Photodissociation dynamics of pyridine

Photodissociation of pyridine, 2,6-d2-pyridine, and d5-pyridine at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C5NH5 --> C5NH4 + H (10%) and five ring opening dissociation channels, C5NH5 --> C4H4 + HCN, C5NH5 --> C3H3 + C2NH2, C5NH5 --> C2H4 +C3NH, C5NH5 --> C4NH2 + CH3 (14%), and C5NH5 --> C2H2 + C3NH3. Extensive H and D atom exchanges of 2,6-d2-pyridine prior to dissociation were observed. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion. The dissociation rate of pyridine excited by 248-nm photons was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 2x10(3) s(-1). Comparisons with potential energies obtained from ab initio calculations and dissociation rates obtained from the Rice-Ramsperger-Kassel-Marcus theory have been made.     

Effect of quantum entanglement on Aharonov-Bohm oscillations, spin-polarized transport and current magnification effect

We present a simple model of transmission across a metallic mesoscopic ring. In one of its arm an electron interacts with a single magnetic impurity via an exchange coupling. We show that entanglement between electron and spin impurity states leads to reduction of Aharonov-Bohm oscillations in the transmission coefficient. The spin-conductance is asymmetric in the flux reversal as opposed to the two-probe electrical conductance which is symmetric. In the same model, in contradiction to the naive expectation of a current magnification effect, we observe enhancement as well as suppression of this effect depending on the system parameters. The limitations of this model to the general notion of dephasing or decoherence in quantum systems are pointed out.     

Fission probabilities of Cu,Nb, Ag,Ho, Au and Bi nuclei excited by stopped antiprotons

The A dependence of nuclear fission induced by stopped antiprotons has been measured. An unambiguous identification of the binary fission decay mode was provided by a coordinate measurement of complementary fission fragments in coincidence using a large-acceptance fission detector based on low pressure multiwire proportional chambers. A deep fissility minimum was observed nearA=100, in agreement with the general behaviour predicted by the liquid-drop model. An unexpectedly low and high fission probability was found for the Ag and Cu nuclei, respectively.     

Chemical dynamics of the formation of the 1,3-butadiynyl radical (C4H(X2Sigma+)) and its isotopomers

The reaction of dicarbon molecules in their electronic ground, C2(X1Sigma(g)+), and first excited state, C2(a3Pi(u)), with acetylene, C2H2(X1Sigma(g)+), to synthesize the 1,3-butadiynyl radical, C4H(X2Sigma+), plus a hydrogen atom was investigated at six different collision energies between 10.6 and 47.5 kJ mol(-1) under single collision conditions. These studies were contemplated by crossed molecular beam experiments of dicarbon with three acetylene isotopomers C2D2(X1Sigma(g)+), C2HD (X1Sigma+), and 13C2H2(X1Sigma(g)+) to elucidate the role of intersystem crossing (ISC) and of the symmetry of the reaction intermediate(s) on the center-of-mass functions. On the singlet surface, dicarbon was found to react with acetylene through an indirect reaction mechanism involving a diacetylene intermediate. The latter fragmented via a loose exit transition state via an emission of a hydrogen atom to form the 1,3-butadiynyl radical C4H(X2Sigma+). The D(infinity)(h) symmetry of the decomposing diacetylene intermediate results in collision-energy invariant, isotropic (flat) center-of-mass angular distributions of this microchannel. Isotopic substitution experiments suggested that at least at a collision energy of 29 kJ mol(-1), the diacetylene isotopomers are long-lived with respect to their rotational periods. On the triplet surface, the reaction involved three feasible addition complexes located in shallower potential energy wells as compared to singlet diacetylene. The involvement of the triplet surface accounted for the asymmetry of the center-of-mass angular distributions. The detection of the 1,3-butadiynyl radical, C4H(X2Sigma+), in the crossed beam reaction of dicarbon molecules with acetylene presents compelling evidence that the 1,3-butadiynyl radical can be formed via bimolecular reactions involving carbon clusters in extreme environments such as circumstellar envelopes of dying carbon stars and combustion flames.     

Mechanism and rate constants of the CH2 + CH2CO reactions in triplet and singlet states: A theoretical study

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH₂ with ketene CH₂CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice–Ramsperger–Kassel–Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH₃ and C₂H₄ + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C₂H₄ + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH₂COCH₂ intermediate or along the pathway of CO elimination from the initial CH₂CH₂CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C₂H₄ + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH₃CHCO and cyclic CH₂COCH₂ intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300–3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.     

Gas-Phase Synthesis of Triphenylene (C18H12)

For the last decades, the hydrogen-abstraction−acetylene-addition (HACA) mechanism has been widely invoked to rationalize the high-temperature synthesis of PAHs as detected in carbonaceous meteorites (CM) and proposed to exist in the interstellar medium (ISM). By unravelling the chemistry of the 9-phenanthrenyl radical ([C₁₄H₉]^.) with vinylacetylene (C₄H₄), we present the first compelling evidence of a barrier-less pathway leading to a prototype tetracyclic PAH – triphenylene (C₁₈H₁₂) – via an unconventional hydrogen abstraction–vinylacetylene addition (HAVA) mechanism operational at temperatures as low as 10 K. The barrier-less, exoergic nature of the reaction reveals HAVA as a versatile reaction mechanism that may drive molecular mass growth processes to PAHs and even two-dimensional, graphene-type nanostructures in cold environments in deep space thus leading to a better understanding of the carbon chemistry in our universe through the untangling of elementary reactions on the most fundamental level.     

Eigenvalue Asymptotics of the Stokes Operator for Fractal Domains

The leading asymptotic term for the function that counts theeigenvalues of the Stokes operator is determined for fairlygeneral underlying bounded domains. Moreover, the remainderis estimated in terms of the fractality of the boundary of thedomain. The results obtained resemble corresponding ones forthe Dirichlet Laplacian. 1991 Mathematics Subject Classification:35P20.     

First detection of the Cs symmetric isomer of carbon hexaoxide (CO6) at 10 K

Carbon oxides of the form CO_n (n = 2–8) have long been known as important molecules in atmospheric and solid state chemical reactions. Here, we report on the first infrared spectroscopic detection of the cyclic (C_s) isomer of carbon hexaoxide (¹²C¹⁶O₆) via its ν₁ vibrational mode centered around 1876 cm⁻¹ under matrix isolation conditions; the identification of the ¹²C¹⁸O₆, ¹³C¹⁶O₆, and ¹³C¹⁸O₆, isotopologues supported by ab initio calculations confirm the assignments. We also discuss possible formation routes of this molecule.     

Ab initio G3-type/statistical theory study of the formation of indene in combustion flames. I. Pathways involving benzene and phenyl radical

Ab initio G3(MP2,CC)//B3LYP calculations of the potential energy surface (PES) for the formation of indene involving hydrocarbon species abundant in combustion, including benzene, phenyl, propargyl, and methyl radicals, and acetylene, have been performed to investigate the build-up of an additional cyclopenta moiety over the existing six-member aromatic ring. They were followed by statistical calculations of high-pressure-limit thermal rate constants in the temperature range of 300-3000 K for all reaction steps utilizing conventional Rice-Ramsperger-Kassel-Marcus (RRKM) and transition-state (TST) theories. The hydrogen abstraction acetylene addition (HACA) type mechanism, which involves the formation of benzyl radical followed by addition of acetylene, is shown to have low barriers (12-16 kcal/mol) and to be a viable candidate to account for indene formation in combustion flames, such as the 1,3-butadiene flame, where this mechanism was earlier suggested as the major indene formation route (Granata et al. Combust. Flame 2002, 131, 273). The mechanism of indene formation involving the addition of propargyl radical to benzene and rearrangements on the C9H9 PES is demonstrated to have higher barriers for all reaction steps as compared to an alternative pathway, which starts from the recombination of phenyl and propargyl radicals and then proceeds by activation of the C9H8 adducts by H abstraction or elimination followed by five-member ring closure in C9H7 and H addition to the 2-indenyl radical. The suggested pathways represent potentially important contributors to the formation of indene in combustion flames, and the computed rate constants can be utilized in kinetic simulations of the reaction mechanisms leading to indene and to higher cyclopentafused polycyclic aromatic hydrocarbons (CP-PAH).