Glyoxal, excited

Siese and Zetzsch (1995) and Bohn and Zetzsch (1998) have studied the OH-C2H2 reaction using FP-RF (see Chapter 5.B.3) and observed biexponential decays of OH. They propose that the adduct I has two channels in its reaction with 02, rather than one as shown above, and that one of the two generates OH and glyoxal, a small portion of which is excited and decomposes to HCO. 

The lowest excited triplet states of a-dicarbonyl compounds are considerably less energetic than those of simple carbonyls. For instance the energy of the vibrationally relaxed triplet of glyoxal is 55 kcal,366 as compared to 72 kcal for formaldehyde. Irradiation of glyoxal at 4358 A populates the lowest vibrational levels of the first excited singlet, 30% of which fluoresce and 70% of which cross over to the triplet manifold.388 Almost all of the triplet molecules then decompose to formaldehyde and carbon monoxide, the phosphorescence yield being only 0.1%. 

It was suggested that vibrationally hot H2CO formed in reaction 19 decomposed to form a H2 molecule and CO which was plausible since this kind of dissociation is known to occur in formaldehyde. They also suggested that there must be another primary process, since their product yields could not be reconciled with just these two primary process. Hepburn et al. (56) did a TOF experiment in which they excited the 8 q band of glyoxal at 439.8 nm. They were able to show from TOF measurements of mass 28 that there was indeed a third primary process, that is, corresponding to the third low energy peak. 

Preliminary measurements on the LIF spectra of the CO produced in the photolysis of glyoxal in the 8 transition have also been reported (57). The results show that CO is formed within in the 0.87 ys lifetime of glyoxal and that the complexity of the observed spectra indicates that the CO is formed vibrationally excited. It will be interesting to see what the final vibrational distribution of the CO will be in light of the previous demonstration that there are three primary processes that lead to CO. 

F. Glyoxal, (CHO)2.—There have been a number of calculations on the ground state of this molecule, and very recently a more detailed study of several excited states. 

With respect to intersystem crossing, it appears that the cutoff between small-molecule behavior and the intermediate case is at six atoms, with formaldehyde totally in the small-molecule category, glyoxal a borderline case depending on excitation energy, and propynal intermediate in class. The five-atom carbonyl ketene unfortunately possesses a unique photochemistry which prevents its use in studies similar to those which have been carried out on formaldehyde and glyoxal. 

Fluorination and chlorination cause a large decrease in the nonradiative decay rates. The intersystem crossing rate constant drops from 4 x 10 s for acetone to 1.5 x 10 s- - for hexafluoroacetone. For the cyclobutanone/perfluorocyclobutanone pair, the rates are 4.2 x 10 s- - and 1 x 10 s l, respectively. A similar decrease is likely for the glyoxal/oxalyl fluoride pair, where zero-pressure tp values of 2.4 ps and 24 ps, respectively, have been measured (19,25,26). The fluorescence yields of these compounds must be measured before the true extent of this effect is known. Mixed chlorofluoroacetones exhibit nonradiative rates of intermediate value, with k jR increasing at low energy excitation with increasing chlorination from 2.7 x 10 s l for chloropentafluoroacetone to 3.9 x 10 s l and 8.0 x 10 s l for dichlorotetrafluoroacetone and trichloro-trifluoroacetone, respectively (94). 

Electronic Spectrum. The A X and B - X absorption spectrum of biacetyl is similar to that of glyoxal. The first absorption band occurs in the 470- to 340-nm region. The 0-0 band is placed at 21,983 cm l (111), and there is some vibrational structure at wavelengths greater than 400 nm. The second absorption band occurs in the 280- to 220-nm region and consists of continuous absorption only. The emission occurs only when the A state is excited. 

There have been some other studies where the PARS technique has been applied, e.g. in a medium-resolution experiment, in which a direct comparison between CARS and PARS spectra of the bands of glyoxal and of methane at pressures of 4 and 6.7 kPa, respectively, excited under similar conditions could be made (Duval et al., 1986). The CARS profiles showed a broader appearance which the authors attribute to contributions of the real part of the nonlinear susceptibility, however, this assumption should be proved by calculation of both profiles. 

Vibrational predissociation of glyoxal.H2 complexes studied for several vibrational levels of the first excited singlet state of glyoxal... 

Elementary Excitation in the Superfluid Clusters. The existence of a roton-type collective excitation spectrum in large ( He)jy clusters N = lO -lO ) at 0.4 K was established from electronic spectroscopy of large molecules (e.g., glyoxal [70]) which manifests coupled electronic-roton excitations [70]. 

Molecules on Excited Electronic Surfaces. Data similar to that discussed here has been obtained for a variety of molecules on excited electronic state surfaces. Examples include rare gases bound to glyoxal (27), benzene (28), tetrazene (29) and stilbene... 

Unlike the case of collision-induced vibrational energy transfer, collision induced rotational energy transfer seems to be free of strong restrictions on the changes in the rotational quantum numbers. When account is taken of the spectral widths of the excitation sources used, the nature of the rotation-vibration structure in the fluorescence and absorption spectra, and the possibility of resonant ener f transfer in the collision, it is concluded that the studies of Bj aniline are the weakest, those of B2 benzene better, and those of glyoxal the best available. With this hierarchy of quality of information kept in mind, the following weaker conclusions can also be obtained from the studies cited. 

Lavolee and Tramer have observed collision-induced intersystem crossing from perturbed A H levels of CO by utilization of synchrotron excitation. These diatomic examples should provide a more quantitative test of the theoretical principles which are also applicable (with some appended summations over coupled states) to larger molecules like glyoxal. 

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