Bond strength change with resonance

With this aim in mind we began the investigation of the thermal stabilities, the thermochemistry and the structures of several series of compounds 5. The substituents X and their characteristic resonance effects on the bond strengths are typical for each series (transition state effect). Within each series the bulk of the alkyl side chains R is changed in order to analyse the steric ground state effect. 

In terms of the structural features that are probed with various analytical methods, solid state nuclear magnetic resonance (SSNMR) may be looked upon as representing a middle ground between IR spectroscopy and X-ray powder diffraction methods. The former provides a measure of essentially molecular parameters, mainly the strengths of bonds as represented by characteristic frequencies, while the latter reflect the periodic nature of the structure of the solid. For polymorphs differences in molecular environment and/or molecular conformation may be reflected in changes in the IR spectrum. The differences in crystal structure that define a polymorphic system are clearly reflected in changes in the X-ray powder diffraction. Details on changes in molecular conformation or in molecular environment can only be determined from full crystal structure analyses as discussed in Section 4.4. 

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