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.     

A matrix isolation study of the C(s) symmetric OCNO(X 2A') radical

Here we report the first experimental detection of the C(s) symmetric nitroformyl radical, OCNO(X 2A') in a nitrogen-carbon dioxide matrix at 10 K using a Fourier transform infrared spectrometer (FTIR). The nu1 vibrational frequency was observed at 2113 cm(-1). This assignment was confirmed by follow-up experiments using isotopically labeled reactant molecules (15N, 18O, 13C). To synthesize this radical, we irradiated solid nitrogen-carbon dioxide ice mixtures with energetic electrons at 10 K. Suprathermal nitrogen atoms in their electronic ground and/or first electronically excited state were generated via the radiation induced degradation of molecular nitrogen; these atoms could then react with carbon dioxide to eventually yield the nitroformyl radical. We also investigated the kinetics of the formation of the nitroformyl radical and support the arguments with computations on the doublet and quartet OCNO potential energy surfaces (PESs).     

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.     

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.     

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.     

Theoretical study of isomerism, structure, and stability of dimers of C-doped aluminide clusters (C@Al12)2 and (C@Al12)(L@Al12) (L = Si, Ge)

The geometric, energetic, and vibrational characteristics of the dimers of carbon-doped aluminum clusters (C@Al₁₂)₂, (C@Al₁₂)(Si@Al₁₂), and (C@Al₁₂)(Ge@Al₁₂) have been calculated by ab initio density functional theory B3LYP method with the basis sets 6-31G* and 6-311+G*. As distinct from the silicon and germanium analogues (Si@Al₁₂)₂ and (Ge@Al₁₂)₂ with an AA lowest-lying structure, in which the Si@Al₁₂ and Ge@Al₁₂ blocks retain the shape of distorted icosahedra A, the symmetric AA structure is not typical of the (C@Al₁₂)₂ dimer (itcorresponds to a transition state) and the most favorable structures are two closely spaced BB and AD isomers, in which the C@Al₁₂ blocks have either a distorted icosahedral (A) or nonicosahedral geometry (B, a dicapped ten-vertex polyhedron; D, a marquee with a centered hexagonal base). The calculated energy of dissociation of the most favorable isomer AD into isolated C@Al₁₂ (I _h ) monomers is ～45 kcal/mol, which is 1.5–2 times as large as the corresponding dissociation energy of (Si@Al₁₂)₂ and (Ge@Al₁₂)₂. For mixed dimers of the (C@Al₁₂)(L@Al₁₂) type with L = Si or Ge, three types of low-lying isomers (AA, BA, and DA) are predicted in which the Si@Al₁₂ and Ge@Al₁₂ blocks retain their shape A and the C@Al₁₂ blocks have geometry A, B, and D, respectively. For all the isomers, the block-block bonds are comparable in length and energy to the bonds inside the blocks and have the same chemical nature. Specific features of the geometry, relative stability, ionization potentials, electron affinity, vibrational spectra, and other characteristics of doped aluminide dimers, which could be used for the identification of their isomers by spectroscopic methods, are discussed.     

Vibrationally Excited Ozone Relaxation by CO

Time profiles of ozone concentration after pulsed UV laser photolysis in the O₂- O₃-Ar-CO mixture, measured using time-resolved absorption spectroscopy, are presented. The experimental results show the dominance of the stabilization channel over the reactive one for the interaction between the vibrationally excited ozone molecule O₃(υ) and carbon monoxide CO. The rate constant of the process O₃(υ)+CO→O₃ + CO, obtained by processing experimental data by the kinetic modeling method is (1.5 ± 0.2) · 10^?13 cm³/s.     

Quantum-chemical study of crystal formation of supramolecular silver compounds with trans-1,2-Bis(4-pyridyl)ethylene and their electronic absorption spectra

The energies of singlet-singlet transitions in different supramolecular compounds formed by interaction of trans-1,2-bis(4-pyridyl)ethylene (DPyEt) with AgNO₃ have been calculated by the TDDFT method. The calculations show that the absorption peaks at 305.7 and 318.3 nm can be assigned to a superposition of the spectra of small suprastructures, e.g., (Ag)(DPyEt)₂(NO₃), (Ag)₂(DPyEt)(NO₃)₂, and (Ag)(DPyEt)(NO₃). These transitions correspond to the excitation of an electron from the bonding orbital into the antibonding orbital with respect to the C=C bond. Strong absorption bands at 350–360 nm, typical of a solid phase, can be assigned to the transition from the MO of the lone pair of the nitrogen of DPyEt into the antibonding MO with respect to the C=C bond. Such bands are present in the calculated excitation spectra of the (Ag)₁(DPyEt)₃(NO₃)₁ and (Ag)₂(DPyEt)₄(NO₃)₂ suprastructures.     

An ab initio/RRKM study of product branching ratios in the photodissociation of buta-1,2- and -1,3-dienes and but-2-yne at 193 nm

Ab initio G2M(MP2)//B3LYP/6-311G** calculations have been performed to investigate the reaction mechanism of photodissociation of buta-1,2- and -1,3-dienes and but-2-yne after their internal conversion into the vibrationally hot ground electronic state. The detailed study of the potential-energy surface was followed by microcanonical RRKM calculations of energy-dependent rate constants for individual reaction steps (at 193 nm photoexcitation and under collision-free conditions) and by solution of kinetic equations aimed at predicting the product branching ratios. For buta-1,2-diene, the major dissociation channels are found to be the single Cbond;C bond cleavage to form the methyl and propargyl radicals and loss of hydrogen atoms from various positions to produce the but-2-yn-1-yl (p1), buta-1,2-dien-4-yl (p2), and but-1-yn-3-yl (p3) isomers of C(4)H(5). The calculated branching ratio of the CH(3) + C(3)H(3)/C(4)H(5) + H products, 87.9:5.9, is in a good agreement with the recent experimental value of 96:4 (ref. 21) taking into account that a significant amount of the C(4)H(5) product undergoes secondary dissociation to C(4)H(4) + H. The isomerization of buta-1,2-diene to buta-1,3-diene or but-2-yne appears to be slower than its one-step decomposition and plays only a minor role. On the other hand, the buta-1,3-diene-->buta-1,2-diene, buta-1,3-diene-->but-2-yne, and buta-1,3-diene-->cyclobutene rearrangements are significant in the dissociation of buta-1,3-diene, which is shown to be a more complex process. The major reaction products are still CH(3) + C(3)H(3), formed after the isomerization of buta-1,3-diene to buta-1,2-diene, but the contribution of the other radical channels, C(4)H(5) + H and C(2)H(3) + C(2)H(3), as well as two molecular channels, C(2)H(2) + C(2)H(4) and C(4)H(4) + H(2), significantly increases. The overall calculated C(4)H(5) + H/CH(3) + C(3)H(3)/C(2)H(3) + C(2)H(3)/C(4)H(4) + H(2)/C(2)H(2) + C(2)H(4) branching ratio is 24.0:49.6:4.6:6.1:15.2, which agrees with the experimental value of 20:50:8:2:2022 within 5 % margins. For but-2-yne, the one-step decomposition pathways, which include mostly H atom loss to produce p1 and, to a minor extent, molecular hydrogen elimination to yield methylethynylcarbene, play an approximately even role with that of the channels that involve the isomerization of but-2-yne to buta-1,2- or -1,3-dienes. p1 + H are the most important reaction products, with a branching ratio of 56.6 %, followed by CH(3) + C(3)H(3) (23.8 %). The overall C(4)H(5) + H/CH(3) + C(3)H(3)/C(2)H(3) + C(2)H(3)/C(4)H(4) + H(2)/C(2)H(2) + C(2)H(4) branching ratio is predicted as 62.0:23.8:2.5:5.7:5.6. Contrary to buta-1,2- and -1,3-dienes, photodissociation of but-2-yne is expected to produce more hydrogen atoms than methyl radicals. The isomerization mechanisms between various isomers of the C(4)H(6) molecule including buta-1,2- and -1,3-dienes, but-2-yne, 1-methylcyclopropene, dimethylvinylidene, and cyclobutene have been also characterized in detail.