Binary and ternary recombination of para-H3(+) and ortho-H3(+) with electrons: state selective study at 77-200 K

Measurements in H(3)(+) afterglow plasmas with spectroscopically determined relative abundances of H(3)(+) ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H(3)(+) ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H(3)(+) recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution.     

Nuclear spin dependence of the reaction of H(3)+ with H2. II. Experimental measurements

The nuclear spin dependence of the chemical reaction H(3)(+)+ H(2) → H(2)?+ H(3)(+) has been studied in a hollow cathode plasma cell. Multipass infrared direct absorption spectroscopy has been employed to monitor the populations of several low-energy rotational levels of ortho- and para-H(3)(+) (o-H(3)(+) and p-H(3)(+)) in hydrogenic plasmas of varying para-H(2) (p-H(2)) enrichment. The ratio of the rates of the proton hop (k(H)) and hydrogen exchange (k(E)) reactions α ≡ k(H)/k(E) is inferred from the observed p-H(3)(+) fraction as a function of p-H(2) fraction using steady-state chemical models. Measurements have been performed both in uncooled (T(kin) ～ 350 K) and in liquid-nitrogen-cooled (T(kin) ～ 135 K) plasmas, marking the first time this reaction has been studied at low temperature. The value of α has been found to decrease from 1.6 ± 0.1 at 350 K to 0.5 ± 0.1 at 135 K.     

Statistical modelling of NH+/ND+ + H2/HD/D2 branching ratios

In this work we study hydrogen isotope fractionation along the proposed gas-phase ammonia formation pathway at temperatures relevant to interstellar modelling. Specifically we study the isotopically substituted variants of the NH(+) + H(2) reaction, for which the primary product is NH(2)(+) + H. Adiabatic capture theory calculations have been performed for the association reaction. A new potential-energy surface has been determined for the NH(2)(+) product. An extensive set of rovibrational energy levels has been calculated for the NH(2)(+) isotopologues described by this potential-energy surface. These rovibrational energy levels have been used to determine energy- and angular-momentum-resolved numbers of accessible product states from the NH(3)(+) isotopologue intermediates, which in turn have been averaged to give statistical branching fractions for all isotopically-substituted NH(+) + H(2) reactions. It is determined that in all cases where both hydrogen and deuterium are present, the NHD(+) product is preferred.     

Primary branching ratios for the low-temperature reaction of state-prepared N2+ with CH4, C2H2, and C2H4

Product branching ratios (BRs) are reported for ion-molecule reactions of state-prepared nitrogen cation (N(2)(+)) with methane (CH(4)), acetylene (C(2)H(2)). and ethylene (C(2)H(4)) at low temperature using a modified ion imaging apparatus. These reactions are performed in a supersonic nozzle expansion characterized by a rotational temperature of 40 ± 5K. For the N(2)(+) + CH(4) reaction, a BR of 0.83:0.17 is obtained for the dissociative charge-transfer (CT) reaction that gives rise to the formation of CH(3)(+) and CH(2)(+) product ions, respectively. The N(2)(+) + C(2)H(2) ion-molecule reaction proceeds through a nondissociative CT process that results in the sole formation of C(2)H(2)(+) product ions. The reaction of N(2)(+) with C(2)H(4) leads to the formation of C(2)H(3)(+) and C(2)H(2)(+) product ions with a BR of 0.74:0.26, respectively. The reported BR for the N(2)(+) + C(2)H(4) reaction is supportive of a nonresonant dissociative CT mechanism similar to the one that accompanies the N(2)(+) + CH(4) reaction. No dependence of the branching ratios on N(2)(+) rotational level was observed. In addition to providing direct insight into the dynamics of the state-prepared N(2)(+) ion-molecule reactions with the target neutral hydrocarbon molecules, the reported low-temperature BRs are also important for accurate modeling of the nitrogen-dominated upper atmosphere of Saturn's moon, Titan.     

Quantum mechanical calculations for the H2O + hnu --> O(1D) + H2 photodissociation process

Quantum mechanical wavepacket calculations for the photodissociation of water in the second absorption band are presented. Using O + H2 Jacobi coordinates, partial cross sections for the O(1D) + H2 channel are calculated for different initial rotational states. Conical intersection and Renner-Teller effects are included. The branching ratios for the four accessible dissociation channels at 121.6 nm are in good agreement with experiment (J. Chem. Phys. 1982, 77, 2432). The calculations predict significant rotational and vibrational excitation of the H2 fragments. Photodissociation of ortho and para water produces predominantly, but not exclusively, ortho and para H2 fragments, respectively.     

Microcanonical statistical study of ortho-para conversion in the reaction H3+ + H2-->(H5+)*-->H3+ + H2 at very low energies

The ortho-para conversion of H(3) (+) and H(2) in the reaction H(3) (+)+H(2)-->(H(5) (+))(*)-->H(3) (+)+H(2) in interstellar space is possible by scrambling the five protons via (H(5) (+))(*) complex formation. The product distribution of the ortho-para conversion reaction can be given by ratios of cumulative reaction probabilities (CRP) calculated by microcanonical statistical theory with conservation of energy, motional angular momentum, nuclear spin, and parity. A statistical method to calculate the state-to-state reaction probabilities for given initial nuclear spin species, rotational states, and collision energies is developed using a simple semiclassical approximation of tunneling and above-barrier reflection. A new calculation method of branching ratios for given total nuclear spins and scrambling mechanisms is also developed. The anisotropic long-range electrostatic interaction potential of H(2) in the Coulomb field of H(3) (+) is taken into account using the first-order perturbation theory in forming the complex. The CRPs and the product distribution of the ortho-para conversion reaction at very low energies with reactants in their ground vibronic and lowest rotational states for given initial nuclear spin species are presented as a function of collision energy assuming complete proton scrambling or incomplete proton scrambling. The authors show that the product distribution at very low energies (or very low temperatures) differs substantially from the high energy (or high temperature) limit branching ratios.     

Accurate partition function for acetylene, 12C2H2, and related thermodynamical quantities

The internal partition function (Q(int)) of ethyne (acetylene), (12)C(2)H(2), is calculated by explicit summation of the contribution of all individual vibration-rotation energy levels up to 15,000 cm(-1). The corresponding energies are predicted from a global model and constants reproducing within 3σ all 18,415 published vibration-rotation lines in the literature involving vibrational states up to 8900 cm(-1), as produced by Amyay et al. [J. Mol. Spectrosc. 267, 80 (2011)]. Values of Q(int), with distinct calculations for para and ortho species are provided from 1 to 2000 K, in step of 1 K. The total internal partition function at 298.15 K is 104.224387(47) or 416.89755(19), with the nuclear degeneracy spin factors taken as 1/4:3/4 (astronomer convention) or 1:3 (atmospheric convention), respectively, for para:ortho species. The Helmholtz function, Gibbs enthalpy function, entropy, and specific heat at constant pressure are also calculated over the same temperature range. Accuracies as well as the missing contribution of the vinylidene isomer of acetylene in the calculations are discussed.     

Two-channel dissociation of chemically and thermally activated n-butylbenzene cations (C10H14+)

The charge-transfer reaction O(2)(+) + n-butylbenzene (C(10)H(14)) --> O(2) + C(10)H(14)(+) was studied in a turbulent ion flow tube at temperatures between 423 and 548 K and pressures between 15 and 250 Torr in the buffer gases He and N(2). Under chemical activation conditions stabilization vs dissociation ratios S/D of vibrationally highly excited C(10)H(14)(+)* as well as branching ratios of the fragments C(7)H(7)(+) (m/z = 91) vs C(7)H(8)(+) (m/z = 92) of the dissociation of C(10)H(14)(+)* were measured. Under thermal activation conditions, the rate constant of the dominating dissociation channel 92 was measured at 498 and 523 K. Employing information on the specific rate constants k(E) of the two channels 91 and 92 and on collisional energy transfer rates from the literature, the measured S/D curves and branching ratios 91/92 could be modeled well. It is demonstrated that the charge transfer occurs approximately equally through resonant transfer and complex-forming transfer. The thermal dissociation experiments provide a high precision value of the energy barrier for the channel 92, being 1.14 (+/-0.02) eV.     

Collision-induced dissociation in (He, H2(+)(v = 0-2; j = 0-3)) system: a time-dependent quantum mechanical investigation

The collision-induced process He + H(2)(+)(v = 0-2; j = 0-3) → He + H + H(+) has been investigated using a time-dependent quantum mechanical wave packet approach, within the centrifugal sudden approximation. The exchange reaction He + H(2)(+) → HeH(+) + H, which has a lower threshold, dominates over the dissociation process over the entire energy range considered in this study. The reaction cross section for both the exchange and dissociation channels and the branching ratio between the two channels have been computed on the McLaughlin-Thompson-Joseph-Sathyamurthy potential-energy surface and compared with the available experimental and quasiclassical trajectory results.     

Millimeter-wave spectroscopy of H(2)C=CD: Tunneling splitting and ortho-para mixing interaction

The H(2)C=CD isotopic species of vinyl radical produced in a supersonic jet expansion by ultraviolet laser photolysis was studied by millimeter-wave spectroscopy. Due to the tunneling motion of the α deuteron, the ground state is split into two components, 0(+) and 0(-). Tunneling-rotation transitions connecting the lower (0(+)) and upper (0(-)) components of the tunneling doublet were observed in the frequency region of 184-334 GHz, including three R- and two Q-branch transitions. Three and two pure rotational transitions in the K(a)=0 and 1 stacks, respectively, were also observed for each of the 0(+) and 0(-) states in the frequency region of 52-159 GHz. Least-squares analysis of the observed frequencies for the tunneling-rotation and pure rotational transitions with well resolved hyperfine structures yielded a set of precise molecular constants, among which the tunneling splitting in the ground state was determined to be ΔE(0)=1187.234(17)?MHz, which is 1/14 that for H(2)C=CH. The potential barrier height derived from the observed tunneling splitting by an analysis of the tunneling dynamics using a one-dimensional model is 1545?cm(-1), consistent with the value 1568?cm(-1) obtained for the normal vinyl. The observed spectrum was found to be perturbed by a hyperfine interaction connecting ortho and para levels. The constant for the interaction, which we call the ortho-para mixing Fermi contact interaction, has been determined to be δa(F) ((β))=68.06(53)?MHz. This is believed to be the first definite detection of such an interaction. By this interaction the ortho and para states of H(2)C=CD are mixed up to about 0.1%. The constant is more than 1000 times larger than spin-rotation interaction constants that cause ortho-para mixing in closed shell molecules and suggests extremely rapid conversion between the ortho and para nuclear spin isomers of H(2)C=CD.     

Geometric phase effects in resonance-mediated scattering: H + H2(+) on its lowest triplet electronic state

We report a quantum dynamics study of H + H(2)(+) (v(0) = 0, j(0) = 0) scattering on its lowest triplet state, for J = 0 total angular momentum and total energies up to 1.85 eV. This provides a benchmark example of indirect resonance-mediated reaction in presence of a conical intersection (CI). Visualization of time-dependent wave packets shows significant "looping" around the CI, which is facilitated by long-lived H(3)(+) scattering resonances, predominant at low energies. State-to-state inelastic transition probabilities exhibit a highly oscillatory structure and pronounced geometric phase effects, which, in contrast to direct reactions, are more strongly marked at lower energies.     

Infrared spectra of C(3)H(3)(+)-N(2) dimers: identification of proton-bound c-C(3)H(3)(+)-N(2) and H(2)CCCH(+)-N(2) isomers.

Mid-infrared photodissociation spectra of mass selected C(3)H(3)(+)-N(2) ionic complexes are obtained in the vicinity of the C-H stretch fundamentals (2970-3370 cm(-1)). The C(3)H(3)(+)-N(2) dimers are produced in an electron impact cluster ion source by supersonically expanding a gas mixture of allene, N(2), and Ar. Rovibrational analysis of the spectra demonstrates that (at least) two C(3)H(3)(+) isomers are produced in the employed ion source, namely the cyclopropenyl (c-C(3)H(3)(+)) and the propargyl (H(2)CCCH(+)) cations. This observation is the first spectroscopic detection of the important c-C(3)H(3)(+) ion in the gas phase. Both C(3)H(3)(+) cations form intermolecular proton bonds to the N(2) ligand with a linear -C-H...N-N configuration, leading to planar C(3)H(3)(+)-N(2) structures with C(2v) symmetry. The strongest absorption of the H(2)CCCH(+)-N(2) dimer in the spectral range investigated corresponds to the acetylenic C-H stretch fundamental (v(1) = 3139 cm(-1)), which experiences a large red shift upon N(2) complexation (Delta(v1) approximately -180 cm(-1)). For c-C(3)H(3)(+)-N(2), the strongly IR active degenerate antisymmetric stretch vibration (v4)) of c-C(3)H(3)(+) is split into two components upon complexation with N(2): v4)(a(1)) = 3094 cm(-1) and v4)(b(2)) = 3129 cm(-1). These values bracket the yet unknown v4) frequency of free c-C(3)H(3)(+) in the gas phase, which is estimated as 3125 +/- 4 cm(-1) by comparison with theoretical data. Analysis of the nuclear spin statistical weights and A rotational constants of H(2)CCCH(+)-N(2) and c-C(3)H(3)(+)-N(2) provide for the first time high-resolution spectroscopic evidence that H(2)CCCH(+) and c-C(3)H(3)(+) are planar ions with C(2v) and D(3h) symmetry, respectively. Ab initio calculations at the MP2(full)/6-311G(2df,2pd) level confirm the given assignments and predict intermolecular separations of R(e) = 2.1772 and 2.0916 A and binding energies of D(e) = 1227 and 1373 cm(-1) for the H-bound c-C(3)H(3)(+)-N(2) and H(2)CCCH(+)-N(2) dimers, respectively.     

Reaction dynamics simulations of the identity S(N)2 reaction H(2)O + HOOH(2)(+)--> H(2)OOH(+)+ H(2)O. Requirements for reaction and competition with proton transfer

Despite the fact that the transition structure of the gas phase S(N)2 reaction H(2)O + HOOH(2)(+)--> HOOH(2)(+)+ H(2)O is well below the reactants in potential energy, the reaction has not yet been observed by experiment. Variational transition state RRKM theory reveals a strong preference for the competing proton transfer reaction H(2)O + HOOH(2)(+)--> H(3)O(+)+ HOOH due to entropy factors. Born-Oppenheimer reaction dynamics simulations confirm these results. However, by increasing the collision energy to around 7.5 eV the probability for nucleophilic substitution increases relative to proton transfer. These observations are explained by the presence of the key common intermediate HOO(H)[dot dot dot]H-OH(2)(+) which leads to effective proton transfer, but can be avoided with increasing collision energy. However, the S(N)2 probability remains below 0.2 since successful passage through the TS requires optimum initial orientation of the reactants, excitation of the relative translational motion and good phase correlation between the O-O vibration and the motion of the incoming water.     

An ab initio based global potential energy surface describing CH5+ --> CH3+ + H2

A full-dimensional, ab initio based potential energy surface (PES) for CH(5)(+), which can describe dissociation is reported. The PES is a precise fit to 36173 coupled-cluster [CCSD(T)] calculations of electronic energies done using an aug-cc-pVTZ basis. The fit uses a polynomial basis that is invariant with respect to permutation of the five H atoms, and thus describes all 120 equivalent minima. The rms fitting error is 78.1 cm(-1) for the entire data set of energies up to 30,000 cm(-1) and a normal-mode analysis of CH(5)(+) also verifies the accuracy of the fit. Two saddle points have been located on the surface as well and compared with previous theoretical work. The PES dissociates correctly to the fragments CH(3)(+) + H(2) and the equilibrium geometry and normal-mode analyses of these fragments are also presented. Diffusion Monte Carlo calculations are done for the zero-point energies of CH(5)(+) (and some isotopologs) as well as for the separated fragments of CH(5)(+), CH(3)(+) + H(2) and those of CH(4)D(+), CH(3)(+) + HD and CH(2)D(+) + H(2). Values of D(0) are reported for these dissociations. A molecular dynamics calculation of CH(4)D(+) dissociation at one total energy is also performed to both validate the applicability of the PES for dynamics studies as well as to test a simple classical statistical prediction of the branching ratio of the dissociation products.     

Charge exchange in tandem mass spectrometry: dissociative single electron capture by doubly-charged toluene cations

Single electron capture by doubly-charged toluene cations upon collision with various target gases has been investigated by sector tandem mass spectrometry. Both non-dissociative and dissociative charge transfer reactions leading to C(7)H(7)(+) + H and to C(5)H(5)(+) + [C(2),H(3)] are detected. Seven atomic or molecular target gases have been used with ionisation energies ranging from 8.8 eV to 14 eV. The branching ratios between the different non-dissociative and dissociative exit channels have been determined as well as the translational energy release on the dissociation products. The experimental data are compared to the predictions of a two-state semi-classical theoretical model that takes into account the non-adiabatic transition responsible for the charge transfer reaction. A wide reaction window shows up but the internal energies of the C(7)H(8)(+) cations produced by single electron capture are observed to be larger than expected. We assign this effect partly to the influence of the large density of vibrational states and to the multichannel nature of the process. Excited states of the dication are also most probably involved in the charge exchange reaction.     

Noncatalytic kinetic study on site-selective H/D exchange reaction of phenol in sub- and supercritical water

The site-selective H/D exchange reaction of phenol in sub- and supercritical water is studied without added catalysts. In subcritical water in equilibrium with steam at 210-240 degrees C, the H/D exchange proceeds both at the ortho and para sites in the phenyl ring, with no exchange observed at the meta site. The pseudo-first-order rate constants are of the order of 10(-4) s(-1); 50% larger for the ortho than for the para site. In supercritical water, the exchange is observed also at the meta site with the rate constant in the range of 10(-6)-10(-4) s(-1). As the bulk density decreases, the exchange slows down and the site selectivity toward the ortho is enhanced. The enhancement is due to the phenol-water interaction preference at the atomic resolution. The site selectivity toward the ortho is further enhanced when the reaction is carried out in benzene/water solution. Using such selectivity control and the reversible nature of the hydrothermal deuteration/protonation process, it is feasible to synthesize phenyl compounds that are deuterated at any topological combination of ortho, meta, and para sites.     

Unusual mechanism for H3(+) formation from ethane as obtained by femtosecond laser pulse ionization and quantum chemical calculations

The formation of H(3)(+) from saturated hydrocarbon molecules represents a prototype of a complex chemical process, involving the breaking and the making of chemical bonds. We present a combined theoretical and experimental investigation providing for the first time an understanding of the mechanism of H(3)(+) formation at the molecular level. The experimental approach involves femtosecond laser pulse ionization of ethane leading to H(3)(+) ions with kinetic energies on the order of 4 to 6.5 eV. The theoretical approach involves high-level quantum chemical calculation of the complete reaction path. The calculations confirm that the process takes place on the potential energy surface of the ethane dication. A surprising result of the theoretical investigation is, that the transition state of the process can be formally regarded as a H(2) molecule attached to a C(2)H(4)(2+) entity but IRC calculations show that it belongs to the reaction channel yielding C(2)H(3)(+) + H(3)(+). Experimentally measured kinetic energies of the correlated H(3)(+) and C(2)H(3)(+) ions confirm the reaction path suggested by theory.     

Isotopic exchange processes in cold plasmas of H2/D2 mixtures

Isotope exchange in low pressure cold plasmas of H(2)/D(2) mixtures has been investigated by means of mass spectrometric measurements of neutrals and ions, and kinetic model calculations. The measurements, which include also electron temperatures and densities, were performed in a stainless steel hollow cathode reactor for three discharge pressures: 1, 2 and 8 Pa, and for mixture compositions ranging from 100% H(2) to 100% D(2). The data are analyzed in the light of the model calculations, which are in good global agreement with the experiments. Isotope selective effects are found both in the surface recombination and in the gas-phase ionic chemistry. The dissociation of the fuel gas molecules is followed by wall recycling, which regenerates H(2) and D(2) and produces HD. Atomic recombination at the wall is found to proceed through an Eley-Rideal mechanism, with a preference for reaction of the adsorbed atoms with gas phase D atoms. The best fit probabilities for Eley-Rideal abstraction with H and D are: γ(ER H) = 1.5 × 10(-3), γ(ER D) = 2.0 × 10(-3). Concerning ions, at 1 Pa the diatomic species H(2)(+), D(2)(+) and HD(+), formed directly by electron impact, prevail in the distributions, and at 8 Pa, the triatomic ions H(3)(+), H(2)D(+), HD(2)(+) and D(3)(+), produced primarily in reactions of diatomic ions with molecules, dominate the plasma composition. In this higher pressure regime, the formation of the mixed ions H(2)D(+) and HD(2)(+) is favoured in comparison with that of H(3)(+) and D(3)(+), as expected on statistical grounds. The model results predict a very small preference, undetectable within the precision of the measurements, for the generation of triatomic ions with a higher degree of deuteration, which is probably a residual influence at room temperature of the marked zero point energy effects (ZPE), relevant for deuterium fractionation in interstellar space. In contrast, ZPE effects are found to be decisive for the observed distribution of monoatomic ions H(+) and D(+), even at room temperature. The final H(+)/D(+) ratio is determined to a great extent by proton (and deuteron) exchange, which favours the enhancement of H(+) and the concomitant decrease of D(+).     

Photoinduced electron-transfer reactions with quinolinic and trimellitic acid imides: experiments and spin density calculations(1)

The regioselectivity of photoinduced electron-transfer (PET) reactions of unsymmetrical phthalimides is controlled by the spin density distribution of the intermediate radical anions. ROHF ab initio calculations were found to be most suitable for atomic spin density analysis. Intramolecular PET reactions of quinolinic acid imides were studied with the potassium butyrate and hexanoate 1a,b and a cysteine derivative 3. The photocyclizations products 2a,b and 4 were formed with moderate regioselectivities (68:32, 57:43, and 81:19) showing preferential ortho cyclization. The intermolecular reaction of potassium propionate and potassium isobutyrate with N-methylquinolinic acid imide (5) yielded as addition products the dihydropyrrolo[3,4-b]pyridines 6a,b with slight ortho regioselectivity (55:45). In contrast to these low regioselectivities, the PET reaction of potassium propionate with the methyl ester of N-methyltrimellitic acid imide (9) yielded solely the para addition product 10. Likewise, the intramolecular photoreaction of the cysteine derivative 7 gave a 75:25 (para/meta) mixture of regioisomeric cyclization products 8. The regioselectivity originates from donor-acceptor interactions prior to electron transfer and differences in spin densities in the corresponding imide radical anions. The results of DFT and ab initio calculations for the radical anions of the quinolinic acid imide (11(*)(-)) and the methyl ester of trimellitic acid imide (12(*)(-))( )()were in agreement with the latter assumption: spin densities in 11(*)(-) were higher for the imido ortho carbon atoms (indicating preferential ortho coupling); for 12(*)(-) the spin densities were higher for the imido para carbon atoms (indicating preferential para coupling). These correlations became more significant when the additional spin densities at the carbonyl oxygen and the adjacent carbon atoms were taken into account. The cyclization selectivities for 2, 4, and 8 deviate from the intermolecular examples probably because of ground-state and solvent effects.     

Formation and decomposition of chemically activated cyclopentoxy radicals from the c-C5H9 + O reaction

The formation and the decomposition of chemically activated cyclopentoxy radicals from the c-C5H9 + O reaction have been studied in the gas phase at room temperature. Two different experimental arrangements have been used. Arrangement A consisted of a laser-flash photolysis set up combined with quantitative Fourier transform infrared spectroscopy and allowed the determination of the stable products at 4 mbar. The c-C5H9 radicals were produced via the reaction c-C5H10 + Cl with chlorine atoms from the photolysis of CFCl3; the O atoms were generated by photolysis of SO2. Arrangement B, a conventional discharge flow-reactor with molecular beam sampling, was used to determine the rate coefficient. Here, the hydrocarbon radicals (c-C5H9, C2H5, CH2OCH3) were produced via the reaction of atomic fluorine with c-C5H10, C2H6, and CH3OCH3, respectively, and detected by mass spectrometry after laser photoionization. For the c-C5H9 + O reaction, the relative contributions of intermediate formation (c-C5H9O) and direct abstraction (c-C5H8 + OH) were found to be 68 +/- 5 and 32 +/- 4%, respectively. The decomposition products of the chemically activated intermediate could be identified, and the following relative branching fractions were obtained: c-C5H8O + H (31 +/- 2%), CH2CH(CH2)2CHO + H (40 +/- 5%), 2 C2H4 + H + CO (17 +/- 5%), and C3H4O + C2H4 + H (12 +/- 5%). Additionally, the product formation of the c-C5H8 + O reaction was studied, and the following relative yields were obtained (mol %): C2H4, 24%; C3H4O, 18%; c-C5H8O, 30%; c-C5H8O, 23%; 4-pentenal, 5%. The rate coefficient of the c-C5H9 + O reaction was determined relative to the reactions C2H5 + O and CH3OCH2 + O leading to k = (1.73 +/- 0.05) x 10(14) cm3 mol(-1) s(-1). The experimental branching fractions are analyzed in terms of statistical rate theory with molecular and transition-state data from quantum chemical calculations, and high-pressure limiting Arrhenius parameters for the unimolecular decomposition reactions of C5H9O species are derived.