Characterization of the Methoxy Carbonyl Radical Formed via Photolysis of Methyl Chloroformate at 193.3 nm

Abstract

This study investigates two features of interest in recent work on the photolytic production of the methoxy carbonyl radical and its subsequent unimolecular dissociation channels. Earlier studies used methyl chloroformate as a photolytic precursor for the CH 3 OCO, methoxy carbonyl (or methoxy formyl) radical, which is an intermediate in many reactions that are relevant to combustion and atmospheric chemistry. That work evidenced two competing C-Cl bond fission channels, tentatively assigning them as producing groundand excited-state methoxy carbonyl radicals. In this study, we measure the photofragment angular distributions for each C-Cl bond fission channel and the spin-orbit state of the Cl atoms produced. The data shows bond fission leading to the production of ground-state methoxy carbonyl radicals with a high kinetic energy release and an angular distribution characterized by an anisotropy parameter, , of between 0.37 and 0.64. The bond fission that leads to the production of excited-state radicals, with a low kinetic energy release, has an angular distribution best described by a negative anisotropy parameter. The very different angular distributions suggest that two different excited states of methyl chloroformate lead to the formation of ground-and excited-state methoxy carbonyl products. Moreover, with these measurements we were able to refine the product branching fractions to 82% of the C-Cl bond fission resulting in ground-state radicals and 18% resulting in excitedstate radicals. The maximum kinetic energy release of 12 kcal/mol measured for the channel producing excitedstate radicals suggests that the adiabatic excitation energy of the radical is less than or equal to 55 kcal/mol, which is lower than the 67.8 kcal/mol calculated by UCCSD(T) methods in this study. The low-lying excited states of methylchloroformate are also considered here to understand the observed angular distributions. Finally, the mechanism for the unimolecular dissociation of the methoxy carbonyl radical to CH 3 + CO 2 , which can occur through a transition state with either cis or, with a much higher barrier, trans geometry, was investigated with natural bond orbital computations. The results suggest donation of electron density from the nonbonding C radical orbital to the σ* orbital of the breaking C-O bond accounts for the additional stability of the cis transition state.

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.…”

Insights into the mechanistic photodissociation of methyl formate

“…[9][10][11][12][13][14][15] Thus, studying the photodissociation dynamics of carbonyl compounds can be used not only to characterize the details of Norrish type I reaction but also to assist us to unveil the unresolved photophysical and photochemical processes of complicated polyatomic molecules. 1-8 Among polyatomic molecules, carbonyl compounds attracted much attention experimentally and theoretically because they only contain one simple chromophore, carbonyl group, and their photodissociation dynamics belongs to Norrish type I reaction, a classical and representative photochemical reaction in organic chemistry.…”

“…1-8 Among polyatomic molecules, carbonyl compounds attracted much attention experimentally and theoretically because they only contain one simple chromophore, carbonyl group, and their photodissociation dynamics belongs to Norrish type I reaction, a classical and representative photochemical reaction in organic chemistry. [9][10][11][12] The exclusive ␣-bond cleavage in the S 1 state is one type, which frequently takes place in acetyl halides ͑X: CH 3 and CH 3 CH 2 ; Y = Cl, Br, and I͒. 16 Photodissociation mechanisms of asymmetrically substituted aliphatic carbonyl compounds, XCOY ͑X: CH 3 and CH 3 CH 2 ; Y: Cl, H, OH, and NH 2 ͒, can be grouped into three types with respect to the ␣-bond fission.…”

“…17,18 The second type comprises acetone, acetaldehyde, and acetamide ͑X and Y: CH 3 and NH 2 ͒ where the ␣-bond fissions mainly occur in the T 1 state via the S 1 → T 1 intersystem crossing. 9,12 Since the state-correlation diagram for photodissociation processes changes from aldehydes to esters, the mechanism in esters might be different from the ones summarized above. 22 These studies mainly involve aliphatic carbonyl compounds, aldehydes/ ketones/acids, and their chloro-substituted derivatives.…”

“…10,15,[19][20][21] Finally, in acetic acid ͑X: CH 3 ; Y: OH͒, there are two comparable dissociation channels: the S 1 direct ␣-bond cleavage and the T 1 channels after the S 1 → T 1 intersystem crossing. 9,12 To understand these differences at the atomistic level, ab initio electronic structure calculations are necessary, even nonadiabatic dynamics simulations. However, photodissociation dynamics of esters and their derivatives ͑X: CH 3 O; H, Cl, and NH 2 ͒ has received less attention until recently.…”