Coupling of intramolecular and intermolecular interactions

1 The theory of resonance-assisted coupling of H-bonding and torsions 

A particularly interesting sensitivity to intermolecular interactions occurs when a torsional group (e.g., R = CH3) is attached to the central atom of a resonance-stabilized allylic-like system, 

Examples of this motif include the carboxylate, amide, and imidazole derivatives shown below  

These moieties (particularly (5.95b)) are prominent in protein chemistry, and mechanisms for controlling their torsional stiffness have obvious potential implications for protein-folding processes. 

As described in Section 3.4.2, hyperconjugative donor-acceptor stabilizations favor conformers in which one of the rotor C—H bonds eclipses an adjacent double bond. (This is equivalent to an ethane-like staggered preference if the double bond is pictured in terms of two bent banana bonds. ) Hence, in the case of a perfectly localized Lewis structure I, the methyl group would be expected to adopt the preferred pseudo-cA conformation la (with in-plane C—H syn to A=C), 

---