1,1‐Dilithioethylene: Toward Spectroscopic Identification of the Definitive Singlet Ground Electronic State of a Peculiar Structure

Abstract

1,1-Dilithioethylene is a prototypical carbon-lithium compound that is not known experimentally. All low-lying singlet and triplet structures of interest were investigated by using high-level theoretical methods with correlation-consistent basis sets up to pentuple ζ. The coupled cluster methods adopted included up to full triple excitations and perturbative quadruples. In contrast to earlier studies that predicted the twisted C2v triplet to be the ground state, we found a peculiar planar Cs singlet ground state in the present research. The lowest excited electronic state of 1,1-dilithioethylene, the twisted Cs triplet, was found to lie 9.0 kcal mol(-1) above the ground state by using energy extrapolation to the complete basis set limit. For the planar Cs singlet and twisted Cs triplet states of 1,1-dilithioethylene, anharmonic vibrational frequencies were reported on the basis of second-order vibrational perturbation theory. The remarkably low (2050 cm(-1) ) C-H stretching fundamental (the C-H bond near the bridging lithium) of the singlet state was found to have very strong infrared intensity. These highly reliable theoretical findings may assist in the long-sought experimental identification of 1,1-dilithioethylene. Using natural bond orbital analysis, we found that lithium bridging structures were strongly influenced by electrostatic effects. All carbon-carbon linkages corresponded to conventional double bonds.

The Nature of Lithium Bonding in C2H2Li2, C6Li6, and Lithium Halide Dimers

“…The natural atomic charges are shown in parentheses, and the Wiberg bond indices are shown in square brackets. Structure 1 was reported earlier in ref .…”

“…The earlier theoretical studies of dilithioethene have been reported by Schleyer, Pople, et al in 1976, then by Schleyer, Schaefer, et al in the 1990s, , and then by Röthlisberger and Klein in 1995 . Recently, we have studied the possible stationary points of 1,1-dilithioethylene (H 2 CCLi 2 ) at the CCSD­(T)/cc-pVQZ level of theory and predicted the energetics at the CCSDT­(Q)/CBS level . The singlet ethylene-like H 2 CCLi 2 structure ( C 2 v ) is actually a transition state, rather than a minimum, with one imaginary vibrational frequency that corresponds to the CCLi bending mode.…”

“…Along the direction of this mode, one finds a local minimum ( C s ) with one lithium atom bridging the C–C bond ( 1 , Figure ). The local minimum 1 has its energy lower than the ethylene-like structure by ∼10 kcal/mol, and other possible H 2 CCLi 2 structures have energies above structure 1 by 5–32 kcal/mol at either singlet or triplet states (Figure 1 in ref ). In isomer 1 (1,1-dilithioethylene), the predicted C–C distance of 1.343 Å is very close to that of the typical CC double bond in ethylene (1.334 Å) .…”

“…For example, the well-known (HF) 2 has a bent-chain H–F···H–F structure, whereas the analogous dimer (LiF) 2 is cyclic . Other examples include dilithioethylene and hexalithiobenzene, which have have been studied by several researchers. − The structures with bridging lithium atoms were found to be significantly lower in energy than their respective ethylene- and benzene-like structures, and furthermore, some anomalous geometries have even lower energies. The peculiar structures of lithium-containing molecules lead us to consider the nature of the lithium bond, where a thorough comparison between the lithium bond and the hydrogen bond will be of significance.…”

The quest for a triplet ground‐state alkene: Highly twisted C═C double bonds

“…Of course, twisted double bonds also may be achieved through electronic effects, ie, by involving electropositive atoms such as lithium or silicon. For example, 1,1‐dilithioethylene was first suggested by Apeloig, Schleyer, and Pople to have a triplet D 2 d structure, and recent studies based on high‐level ab initio calculations document a modest singlet‐triplet gap (9.0 kcal/mol) . Our exploration excludes these examples but focuses on hydrocarbons with twisted double bonds.…”