N-o delocalization

Furthermore, the oxygen atom of the carbonyl group in the amide function has an electron pair oriented antiperiplanar to the polar C-N bond there is therefore an electronic delocalization caused by the overlap of that oxygen electron pair orbital with the antibonding orbital of the C-N sigma bond (o ) as shown in two dimensions by structure 5 and in three dimensions by structure . This additional n-o delocalization is referred to here as a secondary electronic delocalization. Thus, amides are similar to esters because they both have the primary electronic effect and one secondary electronic effect. This is in contrast with Z esters which have two secondary electronic effects besides the primary electronic effect. 

Like 8d [109], the X-ray structure of 8a [108] exhibited a distorted square pyramidal phosphorus. The structural parameters were similar to those obtained by DFT calculations (B3LYP/6-31G ) for 8a and 8b. Calculations on smaller model systems with different substituent pattern, including the tetracoordinate phos-phirenium cation (see Scheme 11), allowed elucidation of the bonding, which was described by a n - o delocalization [108], This result is basically in agreement with the proposal of Regitz and coworkers [109], who used resonance structures to account for delocalization. 

There are only a few fimctional groups that contain an unpaired electron and yet are stable in a wide variety of structural environments. The best example is the nitroxide group, and numerous specific nitroxide radicals have been prepared and characterized. The unpaired electron is delocalized between nitrogen and oxygen in a structure with an N—O bond order of 1.5. 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

Almost all known inorganic heterocychc molecules, where N, O and S atoms with lone pair orbitals are donors while B atoms with vacant p orbitals are acceptors, are classified into discontinuous conjugation. The donors and the acceptors are alternately disposed along the cyclic chain. The thermodynamic stabilities are controlled by the non-cycUc electron delocalization or by the number of neighboring donor-acceptor pairs, but not by the number of % electrons [83]. In fact, both 4n % and 4n + 2% electron heterocycles are similarly known [84,85] (Scheme 33), contradicting the Hueckel rule. 

In the ring molecules containing a n bond, delocalization of tt electrons occurs through the interaction with o orbitals [19]. 

The four leading NRT structures are of approximately equal weighting (17.9%). As seen in Table 3.21, the C—C bond orders are in the range 1.46-1.49 and the N—O bond order is 1.51, which is indicative of strong delocalization in the nitro group as well as in the ring. 

Figure 5.43 The optimized structure of CT-deleted H3N- NO+ (with valence antibonds of NO+ deleted to suppress intermolecular nN-7tNO delocalization). The shortest intermolecular N- O and N- -N distances are 2.99 and 3.11 A, respectively, and the net binding energy is 20.4 kcal mol-1 (cf. Fig. 5.42 and Table 5.23).

The chemistry of nitroxide radicals has been extensively reviewed (e.g. Forrester et al., 1968 Rozantsev, 1970 Rozantsev and Scholle, 1971 Aurich and Heiss, 1976 Berliner, 1976 Keana, 1978). They are -radicals, the unpaired electron occupying a if orbital between the oxygen and nitrogen atoms [4]. Since there is also an N—O o-bond, and two electrons fill a -bonding orbital between these atoms, the effective N—O bond order is 1.5. The nitroxide function is frequently represented as a hybrid of the two structures [5a] and [5b]. The result of this electron delocalization is a relatively stable structure. An alternative view of the absence of any significant tendency... 

The - is delocalized over both O s so that each can be assumed to have a - j charge. Each N—O bond has the same bond length. 

Both (I) and (III) have a double bond not present in (II) that permits delocalization by extended n bonding. Delocalization is more effective in (III) because charge is delocalized to the electronegative O. 

The structural indices of aromaticity, I, of oxadiazoles (145-148), thiadiazoles (150-153) and selenadiazoles (155, 156) are compared with that of the parent furan (144), thiophene (149) and selenophene (154) (Scheme 11). 1,2,3-Oxadiazole (145) is the least stable among them since all attempts to synthesize this compound were unsuccessful, most likely because of its easy isomerization to the acyclic isomer. At the same time its sulfur analogue (150) possesses good stability and has been synthesized. Its 2,4-diaza- (151), 3,4-diaza- (152) and 2,5-diaza-(153) isomers demonstrate even more the extent of n-electron delocalization. There exists a well-known tendency of decreasing aromaticity depending on the type of pyrrole-like heteroatom S > Se > O. However, there is no uniformity in the change in aromaticity in the horizontal rows, i. e., dependence on heteroatom disposition. 

N—0 <—> 0—N—O), nitrile oxide (—C=N—0 <->—C=N—O )—typically has four n electrons in an orbital system delocalized over three centers. These reactions, like the Diels-Alder, are easily accomplished. 

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