THE REMARKABLY STABILIZED TRILITHIOCYCLOPROPENIUM ION, C3LI3+, AND ITS RELATIVES

The structures and energies of lithiated cyclopropenyl cations and the ir acyclic isomers (C3H3-nLin+, n = 0-3) have been calculated employin g ab initio MO (HF/6-31G) and density functional theory (DFT, Becke3L YP/6-311+G) methods. The cyclic isomers (4, Bi, 10, and 14) are alway s 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 bridge d c-C-3(mu-Li)(3)(+) geometry, 14, was preferred over the classical fo rm 3 by 8.7 kcal/mol. A single lithium substituent results in a very l arge (67 kcal/mol) stabilization of the cyclopropenyl cation. The favo rable effects of further lithium substitution are attenuated but are s till large: 48.2 and 40.5 kcal/mol for the second and third replacemen ts, respectively. Comparison with polyamino-substituted cyclopropenyl cations suggest c-C3Li3+ (3 and 14) to be a good candidate for the the rmodynamically most stable carbenium ion. The stabilization of the cyc lopropenyl 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 grou ps, 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 hav e exceptionally low adiabatic ionization energies (3.2-4.3 eV) and eve n 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 carb on, 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 indep endent chemical shifts (NICS, based on absolute magnetic shieldings) r eveal enhanced aromaticity upon increasing lithium substitution. The B 3LYP/6-311+G-computed vibrational frequencies agree closely with expe riment for cyclopropenyl cation and, hence, can be used for the struct ural characterization of the lithiated and amino species.