Acetone, excited states

Yields of excited states from 1,2-dioxetane decomposition have been determined by two methods. Using a photochemical method (17,18) excited acetone from TMD is trapped with /n j -l,2-dicyanoethylene (DCE). Triplet acetone gives i7j -l,2-dicyanoethylene with DCE, whereas singlet acetone gives 2,2-dimethyl-3,4-dicyanooxetane. By measuring the yields of these two products the yields of the two acetone excited states could be determined. The yields of triplet ketone (6) from dioxetanes are determined with a similar technique. 

The peak in the UV VIS spectrum of acetone [(CH3)2C=0] corresponding to the transition appears at 279 nm when hexane is the solvent but shifts to 262 nm in water Which is more polar the ground electronic state or the excited stated... 

A simple aliphatic ketone such as acetone, when promoted to its n,n excited state, undergoes a single unimolecular photochemical reaction in high quantum yield namely a-cleavage giving a methyl and acetyl radical which react further in secondary dark processes. In general, competition... 

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

The colorless tetramer Ccu(py)l] is fairly stable only in non- or weakly coordination solvents such as benzene, CH Cl, or acetone. At room temperature in solution this copper complex shows an intense red photoluminescence ( 0.04 a f nax = nm (54). The emitting state is a metal-centered 3d 4s excited state which is strongly modified by Cu(I)-Cu(I) interaction in the tetramer. This consists of a (Cul) cubane core. 

As to the nature of the electronically excited state, the investigation of the thermolysis of tetramethyl-1.2-dioxetane revealed a high yield (about 50%) of excited triplet acetone 34> ... 

We can illustrate the application of PAC to a simple photochemical reaction. Acetone is readily excited to its singlet excited state which rapidly undergoes efficient intersystem crossing to its triplet state. The triplet state decays in solution primarily by radiationless decay. The PAC experimental waveforms obtained from the photoexcitation of acetone in air and argon-saturated cyclohexane are shown in Fig. 1. In addition, the waveform obtained from the calibration compound 2-hydroxybenzophenone is also shown. 

Nitrofuranes are converted into 3- (or 5-) -hydroxyimino-(3H)-furan-2-ones and 2-nitrop3UTol into 3-hydroxyimino-(3H)-pjuTol-2-one upon irradiation in acetone solution i ). Again, the results are rationalized best by assuming a nitrite intermediate. No experiments to delineate the multiphcity of the excited state responsible for this isomerization have been reported. 

Excitation of acetone with two photons at X = 307 nm delivers 186 kcal/mol of energy, more than enough to break both C—C bonds and give carbon monoxide and two methyl radicals. Following the process with femtosecond mass spectrometry shows that the excited state of acetone of 58 amu rises and falls very quickly, in a spike-like fashion. It is formed and decays with a time constant of 50 fs. For acetone-iig, a similar time dependence is seen The rise and fall of the 64 amu excited species take place in 80 and 80 fs (Fig. 20.5). 

Acetone in the S2 state gives an excited-state linear acetyl radical along with the methyl radical the radical exists in a double-well potential about the CCO angle of 180°. 

Irradiation of biacetyl (1) in isopropyl alcohol gives acetone plus a nearly quantitative yield of a mixture of diastereomeric pinacols in approximately equal amounts (2).44 46 The mechanism of this reaction is shown in the following scheme in which B is biacetyl, AH2 is isopropyl alcohol, and A is acetone. Superscripts indicate the multiplicity of the excited states. 

Figure 1.14 Relalive excited state energies for acetone and penta-1.3-diene...

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