Primary photodissociation pathways of epichlorohydrin and analysis of the C–C bond fission channels from an O(P3)+allyl radical intermediate

Abstract

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H(2)COCH)CH(2)Cl. The three dominant photoproduct channels analyzed are c-(H(2)COCH)CH(2)+Cl, c-(H(2)COCH)+CH(2)Cl, and C(3)H(4)O+HCl. In the second channel, the c-(H(2)COCH) photofission product is a higher energy intermediate on C(2)H(3)O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H(2)CCO. The final primary photodissociation pathway HCl+C(3)H(4)O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H(2)COC)=CH(2); the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C(3)H(5)O radical produced from C-Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O((3)P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C(2)H(4) products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H(2)CO+C(2)H(3) product channel of the O((3)P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H(3)CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C(3)H(5)O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C(2)H(4), and H(2)CO+C(2)H(3) product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels. Using the C(2)H(4) bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C(2)H(4) and HCO yields a value of 11.6+1/-3 Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies.