Excited‐state kinetics of azoalkanes some further studies with azoethane, azo‐n‐propane, and azoisopropane

Abstract

The photochemistry of azo-n-propane is investigated a t 366 nm up to 1 atm pressure, and over a range of temperature from 50 to 190OC. Some additional experiments with azoethane a t room temperature and azoisopropane a t 180 and 190°C are also reported. From a consideration of the pressure dependence of the quantum yields for photodissociation a generalized mechanism is proposed which accounts for the known experimental observations in acyclic azoalkane photochemistry. These observations include the extensive photoisomerization data which were previously obtained for azoisopropane. In the mechanistic scheme dissociation a t low pressures is believed to occur mainly from Sy and Ty, the vibrationally excited and randomized first excited singlet and triplet states. A t high pressures and low temperatures (5 100°C) the major dissociation channel is probably a nonrandom SI state.In direct or singlet sensitized photolysis isomerization occurs predominantly at high pressure and is postulated to occur by internal conversion from S:, the thermalized singlet, to the ground state. During the process partitioning to the cis or trans isomer is equally probable. In triplet sensitized photolysis isomerization occurs via intersystem crossing from TI to the ground state. A t elevated temperatures (>150°C) dissociation from Sy, which has a significant activation energy, can compete with return to the ground state.

Triplet Energy and Lifetime of 2,3-Diazabicyclo[2.2.2]oct-1-ene

“…Photolysis of azoalkanes has long been used to produce radicals in both the gas phase and solution. , The question of which excited state leads to nitrogen loss was addressed beginning in the 1960s. − To evaluate the role of triplet azoalkanes as intermediates in both decomposition and trans−cis isomerization, attempts were made to observe the triplet spectroscopically. However, with two exceptions, these attempts were unsuccessful.…”

Spectra of propylperoxy radicals and rate constants for mutual interaction

“…Indeed, even if hi011(02) were so much less than this that a remained essentially constant, the only result would be to increase k:oll(O~) by less than a factor of 2. Further, since nearly all the dissociation of ANP occurs from the triplet state ( k $ = 4 X. lo8 L/mol s, k~s c = 8 X lo9 s-l [25]), the conclusion is not invalidated by the possibility of an enhanced intersystem crossing rate.…”

“…Values for k$, kz, and kIsc have been given for AIP and A P by Pritchard et al [25], and we have also measured kzand kcoll(Nz) [22]. Assuming k,,11(02) = k,,dNz) = 1.1 X 1O1O L/mol s, it is readily calculated that the presence of 4.5 torr of 0 2 will reduce the yield of isopropyl radicals by <1%.…”

“…The first assumption is readily justified by considering the rate constants for the reactions In discussing the second assumption we consider the relevant part of the mechanism proposed by Pritchard, Marchant, and Steel [25] for the photolysis of azoalkanes:…”