ALBERT

Notes: The reaction of ground state carbon atoms, C(3Pj), with dimethylacetylene, H3CCCCH3, was studied at three collision energies between 21.2 and 36.9 kJmol−1 employing the crossed molecular beam approach. Our experiments were combined with ab initio and RRKM calculations. It is found that the reaction is barrierless via a loose, early transition state located at the centrifugal barrier following indirect scattering dynamics through a complex. C(3Pj) attacks the π system of the dimethylacetylene molecule to form a dimethylcyclopropenylidene intermediate either in one step via an addition to C1 and C2 of the acetylenic bond or through an addition to only one carbon atom to give a short-lived cis/trans dimethylpropenediylidene intermediates followed by ring closure. The cyclic intermediate ring opens to a linear dimethylpropargylene radical which rotates almost parallel to the total angular momentum vector J. This complex fragments to atomic hydrogen and a linear 1-methylbutatrienyl radical, H2CCCCCH3(X2A″), via a tight exit transition state located about 18 kJmol−1 above the separated products. The experimentally determined exothermicity of 190±25 kJmol−1 is in strong agreement with our calculated data of 180±10 kJmol−1. The explicit verification of the carbon versus hydrogen exchange pathway together with the first identification of the H2CCCCCH3 radical represents a third pathway to form chain C5H5 radicals in the reactions of C(3Pj) with C4H6 isomers under single collision conditions. Previous experiments of atomic carbon with the 1,3-butadiene isomer verified the formation of 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X2A″), and H2CCCC2H3(X2A″), respectively. In high-density environments such as combustion flames and circumstellar envelopes of carbon stars, these linear isomers can undergo collision-induced ring closure(s) and/or H atom migration(s) which can lead to the cyclopentadienyl radical. The latter is thought to be a crucial reactive intermediate in soot formation and possibly in the production of polycyclic aromatic hydrocarbon molecules in outflow of carbon stars. Likewise, a H atom catalyzed isomerization can interconvert the 3-vinylpropargyl and the 1-methylbutatrienyl radical. © 2000 American Institute of Physics.

Notes: A new ab initio potential energy surface is generated for the chemical reaction, S(1D)+H2. The quantum chemistry calculations were carried out at the multi-reference configuration interaction (MRCI) level with multi-configuration self-consistent field (MCSCF) reference wave functions. The 1A′, 2A′, 3A′, 1A″, and 2A″ singlet surfaces were computed on a uniform spatial grid of over 2000 points to simulate the full reaction pathway. The results indicate a barrierless insertion pathway along the T-shaped geometry and an 8 kcal/mol barrier to abstraction along the collinear geometry. The lowest surface was fit to a smooth analytical function form based on the reproducing kernel Hilbert space approach and a Carter–Murrell-type expansion. The dynamics of the S(1D)+H2/D2 reactions were simulated using the quasi-classical trajectory method. The results are generally consistent with an insertion mechanism mediated through capture dynamics in the entrance channel followed by the statistical decay of a long-lived complex. Comparison to recent molecular beam experiments shows agreement in the broad pattern of results but also exhibits significant differences in the more finely resolved quantities. © 2001 American Institute of Physics.

Notes: The reaction between ground state carbon atoms, C(3Pj), and 1,3-butadiene, H2CCHCHCH2, was studied at three averaged collision energies between 19.3 and 38.8 kJmol−1 using the crossed molecular beam technique. Our experimental data combined with electronic structure calculations show that the carbon atom adds barrierlessly to the π-orbital of the butadiene molecule via a loose, reactantlike transition state located at the centrifugal barrier. This process forms vinylcyclopropylidene which rotates in a plane almost perpendicular to the total angular momentum vector J around its C-axis. The initial collision complex undergoes ring opening to a long-lived vinyl-substituted triplet allene molecule. This complex shows three reaction pathways. Two distinct H atom loss channels form 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X2A″) and H2CCCC2H3(X2A″), through tight exit transition states located about 20 kJmol−1 above the products; the branching ratio of 1- versus 3-vinylpropargyl radical is about 8:1. A minor channel of less than 10% is the formation of a vinyl, C2H3(X2A′), and propargyl radical C3H3(X2B2). The unambiguous identification of two C5H5 chain isomers under single collision has important implications to combustion processes and interstellar chemistry. Here, in denser media such as fuel flames and in circumstellar shells of carbon stars, the linear structures can undergo a collision-induced ring closure followed by a hydrogen migration to cyclic C5H5 isomers such as the cyclopentadienyl radical—a postulated intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs). © 2000 American Institute of Physics.

Notes: Nonadiabatic coupling matrix elements between the 1 2A′, 2 2A′, and 1 2A″ electronic states of the C2H radical are computed using ab initio full valence active space CASSCF method. The line-integral technique is then applied to study possible geometric phase effects. The results indicate the existence of a unique conical intersection due to CCH bending between the 1 2A′ and 2 2A′ states at the linear configuration in the vicinity of rCC=1.35 Å and rCH=1.60 Å. The line-integral calculations with ab initio nonadiabatic coupling terms confirm that when a path encircles the conical intersection, the line integral always produces the value π for the topological (Berry) phase and when a path encircles the two (symmetrical) conical interactions or none of them, the line integral produces the value of zero for the topological phase. © 2000 American Institute of Physics.

Notes: Crossed molecular beam experiments were conducted to investigate the reaction of ground state carbon atoms, C(3Pj), with 1,2-butadiene, H2CCCH(CH3) (X 1A′), at three collision energies of 20.4 kJ mol−1, 37.9 kJ mol−1, and 48.6 kJ mol−1. Ab initio calculations together with our experimental data reveal that the reaction is initiated by a barrier-less addition of the carbon atom to the π system of the 1,2-butadiene molecule. Dominated by large impact parameters, C(3Pj) attacks preferentially the C2–C3 double bond to form i1 (mechanism 1); to a minor extent, small impact parameters lead to an addition of atomic carbon to the C1–C2 bond yielding i2 (mechanism 2). Both cyclic intermediates i1 and i2 ring open to triplet methylbutatriene complexes i3′ (H2CC*CCH(CH3)) and i3″, (H2CCC*CH(CH3)); C* denotes the attacked carbon atom. i3′ is suggested to decay nonstatistically prior to a complete energy randomization via atomic hydrogen loss forming 1- and 4-methylbutatrienyl CH3CCCCH2 (X 2A″) and HCCCCH(CH3) (X 2A″), respectively. The energy randomization in i3″ is likely to be complete. This isomer decomposes via H atom loss to 3-vinylpropargyl, H2CCCC2H3(X 2A″), as well as 1- and 4-methylbutatrienyl radicals. In high-density environments such as the inner regions of circumstellar envelopes of carbon stars and combustion flames, these linear C5H5 isomers might undergo collision induced isomerization to cyclic structures like the cyclopentadienyl radical. This isomer is strongly believed to be a key intermediate involved in the production of polycyclic aromatic hydrocarbon molecules and soot formation. These characteristics make the reactions of atomic carbon with C4H6 isomers compelling candidates to form C5H5 isomers in the outflow of AGB stars and oxygen-deficient hydrocarbon flames. © 2001 American Institute of Physics.

Notes: The O(1D and 3P)+SiH4 reactions have been studied using ab initio/Rice–Ramsperger–Kassel–Marcus calculations to investigate possible formation mechanisms of various products in combustion and chemical vapor deposition processes. The relative branching ratios for various products formed through the O(1D)+SiH4 reaction involving the long-lived H3SiOH intermediate are calculated as 55.5% for the H2SiO/HSiOH+2H channel, 28.4% for the SiO+2H2 channel, 9.9% for the OH+SiH3 channel, 3.2% for the H2O+SiH2 channel, and 3.0% for the HSiO/SiOH+H2+H channel. These results significantly differ from those obtained in experiment, implying that the O(1D)+SiH4 reaction can take place through a mechanism other than the insertion mechanism. While the O(3P)+SiH4 reaction takes place by the abstraction mechanism, the O(1D)+SiH4 reaction can occur through both insertion and addition/abstraction mechanisms. The addition/abstraction mechanism occurring on the first excited potential energy surface is demonstrated to provide a significant contribution to the reaction products and to account for the forward scattering of the OH products observed in experiment. Finally, heats of formation for various species involving Si atom are computed employing the Gaussian 3 theory. © 2001 American Institute of Physics.

Notes: The three ab initio nonadiabatic coupling terms related to the three strongly coupled states of the C2H molecule, i.e., 2 2A′, 3 2A′, and 4 2A′, were studied applying the line integral technique [M. Baer, Chem. Phys. Lett. 35, 112 (1975)]. The following was verified: (1) Due to the close proximity of the conical intersections between these three states, two-state quantization cannot always be satisfied between two successive states. (2) It is shown that in those cases where the two-state quantization fails a three-state quantization is satisfied. This three-state quantization is achieved by applying the 3×3 nonadiabatic coupling matrix that contains the three relevant nonadiabatic coupling terms. The quantization is shown to be satisfied along four different contours (in positions and sizes) surrounding the relevant conical intersections. © 2002 American Institute of Physics.

Notes: Experimental and theoretical results are combined to show that vibrationally excited C2H radicals undergo photodissociation to produce C2 radicals mainly in the B 1Δg state. Infrared (IR) emissions from the photolysis of acetylene with a focused and unfocused 193 nm excimer laser have been investigated using step-scan Fourier transform infrared (FTIR) emission spectroscopy at both low and high resolution. With an unfocused laser, the low-resolution infrared emission spectra from the C2H radicals show a few new vibrational bands in addition to those previously reported. When the laser is focused, the only emissions observed in the 2800–5400 cm−1 region come from the electronic transitions of the C2 radicals. Most of the emissions are the result of the B 1Δg→A 1Πu transition of C2 although there are some contributions from the Ballik–Ramsay bands C2(b 3Σg−→a 3Πu). A ratio of [B 1Δg]/[b 3Σg−]=6.6 has been calculated from these results. High quality theoretical calculations have been carried out to determine what kind of ratio could be expected if the photodissociation products are formed solely by adiabatic dissociation from the excited states of C2H. To accomplish this, the geometries of different electronic states of C2H (X 2Σ+, A 2Π, 3–6 2A′, and 2–5 2A″) were optimized at the complete active space self consistent field [CASSCF(9,9)/6-311G**] level. The calculated normal modes and vibrational frequencies were then used to compute Franck–Condon factors for a variety of vibronic transitions. In order to estimate the oscillator strengths for transitions from different initial vibronic states of C2H, transition dipole moments were computed at different geometries. The overall Franck–Condon factor for a particular excited electronic state of C2H is defined as the sum of Franck–Condon factors originating from all the energetically accessible vibrational levels of C2H(X,A) states. The adiabatic excitation energies were calculated with the multi-reference configuration interaction/correlation-consistent polarized valence triple zeta [MRCI(9,9)/cc-PVTZ] method. The overall Franck–Condon factors were then multiplied by the corresponding oscillator strengths to obtain the total absorption intensities characterizing the probabilities for the formation of different excited states. Then, the excited states of C2H were adiabatically correlated to various electronic states of C2 (B 1Δg, A 1Πu, B′ 1Σg+, c 3Σu+, and b 3Σg−) to predict the photodissociation branching ratios from the different states of C2H, such as X(0,ν2,0), X(0,ν2,1), A(0,0,0), and A(0,1,0). For C2H produced by 193 nm photodissociation of acetylene, the calculations gave the following B:A:B′:b:c branching ratios of 38:32:10:14:6. This means that the theoretical branching ratio for the [B 1Δg]/[b 3Σg−] is 2.7, which is in excellent agreement with experiment. © 2001 American Institute of Physics.

Notes: The reaction between electronically excited carbon atoms, C(1D), and acetylene was studied at two average collision energies of 45 kJ mol−1 and 109 kJ mol−1 employing the crossed molecular beam technique. The time-of-flight spectra recorded at mass to charge m/e=37(C3H+) and m/e=36(C3+) show identical patterns indicating the existence of a carbon versus atomic hydrogen exchange pathway to form C3H isomer(s); no H2 elimination to the thermodynamically favorable tricarbon channel was observed. Forward-convolution fitting of our data shows that the reaction proceeds via direct stripping dynamics on the 1A′ surface via an addition of the carbon atom to the π-orbital of acetylene to form a highly rovibrationally, short lived cyclopropenylidene intermediate which decomposes by atomic hydrogen emission to c-C3H(X 2B2). The dynamics of this reaction have important impact on modeling of chemical processes in atmospheres of comets approaching the perihelon as photolytically generated C(1D) atoms are present. © 2001 American Institute of Physics.

Notes: In this article we report findings regarding various conical intersections between consecutive pairs of the five lowest 2A′ states of the C2H molecule. We found that conical intersections exist between each two consecutive 2A′ states. We showed that except for small (high-energy) regions in configuration space, the two lowest adiabatic states (i.e., the 1 2A′ and the 2 2A′) form a quasi-isolated system with respect to the higher states. We also revealed the existence of degenerate parabolical intersections, those with a topological (Berry) phase zero, formed by merging two conical intersections belonging to the 3 2A′ and the 4 2A′ states, and suggested a Jahn-Teller-type model to analyze them. Finally, we examined the possibility that the "frozen" locations of the carbons can be considered as points of conical intersection. We found that the relevant two-state topological phase is not zero nor a multiple of π, but that surrounding both carbons yields a zero topological phase. © 2001 American Institute of Physics.