RRKM Rate Research Articles

Photoisomerization Paths of α,ω-Diphenylpolyenes: Reaction Rate Dependence on Temperature, Excitation Wavelength, and Deuteration.

The photoisomerization rate kiso of trans-stilbene (tS) and trans-trans-diphenylbutadiene (ttD) is studied in solution and compared to that in jet/gas. Rice-Ramsperger-Kassel-Marcus (RRKM) theory correctly predicts the tS rate in jet, kRRKM = Am exp(-Ein/kTm) with Ein = 1398 cm-1, and Am = 1.8 ps-1 corresponding to frequency νiso = 60 cm-1 of the reactive mode, Tm being the molecular temperature. However, the behavior in solution cannot be explained by the RRKM rate alone. In solution the rate kiso = AS exp(-Eb/kTS) has a similar form, but depends mainly on solvent temperature TS and proceeds much faster, AS = 19 ps-1, Eb = 1520 cm-1 in n-hexane. Moreover, excitation at high excess energy, resulting in molecular temperature Tm = 607 K, affects the rate only slightly, unlike in jet, and contrary to common theoretical models. The experimental results clearly indicate two isomerization paths in solution: via relatively slow intramolecular activation Am ∼ 1 ps-1, and by much faster solvent activation AS = 18 ps-1 due to solute-solvent interactions (collisions). The data in n-alkanes confirm previously established power dependence kiso ∼ ηα on viscosity η, with α = 0.30 for tS, and α = 0.35 for ttD. With Eη being the viscosity barrier, its contribution to Eb can be isolated, giving the intramolecular barrier Ein = (Eb - αEη), slightly lower than in jet/gas, probably due to the dispersive/induction interactions in solution.

Kinetics and Dynamics of Cyclopentanone Thermal Decomposition in Gas Phase.

Cyclopentanone is a potential bio-fuel which can be produced from bio-mass. Its gas phase dissociation chemistry has attracted several experimental and theoretical investigations. In the photochemical and thermal decomposition studies of cyclopentanone, ethylene and carbon monoxide were found to be dominant reaction products along with several other compounds in smaller quantities. For the formation of ethylene and carbon monoxide, a concerted mechanism has been proposed as primary reaction pathway. In addition, a step-wise mechanism involving ring-opened radical intermediate has also been considered. The present work reports gas phase thermal decomposition of cyclopentanone at high temperatures investigated using electronic structure theory methods, Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant calculations, and Born-Oppenheimer direct classical trajectory simulations. The trajectory calculations were performed on density functional PBE96/6-31+G* potential energy surface using initial conditions selected from fixed energy normal mode distributions. Simulations showed that ethylene and carbon monoxide formed primarily via the concerted mechanism confirming the earlier predictions. In addition, stepwise pathways were also observed for same products in lower fraction of trajectories. Furthermore, several other reaction products in smaller quantities and new mechanistic pathways were observed. The computed RRKM rate constants and simulation data are agreement with experimental results and detailed atomic level dissociation mechanisms presented.

Theoretical investigation on the mechanism and kinetics of the OH•‒initiated atmospheric degradation of p-chloroaniline: Addition of [formula omitted] and isomerization of peroxy radicals

Atmospheric oxidation of the p-chloroaniline-OH• adduct [C6H4ClNH2–OH]• (AD-C2) by ∑g−3O2 and internal isomerization processes of peroxy radical [C6H4ClNH2–OH]•-O2 are theoretically investigated at the M06-2X/aug-cc-pVTZ and CBS-QB3//M06-2X/aug-cc-pVTZ level of theories. Potential energy surfaces (PESs) for the most efficient pathways indicated that the oxidation process begins via the complexation of individual reactants in syn mode forming PRCy-iOO-syn (y = 2,5) in an exothermic and endogenic step. The syn mode addition is favored over the anti one due to the formation of internal hydrogen bond between the hydroxyl and peroxy groups. Formation of new C5–OO bond in PRCy-iOO-syn complex is an unimolecular process which is exothermic and exoergic. This pathway is predominated over other internal conversions due to the presence of stronger intramolecular hydrogen bond. Cyclization of the produced [C6H4ClNH2–OH]•-O2 peroxy radical AD-C2-5OO-syn into the bicyclic peroxy radical AD-C2-5,6OO-syn is the last step which is strongly endothermic and endogenic. The rate coefficients are calculated by means of the RRKM theory over the temperature range 250–350 K and at a pressure range of 0.1 bar to the high-pressure limit. The RRKM rate coefficients at the M06-2X/aug-cc-pVTZ level for the first bimolecular and last unimolecular steps are in order of 10−16 cm3 molecule−1 s−1 and 10−7 s−1, respectively, while the obtained rate coefficients at the CBS-QB3//M06-2X/aug-cc-pVTZ are overestimated about two order of magnitude.

DFT Insight into the Kinetics and Mechanism of OH⋅‐Initiated Atmospheric Oxidation of Catechol: Addition of 3∑g− O2 and Isomerization of Peroxy Radicals

AbstractAtmospheric oxidation mechanism of predominant catechol‐OH adduct [C6H6O2−OH]⋅ (AD2) by oxygen molecule in its triplet electronic ground state and isomerization processes of catechol peroxy radicals [C6H6O2−OH]⋅−O2 into bicyclic peroxy radicals have been studied at the M06‐2X/aug‐cc‐pVTZ level in conjugation with the RRKM theory. The first step begins via the complexation of [C6H6O2−OH]⋅ with oxygen molecule and formation of two pre‐reactive van der Waals complexes in syn and anti modes. Syn mode addition of O2 is more favorable over the anti mode due to the formation of intramolecular hydrogen bond. Kinetically and thermodynamically, addition of O2 at the C5 position in syn mode, namely peroxy radical AD2‐5OO‐syn, is the most efficient process with bimolecular RRKM rate coefficient 2.80×10−16 cm3 molecule−1 s−1 at ambient conditions. Isomerization (cyclization) of peroxy radical AD2‐5OO‐syn into bicyclic peroxy radicals via the formation of −OO− bridge are strongly endothermic and nonspontaneous processes with high activation energies. Ring closure into the bicyclic peroxy radical AD2‐5,6OO‐syn is the most favorable process with unimolecular RRKM rate coefficient 1.41×10−8 s−1 at ambient conditions.

Anomalous lifetimes of ultracold complexes decaying into a single channel

We investigate the lifetimes of complexes formed in ultracold molecule collisions. Employing both transition-state theory and a non-Hermitian microscopic model, we examine processes that can extend the lifetime of complexes beyond that predicted by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. We focus on complexes that possess only one open channel, and find that the extreme distribution of widths for this case favors low decay rates. Thus, decay from a complex into a single energetically available channel can be anomalously slow, even orders of magnitude slower than the RRKM rate, and moreover the decay may be nonexponential in time.

Investigation of Methylcyclopentadiene Reactivity: Abstraction Reactions and Methylcyclopentadienyl Radical Unimolecular Decomposition.

Understanding the reactivities of methylcyclopentadiene and the methylcyclopentadienyl radical is important in order to improve our comprehension of the chemical kinetics leading to the formation, decomposition, and growth of the first aromatic ring, as it has been shown that five-membered-ring species are important intermediates in the reaction kinetics of aromatic species. In this work, the rate constants of some key H-abstraction reactions from methylcyclopentadiene to produce the methylcyclopentadienyl radical and the formation of fulvene and benzene from the latter are theoretically determined. Rate constants are evaluated using the ab initio transition state theory-based master equation approach, determining structures and Hessians of all stationary points at the ωB97X-D/aug-cc-pVTZ level, energies at the CCSD(T) level extrapolated to the complete basis set limit, RRKM rate constants using conventional and variational transition state theory, and phenomenological rate constants through the solution of the one-dimensional master equation. Variational corrections are determined in both internal and Cartesian coordinates, and it is found that the choice of the coordinate system can impact the accuracy of the calculated rate constants by up to a factor of 4 for H-abstraction reactions and 2 for the unimolecular decomposition of the methylcyclopentadienyl radical. The calculated rate constants are in good agreement with the available literature data. Prompt dissociation of methylcyclopentadienyl radicals accessed following H-abstraction from methylcyclopentadiene was also investigated, and the corresponding rate constants were determined; the results show that prompt dissociation plays a key role under combustion conditions. Finally, lumping of theoretically derived rate constants to account for methylcyclopentadiene ⇄ methylcyclopentadienyl tautomerism allowed the derivation of a simplified set of rate constants suitable to be inserted into kinetic mechanisms.

Understanding the kinetics and atmospheric degradation mechanism of chlorotrifluoroethylene (CF2CFCl) initiated by OH radicals.

The atmospheric degradation of chlorotrifluoroethylene (CTFE) by OH˙ was investigated using density functional theory (DFT). The potential energy surfaces were also defined in terms of single-point energies derived from the linked cluster CCSD(T) theory. With an energy barrier of -2.62 to -0.99 kcal mol-1 using the M06-2x method, the negative temperature dependence was determined. The OH˙ attack on Cα and Cβ atoms (labeled pathways R1 and R2, respectively) shows that reaction R2 is 4.22 and 4.42 kcal mol-1, respectively, more exothermic and exergonic than reaction R1. The main pathway should be the addition of OH˙ to the β-carbon, resulting in ˙CClF-CF2OH species. At 298 K, the calculated rate constant was 9.87 × 10-13 cm3 molecule-1 s-1. The TST and RRKM calculations of rate constants and branching ratios were performed at P = 1 bar and in the fall-off pressure regime over the temperature range of 250-400 K. The formation of HF and ˙CClF-CFO species via the 1,2-HF loss process is the most predominant pathway both kinetically and thermodynamically. With increasing temperature and decreasing pressure, the regioselectivity of unimolecular processes of energized adducts [CTFE-OH]˙ gradually decreases. Pressures greater than 10-4 bar are often adequate for assuring saturation of the estimated unimolecular rates when compared to the RRKM rates (in high-pressure limit). Subsequent reactions involve the addition of O2 to the [CTFE-OH]˙ adducts at the α-position of the OH group. The [CTFE-OH-O2]˙ peroxy radical primarily reacts with NO and then directly decomposes into NO2 and oxy radicals. "Carbonic chloride fluoride", "carbonyl fluoride", and "2,2-difluoro-2-hydroxyacetyl fluoride" are predicted to be stable products in an oxidative atmosphere.

HCl elimination in the photolysis of chlorobenzene at 266 nm: An FT-IR spectroscopy and quantum chemical study

The photolysis of chlorobenzene (C6H5Cl) in the gas phase has been studied at 266 nm using repetitive scan FT-IR spectroscopy and density functional theory (DFT) to understand the degradation mechanism relevant to combustion and atmospheric chemistry. Following 266 nm photolysis of C6H5Cl, ro-vibrational lines were observed in the region 3060–2625 cm−1, at 3317.8/ 3262.7 cm−1 and 1346.2/1301.2 cm−1, and at 3341.2 and 1232.7 cm−1. These infrared features are assigned to the hydrochloric acid (HCl), acetylene (C2H2), and 1,3-butadiyne (C4H2), respectively. Identification of C2H2 and C4H2 but not expected HCl co-product ortho-benzyne (o-C6H4) indicates, possibly, o-C6H4 further degraded into C2H2 + C4H2. B3PW91/aug-cc-pVTZ and CBS-QB3 calculated potential energy surfaces for the possible degradation channels of C6H5Cl shows that HCl elimination and C–Cl bond fission are major degradation paths. Their experimental branching ratio was determined to be 1:1. The RRKM rate of HCl elimination of C6H5Cl at 266 nm was found to be 3.8 × 102 s−1 and compared with the HF elimination rate in C6H5F. The possible degradation mechanism is discussed.

Size Dependent Fragmentation Chemistry of Short Doubly Protonated Tryptic Peptides.

Tandem mass spectrometry of electrospray ionized multiply charged peptide ions is commonly used to identify the sequence of peptide(s) and infer the identity of source protein(s). Doubly protonated peptide ions are consistently the most efficiently sequenced ions following collision-induced dissociation of peptides generated by tryptic digestion. While the broad characteristics of longer (N ≥ 8 residue) doubly protonated peptides have been investigated, there is comparatively little data on shorter systems where charge repulsion should exhibit the greatest influence on the dissociation chemistry. To address this gap and further understand the chemistry underlying collisional-dissociation of doubly charged tryptic peptides, two series of analytes ([GxR+2H]2+ and [AxR+2H]2+, x = 2-5) were investigated experimentally and with theory. We find distinct differences in the preference of bond cleavage sites for these peptides as a function of size and to a lesser extent composition. Density functional calculations at two levels of theory predict that the threshold relative energies required for bond cleavages at the same site for peptides of different size are quite similar (for example, b2-yN-2). In isolation, this finding is inconsistent with experiment. However, the predicted extent of entropy change of these reactions is size dependent. Subsequent RRKM rate constant calculations provide a far clearer picture of the kinetics of the competing bond cleavage reactions enabling rationalization of experimental findings. The M06-2X data were substantially more consistent with experiment than were the B3LYP data.

A theoretical investigation on the mechanism and kinetics of the thermal isomerization of Trimethylsilylcyclopropane using CBS-QB3

The kinetics of the thermal isomerization of trimethylsilylcyclopropane in the temperature range of 689.5–751.1 K have been theoretically studied using Rice-Ramsperger-Kassel-Marcus (RRKM) theory and transition state theory (TST) in conjugation with CBS-QB3 calculations. Three possible reaction pathways are identified. Among them, the three-membered ring opening and hydrogen atom transfer to the carbon atom bonded to the SiMe3 group and formation of the allyltrimethylsilane is the main reaction. Our calculated kinetic rate constants appear to be in excellent agreement with the available experimental data. The results show that the most abundant product derived from Trimethylsilylcyclopropane will be the (Z)-1-propenyltrimethylsilane under thermodynamic control, while the most favorable process is isomerization reaction of that reactant into the allyltrimethylsilane from a kinetic viewpoint. The regioselectivity of the reaction decreases with decreasing pressures and increasing temperatures. In proportion to greater barrier heights, pressures P > 10−4 bar are in general enough for confirming saturation of the calculated rate coefficients compared with the high-pressure limit of the RRKM rates.

Quantum-state resolved lifetime of triplet ([formula omitted]) formaldehyde

We report state-resolved lifetimes of formaldehyde in the first excited triplet ã3A2 electronic state. Pump-laser excitation within 2800 cm−1 of the triplet origin is followed by variably delayed F2-laser ionization. Lifetimes fluctuate from >100 μs to ∼100 ns and a large isotope effect is observed. The average decay rate increases exponentially with increasing vibrational energy above the electronic origin, consistent with a mechanism involving tunneling through the barrier on the electronic ground state to the CO+H2/D2 continuum. By accessing the triplet state we are able to investigate the molecular elimination reaction down to ∼25 kJ/mol below the zero-point-energy corrected activation barrier. A model based on empirical RRKM rates allows the intersystem crossing matrix elements to be estimated. The T1–S0 intersystem crossing matrix elements are comparable to the S1–S0 internal conversion matrix elements observed at similar vibrational excitation.

Unimolecular Rate Constants versus Energy and Pressure as a Convolution of Unimolecular Lifetime and Collisional Deactivation Probabilities. Analyses of Intrinsic Non-RRKM Dynamics.

Following work by Slater and Bunker, the unimolecular rate constant versus collision frequency, kuni(ω, E), is expressed as a convolution of unimolecular lifetime and collisional deactivation probabilities. This allows incorporation of nonexponential, intrinsically non-RRKM, populations of dissociating molecules versus time, N( t)/ N(0), in the expression for kuni(ω, E). Previous work using this approach is reviewed. In the work presented here, the biexponential f1 exp(- k1 t) + f2 exp(- k2 t) is used to represent N( t)/ N(0), where f1 k1 + f2 k2 equals the RRKM rate constant k( E) and f1 + f2 = 1. With these two constraints, there are two adjustable parameters in the biexponential N( t)/ N(0) to represent intrinsic non-RRKM dynamics. The rate constant k1 is larger than k( E) and k2 is smaller. This biexponential gives kuni(ω, E) rate constants that are lower than the RRKM prediction, except at the high and low pressure limits. The deviation from the RRKM prediction increases as f1 is made smaller and k1 made larger. Of considerable interest is the finding that, if the collision frequency ω for the RRKM plot of kuni(ω, E) versus ω is multiplied by an energy transfer efficiency factor βc, the RRKM kuni(ω, E) versus ω plot may be scaled to match those for the intrinsic non-RRKM, biexponential N( t)/ N(0), plots. This analysis identifies the importance of determining accurate collisional intermolecular energy transfer (IET) efficiencies.

Sequence Ion Structures and Dissociation Chemistry of Deprotonated Sucrose Anions.

We investigate the tandem mass spectrometry of regiospecifically labeled, deprotonated sucrose analytes. We utilize density functional theory calculations to model the pertinent gas-phase fragmentation chemistry of the prevalent glycosidic bond cleavages (B1-Y1 and C1-Z1 reactions) and compare these predictions to infrared spectroscopy experiments on the resulting B1 and C1 product anions. For the C1 anions, barriers to interconversion of the pyranose [α-glucose-H]-, C1 anions to entropically favorable ring-open aldehyde-terminated forms were modest (41kJmol-1) consistent with the observation of a band assigned to a carbonyl stretch at ~ 1680-1720cm-1. For the B1 anions, our transition structure calculations predict the presence of both deprotonated 1,6-anhydroglucose and carbon 2-ketone ((4S,5S,6R)-4,5-dihydroxy-6-(hydroxymethyl)dihydro-2H-pyran-3(4H)-one) anion structures, with the latter predominating. This hypothesis is supported by our spectroscopic data which show diagnostic bands at 1600, 1674, and 1699cm-1 (deprotonated carbon 2-ketone structures), and at ~ 1541cm-1 (both types of structure) and RRKM rate calculations. The deprotonated carbon 2-ketone structures are also the lowest energy product B1 anions. Graphical Abstract ᅟ.

Effects of vibrational and rotational energies on the lifetime of the pre-reaction complex for the F− + CH3I SN2 reaction

Direct dynamics simulations were performed to investigate unimolecular dynamics of the F−⋯HCH2I pre-reaction complex for the F−+CH3I SN2 reaction. The simulations were performed for a total energy commensurate with the 0.32eV collision energy considered in previous experiments and simulations of this reaction. Two excitation patterns of the complex were considered for the simulations: one with the excitation energy primarily in vibration and the other with the energy primarily in rotation. For the latter equal amounts of energy were added about the three external rotation axes of the complex, giving rise to J and K rotation quantum numbers of 357 and 66 for the initial excitation. For the vibrational excitation the unimolecular dynamics agrees with RRKM theory and dissociation of the complex to the F−+CH3I reactants is negligible, since the barrier for this reaction is 20.2kcal/mol in contrast to the 2.4kcal/mol barrier for accessing the SN2 transition state (TS). The unimolecular dynamics are much different for the rotational excitation simulation. They are non-RRKM and the instantaneous unimolecular rate constant for F−⋯HCH2I decomposition increases with time. Apparently, this arises from K-mixing and vibration/rotation energy redistribution. At the end of the 10ps simulation, the instantaneous simulation rate constant for accessing the SN2 transition state is an order of magnitude smaller than the harmonic RRKM rate constant with the K quantum number treated as an active degree of freedom, indicating that K-mixing and vibration/rotation energy redistribution are not complete on this time scale. For rotational excitation, dissociation to the F−+CH3I reactants is the dominant unimolecular pathway, instead of forming the SN2 products. This is a result of a much higher rotational barrier at the SN2 TS than for the TS leading to F−+CH3I.

OH-Stretching Overtone Induced Dynamics in HSO3F from Reactive Molecular Dynamics Simulations.

The OH-stretch induced dynamics in fluorosulfonic acid (HSO3F) is characterized from a statistically significant number of trajectories using multisurface adiabatic reactive molecular dynamics (MS-ARMD) simulations. The global reactive potential energy surface, which describes H-transfer and HF-elimination, is parametrized at the MP2/6-311G++(2p,2d) level of theory with an accuracy of better than 1 kcal/mol. Excitation along the OH-local mode leads to H-transfer dynamics but elimination of HF is only observed for excitations with ν ≥ 6 for 1 out of 5000 trajectories. This finding differs fundamentally from the situation for vibrationally induced photodissociation of H2SO4 and HSO3Cl, for which, even with excitations of 4 quanta along the OH-stretch mode, elimination of H2O and HCl, respectively, is readily observed on the subnanosecond time scale. RRKM rates for HX-elimination in HSO3X (X = F, Cl) only differ by a factor of 5. The findings from the reactive molecular dynamics simulations together with the RRKM results thus indicate that the origin for a closed HF-production channel is dynamical. This is also consistent with experimental findings for hydrofluoroethanes in shock tubes, which found pronounced non-RRKM behavior.

Calculating RRKM Rate Constants from Vibrational Frequencies and Their Dynamic Interpretation

Calculating RRKM Rate Constants from Vibrational Frequencies and Their Dynamic Interpretation

Theoretical study on reaction mechanism of ground-state cyano radical with 1,3-butadiene: prospect of pyridine formation.

The reaction of ground-state cyano radicals, CN(X(2)Σ(+)), with the simplest polyene, 1,3-butadiene (C4H6(X(1)Ag)), is investigated to explore probable routes and feasibility to form pyridine at ultralow temperatures. The isomerization and dissociation channels for each of the seven initial collision complexes are characterized by utilizing the unrestricted B3LYP/cc-pVTZ and the CCSD(T)/cc-pVTZ calculations. With facilitation of RRKM rate constants, through ab initio paths composed of 7 collision complexes, 331 intermediates, 62 hydrogen atom, 71 hydrogen molecule, and 3 hydrogen cyanide dissociated products, the most probable paths at collision energies up to 10 kcal/mol, and thus the reaction mechanism, are determined. Subsequently, the corresponding rate equations are solved that the concentration evolutions of collision complexes, intermediates, and products versus time are obtained. As a result, the final products and yields are determined. The low-energy routes for the formation of most thermodynamically stable product, pyridine, are identified. This study, however, predicts that seven collision complexes would produce predominately 1-cyano-1,3-butadiene, CH2CHCHCHCN (p2) plus atomic hydrogen via the collision complex c1(CH2CHCHCH2CN) and intermediate i2(CH2CHCH2CHCN), with a very minor amount of pyridine. Our scheme also effectively excludes the presence of 2-cyano-1,3-butadiene, which has energy near-degenerate to 1-cyano-1,3-butadiene, as supported by experimental findings.

Models for Intrinsic Non-RRKM Dynamics. Decomposition of the SN2 Intermediate Cl––CH3Br

Chemical dynamics simulations, based on both an analytic potential energy surface (PES) and direct dynamics, were used to investigate the intrinsic non-RRKM dynamics of the Cl−–CH3Br ion-dipole complex, an important intermediate in the Cl− + CH3Br SN2 nucleophilic substitution reaction. This intermediate may dissociate to Cl− + CH3Br or isomerize to the ClCH3–Br− ion-dipole complex. The decomposition of microcanonical ensembles of the Cl−–CH3Br intermediate were simulated, and the ensuing populations vs. time of the excited intermediate and Cl− + CH3Br and ClCH3–Br− products were fit with multi-exponential functions. The intrinsic non-RRKM dynamics is more pronounced for the simulations with the analytic PES than by direct dynamics, with the populations for the former and latter primarily represented by tri- and bi-exponential functions, respectively. For the analytic PES and direct dynamics simulations, the intrinsic non-RRKM dynamics is more important for the isomerization pathway to form ClCH3–Br− than for dissociation to Cl− + CH3Br. Since the decomposition probability of Cl−–CH3Br is non-exponential, the Cl−–CH3Br unimolecular rate constant depends on pressure, with both high and low pressure limits. The high pressure limit is the RRKM rate constant and for the simulations with the analytic PES the rate constant decreased by a factor of 3.0, 5.6, and 4.3 in going from the high to low pressure limit for total energies of 40, 60, and 80 kcal/mol. For the direct dynamics simulations these respective factors are 2.4, 1.4, and 1.2. A separable phase space model with intermolecular and intramolecular complexes describes some of the simulation results, but overall models advanced for intrinsic non-RRKM dynamics give incomplete representations of the intermediate and product populations vs. time determined from the simulations.

Relationships between DFT/RRKM Branching Ratios of the Complementary Fragment Ions [C5H5O]+ and [M – C5H5O]+ and Relative Abundances in the EI Mass Spectrum of N-(2-Furylmethyl)aniline

The energy-dependent branching ratios of the complementary fragment ions [C(5)H(5)O](+) and [HC(6)H(4)NH](+) ([M - C(5)H(5)O](+)), originating from the N-(2-furylmethyl)aniline molecular ion, [HC(6)H(4)NH-C(5)H(5)O](+•), were obtained from Rice-Ramsperger-Kassel-Marcus (RRKM) rate calculations based on density functional theory (DFT) energy profiles. The UB3LYP/6-311G+(3df,2p)//UB3LYP/6-31G(d) level of theory was used to model the competitive reaction mechanisms by which the molecular ion can be fragmented. Initially, eight pairs of products were taken into account, corresponding to the combination of two isomeric structures for each fragment ion and the concomitant radicals, which can be formed by direct dissociations or through some isomerization-fragmentation pathways. A great deal of the obtained pathways was discarded by looking over the kinetic barrier heights and the individual RRKM rate coefficients calculated for all the steps. This way, the potential energy profiles were simplified to only three reaction channels, two pathways to [C(5)H(5)O](+) and one to [M - C(5)H(5)O](+). The pre-equilibrium and steady-state approximations were then applied to different regions of the remaining potential energy profiles, allowing the branching ratios of the complementary fragment ions to be easily calculated and discriminated among the three rival processes. According to these results, the major fragment ion in the ion source is [C(5)H(5)O](+), which is produced as a mixture of two structures, the furfuryl and pyrylium cations, one formed by a direct C-N bond cleavage and the other through an isomerization-fragmentation channel. In turn, the direct fragmentation is the only mechanism to produce [M - C(5)H(5)O](+). To confront these results with the available experimental information, the model was broadened out to the 4-substituted analogues [4-R-C(6)H(4)NH-C(5)H(5)O](+•) in which R = F, Br, Cl, CH(3), and OCH(3), finding excellent correlations of the calculated branching ratios and the relative abundances in the electron ionization mass spectra.

Structure and energetics of cyclopropane carboxaldehyde

AbstractWe use the energies obtained by a focal point analysis including extrapolation from results with basis sets cc‐pVnZ and aug‐cc‐pVnZ with n up to 4 and correlation corrections through CCSD(T), to estimate thermodynamic functions for the syn and anti isomers of cyclopropane carboxaldehyde (CPCA). These agree with values obtained by well‐established thermochemical schemes CBS‐QB3 and G4. The structures obtained in these studies also conform to the best experimental determination of the rotational constants in the gas phase. The kinetics of gas phase interconversion of the syn‐ and anti‐isomers of CPCA have been studied by a chirped‐pulse dynamic rotational spectroscopy. Computational modeling of the internal rotational potential allows the estimate of the interconversion rates by statistical (RRKM) methods. RRKM rates using a range of barrier heights including a CBS‐Q estimate are more than 10× the rates deduced from the dynamical rotational spectra. This suggests that nonstatistical effects may be limiting the rate. Detailed study of the interconversion potential by a variant of the focal point analysis suggests that previous estimates of the barrier may be too low, and thus, the inferred rice‐ramsperger‐kassel‐marcus (RRKM) rate could be too high. These results cast some doubt on the presence of nonstatistical effects and suggest that molecular dynamics studies should be conducted to characterize the energy flow in detail. © 2013 Wiley Periodicals, Inc.