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

Abstract

The structures and energies of lithiated cyclopropenyl cations and their acyclic isomers (C₃H_3-nLi_n⁺, 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.