Isomerization Channels Research Articles

Formation, Isomerization, and Dissociation of α-Carbon-Centered and π-Centered Glycylglycyltryptophan Radical Cations

Gas phase fragmentations of two isomeric radical cationic tripeptides of glycylglycyltryptophan-[G(*)GW](+) and [GGW](*+)-with well-defined initial radical sites at the alpha-carbon atom and the 3-methylindole ring, respectively, have been studied using collision-induced dissociation (CID), density functional theory (DFT), and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Substantially different low-energy CID spectra were obtained for these two isomeric GGW structures, suggesting that they did not interconvert on the time scale of these experiments. DFT and RRKM calculations were used to investigate the influence of the kinetics, stabilities, and locations of the radicals on the competition between the isomerization and dissociation channels. The calculated isomerization barrier between the GGW radical cations (>35.4 kcal/mol) was slightly higher than the barrier for competitive dissociation of these species (<30.5 kcal/mol); the corresponding microcanonical rate constants for isomerization obtained from RRKM calculations were all considerably lower than the dissociation rates at all internal energies. Thus, interconversion between the GGW isomers examined in this study cannot compete with their fragmentations.

Exploration of structure, potential energy surface, and stability of planar C3B3

The geometrical structures, potential energy surface, stability, and bonding character of low-energy isomers of planar C(3)B(3) were systematically explored and investigated at the B3LYP/6-311+G(d)// CCSD(T)/6-311+G(d) level for the first time. A large number of planar structures for low-energy isomers of C(3)B(3) are located and reported. In particular, isomers 1 (C(s),(2)A') and 2 (C(s),(2)A'), with a belt-like structure corresponding to the lowest-energy structures of planar C(3)B(3), are revealed. Based on molecular orbital (MO) and natural bond orbital (NBO) analyses, delocalized sigma MOs, multi-centered sigma MOs, and delocalized pi MOs play an important role in stabilizing the structures of low-energy isomers of C(3)B(3). It is interesting to note from isomerization analysis that the interconversion of isomers 2 and 7 can be realized through two isomerization channels. The results demonstrate that isomers 1, 2, 3, 4, 7, 9, 12, 17, 19, and 20 of C(3)B(3) are stable both thermodynamically and kinetically at the B3LYP/ 6-311+G(d)//CCSD(T)/ 6-311+G(d) level, and that they are observable in the laboratory, which is helpful for future experimental studies of C(3)B(3).

Theoretical Study on the Reaction of Ground State Cyano Radical with Propylene in Titan’s Atmosphere

The bimolecular reaction of ground state cyano radical with propylene under the condition of single collision is investigated by combining ab initio electronic structure calculations for predicting reaction paths and RRKM theory to yield rate constant for each path. The isomerization and dissociation channels for each of the seven collision complexes are characterized by utilizing the unrestricted B3LYP/cc-pVTZ level of theory and the CCSD(T)/cc-pVTZ calculations. Sifting with the facilitation of RRKM rate constants through web of ab initio paths composed of 8 collision complexes, 37 intermediates, and 12 H-, 23 H(2)-, 3 HCN-, and 4 CH(3)-dissociated products, we identify the most probable paths down to 7-9 species at collisions energies of 0 and 5 kcal/mol as the reaction mechanisms. The rate equations of the reaction mechanisms are solved numerically such that the concentration evolutions for all species involved are obtained. This study predicts that CN + C(2)H(3)CH(3) reaction via any of the seven collision complex, c1-c5, c7, and c8, would produce p1(CH(3)CHCHCN) + H, p2(CH(2)CHCH(2)CN) + H, and mostly p43(vinyl cyanide) + CH(3) for collision energy within 0-5 kcal/mol. In addition to the insertion mechanism through collision complex, the direct H-abstraction of propylene by CN radical might occur. Our investigation indicates that the barrierless and exoergic CN(X(2)Sigma(+)) + C(2)H(3)CH(3)(X(1)A') reaction would be an efficient route for the p1, p2, and p43, and likely HCN formation in cold molecular clouds and in the atmosphere of Saturn's satellite Titan.

Synthesis of interstellar 1,3,5-heptatriynylidyne, C7H(X Π2), via the neutral-neutral reaction of ground state carbon atom, C(P3), with triacetylene, HC6H (X Σ1g+)

The reaction of ground-state carbon atom with a polyyne, triacetylene (HC6H) is investigated theoretically by combining ab initio calculations for predicting reaction paths, RRKM theory to yield rate constant for each path, and a modified Langevin model for estimating capturing cross sections. The isomerization and dissociation channels for each of the five collision complexes are characterized by utilizing the unrestricted B3LYP/6-311G(d,p) level of theory and the CCSD(T)/cc-pVTZ calculations. Navigating with the aid of RRKM rate constants through web of ab initio paths composed of 5 collision complexes, 108 intermediates, and 20 H-dissociated products, the most probable paths, reduced to around ten species at collision energies of 0 and 10 kcal/mol, respectively, are identified and adopted as the reaction mechanisms. The rate equations for the reaction mechanisms are solved numerically such that the evolutions of concentrations with time for all species involved are obtained and their lifetimes deduced. This study predicts that the five collision complexes, c1–c5, would produce a single final product, C7H (p1)+H, via the most stable intermediate, carbon chain HC7H (i1); namely, C+HC6H→HC7H→C7H+H. Our investigation indicates that the title reaction is efficient to form astronomically observed C7H in cold molecular clouds, where a typical translational temperature is 10 K.

Theoretical mechanistic study on the reactions between FO 2 + NO and FO + NO 2

The detailed singlet potential energy surface (PES) of the [FNO 3] system is investigated at B3LYP/6-311+G(2d) and CCSD(T)/6-311+G(2d) (single-point) levels. Six isomers and nine transition states are located. Various possible isomerization and dissociation channels are probed in order to explore the reaction mechanism for FO 2 + NO. The calculated results indicate that the initial association between FO 2 and NO can lead to the adduct FOONO, followed by the concerted F-shift and cleavage of the ON bond leading to the dominant product FNO + O 2. The formation of other products, however, is much less feasible due to energy constraints. The three feasible products and reaction pathways are also analyzed for another reaction, FO + NO 2, on the basis of the [FNO 3] PES. This study may be helpful in understanding the reaction and chemical behaviors of oxygen fluorides.

Theoretical Studies on the Isomerizations of CH3NO2

AbstractThe isomerizations of CH3NO2 (NM) have been investigated by density function theory (DFT) and electron density topological analysis methods. Nine isomers and eight isomerization channels were found. The connection relationship between the transition state (TS) and the isomers was confirmed by intrinsic reaction coordinate (IRC) tracing calculation. The calculation results show that in the CH3NO2→CH3ONOt process, the TS is a tight one (CH3NO2 does not break into CH3 and NO2 throughout the reaction), which is consistent with Arenass' results. A long structure transition region, which contains a four‐numbered ring → a five‐numbered ring → a four‐numbered ring → a five‐numbered ring structure, exists in CH3NOOc→CH2NOOH process. And it is the first time that a five‐numbered ring TS was found in reaction.

Experimental and Theoretical Exploration of the Initial Steps in the Decomposition of a Model Nitramine Energetic Material: Dimethylnitramine

Decomposition of dimethylnitramine (DMNA, (CH(3))(2)NNO(2)) has been studied extensively over the past decades. Although several different mechanisms have been proposed for the initial decomposition of DMNA, the dominant decomposition channel is still far from fully understood. In this report, we collect all the results reported in the literature, along with our new experimental and theoretical results, into a single reference for a sensible comparison in order to reach a general conclusion on DMNA decomposition. In this effort, nanosecond laser, energy resolved spectroscopy and complete active space self-consistent field (CASSCF) calculations are employed. The parent DMNA molecule is electronically excited using two different UV excitation wavelengths, 226 and 193 nm, to initiate the decomposition process. The NO molecule is observed as a major decomposition product with relatively hot (120 K) rotational and cold vibrational distributions by both time-of-flight mass spectrometry and laser induced fluorescence spectroscopy. On the basis of the experimental observations, a nitro-nitrite isomerization mechanism is predicted to be the major channel of decomposition of DMNA in the excited electronic state with a minor contribution from the HONO elimination mechanism. The branching ratio between nitro-nitrite isomerization and HONO elimination channels is estimated to be approximately 1:0.04. CASSCF calculations show that surface crossing (conical intersection) between upper and lower electronic states along the nitro-nitrite isomerization reaction coordinate plays an important role in the overall decomposition of DMNA. Presence of such an (S(2)/S(1))(CI) conical intersection in the nitro-nitrite isomerization reaction coordinate provides a direct nonadiabatic decomposition pathway from the Franck-Condon point of the S(2) surface, which is experimentally accessed by 226 nm photoexcitation. This excited state isomerization takes place through a loose geometry for which the NO(2) moiety interacts with the (CH(3))(2)N moiety from a long distance (approximately 2.8 A); however, in the ground electronic state, a similar (S(1)/S(0))(CI) conical intersection in this nitro-nitrite isomerization reaction coordinate hinders the isomerization exit channel, rendering NO(2) elimination as the major thermal decomposition channel of DMNA.

Theoretical study on reaction mechanism of isocyanate radical NCO with ethene

AbstractThe NCO + C2H4 reaction is simple and prototype for reaction of the NCO radical with unsaturated hydrocarbons, and it is considered to be important in fuel‐rich combustion. In this article, we for the first time perform detailed theoretical investigations for its reaction mechanism based on Gaussian‐3//B3LYP scheme covering various entrance and decomposition channels. The most favorable channel is firstly the NCO and C2H4 approach each other, forming a weakly‐bound complex L1 OCN···C2H4, followed by the formation of isomer L2 OCNCH2CH2 via a small barrier of 1.3 kcal/mol. Transition states of any decomposable or isomeric channels for L2 in energy are much higher than reactants, which indicate that adduct L2 has stabilization effect in this NCO + C2H4 reaction. The direct H‐abstraction channel leading to P1 HNCO + C2H3, might have an important contribution to the eventual products in high temperature. These results can well explain available kinetic experiment. Moreover, reaction mechanism for the title reaction is significantly different from the NCO + C2H2 reaction which proceeds on most favorably to generate the products HCN + HCCO and OCCHCN + H via a four‐membered ring intermediate. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Formation of interstellar 2,4-pentadiynylidyne, HCCCCC(XΠ2), via the neutral-neutral reaction of ground state carbon atom, C(P3), with diacetylene, HCCCCH(XΣg+1)

The interstellar reaction of ground-state carbon atom with the simplest polyyne, diacetylene (HCCCCH), is investigated theoretically to explore probable routes to form hydrogen-deficient carbon clusters at ultralow temperature in cold molecular clouds. The isomerization and dissociation channels for each of the three collision complexes are characterized by utilizing the unrestricted B3LYP/6-311G(d,p) level of theory and the CCSD(T)/cc-pVTZ calculations. With facilitation of RRKM and variational RRKM rate constants at collision energies of 0-10 kcalmol, the most probable paths, thus reaction mechanism, are determined. Subsequently, the corresponding rate equations are solved that the evolutions of concentrations of collision complexes, intermediates, and products versus time are obtained. As a result, the final products and yields are identified. This study predicts that three collision complexes, c1, c2, and c3, would produce a single final product, 2,4-pentadiynylidyne, HCCCCC(X (2)Pi), C(5)H (p1)+H, via the most stable intermediate, carbon chain HC(5)H (i4). Our investigation indicates the title reaction is efficient to form astronomically observed 2,4-pentadiynylidyne in cold molecular clouds, where a typical translational temperature is 10 K, via a single bimolecular gas phase reaction.

Time-Resolved Cavity Ringdown Measurements and Kinetic Modeling of the Pressure Dependences of the Recombination Reactions of SiH2 with the Alkenes C2H4, C3H6, and t-C4H8

Room-temperature rate constants for the pressure-dependent reactions SiH2 + ethene, propene, and t-butene have been determined at total pressures of 3.3 mbar < or = p < or = 300 mbar with Ar as buffer gas. SiH2 was detected by means of time-resolved cavity ringdown spectroscopy (CRDS), and the deconvolution of ringdown, kinetics, and laser bandwidth effect was accomplished with the extended simultaneous kinetics and ringdown model (eSKaR). In this way, pseudofirst-order rate constants could be extracted from nonexponential ringdown profiles. The recombination reactions, including the reaction SiH2 + i-butene, have been modeled based on the simplified statistical adiabatic channel model (SACM) and weak collision energy-grained master equation (ME) simulations. The influence of an interfering fast isomerization channel was investigated based on the Rice, Ramsperger, Kassel, Marcus theory (RRKM) and was found to be only important for the C2H4 reaction. Using ab initio energies (G3) and structures (MP2/6-311G(d,p)) as input parameters for the kinetic models, a consistent description of the pressure and temperature dependences of all four reactions was possible. In the temperature range 295 K < or = T < or = 600 K, the extrapolated limiting high-pressure rate constants, k(infinity)(C2H4)/(cm(3) x mol(-1) x s(-1)) = 1.9 x 10(14) (T/K)(-0.065), k(infinity)(C3H6)/(cm(3) x mol(-1) x s(-1)) = 1.3 x 10(14) (T/K)(0.075), k(infinity)(i-C4H8) = 1.8 x 10(14) cm(3) x mol(-1) x s(-1), and k(infinity)(t-C4H8)/(cm(3) x mol(-1) x s(-1)) = 4.6 x 10(13) (T/K)(0.21), are close to the collision number and are more or less temperature independent. In the case of ethene, probably due to the approximate treatment of rotational effects and/or the interfering isomerization process, the applied model slightly underestimates the falloff and thus yields too high extrapolated rate constants at p < 10 mbar.

Surface Reaction of 1,2,-Dichloroethylene on Si(100)-2 × 1: Importance of Surface Isomerization Channel

The surface reactions of trans-1,2-dichloroethylene (DCE) on Si(100)-2 × 1 were theoretically investigated to study the effects of chlorine substitutions on surface reactions. As in the case of pristine ethylene, diradical as well as π-complex channels leading to [2 + 2] cycloaddition products were found. In general, chlorine substitution makes the former kinetically more favorable and also changes its stereochemistry. In addition to modifying those existing reaction channels, the chlorine substitution opens new subsequent low-energy pathways leading to the experimentally suggested mono- and dichlorinated adstructures. Their large stabilities with low kinetic barriers make them the major surface species. Current study demonstrates that substitutions not only have large effects on existing reaction channels, but also introduces new possibilities, significantly changing the overall surface reactions.

Theoretical Study on Reaction Mechanism of Fulminic Acid HCNO with CN Radical

The HCNO + CN reaction is one potentially important process during the NO-reburning process for the reduction of NOx pollutants from fossil fuel combustion emissions. To compare with the recent experimental study, we performed the first theoretical potential energy surface investigation on the mechanism of HCNO + CN at the G3B3 and CCSD(T)/aug-cc-pVTZ levels based on the B3LYP/6-311++G(d,p) structures, covering various entrance, isomerization, and decomposition channels. The results indicate that the most favorable channel is to barrierlessly form the entrance isomer L1c NCCHNO followed by successive ring closure and concerted CC and NO bond rupture to generate the product P1 HCN + NCO. However, the formation of P4 (3)HCCN + NO, predicted as the only major product in the recent experiment, is kinetically much less competitive. This conclusion is further supported by the master equation rate constant calculation. Future experimental reinvestigations are strongly desired to test the newly predicted mechanism for the CN + HCNO reaction. Implications of the present results are discussed.

Global reaction route mapping on potential energy surfaces of [formula omitted] and [formula omitted

Dissociation and isomerization channels of C 2 H 7 + and C 3 H 9 + have been globally explored by the scaled hypersphere search (SHS) method, which discloses reaction routes in an automatic way by following the anharmonic downward distortion on the potential energy surfaces. New structures have been discovered in addition to all of reported ones. Branching ratios leading to C 2 H 5 + + CH 4 → C 3 H 9 + → C 3 H 7 + + H 2 have been calculated based on microcanonical rate constants of all transition state structures elucidated by the SHS method. Generation of cyclic C 3 H 7 + has been predicted to be responsible for this reaction in a certain collision energy range.

Reaction of Hydrogen Atoms with Propyne at High Temperatures: An Experimental and Theoretical Study

The kinetics of the reaction of hydrogen atoms with propyne (pC3H4) was experimentally studied in a shock tube at temperatures ranging from 1200 to 1400 K and pressures between 1.3 and 4.0 bar with Ar as the bath gas. The hydrogen atoms (initial mole fraction 0.5-2.0 ppm) were produced by pyrolysis of C2H5I and monitored by atomic resonance absorption spectrometry under pseudo-first-order conditions with respect to propyne (initial mole fraction 5-20 ppm). From the hydrogen atom time profiles, overall rate coefficients k(ov) identical with -([pC3H4][H])(-1) x d[H]/dt for the reaction H + pC3H4 --> products ( not equal H) were deduced; the following temperature dependence was obtained: kov = 1.2 x 10(-10) exp(-2270 K/T) cm(3) s(-1) with an estimated uncertainty of +/-20%. A pressure dependence was not observed. The results are analyzed in terms of statistical rate theory with molecular and transition state data from quantum chemical calculations. Geometries were optimized using density functional theory at the B3LYP/6-31G(d) level, and single-point energies were computed at the QCISD(T)/cc-pVTZ level of theory. It is confirmed that the reaction proceeds via an addition-elimination mechanism to yield C2H2 + CH3 and via a parallel direct abstraction to give C3H3 + H2. Furthermore, it is shown that a hydrogen atom catalyzed isomerization channel to allene (aC3H4), H + pC3H4 --> aC3H4 + H, is also important. Kinetic parameters to describe the channel branching of these reactions are deduced.

Theoretical Investigation on Stability and Isomerizations of CH3SO Isomers

The stability and isomerizations of CH3SO isomers have been investigated at B3LYP/6-311G(d,p), MP2/6-311G(d,p), QCISD/6-311G(d,p), and CCSD(T)/6-311G(d,p) levels. Geometries of isomers and transition states (TS) have been optimized at the B3LYP/6-311G(d, p) level. Vibration analysis and the intrinsic reaction coordinate (IRC) calculated at the same level have been applied to validate the connection of the stationary points. The four different methods give similar results: 11 isomers and 9 isomerization channels were found. CH3SO and CH2(S)OH are the most stable species among the 11 isomers. Furthermore, the breakage and formation of the chemical bonds in isomerization reactions have been discussed by the topological analysis method of electronic density. The "energy transition state (ETS)" and the "structure transition state (STS)" of all the isomerizations have been found. The topological analysis shows that the relative positions of ETS and STS are determined by reaction energy. The nonplanar four-member ring structure transition state (STS), which was first found in this paper, extended the concept of ring STS.

Composite C- and H-scrambling and fragmentation of long-lived protonated 6-methylfulvene and 6,6-dimethylfulvene—alternative entry to the gas-phase chemistry of gaseous toluenium (C 7H 9+) and xylenium (C 8H 11+) ions

The fragmentation of metastable 6-methylfulvenium and 6,6-dimethylfulvenium ions, generated by gas-phase protonation of the neutral hydrocarbons in the CI(CH 4) plasma, has been investigated by means of mass-analyzed kinetic energy (MIKE) spectrometry. The C 7H 9 + ions formed from 6-methylfulvene lose H 2 and CH 4, and the C 8H 11 + ions formed from 6,6-dimethylfulvene lose H 2, CH 4 and C 2H 4 in relative amounts and with kinetic energy release characteristics that are reminiscent of the fragmentation behavior of toluenium and xylenium ions, respectively. The isomerization reactions preceding the fragmentation of long-lived protonated 6,6-dimethylfulvene, C 8H 11 +, have been studied in detail by 13C- and 2H-labelling of this cycloolefin and analysis of the isotope distributions observed in the MIKE spectra of the [M + H – methane] + and [M + H – ethene] + ions, in particular. Composite scrambling was found to occur prior to both of these fragmentation reactions, that is, metastable C 8H 11 + ions form two populations that react through different isomerization channels prior to loss of the same neutral fragment. The loss of CH 4 is largely (95%) C-specific but only CH 3-specific for ca. 50% of the ions, whereas the other 50% fraction of the ions suffer complete H scrambling. The loss of C 2H 4 is much less specific: it takes place from a 70–80% fraction of C 8H 11 + ions in which one of the two methyl groups is retained intact, whereas the other is involved in a fast C- and H-scrambling of the remaining C 7H 8 backbone, and also from a 20–30% fraction in which all of the eight C- and the eleven H atoms undergo complete scrambling. The composite scrambling behavior observed through expulsion of ethene is almost identical to that found previously upon protonation of para-xylene by CI(CH 4). Therefore, it is suggested that the C 8H 11 + ions generated by protonation of 6,6-dimethylfulvene readily isomerize to 1,1-dimethylbenzenium and the related xylenium ions. The minor (20–30%) fraction of the latter ions undergoes a further, reversible ring expansion to methyldihydrotropylium ions before re-contracting the seven-membered ring to yield ethylbenzenium ions; the major (70–80%) fraction does so irreversibly before re-contraction to ethylbenzenium ions, from which ethene is expelled eventually.

Computational Investigation of the Conrotatory and Disrotatory Isomerization Channels of Bicyclo[1.1.0]butane to Buta-1,3-diene: A Completely Renormalized Coupled-Cluster Study

The conrotatory and disrotatory mechanisms of the isomerization of bicyclo[1.1.0]butane to trans-buta-1,3-diene have been computationally investigated with the CASSCF, MCQDPT2, (U)B3LYP, CCSD(T), CR-CCSD(T), and CR-CC(2,3) approaches. The coupled-cluster (CC) methods, including the CC approach with singles, doubles, and noniterative triples (CCSD(T)), and its completely renormalized (CR) extensions called CR-CCSD(T) and CR-CC(2,3), and the density functional theory B3LYP approach do an excellent job of correctly predicting the activation barrier for the conrotatory pathway, which corresponds to a weakly biradical transition state (TS), producing values within experimental error bars. In particular, the recently developed CR-CC(2,3) method gives 40.8 or 41.1 kcal/mol, in perfect agreement with the experimental value of 40.6 +/- 2.5 kcal/mol. The complete-active-space self-consistent-field (CASSCF) approach and the second-order multireference perturbation theory (MCQDPT2) are less accurate in describing the conrotatory barrier than CR-CC(2,3). The higher energy disrotatory pathway, which has not been characterized experimentally and which involves a strongly biradical TS, poses a great challenge for many methods. CCSD(T) fails, predicting the activation barrier for the disrotatory pathway significantly below the conrotatory barrier, contradicting the experimental result that the conrotatory pathway describes the mechanism. The strongly biradical character of the disrotatory TS, spin contamination, and the proximity of singlet and triplet potential energy surfaces cause difficulties for B3LYP, which does not link this TS with gauche-buta-1,3-diene. No such difficulties occur in the CASSCF calculations, which offer a proper description of the structure of the disrotatory TS that links it with the reactant and product molecules. The CR-CC(2,3) approach, which accurately balances dynamical and nondynamical correlations in systems containing closed-shell and biradical structures, predicts the activation enthalpy for the disrotatory mechanism of approximately 66 kcal/mol. CR-CCSD(T) gives approximately 69 kcal/mol. In agreement with experiment and earlier multireference configuration interaction calculations of Nguyen and Gordon, CR-CCSD(T) and CR-CC(2,3) favor the conrotatory mechanism. The CASSCF, MCQDPT2, and B3LYP methods correctly place the disrotatory barrier above the conrotatory one, but, on the basis of a comparison with the accurate CR-CC(2,3) results, they underestimate the activation energy for the disrotatory pathway. All CC approaches employed in this study produce very good estimates of the enthalpy of isomerization of bicyclo[1.1.0]butane into buta-1,3-diene, the experimental value of which is -25.9 +/- 0.4 kcal/mol, giving about -28 kcal/mol, when trans-buta-1,3-diene is used as a product, and -25 kcal/mol, when the nearly isoenergetic gauche-buta-1,3-diene rotamer is used as a product. The CC reaction enthalpies are more accurate than those obtained with CASSCF, MCQDPT2, and B3LYP.

An ignored but most favorable channel for NCO+C2H2 reaction

The NCO+C(2)H(2) reaction has been considered as a prototype for understanding the chemical reactivity of the isocyanate radical towards unsaturated hydrocarbons in fuel-rich combustion. It has also been proposed to provide an effective route for formation of oxazole-containing compounds in organic synthesis, and might have potential applications in interstellar processes. Unfortunately, this reaction has met mechanistic controversy both between experiments and between experiments and theoretical calculations. In this paper, detailed theoretical investigations at the Becke's three parameter Lee-Yang-Parr-B3LYP6-31G(d), B3LYP6-311++G(d,p), quadratic configuration interaction with single and double excitations QCISD6-31G(d), and Gaussian-3 levels are performed for the NCO+C(2)H(2) reaction, covering various entrance, isomerization, and decomposition channels. Also, the highly cost-expensive coupled-cluster theory including single and double excitations and perturbative inclusion of triple excitations CCSD(T)/aug-cc-pVTZ single-point energy calculation is performed for the geometries obtained at the Becke's three parameter Lee-Yang-Parr-B3LYP6-311++G(d,p) level. A previously ignored yet most favorable channel via a four-membered ring intermediate with allyl radical character is found. However, formation of P(3) H+HCCNCO and the five-membered ring channel predicted by previous experimental and theoretical studies is kinetically much less competitive. With the new channel, master equation rate constant calculations over a wide range of temperatures (298-1500 K) and pressures (10-560 Torr) show that the predicted total rate constants exhibit a positive-temperature dependence and no distinct pressure dependence effect. This is in qualitative agreement with available experimental results. Under the experimental conditions, the predicted values are about 50% lower than the latest experimental results. Also, the branching ratio variations of the fragments P(2) HCN+HCCO and P(5) OCCHCN+H as well as the intermediates L1 HCHCNCO, r4 cCHCHNC-O, and L5 NCHCHCO are discussed with respect to the temperature and pressure. Future experimental reinvestigations are strongly desired to test the newly predicted channel for the model NCO+C(2)H(2) reaction. Implications of the present results in various fields are discussed.

Acetone: isomerization and aggregation

The advanced experimental and theoretical techniques enable us to obtain information on the rearrangement of atoms or molecules in a reaction nowadays. As an example, we report on our research work on acetone isomerization and aggregation to give an insight into the reaction pathways, the products and their structures, and the growth regularity of aggregation. The evidences on the structural change of acetone and the stability of acetone clusters are found by a laser ionization mass spectrometer and the results are interpreted from theoretical analysis based on the DFT/B3LYP method. Various isomerization channels of acetone have been established and the optimal structures of the neutral clusters (CH3COCH3)n and the protonated acetone clusters (CH3COCH3)nH+ for n=1–7 have been determined.

Quantum dynamics study on multichannel dissociation and isomerization reactions of formaldehyde

We study quantum dynamics of the multichannel reactions of H(2)CO including the molecular and radical dissociation channels as well as the isomerization ones, H(2)CO-->trans-HCOH and trans-HCOH-->cis-HCOH. For this purpose, the previously developed potential energy function [T. Yonehara and S. Kato, J. Chem. Phys. 117, 11131 (2002)] is refined to give accurate transition state energies and to describe the radical dissociation channel. The cumulative reaction probabilities for the molecular dissociation and two isomerization channels are calculated by using the full Watson Hamiltonian. We also carry out wave packet dynamics calculations starting from the transition state region for the molecular dissociation. A contracted basis set for the angular coordinates is constructed to reduce the size of dynamics calculations. The intramolecular vibrational relaxation dynamics is found to be fast and almost complete within 300 fs. Using the energy filtered wave functions, the time propagation of HCOH population is obtained in the energy range from 81 to 94 kcal/mol. The branching ratio of the radical product is estimated by calculating the time dependent reactive fluxes to the molecular and radical dissociation products.