Structure of diacetamide. Reference amido structures for polypeptide conformation analysis

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Abstract
A least squares reduction of the scattered electron intensity pattern, qM(q), for acetamide led to a structure which did not differ appreciably from that in the literature. For diacetamide the optimum fit of the experimental molecular scattering function was obtained for a model in which the atoms in each half of the molecule are coplanar, but the two planes are pivoted around the nitrogen; rg values: 〈C—N〉= 1.402 (2)Å; 〈C—C〉= 1.518 (3)Å; 〈CO〉= 1.210 (2)Å; ∠NCO = 121.3°; ∠CNC = 129.2 (1.0)°; ∠OCC = 123.3 (1.2)°. Overall, diacetamide is nearly planar, with the oxygen atoms trans and a total dihedral angle of 36.1° between the acetyl groups. The angles quoted are thermal averages since the long distances show large amplitudes of vibration. This is clearly indicated by the radial distribution function. Other models tested include an enol-type structure as reported for hexafluoroacetylacetone, and a C2 structure with nearly parallel (CO) bonds, as reported for hexafluoroacetic anhydride. Comparison is made with similar structures found for formic and acetic anhydrides. Crystalline and gas phase structures for various amides and imides are compared and the correlation between internuclear distance and bond stretching frequency confirmed for corresponding changes in these molecular constants. Attention is called to the ambiguous selection of an “unperturbed” structure for the backbone unit in polypeptides used in calculations of minimum energy conformations.

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