lower than the ionization energies ofthe alkali metal atoms Li-Cs (4.0-5.6 eV). The ionizationenergy of C₃Li₃ is thelowest (3.18 eV), followed byC₃(-Li)₃ (3.35 eV). The¹H, ⁶Li, and ¹³C NMR data ofcyclopropenyl cation and its lithium derivatives indicatethe carbon, lithium, and hydrogen chemical shifts to increase withincreasing lithium substitution on the ring. Thecomputed ¹H chemical shifts and the magnetic susceptibilityanisotropies as well as the nucleus independent chemicalshifts (NICS, based on absolute magnetic shieldings) reveal enhancedaromaticity upon increasing lithium substitution.The B3LYP/6-311+G*-computed vibrational frequencies agreeclosely with experiment for cyclopropenyl cationand, hence, can be used for the structural characterization of thelithiated and amino species.