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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.

Notes: The potential energy surface for the unimolecular decomposition of benzene and H+C6H5 recombination has been studied by the ab initio G2M(cc, MP2) method. The results show that besides direct emission of a hydrogen atom occurring without an exit channel barrier, the benzene molecule can undergo sequential 1,2-hydrogen shifts to o-, m-, and p-C6H6 and then lose a H atom with exit barriers of about 6 kcal/mol. o-C6H6 can eliminate a hydrogen molecule with a barrier of 121.4 kcal/mol relative to benzene. o- and m-C6H6 can also isomerize to acyclic isomers, ac-C6H6, with barriers of 110.7 and 100.6 kcal/mol, respectively, but in order to form m-C6H6 from benzene the system has to overcome a barrier of 108.6 kcal/mol for the 1,2-H migration from o-C6H6 to m-C6H6. The bimolecular H+C6H5 reaction is shown to be more complicated than the unimolecular fragmentation reaction due to the presence of various metathetical processes, such as H-atom disproportionation or addition to different sites of the ring. The addition to the radical site is barrierless, the additions to the o-, m-, and p-positions have entrance barriers of about 6 kcal/mol and the disproportionation channel leading to o-benzyne+H2 has a barrier of 7.6 kcal/mol. The Rice-Ramsperger-Kassel-Marcus and transition-state theory methods were used to compute the total and individual rate constants for various channels of the two title reactions under different temperature/pressure conditions. A fit of the calculated total rates for unimolecular benzene decomposition gives the expression 2.26×1014 exp(−53 300/T)s−1 for T=1000–3000 K and atmospheric pressure. This finding is significantly different from the recommended rate constant, 9.0×1015 exp(−54 060/T) s−1, obtained by kinetic modeling assuming only the H+C6H5 product channel. At T=1000 K, the branching ratios for the formation of H+C6H5 and ac-C6H6 are 29% and 71%, respectively. H+C6H5 becomes the major channel at T≥1200 K. The total rate for the bimolecular H+C6H5 reaction is predicted to be between 4.5×10−11 and 2.9×10−10cm3 molecule−1 s−1 for the broad range of temperatures (300–3000 K) and pressures (100 Torr–10 atm). The values in the T=1400–1700 K interval, ∼8×10−11 cm3 molecule−1 s−1, are ∼40% lower than the recommended value of 1.3×10−10 cm3 molecule−1 s−1. The recombination reaction leading to direct formation of benzene through H addition to the radical site is more important than H disproportionations at T〈2000 K. At higher temperatures the recombination channel leading to o-C6H4+H2 and the hydrogen disproportionation channel become more significant, so o-benzyne+H2 should be the major reaction channel at T〉2500 K. © 2001 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 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: 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: 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.