Competing Pathways in the 248 nm Photodissociation of Propionyl Chloride and the Barrier to Dissociation of the Propionyl Radical

Abstract

Photofragment translational spectroscopy was used to study the photodissociation of propionyl chloride at 248 nm. The crossed laser-molecular beam experiment with VUV photoionization showed two primary dissociation channels, C-Cl bond fission and HCl elimination. Following cleavage of the C-Cl bond, unimolecular dissociation of the propionyl radical produced CH 3 CH 2 and CO. The energy barrier to the CH 3 -CH 2 CO f CH 3 CH 2 + CO reaction was estimated to be in the range of 16.3 ( 1.5 kcal/mol by determining the internal energy distribution of surviving propionyl radicals. No other secondary dissociation channels were observed for the propionyl radical. The HCl elimination channel, previously reported only for the condensed phase of propionyl chloride, was observed as the minor primary dissociation channel in the gas phase. The cofragment to the HCl elimination, CH 3 CHCO or CH 2 CH 2 CO, underwent secondary dissociation to produce CO and CH 2 CH 2 with a significant amount of energy partitioned into translational motion.

Investigation of the O+allyl addition/elimination reaction pathways from the OCH2CHCH2 radical intermediate

“…In several earlier studies, we were able to definitively assign the low recoil KE channel to one that produces an electronically excited radical+ Cl, and noted that Cl in this channel was predominantly in the ground spin-orbit state. [47][48][49] Thus, we did consider the possibility that the low recoil KE C-Cl bond fission channel in this study gives electronically excited radicals. However, two-state MCSCF calculations using several active spaces did not reveal any excited states that would be energetically accessible for INT2.…”

Roaming as the dominant mechanism for molecular products in the photodissociation of large aliphatic aldehydes

“…For instance, in the 248 nm photodissociation of the C 2 H 5 COCl molecular beam heated to 153 1C, the major fast dissociation channel of C-Cl bond fission was accompanied by a minor channel of HCl elimination which proceeded in the ground state. 35 In contrast, the HCl elimination was not observed in a supersonic molecular beam at cold temperature. 36 While inspecting the case of CH 3 CHO reported previously, 37-40 the fragments produced via the S 0 pathway are expected to follow a sequential ISC path: S 1 -S 1 /T 1 ISC -T 1 -T 1 /S 0 ISC -S 0 .…”

“…The molecular photodissociation lifetime of CH 3 CHO was reported to be < 5 ns at 312 nm under jet-cooled temperature ∼1 K. 34 35 In contrast, the HCl elimination was not observed in a supersonic molecular beam at cold temperature. 36 While inspecting the case of CH 3 CHO reported previously, 37-40 the fragments produced via the S 0 pathway are expected to follow a sequential ISC path: S 1 →S 1 /T 1 ISC→ T 1 → T 1 /S 0 ISC→S 0 .…”

Characterization of molecular channel in photodissociation of SOCl2 at 248 nm: Cl2 probing by cavity ring-down absorption spectroscopy

“…For instance, in the 248 nm photodissociation of the C 2 H 5 COCl molecular beam heated to 153 1C, the major fast dissociation channel of C-Cl bond fission in the picosecond regime was accompanied by a minor channel of HCl elimination which proceeded on the ground state. 48 In contrast, the HCl elimination was not observed in a supersonic molecular beam at cold temperature. 49 In view of the measurements of Cl 2 at m/z = 70 and its anisotropy parameter of 0.7 AE 0.1 using PTS, Baum et al 5 proposed that the molecular products mainly SO(b 1 S + ) + Cl 2 (X 1 S + g ), contributing o3% of the entire decay process, are dissociated from the prepared 2 1 A 0 state with a decay lifetime comparable to the rotational period or even shorter.…”