Cyclobutanone, singlet excited states

Cyclobutanone and 3-Cyclopenten-l-one in the first singlet excited states. 

Table 3. Main geometry parameters (in A or degrees), barrier heights (in cm" ), and mean values for cyclobutanone and 3-cylopenten-l-one in their fundamental and lowest singlet excited states.

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

Considering the "empirical rules" developed in the previous section, we can develop a simple general hypothesis to help us to understand all the results which conform to these rules direct excitation of cyclobutanones results in the formation of a reactive singlet state which predominantly undergoes a-cleavage to a singlet biradical,... 

The electronic state responsible for reactions II and III is not so well known and it is almost certain that there are essential differences in this respect between the different cyclic ketones. On the basis of the different pressure dependence of propene and cyclopropane formation in the direct photolysis of cyclobutanone, McGee concluded that cyclopropane is formed from the excited singlet state, while propene is considered to be a triplet product. 

The effect of substituents on the temperature dependence of a-carbonyl-carbene reactivity has been examined using carbenes generated by low-temperature photolysis of methyl diazophenylacetate. A correction to the literature on the photoreaction of isopropylidene diazomalonate (98) with 1,3,3-trimethyl-cyclohexane (99) has been reported. The photoproduct, originally thought to be a cyclopropane derivative, has now been shown to be the cyclobutanone (100), the formation of which presumably involves a photo-Wolff rearrangement as illustrated in Scheme 11. Substituent effects observed in the product distribution of diazo-amide photochemistry have been ascribed to conformational factors the jS-lactam, oxindole, and Wolff rearrangement products appear to arise directly from the excited singlet state of the sym-Z form of the diazo-amide itself. 

Cyclobutanone was considered in both the singlet fundamental and the first excited (n — tt ) state [16]. The geometry was fully optimized, at the RHF and HPHF levels, respectively, using a dummy atom at the center of the molecule X), and the minimal basis set [7s,3p/2s,lp] [28]. 

The model permits to determine spectroscopic constants, as well as to optimize the geometry in the lowest excited singlet state. The theoretical structures, inversion angles and barrier heights obtained for cyclobutanone and... 

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