Dots, pairs

The simplest oxidative addition reaction in organic chemistry is the bond activation reaction of, for example, a C—H by a carbenoid reagent X2E , for example, E = C, Si, Ge, Sn, Pb, while X = H or any other monovalent groups. Scheme 6.5a shows the R and R states for this bond activation process, using both electron-dot pairing schemes as well as FO—VB bond-pairing diagrams. 

It is a convention that the first four dots are placed one on each of the four sides of the symbol, with any remaining dots paired up with the first dots. 

Lewis structure electron dot structure dot structure. A model pioneered by Gilbert N. Lewis and Irving Langmuir that represents the electronic structure of a molecule by writing the valence electrons of atoms as dots. Pairs of dots (or lines) wedged between atoms represent bonds dots drawn elsewhere represent nonbonding electrons. 

The Lewis theory makes no prediction about molecular shapes, so it is permissible to arrange the dot pairs and other atoms around the central atom in an arbitrary way. Usually, a more or less symmetrical arrangement is written. 

Chemical bonding usually involves only the outermost electrons of atoms, also called valence electrons. In Lewis dot representations, only the electrons in the outermost occupied r and p orbitals are shown as dots. Paired and unpaired electrons are also indicated. Table 7-1 shows Lewis dot formulas for the representative elements. All elements in a given group have the same outer-shell electron configuration. It is somewhat arbitrary on which side of the atom symbol we write the electron dots. We do, however, represent an electron pair as a pair of dots and an unpaired electron as a single dot. 

Keep adding dots, pairing the dots until all are used up. 

These representations are all Lewis structures, formulas in which atomic symbols represent nuclei and inner-shell electrons, dot-pairs or dashes between two atomic symbols represent electron pairs in covalent bonds, and dots adjacent to only one atomic symbol represent unshared electrons. It is... 

Fig. 18 The new strategy outlined in Fig. 17 was evaluated in the analysis of protein-affinity pull-down experiments, to distinguish specifically enriched sample components from experimental artifacts. The carboxy terminus of Hsp70-interacting protein (CHIP) was expressed as GST fusion protein in E. coli and used as bait to affinity purify potential interaction partners from a crude human brain protein extract, a Peptide profile obtained from the LC-MALDl MS analysis of the protein isolate obtained from the affinity pull-down experiment. Blue dots-, unpaired peptides, red dots paired signals, black dots all detected monoisotopic peaks, b Mass spectrum acquired from fraction 89. The signal of m/z 1253.62 is, apart from the internal calibrants, the only unpaired signal in the spectrum, indicating that it originates from a protein that specifically interacts with CHIP, c MS/MS spectrum of m/z 1253.62. A database search identified the peptide FEEL-NADLFR of heat shock cognate 71-kDa protein. Reproduced from [316] with permission from American Chemical Society, 2006...

The most stable nuclear configuration of this system is a pair of H2 molecules. There are three possible spin coupling combinations for H4 corresponding to three distinct stable product H2 pairs H1 H2 with H3 H4, H1 H3 with H2 H4, and H1 H4 with H2 H3. Each H atom contributes one electron, the dot diagrams indicate spin pairing. The three combinations are designated as Hfl), HOT), and H(III), respectively. They may be interconverted via square transition states, Figure 2. 

Figure 2.6. The tetrahedral structures of ice (a), (fc) are planes through sheets of selected oxygen nuclei (open circles), hydrogen nuclei (shotm in the insert as solid circles) are not shown in the main drawing. The insert illustrates the overlap of oxygen line pairs and the hydrogen nuclei, thus forming the hydrogen bonds (dotted lines)...

This is known as a hydrogen-bridge structure. There are not enough electrons to make all the dotted-line bonds electron-pairs and hence it is an example of an electron-deficient compound. The structure of diborane may be alternatively shown as drawn in... 

The same G N Lewis who gave us electron dot formulas also suggested a way of think mg about acids and bases that is more general than the Brpnsted-Lowry approach Where Brpnsted and Lowry viewed acids and bases as donors and acceptors of protons (positively charged) Lewis took the opposite view and focused on electron pairs (negatively charged) According to Lewis an acid is an electron pair acceptor and a base is an electron pair donor... 

The easiest way to proceed is to use vectors to describe this part of the problem. We represent the distance between the pair of scattering sites by the vector OP the length of which is simply r. To express di and d2 in terms of OP we construct the unit vectors a and b which are parallel to the incident and scattered directions, respectively. The projection of OP into direction a, given by the dot product of these two vectors, equals dj. Likewise, the projection of OP into direction b gives d2. Therefore we can write... 

Palindromic base pairs that are most frequent at the two ends are green, and the pseudo-twofold symmetry axis is Indicated by a red dot. 

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

Fig. 3. The 2D graphene sheet is shown along with the vector which specifies the chiral nanotube. The pairs of integers ( , ) in the figure specify chiral vectors Cy, (see Table I) for carbon nanotubes, including zigzag, armchair, and chiral tubules. Below each pair of integers (n,m) is listed the number of distinct caps that can be joined continuously to the cylindrical carbon tubule denoted by (n,wi)[6]. The circled dots denote metallic tubules and the small dots are for semiconducting tubules.

Fig. 8. Planar representation of the (9M,0)-(5n,5n) knees, having a 36° bend angle produced by a heptagon-pentagon pair on the equatorial plane. The arrows show the dotted line of bonds where the knee N or N is connected to the corresponding straight tubules (a) knee N for n= 1 (b) stretched knee N , . for h = 1 and c=38 (c) general knees N and jV f.

Figure 1. The effective pair interactions as functions of alloy composition for the alloy system Al-Ni. The results of the CWIS based on the FP-LAPW calculations for 5 ordered structures (full line) and on the TB-LMTO-CPA for 5 disordered alloys (dash line) are compared with the the results of the GPM (dotted line).

Figure 1 Disorder-Llo-Ll2 phase diagram [9]. The broken line indicates the (100) Spin-odal ordering locus and dotted lines e metastable phase boundaries. The temperature axis is normalized with respect to the neaiest neighbor pair interaction energy. The ordering transition temperatures of L q and LI2 phases are 1.89 and 1.92, respectively.

Complete the electron-dot structure of caffeine, showing all lone-pair electrons, and identify the hybridization of the indicated atoms. 

Draw an electron-dot structure for each of the molecules in Problem 2.38, indicating any unshared electron pairs. 

An unshared pair of electrons, owned entirely by one atom, is shown as a pair of dots on that atom. (An unshared pair is often referred to, more picturesquely, as a lone pair.)... 

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