Delocalization electrons, resonance structures

Diffuse reflectance spectroscopy (DRS) of VO-porphyrins on reduced and sulfided catalysts exhibit shifts in the porphyrinic electronic spectra (Soret, a, (3 bands) to higher frequencies. Adsorption results in modification of the delocalized electronic resonance structure not observed on the oxide form of the catalyst. X-ray photoelectron spectroscopy reveals shifts to higher Mo binding energies on reduced and sulfided catalysts following VO-porphyrin adsorption, consistent with transfer of electrons from Mo electron donor sites to the V02+ ion. Interaction at the electron donor sites is stronger than interaction at electron acceptor sites typical of the oxide catalyst. This gives rise to the possibility of lower VO-porphyrin diffusion rates on sulfided catalysts, but this effect has not been experimentally demonstrated. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

Each peptide unit lies in a plane because it consists of a delocalized system of n electrons associated with n orbitals of the C and O atoms together with the lone electron-pair orbital of the N atom. Such an electron resonance structure is sufficient to produce significant diamagnetic anisotropy in the protein. " Some two dozen amino-acid residues make up polypeptides, but only glycine and proline have a first atom in the side chain R which is not a carbon atom. Thus, many features of regularity are present which may cooperate in forming energy bands, particularly in the extended arrays of a helices and ) -pleated sheets. In fact, spectra of DNA... 

Electron delocalization can be important in ions as well as in neutral molecules Using curved arrows show how an equally stable resonance structure can be generated for each of the following anions... 

These are the most important rules to be concerned with at present Additional aspects of electron delocalization as well as additional rules for Its depiction by way of resonance structures will be developed as needed in subsequent chapters... 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

In resonance terms electron delocalization map unsaturated carbonyl compounds IS represented by contributions from three principal resonance structures... 

Protonation of imidazole yields an ion that is stabilized by the electron delocalization represented in the resonance structures shown... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Most MO methods find a bond alternation pattern in the minimum-energy structure, but calculations that include electron correlation lead to a delocalized minimum-energy structure. Thus, although the n system in 1 is not completely planar, it appears to be sufficiently close to provide a delocalized 10-electron Ji system. A resonance energy of 17.2 kcal has been obtained on the basis of an experimental heat of hydrogenation. ... 

The stabilizing role of other functional groups can also be described in resonance terms. Both electron-attracting groups such as carbonyl and cyano and electron-donating groups such as methoxy and dimethylamino have a stabilizing etfect on a radical intermediate at an adjacent carbon. The resonance structures which depict these interactions indicate delocalization of the unpaired electron onto the adjacent substituents ... 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

Molecular orbitals are useful tools for identifying reactive sites in a molecule. For example, the positive charge in allyl cation is delocalized over the two terminal carbon atoms, and both atoms can act as electron acceptors. This is normally shown using two resonance structures, but a more compact way to see this is to look at the shape of the ion s LUMO (the LUMO is a molecule s electron-acceptor orbital). Allyl cation s LUMO appear s as four surfaces. Two surfaces are positioned near- each of the terminal car bon atoms, and they identify allyl cation s electron-acceptor sites. 

Electrons that are shown in different positions in a set of resonance structures are said to be delocalized. Delocalization means that a shared electron pair is distributed over several pairs of atoms and cannot be identified with just one pair of atoms. 

A clue to the nature of the third itt MO can be found in the placement of electrons in the two resonance structures for ozone, which are shown with color highlights in Figure 10-36a. Notice that in one resonance structure, the left outer atom has three lone pairs and a single bond, while the right outer atom has two lone pairs and a double bond. In the other resonance structure, the third lone pair is on the right outer atom, with the double bond to the left outer atom. The double bond appears in different positions in the two stmctures, and one of the lone pairs also appears in different positions. These variations signal delocalized orbitals. 

If the molecule contains multiple bonds, construct the n bonding system using molecular orbital theory, as described in this section and in the remaining pages of Chapter 10. Watch for resonance structures, which signal the presence of delocalized electrons. 

Resonance structures result from a phenomenon known as electron delocalization. The electron pairs in the three double bonds in a benzene ring are delocalized. These are electrons that belong to no particular atom or bond. As a consequence, no ordinary double bonds exist in a benzene ring. The electrons are in an orbital that extends across adjacent atoms. This smear of electrons is usually represented as a circle within the ring. 

The concept of electrons not belonging to any particular atom in a molecule brings us back to resonance structures. The electrons in a metal are also delocalized. An electron in a bar of sodium is not associated with any particular atom, just as the electrons in the double bonds of benzene are not associated with any particular atom. 

Resonance Molecules with two or more valid structures are said to be resonant. The actual structure is neither of the alternatives but a lower-energy molecule with delocalized valence electrons. Benzene with its alternating double and single bonds is an example of a resonant structure. Benzene actually has no single... 

In the nonbonding orbital two electrons are delocalized over the two fluorine atoms and do not contribute to the bonding, which is due only to the two electrons in the bonding orbital. This type of 3c, 4e bond is often denoted by a dashed line, as shown in Figure 2. Each P—F bond is effectively a half-bond, so this description of the bonding is roughly equivalent to the two resonance structures 1 and 2 ... 

The ESR spectrum of triplet anthronylidene lp was recorded after UV irradiation of 2p at 77 K.95-97 At 0.365 cm-1 the zero field parameter D in lp is larger than that in la, which indicates that the delocalization of the unpaired n -electron in lp is less efficient — despite the larger 7t-system — than in la. Obviously the phenoxyl resonance structure in la is more favorable than in lp. 

---