The study of barriers to bond rotation typically begins with simple single-bond rotors such as the carbon-carbon sigma bonds found in ethane and butane. Those principles are then used to estimate rotational behavior in more complex molecular structures. Studies of amide bond rotation may begin in undergraduate organic chemistry and an understanding of this phenomenon is required to describe protein (polyamide) structure. Resonance arguments rationalize the double-bond (pi-bond) character of the carbonyl carbon-to-nitrogen bond that is typically drawn as a single bond in Lewis or line-angle structures. Experimental data interpreted through the lens of computational studies have provided an analytical description of the thermodynamic and kinetic parameters governing amide bond rotation in a variety of structural frameworks. This presentation aims to build on these approaches with a computational analysis of barriers to rotation in a homologous series of amides, esters, and related functional groups. Effects of conjugation on bond rotation are described not only for the amide nitrogen or ester oxygen but also for the second group bonded to the carbonyl carbon. Importantly, the effects of acid and base catalysis are contrasted with the barriers for uncatalyzed systems. Control experiments probing nonpolar π-π (1,3-diene) conjugation as well as steric bulk are included to estimate the effects of these contributing factors. In summary, a global picture of barriers to rotation that correlates the contributions from individual structural components emerges by directly comparing the behavior of amides, esters, and related functional groups studied under parallel computational conditions.

Barriers to rotation in amides, esters, and related functional groups

ACS Spring 2022

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