Planar and pyramidal conformers of the alkali–ethylene complexes

Solid Parahydrogen Infrared Matrix Isolation and Computational Studies of Lin–(C2H4)m Complexes

“…Examination of Figure shows this explicitly; the B3LYP functional reproduces the large stabilization energy for the 2 B 2 electronic state but only shows the 2 A 1 state to be repulsive (no van der Waals well). The most recent ab initio calculations predict that for the relaxed potential energy curves (relaxed from planar C 2 H 4 geometries) the 2 A 1 state is 0.06 eV lower in energy than the 2 B 2 state . It would be interesting if newer long-range corrected hybrid density functionals could be applied to this problem to more fully map out the long-range interaction of the 2 A 1 state.…”

“…Calculating which is the true ground electronic state of the complex is made more difficult because the weak van der Waals interaction correlates asymptotically with the lower energy Li­(2s) atomic orbital whereas the stronger electron-transfer interaction correlates with the Li­(2p) orbital, which is 42.6 kcal mol –1 (14 904 cm –1 ) higher in energy. The most recent ab initio studies predict in the gas phase, in the absence of solvent, the van der Waals 2 A 1 state is 1.4 kcal mol –1 lower than the 2 B 2 state. If this were the case, then these 1:1 complexes would be weakly bound by only van der Waals forces and would therefore not make good hydrogen storage materials.…”

“…These recent computational studies predict that 0.47 of an electron is transferred from the Li atom to the Π* antibonding molecular orbital of C 2 H 4 in the 1:1 complex, and this metal-to-ligand charge-transfer bonding results in a complex that can bind H 2 electrostatically through strong electric dipole–induced dipole and electric dipole–quadrupole intermolecular interactions . However, these and earlier ab initio calculations have produced conflicting results for what is the true ground electronic state of the complex in the gaseous state. − As first pointed out by Manceron in 1993, there are two possibilities for the interaction of a Li atom with an ethylene molecule with a filled Π molecular orbital. In the first, the atomic orbitals on Li do not interact strongly with the Π-system of C 2 H 4 , resulting in a 2 A 1 ground electronic state (in the C 2 v geometry) that is purely repulsive and the complex is only bound by weak van der Waals forces .…”

“…4 Modern ab initio calculations with diffuse-augmented basis sets predict the ground state in vacuo is the 2 A 1 van der Waals state; however, the 2 B 2 state is only predicted to be 1−3 kcal mol −1 higher in energy. 5,6 Some have suggested that solvation of the complex even by the nonpolar but polarizable Ar matrix shifts this balance toward the 2 B 2 charge-transfer state which will have the greater solvation energy. 3,5 In computational studies that included the effects of the Ar solvent using a polarizable continuum model, the 2 B 2 state was predicted to be lower in energy by roughly 3 kcal mol −1 , but these differences are still rather small.…”

“…Indeed, a variety of Li n –(C 2 H 4 ) m complexes have been observed in Ar matrix isolation experiments which have shown through ESR, UV, and IR spectroscopy that the 1:1 complex is unequivocally in the 2 B 2 charge-transfer state. − Of all the alkali atoms only Li is predicted to show this behavior whereas Na and K only form weak van der Waals complexes . Modern ab initio calculations with diffuse-augmented basis sets predict the ground state in vacuo is the 2 A 1 van der Waals state; however, the 2 B 2 state is only predicted to be 1–3 kcal mol –1 higher in energy. , Some have suggested that solvation of the complex even by the nonpolar but polarizable Ar matrix shifts this balance toward the 2 B 2 charge-transfer state which will have the greater solvation energy. , In computational studies that included the effects of the Ar solvent using a polarizable continuum model, the 2 B 2 state was predicted to be lower in energy by roughly 3 kcal mol –1 , but these differences are still rather small . We decided to study these Li n –(C 2 H 4 ) m complexes solvated in solid parahydrogen (pH 2 ) to see if the 2 B 2 charge-transfer state is formed similarly to the Ar matrix results and to study the complexes under conditions that can directly probe their hydrogen storage capabilities.…”