Representing Organic Molecules

You ve learned previously how to represent molecules using molecular and structural formulas [W Section 2.6], as well as using Lewis structures [H4 Section 8.3], In this section, we will learn several additional ways to represent molecules— ways that are particularly useful in the study of organic chemistry. 

The necessary nurhber of H atoms B the number necessary to comfitete the octet of each CatomlMt Section 8.3]. 

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The rate constants for unimolecular photophysical processes in few representative organic molecules are given in the Table 5.4. These give an idea of the order of magnitude expected for various processes. 

Figure 23-14 Potential energy diagram for the ground state S0 and the first excited singlet S, and triplet Tj states of a representative organic molecule in solution. G is a point of intersystem crossing Sj —> T,. For convenience in representation, the distances r were chosen rS() < rSj < rT thus, the spectra are spread out. Actually, in complex, fairly symmetric molecules, rS(. rs < rT and the 0-0 absorption and fluorescence bands almost coincide, but phosphorescence bands are significantly displaced to the lower wavelengths. From Calvert and Pitts,2 p. 274.

A convenient way of representing organic molecules in three dimensions, which shows the tetrahedral relationships of the atoms very clearly, uses the so-called ball-and-stick models 2. The sticks that represent the bonds or valences form the tetrahedral angles of 109.47°. 

Fig 1. Sample from the manifold of excited states of a representative organic molecule. Straight lines represent radiative and absorptive processes wavy lines show nonradiative processes chemical transformations are not shown in this figure. 

In this experiment, we will further investigate stereoisomerism by examining a cyclic system, cyclohexane, and several acyclic tetrahedral carbon systems. The latter possess more subtle characteristics as a result of the spatial arrangement of the component atoms. We will do this by building models of representative organic molecules, then studying their properties. 

The four bonds with which carbon attaches to other atoms are equally distributed in a singly bonded carbon atom. Picture, then, that the bond sites of carbon are like the comers of a tetrahedron. Organic molecules, therefore, are three dimensional. Because it is difficult to draw complex, three-dimensional figures, we represent organic molecules by convention with a two-dimensional system of notation. 

Note No hydrogen atoms are shown in structures 1-12 and 1-13. In representing organic molecules, it is assumed that the valence requirements of carbon are satisfied by hydrogen unless otherwise specified. Thus, in structures 1-12 and 1-13, it is understood that there are six hydrogen atoms, three on each carbon. 

Fig. 8 Photophysical properties of a representative organic molecule compared to two or-gano-transition-metal emitters. The emissive triplets of Pd(thpy)2 and Pt(thpy)2 exhibit small and significant MLCT admixtures to the LC(7ztz ) states, respectively. The positions of the compounds in the ordering scheme and the molecular structures are found in Fig. 7. Photophysical properties of Pd(thpy)2 and Pt(thpy)2 are discussed in detail in [79]. lig. vibr.=ligand vibrations, M-L vibr.=metal-ligand vibration...

Models and hydrocarbons Chemists represent organic molecules in a variety of ways. Figure 21.4 shows four different ways to represent a methane molecule. Covalent bonds are represented by a single straight line, which denotes two shared electrons. Most often, chemists use the type of model that best shows the information they want to highlight. 

The decay-induced fragmentation pattern of a number of representative organic molecules has been determined with appropriate mass spectrometric techniques (7), showing that, in most cases, the daughter organic cation escapes further dissociation, for example, the methyl cation from Equation 1 is obtained with a 82% yield, being formed in its ground electronic state with little or no vibrational excitation. 

Also, in further developments of this kind of treatment, specific structureforming and structure-breaking effects of the organic adsorbate must be considered in relation to the orientation of solvent molecules, e.g., as indi-cated by entropies and heats of adsorption of pyridine at Hg electrodes. Analysis of various types of isotherms which represent organic molecule adsorption has been made by Kovac and Bockris and by Wroblowa and Mueller in relation to the theory presented above. 

Electrostatic Potential Surfaces for Representative Organic Molecules... 

What is organic chemistry Some representative organic molecules... 

We can represent organic molecules by a variety of different types of formula. 

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