N/rr* state

Both reactions proceed via triplet excited species and, to some extent, are controlled by whether the ti-tt (path A) or n-rr states are involved. The di-rr-methane rearrangement pathway is restricted to 4-aryl- or 4-vinylcyclohexenones. At the most basic level of... 

This raises the question as to why one should observe any photoreduction at all with those compounds in which the lowest triplet clearly is of it -> rr character. The answer to this question lies in the fact that the electronic states are not pure in character. That is, an n - it state has some character obtained from a mixing of other electronic levels. This mixing results from vibronic and spin-orbit coupling/51-52 Likewise a it - w state has a degree of n —> w character obtained by mixing. It is this degree of n—> ir character which is responsible for the observation of photoreduction... 

Turro69 has reported that several alkyl ketones react with 1,2-dicyano-ethylene (12) by nucleophilic attack of the ketone (rr, n) singlet state to give oxetanes stereospecifically. Addition of diene quenchers which are known to deactivate ketone triplets do not affect the rate of oxetane formation. This... 

In some transitions, the polarity of the chromophore is weaker after absorption of radiation. One case of this is the n — jt absorption due to the carbonyl present in ketones in solution. Before absorption, the C+-0 polarisation stabilises in the presence of a polar solvent whose molecules will be clustered around the solute because of electrostatic effects. Thus, the n —> -rr electronic transition will require more energy and its maximum will be displaced towards a shorter wavelength, contrary to what would be observed in a nonpolar solvent. This is the hypsochromic effect. Because the excited state is readily formed, the solvent shell around the... 

Thus far the discussion has centered on n-ir excited states and their reactions, t-t photochemistry is equally intriguing but more difficult to discuss from a mechanistic viewpoint. In the n-rr excited state the localized orbital from whence the electron is promoted and the ir system receiving the promoted electron are separated from one another and each is subject to qualitative valence bond description in fact, the ir system becomes that of a metal ketyl which is a well-known species in organic chemistry. Furthermore, in the ir system of the excited state there are no vacant low-energy molecular orbitals. 

Some possible dispositions of (n, rr ) and (ir, n ) states are given in Figure 5.8 for a few representative molecule types. 

In most simple aldehydes and ketnoes, including benzophenone, the longest wavelength absorption is a low intensity n - rr transition. The promotion of a n-electron, localized on O-atomto a re-orbital, leaves behind a positive hole on this atom. The charge density on C-atom is increased creating a bipolar state. The dipole moment of >C = O bond is reduced. Three primary processes are commonly encountered for this electrophilic centre ... 

Hydrogen abstraction can occur from a position within the ketone molecule, and this generates a biradical that may cyclize bv combination ol the radical centres. The overall photocydization process is observed for a wide variety of compound types, and it has been used extensively to make cyclic or polycyclic systems. In an unconstrained system a ketone (n,rr ) excited state shows a preference for abstraction from the y-position (4.42), which can be understood on... 

Photoreduction of the excited state of phenazine by THF occurs via the (n, TT ) and (rr, rr ) states of phenazine. The photodimerization of deazaflavin has been investigated, in particular its excitation in the presence of oxalate anion, which causes photodimerization with quantum yield Both electrochemical reduction and photochemical reaction of some i-(2-... 

Co(L-proma)(L-promp)], [Co L-proma)(ida)] and [Co(L-promp)(ida)] (L-promp = L-proline-iV-monopropionate), and it was found that all the isomers obtained in solid state have tm/i5(N)-RR or trotts(N)-R configuration. 

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