1,1-Dilithioethylene. A ground-state triplet olefin with nearly free rotation about the double bond

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

“…With planar C 2v triplet 8 and twisted C s triplet 6 lying only 0.12 and 0.45 kcal mol À1 above triplet structure 7,r espectively,o ther structures are much higher in Hartree-Fork theory.T wisted C 2v triplet structure 9 has the highest relative energy,3 7.0 kcal mol À1 .O ur SCF relative energies generally reproduce the order of those of Laidig and Schaefer. [15] In fact, the differenceb etween their triplet twisted and planar C 2v structures (1.2 kcal mol À1 )i so nly slightly greater than the corresponding result (0.12 kcal mol À1 )i nt he present research. Between our work and the predictions of Laidig and Schaefer, [15] the only qualitative difference was that we found new twisted C s triplet structure 6 that lies among the three lowest energy conformers 6-8 at the SCF level of theory.…”

“…[15] In fact, the differenceb etween their triplet twisted and planar C 2v structures (1.2 kcal mol À1 )i so nly slightly greater than the corresponding result (0.12 kcal mol À1 )i nt he present research. Between our work and the predictions of Laidig and Schaefer, [15] the only qualitative difference was that we found new twisted C s triplet structure 6 that lies among the three lowest energy conformers 6-8 at the SCF level of theory.…”

“…Singlet structure 1 is now 8.1 kcal mol À1 lower than twisted C s triplet 6. Comparison with the prediction of Laidig and Schaefer [15] (in Ta ble 2) indicates that planar C s singlet state 1 rather than [a] All computations used the cc-pVQZbasis set.…”

“…The simple lithium-substituted alkene 1,1-dilithioethylene is at ypical example of much theoretical investigation. [8,15,25] In 1976, ap ioneering ab initio study of C 2v structures A and B (Scheme1), that is, singlet and triplet 1,1-dilithioethylene, were reported by Apeloig, Schleyer,a nd Pople. [8] Their research offered the first prediction that dilithioalkenesm ight prefer twisted geometries.…”

“…For both the singlet and triplet H 2 C=CLi 2 molecule, twisted structure B is preferred energeticallyt o planar A with the Hartree-Fock( HF)/4-31G//HF/STO-3G method. In 1979, Laidig [15] reported resultsr elatedt ot hose of Apeloig, Schleyer,a nd Pople by using the configuration interaction methodw ith single and double excitations( CISD). In 1995, Bolton,L aidig, and Schleyer [25] found monobridged lithium planar C s structure C to be the lowest-lying singlet 1,1-dilithioethylene structure, plus singlet C 2v structure D.T heir research used the coupled-cluster method including all single and double excitationsw ith double-z plus polarization basis 1,1-Dilithioethylene is aprototypical carbon-lithium compound that is not known experimentally.A ll low-lying singlet and triplet structures of interestw ere investigated by using high-level theoreticalm ethods with correlation-consistent basis sets up to pentuple z.T he coupled cluster methods adopted included up to full triple excitationsa nd perturbative quadruples.…”