Ketene photodissociation

Photolysis of dimethylketene, which underwent thermal dimerization into tetramethyl-cyclobutane-1,3-dione, gave tetramethylcyclopropanone 1, together with 2,3-dimethylbut-2-ene (from dimethylcarbene dimerization) and carbon monoxide (from ketene photodissociation). Formation of the three-membered ring most probably arises from the addition of dimethylcarbene to dimethylketene. ... 

The simplest way to model a photodissociation reaction is by using a one-dimensional picture, such as the dissociation of a diatomic molecule into two atoms. The molecule starts out in the bound ground-state potential with the bond distance constrained near the equilibrium separation. One can also consider the dissociation of a polyatomic molecule in this way instead of using an interatomic distance, one can use either the length of the breaking bond or the distance between the centers of mass (the line of centers) of the two fragments as the dissociation coordinate. Two examples of such a model are shown in Figure 1 for methyl iodide and ketene photodissociation. 

The first correlated measurement of ketene photodissociation found in the literature was that of the Brookhaven group, using absorption spectroscopy to measure Doppler profiles of singlet CH2 fragments in low rota-... 

Carbon monoxide metastable TOF spectra for ketene photodissociation at 351 nm were taken from jco = 3-18. There was no difference in the shapes of the TOF spectra when the polarization of the probe laser was changed with respect to the flight path. Thus, our results are not sensitive to whatever vector correlations may exist in this photodissociation and the line strength factor of Eq. 38 can be ignored. The resulting F( lrans) functions reveal that... 

Figure 20. The total CH2 rotational distribution from ketene photodissociation at 351 nm as measured by the metastable TOF method is shown as a solid line with error bars. The symbols show the contributions to the total CH2 rotational distribution from each experimentally derived state-specific P( ,ran3) measured (Oico = 3, DJco = C Oico = x ico = +7co = 15, and Z Zco = 18). These contributions have been weighted by the experimentally determined population of each jco state measured.

Recent studies indicate that the mechanism of ketene photodissociation is wavelength dependent as the energy of the exciting radiation increases, new primary processes become energetically allowed [W. L. Hase and P. M. Kelley, J. Chem. Phys. 66, 5093 (1977)]. 

IX-E-6.5. Estimation of the Photolysis Erequencies for Ketene Photodissociation within the Lower Troposphere... 

Hase, W.L., and P.M. Kelley (1977), A bimolecular mechanism for ketene photodissociation in the near ultraviolet, J. Chem. Phys., 66, 5093-5099. 

Morgan, C.G, M. Drabbels, and A.M. Wodtke (1996), State-specific neutral time-of-flight of CO from ketene photodissociation at 351 nm The internal energy distribution of CH2(X Bi), J. Chem. Phys., 105, 4550-4555. 

Gezelter J D and Miller W H 1996 Dynamics of the photodissociation of triplet ketene J. Chem. Phys. 104 3546-54... 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

Zabransky and Carr (257,258) have also studied the photodissociation of ketene between 313 and 214 nm. Between 313 and 250 nm, CH2(3Bq) and CH2(1A1) are again produced in an approximately constant ratio. No wavelengths between 250 and 214 nm were studied. At the latter wavelength, it is postulated that a third state, the CH2(b3B ) methylene state, is also formed in addition to the 3Bi and 3Aq states. 

CO i.r. fluorescence has been used to determine the energy disposal in CO following the 193, 248, and 308 nm photodissociation of ketene. Vibration-ally excited CO was found to be formed only in the 193 nm photolysis, and its vibrational and rotational distributions could be characterized by the temperatures Tv = 3750 K and Tr=6700K, A non-linear photoexcitation mechanism was proposed for photolysis at this wavelength. 

Laser photodissociation of ketene at 230 nm has been investigated in molecular beams. The experimental rovibrational population distribution has been compared to predictions from phase-space theory for the channels leading to CO + CH2(a Ai) and CO + CH2(b Bi). The calculations are not compatible with the latter channel, suggesting that it does not contribute significantly to the dissociation process. The photodissociation of singlet ketene by two-step IR + UV excitation has been studied using state-selective detection of CH2 by laser-induced fluorescence, and the results compared with... 

Miscellaneous Reactions. A study of the photodissociation of ketene under jet-cooled conditions in the 193-215 nm wavelength range has shown that bond fission to afford H atoms is a principal reaction.Acryloyl chloride undergoes C-Cl bond fission on irradiation at 193 nm. The photodecarbon-ylation of the cyclopropenones (54) occurs quantitatively on irradiation in methanol solution to yield the corresponding alkynes. The quantum yield for decarbonylation for the alkyl substituted derivatives is in the range 0.2-0.3, while for the aryl derivatives = 0.7. A determination of the reaction dynamics for the photodissociation of diphenylcyclopropenone has shown that the dissociation occurs from the So state. 

A new statistical quantum mechanical approach - the statistical adiabatic product distribution method has been applied to the photodissociation of ketene in parallel theoretical and experimental studies. The main focus of the work, however, was H-atom production via C-H cleavage rather than the elimination of CO. The photodecomposition of formohydroxamic acid (HCONHOH) has been investigated in matrix-isolation FTIR and DFT stud-ies. " Irradiation of the acid in Ar or Xe matrices with the full output of a xenon arc lamp generated H-bonded HNCO- H2O and NH20H- CO complexes. In the latter case, the IR spectra also suggest the existence of a structure with the NH2 group interacting with the carbon atom. 

Photodissociation lasers based on vibrationally excited HF and CO produced in the far-u.v. flash photolysis of FCHO and CH2CO respectively, have been reported. In formyl fluoride there was no evidence for high vibrational excitation in the CO since no laser action was observed, and the vibrational energy in the HF only accounted for ca. 7% of the available excess energy. There was evidence of rotational excitation in the HF but it is likely that the deficit is associated with electronic excitation of the CO at the wavelengths employed (A > 165 nm). In the photodissociation of ketene... 

The mechanism of photodissociation of ketene as indicated by the equations of processes (I) and (II) hides some interesting molecular contortions that occur during photolysis. An unexpected observation of Russell and Rowland (1968) showed that up to a 10% exchange of carbon atoms in ketene occurred when " CH2=C=0 was photolyzed at 334 nm. They suggested that this occurred through the intermediate formation of oxirene-like intermediate ... 

IX-E-6.4. Quantum Yields of Photodissociation of Ketene in the Lower Troposphere... 

Drabbles, M., C.G Morgan, D.S. McGuire, and A.M. Wodtke (1995), The first vibronically resolved measurement of correlated-prodnct-state distributions in ultraviolet photodissociation ketene at 308 nm, J. Chem. Phys., 102, 611-614. 

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