Lithium molecule, molecular orbitals

Compare atomic charges for the enolate anion and the lithium salt. Are there major differences, in particular, for the oxygen and the a carbon Also compare the highest-occupied molecular orbital (HOMO) in the two molecules. This identifies the most nucleophilic sites, that is, the most likely sites for attack by electrophiles. Are the two orbitals similar or do they differ substantially Elaborate. 

In 1998, Hasanayn and Streitwieser reported the kinetics and isotope effects of the Aldol-Tishchenko reaction . They studied the reaction between lithium enolates of isobu-tyrophenone and two molecule of beuzaldehyde, which results iu the formation of a 1,3-diol monoester after protonation (Figure 28). They analyzed several aspects of this mechanism experimentally. Ab initio molecular orbital calculatious ou models are used to study the equilibrium and transition state structures. The spectroscopic properties of the lithium enolate of p-(phenylsulfonyl) isobutyrophenone (LiSIBP) have allowed kinetic study of the reaction. The computed equilibrium and transition state structures for the compounds in the sequence of reactions in Figure 28 are given along with the computed reaction barriers and energy in Figure 29 and Table 6. 

Ztg—Each atom of lithium has three electrons, two and one 2s. However, we have shown above that the inner orbitals do not take part in bond formation and retain their atomic character in the molecule and only the electrons in the outermost shell need be considered in the formation of the molecular orbital. The two valence electrons in Lig will occupy the a2s or in the abbreviated terminology, the zer molecular orbital. 

In thissection we cons de [ homonuclear diatomic molecules (those composed of two identical atoms) of elements in Period 2 of the periodic table. Since the lithium atom has a 15 25 electron configuration, it would seem that we should use the Li I5 and 2s orbitals to form the molecular orbitals of the Ll2 molecule. However, the I5 orbitals on the lithium atoms are much smaller than the 2s orbitals and therefore do not overlap in space to any appreciable extent (see Fig. 9.31). Thus the two electrons in each I5 orbital can be assumed to be localized and not to participate in the bonding. To participate in molecular orbitals, atomic orbitals must overlap in space. This means that only the valence orbitals of the atoms contribute significantly to the molecular orbitals of a particular molecule. 

In this chapter we shall use lithium hydride, LiH, to discuss the application of the molecular orbital model to a heteronuclear diatomic molecule, and begin by outlining a very simple computational procedure that yields an approximate description of the molecular orbital containing the two valence electrons. We then go on to outline the application of Hartree-Fock (HF) calculations based on a wavefuntion for both the two valence and the two inner-shell electrons. The wavefunction obtained by such calculations indicate that the bonding molecular orbital must be written as a linear combination of the H I5 with both 2s and 2pa atomic orbitals on the Li atom. 

Raptis, Papadopoulos and Sadlej have calculated the electronic structure of fiilly lithiated benzene and find very large increases in the dipole polarizability and y-hyperpolarizability, which they attribute to the effect on the molecular orbital structure of the molecule arising from the electronic valence structure of the lithium atom. It is suggested that several known stable organometalhc systems involving alkah metals should be investigated. 

Write formulas for the lowest energy and highest energy molecular orbitals of the linear molecule LiH that can be formed from sums of the Is orbitals. Use Isa and Isb to represent the orbitals on the two hydrogens, and Isu for the Is on the lithium. The lithium is between the H atoms. Don t worry about normalization. 

L12. (atomic niunber of Li = 3) Six electrons (2 X 3) must be added to the MO s. The four Is inner-shell electrons (2 horn each atom) fill the cTi, and cti, molecular orbitals and so contribute nothing to the bond energy. The two 2s electrons enter the erg, bonding molecular orbital, and therefore Li should be stable. The molecule has been detected in the vapor state of lithium. This MO configuration of Li may be represented as... 

We begin with Li2. Even though lithium is normally a metal, we can use MO theory to predict whether or not the Li2 molecule should exist in the gas phase. We approximate the molecular orbitals in Li2 as linear combinations of the 2s atomic orbitals. The resulting molecular orbitals look much like those of the H2 molecule. The MO diagram for Li2 therefore looks a lot like the MO diagram for H2 ... 

FIGURE 11.59 Energy Levels of Molecular Orbitals In Lithium Molecules... 

The 2 basis function can combine with the 2py function on the oxygen, and the 2px function on the oxygen cannot combine with any of the other basis functions. Knowing the symmetry species of the basis orbitals and including only those that have the same symmetry species shortens the calculation, as did exclusion of the 2px and 2py orbitals from the sigma molecular orbitals that we formed for the lithium hydride molecule in Section 20.4. 

It is also possible to apply the same idea of using radiation to control the electron charge distribution in open-shell molecules. For example, the lithium dimer (Li2) has a closed-shell electronic structure in its X E+ ground state in which both valence electrons are spin paired in tire same a molecular orbital. Witii visible radiation, Li2 is readily excited to its A H state in which one electron... 

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