The remarkably stabilized trilithiocyclopropenium ion, C3Li3+, and its relatives

Jemmis, Eluvathingal D. ; Subramanian, Govindan ; Kos, Alexander J. ; Schleyer, Paul V. R. (1997) The remarkably stabilized trilithiocyclopropenium ion, C3Li3+, and its relatives Journal of the American Chemical Society, 119 (40). pp. 9504-9512. ISSN 0002-7863

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The structures and energies of lithiated cyclopropenyl cations and their acyclic isomers (C3H3-nLin+, n = 0-3) have been calculated employing ab initio MO (HF/6-31G) and density functional theory (DFT, Becke3LYP/6-311+G) methods. The cyclic isomers (4, 6, 10, and 14) are always favored, but when lithium is substituted sequentially along the C3H3+, C3H2Li+, C3HLi2+, and C3Li3+ series, the acyclic forms (5, 7, 11, 16) become progressively less competitive energetically. A triply bridged c-C3(μ-Li)3+ geometry, 14, was preferred over the classical form 3 by 8.7 kcal/mol. A single lithium substituent results in a very large (67 kcal/mol) stabilization of the cyclopropenyl cation. The favorable effects of further lithium substitution are attenuated but are still large: 48.2 and 40.5 kcal/mol for the second and third replacements, respectively. Comparison with polyamino-substituted cyclopropenyl cations suggest c-C3Li3+ (3 and 14) to be a good candidate for the thermodynamically most stable carbenium ion. The stabilization of the cyclopropenyl cation afforded by the excellent p-donor substituent NH2 (42.8, 33.4, and 23.7 kcal/mol for the first, second and third NH2 groups, respectively) is uniformly lower than the corresponding values for Li substitution. The total stabilization due to two NH2 groups, and a Li (128.2 kcal/mol) is higher than that due to three NH2 groups (99.8 kcal/mol). All the lithiated cyclopropyl radicals are computed to have exceptionally low adiabatic ionization energies (3.2-4.3 eV) and even lower than the ionization energies of the alkali metal atoms Li-Cs (4.0-5.6 eV). The ionization energy of C3Li3 is the lowest (3.18 eV), followed by C3(μ-Li)3 (3.35 eV). The 1H, 6Li, and 13C NMR data of cyclopropenyl cation and its lithium derivatives indicate the carbon, lithium, and hydrogen chemical shifts to increase with increasing lithium substitution on the ring. The computed 1H chemical shifts and the magnetic susceptibility anisotropies as well as the nucleus independent chemical shifts (NICS, based on absolute magnetic shieldings) reveal enhanced aromaticity upon increasing lithium substitution. The B3LYP/6-311+G-computed vibrational frequencies agree closely with experiment for cyclopropenyl cation and, hence, can be used for the structural characterization of the lithiated and amino species.

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