Amides resonance structures

Problem What is the critical weighting i/>dip of the alternative dipolar amide resonance structure in (5.102) that would reverse the preference for pseudo-c/s over pseudo-trans geometry at Cal What are the corresponding bond lengths Rco and Rcn at this critical resonance weighting ... 

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

The role of IR spectroscopy in the early penicillin structure studies has been described (B-49MI51103) and the results of more recent work have been summarized (B-72MI51101). The most noteworthy aspect of a penicillin IR spectrum is the stretching frequency of the /3-lactam carbonyl, which comes at approximately 1780 cm" This is in contrast to a linear tertiary amide which absorbs at approximately 1650 cm and a /3-lactam which is not fused to another ring (e.g. benzyldethiopenicillin), which absorbs at approximately 1740 cm (the exact absorption frequency will, of course, depend upon the specific compound and technique of spectrum determination). The /3-lactam carbonyl absorptions of penicillin sulfoxides and sulfones occur at approximately 1805 and 1810 cm respectively. The high absorption frequency of the penicillin /3-lactam carbonyl is interpreted in terms of the increased double bond character of that bond as a consequence of decreased amide resonance, as discussed in the X-ray crystallographic section. Other aspects of the penicillin IR spectrum, e.g. the side chain amide absorptions at approximately 1680 and 1510 cm and the carboxylate absorption at approximately 1610 cm are as expected. 

The reported systems are always hydrogenated derivatives or compounds which exhibit full conjugation only in one mesomeric resonance structure, e.g. amides. 

The decomposition of dioxetanone may involve the chemically initiated electron-exchange luminescence (CIEEL) mechanism (McCapra, 1977 Koo et al., 1978). In the CIEEL mechanism, the singlet excited state amide anion is formed upon charge annihilation of the two radical species that are produced by the decomposition of dioxetanone. According to McCapra (1997), however, the mechanism has various shortfalls if it is applied to bioluminescence reactions. It should also be pointed out that the amide anion of coelenteramide can take various resonance structures involving the N-C-N-C-O linkage, even if it is not specifically mentioned. 

Reactive trajectories, 43-44,45, 88,90-92,215 downhill trajectories, 90,91 velocity of, 90 Relaxation processes, 122 Relaxation times, 122 Reorganization energy, 92,227 Resonance integral, 10 Resonance structures, 58,143 for amide hydrolysis, 174,175 covalent bonding arrangement for, 84 for Cys-His proton transfer in papain, 141 for general acid catalysis, 160,161 for phosphodiester hydrolysis, 191-195,... 

Show how resonance can occur in the following organic ions (a) acetate ion, CH,CO, (b) enolate ion, CH,COCH5, which has one resonance structure with a C=C double bond and an —O group on the central carbon atom (c) allyl cation, CH,CHCH,+ (d) amidate ion, CH,CONH (the O and the N atoms are both bonded to the second C atom). 

The RAHB effect may be illustrated by the ubiquitous C=0- -H—N hydrogen bond of protein chemistry. As shown in Section 5.2.2, the simplest non-RAHB prototype for such bonding, the formaldehyde-ammonia complex (5.31c), has only a feeble H-bond (1.41 kcalmol-1). However, when the carbonyl and amine moieties are combined in the resonating amide group of, e.g., formamide, with strong contributions of covalent (I) and ionic (II) resonance structures,... 

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

The mechanism of acid hydrolysis is also different in acyclic amides and /1-lactams acid catalysis of acyclic amides proceeds via O-protonation (see Chapt. 4), whereas that of /1-lactams appears to be a unimolecular A1 type process, involving V-protonation (Fig. 5.6,b) [76], A-Protonation is not the result of reduced amide resonance but an intrinsic property of the /1-lactam structure, since bicyclic /1-lactams and monocyclic /1-lactams exhibit similar reactivity and behavior [76],... 

The ring torsional angles of the amide and O-protonated amide moieties in these structures are close to zero and the C(O)—N bond length is shorter in the protonated lactam (445) (1.298 A) than it is in the lactam itself (444) (1.337 A), as expected from the dominant resonance structures in these two compounds. The room temperature 15N and 13C NMR spectra of the lactam (444) and its O-trimethylsilyl derivative have been determined but do not give much information about the solution conformation of these compounds (76JA5082, 82JMR(46)163>. 

The amino groups of the nucleic acid bases may, in principle, undergo relatively free rotation. In fact, however, they must be considered as having a significant amide character as a result of a partial double-bond character of the C-N bond due to resonance structures of types 25b and 25c. 

Amide resonance is a powerful stabilizing force and gives rise to a number of structural effects. Unlike the pyramidal arrangement of bonds in ammonia and amines, the bonds to nitrogen in amides lie in the same plane. The carbon-nitrogen bond has considerable double-bond character and, at 135 pm, is substantially shorter than the normal 147-pm carbon-nitrogen single-bond distance observed in amines. 

From simple one-dimensional experiments, qualitative information regarding secondary structure can be obtained. Shown in Figure 5a are the regions of the ID NMR spectrum containing the amide and aromatic proton resonances for GSM (2) and one GS14 diaste-reomer, GS14K2J12 The GSM spectrum shows 12 well-resolved amide resonances (each is a... 

As in tertiary amides, the primary electronic effect (n-i ) in imidates corresponds to the delocalization of two electron pairs, one from the nitrogen and one from the oxygen atom, which is normally represented by resonance structures 46, 46, and 47. The central atoms 1C, N, and 0) of the imidate function are therefore sp hybridized and this is confirmed by X-ray analysis (32) which shows that this function is planar. As a consequence, an imidate function can exist in two different conformations, the anti or the syn form. 

In amides, the nitrogen electron pair is n-r conjugated with the carbonyl group and this electronic delocalization is normally expressed by resonance structures 1, 2, and 3. As a result, the amide function is essentially planar and it is assumed that the three atoms (C, ii, and 0) of this function are sp hybridized. The amide function can be illustrated in three dimensions by structure 4. The electronic distribution can also be viewed as the result of the delocalization of two n electron pairs, one from the oxygen atom and one from the nitrogen atom (cf. 1 and 3 versus 2) and on that basis, it is referred to here as the primary electronic delocalization of the amide function. 

Rotation about the carbon-nitrogen bond is required to average the environments of the two methyl groups, but this rotation is relatively slow in amides as the result of the double-bond character imparted to the carbon-nitrogen bond, as shown by these two resonance structures. 

An understanding of the internal rotation about the amide bond is important because of its relevance to protein structure. Formamide is the simplest amide. The coplanarity and the remarkable rotational barrier about the C-N bond in formamide can be rationalized by resonance between the n electrons of the carbonyl group and the lone pair of the nitrogen atom [1, 50]. According to VB theory, the Jt electronic structure of formamide may be described by six resonance structures. 

Contribution from resonance structure 3, which contains a formal double bond between carbon and nitrogen, is considered to be primarily responsible for the coplanarity and the high rotational barrier about the amide bond [58], The introduction of resonance structure 3 also implies that there is significant charge-delocalization from the nitrogen lone pair to the carbonyl oxygen. 

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