Orbitals delocalised

Ferrocene Figure 2-47) provides a prime ex.ample of multi-haptic bonds, i.e, a situation where the electrons that coordinate the cyclopentadienyl rings with the iron atom arc contained in molecular orbitals delocalised over the iron atom and the 10 carbon atoms of the cyclopentadienyl rings [82. ... 

The schematic model is depicted in Fig. 8. As the bias voltage increases, the number of the molecular orbitals available for conduction also increases (Fig. 8) and it results in the step-wise increase in the current. It was also found that the conductance peak plotted vs. the bias voltage decreases and broadens with increasing temperature to ca. 1 K. This fact supports the idea that transport of carriers from one electrode to another can take place through one molecular orbital delocalising over whole length of the CNT, or at least the distance between two electrodes (140 nm). In other words, individual CNTs work as coherent quantum wires. 

The number of configurations required to construct a VB wavefunction of reasonable quality is considerably smaller in approaches implementing the Coul-son-Fischer idea to employ orbitals delocalised over more than one atom. Two of the more widely used modern VB methods, spin-coupled (SC) theory " and the generalised VB (GVB) approach, make full use of this idea by using orbitals constructed as LCAOs, just as in MO theory. Both of these methods do not include any ionic structures in the wavefunction for a neutral system. Other implementations... 

A totally different description seems in order, using orbitals delocalised over a bridging carbon and the bridged metal atoms [40, 1 (g)). 

DFT calculations of the DCNDBQT molecule revealed that the degree of 7i-orbital delocalisation of the cyano groups is very low compared to the thiophene ring system. Depending on the used calculation method, approx. 4.6% to 7.1% of the 7i-orbitals are situated at the CN-groups [23]. Thus, the n-system of the DCNDBQT is not weakened by the cyano groups. 

As described above, metallic CNTs are of great interest because they possess molecular orbitals which are highly delocalised. However, metallic CNTs are very difficult to use in actual devices because they require very low temperatures to control their carrier transfer. On the contrary, even at room temperature, the nonlinear /-V jas curve and the effective gate voltage dependence have been presented by using individual semiconducting SWCNTs [29]. 

Benzene has often been used as a test system for vibrational calculations using a variety of different electronic structure algorithms. The molecule exhibits regular hexagonal planar symmetry with six carbon atoms joined by a bonds and six remaining p-orbitals which overlap to form a delocalised n electron over all six carbon atoms. Table 1 shows comparisons of several different methods for benzene. 

According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

The special feature of the dithiocarbamato ligand is an additional 7r-electron flow from the nitrogen atom to the sulfur atoms via a planar delocalised rr-orbital system. The net effect is a strong electron donation, resulting in a high electron density on the metal. )... 

Fluorimetric methods of analysis make use of the natural fluorescence of the analyte, the formation of a fluorescent derivative or the quenching of the fluorescence of a suitable compound by the analyte. Fluorescence cannot occur unless there is light absorption, so that all fluorescent molecules absorb, but the reverse is not true only a small fraction of all absorbing compounds exhibits fluorescence. The types of molecule most likely to show useful fluorescence are those with delocalised ji-orbital systems. Often, the more rigid the molecule the stronger the fluorescence intensity. Naturally fluorescent compounds include Vitamin A, E (tocopherol). 

The stabilisation that can result by delocalisation of a positive or negative charge in an ion, via its n orbitals, can be a potent feature in making the formation of the ion possible in the first place (cf. p. 55). It is, for instance, the stabilisation of the phenoxide anion (23), by delocalisation of its charge via the delocalised n orbitals of the nucleus, that is largely responsible for the acidity of phenol (cf. p. 56) ... 

Mesomeric, like inductive, effects are permanent polarisations in the ground state of a molecule, and are therefore manifested in the physical properties of the compounds in which they occur. The essential difference between inductive and mesomeric effects is that while inductive effects can operate in both saturated and unsaturated compounds, mesomeric effects can operate only in unsaturated, especially in conjugated, compounds. The former involve the electrons in a bonds, the latter those in tt bonds and orbitals. Inductive effects are transmitted over only quite short distances in saturated chains before dying away, whereas mesomeric effects may be transmitted from one end to the other of quite large molecules provided that conjugation (i.e. delocalised tt orbitals) is present, through which they can proceed. 

With alcohols there is no such factor stabilising the alkoxide anion ROe, relative to the alcohol itself, and alcohols are thus very much less acidic than carboxylic acids. With phenols, however, there is again the possibility of relative stabilisation of the anion (2), by delocalisation of its negative charge through interaction with the n orbitals of the aromatic nucleus ... 

The exact reverse of the above is seen with aniline (13), which is a very weak base (pKa = 4-62) compared with ammonia (pKa = 9-25) or cyclohexylamine (pKa = 10-68). In aniline the nitrogen atom is again bonded to an sp2 hybridised carbon atom but, more significantly, the unshared electron pair on nitrogen can interact with the delocalised 7r orbitals of the nucleus ... 

A group with a more powerful (electron-withdrawing) inductive effect, e.g. NOa, is found to have rather more influence. Electron-withdrawal is intensified when the nitro group is in the o- or p-position, for the interaction of the unshared pair of the amino nitrogen with the delocalised it orbital system of the benzene nucleus is then enhanced. The neutral molecule is thus stabilised even further with respect to the cation, resulting in further weakening as a base. Thus the nitro-anilines are found to have related p a values ... 

This reflects the greater stability of a secondary rather than a primary carbocation shifts in the reverse direction can, however, take place where this makes available the greater delocalisation possibilities of the ir orbital system of a benzene ring (i.e. tertiary — secondary) ... 

Reference has already been made to the structure of benzene and, in particular, to its delocalised -ir orbitals (p. 15) the concentration of negative charge above and below the plane of the ring-carbon atoms is thus benzene s most accessible feature ... 

It might be expected that the first phase of reaction would be interaction between the approaching electrophile and the delocalised n orbitals and, in fact, so-called n complexes such as (1) are formed ... 

Pyridine (62), like benzene, has six n electrons (one being supplied by nitrogen) in delocalised n orbitals but, unlike benzene, the orbitals will be deformed by being attracted towards the nitrogen atom because of the latter s being more electronegative than carbon. This is reflected in the dipole of pyridine, which has the negative end on N and the positive end on the nucleus ... 

---