Singlet form

Sensitizer singlet formed Intersystem crossing of sensitizer Energy transfer to reactant molecule... 

Photosensitized decomposition of 9b in substituted benzenes led to similar results [83DIS(B)(44)1113]. Therefore, either spin inversion from the triplet to the singlet form of 4-diazoimidazole is faster than the decomposition of the excited diazo compound, or intersystem crossing from the triplet to singlet carbene is easier and faster than the reaction of the triplet state with substrates. 

Several explanations can be offered for the reactivity of the naph-thaldehydes. The reaction could be taking place with (/) the 77,77 triplet (2) the n,ir triplet, which could be formed on the way to, or in some cases, be in thermal equilibrium with, the 77,77 triplet or (5) the h,tt singlet, formed either directly upon absorption or on the way to the triplet. It is not possible, at this time, to definitely rule out any of these possibilities. However, there is evidence indicating that, in the case of naphthaldehyde, the 77,77 triplet can be the reactive state involved, since the reduction is sensitized by triplet-triplet transfer with sensitizers having a triplet energy such that only this state is activated.47... 

This reaction can be viewed as an internal hydrogen abstraction which takes place through a highly strained four-membered transition state instead of the usual six-mem bered one. The formation of the enol is wavelength dependent and is retarded by triplet quenchers.58-80 In water, excitation into the second excited singlet of biacetyl formed the enol with a quantum yield of 0.10, while excitation into the first excited singlet formed the enol with a quantum yield of 0.01, possibly via a higher excited state formed by a triplet-triplet reaction. Therefore, the authors conclude that the second excited triplet is the state that isomerizes to the enol. 

The confusion generated in the initial report185 on photoracemization of sulfoxides has recently been removed with the postulate that naphthalene singlet forms an excited complex with sulfoxides.186 Thus, despite the fact that the singlet state of 36 lies at 113 kcal, some 23 kcal above that of naphthalene, 36 quenches the fluorescence of this hydrocarbon with kes = 3.2 x 107Af -1 sec-1. From the dependence of the quantum yield of racemization on sulfoxide concentration (Eq. 34), a value of kes = 2.3 x 107Af-1 sec-1 was deduced. Since these values are the same within experimental error, it follows that the singlet state of naphthalene is responsible for photoracemization. 

Life with carbenes is substantially complicated by the fact that there are two different forms (singlet and triplet) of CH3 and presumably of all other carbenes. The two forms of CH2 differ considerably in their reactivity. One is the singlet, which has its unshared electrons paired, while the other is the triplet with the same electrons unpaired. For CH2, the singlet form is the less stable and more reactive, whereas with CC12, the triplet is the less stable and more reactive. 

Exercise 14-25 Devise atomic-orbital models of the singlet and triplet forms of CH2. Of these one has a much greater H—C—H angle than the other. Deduce whether the triplet or the singlet form should have the wider H—C—H angle. (Remember the Pauli principle, Section 6-1.)... 

It should be noted that for aqueous systems a suitable dye was not easily found, since many water-soluble dyes apparently undergo a photoreaction in the presence of triplet oxygen, which is photochemically transformed into its reactive singlet form due to sensibilization by the dye [41]. After degassing with argon, good results for aqueous systems have been achieved with Orange I [42]. 

In the absence of flow, the aggregation kinetics of interacting dipolar particles has been studied by See and Doi (1991) (see Fig. 8-5). The time-dependence of the aggregation process seems to be reasonably well described by a hierarchical model, in which singlets form doublets after a time ti, doublets form quadruplets after a time t2, and so on. By... 

Such endoperoxides generated from anthracene and its homologues release oxygen in singlet form when heated [17, 18, 19, 1116]. This reversible reaction is typical of rubrene (tetraphenylnaphthacene), a red compound that yields a colorless endoperoxide (equation 136) [18, 19, 1116]. 

It is believed that, because of the presence of the halogen atoms, the singlet form, with the electrons paired, is the.more stable form of dichlorocarbene, and is the one adding to the double bond. (Stabilization by the halogen atoms is presumably one reason why dihalocarbenes do not generally undergo the insertion reaction that is so characteristic of unsubstituted singlet methylene.)... 

The singlet form of carbenes substituted with a carbonyl group is much lower in energy than the triplet, so the photolysis of a-diazocarbonyl compounds gives singlet carbenes. The photo-Wolff rearrangement (Chapter 2) of a-diazoketones occurs upon photolysis of these compounds. 

The structures of 2,5-dihydro-1,2,3-triazines were supported by H-NMR spectroscopy. Thus the proton attached to N-2 appears in the range <5 8.3-9.02 and methylene protons at position 5 appear at <5 2.45-2.80 ppm. The singlet form of the signal of these protons indicates fast ring inversion of the compound. 

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