Anisole singlet

Equivalent protons. All hydrogens which are in identical environments have the same chemical shift and therefore absorb at the same frequency they are said to be chemically equivalent. This can arise in two ways. Firstly, the protons are equivalent if they are bonded to the same carbon atom which is also free to rotate. For example, the three protons in a methyl group are equivalent and appear as a singlet (see the spectra of toluene, anisole or acetophenone above), and the two protons of a methylene group, provided that it can rotate freely, are identical and appear as a singlet (see the spectrum of phenylacetic acid above) frequently this is not the case with methylene groups in cyclic systems where rotation is restricted. 

In this respect, it is of interest that the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile proceeds with retention of ( )-olefin geometry in acetonitrile from the first excited singlet state of the anisole chromophore [160] (Scheme 40). The ortho adduct undergoes further... 

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

Quenching of the exciplex fluorescence by a third molecule may lead to a termolecular complex called triplex. It was first observed by Beens and Weller [47] and has since been shown to be a general phenomenon [48-53]. Each exciplex shows a marked preference for quenching by donor (DQ) or acceptor (Aq). The species approximately represented by Ds+. .. A8+ should be quenched from the D side by Dq, i.e., Dq+. .. D8+. .. A8-, or from the A side by Aq, i.e., Ds+. .. A8-... F Aq Caldwell s results showed a quenching preference when the interaction occurred with the exciplex component of lower singlet energy. Thus the exciplex of 9-cyanophenanthrene (9-(NP) and p-(isobutenyl) anisole (p-BA) clearly prefers Aq, while the exciplex of phenanthrene and FN prefers DQ. 

Figure 8. Charge distribution of anisole in the ground state and in singlet and triplet excited states (see Lodder and Havinga, 1972).

A closely related tetrad featuring two porphyrin moieties and a single naphthoquinone acceptor has also been reported [13]. Excitation of either porphyrin moiety of C-P-P-Q in anisole solution is followed by rapid (>10 s ) singlet-singlet energy transfer between the two porphyrins, whose absorption and emission spectra are essentially identical. C-P- P-Q decays by photoinduced electron transfer to the quinone with a rate constant of 2.4 x 10 s. Sequential transfer of the radical cation hole to the second porphyrin, and then to the carotenoid yields a final C +-P-P-Q state with a quantum yield of 0.25 and a lifetime of 2.9 ps. 

N-CIDNP was used to investigate photoinduced nitrations of aromatic compoimds ArH with tetranitromethane. The polarizations invariably arise from radical pairs ArH NO, but the experiments reveal different pathways of formation of these pairs. With a substrate such as 1,2-dimethoxybenzene, the precursor multiplicity is triplet and the pairs are produced by a dissociative electron transfer from the aromatic compound to tetranitromethane, which then cleaves into an NO2 radical and C(N02)3. In contrast, the very similar substrate anisole (methoxybenzene) exhibits polarizations indicating a singlet precursor, and the radical pair is thought to be formed by decomposition of a preceding unstable diamagnetic intermediate, most probably a nitro-trinitromethyl adduct. ... 

The role of singlet and triplet nitrenium ions in these reactions has been evaluated.228,229 Convincing evidence for the participation of singlet ground-state 2-acetylphenyl nitrenium ions during the photodecomposition of 3-methylanthranils in concentrated sulfuric acid has been obtained.229 For example, in the presence of arenes or anisole, in addition to the usual products, substantial amounts of diphenylamines and 3- and 5-(p-methoxy-phenyl)-2-aminoacetophenones, respectively, are formed by electrophilic attack of the nitrenium ion on the added substrate. 

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