Triple bonds resonance structures

For NRT bond order calculations reported in Tables 11.2 and 11.3, we employ SNRTSTR keylists to force consistent inclusion of alternative single-, double-, and triple-bonded resonance structures in each state. A sample Gaussian input file to compute the final entries of Table 11.3 for the C-state at / = 1.60 A is given below ... 

Isocyanides, also known as isonitriles or carbylamines, are characterized by a primary isocyanide (R-NC) functional group and pungent odor. According to the Hiickel, MNDO, and ab initio molecular orbital calculations, the electronic configuration of isocyanide is represented by an iminocarbene resonance hybrid (Eq. 7.1) [1]. Experimental studies have confirmed that the resonance hybrid is closely represented by the triple-bonded resonance structure [2], Electronically, isocyanides resemble that of carbon monoxide (Eq. 7.2) and recently, isocyanides have been used as substitutes for carbon monoxide in organometallic transformations [3]. 

Following the synthesis of metal carbyne complexes, the first metalladiyne derivative was prepared by treatment of W =C(OEt)C=CPh (CO>5 with BX3 (X = C1, Br, I) (pentane, -45°C) to give rranj-W(=CC=CPh)(X)(CO)4 (334 Scheme 77) in good yields (30-60%). Subsequent reactions with NHMea give W sCCH=CPh(NMc2) (X)(CO)4 by addition to the C=C triple bond, the structure of which indicates a contribution from the vinylidene resonance form. ... 

Nitrous oxide is a good example of a molecule that shows resonance. The structure on the left in Figure 7.6 has two double bonds, with two lone pairs on each of the distal atoms. The structure on the right has a triple bond. Neither structure fully describes nitrous oxide, nor does either structure actually exist. The real nitrous oxide molecule is a resonance hybrid of the two Lewis structures. 

Pristine poly-DCH. Fig. 2 shows the C CP-MAS spectra of unreacted N-dicarbazolyl-2,4-hexadiyne and of the polymerized material. The appearance of a new peak at 129 ppm in the spectrum of poly-DCH, resulting from the formar tion of a double bond, is evident from the spectra. Furthermore, polymerization causes a marked downfield shift of the triple bond resonance(s) from 68.3 and 75.0 ppm in the diacetylenic monomer to 104.3 ppm in poly-DCH. This effect is well-documented for many polycoi jugated systems containing triple bonds (12,21), and presumably reflects an increasing cumulene contribution to the original enyne electronic structure of such systems. 

Carbon dioxide has a linear structure. The simple double-bonded formula, however, does not fully explain the structure since the measured carbon-oxygen bond lengths are equal but intermediate between those expected for a double and a triple bond. A more accurate representation is, therefore, obtained by considering carbon dioxide as a resonance hybrid of the three structures given below ... 

The correction term in Eq. (9) shows that the basic assumption of additivity of the fragmental constants obviously does not hold true here. Correction has to be appHed, e.g., for structural features such as resonance interactions, condensation in aromatics or even hydrogen atoms bound to electronegative groups. Astonishingly, the correction applied for each feature is always a multiple of the constant Cu, which is therefore often called the magic constant . For example, the correction for a resonance interaction is +2 Cj, or per triple bond it is -1 A detailed treatment of the Ef system approach is given by Mannhold and Rekker [5]. 

STRATEGY Write a Lewis structure for the molecule by using the method outlined in Toolbox 2.1. Decide whether there is another equivalent structure that results from the interchange of a single bond and a double or triple bond. Write the actual structure as a resonance hybrid of these Lewis structures. 

The 1,3-dipolar cycloaddition reactions to unsaturated carbon-carbon bonds have been known for quite some time and have become an important part of strategies for organic synthesis of many compounds (Smith and March, 2007). The 1,3-dipolar compounds that participate in this reaction include many of those that can be drawn having charged resonance hybrid structures, such as azides, diazoalkanes, nitriles, azomethine ylides, and aziridines, among others. The heterocyclic ring structures formed as the result of this reaction typically are triazoline, triazole, or pyrrolidine derivatives. In all cases, the product is a 5-membered heterocycle that contains components of both reactants and occurs with a reduction in the total bond unsaturation. In addition, this type of cycloaddition reaction can be done using carbon-carbon double bonds or triple bonds (alkynes). 

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

C—All the other answers involve species containing only single bonds. Substances without double or triple bonds seldom need resonance structures. 

A comparison of the calculated and observed bond lengths shows that neither structure is correct. Nevertheless, these contributing (resonance) structures tell us that the actual resonance hybrid has some double-bond character between N and O, and some triple-bond character between N and N. This state of affairs is described by the non-Lewis structure... 

Sixteen-Electron. Those for which the dipolar canonical form has a double bond on the sextet atom and the other resonance structure has a triple bond. Examples are azides (R—N3), diazoalkanes (R2C=N=N), and nitriloxides (R—C= N—O). These have also been labeled as propargyl/allenyl anion type [270]. 

The same situation is encountered with a,/ -alkynic phosphonium salts in which C(a) carbons are strongly shielded vs C p) carbons 47.8-73.0 vs 119-127 ppm36,97. The linear correlation of the C(a) and C(P) chemical shifts with a para Hammet constant was illustrative of the polarization of the 7r-electron system in the C=C triple bond (Figure 8). In the same sense, the observed C(flt) and C(/J) chemical shifts were used to argue for the existence of either resonance hybrid structures as in the case of enaminophosphonium salts98,99 or incipient ylide contribution40. 

A decision cannot be made between structures A, B, and C, which are, indeed, so closely similar in nature that there can be no large energy difference between them. Moreover, they satisfy the other conditions for resonance they involve the same number of unpaired electrons (zero), and they correspond to about the same equilibrium configuration of the nuclei (linear, for a central tetrahedral atom forming either two double bonds or a single bond and a triple bond). We accordingly expect the normal state of the molecule to correspond to resonance among structures A, B, and C, with small contributions by the other leas... 

In benzaldehyde and many other similar molecules, on the other hand, the resonance effect directs toward the meta positions, this resulting whenever the substituted group R contains an electronegative atom and a double or triple bond conjugated with the benzene ring (R = COOH, CHO, NO, COCHa, SOaH, CN, etc.). The structures leading to this effect, F, G, and H, are of the types... 

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