Photodissociation of a bicyclic azoalkane: Time-resolved coherent anti-stokes Raman spectroscopy studies of vapor-phase 2,3-diazabicyclo[2.2.1]hept-2-ene

Adams, J. Stephen,Weisman, R. Bruce,Engel, Paul S.

, p. 9115 - 9121 (1990)

The photodissociative mechanism of vapor-phase 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) has been studied with nanosecond-regime transient spectroscopic methods. Following excitation of the vibrationless S1 level at 338.5 nm, data from time-resolved CARS (a vibrational spectroscopy) show the appearance rate for formation of N2 to be 4 × 107 s-1. This value is significantly slower than the 5 × 108 s-1 principal component observed in S1 fluorescence decay, establishing that the dissociating state is not S1. CARS measurements on the nascent N2 photofragments reveal a vibrational distribution (84% v = 0, 12% v = 1) very similar to that observed earlier for the nitrogen formed in the stepwise photodissociation of azomethane. This result and the low level of nascent rotational excitation suggest that dissociation into N2 plus 1,3-cyclopentanediyl biradical occurs from an excited state of the diazenyl biradical that has a linear CNN bond angle. Transient CARS probing has also revealed the subsequent appearance of bicyclo[2.1.0]pentane formed by ring closure of the 1,3-cyclopentanediyl biradicals. Formation kinetics of this ring closure product shows a single first-order component with a rate coefficient of approximately 5.1 × 106 s-1. This observation implies that S1 excitation of vapor-phase DBH produces 1,3-cyclopentanediyl biradicals only in their ground triplet state. Mechanistic differences between the gas-phase photochemistries of DBH and acyclic azoalkanes are attributed to a low-lying excited state of the diazenyl biradical that becomes accessible in DBH through the release of ring strain energy.

Time-resolved infrared studies of triplet 1,3-cyclopentanediyl

Showalter, Brett M.,Bentz, Timothy C.,Ryzhkov, Lev R.,Hadad, Christopher M.,Toscano, John P.

, p. 309 - 312 (2000)

Triplet-sensitized photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (1) in argon- or oxygen- saturated acetonitrile-d3 solutions results in the formation of bicyclo[2.1.0]pentane (3), a ring closure product arising from an intermediate 1,3-cyclopentanediyl triplet biradical (2). Time-resolved infrared (TRIR) spectroscopy was used to monitor the kinetics of bicyclopentane 3 production. This analysis provides a measurement of the triplet biradical lifetime and an estimate of the bimolecular reaction rate between biradical 2 and oxygen, both in good agreement with previous investigations. Our studies also indicate that certain IR bands due to 3 in the C-H stretching region overlap with corresponding bands in biradical 2. This interpretation is supported by computational investigations. Copyright

Determination of the Enthalpy and Reaction Volume Changes of Organic Photoreactions Using Photoacoustic Calorimetry

Herman, Michael S.,Goodman, Joshua L.

, p. 1849 - 1854 (1989)

Photoacoustic calorimetry (PAC) can be used to measure both the thermal and reaction volume changes for photoinitiated reactions.The photoreactions of 2,3-diazabicyclohept-2-ene (DBH), diphenylcyclopropenone (DPC), and trans-stilbene (TS) are investigated by PAC.The resolution of these experimental volume changes is accomplished by either a temperature dependence or a binary solvent mixture method.The thermal volume changes yield the enthalpies of reaction in solution, which can be compared to literature values.In two cases (DPH and DPC), the values are moreendothermic than those predicted from gas-phase heats of formation.The differences can possibly be attributed to differential solvation of the reactants and products in the polar solvents employed.Absolute reaction volume changes for the photoreactions are also obtained for the photoreactions.PAC is a useful alternative technique to pressure-dependence studies to obtain this information.These volume changes can further be time-resolved to provide kinetic information about the photoprocesses.

Meinwald et al.

, p. 731 (1967)

Viscosity dependence of the denitrogenation quantum yield in azoalkane photolysis: Experimental evidence for reversible formation of the diazenyl diradical

Adam, Waldemar,Corley, David A.,Trofimov, Alexei V.,White, Rick C.

, p. 4277 - 4280 (2002)

(equation presented) Experimental evidence is reported for the reversible formation of the singlet diazenyl diradical (1DZ), photolytically generated from the structurally elaborate DBH-type azoalkane. Reversiblity of the 1DZ formation manifests itself through the decrease of the photodenitrogenation quantum yield over a ca. 40-fold viscosity variation (from 0.5 to 19.3 cP). This viscosity behavior is interpreted in terms of frictional effects on the competitive reaction modes of the diazenyl diradical.

On the Mechanism of the Benzophenone-Sensitized Photolysis of 2,3-Diazabicyclohept-2-ene in the Laser Jet: Evidence for Intermolecular Triplet Diradical Reactions

Adam, Waldemar,Finzel, Ralf,Walther, Barbara

, p. 2137 - 2142 (2007/10/02)

The benzophenone-sensitized laser jet photolysis of 2,3-diazabicyclohept-2-ene (1) affords, besides the previously reported cyclopentene and housane (2), also cyclopentane, cyclopentadiene, and the dimers bicyclopent-2-en-1-yl (7), 3-cyclopentylcyclopent-1-ene (8), and 1,1'-bicyclopentyl (9).As a model reaction, the pyrolysis of dimer 8 at 600 deg C/ 20 Torr leads to the other dimers 7 and 9 together with cyclopentadiene, cyclopentene, and traces of cyclopentane.Control experiments showed that H abstraction by the cyclopentane-1,3-diyl diradical (3) from cyclohexene (as model substrate for cyclopentene) and addition to housane (2) with formation of diradical 6 are unlikely pathways.Instead, the product data available can be best explained in terms of an intermolecular disproportionation of two diradicals 3 to give the cyclopent-2-en-1-yl (4) and cyclopentyl (5) radical pair, which is subsequently converted to the observed products by in-cage and out-of-cage coupling and H transfer reactions.Such intermolecular diradical chemistry becomes feasible due to the high steady-state concentrations (ca. micromolar) generated in the laser jet.Two-photon processes take place, but are of subordinate importance. - Key Words: Laser jet/ Azoalkane/ Diradical/ Radical coupling