Ji-orbital

Figure B3.1.7. Left- and right-polarized orbital pairs involving ti and ji orbitals.

Highest occupied ji orbital of a photochemically excited olefin... 

Carbon-centered organic radicals are highly reactive trivalent species with only one nonbonding electron. While most known radicals have their unpaired electron in a pure p- or a delocalized Ji-orbital, there are also examples of radicals centered in s/t" hybrid o-orbitals, such as the well known phenyl and cyclopropyl radicals. The first radical reported in the literature is credited to Gomberg s landmark paper in 1900 when he postulated the formation of triphenylmethyl radical 36, also known as tri-fyj 99,100 jj-jjyj j-adical is an example of a persistent radical that exists in equilibrium... 

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). 

Through an interaction of the two singly occupied ji orbitals, a transannular six-center four-electron 71 -ji bond with a formal bond order of 0.67 results... 

A systematic route into non-fused derivatives appears to be from the reactivity of [S4][AsF6]2 and [Sg][AsF6]2 with alkynes.87 The equi-molar mixture of S42+ and Sg2+ appears to act as if it were S3+ although there is little evidence of this species in solution itself. The reactivity of this hypothetical S3+ radical appears to mimic that of the closed-shell SNS+ cation but with an additional electron in a ji orbital. Using this method Passmore has isolated 7 (R=CF3, R=C02Me). 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

A bonding orbital always is lower than the antibonding orbital if they are derived from the same atomic orbitals. 

Figure 2.13 n and k molecular orbitals associated with the >C=C< chromophore. Both the n- and Ji -orbitals lie in the plane of the paper... 

No photoreactions, except incidental photodestruction are known for hexa- and higher helicenes. As already mentioned in 2.1.1. pentahelicene forms benzo[g,h,i]perylene on irradiation in the presence of iodine as an oxidizing agent. Remarkably, the symmetrical benzoderivative (111) does not yield a similar cyclization product. This is ascribed to the antibonding character of the ji-orbitals at C-(l) and C-(14) involved in the expected photoreaction as appeared from EHMO calculation 17b). 

Fig. 25. Interactions of the 3dyz (3dxz) atomic orbital of the heme iron with the ji-orbitals of the ligand nitrogen atoms. The arrows indicate likely pathways for electron spin delocalization from the iron to the ligands...

Nucleophilic attack involves electron donation to the ji orbital of the carbonyl however, the relative reactivity does not reflect the order of these LUMO energy levels. To account for the order it is necessary to consider both the bond-making and bondbreaking processes, that is we need to mix the LUMO with the LUMO + 1 The... 

The 2-chloroallyl cation generated by matrix isolation has been studied by FT-IR spectroscopy and calculations [MP2(fc)/6-311G(d,p) level].235 Structure 78 of Cs symmetry with bridging chlorine, proposed earlier,162 was found to be less stable by 7.5 kcal mol 1 than cation 79 of C2v symmetry and could not be found by spectroscopy. Furthermore, in contrast to 1- or 3-chloroallyl cations, the centrally positioned chlorine does not contribute to charge delocalization through back-donation as a consequence of the Ji-orbital noninteraction between the n electrons of Cl and the LUMO of the allyl cation.236... 

Stevens and Lipscomb s method, for diatomics, avoids the self-consistency error by writing the first-order perturbed wavefunction in terms of the Ji-orbitals, since the ground-state wavefunction involves only [Pg.96]

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