Bonding in complex molecules

Morikawa, K. Benedict, W.S. and Taylor, H.S. "The activation of specific bonds in complex molecules at catalytic surfaces. II. The carbon-hydrogen and carbon-carbon bonds in ethane and ethane-[Pg.29]

The HSAB theory of Pearson has been one of the key organizing concepts in the study of nucleophiles. This theory is applied and examined in Chapters 15 and 16. In Chapter 15, Fuji applies the HSAB principles to design nucleophilic reagents for cleaving C-X bonds. Fuji notes that all bonds are made of a combination of Lewis acid and Lewis base and have hard-soft dissymmetry for the typical C-X bond, the carbon is a soft acid and the X is a hard base. Thus, in accord with the HSAB principles, a soft base (the nucleophile) and a hard acid are required to cleave this bond selectively. Applying these ideas, Fuji then shows the utility of several soft base-hard acid reagents for cleaving various C-X bonds in complex molecules. 

Although useful for quantitative calculations on computers, molecular orbital theory alone does not provide an intuitive understanding of a bonds in complex molecules. 

Among MCRs, the Ugi reaction is a powerful three- or four-component coupling reaction with isocyanide as one of the reactants to construct multiple bonds in complex molecules. Applications of Ugi reactions in total synthesis are rather rare. One of the most advanced examples is the use in the total synthesis of ecteinascidin 743 48 by Fukuyama and coworkers, as depicted in Scheme 6.5 [22]. 

VB and MO theory are sometimes jointly invoked to describe the bonding in complex molecules. 

Hybrid Orbitals Bonding in Complex Molecules CHAPTER 1... 

The benzene complexes have silver bound rf to two benzene rings in the perchlorate but only to one in the tetrachloro aluminate (Figure 4.31), while in the COT complex, each silver is bound to two double bonds in one molecule. 

It gets a little more complicated when we realize that every single bond in every molecule can freely rotate, giving rise to a very large number of conformations for molecules. We can avoid that kind of complexity by focusing just on one particular bond, and the various conformations that arise from free rotation of just that bond. If we can learn how to do that, then we can do that for the part of the molecule that is undergoing a reaction and we do not need to concern ourselves with the rest of the molecule. 

Fourier-transform infrared (FTIR) spectroscopy Spectroscopy based on excitation of vibrational modes of chemical bonds in a molecule. The energy of the infrared radiation absorbed is expressed in inverse centimeters (cm ), which represents a frequency unit. For transition-metal complexes, the ligands -C N and -C=0 have characteristic absorption bands at unusually high frequencies, so that they are easily distinguished from other bonds. The position of these bonds depends on the distribution of electron density between the metal and the ligand an increase of charge density at the metal results in a shift of the bands to lower frequencies. 

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