Lone pairs, hydrogen bonding

Figure 1.100 Illustration of water structure in solid (ice) and liquid state, (a) water has two 0—H bonds and two lone pairs each water molecule may form up to four short hydrogen bonds (b), two from 0—H hydrogen bond donors d and two from 0 lone pair hydrogen bond acceptors a (c) together these water molecules create "adamantane-like" Ih hydrogen bonded structures (d).

How do we know the 0 has its lone pairs in sp orbitals Well whenever carbonyl compounds form bonds using those lone pairs—hydrogen bonds, for example—they prefer to do so in a direction corresponding to where the lone pairs are expected to be. 

That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

In general each atom will be surrounded by four electron pairs (octet theory), either bonding or lone-pair (hydrogen by two electrons). A bond stroke in the formulae always represents a common electron pair while the lone pairs are likewise represented by transverse strokes. The older representation by dots is, however, still largely employed. 

Verify this with the aid of a molecular model. Certain types of molecular models contain oxygen lone pairs. If such a model set is not available, use a carbon atom instead and paint (or mark) two hydrogen atoms to represent lone pairs. The bond lengths and angles at carbon are slightly, but not significantly, different from those at oxygen. 

Shortcut The shortcut to drawing Lewis structures described in Section 3.3 can often be used for uncharged molecules such as CH3Br. Carbon atoms usually have four bonds and no lone pairs, hydrogen atoms always have one bond, and bromine atoms most commonly have one bond and three lone pairs. The only way to give these atoms their most common bonding patterns is with the following Lewis structure, which is the same Lewis structure we arrived at with the stepwise procedure. 

Protein chains in hair are held together by hydrogen bonds, ionic bonds, and disulfide bonds. Hydrogen bonds form between H atoms attached to very electronegative atoms (N and O) and the electron rich atoms with lone pairs. Ionic bonds result when carboxylate ions (RCOO ) and the protonated amino groups —NHj are attracted to one another. Disulfide bonds form between sulfur atoms in cysteine fragments in adjacent strands. 

Molecular hydrogen has neither a lone pair nor a tt bond, yet it also binds as an intact molecule to metals in such complexes as [W(t -H2)(CO)3L2]. The only available electron pair is the H—H a bond, and this becomes the donor ( 3 in Fig. 1,9b). Back donation in this case ( 4 in Fig. 1.96) is accepted by the H2 (x orbital. The metal binds side-on to H2 to maximize a-d overlap. Related o-bond complexes are formed with C—H, Si—H, B—H, and M—H bonds. In general, the basicity of electron pairs decreases in the following order lone pairs > w-bonding pairs > o-bonding pairs, because being part of a bond stabilizes electrons. The usual order of binding ability is therefore as follows lone pair donor > ir-bond donor > o-bond donor. 

Notice that if both lone pairs and bonding electron pairs are counted, the oetet rule is satisfied for all atoms except hydrogen in all Lewis diagrams in this section. Hydrogen, as usual, duplicates the two-electron count of the noble gas helium. 

---