Photodissociation of Propionaldehyde at 248 nm: Roaming Pathway as an Increasingly Important Role in Large Aliphatic Aldehydes

Abstract

Time-resolved Fourier transform infrared emission spectroscopy is employed in the photolysis of propionaldehyde (CH3CH2CHO) at 248 nm to characterize the role of the roaming pathway. High-resolution spectra of CO are analyzed to yield a single Boltzmann rotational distribution for each vibrational level (ν = 1-4) with small rotational and large vibrational energy disposals. A roaming saddle point is found containing two far separated moieties of HCO and CH3CH2 with a weak interaction between them. Quasiclassical trajectory calculations on this configuration yield the CO energy flow behavior, consistent with the findings. The rate constant along the roaming pathway is evaluated to be larger by >1-2 orders of magnitude than those along tight transition state or three-body dissociation pathways. This work implies that the roaming mechanism plays an increasingly important role in aliphatic aldehydes as the molecular size becomes larger.

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193 nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

“…Such roaming results exhibit a consistent trend as photolysis of propionaldehyde (CH 3 CH 2 CHO) leading to CO + C 2 H 6 molecular products, in which the CO roaming behavior was theoretically recognized to be rotational cold but vibrational hot 27 . Nevertheless, these two roaming types are different from each other.…”

“…However, the roaming dynamics turns out to be more complicated to render studies oriented in varied directions 12-25 . As an example using carbonyl compounds in the photodissociation, aliphatic aldehydes showed a loose roaming saddle point along the roaming dissociation coordinate 26,27 , whereas the roaming signature in methyl formate (HCOOCH 3 ) was verified to involve multiple energy states via a conical intersection 17,18 . These two types of roaming dynamics apparently behave differently.Formic acid (HCOOH) belongs to the family of carbonyl compounds, and its photodissociation dynamics has been popularly investigated for more than two decades [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] .…”

Roads leading to roam. Role of triple fragmentation and of conical intersections in photochemical reactions: experiments and theory on methyl formate

“…In current debates 1 such structure has been attributed to either direct versus delayed or transition-state versus roaming-mediated mechanisms. [2][3][4][5][6][7][8][9][10][11][12][13][14] This work contributes to defining the strength and weakness of the concept of transition state, pervasive in chemistry along with the related ones of reaction coordinates and minimum energy paths. Accordingly, chemical reactions are modeled to proceed along a path that the system walks on a potential energy surface, going through the transition state (the ''mountain'' pass along this path), eventually reaching the products' region.…”

Transport and roaming on the double van der Waals potential energy surface

“…In a roaming reaction, however, the molecule splits into two fragments, but one part moves around the other one until the two fragments collide, recombine, and finally two new stable molecules separate. After pioneering work on formaldehyde (H 2 CO) dissociation [4,5], roaming has been reported for other molecules [3,[6][7][8][9][10][11][12] and suggested for many bimolecular systems too [13][14][15][16][17][18][19][20][21]. Currently, roaming is a widely recognised chemical reaction mechanism.…”