Major Product Channels Research Articles

Calculations of Dissociation Dynamics of CH3OH on a Global Potential Energy Surface Reveal the Mechanism for the Formation of HCOH; Roaming Plays a Role.

The experimental observation of hydroxymethylene, HCOH, following excitation of methanol at 193 nm, was reported recently (Hockey, E. K.; McLane, N.; Martí, C.; Duckett, L.; Osborn, D. L.; Dodson, L. G. Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation. J. Am. Chem. Soc. 2024, 146, 14416-14421, 10.1021/jacs.4c03090). This stimulated us to investigate a dynamic mechanism for its formation using a global potential energy surface for the ground electronic state (S0) of methanol. We show via quasi-classical trajectory calculations that hydroxymethylene is indeed formed under the reasonable assumption that the initially excited state undergoes rapid internal conversion to S0. From the trajectories, fractional yields of the six major product channels are determined as a function of excitation energy, and the rate of production of each is determined from the fractions and rate of methanol disappearance. Roaming is observed in trajectories leading to the OH and CH3 products as well as in those leading to CH2OH + H and non-reactive trajectories. A "frustrated" roaming accounts for roughly 20% of the HCOH production.

Mechanistic Insight and Intersystem Crossing Dynamics of the C(3P) + H2CO/D2CO Reaction

The reaction of atomic carbon, C(3P), with H2CO has been investigated using the direct dynamics trajectory surface hopping (DDTSH) method with Tully's fewest switches algorithm. The lowest lying ground triplet and single states are considered for the dynamics study at a reagent collision energy of 8.0 kcal/mol. From the trajectory calculations, we observed that CH2 + CO and H + HCCO are the two major product channels for the title reaction. The insertion mechanism of the C(3P) + H2CO reaction is rather complex and is followed by three distinct intermediates with no entrance channel barrier to the reaction on the B3LYP/6-31G(d,p) potential energy surfaces. The triplet insertion complexes are formed by three different approaches; "Sideways", "End-on" and "Head-on" attack of the triplet carbon atom toward H2CO molecule. Our dynamics calculations predict a new product channel (H + HCCO(X 2A'')) with a contribution of ∼46% of the overall products formation via ketocarbene intermediate through "Head-on" approach. Despite the weak spin-orbit coupling (SOC) interactions, intersystem crossing (ISC) via a ketocarbene intermediate has a small but significant contribution, about 2.3%, for the CH2 + CO channel. To understand the kinetic isotope effects on the reaction dynamics, we have extended our study for the C(3P) + D2CO reaction. It is seen that isotopic substitution of both the H atoms has a small reduction in the extent of ISC dynamics for the carbene formation. Our results, certainly, reveal the importance of the ketocarbene intermediate and the H + HCCO products channel as one of the major product formation channels in the title reaction, which was not reported earlier.

The reactions of 2-furfuryl alcohol with hydrogen atom: A theoretical calculation and kinetic modeling analysis

The rate constants for H-initiated reactions of 2-furfuryl alcohol (2FFOH) were calculated for the first time by a high-level quantum chemical method combined with the Rice–Ramsperger–Kassel–Marcus (RRKM) theory/Master equation method. A detailed potential energy surface was constructed by the CCSD(T)/CBS//M06–2X/def2-TZVP method, involving H-abstraction, initial H-addition, adduct-subsequent pathways and interconnected conversion pathways. The H-addition reaction on the β-carbon and δ-carbon sites to form bimolecular products 2-methylene-2,3-dihydrofuran plus OH and 2-methylene-2,5-dihydrofuran plus OH were found to be the major product channels at high temperatures. No significant pressure dependence is observed for the total rate constants of 2FFOH + H. However, the overall kinetics behavior does not reflect the site characteristics, i.e., the site-specific rate constants for each reaction channel show different temperature and pressure dependences. The total rate constants via the H-addition mechanism are higher by factors of 3.3–10 than those via H-abstraction channels under all studied conditions. The present updated kinetic model obtained by applying these new calculated rate constants can better predict 2FFOH decomposition, especially at 30 Torr, and it can also lead to good predictions of the speciation profiles (e.g., vinyl ketene) during 2FFOH pyrolysis. The data calculated in the present work can provide valuable information for the development of combustion models of furan-based biofuels.

UV photodissociation action spectra of protonated formylpyridines.

The first ππ* transition for protonated 2-, 3-, and 4-formylpyridine (FPH+) (m/z 108) is investigated by mass spectrometry coupled with photodissociation action spectroscopy at room temperature and 10K. The photoproduct ions are detected over 35000-43000cm-1, and the major product channel for 3-FPH+ and 4-FPH+ is the loss of CO forming protonated pyridine at m/z 80. For 2-FPH+, the CO loss product is present but a more abundant photoproduct arises from the loss of CH2O to form m/z 78. Plausible potential energy pathways that lead to dissociation are mapped out and comparisons are made to products arising from collision-induced dissociation. Although, in all cases, the elimination of CO is the overwhelming thermodynamically preferred pathway, the protonated 2-FPH+ results suggest that the CH2O product is kinetically driven and competitive with CO loss. In addition, for each isomer, radical photoproduct ions are detected at lower abundances. SCS-CC2/aug-cc-pVTZ Franck-Condon simulations assist with the assignment of vibrionic structure and adiabatic energies (0-0) for 2-FPH+ at 36 560cm-1, 37 430cm-1 for 3-FPH+, and 36 140cm-1 for 4-FPH+, yielding an accurate prediction, on average, within 620cm-1.

An Experimental and Theoretical Investigation of the Gas-Phase C(3P) + N2O Reaction. Low Temperature Rate Constants and Astrochemical Implications.

The reaction between atomic carbon in its ground electronic state, C(3P), and nitrous oxide, N2O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(3P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser-induced fluorescence at 115.8 nm. Additional measurements allowing the major product channels to be elucidated were also performed. On the theoretical side, statistical rate theory was used to calculate low temperature rate constants. These calculations employed the results of new electronic structure calculations of the 3A″ potential energy surface of CNNO and provided a basis to extrapolate the measured rate constants to lower temperatures and pressures. The rate constant was found to increase monotonically as the temperature falls (kC(3P)+N2O (296 K) = (3.4 ± 0.3) × 10-11 cm3 s-1), reaching a value of kC(3P)+N2O (50 K) = (7.9 ± 0.8) × 10-11 cm3 s-1 at 50 K. As current astrochemical models do not include the C + N2O reaction, we tested the influence of this process on interstellar N2O and other related species using a gas-grain model of dense interstellar clouds. These simulations predict that N2O abundances decrease significantly at intermediate times (103 - 105 years) when gas-phase C(3P) abundances are high.

Experimental and Theoretical Investigations of the Radical-Radical Reaction: N2H3 + NO2.

The N2H3 + NO2 reaction plays a key role during the early stages of hypergolic ignition between N2H4 and N2O4. Here for the first time, the reaction kinetics of N2H3 in excess NO2 was studied in 2.0 Torr of N2 and in the narrow temperature range 298-348 K in a pulsed photolysis flow-tube reactor coupled to a mass spectrometer. The temporal profile of the product, HONO, was determined by direct detection of the m/z +47 amu ion signal. For each chosen [NO2], the observed [HONO] trace was fitted to a biexponential kinetics expression, which yielded a value for the pseudo-first-order rate coefficient, k', for the reaction of N2H3 with NO2. The slope of the plot of k' versus [NO2] yielded a value for the observed bimolecular rate coefficient, kobs, which could be fitted to an Arrhenius expression of (2.36 ± 0.47) × 10-12 exp((520 ± 350)/T) cm3 molecule-1 s-1. The errors are 1σ and include estimated uncertainties in the NO2 concentration. The potential energy surface of N2H3 + NO2 was investigated by advanced ab initio quantum chemistry theories. It was found that the reaction occurs via a complex reaction mechanism, and all of the reaction channels have transition state energies below that of the entrance asymptote. The radical-radical addition forms the N2H3NO2 adducts, while roaming-mediated isomerization reactions yield the N2H3ONO isomers, which undergo rapid dissociation reactions to several sets of distinct products. The RRKM multiwell master equation simulations revealed that the major product channel involves the formation of trans-HONO and trans-N2H2 below 500 K and the formation of NO + NH2NHO above 500 K, which is nearly pressure independent. The pressure-dependent rate coefficients of the product channels were computed over a wide pressure-temperature range, which encompassed the experimental data.

Dynamics of H(2S) + CH(X2Π) reactions based on a new CH2() surface via extrapolation to the complete basis set limit

A grid of 5285 ab initio points is utilized to construct a 3D potential energy surface (PES) of the system. In the calculation procedure, the aug-cc-pVXZ (X = Q and 5) basis sets with Davidson correction are employed. The reference wave function for the multi-reference configuration interaction calculations is composed of a full valence complete-active-space self-consistent field wave function. In order to get a more accurate PES, the complete basis set (CBS) limit proposal and the many-body expansion form are used, with the total root mean square deviation of the final CBS-PES being 0.0349 eV. Based on the accurate CBS-PES, the stationary points and vibrational energy levels are obtained and examined in detail, which agree well with other theoretical results. Then, utilizing the CBS-PES and quasi-classical trajectory method, the integral cross-sections (ICSs) and rate constants of the → / reactions are calculated. It is found that the present ICSs are in good agreement with other theoretical results, and is the major product channel. For this channel, the rate constants calculated in this work agree well with experimental and other theoretical results in the high-temperature range. It is worth noting that at room temperature, the theoretical results, including the present work, are consistent with each other, but they are all higher (about 7–10 times) than the experimental result, which implies that a new measurement for the rate constant at room temperature is necessary.

Temperature and pressure dependent rate coefficients for the reaction of C2H4 + HO2 on the C2H4O2H potential energy surface.

The potential energy surface (PES) for reaction C2H4 + HO2 was examined by using the quantum chemical methods. All rates were determined computationally using the CBS-QB3 composite method combined with conventional transition state theory(TST), variational transition-state theory (VTST) and Rice-Ramsberger-Kassel-Marcus/master-equation (RRKM/ME) theory. The geometries optimization and the vibrational frequency analysis of reactants, transition states, and products were performed at the B3LYP/CBSB7 level. The composite CBS-QB3 method was applied for energy calculations. The major product channel of reaction C2H4 + HO2 is the formation C2H4O2H via an OH(···)π complex with 3.7 kcal/mol binding energy which exhibits negative-temperature dependence. We further investigated the reactions related to this complex, which were ignored in previous studies. Thermochemical properties of the species involved in the reactions were determined using the CBS-QB3 method, and enthalpies of formation of species were compared with literature values. The calculated rate constants are in good agreement with those available from literature and given in modified Arrhenius equation form, which are serviceable in combustion modeling of hydrocarbons. Finally, in order to illustrate the effect for low-temperature ignition of our new rate constants, we have implemented them into the existing mechanisms, which can predict ethylene ignition in a shock tube with better performance.

Theoretical Kinetics Studies on the Reaction of CF3CF═CF2 with Hydroxyl Radical

The potential energy surface for the reaction of hexafluoropropene with hydroxyl radical is explored by using BB1K density functional method. Single-point energy calculations are performed at CBS-Q level of theory. Semiclassical transition state theory and a modified strong collision/RRKM model are employed to calculate the thermal rate coefficients for the formation of major products as a function of temperature and pressure. It is revealed from the computed rate constants that the major product channels at low temperatures and high pressures are the formation of the primary adducts formed through OH addition to the double bond of CF3═CFCF2. At high temperatures and low pressures, however, many products arising from unimolecular decomposition of the chemically activated intermediates become important. P9A (CF3CFCOF + HF) and P7B (CF3COCF2 + HF) are dominant products at elevated temperatures. Semiclassical transition state theory is used to compute the overall high-pressure rate constants over the temperature range of 200-1500 K. The computed rate constants are in accordance with the available experimental data.

Theoretical study on the mechanism and kinetics for the ozonolysis of vinyl propionate

The O3-initiated oxidation of vinyl propionate is studied using quantum chemistry calculations. Detailed and complete reaction mechanisms are presented which involve the formation of the primary ozonide (POZ), the subsequent decomposition of POZ, the secondary reactions of CH3CH2C(O)OCHO2 (IM4) in the presence of H2O or NO as well as the generation of the secondary ozonide (IM6). Based on the above PESs calculations, the Multichannel Rice–Ramsperger–Kassel–Marcus theory is employed to calculate the total and individual rate constants for major product channels. The rate constants and branching ratios of main products are obtained. The total rate constants are temperature dependent over the whole study temperature range (200–2,000 K), but pressure independent over the range of 0.01–10,000 Torr. In addition, the atmospheric lifetime is estimated in accordance with rate constants.

Unimolecular decomposition of 2,5-dimethylfuran: a theoretical chemical kinetic study

The unimolecular decomposition of 2,5-dimethylfuran (DMF), a promising next-generation biofuel, was studied at the CBS-QB3 level of theory. As most of its decomposition routes remain unknown, a large number of pathways were explored: initial C-H bond fission, biradical ring opening, H-atom and CH(3)-group transfers involving carbene intermediates. Based on the computed potential energy surfaces, thermochemical data and high-pressure limit rate constants were determined and included in a small detailed kinetic mechanism for DMF unimolecular decomposition. Simulations performed under the conditions of recent experimental data focusing on the initial steps in DMF thermal decomposition showed that the unimolecular decomposition of DMF is dominated by two product channels. For typical combustion conditions, the major product channel (~70% at 1500 K and 1 bar) is reached by a 3,2-hydrogen transfer in DMF leading to a carbene intermediate that rearranges to hexa-3,4-dien-2-one which in turn decomposes into CH(3)CO and C(4)H(5) by initial C-C bond fission. In contrast with previous studies, the initial C-H bond fission in DMF has not been found to be the major decomposition channel (~30% at 1500 K and 1 bar). Pressure effects were probed using a semi-quantitative approach and were found to be negligible above 1 bar. Below atmospheric pressure, initial C-H bond cleavage yields increase while CH(3)CO + C(4)H(5) branching ratios decrease. This study brings a new understanding of the nature of the initial radicals and molecules created upon thermal activation of DMF.

Chemically activated formation of organic acids in reactions of the Criegee intermediate with aldehydes and ketones

Reactions of the Criegee intermediate (CI, ˙CH2OO˙) are important in atmospheric ozonolysis models. In this work, we compute the rates for reactions between ˙CH2OO˙ and HCHO, CH3CHO and CH3COCH3 leading to the formation of secondary ozonides (SOZ) and organic acids. Relative to infinitely separated reactants, the SOZ in all three cases is found to be 48-51 kcal mol(-1) lower in energy, formed via 1,3-cycloaddition of ˙CH2OO˙ across the C=O bond. The lowest energy pathway found for SOZ decomposition is intramolecular disproportionation of the singlet biradical intermediate formed from cleavage of the O-O bond to form hydroxyalkyl esters. These hydroxyalkyl esters undergo concerted decomposition providing a low energy pathway from SOZ to acids. Geometries and frequencies of all stationary points were obtained using the B3LYP/MG3S DFT model chemistry, and energies were refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. RRKM calculations were used to obtain microcanonical rate coefficients (k(E)) and the reservoir state method was used to obtain temperature and pressure dependent rate coefficients (k(T, P)) and product branching ratios. At atmospheric pressure, the yield of collisionally stabilized SOZ was found to increase in the order HCHO < CH3CHO < CH3COCH3 (the highest yield being 10(-4) times lower than the initial ˙CH2OO˙ concentration). At low pressures, chemically activated formation of organic acids (formic acid in the case of HCHO and CH3COCH3, formic and acetic acid in the case of CH3CHO) was found to be the major product channel in agreement with recent direct measurements. Collisional energy transfer parameters and the barrier heights for SOZ reactions were found to be the most sensitive parameters determining SOZ and organic acid yield.

Metastable fragmentation of photoionized styrene cluster ions: Implications for cluster ion structure

Styrene clusters are produced by supersonic expansion of a styrene/Ar mixture, photoionized with 193 or 248nm light, and analyzed in a reflectron time-of-flight mass spectrometer. The styrene cluster ions Stn+, n≤10, show an alternation in intensity that favors cluster ions containing an even number of molecules. Mass-selected cluster ions, n=2–8, are allowed to undergo metastable fragmentation, and the fragment ions are separated from the parent ions in the reflectron. For n=4–8, fragmentation proceeds exclusively by elimination of a series of styrene molecules. Loss of as many as five styrene molecules is observed for n=6–8. For n=4, the St+ fragment signal is very small, and for n=3 it is nonexistent. A robust signal for St2+ indicates the major product channel in both cases. These results are interpreted in terms of formation of covalently bound styrene dimer ions within the ionized clusters, especially n=3 and 4. The present fragmentation results are also discussed in relation to earlier work. Internal energy considerations show that there should be sufficient energy present to cause fragmentation of the structures proposed in the prior work, but no fragments corresponding to such fragmentation are observed. Thus the present work does not support the cluster ion structures proposed earlier, though it also does not completely rule them out.

Theoretical study of the reaction 2,5-dimethylfuran + H → products

The reaction of 2,5-dimethylfuran (DMF) with H-atoms was studied using a potential energy surface calculated at the CBS-QB3 level of theory and master equation/RRKM modeling. Hydrogen abstraction by H-atom and hydrogen additions on DMF were considered. As the decomposition pathways of the initial adducts were unknown, a large number of decomposition routes was explored for these adducts. An important number of interconnected product channels were found and preliminary master equation calculations were performed to select the crucial wells and exit channels. The ipso substitution DMF+H→methylfuran (MF)+CH3 and the formation of 1,3-butadiene and acetyl radical (CH3CO) were found to be the major product channels in the addition process. The total calculated rate constant was found in good agreement with experimental data and is nearly pressure-independent. A small sensitivity to pressure was found for the computed branching ratios. At 1bar, the yields of the two product channels of the addition process are maximal at 1100K with computed branching ratios of 39% (MF+CH3) and 27% (1,3-C4H6+CH3CO). Above 1300K, hydrogen abstraction by H-atom becomes dominant and reaches a branching ratio of 56% at 2000K.

Theoretical Study on the Mechanism of NH + HCNO Reaction

DFT B3LYP calculations with the 6-311G(d, p) basis set were carried out to explore the mechanism of the NH (X3Σ-) + HCNO reaction. On the basis of calculated reaction paths, the three reaction channels are predicted to occur via the following reaction steps. The NH radical initially attacks C atom of the HCNO radical, leading to an intermediate HC(NH)NO (a1), followed by formation of a bond between the H atom of NH (X3Σ-) radical and the N atom of HCNO, leading to the formation of product HNO + HCN. In addition to the H atom of NH (X3Σ-) radical migration in the intermediate HC(NH)NO (a1), the H atom migration from C atom to N atom leads to an intermediate HN(H)CNO (b), followed by rupture of H2N－CNO bond, leading to the products NH2 + CNO. The NH radical initially attacks N atom of the HCNO radical, leading to an intermediate HCN(NH)O (a3), followed by formation of the products CH2O + N2, through the intermediates d1, d2, d3, d4, e1, e2 and f. The CCSD(T)/ 6-311G(d,p) energetic results indicated that the total barrier of product 1, product 2 and product 3 is 32.8 kcal/mol, 89.5 kcal/mol, 40.0 kcal/mol, respectively. It is shown that P1(CH2O + N2), P3 (HCN + HNO) are the major product channels with a minor contribution from P2 (NH2 + CNO).

Mechanistic and kinetic study on the ozonolysis of ethyl vinyl ether and propyl vinyl ether

The reaction mechanisms for ozonolysis of ethyl vinyl ether (EVE) and propyl vinyl ether (PVE) have been investigated using the density functional theory (DFT) and ab initio method. Cycloaddition reactions of O3 to EVE and PVE are highly exothermic by 52.91 and 53.17 kcal/mol, respectively. Major products (formaldehyde, ethyl formate, and propyl formate) resulting from the both reactions are identified by comparing them with the experimental results. Further reactions of the most energy-rich Criegee intermediates (C2H5OCHOO and C3H7OCHOO) have been proposed in the presence of NO and H2O in which the main products are ethyl formate and propyl formate. The Multichannel Rice–Ramsperger–Kassel–Marcus (RRKM) approach is employed to calculate the total and individual rate constants for major product channels over a wide range of temperatures and different pressures. In the temperature range of 200–2500 K, the main path is the production of ethyl formate with kEVE+O3 = 4.67 × 10−12 exp(−3029/T), for the EVE with O3 reaction and kPVE+O3 = 3.58 × 10−12 exp(−2858/T) for the PVE with O3 reaction. At 298 K and 760 torr, the rate constants calculated are 1.80 × 10−16 and 2.45 × 10−16 cm3 molecule−1 s−1 for ozonolysis of EVE and PVE, which are consistent with the experimental results. The total rate constants show positive temperature dependence over the temperature range of 200–2000 K but pressure independence in the range of 0.01–10000 Torr. Estimation of branching ratios of several products is also performed. The influence of carbon chain length on reactivity toward ozone is examined.

Theoretical Study on the Mechanism of CH + O&lt;sub&gt;2&lt;/sub&gt; Reaction

A detailed mechanistic study of the CH + O2reaction, in which the products are involved, is carried out by means of CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) + ZPVE computational method to determine a set of reasonable pathways. The reaction is shown to proceed with an exothermic barrierless addition of O2to the methylidyne (CH) radical to form intermediate HCOO (a2). The HCOO radical was found to have two dissociation channels, giving products P1 HCO and O radical through the nonplanar transition states, and giving intermediate CO2H (b1) through the planar transition states, in addition to the backward reaction forming the products radical (channel A, B, C, D, E). It is shown that B (HCO + O), E (CO + OH) are the major product channels with a minor contribution from D (CO2 + H), whereas the other channels for are less favorable.

ChemInform Abstract: Ab Initio Chemical Kinetics for ClO Reactions with HOx, ClOx and NOx (x = 1, 2): A Review

AbstractReview: 180 refs.

Thermal Decomposition of Ethanol. 4. Ab Initio Chemical Kinetics for Reactions of H Atoms with CH3CH2O and CH3CHOH Radicals

The potential energy surfaces of H-atom reactions with CH(3)CH(2)O and CH(3)CHOH, two major radicals in the decomposition and oxidation of ethanol, have been studied at the CCSD(T)/6-311+G(3df,2p) level of theory with geometric optimization carried out at the BH&HLYP/6-311+G(3df,2p) level. The direct hydrogen abstraction channels and the indirect association/decomposition channels from the chemically activated ethanol molecule have been considered for both reactions. The rate constants for both reactions have been calculated at 100-3000 K and 10(-4) Torr to 10(3) atm Ar pressure by microcanonical VTST/RRKM theory with master equation solution for all accessible product channels. The results show that the major product channel of the CH(3)CH(2)O + H reaction is CH(3) + CH(2)OH under atmospheric pressure conditions. Only at high pressure and low temperature, the rate constant for CH(3)CH(2)OH formation by collisonal deactivation becomes dominant. For CH(3)CHOH + H, there are three major product channels; at high temperatures, CH(3)+CH(2)OH production predominates at low pressures (P < 100 Torr), while the formation of CH(3)CH(2)OH by collisional deactivation becomes competitive at high pressures and low temperatures (T < 500 K). At high temperatures, the direct hydrogen abstraction reaction producing CH(2)CHOH + H(2) becomes dominant. Rate constants for all accessible product channels in both systems have been predicted and tabulated for modeling applications. The predicted value for CH(3)CHOH + H at 295 K and 1 Torr pressure agrees closely with available experimental data. For practical modeling applications, the rate constants for the thermal unimolecular decomposition of ethanol giving key accessible products have been predicted; those for the two major product channels taking place by dehydration and C-C breaking agree closely with available literature data.

Ion-Molecule Reactions of CH5+, C2H5+, and C3H5+ with Halogenated Benzenes in an Ion-Trap Type of a Mass Spectrometer

Gas-phase reactions of CH5+, C2H5+, and C3H5+ ions with monohalogenated benzenes (PhX=F, Cl, Br, and I) were studied using an ion-trap type of GC/MS at a low chemical-ionization gas pressure of CH4. The major product channel involves dissociative and non-dissociative proton transfer leading to [M+H-HX+CH4]+=[PhCH3+H]+ for PhF, [M+H]+ for PhCl and PhBr, and [M+H-X]+=PhH+ for PhI. Adduct ions and their decomposition products are also found in reactions of C2H5+ and C3H5+. The reaction mechanism is discussed based on the reaction-time dependence of product-ion distributions and semi-empirical calculations of the heat of reaction for each process.