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

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, Bi, 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-C-3(mu-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 pi-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 C-3(mu-Li)(3)* (3.35 eV). The H-1, Li-6, and C-13 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 H-1 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.