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

Abstract

This 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.

Concerted stereodynamics of chemical changes: The branching selectivity in the photodissociation of asymmetric‐top molecules, formic acid, and the cold reactivity of the H + H2 exchange reaction

“…The authors have shared the scientific challenges of reaction dynamics together with him by combining stereodynamical features with roaming-type of photo-excitation/ dissociation events into focus. Thus, we would review those works with a notion based on the concerted stereodynamics of chemical changes [1][2][3][4][5][6]. This reaction scheme would be helpful in understanding about branching processes in the non-adiabatic photodissociation of complex molecules such as asymmetric-top molecules, also the roaming signature in the formic acid, Further, the cold reactivity in the H + H 2 exchange reaction at a similar cold temperature as in the interstellar clouds as you see in the next section.…”

“…Since decades ago, the roaming mechanism has become a hot topic in photodissociation studies, and its dynamics are quite interesting but complicated [7,8]. The photodissociation of aliphatic aldehydes (RCOH, R = H, CH 3 , and large alkyl group) shows a loose roaming saddle point along the dissociation coordinate [5,6], whereas the roaming of methyl formate (HCOOCH 3 ) includes multiple energy states via a conical intersection. This roaming route is recognized by quasi-classical trajectory (QCT) calculations when the corresponding initial configuration of trajectories is set at the conical intersection [9].…”

“…This roaming route is recognized by quasi-classical trajectory (QCT) calculations when the corresponding initial configuration of trajectories is set at the conical intersection [9]. We have learned that the roaming mechanism becomes dominant with the size of aliphatic aldehydes [5,6]. Nowadays, it seems clear based on many experimental and theoretical evidences that the venerable chemical reaction scheme of the transition-state theory with one reaction coordinate is no more sufficient to explain the diversity of chemical reaction pathways.…”

Pattern analysis of the impact‐parameter dependent trajectories for the H + H2 exchange reaction at T = 3 and 300 K: A characteristic propensity for reactive versus nonreactive trajectories found in the time‐dependent interaction potential and a roaming‐like libration motion at cold temperature

“…[36] Reaction pathways, which bypass the TSs, such as roaming pathways are discussed in the previous studies. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] We learned that the classical trajectory calculations serve us the detailed reaction dynamics. [20][21][22][23][24][25][26][27]49] The Polanyi rule is a notable example of the usefulness of the trajectory calculation in which how the location of the energy barrier along the reaction coordinate determines chemical reaction dynamics.…”