Regulation of nonadiabatic processes in the photolysis of some carbonyl compounds

Abstract

Carbonyl compounds studied are confined to acetyl halide (CH3COCl), acetyl cyanide (CH3COCN), acetyl sulfide (CH3COSH), acetaldehyde (CH3CHO), and methyl formate (HCOOCH3). They are asymmetrically substituted, but do not follow the well-known Norrish type I reactions. Each compound ejected in an effusive beam at about 300 K is commonly excited to the (1)(n, π*)CO lower state; that is, a nonbonding electron on O of the C[double bond, length as m-dash]O group is promoted to the antibonding orbital of π*CO. The photolysis experiments are conducted in the presence of Ar gas and the corresponding fragments are detected using time-resolved Fourier-transform Infrared (FTIR) emission spectroscopy. The enhancement of the collision-induced internal conversion or intersystem crossing facilitates the dissociation channels via highly vibrational states of the ground singlet (So) or triplet (T1) potential energy surfaces. In this manner, an alternative nonadiabatic channel is likely to open yielding different products, even if the diabatic coupling strength is strong between the excited state and the neighboring state. For instance, the photodissociation of CH3COCl at 248 nm produces HCl, CO, and CH2 fragments, in contrast to the supersonic jet experiments showing dominance of the Cl fragment eliminated from the excited state. If the diabatic coupling strength is weak, dissociation proceeds mainly through internal conversion, such as the cases of CH3COCN and CH3COSH. The photodissociation of CH3COCN at 308 nm has never been reported before, while for CH3COSH matrix-isolated photodissociation was conducted that shows a distinct spectral feature from the current FTIR method. The CH3CHO and HCOOCH3 molecules belong to the same type of carbonyl compounds, in which the molecular products, CO + CH4 and CO + CH3OH, are produced through both transition state and roaming pathways. Their products are characterized differently between molecular beam and current FTIR experiments. For instance, the photodissociation of HCOOCH3 at 248 nm yields CO with the vibrational state v ≥ 4, in contrast to the molecular beam experiments producing CO at v = 1. The photodissociation of CH3CHO at 308 nm intensifies a low energy component in the CH4 vibrational distribution, thus verifying the transition state pathway for the first time.

Photodissociation Dynamics of CO-Forming Channels on the Ground-State Surface of Methyl Formate at 248 nm: Direct Dynamics Study and Assessment of Generalized Multicenter Impulsive Models

“…We have examined five channels characterized by different energy distributions: four of them with methanol + CO as products including a pathway characterized by the roaming saddle point and a fifth channel that produces H 2 + CO + H 2 CO. We have also analyzed and compared the results of the present work with those obtained from velocity map imaging ,, and FTIR experiments. − , The results show that initiating the direct dynamics from the saddle points, none of the CO-forming pathways can provide satisfactory explanations to all of the experimental evidence. Such a circumstance further strengthens the hypothesis about the role played by the S 1 /S 0 conical intersection, as mentioned in previous works. − ,, Indeed, starting trajectories from the conical intersection provides alternative dissociation routes − and consequently alternative product energy disposal. Combining the results of both this work and the experiments, ,,, the TS2 pathway, which was overlooked in previous studies, − , might also contribute to the experimental observation.…”

“…Details on the trajectory simulations are reported in Table S2 of the Supporting Information. A detailed comparison has been made between results of direct dynamics and experimental outcomes reported in previous studies. ,,, The state-resolved translational and ro-vibrational distributions of CO products are analyzed and compared with experiments (Figures – ). To further explore the possibility of discriminating the dissociation mechanism of the products with multiple forming pathways, several combinations of product states are exhibited in terms of correlation diagrams (Figures – ).…”

“…Recent experimental studies on methyl formate at 248 nm-wavelength photolysis reported the kinetic energy and ro-vibrational energy distribution of CO­(v, J ) using velocity mapping ion imaging , (VMI) and time-resolved infrared emission measurements, ,, respectively. To compare the results of direct dynamics with these state-resolved data, we have chosen the outcomes with proper quantum states of the CO products.…”

“…Inspired by experimental studies carried out in these last few years on the photodissociation of methyl formate, − ,, we performed an investigation on the dynamics of the photoinitiated processes, with a wavelength of 248 nm, which characterizes this ester, aimed at providing tools to identify the various channels in the ground state S 0 with the production of CO. Dynamics simulations are based on the assumptions of the RRKM theory and the minimum energy path and are initiated from the saddle points of the investigated dissociation pathways. We have examined five channels characterized by different energy distributions: four of them with methanol + CO as products including a pathway characterized by the roaming saddle point and a fifth channel that produces H 2 + CO + H 2 CO. We have also analyzed and compared the results of the present work with those obtained from velocity map imaging ,, and FTIR experiments. − , The results show that initiating the direct dynamics from the saddle points, none of the CO-forming pathways can provide satisfactory explanations to all of the experimental evidence.…”

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

“…The addition of Ar plays a role to facilitate the total rate of level‐to‐level coupling between two electronic states to enhance the radiationless relaxation via internal conversion (IC) or intersystem crossing (ISC) that increase the signal‐to‐noise ratio to allow for detailed analysis of the CO spectrum even with a small Einstein emission coefficient in the IR region. [ 37–40 ]…”