Rice – Ramsperger – Kassel – Marcus Research Articles

Atmospheric chemistry of CHBr2O2: a theoretical study on mechanisms and kinetics of the CHBr2O2 + ClO reaction

CCSD(T)//B3LYP calculations of the potential energy surfaces (PESs) are associated with the rate constants and branch ratio of products using the RRKM (Rice–Ramsperger–Kassel–Marcus) theories to research the mechanism and product distribution of the CHBr2O2 with ClO reaction. The RRKM-computed theory reveals that IM1 (CHBr2OOOCl) are estimated to stable at T ≤ 500 K; the generating P1 (CBr2O + HO2 + Cl) was the primary product at T > 500 K. The lifetimes of CHBr2O2 in the presence of ClO are predicted to 27.8 h. Time-dependent density functional theory (TDDFT) calculations suggest that IM1 (CHBr2OOOCl), IM2 (CHBr2OOClO), and IM3 (CHBr2OClO2) will photolyze under the sunlight.

Mechanistic and Kinetic Study of Atmospheric Oxidation of Chlordane Initiated by OH Radicals

Chlordane, one of the extremely hazardous Persistent Organic Pollutants (POPs), was widely used as pesticides all over the world and its residues have been detected at high concentrations in many areas. As a species of Semi-Volatile Organic Compounds (SVOCs), chlordane exists mainly in the atmosphere where it can be migrated and transformed. Due to the carcinogenic and mutagenic properties, understanding its atmospheric fate is of great significance. In the present work, the oxidation mechanism of chlordane initiated by OH radicals under the atmospheric conditions was investigated by using Density Functional Theory (DFT). The geometrical structures were optimized at the M06- 2X/6-311+g(d,p) level and single-point energies were calculated at the M06-2X/6-311+g(3df,2p) level. The relevant rate constants of the key elementary reactions were calculated by using Rice-Ramsperger- Kassel-Marcus (RRKM) theory at 298 K and 1 atm. All of the energetically favorable pathways were discussed in detail, and theoretical results showed that the oxidation products are dichlorochlordene, hydroxychlrodane, cycloketone and dichloracyl. Combined with available experimental observation, this study can, therefore, help to clarify the atmospheric fate of chlordane.

Theoretical investigation of the mechanism and kinetics of the CH3NH2 +O(3P) reaction

We present the kinetics and mechanism of the reaction of O (3P) with CH3NH2. Geometries of all the stationary points involved in the reaction have been investigated at the B3LYP/6-311+G (d, p) level. We report the potential energy surface for all of the possible pathways by the Gaussian-2 G3B3 method. This is the first time one has gained a conclusive insight into the detailed mechanism and kinetics for this reaction. Using the Rice–Ramsperger–Kassel-Marcus (RRKM) theory, we have computed the rate constants for channels leading to several products over the temperature range 298–440 K. The main reaction channel is found to be addition of an oxygen atom to CH3NH2 followed by migration of an H atom from N to O atoms and loss of H2O, which agrees with experimental results (Slagle, Dudich, and Gutman (1979) J. Phys. Chem., 83). As experimental data are available for this reaction, we are able to conclude that our thermal rate constants are in agreement over a wide range of temperatures.

On some characteristics of Marcus’ work in the light of the history of science

Professor Rudolph A. Marcus, recipient of the 1992 Nobel Prize in Chemistry, is a distinguished theoretical chemist. Two important theories happen to bear his name: the Rice Ramsperger Kassel Marcus (RRKM) theory of unimolecular reactions and the Marcus theory of electron transfer reactions. When considering Marcus’ work, one finds characteristics of it that bear striking similarity to those that can be found in the work of some famous scientists. Such characteristics appear then as common recurring patterns in the work of theoreticians. Among them we find the following: (1) the creation of new theories, driven by experiments that preceding theories were unable to explain; (2) the use of thought experiments; (3) the generalization and extensions from one realm of science to another; (4) the making of predictions and the finding of new properties from correlations among series of data; (5) the use of variational principles; (6) the providing of different demonstrations of an equation; (7) the introduction of new types of coordinates; (8) the innovative capacity to use, by somehow recreating it under the pressure of the physical problem, a mathematical method previously unknown to the author; and finally, (9) the presentation of formulations of the same theory in different mathematical and physical languages. Before discussing in some detail the above points, a brief description of Marcus’ work is first presented for the convenience of the reader.

Influence of the Charge State on the Structures and Interactions of Vancomycin Antibiotics with Cell‐Wall Analogue Peptides: Experimental and Theoretical Studies

Charge matters! The charge state significantly influences the conformation and the binding energy between vancomycin antibiotic and bacterial cell-wall analogue peptides (see figure). Surface-induced dissociation (SID) studies provide a quantitative comparison between the stabilities of different charge states of the complex.In this study we examined the effect of the charge state on the energetics and dynamics of dissociation of the noncovalent complex between the vancomycin and the cell-wall peptide analogue N(alpha),N(epsilon)-diacetyl-L-Lys-D-Ala-D-Ala (V-Ac(2)LKdAdA). The binding energies between the vancomycin and the peptide were obtained from the RRKM (Rice, Ramsperger, Kassel, Marcus) modeling of the time- and energy-resolved surface-induced dissociation (SID) experiments. Our results demonstrate that the stability of the complex towards fragmentation increases in the order: doubly protonated<singly protonated<deprotonated. Dissociation of the singly protonated and singly deprotonated complex is characterized by very large entropy effects, which indicate a substantial increase in the conformational flexibility of the resulting products. The experimental threshold energies of (1.75+/-0.08) eV ((40.3+/-1.8) kcal mol(-1)) and (1.34+/-0.08) eV ((30.9+/-1.8) kcal mol(-1)) obtained for the deprotonated and singly protonated complexes, respectively, are in excellent agreement with the results of density functional theory calculations. The increased stability of the deprotonated complex observed experimentally is attributed to the presence of three charged sites in the deprotonated complex, as compared with only one charged site in the singly protonated complex. The low binding energy of (0.93+/-0.04) eV ((21.4+/-0.9) kcal mol(-1)) obtained for the doubly protonated complex suggests that this ion is destabilized by Coulomb repulsion between the singly protonated vancomycin and the singly protonated peptide comprising the complex.

Time-resolved gas-phase kinetic study of the reaction of germylene with propene over the temperature range 293–415 K: the thermal stabilities of germiranes

Absolute rate constants have been obtained for the title reaction over the pressure range 1–100 Torr (in SF 6 bath gas) and the temperature range 293–415 K, by means of laser flash photolysis to generate and monitor germylene, GeH 2. The reaction showed the characteristic pressure dependence of a third-body assisted association reaction. The high pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k ∞ / cm 3 molecule −1 s −1 )=( −10.86±0.19)+(7.16 ±1.23 kJ mol −1 )/RT ln 10 The parameters correspond to a fast reaction occurring at ca. 59% of the collision rate at room temperature. Collision efficiencies for GeH 2 π-additions are not much less than those of SiH 2. RRKM (Rice, Ramsperger, Kassel, Marcus) modelling, based on a variational transition state consistent with the kinetics, gave a good fit to the data, with the value of E o=105 kJ mol −1 corresponding to Δ H°=122±12 kJ mol −1 for 2-methylgermirane decomposition. This result in conjunction with other work shows that C-methyl substitution lowers the thermal stability of germirane, and that germiranes are ca. 50–60 kJ mol −1 less stable to decomposition than their silirane counterparts.

Strange and unconventional isotope effects in ozone formation.

The puzzling mass-independent isotopic enrichment in ozone formation contrasts markedly with the more recently observed large unconventional mass-dependent ratios of the individual ozone formation rate constants in certain systems. An RRKM (Rice, Ramsperger, Kassel, Marcus)-based theory is used to treat both effects. Restrictions of symmetry on how energy is shared among the rotational/vibrational states of the ozone isotopomer, together with an analysis of the competition between the transition states of its two exit channels, permit the calculation of isotope effects consistent with a wide array of experimental results.

Vibrational mode and collision energy effects on proton transfer in phenol cation–methylamine collisions

Mass-analyzed threshold ionization has been used to prepare vibrationally state-selected phenol cations, that were then reacted with methylamine at collision energies ranging from 0.1 to 2 eV. Integral cross sections and product recoil velocity distributions are reported. Ab initio calculations of stationary points on the surface and RRKM (Rice–Ramsperger–Kassel–Marcus) analysis of complex lifetimes are also presented for comparison. The only reaction observed over the entire energy range is exoergic proton transfer (PT). For ground-state reactants, the PT cross section is reduced by increasing collision energy, such that the reaction efficiency declines from ∼71% at low Ecollision to ∼50% at 2 eV. Excitation of either v6a or v12 vibrations inhibits reaction over the entire collision energy range, with the effect being somewhat mode-specific and increasing with increasing Ecollision. At low Ecollision, both vibrational and collision energy inhibit reaction with similar efficiency. Collision energy effects diminish at high Ecollision, while vibration continues to have a strong effect. Product ion velocity distributions are approximately forward–backward symmetric at Ecollision⩽1 eV, but are backward peaked at high energies. Mechanistic implications of these results are discussed.

Ultrafast photodissociation dynamics of the S1 and S2 states of acetone

The photodissociation dynamics for the two lowest excited electronic states (S1 valence state and the S2, 3s Rydberg state) of acetone (h6 and d6) have been studied using femtosecond mass-resolved photoionization spectroscopy. The S1 state dynamics was investigated by near ultraviolet (UV) pump (∼265 nm) and deep UV (205 nm) or visible (410 nm) probe. The primary dissociation time is instrument-limited, providing a 200 fs upper limit to the lifetime. The acetyl ion signal exhibits a subpicosecond decay and a persistent signal. The fast decay is consistent with results from Kim et al. [J. Chem. Phys. 103, 477 (1995)] for two-photon excitation to near the 4s state. The persistent signal is due to probe-induced ionization of acetyl radicals that are stable with respect to secondary dissociation. The S2 excited state lifetime measured for acetone-d6 using 194 pump and 259 nm probe is 13.5±1.0 ps. This is almost three times longer than we previously determined for this state in acetone-h6, 4.7±0.2 ps. The secondary dissociation time for acetyl-d3 measured with two-photon ionization probe at 388 nm is 3.0±1.0 ps. This is the same (within the experimental uncertainty) as our result for acetyl-h3 (3.1±0.5 ps), so that there is no apparent isotope effect. The calculated RRKM (Rice–Ramsperger–Kassel–Marcus) rate, however, is significantly faster for acetyl-h3 at the same internal energy, so that the isotopic dependence of the rate deviates from the RRKM predictions. Consequently, there is either an isotope dependence in the energy partitioning for primary dissociation or a reverse isotope effect in the secondary dissociation, or both. In the latter, more likely case, this indicates that the secondary dissociation does not conform to a statistical, RRKM-type unimolecular dissociation.

Toward the understanding of ethylene photodissociation: Theoretical study of energy partition in products and rate constants

The energy partition in the products of ethylene photodissociation (including C2H4, C2D4, D2CCH2, cis- and trans-HDCCDH) at 193 and 157 nm and the rate constants of H loss channels were computed based on ab initio ethylene ground-state surfaces of which most were reported earlier. In the calculations of the energy partitions, a simple model was used in which the excess energy above the transition state is distributed statistically and the energy released by the exit barrier is described by the modified impulsive model. The rate constants of the ethylene H(D) elimination were calculated according to the variational RRKM (Rice–Ramsperger–Kassel–Marcus) theory, and the RRKM rate constants with tunneling corrections were obtained for vinyl decomposition at 193 nm. In contrast with previous conclusions drawn by LIF (laser induced fluorescence) studies, the rate constant calculations suggest that the H loss may be a nonstatistical process. However, the computed variational transition states for H loss appear reasonable as indicated by the translational energy. That with present investigation indicates that the atomic elimination proceeds via the predicted transition states though the process is nonstatistical. Analysis of the H2 translational energy measured at 193 and 157 nm by molecular beam experiments gives evidence that the overall mechanisms of the molecular elimination are different at the two wavelengths, which is also in disagreement with previous belief. At 193 nm, both H2 elimination channels may occur through the predicted transition states. On the other hand, further comparison of the theoretical and experimental translational energy of hydrogen molecule at 157 nm suggests that the observed (1,1E) reaction path is most likely of much higher “exit barrier” than the one computed. For the (1,2E) channel, the calculations are still in support of the computed transition state being the one along the experimentally observed pathway at 157 nm.

Ultrafast studies of the photodissociation of the acetone 3s Rydberg state at 195 nm: Formation and unimolecular dissociation of the acetyl radical

Ultrafast deep UV mass-resolved photoionization spectroscopy has been used to investigate the photodissociation dynamics of the 3s Rydberg state of acetone. Single photon excitation at 193–195 nm is followed by single photon (at 260 nm) and two photon (at 390 nm) ionization and the signal is measured for both the acetone and acetyl photoions. The acetone Rydberg state lifetime determined from both single and two photon detection is surprisingly long, 4.7±0.2 ps. The higher probe energy for two photon ionization results in a lower minimum acetyl internal energy for ionization, so that part of the measured signal is due to neutral acetyl dissociation dynamics rather than only dissociative ionization of excited state acetone (which is the case for single photon ionization at 260 nm). The secondary dissociation rate of the neutral acetyl intermediate is measured, clearly establishing that photodissociation via the first Rydberg state of acetone occurs by a sequential dissociation mechanism. The acetyl dissociation occurs with a characteristic time of 3.1±0.5 ps. Based on RRKM (Rice–Ramsperger–Kassel–Marcus) calculations, this suggests an average acetyl internal energy of ∼25 kcal/mole. The long lifetime of the 3s Rydberg state suggests that the dissociation dynamics may be described in terms of a fully statistical dissociation mechanism.

Sensitivity of unimolecular lifetime distributions and energy dependent rate constants to fluctuations in state specific rate constants

Different distribution functions are used to describe fluctuations in state-specific unimolecular rate constants, for states which lie within a narrow energy interval. The effect of these distributions on the reactant population vs time N(τ,E) and the time-dependent unimolecular rate constant k(τ,E) are considered. Comparisons are made with the exponential decay predicts by RRKM (Rice–Ramsperger–Kassel–Marcus) theory of unimolecular kinetics. The collision-averaged unimolecular rate constant k(ω,E) is also determined for each of the distributions of state-specific unimolecular rate constants. Overall, k(ω,E) is found to be relatively insensitive to fluctuations in state-specific unimolecular rate constants.

Kinetics of the reaction of silylene with hydrogen and a possible resolution of discrepancies over ΔHf⊖(SiH2)

RRKM (Rice–Ramsperger–Kassel–Marcus) calculations, carried out to fit some new experimental data, are found to reconcile apparent differences in two recent studies of the fast reaction between SiH2 and H2: the calculations lead to ΔHf⊖(SiH2)= 65.3 ± 1.5 kcal mol–1.

Some Comparisons of the Classical RRK and the RRKM Theoretical Rate Formulations

It is shown by comparison with RRKM (Rice—Ramsperger—Kassel—Marcus) calculations for unimolecular reactions that the classical Kassel empirical fit parameter s is a complex function of temperature, the critical energy ε0, and of the vibrational pattern of the molecule and activated complex considered. The comparisons have afforded a delineation of quantal molecular behavior and of the empirical classical interpretation. Some detailed application is made to the methane and fluoroform decomposition reactions. Possible discrepancies in shock-tube measurements are discussed.