Molecular Structure Molecules with Double Bonds

Step 4 The molecular structure is called V-shaped or bent. 

EXERCISE 12.5 Predict the arrangement of electron pairs around the central atom. Then sketch and name the molecular structure for each of the following molecules 

The various molecules we have considered are summarized in Table 12.4 on the following page. Note the following general rules. 

Rules for Predicting Molecular Structure Using the VSEPR Model 

Two pairs of electrons on a central atom in a molecule are always placed 180° apart. This is a linear arrangement of pairs. 

Electron Electron Pair Ball-and- Between Molecular Lewis Ball-and- 

Case Pairs Bonds Arrangement Stick Model Pairs Structure Structure Stick Model Example 

Molecular Structure Molecules with 11.10 Double Bonds 

Up to this point we have applied the VSEPR model only to molecules (and ions) that contain single bonds. In this section we will show that this model applies equally well to species with one or more double bonds. We will develop the procedures for dealing with molecules with double bonds by considering examples whose structures are known. 

First we will examine the structure of carbon dioxide, a substance that may be contributing to the warming of the earth. The carbon dioxide molecule has the Lewis structure 

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Molecular Structure Molecules with Double Bonds 387... 

Orbital is the highest occupied molecular orbital (HOMO) in the ground state. It corresponds to the structural formula of the molecule, with double bonds between and C2, and between C3 and C4. Orbital is likewise the lowest unoccupied molecular orbital (LUMO) in the ground state and corresponds to a biradical structure of the molecule with unpaired electrons on Gj and C4. Such a biradical structure can be a very simple but sometimes useful representation of the excited molecule (in states Si or T. ... 

D. A conjugated molecule is a molecule with double bonds on adjacent atoms such as the molecule shown in A. Choice B and C give the definition of sigma and pi molecular orbitals. D is false because a resonance form is one of multiple equivalent Lewis structures, but these structures do not describe the actual state of the molecule. The anion will exist in a state between the two forms. 

Molecular oxygen combines with double bonds, at ordinary or moderate temperatures, to form peroxides.-3 These peroxides are, in general, but short lived and decompose into aldehydes or ketones or both, depending on the structure of the molecule ... 

In this chapter, we develop a model of bonding that can be applied to molecules as simple as H2 or as complex as chlorophyll. We begin with a description of bonding based on the idea of overlapping atomic orbitals. We then extend the model to include the molecular shapes described in Chapter 9. Next we apply the model to molecules with double and triple bonds. Then we present variations on the orbital overlap model that encompass electrons distributed across three, four, or more atoms, including the extended systems of molecules such as chlorophyll. Finally, we show how to generalize the model to describe the electronic structures of metals and semiconductors. 

Think About It Often the bond order determined from a molecular orbital diagram corresponds to the number of bonds in the Lewis structure of the molecule. In the case of NO, though, the Lewis structure contains a double bond whereas the molecular orbital approach gives a bond order of 2.5. In fact, the molecular orbital approach gives a bond order that is more consistent with experimental data. The experimental bond enthalpy in NO is 631 kJ/mol, stronger than the tabulated value for a nitrogen-oxygen double bond [M Section 8.9]. 

The molecular structure of DNA (double helix) consists of two inter-twined spirals of sugar and phosphate molecules linked by hydrogen-bonded base pairs (see Fig. 1). The phosphate backbone is negatively charged with H- - or Na-t-to balance the neutrality. The width of the double helix is about two nanometres and the length of the DNA molecule depends on the number of base pairs (about a third of a nanometre per base pair). For practical use, natural DNA, derived from salmon milt, which is normally a waste product of the salmon-fishing industry is attractive... 

The Chemical Abstracts Service (CAS) chemical registry system handles tautomers and molecules with alternating bonds by replacing the explicit single and double bonds with normalized bonds. The system takes as input chemically equivalent representations of a chemical compound and offers as output a unique structural representation. Such a technique to generate unique molecular representations has to be incorporated in any chemical coding system. 

The developments in molecular biology opened up further doors within this area. More and more maCTomolecules were discovered that were linked to very specific functions inside cells. Another landmark was setup when Watson and Crick disclosed the complex structure of DNA aud how it could explain the mechanism of hereditary conservation of characteristics from parents to children (Watson and Crick, 1953). DNA is not a single polymer chain as many polymers are. DNA is a double-stranded molecule with hydrogen bonds between nitrogen bases, these bases being placed in a strict matching mechanism. 

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