Theoretical study of isomerization and decomposition of propenal

Abstract

We investigated the dynamics of isomerization and multi-channel dissociation of propenal (CH(2)CHCHO), methyl ketene (CH(3)CHCO), hydroxyl propadiene (CH(2)CH(2)CHOH), and hydroxyl cyclopropene (cyclic-C(3)H(3)-OH) in the ground potential-energy surface using quantum-chemical calculations. Optimized structures and vibrational frequencies of molecular species were computed with method B3LYP∕6-311G(d,p). Total energies of molecules at optimized structures were computed at the CCSD(T)∕6-311+G(3df,2p) level of theory. We established the potential-energy surface for decomposition to CH(2)CHCO + H, CH(2)CH + HCO, CH(2)CH(2)∕CH(3)CH + CO, CHCH∕CH(2)C + H(2)CO, CHCCHO∕CH(2)CCO + H(2), CHCH + CO + H(2), CH(3) + HCCO, CH(2)CCH + OH, and CH(2)CC∕cyclic-C(3)H(2) + H(2)O. Microcanonical rate coefficients of various reactions of trans-propenal with internal energies 148 and 182 kcal mol(-1) were calculated using Rice-Ramsperger-Kassel-Marcus and Variational transition state theories. Product branching ratios were derivable using numerical integration of kinetic master equations and the steady-state approximation. The concerted three-body dissociation of trans-propenal to fragments C(2)H(2) + CO + H(2) is the prevailing channel in present calculations. In contrast, C(3)H(3)O + H, C(2)H(3) + HCO and C(2)H(4) + CO were identified as major channels in the photolysis of trans-propenal. The discrepancy between calculations and experiments in product branching ratios indicates that the three major photodissociation channels occur mainly on an excited potential-energy surface whereas the other channels occur mainly on the ground potential-energy surface. This work provides profound insight in the mechanisms of isomerization and multichannel dissociation of the system C(3)H(4)O.

Molecular-beam experiments for photodissociation of propenal at 157 nm and quantum-chemical calculations for migration and elimination of hydrogen atoms in systems C3H4O and C3H3O

“…In the present work, the C 3 H 4 O system includes three isomers -propenal CH 2 CHCHO ( 1 A ), methyl ketene CH 3 CHCO ( 1 A ), and hydroxyl propadiene CH 2 CCHOH ( 1 A ) -and two transition structures, TS1 and TS2, of isomerization. Since the isomerization of propenal has been reported in the previous work, 16 the relevant description is omitted here. The mechanisms of elimination of hydrogen atoms from the three C 3 H 4 O isomers are addressed in the following.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Three primary dissociation pathways (1)-(3) were observed upon optical excitation to the S 1 (n-π *) state of propenal with wavelengths 300, 313, and 334 nm, 3,7,11 CH 2 CHCHO → CH 2 CHCO + H H 0K = 87.5 (86.9) kcal mol −1 , …”

“…H 0K denotes the enthalpy of a reaction at 0 K calculated with the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level of theory; 16 shown in a parenthesis is an experimental value. 8 propenal on S 0 , S 1 , and T 1 potential-energy surfaces (PES) for reactions (1)-(3) using complete-active-space multiconfiguration self-consistent-field (CASSCF) methods.…”

“…14,16 Rate coefficients and branching ratios for various dissociation channels of propenal with internal energies 148 and 182 kcal mol −1 , corresponding to one-photon wavelengths 193 and 157 nm, were performed using Rice-Ramsperger-Kassel-Marcus (RRKM) theory based on an ab initio PES. 16 Spontaneous decomposition of internally excited C 3 H 3 O via reaction (1 ) or (1 ) and CH 2 CH via reaction (2 ) might occur due to large excitation energy, …”

“…Propenal can rearrange to methyl ketene (1-propen-1-one, CH 3 CHCO) and hydroxyl propadiene (1hydroxy-1,2-propadiene, CH 2 CCHOH) followed by reactions (6) and (7) and reactions (8) and (9), respectively. 15,16 Thus, elimination of a hydrogen atom from intermediates methyl ketene and hydroxyl propadiene could produce distinct C 3 H 3 O isomers. Some isomers of C 3 H 3 O might have isomerization or dissociation barriers higher than that of CH 2 CHCO (X 2 A ) and thus could survive in the photodissociation of propenal at 193 nm.…”