The amide functional group is a fundamental building block in biological systems.  Numerous investigations have been carried out over the years in an attempt to better elucidate the electronic structure of the amide bond, the barrier to rotation around the C-N bond, and the overall dynamics associated with chemical exchange of the groups bonded to the nitrogen.  Dynamic NMR has been a useful experimental tool for measuring rotational barriers in these systems.  Various computational methods have also been employed in an attempt to better assess the role delocalization plays in the structure of amides.  Despite the volume of prior work, some controversy still exists about how amides are able to overcome the relatively high C-N rotational energy barrier.  A density function theory (DFT) computational investigation is reported here that is aimed at better quantifying how much inversional motion around the nitrogen center occurs during C-N bond rotation.  Two-dimensional ‘relaxed optimization’ potential energy surface (PES) scans were carried out on formamide and a number of related derivatives using both the B3LYP and MO5 functionals in combination with the 6-31g(d,p) and 6-311+g(2df,2p) basis sets.  Two geometric coordinates were scanned corresponding to (1) ‘pure’ rotation around the C-N bond and (2) inversional (puckering) of the nitrogen out of the plane of the ground state geometry.  PES scan results reveal distinct saddle points in the energy surface whereby the C-N bond is rotated 90 degrees and the nitrogen is simultaneously puckered approximately 40 degrees so that the nitrogen center takes on a mostly sp<sup>3</sup> hybridized geometry.  The saddle points are positioned approximately 25% lower in energy than the corresponding barrier to rotation that involves no puckering of the nitrogen.  Similar PES scan results were obtained for formamide, various methyl substituted derivatives of formamide, and benzamide.<br/>

Two-dimensional DFT potential energy surface scans for formamide and related derivatives: Studying the degree of nitrogen inversion during C-N bond rotation

255th National Meeting (2018)

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