H₂ Production in the 440-nm Photodissociation of Glyoxal

Abstract

H 2 has been detected following the photolysis of glyoxal at 440 nm using the techniques of vacuum ultraviolet laser-induced fluorescence and (2 + 1) resonance enhanced multiphoton ionization. It is thus confirmed that a fraction of the glyoxal excited at this wavelength dissociates into three photofragments: HCOCOH f H 2 + 2 CO. The most populated vibrational level of those observed was H 2 (V ) 1), and in this level rotational states from J ) 0-9 were detected. Doppler profiles of these lines provide estimates of the translational energy and show a v||J correlation. Of the available energy to the H 2 + 2CO products, 3.1% appears as rotational energy in H 2 (V ) 1), 17.8% appears as the H 2 (V ) 1) vibration, and 46.8% appears as H 2 (V ) 1) translation. Excitation of the 7 0 2 band produces somewhat more of the H 2 + 2CO channel than does excitation of the 0 0 0 , 5 0 1 , or 8 0 1 bands. These observations are consistent with a model in which transfcis isomerization precedes dissociation. Rotational excitation with v||J is caused by the V 7 torsional motion. The small degree of rotational excitation, the production of H 2 in V ) 1 and V ) 2, and the translational energy distribution are all consistent with ab initio calculations of the transition state structure.

Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals. III. Comparison with Born–Oppenheimer dynamics

“…The photofragmentation of glyoxal, C 2 H 2 O 2 , is one of the best studied examples of a synchronous three-body fragmentation ͑see Ref. 74 for a summary of the experimental work͒. Intersystem crossing from the first excited state, S 1 , is induced by collisions and the triplet dissociates to H 2 CO ϩCO.…”

Photodissociation of glyoxal: Resolution of a paradox

“…[1][2][3][4][5][6][7][8][9][10][11][12] The observation of H 2 as a product confirms this pathway, since there is not enough energy to produce H 2 by secondary fragmentation of HCOH or H 2 CO ͑barrier of Ϸ80 kcal/mol or more͒. [1][2][3][4][5][6][7][8][9][10][11][12] The observation of H 2 as a product confirms this pathway, since there is not enough energy to produce H 2 by secondary fragmentation of HCOH or H 2 CO ͑barrier of Ϸ80 kcal/mol or more͒.…”

Glyoxal photodissociation. II. An ab initio direct classical trajectory study of C2H2O2→CO+H2CO

“…[1][2][3][4][5][6][7][8][9][10][11][12] In 1980, Parmenter and co-workers presented evidence suggesting that glyoxal dissociated in the absence of collisions. 5 This work was followed by extensive experimental as well as theoretical efforts.…”

“…50% with the remainder of the glyoxal undergoing internal conversion to S 0 with a high degree of vibrational excitation. 7,8,11,12 The long lifetime of the S 1 state ͑order of 10 Ϫ6 s͒ 1,10 provides ample time for vibrational energy redistribution within the energized molecule. Internal conversion from S 1 to S 0 yields an excess energy of 63-65 kcal/mol.…”