Strongly Polar Electron Pair Bonding

However, a detailed reexamination within the framework of Kohn-Sham MO theory leads us to a view that differs in a number of ways.51 In the following, we show that the C—Li bond in CH3Li (25) may very well be envisaged as an electron pair bond (24b), although a rather polar one, of course. But our point is not just the shift of the bonding picture back to the more covalent side of the 24a-24b spectrum. In particular, if we go to the oligomers (e.g., the methyllithium tetramer 27), a fundamentally new phenomenon occurs in the C—Li bonding mechanism. This phenomenon emphasizes the presence of dis- 

The Polar C—Li Electron Pair Bond in Monomeric CH3Li 

Furthermore we have quantitatively analyzed the C—Li bonding mechanism in the CH3Li monomer 25, dimer 26 (not discussed here), and tetramer 27 (see later) through a decomposition of the overall bond energy AE. The latter corresponds to the formation of (CH3Li) from the corresponding methyl and lithium radicals and is made up of two major components (Eq. [34]). 

The interaction energy is further split up into three physically meaningful components as discussed earlier (1) the classical electrostatic interaction A Vc stat between the unperturbed charge distributions of the prepared fragments which is usually attractive, (2) the Pauli repulsive orbital interactions AEpauii, and (3) the stabilizing orbital interaction AEoi (Eq. [38])  

The orbital interactions can be further split up into the contributions from each irreducible representation T of a (CH3Li) system (Eq. [39])  

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The chemical properties of solvents have obviously a strong bearing on their applicability for various purposes. The solvents should selectively dissolve the desired solutes and not some others, they should be inactive in the chemical reactions undergone by the solutes, but solvate, again selectively, reactants, transition states, intermediates, and products. These aspects of the behaviour can be achieved by the proper blend of the chemical properties of structuredness, polarity, electron-pair and hydrogen bond donation and acceptance ability, softness, acidity and basicity, hydrophilicity or hydrophobicity, and redox properties, among others. Such chemical characteristics can often be derived from physical properties, but in other cases must be obtained from chemical interactions, for instance by the use of chemical probes ( indicators ). 

Because of favorable dipole-dipole attractions between solvent molecules and solute molecules, polar liquids tend to dissolve in polar solvents. Water is both polar and able to form hydrogen bonds. (Section 11.2) Thus, polar molecules, especially those that can form hydrogen bonds with water molecules, tend to be soluble in water. For example, acetone, a polar molecule with the structural formula shown in the margin, mixes in all proportions with water. Acetone has a strongly polar C = O bond and pairs of nonbonding electrons on the O atom that can form hydrogen bonds with water. 

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