Prediction of product branching ratios in the C(P3)+C2H2→l-C3H+H∕c-C3H+H∕C3+H2 reaction using ab initio coupled clusters calculations extrapolated to the complete basis set combined with Rice-Ramsperger-Kassel-Marcus and radiationless transition theories

Intersystem crossing in the entrance channel of the reaction of O(3P) with pyridine

Two quantum effects can enable reactions to take place at energies below the barrier separating reactants from products: tunnelling and intersystem crossing between coupled potential energy surfaces. Here we show that intersystem crossing in the region between the pre-reactive complex and the reaction barrier can control the rate of bimolecular reactions for weakly coupled potential energy surfaces, even in the absence of heavy atoms. For O(3P) plus pyridine, a reaction relevant to combustion, astrochemistry and biochemistry, crossed-beam experiments indicate that the dominant products are pyrrole and CO, obtained through a spin-forbidden ring-contraction mechanism. The experimental findings are interpreted-by high-level quantum-chemical calculations and statistical non-adiabatic computations of branching fractions-in terms of an efficient intersystem crossing occurring before the high entrance barrier for O-atom addition to the N-atom lone pair. At low to moderate temperatures, the computed reaction rates prove to be dominated by intersystem crossing.

Computational study of the reaction of P+ with acetylene: does spin-crossing play a significant role?

A computational study of the reaction of P(+)((3)P) with acetylene has been carried out. The only exothermic products correlating with the reactants are PCCH(+)((2)Π) + H((2)S). Two different pathways leading to these products that are apparently barrier-free have been found. Both pathways involve isomerization into open-chain intermediates followed by direct elimination of a hydrogen atom. The possibility of spin-crossing has been considered because the species on the singlet surface are considerably more stable than those on the triplet one. On the singlet surface, there are other possible channels for the reaction, namely, cyclic PC(2)H(+)((2)A') + H((2)S) and CCP(+)((1)Σ) + H(2) ((1)Σ(g)(+)). A computational kinetic study shows that, in agreement with the experimental evidence, the major products are PCCH(+)((2)Π) + H((2)S) at all temperatures. Only at very high temperatures is CCP(+)((1)Σ) + H(2) ((1)Σ(g)(+)) formed in non-negligible amounts. Therefore, only PCCH(+) should be formed in the interstellar medium.

Finding Minimum Structures on the Seam of Crossing in Reactions of Type A + B → X: Exploration of Nonadiabatic Ignition Pathways of Unsaturated Hydrocarbons

A new theoretical approach is proposed for finding automatically minimum structures on the seam of crossing (MSX) in reactions of type A + B → X, where the artificial-force-induced reaction (AFIR) method is combined with the seam model function (SMF) approach. Its application to reactions between triplet dioxygen and unsaturated hydrocarbons provided many MSX structures. In addition to known ignition pathways, we discovered a pathway through a new type of MSX in the reaction of dioxygen with aromatic hydrocarbons; for benzene, this new pathway requires a lower energy than those of three known ignition pathways and is likely to be the most important. This demonstrates that the AFIR-SMF approach has the ability to discover unknown/unexpected MSX structures without prejudice for presumed pathways or mechanisms.

Triplet states of cyclopropenylidene and its isomers

The unique importance of the cyclopropenylidene molecule conveys significance to its low-lying isomers. Eleven low-lying electronic triplet stationary points as well as the two lowest singlet structures of C(3)H(2) have been systematically investigated. This research used coupled cluster (CC) methods with single and double excitations and perturbative triple excitations [CCSD(T)] and Dunning's correlation-consistent polarized valence cc-pVXZ (where X=D, T, and Q) basis sets. Geometries, dipole moments, vibrational frequencies, and associated infrared intensities of the targeted molecules have been predicted. The electronic ground state of cyclopropenylidene (3S, the global minimum) is the X (1)A(1) state with C(2v) point group symmetry. Among the 11 triplet stationary points, 7 structures are found to be minima, 2 structures to be transition states, and 2 structures to be higher-order saddle points. For the six lowest-lying triplet structures and singlet propadienylidene (2S), relative energies (zero-point vibrational energy corrected values in parentheses) with respect to the global minimum [ X (1)A(1) (3S)] at the cc-pVQZ-UCCSD(T) level of theory are predicted to be propynylidene (3)B(1aT)15.5(11.3)<propadienylidene[(1)A(1)(2S)]14.2(13.3)<propadienylidene[(3)B(1)(2T)]45.0(43.8)<cyclopropenylidene[(3)A(3aT)]53.8(52.5)<cyclopropyne[(3)B(2)(4aT)]70.0(70.6)<cyclopropenylidene [(3)B(1)(3cT)]71.2(69.9)<trans-propenediylidene [(3)A(")(5T)]73.6(70.7) kcal mol(-1). The combined energy for the [C((3)P)+C(2)H(2)(X (1)Sigma(g) (+))] system with respect to the global minimum (3S) is determined to be 107.4(103.8) kcal mol(-1). Therefore, these six triplet equilibrium structures are all energetically well below the dissociation limit to [C((3)P)+C(2)H(2)(X (1)Sigma(g) (+))].

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.

Probing the dynamics of polyatomic multichannel elementary reactions by crossed molecular beam experiments with soft electron-ionization mass spectrometric detection

In this Perspective we highlight developments in the field of chemical reaction dynamics. Focus is on the advances recently made in the investigation of the dynamics of elementary multichannel radical-molecule and radical-radical reactions, as they have become possible using an improved crossed molecular beam scattering apparatus with universal electron-ionization mass spectrometric detection and time-of-flight analysis. These improvements consist in the implementation of (a) soft ionization detection by tunable low-energy electrons which has permitted us to reduce interfering signals originating from dissociative ionization processes, usually representing a major complication, (b) different beam crossing-angle set-ups which have permitted us to extend the range of collision energies over which a reaction can be studied, from very low (a few kJ mol(-1), as of interest in astrochemistry or planetary atmospheric chemistry) to quite high energies (several tens of kJ mol(-1), as of interest in high temperature combustion systems), and (c) continuous supersonic sources for producing a wide variety of atomic and molecular radical reactant beams. Exploiting these new features it has become possible to tackle the dynamics of a variety of polyatomic multichannel reactions, such as those occurring in many environments ranging from combustion and plasmas to terrestrial/planetary atmospheres and interstellar clouds. By measuring product angular and velocity distributions, after having suppressed or mitigated, when needed, the problem of dissociative ionization of interfering species (reactants, products, background gases) by soft ionization detection, essentially all primary reaction products can be identified, the dynamics of each reaction channel characterized, and the branching ratios determined as a function of collision energy. In general this information, besides being of fundamental relevance, is required for a predictive description of the chemistry of these environments via computer models. Examples are taken from recent on-going work (partly published) on the reactions of atomic oxygen with acetylene, ethylene and allyl radical, of great importance in combustion. A reaction of relevance in interstellar chemistry, as that of atomic carbon with acetylene, is also discussed briefly. Comparison with theoretical results is made wherever possible, both at the level of electronic structure calculations of the potential energy surfaces and dynamical computations. Recent complementary CMB work as well as kinetic work exploiting soft photo-ionization with synchrotron radiation are noted. The examples illustrated in this article demonstrate that the type of dynamical results now obtainable on polyatomic multichannel radical-molecule and radical-radical reactions might well complement reaction kinetics experiments and hence contribute to bridging the gap between microscopic reaction dynamics and thermal reaction kinetics, enhancing significantly our basic knowledge of chemical reactivity and understanding of the elementary reactions which occur in real-world environments.

Unraveling the dynamics of the C(3P,1D) + C2H2 reactions by the crossed molecular beam scattering technique

A detailed investigation of the dynamics of the reactions of ground- and excited-state carbon atoms, C(3P) and C(1D), with acetylene is reported over a wide collision energy range (3.6-49.1 kJ mol-1) using the crossed molecular beam (CMB) scattering technique with electron ionization mass spectrometric detection and time-of-flight (TOF) analysis. We have exploited the capability of (a) generating continuous intense supersonic beams of C(3P, 1D), (b) crossing the two reactant beams at different intersection angles (45, 90, and 135 degrees ) to attain a wide range of collision energies, and (c) tuning the energy of the ionizing electrons to low values (soft ionization) to suppress interferences from dissociative ionization processes. From angular and TOF distribution measurements of products at m/z=37 and 36, the primary reaction products of the C(3P) and C(1D) reactions with C2H2 have been identified to be cyclic (c)-C3H + H, linear (l)-C3H + H, and C3 + H2. From the data analysis, product angular and translational energy distributions in the center-of-mass (CM) system for both the linear and cyclic C3H isomers as well as the C3 product from C(3P) and for l/c-C3H and C3 from C(1D) have been derived as a function of collision energy from 3.6 to 49.1 kJ mol-1. The cyclic/linear C3H ratio and the C3/(C3 + c/l-C3H) branching ratios for the C(3P) reaction have been determined as a function of collision energy. The present findings have been compared with those from previous CMB studies using pulsed beams; here, a marked contrast is noted in the CM angular distributions for both C3H- and C3-forming channels from C(3P) and their trend with collision energy. Consequently, the interpretation of the reaction dynamics derived in the present work contradicts that previously proposed from the pulsed CMB studies. The results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 ab initio potential energy surfaces (PESs). In particular, the branching ratios for the C(3P) + C2H2 reaction have been compared with the available theoretical predictions (approximate quantum scattering calculations and quasiclassical trajectory calculations on ab initio triplet PESs and, very recent, statistical calculations on ab initio triplet PESs as well as on ab initio triplet/singlet PESs including nonadiabatic effects, that is, intersystem crossing). While the experimental branching ratios have been corroborated by the statistical predictions, strong disagreement has been found with the results of the dynamical calculations. The astrophysical implications of the present results have been noted.