Coplanar atoms, partial double-bond

The partial double-bond character of the bond that links the carbonyl carbon and the nitrogen of a peptide renders four atoms of the peptide bond coplanar and restricts the number of possible peptide conformations. 

In allylic systems, favorable overlap of the p orbitals of the n system should require a coplanar arrangement of the three sp2 carbons and their five substituent atoms evidence that such a structure is indeed preferred comes, for example, from proton magnetic resonance observations that demonstrate barriers to bond rotation in the isomeric dimethylallyl ions 21, 22, and 23. These ions form stereo-specifically from the three dimethylcyclopropyl chlorides (Section 12.2), and barriers to rotation about the partial double bonds are sufficiently high to prevent interconversion at low temperature. At — 10°C, 21, the least stable isomer,... 

Peptide bond structure. The peptide bond structure favors coplanar N, C, and O atoms. Although a peptide bond is formally a carbon-nitrogen single bond, the unpaired electrons on the carboxyl oxygen and on the nitrogen can overlap through their pi orbitals to make the three-atom system partially double-bonded in character. The partially double-bonded system makes it harder to rotate the peptide bond in solution. As a result, peptide bonds can exist in one of two conformational isomers, with the two carbons either cis or trans to each other. 

The X-ray determination of the crystal structures of several carbene complexes have confirmed the bonding scheme outlined above (Cardin et a ., 1972a, 1973 Cotton and Lukehart, 1972). In the crystal structure of (CO)5CrC(OCH3)C6Hs (Mills and Redhouse, 1968), the carbene carbon atom and the three atoms (Cr, C, O) attached to it are coplanar. The chro-mium-carbene distance of 2.04 A is shorter than the predicted single-bond distance of 2.21 A and indicates partial double-bond character of the chro-mium-carbene bond. The amount of double-bond character in the chromium carbene bond is, however, less than the double-bond character of the Cr-CO... 

The sp hybridization of nitrogen and carbonyl carbon and the partial double bond character of the C-N bond, derived from the delocalization of the nitrogen lone pair electrons (pz-dectrons) into the n system of C=0, require a coplanar displacement for all atoms connected to C and N of the amide group (six atoms, in total). Twisting or other distortion of the amide group inhibits this ddocalization and thus modifies the dectronic stmcture and the chemical properties of the group, such as, for example, the spectroscopic properties. ... 

Ren, et al, observed dielectric spectra of polylactic acid fractions in benzene solution(lO). The polymer s backbone repeat unit is (-C-CO-0-C-). The partial-double-bonding resonance nature of the ester linkage forces the backbone atoms in each backbone unit to be coplanar, so for poly-(D,L)-lactic acid the total dipole... 

A peptide chain consists of a succession of -C(=0)-N-C - atoms that are coplanar, with the C-N bond being shorter than that of a normal amide. There is a partial sharing of the 7t-electrons between the C=0 and the C-N bond, giving the latter double-bond character (-40%), so that it is unable to rotate freely. The NH-proton and the oxygen atom are in the same plane but in a trans relationship (Figure 6.24). 

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