Amides planar structure

Methodical details of crystal calculations have been published by several authors (8, 95-98). As applications we only mention the calculation of the crystal structure of diphenyl (98, 99) which has a planar structure in the solid state (100) whereas gaseous diphenyl is twisted around the CC-bond, which links both benzene rings, by about 40° (101). The calculations reproduce satisfactorily the experimental findings. A further application is the calculation of the crystal structures of a series of amides (102), with the object of deriving suitable functions for the description of hydrogen bonds (cf. previous Section). 

Amides possess planar or almost planar structures (1 and 2). Their rotational ground state is stabilized because the amino group is a strongly electron-donating group and the carbonyl function is strongly electron accepting. Excellent reviews on this topic have been published (28,29,30), and should be consulted by readers interested in amide rotation. 

This resonance representation correctly predicts a planar amide nitrogen atom that is sp2 hybridized to allow pi bonding with the carbonyl carbon atom. For example, formamide has a planar structure like an alkene. The C—N bond has partial double-bond character, with a rotational barrier of 75 kJ/mol (18 kcal/mol). 

How do we know the amide group is planar X-ray crystal structures are the simplest answer. Other techniques such as electron diffraction also show that simple (noncrystalline) amides have planar structures. N,N- dimethyl for mamide (DMF) is an example. 

Amides have structures that formally resemble acids, and H bonded ring dimers could form. Dielectric measurements show that only in special instances do these cyclic structures exist to any significant extent. Worsham and Hobbs (2197) interpreted their experiments on JV-substituted benzamides to show that the —CONH— group is planar with the O and H in the trans arrangement. As a consequence, H bonded chains similar to alcohols result, and the dielectric constant is high. 

Planar cis and trans isomers (Figure 2.1(a)) are the most stable configurations, because the planar structure involves maximum orbital overlap. For the majority of peptides built up from a-amino acids, the amide bond adopts the trans geometry, a-Imino acids (notably proline but also /V-methylamino acids), as well as a-methyl-a-... 

Many enhancements have been made to the basic distance geometry method. Some of the most useful enhancements result from the incorporation of chemical information. For example, if the lower bound for the 1,4 distances is set to a value equivalent to a torsion angle of 60° rather than one of 0° then eclipsed conformations can be avoided. Similarly, amide bonds can be forced to adopt a nearly planar structure by an appropriate choice of distance bounds and chiral constraints. 

Tetranuclear copper complexes have tetrahedral, square-planar, or butterfly structures. The compound Cu4 (Pr 0)2PS2 4 has a distorted tetrahedral skeleton of the metal atoms with one sulfur atom symmetrically bonded to two copper atoms, while the other sulfur atom forms a bond with the third copper atom. Copper and silver form many clusters containing alkyl and aryl ligands (See Chapter 4). The complex Cu4(CH2SiMe3)4, like Cu4 N(SiMe3)2 4, has a square-planar structure with bridging alkyl ligands. In the alkyl compound, in contrast to the amide one, there are Cu —Cu bonds. 

An amide (peptide) bond connects two amino acid residues. The amide unit is quite interesting in that it is essentially planar. Structure 125 shows an amide bond fragment and the electrons are delocalized as shown by the two resonance structm-es, 125A and 125B. This delocalization leads to the C-N unit having partial double-bond character, which is normal for the C-N unit in simple amides such as acetamide (ethanamide see Chapter 16, Section 16.7, and Chapter 20, Section 20.6). This phenomenon is observed in the infrared spectrum of primary and secondary amides, which exhibit two absorptions (1640 cm"i [C-0 stretch] and 1650-1515 cm [imine N-H bend of NH2 or NH] for the amide I and amide II bands see Chapter 14, Section 14.3.4). 

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