Isomerization Channels Research Articles

The infrared-driven cis-trans isomerization of HONO. II: Vibrational relaxation and slow isomerization channel

In a recent paper [R. Schanz et al., J. Chem. Phys. 122, 044509 (2005)], we investigated the IR-driven cis-trans isomerization of HONO in a Kr matrix with the help of femtosecond IR spectroscopy. We found that isomerization occurs on a 20 ps time scale, however, with a cis-->trans quantum yield of only 10% that is significantly below the value reported in the literature (close to 100%). At the same time, we concluded that vibrational energy has not completely dissipated out of the molecule at the maximum delay time we reached in this study (500 ps). In order to verify whether additional, slower reaction channels exist, we extend the study here to delay times up to 100 ns. At a temperature of 32 K, we indeed find an additional isomerization channel on a 2 ns timescale, which increases the total cis-->trans quantum yield to approximately 30%. The trans-->cis quantum yield is approximately 7%. There is still a discrepancy between the quantum yields we observe and the literature values, however, we provide experimental evidence that this discrepancy is due to the different temperatures of our study. Vibrational cooling occurs on a 20 ns time scale, and cascades in a highly nonstatistical manner through one single normal mode (most likely the ONO bending mode nu(5)). Intermolecular energy dissipation into the rare gas matrix is more efficient than intramolecular vibrational energy redistribution and the matrix environment can certainly not be considered a weak perturbation.

Quantum mechanical analysis of decomposition pathways of chloromethyl hypochlorite

The most important decomposition pathways of chloromethyl hypochlorite, ClCH 2OCl, are examined using molecular structure quantum mechanical techniques. Specifically, the 1,1 and 1,2 elimination reactions, the isomerization channel and the C–O, O–Cl bond scissions are considered. The theoretical investigation yields high barriers for all production channels, which emphasize the high stability of ClCH 2OCl. Among the various reaction pathways, only the 1,2 eliminations to CH 2O+Cl 2 and ClCHO+HCl and the isomerization to dichloromethanol lead to more stable species. The photolytic O–Cl bond scission presents the lowest critical energy for reaction at the CCSD(T) level. Comparison with literature reports about the decomposition scheme of simple methyl hypochlorite, CH 3OCl, and fluoromethyl hypochlorite, FCH 2OCl, indicates a moderate effect of the different substitute on the dissociation pattern of these compounds.

Potential energy surfaces in Coulomb explosion of polyatomic molecules: Benzene and cyclohexane trications and acetylene dication

AbstractWe report density functional and ab initio calculations of potential energy surfaces (PES) for benzene and cyclohexane trications relevant to the Coulomb explosion mechanisms of C6H6 and C6H12. Numerous isomerization and dissociation mechanisms on these PES have been considered, and the products most favorable thermodynamically and kinetically have been predicted. We also demonstrate applications of RRKM theory to the calculation of product branching ratios in decomposition of multiply charged cations. Significant common features of PES related to Coulomb explosion experiments on hydrocarbon molecules have been described. Calculations along minimal energy reaction paths for some representative isomerization and dissociation channels have been carried out in the presence of strong external electric field for C6H and C2H and the influence of the field on molecular orientation and PES have been analyzed. The results show that proton mobility in nonaromatic hydrocarbon cations is high due to nearly zero barriers for the H+ migrations. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Multiconfigurational second-order perturbation study of the decomposition of the radical anion of nitromethane

The doublet potential energy surfaces involved in the decomposition of the nitromethane radical anion (CH(3)NO(2) (-)) have been studied by using the multistate extension of the multiconfigurational second-order perturbation method (MS-CASPT2) in conjunction with large atomic natural orbital-type basis sets. A very low energy barrier is found for the decomposition reaction: CH(3)NO(2) (-)-->[CH(3)NO(2)](-)-->CH(3)+NO(2) (-). No evidence has been obtained on the existence of an isomerization channel leading to the initial formation of the methylnitrite anion (CH(3)ONO(-)) which, in a subsequent reaction, would yield nitric oxide (NO). In contrast, it is suggested that NO is formed through the bimolecular reaction: CH(3)+NO(2) (-)-->[CH(3)O-N-O](-)-->CH(3)O(-)+NO. In particular, the CASSCF/MS-CASPT2 results indicate that the methylnitrite radical anion CH(3)ONO(-) does not represent a minimum energy structure, as concluded by using density functional theory (DFT) methodologies. The inverse symmetry breaking effect present in DFT is demonstrated to be responsible for such erroneous prediction.

Direct ab Initio Dynamics Study of the Unimolecular Reaction of CH2FO

A direct ab initio dynamics study is carried out for three channels of the unimolecular reaction of fluoromethoxy, CH2FO. The minimum energy paths (MEPs) of the three channels are computed with the MPW1K/6-31+G(d,p) method. The geometries, harmonic vibrational frequencies, energies, and enthalpies of all stationary points are calculated at the MPW1K and QCISD levels of theory in conjunction with the 6-31+G(d,p) basis set. The energies of stationary points and the points along MEP for each channel are further refined by using single-point multilevel energy calculations. The rate constants are evaluated by means of the conventional transition state theory (TST), the canonical varitional transition state theory (CVT), and the canonical varitional transition state theory with small-curvature tunneling correction (CVT/SCT) in the temperature range of 200−2500 K. The fitted Arrhenius expressions of the calculated CVT/SCT rate constants are k1CVT/SCT(T) = (1.26 × 106)T2.19e-7150.6/T s-1, k2CVT/SCT(T) = (3.68 × 102)T3.23e-12327.7/T s-1, k3CVT/SCT(T) = (9.97 × 10-26)T10.90e-3873.3/T s-1 for the H elimination, HF elimination, and isomerization channels, respectively.

Photodissociation of simple aromatic molecules in a molecular beam

An overview is presented of our recent experimental studies of aromatic molecules using multimass ion imaging techniques. Photodissociation of benzene, fluorobenzene, toluene, m-xylene, ethylbenzene, propylbenzene and chlorotoluene at 248 nm or 193 nm was investigated under collisionless conditions. The photofragment translational energy distribution and dissociation rate of each dissociation channel were recorded. New isomerization and dissociation channels were observed from these studies. The data are discussed with reference to the ab initio potential energy surface and statistical theory results.

Ab initio molecular orbital and RRKM calculations of the thermal decomposition of CH2BrO radical

A theoretical study of the thermal decomposition and isomerization channels of bromomethoxy radical is carried out using ab initio molecular orbital methods and RRKM theory. Three kinds of reaction pathways are examined, bond scission, intramolecular three-center HBr elimination and isomerization. Energy-specific rate coefficients k ( E ) and thermal rate constants k ( T , P ) are evaluated using the ab initio data and RRKM theory. Relevance to existing experimental evidence is discussed.

Theoretical study of the thermal decomposition and isomerization of α-bromoethoxy radicals

A theoretical study of the thermal decomposition and isomerization channels of α-bromoethoxy radicals is reported. Geometry optimizations were carried out at the MP2(full)/6-311G(d,p) level of theory and energetics were calculated using G2MP2 theory. Bond scission, intramolecular three-center HBr elimination and isomerization processes have been examined. Energy-specific rate constants k( E) and thermal rate constants k( T) under atmospheric conditions, were evaluated using Rice–Ramsberger–Kassel–Marcus theory. The calculations indicate that C–Br bond scission is the dominant decomposition channel, providing theoretical evidence for the previous experimental result.

Reaction of Phenyl Radicals with Acetylene: Quantum Chemical Investigation of the Mechanism and Master Equation Analysis of the Kinetics

The mechanism of the C(6)H(5) + C(2)H(2) reaction has been investigated by various quantum chemical methods. Electrophilic addition to the CC triple bond is found to be the only important mode of phenyl radical attack on acetylene. The initially formed chemically activated C(6)H(5)C(2)H(2) adducts may follow several isomerization pathways in competition with collisional stabilization and H-elimination. Thermochemistry of various decomposition and isomerization channels is evaluated by the G2M method. For key intermediates, the following standard enthalpies of formation have been deduced from isodesmic reactions: 94.2 +/- 2.0 kcal/mol (C(6)H(5)CHCH), 86.4 +/- 2.0 kcal/mol (C(6)H(5)CCH(2)), and 95.5 +/- 1.8 kcal/ mol (o-C(6)H(4)C(2)H(3)). The accuracy of theoretical predictions was examined through extensive comparisons with available experimental and theoretical data. The kinetics and product branching of the C(6)H(5) + C(2)H(2) reaction have been evaluated by weak collision master equation/Rice-Ramsperger-Kassel-Marcus (RRKM) analysis of the truncated kinetic model including only kinetically important transformations of the isomeric C(8)H(7) radicals. Available experimental kinetic data can be quantitatively reproduced by calculation with a minor adjustment of the C(6)H(5) addition barrier from 3.7 to 4.1 kcal/mol. Our predicted total rate constant, k(R1) = (1.29 x 10(10))T(0.834) exp(-2320/T) cm(3) mol(-)(1) s(-)(1), is weakly dependent on P and corresponds to the phenylation process under combustion conditions (T > 1000 K).

Photodissociation dynamics of azulene

Photodissociation of azulene at 193 nm was studied in a molecular beam using multimass ion imaging techniques. Most of the azulene molecules excited by 193 nm photon quickly relax to the ground electronic state through internal conversion, then isomerize to naphthalene, and eventually dissociate through the H atom elimination channel with a rate of 5.1×104 s−1. A small amount of azulene entering different isomerization channels was found. The effect of dissociation in the energy transfer experiments using azulene as a vibrationally highly excited molecule and the existence of azulene in an interstellar medium is discussed.

Role of isomerization channel in unimolecular dissociation reaction H2CO→H2+CO: Ab initio global potential energy surface and classical trajectory analysis

We constructed a full dimensional potential energy function of H2CO that can describe both the dissociation and isomerization channels by the modified Shepard interpolation method. Ab initio calculations at the MP2/cc-pVTZ level were carried out to obtain the local potential functions at about 4700 points. The interpolant points were sampled by classical trajectory calculations and by the grid searches in the internal coordinate space. Classical trajectory calculations were performed to examine the intramolecular dynamics associated with the dissociation as well as the product state distributions. The time scale of intramolecular vibrational energy randomization was much faster than that of the dissociation reaction. The dissociation rate was obtained from the classical trajectory results and the effect of the isomerization channel on the dissociation was estimated. The calculated rate constants were compared with those by Rice–Ramsperger–Kassel–Marcus theory.

Caging ultrafast proton transfer and twisting motion of 1-hydroxy-2-acetonaphthone

We report on emission studies of 1-hydroxy-2-acetonaphthone (HAN) in solution nanocavities. In neutral water solutions, intramolecular proton transfer in the enol (E*) structure of HAN and a subsequent internal rotation in the formed keto-type tautomer (K*) occur in few tens of ps, to produce a ns-living rotamers (KR*). When caging HAN in cyclodextrins, the dynamics of twisting is greatly affected. Depending on the size of the cage, the isomerization channel leading to KR* can be closed, and emission occurs from K*. In a larger caging cavity, the fast component of the anisotropy decay becomes shorter while the slow one becomes longer.

Chlorination Chemistry 4. Ab Initio Study of the Addition, Metathesis, and Isomerization Channels Governing the Reaction of Chlorine Atom with Propargyl Chloride

Highly correlated ab initio molecular orbital calculations have been used to map out the potential energy surface corresponding to the reaction of Cl + propargyl chloride (C3H3Cl) in the gas phase. Ten transition state structures governing the mechanism of the title reaction were computed at seven different levels of theory up to QCISD(T)/6-311+G(d,p)//QCISD(T)/6-31+G(d,p). Chlorine atom additions at the center and end unsaturated carbons are barrierless processes forming incipient, chemically activated 2,3-dichloro-1-propen-1-yl and 1,3-dichloro-1-propen-2-yl radicals, respectively. The incipient radicals cannot isomerize by transferring a hydrogen atom because the governing transition state structures reside at ≥ 95 kJ·mol-1 above the initial reactants. The presence of a transition state for chlorine atom transfer below the energy of the initial reactants enables isomerization between the incipient radicals. A second accessible chlorine transfer transition state enables the 1,3-dichloro-1-propen-2-yl radical to isomerize into the more stable 1,2-dichloroallyl radical. The chemically activated 1,3-dichloro-1-propen-2-yl radical also undergoes bond scission of the chlorine atom in the chloromethyl group forming chloroallene and Cl atom. Chlorine atom attack on the chloromethyl group encounters metathesis transition states for HCl and Cl2 formation at 5 kJ·mol-1 and 124 kJ·mol-1 above the initial reactants, respectively. The accord demonstrated between master equation calculations, based on the ab initio results, and experimental data validate the proposed reaction mechanism and predict that the dominant products of chlorine atom addition to propagryl chloride are chloroallene and the 1,2-dichloroallyl radical at 298 K and HCl, chloropropargyl radical, and chloroallene at 1000 K.

Theoretical study on the mechanism of the 1CHCl + NO reaction.

The complex doublet potential energy surface of the CHClNO system, including 31 minimum isomers and 84 transition states, is investigated at the QCISD(T)/6-311G(d, p)//B3LYP/6-31G(d, p) level in order to explore the possible reaction mechanism of the singlet CHCl with NO. Various possible isomerization and dissociation channels are probed. The initial association between 1CHCl and NO at the terminal N-site can almost barrierlessly lead to the chainlike adducts HClCNO a (a1, a2) followed by the direct Cl-extrusion to product P9 Cl + HCNO, which is the most feasible channel. Much less competitively, a (a1, a2) undergoes a ring-closure leading to the cyclic isomer c-C(HCl)NO d followed by a concerted Cl-shift and N-O cleavage of d to form the branched isomers ClNC(H)O f (f1, f2). Eventually, f (f1, f2) may take a direct H-extrusion to produce P7 H + ClNCO or a concerted 1,2-H-shift and Cl-extrusion to form P1 Cl + HNCO. The low-lying products P2 HCl + NCO, P3 Cl + HOCN, P14 HCO + 3NCl, P6 ClO + HCN, and P13 ClNC + OH may have the lowest yields observed. Our calculations show that the product distributions of the title reaction are quite different from those of the analogous 1CHF + NO reaction, yet are similar to those of another analogous 3CH2 + NO reaction. The similarities and discrepancies among the three reactions are discussed in terms of the substitution effect. The present article may assist in future experimental identification of the product distributions for the title reaction and may be helpful for understanding the halogenated carbene chemistry.

Unimolecular reaction dynamics of HSO. Analysis of the influence of different barrier samplings on the product energy distributions

Classical dynamics calculations are reported for the lowest energy unimolecular reaction channels of HSO. A recently published single-valued DMBE potential energy surface for HSO was employed in the dynamics calculations. The results predict that the isomerization channel HSO→HOS is the preferred one at the lowest energies of the study, while dissociation to give H+SO is increasingly important as the energy rises. For the energies selected, the vibrational coupling is not strong enough to keep the phase space uniform during the unimolecular dynamics. In addition, strong mode specific effects were found under initial excitation of the three normal modes of vibration. The product energy distributions obtained with classical and quasi-classical barrier sampling techniques are very similar. The results predicted by EMS (with and without ZPE restrictions) initialization at the barrier show a much higher vibrational energy content of the products than those obtained by the above sampling techniques. This arises from the separability of the reaction coordinate mode inherent to the QCBS and CBS models.

1,2‐Pentadiene decomposition

Abstract1,2‐Pentadiene has been decomposed in single pulse shock tube experiments. There appear to be a large number of parallel decomposition and isomerization channels. The following rate expressions for isomerization in the temperature range 1100–1250 K and pressures between 200 and 300kPa have been obtainedk(1,2‐pentadiene → 1‐pentyne) = 1.0 × 1013exp(−32300 ± 900 K/T) s−1k(1,2‐pentadiene → cis/trans‐1,3‐pentadiene = 2.2 × 1014exp(−33900 ± 900 K/T) s−1k(1,2‐pentadiene → cyclopentene) = 1.8 × 1011exp(−26080 ± 900 K/T) s−1Overall, we estimate the 2σ uncertainty in the absolute rate constants to be about 30%. From detailed balance the rate expression for 1,3 to 1,2‐pentadiene isomerization has been found to bek(1,3‐pentadiene → 1,2‐pentadiene) = 1.2 × 1015exp(−42070 ± 900 K/T) s−1The significance of the 1,3 to 1,2 isomerization process as a bridge between the two‐carbon and three‐carbon species in high temperature hydrocarbon decomposition systems is discussed.The decomposition processes involve bond breaking and retroene reactions. The rate expression for the latter has been found to bek(1,2‐pentadiene → propyne + ethylene) = 6.6 × 1012exp(−29240 ± 900 K/T) s−1The rate constants for fission of the C3C4and C4C5bonds appear to be close to each other. Our results indicate that the resonance energy of the 1,3‐butadiene‐2‐yl radical is smaller than that of allyl radical by an amount equal to the π bond conjugation energy of butadiene. © 2001 John Wiley & Sons, Inc.*This article is a US Government work and, as such, is in the public domain in the United States of America.Int J Chem Kinet 33: 755–767, 2001

Competing isomeric product channels in the 193 nm photodissociation of 2-chloropropene and in the unimolecular dissociation of the 2-propenyl radical

This paper presents product translational energy spectroscopy measurements of the primary photofragmentation channels of 2-chloropropene excited at 193 nm and of the unimolecular dissociation of the 2-propenyl radical. Tunable vacuum ultraviolet (VUV) photoionization of the products allows us to distinguish between the various product isomers formed in these processes. The data show evidence for three significant primary reaction channels in the dissociation of 2-chloropropene: An excited-state C–Cl fission channel producing fast Cl atoms, a C–Cl fission channel producing slow Cl atoms, and HCl elimination. A minor C–CH3 fission channel contributes as well. The measured branching of the major primary product channels is: [fast C–Cl]:[slow C–Cl]:[HCl elimination]=62%:23%:15%. The experiments also allow us to resolve selectively the product branching between the unimolecular dissociation channels of the 2-propenyl radical, a high energy C3H5 isomer; we measure how the branching ratio between the two competing C–H fission channels changes as a function of the radical’s internal energy. The data resolve the competition between the unimolecular H+allene and H+propyne product channels from the radical with internal energies from 0 to 18 kcal/mol above the H+propyne barrier. We find that the barrier to H+allene formation from this high-energy C3H5 radical is higher than the barrier to H+propyne formation, in agreement with recent theoretical calculations but in sharp contrast to that predicted for the most stable C3H5 isomer, the allyl radical. The experiments demonstrate a general technique for selectively forming a particular CnHm isomer dispersed by internal energy due to the primary photolysis, thus allowing us to determine the branching between unimolecular dissociation channels as a function of the selected radical isomer’s internal energy.

Internal Energy Dependence of the H + Allene/H + Propyne Product Branching from the Unimolecular Dissociation of 2-Propenyl Radicals

This Letter introduces a new technique to probe the competing unimolecular dissociation channels of isomerically-selected hydrocarbon radicals as a function of internal energy in the radical. The crossed laser−molecular beam scattering experiments produce 2-propenyl radicals by photolysis of 2-chloropropene and disperse the radicals by the neutral velocity imparted in the photolysis, thus dispersing them by internal energy in the neutral time-of-flight spectrum. For the unstable radicals, the experiments then measure the branching between the two competing C-H bond fission product channels via tunable vacuum-UV photoionization of the products. Dispersing the neutral products by arrival time at the detector allows us to measure the branching between isomeric product channels as a function of internal energy in the dissociating radical isomer. The data resolve the competition between the unimolecular H + allene and H + propyne product channels from the radical with internal energies from 0 to 18 kcal/mol above the H + propyne barrier. We find that the barrier to H + allene formation from this high-energy C3H5 radical is slightly higher than the barrier to H + propyne formation, in agreement with recent theoretical calculations but in sharp contrast to that predicted for the most stable C3H5 isomer, the allyl radical. The dominance of the branching to H + propyne formation over H + allene formation for this isomer persists at the higher internal energies in agreement with RRKM predictions that take into account the freezing of the methyl rotor in the H + allene channel and predict a concomitant reduction in the A factor for that channel.

Mechanism and rate constants for 1,3-butadiene decomposition

Data on the decomposition of 1,3-butadiene have been analyzed. The numerous isomerization processes that have recently been proposed as additional channels for decomposition have been considered. Energy transfer effects have been taken into account through the solution of the time-dependent master equation. We confirm recent supposition that direct formation of ethylene and acetylene is a major reaction channel during the thermal decomposition of 1,3-butadiene. The isomerization to 1,2-butadiene and its subsequent decomposition to form propargyl and methyl is the next most important process. Almost no vinyl radicals are produced. The formation of the isomerization products, in contrast to those from the initial reactant, is generally characterized by long transients before the attainment of a steady state. Standard methods of treating processes where such reactions are important must be carefully analyzed. The calculations are calibrated with experimental results. It proved not to be possible to fit simultaneously all the experimental observations with the model. Rate expressions for the major decomposition and isomerization channels are presented.

Multiphoton ionisation of acetone at 355 nm: a time-of-flight mass spectrometry study

Multiphoton ionisation/fragmentation of acetone has been studied at λ=355 nm using time-of-flight mass spectrometry. A broad peak at m/ e=43 is observed which shows a three-photon laser power dependence. The results suggest that three-photon ionisation at 355 nm produces vibrationally excited acetone ions which undergo fast dissociation. The broadening of the m/ e=43 peak is found to depend on the laser power in the low laser intensity region. Under high laser intensity conditions, a bimodal distribution of m/ e=43 peak was observed which has been assigned to two different isomeric product channels. The results provide clear evidence for partial isomerisation of the keto form of the acetone radical cation to the enol form. On ionisation at 355 nm, CH 3COC 2H 5 fragments into two channels corresponding to (CH 3CO +) and (C 2H 5CO +) in the ratio of approximately 3:1.