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Carbon-rich radicals are important for understanding chemistry of flames as well as astrochemistry. However, they are elusive for both experiments and computations, due to the peculiarity of radicals. Here, we calculated excited states of the 1-buten-3-yn-2-yl (HCCCCH2 , i-C4H3 ) and n-C4H3 (HCCCHCH) radicals with the state-of-the-art complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) methods using the 6-311 + G(3df,2p) and aTVZ basis sets. The vertical energies for the seven excited states of each of the two isomers were obtained with the MRCI method at the geometries optimized at the CASSCF/6-311G(d,p) level. The result using the aTVZ basis set has been more reliable than predicted with the 6-311 + G(3df,2p) basis set with the relative stability of the ground states of the two isomers better predicted in comparison to existing data. We believe that our results will be useful for understanding the chemistry of carbon-rich radicals.

Ab initio calculations of the potential energy surface for the C3(1Sigmag+)+C2H2(1Sigmag+) reaction have been performed at the RCCSD(T)/cc-pVQZ//B3LYP/6-311G(d,p) + ZPE[B3LYP/6-311G(d,p)] level with extrapolation to the complete basis set limit for key intermediates and products. These calculations have been followed by statistical calculations of reaction rate constants and product branching ratios. The results show the reaction to begin with the formation of the 3-(didehydrovinylidene)cyclopropene intermediate i1 or five-member ring isomer i7 with the entrance barriers of 7.6 and 13.8 kcal/mol, respectively. i1 rearranges to the other C5H2 isomers, including ethynylpropadienylidene i2, singlet pentadiynylidene i3, pentatetraenylidene i4, ethynylcyclopropenylidene i5, and four- and five-member ring structures i6, i7, and i8 by ring-closure and ring-opening processes and hydrogen migrations. i2, i3, and i4 lose a hydrogen atom to produce the most stable linear isomer of C5H with the overall reaction endothermicity of approximately 24 kcal/mol. H elimination from i5 leads to the formation of the cyclic C5H isomer, HC2C3, +H, 27 kcal/ mol above C3+C2H2. 1,1-H2 loss from i4 results in the linear pentacarbon C5+H2 products endothermic by 4 kcal/mol. The H elimination pathways occur without exit barriers, whereas the H2 loss from i4 proceeds via a tight transition state 26.4 kcal/mol above the reactants. The characteristic energy threshold for the reaction under single collision conditions is predicted be in the range of approximately 24 kcal/mol. Product branching ratios obtained by solving kinetic equations with individual rate constants calculated using RRKM and VTST theories for collision energies between 25 and 35 kcal/mol show that l-C5H+H are the dominant reaction products, whereas HC2C3+H and l-C5+H2 are minor products with branching ratios not exceeding 2.5% and 0.7%, respectively. The ethynylcyclopropenylidene isomer i5 is calculated to be the most stable C5H2 species, more favorable than triplet pentadiynylidene i3t by approximately 2 kcal/mol.

Density functional B3LYP /6-31 G (d,p) calculations have been performed in order to investigate isomerization and dissociation of [Formula: see text] in the ground electronic state, which are relevant to the Coulomb explosion mechanism of benzene. The results demonstrate that the benzene-like isomer of [Formula: see text], 1, can decompose through various pathways leading to distinct fragmentation products. The most kinetically favorable channel involves ring opening in 1 accompanied with 1,2-shifts of two hydrogen atoms followed by rotation around the middle C–C bond and dissociation to H 2 CCCH 2+ + H 2 CCCH + with exothermicity and the highest barrier of 100.2 and 38.4 kcal/mol, respectively. Several other product channels, including [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text], share the same highest barrier for the initial ring opening step with the pathway producing H 2 CCCH 2+ + H 2 CCCH + but exhibit higher second highest barriers. Elimination of [Formula: see text] to form [Formula: see text] is 137.9 kcal/mol exothermic but the highest barrier on the pathway leading to these products is 53.0 kcal/mol. A simple proton elimination to produce [Formula: see text] is computed to be 82–84 kcal/mol exothermic and to depict a 64–65 kcal/mol barrier. RRKM calculations of rate constants for individual reaction steps assuming that the initial internal energy of 1 is 110 kcal/mol and solving kinetic master equations to obtain relative branching ratios show that H 2 CCCH 2+ + H 2 CCCH + are the dominant products (81.5%) followed by [Formula: see text] (13.2%) and the other minor products include [Formula: see text] (2.6%), [Formula: see text] (1.1%), [Formula: see text] (0.55%), [Formula: see text] (0.49%), [Formula: see text] (0.20%), and [Formula: see text] (0.14%). The fragments are expected to be produced with high translational energy due to high Coulomb repulsion energy barriers.

Calculations of the lowest triplet state potential energy surface for the C(3P) + CH4 reaction have been performed using the CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p) method, and the microcanonical RRKM approach has been used to compute rate constants for individual reaction steps and product branching ratios. The results show that the reaction can occur by abstraction and insertion mechanisms. The abstraction pathway producing CH(2Π) + CH3(2A2‘ ‘) has a barrier of 26.9 kcal/mol relative to the reactants. The insertion leading to the HC−CH3(3A‘ ‘) intermediate via a 12.2 kcal/mol barrier followed by its isomerization to H2C−CH2(3A1) (through a 1,2 H shift) and/or by dissociation with an H-atom loss is found to be a more favorable mechanism. At a low excess internal energy originating from the collision energy (12.2 kcal/mol), the sole reaction products are C2H3 + H, where 90% of them are formed through the fragmentation of HC−CH3 and the rest (10%) are produced via the H2C−CH2 intermediate. At the higher...

Geometries, vibrational frequencies, excitation energies, and vibronic spectra of seven excited states of C6H5 have been studied by multireference ab initio calculations. For the 1 2 B 1 ,1 2 B 2 ,2 2 B 2 , and 2 2 B 1 states the most intense vibronic peaks are calculated at 17952, 37744, 43380, and 43251 cm −1 , respectively, and for 1 2 A 2 , 2 2 A 1 , and 3 2 A 1 upper estimates for adiabatic excitation energies are 22176, 30988, and 42543 cm −1 . We have assigned the 530–440 nm band in the absorption spectra to 1 2 B 1 with possible contribution from 1 2 A 2 , the 265–230 nm band to 1 2 B 2 and 3 2 A 1 , and intense peaks starting from ∼212 nm to 2 2 B 1 and 2 2 B 2 .

This paper presents a comprehensive theoretical treatment of the low-lying electronic states of the molecular ion AsCl+ correlating with the lowest dissociation asymptote As+ (3P) + Cl (2P). All-electron CASSCF + CI calculations have been made with averaged atomic natural orbital basis sets. There are four bound states, namely X2Π, A2Π, 1 4 Σ − and 1 4 Π . Spectroscopic constants are calculated for the bound states and are in good agreement with experimental data for the doublet states.

Ab initio G2M calculations have been performed to investigate the potential energy surface for the reaction of C6H5 with O2. The reaction is shown to start with an exothermic barrierless addition of O2 to the radical site of C6H5 to produce phenylperoxy (1) and, possibly, 1,2-dioxaspiro[2.5]octadienyl (dioxiranyl, 8) radicals. Next, 1 loses the terminal oxygen atom to yield the phenoxy + O products (3) or rearranges to 8. The dioxiranyl can further isomerize to a seven-member ring 2-oxepinyloxy radical (10), which can give rise to various products including C5H5 + CO2, pyranyl + CO, o-benzoquinone + H, and 2-oxo-2,3-dihydrofuran-4-yl + C2H2. Once 10 is produced, it is unlikely to go back to 8 and 1, because the barriers separating 10 from the products are much lower than the reverse barrier from 10 to 8. Thus, the branching ratio of C6H5O + O against the other products is mostly controlled by the critical transition states between 1 and 3, 1 and 8, and 8 and 10. According to the calculated barriers, the most favorable product channel for the decomposition of 10 is C5H5 + CO2, followed by pyranyl + CO and o-benzoquinone + H. Since C6H5O + O and C5H5 + CO2 are expected to be the major primary products of the C6H5 + O2 reaction and thermal decomposition of C6H5O leads to C5H5 + CO, cyclopentadienyl radicals are likely to be the major product of phenyl radical oxidation, and so it results in degradation of the six-member aromatic ring to the five-member cyclopentadienyl ring. Future multichannel RRKM calculations of reaction rate constants are required to support these conclusions and to quantify the product branching ratios at various combustion conditions.