RRKM Research Articles

Mechanism and kinetic of nitrate radical-initiated atmospheric reactions of guaiacol (2-methoxyphenol)

Abstract Guaiacol is the main representative of methoxyphenols generated from the pyrolysis of ligin. The gas-phase reaction mechanism of guaiacol with NO3 radical was researched by using density functional theory at M06-2X/6-31+g(d,p)//M06-2X/6-311+g(3df,2p) level in this work, and the reaction rate constants were calculated by using RRKM theory. The relative complete gas phase reaction mechanism of guaiacol with nitrate radicals was obtained. The calculated results showed that, NO3 addition and H abstraction are two dominant progresses for the initial reaction of guaiacol with NO3 radicals. The total reaction rate constant is 3.11 × 10−11 cm3 molecule−1 s−1 at 298 K and 1 atm. The atmospheric lifetime is 64 s under typical tropospheric concentration of NO3 radicals. The possibility of products (methoxybenzoquinone and nitro-guaiacol) is verified and new products (dialdehyde and diketone) are predicted.

Infrared Population Transfer Spectroscopy of Cryo-Cooled Ions: Quantitative Tests of the Effects of Collisional Cooling on the Room Temperature Conformer Populations.

The single-conformation spectroscopy and infrared-induced conformational isomerization of a model protonated pentapeptide [YGPAA + H]+ is studied under cryo-cooled conditions in the gas phase. Building on recent results ( DeBlase , A. F. ; J. Am. Chem. Soc. 2017 , 139 , 5481 - 5493 ), firm assignments are established for the presence of two conformer families with distinct infrared and ultraviolet spectra, using IR-UV depletion spectroscopy. Families (A and B) share a similar structure near the N-terminus but differ in the way that the C-terminal COOH group configures itself (cis versus trans) in forming H-bonds with the peptide backbone. Infrared population transfer (IR-PT) spectroscopy is used to study the IR-induced conformational isomerization following single-conformer infrared excitation. IR-induced isomerization is accomplished in both directions (A → B and B → A) in the hydride stretch region and is used to determine fractional abundances for the two conformer families (FA = 0.65 ± 0.04, FB = 0.35 ± 0.04, 2σ error bars). The time scale for collisional cooling of the room-temperature ions to Tvib = 10 K by cold helium in the octupole trap is established as 1.0 ms. Key stationary points on the isomerization potential energy surface are calculated at the DFT B3LYP/6-31+G(d) G3DBJ level of theory. Using RRKM theory, the energy-dependent isomerization rates and populations are calculated as a function of energy. According to the model, the observed population distribution after collisional cooling is close to that of the 298 K Boltzmann distribution and is in near-quantitative agreement with experiment. On the basis of this success, inferences are drawn for the circumstances that govern the population distribution in the trap, concluding that, in ions the size of [YGPAA + H]+ and larger, the observed distributions will be near those at 298 K.

Computational study on the mechanisms and kinetics of the CH2=CHCH2I with OH reaction

The potential energy surface for the reaction of OH with CH2═CHCH2I has been studied at the CCSD(T)//M06-2X/6-311++G(d,p) level of theory. Three different reaction entrances were revealed, namely, terminal-C addition, central-C addition, and H-abstraction, leading to CH2OHCHCH2I (IM1), CH2CHOHCH2I (IM2), and H2O + C3H4I, respectively. Several conceivable decomposition and isomerization channels were also examined for IM1 and IM2. The total and individual rate constants were calculated by using multichannel RRKM and TST theories over a wide range of temperatures (200–3000 K) and pressures(10−14–1014 Torr).

Real-time observation of the photoionization-induced water rearrangement dynamics in the 5-hydroxyindole-water cluster by time-resolved IR spectroscopy.

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room-Temperature Flow Reactor

Chirped-pulse Fourier transform millimeter-wave spectroscopy is a potentially powerful tool for studying chemical reaction dynamics and kinetics. Branching ratios of multiple reaction products and intermediates can be measured with unprecedented chemical specificity; molecular isomers, conformers, and vibrational states have distinct rotational spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide photoproducts in a flow tube reactor at ambient temperature of 295 K and pressures of 1-10 μbar. This in situ and time-resolved experiment illustrates the utility of this novel approach to investigating chemical reaction dynamics and kinetics. Following 193 nm photodissociation of CH2CHCN, we observe rotational relaxation of energized HCN, HNC, and HCCCN photoproducts with 10 μs time resolution and sample the vibrational population distribution of HCCCN. The experimental branching ratio HCN/HCCCN is compared with a model based on RRKM theory using high-level ab initio calculations, which were in turn validated by comparisons to Active Thermochemical Tables enthalpies.

Experimental and RRKM Investigations on the Degradation of Ethyl Formate

AbstractA Single pulse shock tube was used to investigate the thermal decomposition of ethyl formate over the temperature range of 909–1258 K and pressure range of 9–17 atm. RRKM (Rice‐Ramsperger‐Kassel‐Marcus) theory was used to estimate the temperature (500–2500 K) and pressure dependent (0.01–100 atm) rate coefficients. The products identified were methane, ethylene, and propane. The initial elementary step for the decomposition of ethyl formate is the unimolecular elimination of ethylene via the reaction CH3CH2OC(O)H → CH2=CH2 + HCOOH. The formed formic acid is further decomposed within the studied temperature range. A detailed reaction mechanism proposed with 21 species and 22 elementary reactions was used to simulate reactant and product distribution over the investigated temperature range. The temperature dependent rate coefficient for the total decomposition of ethyl formate was found to be (909–1258 K)=2.68 × 1011 (s−1) exp (‐42.9 ± 2.2 kcal mol−1/RT). The bond dissociation energies were calculated for the various types of bonds. The calculated total rate coefficient for the thermal decomposition of ethyl formate is (500–2500 K)=3.61 × 1013 (s−1) exp (‐53.4 kcal mol−1/RT) at high pressure limit.

On the theory of time dilation in chemical kinetics

The rates of chemical reactions are not absolute but their magnitude depends upon the relative speeds of the moving observers. This has been proved by unifying basic theories of chemical kinetics, which are transition state theory, collision theory, RRKM and Marcus theory, with the special theory of relativity. Boltzmann constant and energy spacing between permitted quantum levels of molecules are quantum mechanically proved to be Lorentz variant. The relativistic statistical thermodynamics has been developed to explain quasi-equilibrium existing between reactants and activated complex. The newly formulated Lorentz transformation of the rate constant from Arrhenius equation, of the collision frequency and of the Eyring and Marcus equations renders the rate of reaction to be Lorentz variant. For a moving observer moving at fractions of the speed of light along the reaction coordinate, the transition state possess less kinetic energy to sweep translation over it. This results in the slower transformation of reactants into products and in a stretched time frame for the chemical reaction to complete. Lorentz transformation of the half-life equation explains time dilation of the half-life period of chemical reactions and proves special theory of relativity and presents theory in accord with each other. To demonstrate the effectiveness of the present theory, the enzymatic reaction of methylamine dehydrogenase and radioactive disintegration of Astatine into Bismuth are considered as numerical examples.

Anharmonic effect of the rate constant of the reactions of CH3SCH2OO system in high-temperature combustion

The study mainly focuses on the anharmonic effect of the reactions of CH3SCH2OO system. The geometries of the reactants and the transition states are optimized with Gaussian 09. The barrier heights are calculated with the energy of the reactants and the transition states. The RRKM theory is utilized to calculate the anharmonic and harmonic rate constants of the reactions. The anharmonic effect of these reactions can be clearly demonstrated by our results. Generally speaking, in the study, for most reactions, the rate constants increase with the temperature in the canonical case and the total energy in the microcanonical case, and the anharmonic effect of these reactions is significant and should not be neglected in high-temperature combustion. In CH3SCH2OO system, CH3SCH2OO → CH2SCH2OOH → CH2S + CH2O + OH is the main reaction channel. After a series of calculations, the anharmonic effect is remarkable, especially in high-temperature combustion. By analyzing other meaningful reactions that followed that channel above, the anharmonic effect of these reactions is generally obvious enough, especially for those reactions whose barrier heights are relatively low.

Shock tube study and RRKM calculations on thermal decomposition of 2-chloroethyl methyl ether

The thermal decomposition of 2-chloroethyl methyl ether (2-CEME) was studied in the temperatures between 1175 and 1467 K. The decomposition of 2-CEME happens predominantly via molecular elimination reactions than via CC and CO bond fission channels. The major decomposition products are methane, ethylene and methanol. The minor are acetaldehyde and ethane. The Arrhenius expression for the overall decomposition of 2-CEME was obtained to be ktotalexp(1175−1467K)=(4.12±0.42)×1011exp(−(52.2kcalmol−1±2.6)/RT)s−1. To simulate the distribution of reactant and products over the experimentally studied temperatures between 1175 and 1467 K, a reaction scheme was constructed with 45 species and 71 elementary reactions. The pressure and temperature dependent rate coefficients were calculated for various unimolecular dissociation pathways in 2-CEME using RRKM theory. The high pressure limit temperature dependent rate coefficient for the total decomposition of 2-CEME was obtained to be ktotalCCSDT//M06−2X(500–2000 K) = (2.55 ± 0.21) × 1014 exp (−(67.6 kcal mol−1 ± 3.0)/RT) s−1.

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.

The gas phase aldose‐ketone isomerization mechanism: Direct interconversion of the model hydroxycarbonyls 2‐hydroxypropanal and hydroxyacetone

We report a novel mechanism for the interconversion of 2-hydroxypropanal with its more-stable ketone isomer hydroxyacetone. Reaction proceeds via concerted transfer of two H atoms, requires a barrier of only ∼40 kcal mol−1, bypasses the enediol intermediate, and is general for α-hydroxy carbonyls. A similar isomerization mechanism is shown to persist for β, γ, and δ-hydroxy carbonyls; these compounds are skeletal forms of the monosaccharides and this work, therefore, discloses the gas-phase mechanism for aldose-ketose isomerization. As an example, the isomerization of glyceraldehyde to dihydroxyacetone is shown to proceed via this mechanism with a barrier of 31 kcal mol−1. Rate coefficients and thermochemical properties are reported for the isomerization of 2-hydroxypropanal and hydroxyacetone for use in detailed kinetic models. Additionally, RRKM theory k(E) values for this reaction suggest that it may transpire in the troposphere following solar excitation.

Ab Initio Chemical Kinetics for the Thermal Decomposition of SiH4 + Ion and Related Reverse Ion–Molecule Reactions of Interest to PECVD of a-Si:H Films

The thermal unimolecular decomposition of SiH4 + ion and its related reverse reactions, SiH3 + + H and SiH2 + + H2, have been investigated by ab initio molecular orbital and quantum statistical variational RRKM theory calculations. The potential energy surface has been calculated at different levels of theory; the results at the highest level, CCSD(T)/CBS//CCSD(T)/6-311++G(3df,2p), show that the decomposition of SiH4 + can mainly occur via a barrierless channel giving SiH2 + + H2 lying 11.8 kcal/mol above the reactant, or via a transition state forming SiH3 +···H complex to be followed by a barrierless decomposition yielding SiH3 + + H lying 23.5 kcal/mol above the reactant. Barrierless processes were calculated using the CASPT2 and CASSCF methods with the 6-311++G(3df,2p) basis set and fitted with Morse potentials. The rate constants were predicted by solving master equations based on the RRKM theory at the E,J-resolved level; the results show that the channel SiH4 + → SiH2 + + H2 is predominate under PEVCD conditions. For H- and H2-capturing by SiH3 + and SiH2 + ions, respectively, the rate constants were found to be weakly dependent on temperature at the high-pressure limit and decrease rapidly with pressure. The calculated heats of formation of the SiH x + (x = 2–4) ions are in close agreement with available thermochemical data.

Mechanistic and kinetic study on the reaction of methylperoxy radical with atomic iodine

Singlet and triplet potential energy surfaces for the CH3O2 with I reaction have been investigated computationally to propose the reaction mechanisms and possible products. Multichannel RRKM theory and transition-state theory have been used to compute the overall and individual rate constants at 200–3000K and 10−14–1014Torr. On the singlet PES, addition-elimination, substitution and H-abstraction mechanisms are located, and the addition-elimination mechanism is dominant. At 70Torr with N2 as bath gas, IM1(CH3OOI) formed by collisional stabilization is dominated at 200–300K, whereas CH2O and HIO are the major products at the temperatures between 350 and 3000K; The title reaction exhibits the typical falloff behavior. The results show that temperature and pressure affect the yield of products.Furthermore, the predicted rate constants at 298K 70Torr of N2 agree well with the available experimental values. On the triplet PES, the most favorable product should be CH3I+O2(3Σ) at atmospheric condition. Other two pathways on the triplet PES will not compete with the pathways on the singlet PES in kinetically and thermodynamically.

Perspective: chemical dynamics simulations of non-statistical reaction dynamics.

Non-statistical chemical dynamics are exemplified by disagreements with the transition state (TS), RRKM and phase space theories of chemical kinetics and dynamics. The intrinsic reaction coordinate (IRC) is often used for the former two theories, and non-statistical dynamics arising from non-IRC dynamics are often important. In this perspective, non-statistical dynamics are discussed for chemical reactions, with results primarily obtained from chemical dynamics simulations and to a lesser extent from experiment. The non-statistical dynamical properties discussed are: post-TS dynamics, including potential energy surface bifurcations, product energy partitioning in unimolecular dissociation and avoiding exit-channel potential energy minima; non-RRKM unimolecular decomposition; non-IRC dynamics; direct mechanisms for bimolecular reactions with pre- and/or post-reaction potential energy minima; non-TS theory barrier recrossings; and roaming dynamics.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Theoretical Study on the Gas Phase Reaction of Allyl Bromide with Hydroxyl Radical

Mechanisms and reaction channels of the allyl bromide (CH2CHCH2Br) with OH reaction are studied using quantum chemistry. It is predicted that the H(or Br)-abstraction and addition/elimination mechanisms have been revealed on potential energy surface (PES). Direct H-abstract from the CH2Br group of CH2CHCH2Br leading to h-P1 (CH2CHCHBr+H2O) is dominant. As for addition/elimination mechanism, it is shown that the reaction is initiated by the addition of OH radical to the CC bond of CH2CHCH2Br to barrierlessly generate the intermediates IM1 and IM2, respectively. Multichannel RRKM theory and variational transition-state theory (VTST) are employed to evaluate the rate constants of temperature- and pressure-dependent. The calculated rate constants are in good agreement with the available experimental data. At 100Torr with helium as bath gas, IM3 (CH2OHCHBrCH2) formed by collisional stabilization and the final products P1 (CH2OH+CH2CHBr) are the major product in the temperature range of 200–700K and 700–1000K, respectively. The production of CH2CHCHBr via hydrogen abstraction becomes dominant at high temperatures (T⩾1000K). Time-dependent DFT (TD-DFT) calculations indicate that IM1-IM4, IM6, IM8 and IM9 take photolysis easily in the sunlight at the wavelength of 333nm, 244nm, 327nm, 313nm, 298nm, 305nm and 428nm, once they are generated.

The mechanistic and kinetic investigation on the atmospheric reaction of atomic O(3P) with crotononitrile

The mechanism and kinetics for the O(3P)+CH3CHCHCN reaction has been investigated firstly. The BHandHLYP and M05-2X methods were employed to obtain the initial geometries. The triplet/singlet potential energy surfaces (PESs) were constructed with high-level BMC-CCSD method. The conventional transition-state theory (CTST) and multichannel RRKM theory were employed to calculate the total and individual rate constants over a wide range of temperatures under high-pressure limit. Our computed rates agree well with the available experimental results. The yield of IM1 is 0.9–0.5 from 200 to 1000K, and the construction of h-P1(OH+CH2CHCHCN) is 0.42 at 2000K under high-pressure limit. The yields of the predicted decomposition products P1(CH3+HCOHCCN), P2(HCCN+CH3CHO) and P3(H+CH3COHCCN) at 298K and 1 atom are 0.81, 0.07, and 0.09, respectively.

Mechanism and selectivity of X− + CH3ONO2 (X = NCCH2, CH3C(O)CH2, and PhCH2) multichannel gas phase reactions

The mechanisms and ionic product distributions related to multiple channels of the X−+CH3ONO2 (X=NCCH2, CH3C(O)CH2, and PhCH2) gas phase reactions were investigated at the B2PLYP/6–311+G(3df,2p)//MP2/6–31+G(d,p) level of theory. The reaction channels are bimolecular nucleophilic displacements at either the carbon (SN2@C) or nitrogen (SN2@N) centers and a proton abstraction followed by dissociation (ECO2). For the ambident NCCH2− and CH3C(O)CH2− nucleophiles, two additional pathways for each reaction channel become available through attacks via the methylenic carbon or nitrogen/oxygen centers. The large number of reaction channels for these ambident nucleophiles precludes the unique determination of the ECO2:SN2@C:SN2@N ratios and of the carbon or nitrogen/oxygen regioselectivity by mass spectrometry measurements. Thus, the relevance of performing detailed, accurate and reliable computational modeling, including determination of unimolecular rate constants with the RRKM theory. The nucleophilic displacements have the largest rate constants, which are: SN2@C for the NCCH2− and CH3C(O)CH2− nucleophiles via methylenic carbon and oxygen centers attacks, respectively, and SN2@N for the PhCH2− anion. The ionic products distributions 10:82:8, 2:97:1, and 0:18:82 related to the ECO2:SN2@C:SN2@N reaction pathways were calculated for the reactions with NCCH2−, CH3C(O)CH2−, and PhCH2−, respectively, which are in excellent quantitative agreement with the experimental data. These agreements suggest that these reactions with large nucleophiles and/or (de)localized charge have a statistical behavior, unlike the same reaction with smaller and localized charge nucleophiles (F− and OH−).

Thermal Decomposition of 2-Pentanol: A Shock Tube Study and RRKM Calculations.

A single pulse shock tube was used to study the thermal decomposition of 2-pentanol in the temperatures between 1110 and 1325 K. Three major decomposition products are methane, ethylene, and propylene. The minor products detected in lower concentration are ethane, acetylene, acetaldehyde, 1-pentene, and 2-pentene. The rate coefficient for the overall decomposition of 2-pentanol was found to be ktotalexp(1110-1325 K) = (4.01 ± 0.51) × 109 exp(-(36.2 ± 4.7)/RT) s-1, where the activation energies are given in kcal mol-1. To simulate reactant and product distribution over the experimentally studied temperatures between 1110 and 1325 K, a reaction scheme was constructed with 34 species and 39 reactions. In addition to this, the temperature and pressure dependent rate coefficients were computed for various unimolecular dissociation pathways using RRKM theory. The high pressure limit rate coefficient for overall decomposition of 2-pentanol was obtained to be ktotaltheory(500-2500 K) = (9.67 ± 1.11) × 1014 exp(-(67.7 ± 2.9)/RT) s-1. The calculated high pressure rate coefficients and experimentally measured rate constants are in good agreement with each other. The reaction is primarily governed by the unimolecular elimination of water.

Theoretical Study of the Reactions of Methane and Ethane with Electronically Excited N2(A(3)Σu(+)).

Comprehensive quantum chemical analysis with the usage of density functional theory and post-Hartree-Fock approaches were carried out to study the processes in the N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 systems. The energetically favorable reaction pathways have been revealed on the basis of the examination of potential energy surfaces. It has been shown that the reactions N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 occur with very small or even zero activation barriers and, primarily, lead to the formation of N2H + CH3 and N2H + C2H5 products, respectively. Further, the interaction of these species can give rise the ground state N2(X(1)Σg(+)) and CH4 (or C2H6) products, i.e., quenching of N2(A(3)Σu(+)) by CH4 and C2H6 molecules is the complex two-step process. The possibility of dissociative quenching in the course of the interaction of N2(A(3)Σu(+)) with CH4 and C2H6 molecules has been analyzed on the basis of RRKM theory. It has been revealed that, for the reaction of N2(A(3)Σu(+)) with CH4, the dissociative quenching channel could occur with rather high probability, whereas in the N2(A(3)Σu(+)) + C2H6 reacting system, an analogous process was little probable. Appropriate rate constants for revealed reaction channels have been estimated by using a canonical variational theory and capture approximation. The estimations showed that the rate constant of the N2(A(3)Σu(+)) + C2H6 reaction path is considerably greater than that for the N2(A(3)Σu(+)) + CH4 one.

Rotamers and Migration: Investigating the Dissociative Photoionization of Ethylenediamine.

The unimolecular dissociation of energy-selected ethylenediamine cations was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) in the photon energy range of 8.60-12.50 eV. Modeling the breakdown diagram and time-of-flight distributions with rigid activated complex RRKM theory yielded 0 K appearance energies for eight dissociation channels, leading to NH2CHCH2(+)(•) at 9.120 ± 0.010 eV, CH3C(NH2)2(+) at 9.200 ± 0.012 eV, NH2CHCH3(+) at 9.34 ± 0.08 eV, CH2NH2(+) at 9.449 ± 0.025 eV, CH2NH3(+) at 9.8 ± 0.1 eV, c-C2H4NH2(+) at 10.1 ± 0.1 eV, CH3NHCHCH2(+) at 10.2 ± 0.1 eV, and the reappearance of CH2NH2(+) at 10.2 ± 0.1 eV. The CBS-QB3-calculated pathways highlighted the influence of intramolecular hydrogen attractions on the dissociation processes, presenting novel isomers and low-energy van der Waals intermediates that led to fragments in good agreement with experimental results. While most of the dissociation channels take place through reverse barriers, the 0 K heat of formation of (•)CH2NH2 was determined to be 147.6 ± 3.7 kJ mol(-1), in excellent agreement with literature, and the 0 K heat of formation of CH2NH3(+) at 844 ± 10 kJ mol(-1) is the first experimentally measured value available and is in good agreement with theory.