Prediction of absolute rate coefficients and product branching ratios for the C(3P)+allene reaction system

Radical-molecule reaction C(3P) + C3H6: mechanistic study

The complex triplet potential energy surface for the reaction of ground-state atomic carbon C(3P) with propylene C3H6 is explored at the B3LYP/6-311G(d,p), QCISD/6-311G(d,p), and G3B3 (single-point) levels. Various possible reaction pathways are probed. It is shown that the reaction is initiated by the addition of C(3P) to the C=C bond of C3H6 to generate barrierlessly the three-membered ring isomer 1 CH3-cCHCCH2, followed by the ring-opening process to form 2a trans-CH3CHCCH2, which can easily interconvert to 2b cis-CH3CHCCH2. Starting from 2 (2a, 2b), the most feasible pathway is the internal C-H bond rupture of 2a leading to P4(2CH3CCCH2 + 2H), terminal C-H bond cleavage of 2 (2a,2b) to form P5(2CH3CHCCH + 2H), or direct C-C bond fission of 2b to form P7(2CH2CCH + 2CH3), all of which may have comparable contributions to the title reaction. Much less competitively, 2a takes a 1,2-H-shift to form 5a trans-cis-CH3CHCHCH, followed by a C-C bond rupture leading to P6(1C2H2 + 3CH3CH). Because the intermediates and transition states involved in the feasible pathways all lie below the reactant, the title reaction is expected to be rapid, which is consistent with the measured large rate constant. The present article may provide some useful information for future experimental investigation of the title reaction.

Theoretical study of the C(3P) + trans-C4H8 reaction

The complex triplet potential energy surface for the reaction of ground-state carbon atom C((3)P) with trans-C(4)H(8) is theoretically investigated at the B3LYP/6-311G(d,p) and G3B3(single-point) levels. Various possible isomerization and dissociation pathways are probed. The initial association between C((3)P) and trans-C(4)H(8) is found to be the C((3)P) addition to the C=C bond of trans-C(4)H(8) to barrierlessly generate the three-membered cyclic isomer 1 CH(3)-cCHCCH-CH(3). Subsequently, 1 undergoes a ring-opening process to form the chainlike isomer 3a cis-trans-CH(3)CHCCHCH(3), which can either lead to P(6)((2)CH(3)CHCCCH(3) + (2)H) via the C-H bond cleavage or to P(7)((2)CH(3)CHCCH + (2)CH(3)) via C-C bond rupture. These two paths are the most favorable channels of the title reaction. Other channels leading to products P(1)((2)CH(3)-cCHCCH + (2)CH(3)), P(2)((2)CH(3)-cCHCC-CH(3) + (2)H), P(3)(trans-(2)CH(3)CHCH + (2)C(2)H(3)), P(4)(cis-(2)CH(3)CHCH + (2)C(2)H(3)), P(5)((3)CH(3)CH + (1)CH(3)CCH), P(8)(cis-(2)CH(3)CHCHCCH(2) + (2)H), P(9)(trans-(2)CH(3)CHCHCCH(2) + (2)H), P(10)((2)CH(3)CCCH(2) + (2)CH(3)), and P(11)((2)CH(3)CHCCHCH(2) + (2)H), however, are much less competitive due to either kinetic or thermodynamic factors. Because the intermediates and transition states involved in the C((3)P) + trans-C(4)H(8) reaction all lie below the reactant, the title reaction is expected to be rapid, as is consistent with the measured large rate constant. Our results may be helpful for future experimental investigation of the title reaction.

Quantum Chemical Evaluation of the Astrochemical Significance of Reactions between S Atom and Acetylene or Ethylene

Addition-elimination reactions of S atom in its 3P ground state with acetylene (C2H2) and ethylene (C2H4) were characterized with both molecular orbital and density functional theory calculations employing correlation consistent basis sets in order to assess the likelihood that either reaction might play a general role in astrochemistry or a specific role in the formation of S2 (X3Sigmag-) via a mechanism proposed by Saxena, P. P.; Misra, A. Mon. Not. R. Astron. Soc. 1995, 272, 89. The acetylene and ethylene reactions proceed through C2H2S (3A' ') and C2H4S (3A' ') intermediates, respectively, to yield HCCS (2Pi) and C2H3S (2A'). Substantial barriers were found in the exit channels for every combination of method and basis set considered in this work, which effectively precludes hydrogen elimination pathways for both S + C2H2 and S + C2H4 in the ultracold interstellar medium where only very modest barriers can be surmounted and processes without barriers tend to predominate. However, if one or both intermediates are formed and stabilized efficiently under cometary or dense interstellar cloud conditions, they could serve as temporary reservoirs for the S atom and participate in reactions such as S + C2H2S --> S2 + C2H2 or S + C2H4S --> S2 + C2H4. For formation and stabilization to be efficient, the reaction must possess a barrier height small enough to be surmountable at low temperatures yet large enough to prevent redissociation to reactants. Barrier heights computed with B3LYP and large basis sets are very low, but more rigorous QCISD(T) and RCCSD(T) results indicate that the barrier heights are closer to 3-4 kcal/mol. The calculations therefore indicate that S + C2H2 or S + C2H4 could contribute to the formation of S2 in comets and may serve as a means to gauge coma temperature. The energetics of the ethylene reaction are more favorable.