Transient CARS studies of the vapor phase photodissociation of azoalkanes

[摘要] 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 $pm$ 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 $pm$ 50 ns. The nitrogen photoproduct shows an appearance that has a risetime of 20 $pm$ 5 ns. The nascent vibrational distribution of the nitrogen photoproduct was found to be 82 ($pm$4)% in v = 0, 13 ($pm$4)% in v = 1, and 5 (+2/$-$4)% in v = 2, giving a value of 535 cm$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.

[发布机构] Rice University