Carbon-nitrogen bond length

Table 2 Average Carbon-Nitrogen Bond Lengths...

Structural parameters and interatomic distances derived from electron diffraction (7) (77JST(42)l2i) and X-ray diffraction (8) studies (76AX(B)3178) provide unequivocal evidence that pyrazine is planar with >2a symmetry. There is an increased localization of electron density in the carbon-nitrogen bonds, with carbon-carbon bonds being similar in length to those in benzene. ... 

The carbon-nitrogen bond is referred to as the peptide bond. A polypeptide chain is the result of a number of such reactions, and the characteristic of all peptides, like the amino acids from which they have been formed, is that one end of the molecule is an amino group and the other end is a carboxylic acid. Different peptides are characterized by their lengths and their side chains (R groups). In fact, only twenty types of side chains, with different sizes, shapes, charges, chemical reactivity, and hydrogenbonding capacity, have so far been found in the proteins on all species on earth, from bacteria to man. 

Problem 20.19. The amide bond in peptides does not have the properties of a single bond. There is no free rotation about the carbon-nitrogen bond and the bond length is shorter than expected for a single bond. The CO — N bond is said to have considerable double-bond character. Describe how resonance theory rationalizes the double-bond properties of the amide bond. 

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