Photoproduct Channels from BrCD₂CD₂OH at 193 nm and the HDO + Vinyl Products from the CD₂CD₂OH Radical Intermediate

Abstract

We present the results of our product branching studies of the OH + C 2 D 4 reaction, beginning at the CD 2 CD 2 OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD 2 CD 2 OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD 2 CD 2 OH), and of the secondary dissociation of vibrationally excited CD 2 CD 2 OH radicals (OH, C 2 D 4 / CD 2 O, C 2 D 3 , CD 2 H, and CD 2 CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD 2 CD 2 O and DBr + CD 2 CDOH, and measure the branching ratio between the C−Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at m/e = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C 2 D 3 ) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C 2 D 3 product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the β-hydroxyethyl radicals.

Predicting the effect of angular momentum on the dissociation dynamics of highly rotationally excited radical intermediates

“…The resulting angular momenta were used to predict the portions of the total P(E T ) corresponding to stable and unstable radicals, which agreed quite well with the experimentally measured speeds of the stable radicals. In these experiments, an unexpectedly high branching to the vinyl + H 2 O channel was observed 21,22 in comparison to the expected branching to OH + ethene (the lowest barrier dissociation channel). The vinyl + H 2 O channel was proposed to occur via a "frustrated" dissociation or "roaming" pathway by Kamarchik et al 23 who performed quasiclassical trajectory calculations on vibrationally excited β-hydroxyethyl radicals.…”

“…The vinyl + H 2 O channel was proposed to occur via a "frustrated" dissociation or "roaming" pathway by Kamarchik et al 23 who performed quasiclassical trajectory calculations on vibrationally excited β-hydroxyethyl radicals. Womack et al 22 proposed that the unexpected branching to vinyl + H 2 O may be the result of the change in inertial tensor en route to the transition state. This inertial change has the effect of decreasing the energy difference between the respective OH and H 2 O loss transition states as the rotational energy increases, and as the radicals in those experiments were highly rotationally excited, that may be an explanation for the higher-than-expected branching to vinyl + H 2 O.…”

“…The ground electronic state dissociation of each of these radicals was studied in prior work 16,22 on the photodissociation of 2-bromoethanol-d 4 and a 70/30 mixture of 1-bromo-2propanol and 2-bromo-1-propanol at 193 nm. High-level calculations [25][26][27][28][29][30][31][32][33][34][35] of the minima and transition states on the potential energy surfaces for these radicals show that the dissociation to OH + ethene-d 4 (from the β-hydroxyethyl-d 4 radicals) or OH + propene (from the 1-hydroxy-2-propyl and 2-hydroxy-1-propyl radicals) proceeds via a transition state with no barrier beyond the endoergicity.…”

Radical Intermediates in the Addition of OH to Propene: Photolytic Precursors and Angular Momentum Effects

“…The first model is an extension of one used by Ratliff et al 42 and Womack et al, 43 which is used to predict the rotational energy imparted to the nascent radicals from the C−Br photofission. As in the prior work, the model uses the measured recoil kinetic energy distribution and the geometry of the precursor in the Franck−Condon region of the repulsive excited state to estimate the distribution of angular momenta in the nascent radicals.…”

“…Consequently, there is less energy available to the vibrational modes of the molecule, resulting in a higher fraction of stable radicals than in the absence of any rotation. To predict the rotational energy imparted to the nascent recoiling C 3 H 6 OH radicals we use a method similar to that used by Ratliff et al 42 and Womack et al, 43 but while their model used the scalar value of inertia, our model includes the full tensor of inertia, as the former is correct only for rotation about a principle axis. 44 This method uses conservation of angular momentum to determine the amount of rotational energy imparted to the recoiling C 3 H 6 OH fragments as a function of the recoil velocity vector along the C−Br bond.…”