Ab initio/RRKM approach toward the understanding of ethylene photodissociation

Abstract

The optimized structures and harmonic frequencies for the transition states and intermediates on the ground state potential energy surfaces of ethylenes, including C 2 H 4 , C 2 D 4 , D 2 CCH 2 , and cis-and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G͑d,p͒ level. The coupled cluster method, CCSD͑T͒/6-311ϩG͑3df,2p͒, was employed to calculate the corresponding energies with the zero-point energy corrections by the B3LYP/6-311G͑d,p͒ approach. Ethylidene was found to be an intermediate in the 1,2-H 2 elimination channel. The barrier for the 1,1-H 2 elimination was computed to be the lowest ͑4.10-4.16 eV͒, while the 1,2-H 2 elimination and H loss channels have barriers of a similar height ͑4.70-4.80 eV͒. The rate constant for each elementary step of ethylene photodissociation at 193 and 157 nm was calculated according to the RRKM theory based on the ab initio surfaces. The rate equations were subsequently solved, and thus the concentration of each species was obtained as a function of time. The concentrations at t→ϱ were taken for calculating branching ratios or yields. In accord with previous experimental findings, the calculated branching ratio for the 1,1-H 2 elimination process is higher than that for the 1,2-H 2 elimination, and the atomic elimination channel is predicted to be favored at increasing excitation energy when competing with the molecular elimination. The significant discrepancies between theoretical and experimental results in the magnitude of the yields and their dependence on the wavelength for the molecular elimination channels suggest the dynamics of either 1,2-H 2 , or 1,1-H 2 elimination, or both channels may be nonstatistical in nature.

Photodissociation of 1,2‐Dibromoethylene at 248 nm: Br2 Molecular Elimination Probed by Cavity Ring‐Down Absorption Spectroscopy

“…RRKM Rate-Constant Calculations: The Rice-Ramsperger-KasselMarcus (RRKM) and variational RRKM rate constants [15,34] for the molecular and atomic dissociations at 248 nm, on both the singletground-state and first-triplet-state potential energy surfaces of 1,2-C 2 H 2 Br 2 , were predicted. The molecule was assumed to be a collection of harmonic oscillators whose harmonic frequencies and energies were obtained as described above.…”

“…The H 2 molecule can be eliminated via 1,1-, 1,2-cis, and 1,2-trans mechanisms. [14,15] Following the 1,1-site elimination, vinylidine CCH 2 rapidly undergoes isomerization (through a barrier of 5.8 kJ mol…”

“…[15] When chlorine atoms are substituted, the photodissociation of vinyl chloride and dichloroethylene may further cause elimination of Cl and HCl, apart from the H and H 2 fragments; for instance, Blank et al investigated the photodissociation of vinyl chloride at 193 nm, thereby observing five primary dissociation channels for H, Cl, H 2 , and HCl elimination. The majority of the Cl-atom release with large translational energy follows a dissociation channel that occurs from the initially excited ðp; p*Þ state crossing to a repulsive [p(Cl), s*A C H T U N G T R E N N U N G (C-Cl)] state.…”

Probing the Ignored Elimination Channel of Br2 in the 248 nm Photodissociation of 1,1‐Dibromoethylene by Cavity Ring‐Down Absorption Spectroscopy

“…The Gaussian 98 programs were utilized in the electronic structure calculations. [32] The Rice-Ramsperger-Kassel-Marcus (RRKM) rate constants [33] for Br 2 dissociations at 248 nm, on both the singlet ground-state and first triplet-state potential energy surfaces of 1,1-C 2 H 2 Br 2 , were predicted. If the rate of the energy equilibration is faster than the reaction rate, the rate constant can be explained statistically.…”

Evidence for Synchronous Concerted Three-Body Dissociation of Propenal to C2H2+CO+H2

“…Considering the sequential dissociation processes C 3 H 4 O!C 2 H 4 + CO, C 2 H 2 + H 2 CO and C 3 H 2 O + H 2 ! C 2 H 2 + CO + H 2 , reactions C 2 H 4 !C 2 H 2 + H 2 , H 2 CO!H 2 + CO, and CHCCHO!C 2 H 2 + CO have barrier heights of 94 kcal mol À1 , [23] 79 kcal mol À1 , [24] and 76 kcal mol…”