Spectroscopic Probes of n-o Interactions

It was found that the C—Cl bond adjacent to 0 is longer than that adjacent to S as expected from greater charge transfer to a ac ci orbital in the case of oxygen. More examples can be found in the structural data shown below 339-341) 

In Part II we examined in detail various experimental tests of nonbonded interactions. Here, we again focus on speciOc physical and reactivity probes in order to test the importance of n—o interactions in organic problems. Specifically, we shall examine the following two areas  

In this section, results of Photo Electron Spectroscopy (PES) as well as infrared spectroscopy shall be examined in terms of n—a interactions. 

The ionization potential of lone pair AO s is easily obtained by PES Consequently, this method can be a delicate probe of n—a conjugative interactions. For example, consider the ionization energy of the nitrogen lone pair AO as a function of the substituent X in the model system shown below  

In the above molecule, we need only consider the effect of the nisj-ocx attd nff—Ocx interactions on the energy of Hn- In most cases, the interaction which perturbs the hn AO the most is the hn-Ocx interaction due to the energy proximity of the orbitals. We can make the following predictions  

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