Introduction to Resonance

We have seen that bond-hne structures are generally the most efficient and preferred way to draw the structure of an organic compound. Nevertheless, bond-Une structures suffer from one major defect. Specifically, a pair of bonding electrons is always represented as a fine that is drawn between two atoms, which implies that the bonding electrons are confined to a r ion of space directly in between two atoms. In some cases, this assertion is acceptable, as in the following 

In this case, the Tt electrons are in fact located where they are drawn, in between the two central carbon atoms. But in other cases, the electron density is spread out over a larger region of the molecule. For example, consider the following ion, called an allyl carbocatiom 

The molecular orbitals associated with the TT electrons of an allylic system. 

This image focuses our attention on the continuous system of p orbitals, which functions as a conduit, allowing the two it electrons to be associated with all three carbon atoms. Valence bond theory is inadequate for analysis of this system because it treats the electrons as if they were confined between only two atoms. A more appropriate analysis of the allyl cation requires the use of molecular orbital (MO) theory (Section 1.8), in which electrons are associated with the molecule as a whole, rather than individual atoms. Specifically, in MO theory, the entire molecule is treated as one entity, and all of the electrons in the entire molecule occupy regions of space called molecular orbitals. Two electrons are placed in each orbital, starting with the lowest energy orbital, until all electrons occupy orbitals. 

There should be an electron occupying this nonbonding MO, but the electron is missing. Therefore, the colored lobes are empty and represent regions of space that are electron deficient. In conclusion, MO theory suggests that the positive charge of the allyl carbocation is associated with the two ends of the system, rather than just one end. 

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A beautiful, easy-to-read introduction to wavepackets and their use in interpreting molecular absorption and resonance Raman spectra. 

Goez M 1995 An introduction to chemically induced dynamic nuclear polarization Concepts Magn. Reson. 7 69-86... 

Carrington A and McLachian A D 1979 Introduction to Magnetic Resonance, with Applications to Chemistry and Chemical Physics (New York Wiiey)... 

An exceiient beginner s introduction to magnetic resonance, spin operators and their manipuiation to predict and anaiyze spectra. 

M. Bersohn and J. C. Baird, An Introduction to Electron Paramagnetic Resonance, W. A. Benjamin, New York,... 

Electron paramagnetic resonance spectroscopy (HER), also called electron spin resonance spectroscopy (ESR), may be used for direct detection and conformational and structural characterization of paramagnetic species. Good introductions to F.PR have been provided by Fischer8 and I.effler9 and most books on radical chemistry have a section on EPR. EPR detection limits arc dependent on radical structure and the signal complexity. However, with modern instrumentation, radical concentrations > 1 O 9 M can be detected and concentrations > I0"7 M can be reliably quantified. 

For monographs, see Wertz, J.E. Bolton, J.R. Electron Spin Resonance McGraw-Hill NY, 1972 (reprinted by Chapman and Hall NY, and Methuen London, 1986) Assenheim, H.M. Introduction to Electron Spin Resonance Plenum NY, 1967 Bersohn, R. Baird,... 

An Introduction to Time-Resolved Resonance Raman Spectroscopy and Its Application to Reactive Intermediates... 

AN INTRODUCTION TO TIME-RESOLVED RESONANCE RAMAN SPECTROSCOPY... 

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