Beyond the rule of transition state: Identification of roaming routes in some cases of carbonyl compounds

Abstract

AbstractThis account focuses on photodissociation of carbonyl compounds to verify a so‐called roaming route, alternative to those passing through a transition state (TS) invoked in the realm of the kinetics of elementary chemical processes. Such a roaming route bypasses the minimum energy path but produces the same molecular products. The photodissociation of the carbonyl compounds of study includes methyl formate (HCOOCH3), formic acid (HCOOH), and aliphatic aldehydes (RCOH, RH, CH3, and large alkyl group). Methyl formate and formic acid were promoted to the excited state, followed by internal conversion via a conical intersection. Then, the energetic precursor dissociated to fragments, which proceeded along either TS or roaming path. As the excitation energy increases, the roaming fraction in methyl formate is significantly interfered with by an open channel of triple fragmentation (H + CO + CH3O), whereas such a channel may not occur in formic acid even at an excitation wavelength of 193 nm. As a distinct roaming type, aliphatic aldehydes are found to possess the structure of the roaming saddle point comprising two moieties weakly bound at a distance. The energy difference between the TS barrier and the roaming saddle point is indicative of the extent for the roaming contribution. The roaming route becomes increasingly dominant when the aliphatic aldehyde is larger. Experimentally, ion imaging was employed to visualize the rotational‐level dependence of the roaming route, while time‐resolved Fourier‐transform infrared emission spectroscopy was applied to determine the vibrational‐state dependence. The roaming signature was verified theoretically by quasi‐classical trajectory calculations. As an alternative, a multi‐center impulsive model was developed to simulate the roaming scalar and vector properties without involving the global energy surface.