Derived Rate Constants Research Articles

Involvement of proton transfer in the reductive repair of DNA guanyl radicals by aniline derivatives

The most easily oxidized sites in DNA are the guanine bases, and major intermediates produced by the direct effect of ionizing radiation (ionization of the DNA itself) are electron deficient guanine species. By means of a radiation chemical method (gamma-irradiation of aqueous thiocyanate), we are able to produce these guanyl radicals in dilute aqueous solutions of plasmid DNA where the direct effect would otherwise be negligible. Stable modified guanine products are formed from these radicals. They can be detected in the plasmid conversion to strand breaks after a post-irradiation incubation with a DNA base excision endonuclease enzyme. If aniline compounds are also present, the yield of modified guanines is strongly attenuated. The mechanism responsible for this effect is electron donation from the aniline compound to the guanyl radical, and it is possible to derive rate constants for this reaction. Aniline compounds bearing electron withdrawing groups (e.g., 4-CF3) were found to be less reactive than those bearing electron donating groups (e.g., 4-CH3). At physiological pH values, the reduction of a guanyl radical involves the transfer of a proton as well as of an electron. The mild dependence of the rate constant on the driving force suggests that the electron is not transferred before the proton. Although the source of the proton is unclear, our observations emphasize the importance of an accompanying proton transfer in the reductive repair of oxidative damage to guanine bases which are located in a biologically active double stranded plasmid DNA substrate.

A note on chemiluminescence in low-pressure hydrogen and methane–nitrous oxide flames

Absolute OH(A) and CH(A) concentrations were determined in low-pressure H 2–air and CH 4–N 2O flames, respectively, by measuring absolute chemiluminescence yields at 310 and 430 nm. From spatial profiles and intensities in these and other flames, we deduce that two reactions are responsible in each case, and derive rate constants for all.

Electron-Transfer Kinetics of Nitroxide Radicals as an Electrode-Active Material

Abstract The electron-transfer rate constants of nitroxide derivatives, 4-(N-t-butyl-N-oxylamino)-t-butylbenzene, 4-(N-t-butyl-N-oxylamino)methoxybenzene, and 2,2,6,6-tetramethylpiperidinyl-N-oxyl, were investigated by electrochemical methods, which demonstrated that these radicals can potentially be used as a high power-rate electrode-active material due to their fast electron-transfer process.

Metal ion promoted transesterifications of carboxylate esters. A structure/activity study of the efficacy of Zn2+ and La3+ to catalyze the methanolysis of some aryl and aliphatic esters

The methanolysis of various aryl and aliphatic carboxylate esters promoted by methoxide, 1,5,9-triazacyclododecane : Zn2+(-OCH3) and La3+(-OCH3), were studied and the derived rate constants (kOCH3, kcat3:Zn(OCH3) and kcatLa(OCH3)) correlated in various ways. The metal ion catalyzed reactions are very much faster than the background reactions in some cases reaching up to 7 x 10(6)-fold acceleration when present at concentrations of 5 mmol dm(-3). The data for both metals exhibit non-linear Bronsted correlations with the pKa of the leaving group which are analyzed in terms of a change in rate limiting step from formation to breakdown of a metal-coordinated tetrahedral intermediate as the pKa increases above values of approximately 14.7. Plots of the log kOCH3 reaction vs. the log kcat values for each metal ion indicate low sensitivity for aryl esters and a higher sensitivity for the aliphatic esters. A mechanistic rationale for the observations is presented.

Collisional Stabilization and Thermal Dissociation of Highly Vibrationally Excited C9H12+ Ions from the Reaction O2+ + C9H12 → O2 + C9H12+

Highly vibrationally excited n-propylbenzene cations, C9H12+*, were prepared by the charge transfer reaction O2+ + C9H12 → O2 + C9H12+* in a turbulent ion flow tube. The subsequent competition between fragmentation of C9H12+* into C7H7+ + C2H5 and stabilization in collisions with N2 was studied at temperatures in the range 423−603 K and at pressures between 15 and 200 Torr. Most of the C7H7+ is the aromatic benzylium isomer, while the fraction of the minor species, seven-membered-ring tropylium, increases with T, from 5 to 20%. Minor fragments are C6H6+, C7H8+, and C8H9+. Energy-transfer step sizes 〈ΔE〉 for collisional deactivation are obtained by combining the stabilization versus fragmentation ratios measured as a function of pressure in this study with fragmentation rates from the literature. The values are compared with related information for other excited molecular ions and are similar to those of their neutral analogues. At the highest temperatures, C9H12+ was also observed to pyrolyze after collisional stabilization. Employing unimolecular rate theory, the derived rate constants for thermal dissociation of C9H12+ are related to values derived from the specific rate constants k(E,J) for fragmentation. Good agreement is found between measured and predicted pyrolysis rate constants. This allows us to confirm the dissociation energy of C9H12+ into C7H7+ (benzylium) and C2H5 as 166.9 (±2.2) kJ mol-1 (at 0 K).

A Shock-Tube, Laser-Schlieren Study of the Dissociation of 1,1,1-Trifluoroethane: An Intrinsic Non-RRKM Process

We report a shock-tube, laser-schlieren investigation of the molecular dissociation of the title trifluoroethane, CF 3 CH 3 → CH 2 CF 2 + HF, over very high temperatures, 1600-2400 K, and a wide range of sub-atmospheric pressures, 15-550 Torr. The density gradients are well fit by a simple two-reaction mechanism and accurate dissociation rates obtained. The results are compared with a k ∞ calculated from a G3 TS for this molecular elimination, which is a superb fit to the available lower-T data and a reliable extrapolation of k ∞ to high temperatures. The derived rate constants show a very deep falloff from this extrapolation but surprisingly little variation with pressure. This peculiarity is so severe that RRKM calculations dramatically fail to account for the behavior. The dissociation seems to be a clear example of an intrinsic non-RRKM process (nonstatistical dissociation). This conclusion is strongly supported by the observation of double vibrational relaxation at both dissociating and nondissociating temperatures, an unambiguous demonstration of slow IVR. Using a simple model with division into two groups of states, the deep falloff is found to be consistent with a rate-controlling slow IVR, not with low collision efficiency. The model suggests an IVR rate of ∼10 8 s - 1 for dissociation energies.

Unbiased spectrophotometric method for estimating phenol or o-cresol in unknown water samples.

The generalised H-point standard addition method (GHPSAM) was applied to quantify phenol and o-cresol in waters and compared with other analytical approaches. The method is based on spectroscopic and kinetic measurement of the formation of derivatives with p-aminophenol and KIO(4) in presence of NaOH. First, pseudo-first order kinetic behaviour of the reaction products was demonstrated. The unbiased formation rate constants of phenol and o-cresol derivatives were calculated. The analytical figures of merit were determined using the GHPSAM as calibration method. Detection limits achieved were 0.2 microg L(-1) of phenol and 0.2 microg L(-1) of o-cresol using a preconcentration factor of 10. Styrene-divinyl benzene cartridges were used in the preconcentration step. Repeatability values were 3.7% for phenol and 2.0% for o-cresol; reproducibility values were 6.9% and 3.5%, respectively. Accurate and precise results were obtained when the method was applied to real samples of natural water.

Modeling the kinetics of step homopolymerization reactions––an assessment of different approaches

A critical analysis of two models used to describe the kinetics of step homopolymerization has been undertaken. The classical second-order kinetic model and the more widely adopted Mathew et al. oligomer precipitation model were tested against data published in the literature. The classical model, which predicts an identical molecular weight distribution to that obtained by Flory using a statistical approach, is based on a single rate constant and provided an excellent fit ( R 2>0.99) to the experimental data. Derived rate constants exhibited logical trends. The Mathew et al. oligomer precipitation model, on the other hand, requires three fitting parameters. This model also fitted the data well but in many cases yielded either negative rate constants for the purported termination step or illogical parametric trends. It was concluded that the classical model was superior to the precipitation model in describing the kinetics and reaction mechanisms of step homopolymerization.

Characterization of the nucleation step and folding of a collagen triple-helix peptide.

Peptide T1-892 is a triple-helical peptide designed to include two distinct domains: a C-terminal (Gly-Pro-Hyp)(4) sequence, together with an N-terminal 18-residue sequence from the alpha1(I) chain of type I collagen. Folding experiments of T1-892 using CD spectroscopy were carried out at varying concentrations and temperatures, and fitting of kinetic models to the data was used to obtain information about the folding mechanism and to derive rate constants. Proposed models include a heterogeneous population of monomers with respect to cis-trans isomerization and a third-order folding reaction from competent monomer to the triple helix. Fitting results support a nucleation domain composed of all or most of the (Gly-Pro-Hyp)(4) sequence, which must be in trans form before the monomer is competent to initiate triple-helix formation. The folding of competent monomer to a triple helix is best described by an all-or-none third-order reaction. The temperature dependence of the third-order rate constant indicates a negative activation energy and provides information about the thermodynamics of the trimerization step. These CD studies complement NMR studies carried out on the same peptide at high concentrations, illustrating how the rate-limiting folding step is affected by changes in concentration. This sequence preference of repeating Gly-Pro-Hyp units for the initiation of triple-helix formation in peptide T1-892 may be related to features in the triple-helix folding of collagens.

In vivo metabolism and partitioning of 6-[18F]fluoro-L-meta-tyrosine in whole blood: a unified compartment model

Physiological quantification of dynamic PET data requires the determination of an input function, preferably from plasma. A compartmental model relating a parent radiotracer, its radiolabelled metabolites and their exchange between plasma and erythrocytes is presented. This model allows for the time course of radioactivity measured in whole blood to be transformed into the time course of the radiotracer in plasma. The utility of this approach is illustrated with blood data collected on 30 human subjects injected with 6-[18F]fluoro-L-meta-tyrosine (FmT), a pre-synaptic dopaminergic radiotracer. A three-compartment four-parameter model is shown to yield significantly better fits to the blood data than related lower and higher order models. This model is found to be robust to measurement noise, and yet sensitive to metabolic changes induced by pretreatment with carbidopa. For FmT, the between-subject variations are shown to be small enough to warrant the use of a population-based correction;; tissue time–activity curves were simulated to verify that this correction does not significantly affect the precision and accuracy of the derived rate constants. The unified blood model can be adapted for radiotracers other than FmT as long as the blood partition ratio of the parent radiotracer differs from that of its metabolites and/or the rate at which they equilibrate between plasma and erythrocytes is different.

Reactions of Silylene with Unreactive Molecules. I: Carbon Dioxide; Gas-Phase Kinetic and Theoretical Studies

Time-resolved studies of the reaction of silylene, SiH2, with CO2 have been carried out over the temperature range 298−681 K, using laser flash photolysis to generate and monitor SiH2. Between 339 and 681 K the reaction obeys second-order kinetics and the derived rate constants gave the following Arrhenius parameters: log(A/cm3 molecule-1 s-1) = −11.89 ± 0.13, Ea = +16.36 ± 1.23 kJ mol-1, where the uncertainties are single standard deviations. The reaction shows no overall pressure dependence. This reaction is unusual for SiH2 in being extremely slow and having a positive activation energy. Ab-initio calculations at the G2 level suggest a mechanism occurring via the intermediacy of siloxiranone (formed via addition of SiH2 to one of the CO double bonds) leading to overall formation of H2SiO + CO, consistent with kinetic findings. Direct abstraction of an O atom appears to be ruled out, as are other potential pathways. The mechanism resembles that of CH2(1A1) + CO2.

General base catalysis vs. medium effects in the hydrolysis of an RNA modelElectronic supplementary information (ESI) available: tables of primary kinetic data and derived rate constants. See http://www.rsc.org/suppdata/p2/b1/b109325h/

The interpretation of buffer catalysis data is always subject to a degree of uncertainty, because changing the concentration of the catalyst necessarily changes the medium. The history of the imidazole-catalysed reactions of ribonucleotide derivatives is a classic illustration of the potential problems, which are complicated further by significant sensitivity to ionic strength and specific salt effects. Results are reported for the hydrolysis of 4-nitrophenoxymethyl uridine 3′-phosphate, which releases the chromophoric p-nitrophenolate on cleavage of the P–OCH2OAr bond. In the case of catalysis by imidazole, a bell-shaped dependence on the buffer ratio is explained in terms of a solvent effect on the background, hydroxide-catalysed reaction. Results for six other amine bases with pKa = 7 ± 1 show that, in this system, non-linear behaviour is the norm rather than the exception.

Inhibition and sensitization of fuel oxidation by SO 2

An experimental and theoretical study of the interaction of SO 2 with the radical pool under combustion conditions has been carried out. Experiments on moist CO oxidation were conducted in an isothermal quartz flow reactor at 1 atm; temperature ranged from 800 to 1,500 K and stoichiometries from fuel-rich to very lean. In addition, literature data on sulfur species concentration profiles and H atom decay in fuel-rich H 2/O 2 flames doped with SO 2 were analyzed. The results show that under flow-reactor conditions SO 2 may inhibit or promote oxidation of fuel, depending on conditions. In a narrow range of operating conditions close to stoichiometric SO 2 promotes oxidation through the sequence: SO 2 + H ⇌ SO + OH, SO + O 2 ⇌ SO 2 + O. Inhibition of oxidation by removal of radicals can be explained in terms of the SO 2+O+M reaction, even under fuel-rich conditions. From the shift in temperature for the onset of CO oxidation because of SO 2 addition under reducing conditions an upper limit of 3.0 × 10 14 cm 6 mol −2s −1 at 1,060 K can be estimated for the rate constant of H + SO 2 + N 2 ⇌ HOSO + N 2. This value is consistent with a significant barrier to reaction as proposed theoretically, but an order of magnitude lower than indicated by both ab initio calculations (Marshall and co-workers) and reaction rates derived from flames. However, we find that data on H atom decay in flames doped with SO 2 are not suitable for deriving rate constants because of uncertainty in important side reactions involving SO. Furthermore, we propose that the enhanced H atom decay observed in these flames may be attributed to recombination of H atoms with SO and S 2 species, rather than to a mechanism initiated by the H + SO 2 + M reaction.

Fate of the herbicides 2,4,5-T, atrazine, and DNOC in a shallow, anaerobic aquifer investigated by in situ passive diffusive emitters and laboratory batch experiments.

The fate of the three herbicides 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), atrazine (6-chloro-N-ethyl-N'-[1-methyl-ethyl]-1,3,5-triazine-2,4-diamine), and DNOC (4,6-dinitro-2-methylphenol) in an anaerobic sandy aquifer was investigated. In the field, each of the herbicides was released simultaneously with tritiated water (HTO) as tracer in the depth interval 3 to 4 mbs (meters below surface) by use of passive diffusive emitters. Atrazine and 2,4,5-T were persistent during the approximately 18 days residence time in the aquifer. In contrast, DNOC was rapidly removed from the water phase following first-order kinetics. The removal mechanism was likely an abiotic reduction. At day 25, the first-order rate constant was 1.47 d(-1), but it decreased with time and seemed to stabilize at 0.35 d(-1) after 150 to 200 days. In the laboratory, batch experiments were conducted with sediments from 3 to 4 mbs and from 8 to 9 mbs. In these incubations, formation of Fe2+ and depletion of sulfate showed iron and sulfate reduction in sediment from 3 to 3.5 mbs and sulfate reduction in 3.5 to 4 mbs sediment. In sediment from 8 to 9 mbs, the dominant redox process was methane formation. In sediment from 3 to 3.5 mbs, only 27% to 52% of the 2,4,5-T remained after 196 days. 2,4,5-trichlorophenol was identified as the major metabolite. A lag period of at least 50 days was observed, and no degradation occurred in HgCl2 amended controls, verifying that the process was microbially mediated. In the other 2,4,5-T incubations and all the atrazine incubations, concentrations decreased linearly, but less than 25% was removed within 200 to 250 days. No degradation products could be detected, and slow sorption was the likely explanation. In all the laboratory incubations DNOC was degraded, following first-order kinetics, and when normalized to the sediment/water-ratio, the field and laboratory derived rate constants compared well. The DNOC degradation in the methanogenic incubations (8 to 9 mbs) was up to 50 times faster than in the sediments from 3 to 4 mbs, likely due to the low redox potential.

Study of the Recombination Reaction CCl3 + O2 (+M) → CCl3O2 (+M) at Pressures of 2−900 bar and Temperatures of 260−346 K

The recombination reaction CCl3 + O2 (+M) → CCl3O2 (+M) was studied in the upper part of the falloff curve at 260, 300, and 346 K over the pressure range of 2−900 bar. CCl3 radicals were generated by photolysis of CCl3Br at 248 nm; the temporal decay of the absorption from CCl3 at 223.5 nm was monitored in the presence of O2; falloff curves of the rate constants were determined in N2 and He as bath gases. The falloff curves could well be represented by limiting high-pressure rate constants k∞ = (5.2 ± 0.2) × 10-12 (T/300 K)-1.4±0.4 cm3 molecule-1 s-1 from the present work, limiting low-pressure rate constants k0 = [N2] (1.1 ± 0.3) × 10-30 (T/300 K)-6.3±1.0 cm3 molecule-1 s-1 or k0 = [He] (4.2 ± 0.7) × 10-31 (T/300 K)-6.9±1.0 cm3 molecule-1 s-1 from the literature, and center-broadening factors Fc (N2) = (0.35 ± 0.03) (T/300 K)-0.35 or Fc (He) = (0.30 ± 0.03) (T/300 K)-0.48 derived from unimolecular rate theory. An onset of diffusion-controlled kinetics at pressures above about 300 bar of N2 was observed, in agreement with predictions from simple diffusion models. The derived rate constants are analyzed in the framework of unimolecular rate theory.

Degradation of soil fumigants as affected by initial concentration and temperature.

Soil fumigation using shank injection creates high fumigant concentration gradients in soil from the injection point to the soil surface. A temperature gradient also exists along the soil profile. We studied the degradation of methyl isothiocyanate (MITC) and 1,3-dichloropropene (1,3-D) in an Arlington sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralf) at four temperatures and four initial concentrations. We then tested the applicability of first-order, half-order, and second-order kinetics, and the Michaelis-Menten model for describing fumigant degradation as affected by temperature and initial concentration. Overall, none of the models adequately described the degradation of MITC and 1,3-D isomers over the range of the initial concentrations. First-order and half-order kinetics adequately described the degradation of MITC and 1,3-D isomers at each initial concentration, with the correlation coefficients greater than 0.78 (r2> 0.78). However, the derived rate constant was dependent on the initial concentration. The first-order rate constants varied between 6 and 10x for MITC for the concentration range of 3 to 140 mg kg(-1), and between 1.5 and 4x for 1,3-D isomers for the concentration range of 0.6 to 60 mg kg(-1), depending on temperature. For the same initial concentration range, the variation in the half-order rate constants was between 1.4 and 1.7x for MITC and between 3.1 and 6.1x for 1,3-D isomers, depending on temperature. Second-order kinetics and the Michaelis-Menten model did not satisfactorily describe the degradation at all initial concentrations. The degradation of MITC and 1,3-D was primarily biodegradation, which was affected by temperature between 20 and 40 degrees C, following the Arrhenius equation (r2 > 0.74).

Formation and reactions of S-nitroso proteins

Nitrosation of Bovine Serum Albumin (BSA) by nitrous acid occurs in two stages: a rapid step, which is consistent with an S-nitrosation reaction of a free cysteine unit, followed by a slower reaction which is consistent with an N-nitrosation reaction of a tryptophan residue. The fast reaction is catalysed by chloride and bromide ions (as is the reaction of free cysteine), whereas the slower reaction is not halide ion catalysed (like the reaction of tryptophan). Kinetic results were obtained for both reactions. The derived rate constants for the first stage for the reaction of ClNO and BrNO are reasonably close to the reported values for the reactions of cysteine. The second stage is a reversible process and we can estimate from measured infinity values and also from the variation in the measured rate constant for reactions at different [HNO2], values for the equilibrium constant of 3500 ± 200 and 2600 ± 200 dm3 mol−1 which compare reasonably with the reported value for the reaction of tryptophan of 850 dm3 mol−1. The pKa values of the cysteine residue in both BSA and Human Serum Albumin (HSA) were determined from rate measurements of the reactions with dipyridin-4-yl disulfide (4-aldrithiol), and yielded values of 8.32 and 8.18 respectively, which are close to the accepted value of 8.4 for cysteine itself, and which are substantially higher than the much quoted literature values. The S-nitroso derivatives of both BSA and HSA generated in solution (at ∼1 × 10−4 mol dm−3) showed very little sign of decomposition at pH 7.4 when measured spectrophotometrically, even in the presence of added Cu2+. There was, however, clear evidence of rapid decomposition at much lower reactant concentration (∼1 × 10−6 mol dm−3) yielding nitric oxide in increasing amounts and rates, as Cu2+ is added. These results are discussed in terms of the complexation of the Cu2+ catalyst at different albumin derivative concentrations. Transnitrosation between the S-nitroso derivatives of both BSA and HSA and excess cysteine occurred very readily. These experiments were carried out in the presence of added Cu2+, and the decomposition of the resulting S-nitrosocysteine (S-NOCys) was monitored. The results support the suggestion that S-nitroso proteins can, in theory, act as a reservoir for NO which can readily be made available, either by the Cu2+-catalysed reaction, or more readily by a direct NO+ transfer to a low molecular weight thiol, such as cysteine, which is much more prone to yield NO rapidly and quantitatively by the Cu2+-catalysed pathway. The S-nitroso derivative of BSA also underwent a relatively slow decomposition reaction initiated by cysteine, even in the presence of EDTA. This is discussed in terms of a reaction yielding nitrite anion which is not derived from NO.

Modeling Arsenic(III) Adsorption and Heterogeneous Oxidation Kinetics in Soils

Arsenite [As(III)] is a soluble and toxic species of arsenic that can be introduced into soil by geothermal waters, mining activities, irrigation practices, and disposal of industrial wastes. We determined the rates of As(III) adsorption, and subsequent oxidation to arsenate [As(V)] in aerobic soil–water suspensions using four California soils. The rate of As(III) adsorption on the soils was closely dependent on soil properties that reflect the reactivity of mineral surfaces including citrate–dithionite (CD) extractable metals, soil texture, specific surface area, and pH. Heterogeneous oxidation of As(III) to As(V) was observed in all soils studied. The recovery of As(V) from As(III)‐treated soils was dependent on levels of oxalate‐extractable Mn and soil texture. After derivation of rate equations to describe the changes in soluble and recoverable As(III) and As(V) in soil suspensions, soil property measurements were used to normalize the empirically derived rate constants for three soils. The fourth soil, which had substantially different soil properties from the other three soils, was used to independently test the derived soil property–normalized model. The soil property–normalized consecutive reaction model gave a satisfactory description of the trends seen in the experimental data for all four soils. Understanding the effects of soil properties on the kinetics of chemical reactions of As(III) and As(V) in soils will be essential to development of quantitative models for predicting the mobility of As in the field.

Rate constants for the reactions of Ar + with CO 2 and SO 2 as a function of temperature (300–1500 K)

We have measured the rate constants for the reactions of Ar + with CO 2 and SO 2 from 300 to 1500 K in a high temperature flowing afterglow. For the reaction with CO 2, we have found that all modes of energy, i.e., translation, rotation, and vibration, affect the rate constant to the same degree up to a total energy of 0.4 eV. Above 0.4 eV total energy, internal energy decreases the rate constants more effectively than does translational energy. For the reaction of Ar + with SO 2, the rate constants go through a minimum at about 900 K. By comparing our results to drift tube data, we derive rate constants for reaction from the v = 0 and v > 0 vibrational levels. At low energy, the vibrationally excited SO 2 molecules react with Ar + approximately twice as fast as the ground state molecules. Both vibrational modes have similar temperature dependences.

Rotational state-to-state rate constants and pressure broadening coefficients for He–C2H2 collisions: Theory and experiment

Converged close-coupling and coupled-states calculations were used to obtain state-to-state rate constants and pressure broadening coefficients for the collisional rotational (de-)excitation of C2H2 by He. The ab initio potential used in these calculations was previously computed by symmetry-adapted perturbation theory. The computed pressure broadening coefficients and total rate constants agree well with the available experimental data. In the experimental part of the paper stimulated Raman-pumping has been used to prepare acetylene in selected rotational states (ji=2 to 18 and ji=1 to 19 of the C≡C stretching mode). The population decay in the prepared state and the transfer to other rotational states was monitored by laser induced fluorescence. The experimental data can be described by an infinite-order-sudden power law (IOS-P) or directly compared with the ab initio derived rate constants. The influence of multiple collisions possible at the relatively large pressure-delay-products employed has been taken into account by simulating the rotational energy transfer with a master equation. Experimentally we obtain a total rate constant for depopulation of ktot=10.89±0.07μs−1 Torr−1 and IOS-P fitting parameters of A=5.58 and γ=0.96 in very good agreement with the ab initio calculated values. There we obtain ktot=10.69±0.09μs−1 Torr−1 and IOS-P fitting parameters of A=6.18 and γ=0.96. Also the experimental state-to-state rotational energy transfer constants, which vary from 5.83 to 2.32 μs−1 Torr−1 for endothermic Δj=2 collisions, agree very well with the ab initio data.