Double bond representation

In azole N-oxides the bond between the nitrogen and the oxygen atom is formed by an overlap of a lone pair orbital at the N-atom with an empty p-orbital at the oxygen atom. In the literature the N-O bond has been depicted as a dipolar single bond, a double bond, or as an arrow as shown in Scheme 2. The dipolar representation is used here. The double bond representation is usually applied in literature search engines. 

In tetragonal traws-dioxometal complexes with 0, 1, or 2 -electrons, 4 bonds (two a and two ji) will be divided between two MO interactions. The average MO bond order is two, such that a double bond representation is not incorrect, although a structure with one full bond and two half bonds is a more accurate depiction of the interaction. 

This representation of the double bond applies to other double bonds also, for example C==0, S=0, P=0, and so on. 

Carbon dioxide has a linear structure. The simple double-bonded formula, however, does not fully explain the structure since the measured carbon-oxygen bond lengths are equal but intermediate between those expected for a double and a triple bond. A more accurate representation is, therefore, obtained by considering carbon dioxide as a resonance hybrid of the three structures given below ... 

In the case of 1,3-butadiene, RAMSES combines the two double bonds to form a single, delocalized r-electron system containing four electrons over all four atoms (Figure 2-50a). The same concept is applied to benzene. As shown in Figure 2-50b, the three double bonds of the Kekule representation form one electron system with six atoms and six electrons. 

Figure 2-50. Representations of a) 1,3-butadIene and b) benzene, as examples of conjugated double bonds in RAMSES.

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

Stereoisomerism at double bonds is indicated in SMILES by / and . The characters specify the relative direction of the connected atoms at a double bond and act as a frame. The characters frame the atoms of a double bond in a parallel or an opposite direction. It is therefore only reasonable to use them on both sides Figure 2-78). There are other valid representations of cis/trans isomers, because the characters can be written in different ways. Further details are listed in Section 2,3.3, in the Handbook or in Ref, [22]. 

All the double bonds are cis and the absolute configuration of the chirality center is S Wnte a stereochemically accurate representation of ectocarpene... 

There are variations in representation of rings in different disciplines. The dye industry does not designate aromaticity or double bonds in rings. AH double bonds and aromaticity are shown in the Encyclopedia as a matter of course. For example, tetralin has an aromatic ring and a saturated ring and its stmcture appears in the Encyclopedia with its common name. Registry Number enclosed in brackets, and parenthetical CA index name, ie, tetralin [119-64-2] (1,2,3,4-tetrahydronaphthalene). With names and stmctural formulas, and especiaHy with CAS Registry Numbers, the aim is to help the reader have a concise means of substance identification. 

Fig. 15. Growth of a (5ii,5n) tubule on the catalyst surface, illustrated by that of the (5,5) tubule. The central grey circle represents the catalyst particle with 10 coordination sites, and the small grey circles represent the other 10 catalyst coordination sites. The normal and bold lines represent single and double bonds, respectively, while coordinative bonds are represented by dotted lines [(a), (b) and (c)] (a ), (b ) and (c ) are the corresponding planar representations.

On the basis of these values one can conclude that, with increasing bond orders, the force constants rise, suggesting that the S—O bond of sulphoxides should have more semipolar character than that of sulphones. Furthermore, molecular diffraction measurements20 and Parachors21 for sulphoxides also suggest that the S—O bond in sulphoxides should have a semipolar single-bond representation while the S—O bond in sulphones is described by double bonds or better as the resonance hybride shown in Scheme 1. 

Representations showing electrons in molecules seem to suggest localisation of the valence electrons, but there are problematic issues in this regard. For example, we might ask if dioxygen has a double bond and two lone pairs on each O atom (as in Table 1.1) - a stmcture that does not reconcile with the paramagnetic nature of the substance - or a single bond and an odd number of electrons localised on each atom, as shown here ... 

A range of symbolic conventions is used in representing atomic and molecular stractures at the electronic level. So for example double and triple lines are used for multiple bonds. This seems a clear convention, which helps keep check of valency rales. However the symbol = for a double bond is not intended to imply two equal bonds (which the symmetry of the symbol could seem to suggest) as u and TT components have different geometries, contributions to bond strength , and consequences for chemical properties. The novice learner may well find interpreting such representations a considerable challenge. 

Which of these options is the best Lewis structure Actually, no single Lewis structure by itself is an accurate representation of NO3. Any single structure of the anion shows nitrate with one NDO double bond and two N— O single bonds. In Section 9 1, we show that single and double bonds between the same types of atoms have different lengths and different energies. In contrast, experiments show that the three nitrate N—O bonds are identical. To show that the nitrate N—O bonds are all alike, we use a composite of the three equivalent Lewis structures. These are traditionally called resonance structures. Resonance stmctures are connected by double-headed arrows to emphasize that a complete depiction requires all of them. 

Figure 3.19 Bent-bond representation of the double bond in ethene. The overlap of sp3 orbitals on each carbon atom produces to bend bond (r) orbitals.

The best Lewis-type representation of the bonding in OCF3 would therefore appear to be as in 4, even though the carbon atom does not obey the octet rule. This molecule can be considered to be a hypervalent molecule of carbon just like the hypervalent molecules of the period 3 elements, such as SFfi. We introduced the atom hypervalent in Chapter 2 and we discuss it in more detail in Chapter 9. But it is important to emphasize that the bonds are very polar. In short, OCF3 has one very polar CO double bond and three very polar CF single bonds. A serious limitation of Lewis structures is that they do not give any indication of the polarity of the bonds, and much of the discussion about the nature of the bonding in this molecule has resulted from a lack of appreciation of this limitation. 

In our laboratory we have examined the reactivity pattern of [0s3(y-H)2(C0)10], an unsaturated cluster which can be represented as possessing an osmium-osmium double bond in its classical valence bond representation. We find (2,3) that this compound undergoes a number of reactions with metal carbonyls which in some cases can be formulated as proceeding through intermediates analogous to metal olefin complexes ... 

If you ever have to interconvert between these two nomenclatures, it may be easier for you to write down a representation of the structure rather than to try to figure out the relationships between the length, number of double bonds, and how you add and subtract what to get whatever. 

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