The roaming atom pathway in formaldehyde decomposition

Abstract

We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H(2) and CO following excitation to the 2(1)4(1) and 2(1)4(3) transitions in S(1). The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, j(CO)=5-45). These high-resolution measurements reveal the correlated internal state distribution in the H(2) cofragments. The results show that rotationally hot CO (j(CO) approximately 45) is produced in conjunction with vibrationally "cold" H(2) fragments (v=0-5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H(2)CO-->H+HCO) we also find formation of rotationally cold CO (j(CO)=5-28) correlated to highly vibrationally excited H(2) (v=6-8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this "roaming" mechanism are the focus of this paper. The correlations between the vibrational states of H(2) and rotational states of CO formed following excitation on the 2(1)4(3) transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel.

Skirting the transition state, a new paradigm in reaction rate theory

“…The ''roaming'' mechanism was discovered in a recent combined experimental͞theoretical study of formaldehyde photodissociation (2,3). (Formaldehyde is acetaldehyde with the methyl group replaced by hydrogen.)…”

State-Resolved Dynamics of the CN(B2Σ+) and CH(A2Δ) Excited Products Resulting from the VUV Photodissociation of CH3CN

“…This mechanism is reminiscent of the "roaming" atom scheme in formaldehyde, whose experimental signature is the formation of vibrationally excited H 2 (v = 6 -8). [27][28][29][30] Positive identification of this pathway would provide additional evidence for a new class of dynamics that is not fully described using conventional transition state theory. Since higher K′ values, a similar to statistical partitioning of the rotational energy, combined with much higher rotational temperatures, suggests that a bent transition state is likely in these cases.…”

“…This loosely bound hydrogen turns back towards HCO and abstracts the other H atom. The scheme detailed above is deemed the "roaming" atom mechanism; 27,28 note that more conventional dissociation pathways leading to H + HCO and H + H + CO are also possible as a result of UV photolysis. Similarly, in the reaction of CH 3 + O, an unexpected pathway leading to CO + H 2 + H via HCO + H 2 is observed, thus departing from the typical minimum energy path that leads to H 2 CO + H. 29 These unorthodox processes have also been shown to occur in CH 3 CHO, where the methyl group is the "roaming" moiety.…”

Experimental and Theoretical Studies of Roaming Dynamics in the Unimolecular Dissociation of CH3NO2to CH3O+NO

“…This second pathway thus deviates dramatically from the conventional tight TS pathway. Representations of this roaming pathway and detailed analysis of new experi-ments have appeared in the literature along with reviews of this phenomenon [19][20][21][22][23][24][25][26]. There are now numerous examples of roaming in unimolecular reactions.…”

“…There are now numerous examples of roaming in unimolecular reactions. The following is a list of reactions where roaming has been identified as a reaction pathway and a partial list of references for each: H 2 CO [20][21][22][23][24][25][26][27] [43], HCOOH [44], and CH 3 NO 2 [45,46]. Roaming has also been reported in bimolecular reactions, H + HCO [47] and in a quantum scattering calculation of the reaction MgH + H → Mg + H 2 [48].…”