N, tt* excited state

Free-radical chemistry is important in organic photochemistry as, generally speaking, all organic photochemical reactions of in, ) or 3(n,tt ) excited states and all reactions of 3(tt,Jt ) excited states result in the formation of either a pair of radicals or a biradical. We shall consider... 

One example of interest is the hydrogen abstraction process by the triplet n-tt excited state of a ketone. The correlation diagram is shown in Fig. 11. 

A further distinction between (n, n ) and n, tt ) excited states results from the selection rules for electronic transitions which, as can be seen in the orbital diagram above, classify the former as partially forbidden — essentially because of the poor spatial overlap between n and n orbitals, and the latter as allowed. In consequence molar extinction coefficients for typical n- n transitions lie in the range 10 — 10 M cm whereas those for n n transitions range from 10 —10 cm, a matter... 

In the same way, 3-cyclopenten-l-one was considered in both the singlet fundamental and the first (n — tt ) excited states. The geometries were also optimized at RHF and HPHF levels, respectively, using the same basis set [28. ... 

Resonance representation of a carbonyl n.TT excited state. (Adapted from reference 92.)... 

Spectroscopic studies show that for benzophenone and most other ketones, the n-TT excited states and Tj are of lower energy than the tt-tt excited states. An energy diagram depicting the excited states of benzophenone (along with one that depicts those of naphthalene) is shown. 

Table 6 Energy values (in a.u.) of A(n, tt ) excited state of formic acid for the two minima and two planar conformations.

Generalized Photophysical and Photochemical Properties of Ketones and c/s-Azoalkanes with Lowest n,tt Excited-State Configuration ... 

In view of recent findings regarding the ease of hydrogen abstraction by n TT relative to tt ir excited states (Hammond and Leermakers, 1962b), it is tempting to propose that an n —> tt excited state (S or T ) is involved in the initial abstraction step of Reaction (116). A plausible mechanism for the reaction might involve formation of the enol as an intermediate, Eq. (117). 

The question of reactivity of it,it versus n,tt excited triplets has provoked considerable speculation and diverse interpretations of existing data. One fairly common hypothesis is that only n,n states react, while tt,tt states are totally unreactive. At the other extreme stands Porter s analysis,365 with tt,-tt states being perhaps one-tenth as... 

As observed in many theoretical studies of symmetric radicals (92), a HF wavefunction describing it ->- tt excited states preferentially localizes the hole on one moiety leading to a broken symmetry structure of the states with unequal bond lengths for the minimum energy structure. Similarly, for the n tt ... 

W. M. Nau, G. Greiner, J. Wall, H. Rau, M. Olivucci, and M. A. Robb, Angew. Chem. Int. Ed. Engl., 37,98 (1998). The Mechanism for Hydrogen Abstraction by n,tt Excited Singlet States Evidence for Thermal Activation and Deactivation Through a Conical Intersection. 

The involvement of upper excited states following irradiation of the phthalocyanine complexes Rh(Pc)(MeOH)X (where X = Cl, Br, or I) has been investigated. All compounds have the same action spectrum for photoinduced H-abstraction and the emission at 420 nm is attributed to relaxation of an upper tt, tt ) excited state. At high photonic fluxes it has been shown that biphotonic photochemistry involves the n, tt state. A study of the photoaquation of [RhCNH,), ] in fully and partially hydrated zeolite Y has been reported. Reaction within the cavities occurs with a quantum yield that is only 15—20% of that in aqueous solution this is attributed to the decrease in mobility of the water in the zeolite and to the exclusion of water from the ligand-exchange site by the zeolite lattice. ... 

On the other hand, very new calculations by Ishikawa et al. reject the possibility of inversion. They predict fast isomerization on n Tt excitation via a conical intersection with the ground state near 90 on the rotational coordinate. These authors agree with Cattaneo and Persico On exci-... 

Unlike alkenes, the unsaturated azo group in N=N or C=N derivatives possess an inplane lone electron pair in an n-orbital. As a result, both the n,Jt and Tt.Tt excited states can be involved in two limiting mechanisms of photoisomerization.1062 1080 1081 The first mechanism is a 180° rotation (twist) about the former double bond in conjunction with a reduced bond order, similar to that observed in alkenes (Scheme 6.157). In the second case, an in-plctne inversion proceeds due to rehybridization of one nitrogen without... 

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