Transient CARS studies of the vapor phase photodissociation of azoalkanes

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A fundamental question in the chemistry of azoalkanes concerns the nature of their primary bond cleavage: do the two C-N bonds break in a synchronous or a sequential fashion? The photodissociation of an acyclic, unsymmetrical azoalkane, 3-(methylazo)-3-methyl-1-butene (MAMB), was studied using time-resolved coherent anti-Stokes Raman spectroscopy (CARS) to probe for product formation. Appearance kinetics were measured for all three primary photoproducts following excitation. The 3-methyl-1-buten-3-yl radical fragment appeared within 2 ns of excitation, while the methyl radical and N\sb2 were formed through decay of a reaction intermediate (the methyldiazenyl radical) having a lifetime of 12 ± 2 ns. These results directly demonstrate sequential photodissociation. Similar investigations of photodissociation in the bicyclic azoalkane, 2,3-diazabicyclo (2.2.1) hept-2-ene (DBH), were also performed. After excitation to S\sb1, DBH dissociated to form N\sb2 and a biradical, 1,3-cyclopentadiyl which was observed to decay through ring closure with a lifetime of 235 ± 50 ns. The nitrogen photoproduct shows an appearance that has a risetime of 20 ± 5 ns. The nascent vibrational distribution of the nitrogen photoproduct was found to be 82 ($\pm4)\pm4)-4)\sp{-1}$ for its average vibrational energy content. Observations of the vibration-rotation contour of the precollisional nitrogen revealed a rotational population distribution peaked at low values of J. Such a distribution suggests a transition state in which both C-N bonds are equally stretched and symmetric forces act on the nitrogen centers as the fragments separate. Fluorescence from DBH vapor excited to its S\sb1 origin was directly observed to have a rapid dual exponential decay, implying that DBH undergoes an intermediate-case radiationless transition to discrete levels of T\sb1 within 5 ns. A photochemical mechanism proposed to explain these findings involves single-step electronic predissociation of DBH from its T\sb1 surface to form N\sb2 plus triplet 1,3-cyclopentadiyl biradical. By contrast, acyclic azoalkanes are suggested to dissociate through a competing two-step process whose initial step occurs from a twisted T\sb1 structure that is not accessible in DBH.

Doctor of Philosophy
Physical chemistry

Adams, James Stephen. "Transient CARS studies of the vapor phase photodissociation of azoalkanes." (1989) Diss., Rice University.

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