Partial double bond character

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

In resonance terms the shorter carbon-oxygen bond distance in phenol is attrib uted to the partial double bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring... 

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

The Peptide Bond Has Partial Double Bond Character... 

On boiling the methiodide with 70% sulfuric acid an N-methyl-oxo derivative was obtained, and this in turn gave 3-amino-2-phenyl-quinoline, methylamine, and ammonia on fusion with soda lime. The bulk of the evidence therefore favors quaternization at N-2 (cf, 154), in which case the acid-hydrolysis product is 155. Quaternization at N-2 would be expected because of the steric influence of the 10-phenyl group and the influence of the 4-amino group (cf. 4-hydroxy-pyridazine ) in the pyridazine-type ring, although the partial double-bond character of that ring is probably different from that in pyridazine itself. 

Figure 14.3 Electrostatic potential maps of 1,3-butadiene (conjugated) and 1,4-pentadiene (nonconjugated) show additional electron density (red) in the central C—C bond of 1,3-butadiene, corresponding to partial double-bond character.

There is some experimental data from nuclear magnetic resonance5 25 and sonic30 observations which indicate that a bond which would be expected to have a partial double bond character because of conjugation with an adjacent double bond will show a barrier intermediate between single and double bond values. 

Although it is rather certain that electrostatic interactions of polar groups, steric hindrance, and partial double bond character due to conjugation will all be of importance in selected molecules, the explanation of the barrier in ethane probably requires something else. Though far from being proven and certainly not now useful for prediction, the idea that the ethane barrier arises from repulsion of C—H bond orbitals on the carbons, due to their being more concentrated than sp hybrids, seems the most plausible picture available. 

The effect of this partial double bond character on the chemical properties of chlorine atoms conjugated to double bonds is well known it corresponds in the main to a diminution in reactivity. The correlation with bond angles is discussed in a later section of this paper. 

The unusual carbon-halogen bond strength in vinyl halides compared to saturated alkyl halides has been ascribed to partial double-bond character (103, 104) coupled with increased a-bond strength (105) due to differences in... 

Structural data from X-ray- and electron-diffraction studies, mass spectra, and MO calculations were analyzed for R3SiCo(CO)4 (R = H, F, Cl) and Me3SiMn(CO)5 it was believed that there is partial double-bond character between Si and Co and that there may be intramolecular interaction between the axial silicon atom and the equatorial carbonyl groups 30, 212). 

Although peptides are written as if a single bond finked die a-carboxyl and a-nitrogen atoms, this bond in fact exhibits partial double-bond character ... 

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. 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

The question of electronic conductivity in the polyphosphazenes inevitably raises questions regarding the electronic structure of the phosphazene linkage.7-12 This matter has been the subject of controversy in the literature, but experimentally the situation is now well known.4,13 In spite of the fact that the phosphazene backbone is fully conjugated, bond equalized and possesses bond lengths which are indicative of partial double bond character, the evidence suggests that these are localized systems. 

Alkyl-CON H2/-CON H R 9-7 Often broad but frequently couple. Primary amides often appear as two broad signals due to partial double bond character of amide bond. Often slow to exchange and may require warming/mild base... 

Certain types of bond, whilst nominally being considered as single , have in fact, sufficient double bond character , to render rotation about their axis, restricted . The one you are most likely to encounter, is the amide bond. Partial double bond character exists between the carbonyl, and the nitrogen, and may be represented as in Structure 6.12 ... 

The peptide bond is characterized by a fixed planar structure, as was discovered by X-ray crystallography of peptides more than 60 years ago. The arrangement of the atoms in the peptide bond is due to resonance stabilisation the lowest-energy state of the system is that in which the four atoms forming the peptide linkage lie in a plane, while the C-N bond has partial double bond character. 

The C2-C3 overlap gives the central bond partial double bond character and allows the four tt electrons of 1,3-butadiene to be delocalized over all four atoms. 

Peptides and proteins are composed of amino acids polymerized together through the formation of peptide (amide) bonds. The peptide bonded polymer that forms the backbone of polypeptide structure is called the a-chain. The peptide bonds of the a-chain are rigid planar units formed by the reaction of the oc-amino group of one amino acid with the a-carboxyl group of another (Figure 1.1). The peptide bond possesses no rotational freedom due to the partial double bond character of the carbonyl-amino amide bond. The bonds around the oc-carbon atom, however, are true single bonds with considerable freedom of movement. 

These dimeric complexes involve, in their neutral state, two metal atoms in the (III) oxidation state. In the vanadium complexes such as [CpV(bdt)]2 and [CpV(tft)]2, the V—V bond length, 2.54 A in [CpV(bdt)]2, are shorter than observed in model complexes with a single V—V bond, indicating a partial double-bond character, also confirmed by a measured magnetic moment of 0.6 fiB in [CpV(tfd)]2, lower than expected if the two remaining unpaired electrons contribute to the magnetic susceptibility [20, 49]. This class of complexes most probably deserves deeper attention in order to understand their exact electronic structure. 

The first systematic studies of polyene radical cations were carried out by Shida and coworkers230 using the above methods. In this connection an important feature of these species was discovered, namely that they exist in the form of multiple rotamers which do not interconvert easily due to the partial double-bond character which all polyene C—C bonds attain upon ionization (see below). 

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