Reliable Rate Constants Research Articles

Automatization of theoretical kinetic data generation for tabulated TS models building - Part 1: Application to 1,3-H-shift reactions

Estimation of kinetic parameters is a key aspect of chemical combustion modeling and several approaches were developed to approximate unknown data. In this work, a code in Python was developed to build tables of transition state (TS) models automatically for intramolecular H-shift reactions in alkyl radicals. The code generates the kinetic rules for all the possible combinations of methyl-substituted reactions based on the structures of the minimal, non-substituted, reactant, TS, and product. The code is able to create and differentiate multiple transition state configurations, considering the axial and equatorial positions for the cyclic substituents and including all the possible pathways for the reactions, which is shown to be an important feature in performing accurate automatic kinetic calculations. Each structure is automatically submitted to geometry optimization and electronic energy calculations, as well as the relaxed scans of the torsional modes identified by the code. From the results of electronic calculations, the rate constants for each pathway are obtained automatically by the application of the transition state theory with tunneling corrections, in a defined temperature range. The kinetic coefficients, as well as the modified Arrhenius parameters, are then assembled and organized to create a final table that connects the kinetic data with TS structure characteristics. These tables can be directly applied as a kinetic data source for reaction mechanism development. The ability of the code to generate reliable rate constants was tested for 1,3-H-shift reactions and the results were compared with theoretical data manually produced, and showed a good agreement. In particular, the code was able to create all the transition state configurations, with an exhaustive description of all possible reaction pathways, using a rigorous and systematic counting based on symmetry, stereocenters, and diastereomers. The proposed method leads to more accurate results on these aspects, compared to repetitive hand calculations of dozens of rate constants.

Quantum dynamics of the P(2D) + H2 (X1[formula omitted]) → PH (X3Σ-) + H(2S) reaction

The title reaction is thought to be important for the astronomical PO formation, which may contribute to the generation of bioavailable H3PO3. Using quasi-classical trajectory (QCT) method, the state-specified (v = 0, j = 0) rate constant of this reaction was calculated. The comparison with previous temperature-averaged QCT rate constant shows that the effect of ro-vibrational excitation is relatively small below 2000 K. Then, the time-dependent wave packet (TDWP) method was applied to investigate the quantum reaction mechanisms, obtaining the state-specified (v = 0, j = 0) reaction probability, integral cross sections and rate constants. The number of partial waves and the integral interval of collision energy considered were rigorously tested to ensure the convergence of the TDWP rate constant below 2000 K. The TDWP rate constant agrees better to the latest experimental results at 291 to 685 K than those from the two QCT calculations. Also, the TDWP calculations can produce relatively reliable rate constants at 700 to 2000 K, where the Arrhenius formula fitted by experimental results ignores the temperature dependence of the activation energy. The calculated rate constants are helpful for modelling the phosphorus chemistry and determining the abundances of the P-bearing species in interstellar media (ISM).

Sensitivity-enhanced NMR 15N R1 and R1ρ relaxation experiments for the investigation of intrinsically disordered proteins at high magnetic fields

NMR relaxation experiments provide residue-specific insights into the structural dynamics of proteins. Here, we present an optimized set of sensitivity-enhanced 15N R1 and R1ρ relaxation experiments applicable to fully protonated proteins. The NMR pulse sequences are conceptually similar to the set of TROSY-based sequences and their HSQC counterpart (Lakomek et al., J. Biomol. NMR 2012). Instead of the TROSY read-out scheme, a sensitivity-enhanced HSQC read-out scheme is used, with improved and easier optimized water suppression. The presented pulse sequences are applied on the cytoplasmic domain of the SNARE protein Synpatobrevin-2 (Syb-2), which is intrinsically disordered in its monomeric pre-fusion state. A two-fold increase in the obtained signal-to-noise ratio is observed for this intrinsically disordered protein, therefore offering a four-fold reduction of measurement time compared to the TROSY-detected version. The inter-scan recovery delay can be shortened to two seconds. Pulse sequences were tested at 600 MHz and 1200 MHz 1H Larmor frequency, thus applicable over a wide magnetic field range. A comparison between protonated and deuterated protein samples reveals high agreement, indicating that reliable 15N R1 and R1ρ rate constants can be extracted for fully protonated and deuterated samples. The presented pulse sequences will benefit not only for IDPs but also for an entire range of low and medium-sized proteins.

Electron Transfer Rates in Solution: Toward a Predictive First Principle Approach

Using a very recently proposed theoretical model, electron transfer rates in solution are calculated from first principles for different donor-acceptor pairs in tetrahydrofuran. We show that this approach, which integrates tunneling effects into a classical treatment of solvent motion, is able to provide reliable rate constants and their temperature dependence, even in the case of highly exergonic reactions, where Marcus’ theory usually fails.

Theoretical kinetics of HO2 + C5H5: A missing piece in cyclopentadienyl radical oxidation reactions

The resonantly-stabilized cyclopentadienyl radical (C5H5) is a key species in the combustion and molecular growth kinetics of mono and poly-aromatic hydrocarbons (M/PAHs). At intermediate-to-low temperatures, the C5H5 reaction with the hydroperoxyl radical (HO2) strongly impacts the competition between oxidation to smaller products and growth to PAHs, precursors of soot. However, literature estimates for the HO2 + C5H5 reaction rate are inaccurate and inconsistent with recent theoretical calculations, thus generating discrepancies in global combustion kinetic models. In this work, we perform state-of-the-art theoretical calculations for the HO2 + C5H5 reaction including variable reaction coordinate transition state theory for barrierless channels, accurate thermochemistry, and multi-well master equation (ME) simulations. Contrary to previous studies, we predict that OH + 1,3-C5H5O is the main reaction channel. The new rate constants are introduced in two literature kinetic models exploiting our recently developed ME based lumping methodology and used to perform kinetic simulations of experimental data of MAHs oxidation. It is found that the resonantly-stabilized 1,3-C5H5O radical is the main C5H5O isomer, accumulating in relevant concentration in the system, and that the adopted lumping procedure is fully consistent with results obtained with detailed kinetics. The reactivity of C5H5O with OH and O2 radicals is included in the kinetic mechanisms based on analogy rules. As a result, C5H5O mostly reacts with O2 producing smaller C3/C4 species and large amounts of C5H4O, suggesting that further investigations of the reactivity of both C5H5O and C5H4O with oxygenated radicals is necessary. Overall, this work presents new reliable rate constants for the HO2 + C5H5 reaction and provides indications for future investigations of relevant reactions in the sub-mechanisms of cyclopentadiene and MAH oxidation.

Easy and Rapid Approach to Obtaining the Binding Affinity of Biomolecular Interactions Based on the Deep Learning Boost.

Recently, the deep learning (DL) dimension of artificial intelligence has received much attention from biochemical researchers and thus has gradually become the key approach adopted in the area of biosensing applications. Studies have shown that the use of DL techniques for sensing can not only shorten the time of data analysis but also significantly increase the accuracy of data analysis and prediction, resulting in the performance improvement of biosensing systems in comparison to conventional methods. However, obtaining reliable equilibrium and rate constants of biomolecular interactions during the detection process remains difficult and time-consuming to date. In this study, we propose a transformed model based on the deep transfer learning and sequence-to-sequence autoencoder that can successfully transfer the SPR sensorgram to the protein-binding constants, that is, the association rate constant (ka) and dissociation rate constant (kd), which provide crucial information to understand the mechanisms of drug action and the functional structures of biomolecules. Experimentally, we first trained and tested the pre-trained model using the Langmuir model which generated ideal SPR sensorgrams and then we fine-tuned the pre-trained model through the augmented SPR sensorgrams which were synthesized by using the synthesized minority oversampling technique (SMOTE) through the moderate-scale experiment. Next, the fine-tuned model was inputted with a short experimental SPR sensorgram that only needs 110 s, and the sensorgram was directly transformed into a reconstructed ideal sensorgram. Finally, the binding kinetic constants, that is, ka and kd, as outputs, were obtained through fitting the reconstructed ideal sensorgram. The results showed that the prediction errors of ka and kd obtained by our model were less than 12 and 24%, respectively. Based on the convenience, accuracy, and reliability of the proposed DL approach, we believe our strategy significantly boosts the feasibility to monitor the binding affinity of antibodies online during production.

Theoretical Study on Reactions of α-Site Hydroxyethyl and Hydroxypropyl Radicals with O2.

α-Site alcohol radicals are the most important products of H-abstract reactions from alcohols since the hydroxyl moiety weakens the α-site C-H bond. Reactions between α-site alcohol radicals and O2 play an important role in combustion of alcohols, especially at relatively low temperatures. However, reliable reaction pathways and rate constants for these reactions are still lacking. Theoretical studies on reactions in α-hydroxyethyl radical (CH3C•HOH) + O2 and α-hydroxypropyl radical (C2H5C•HOH and CH3C•OHCH3) + O2 reaction systems are performed in this work. Pressure-dependent rate constants for the involved reactions in a wide range of temperatures are determined using the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) method. Our results show that rate constants for reactions in the α-hydroxypropyl radical + O2 system are quite different from those in the CH3C•HOH + O2 system. Detailed reaction pathways for these reaction systems are clarified, although combustion characteristics of ethanol and propanol do not change much with the obtained rate constants for these reactions. Important reaction channels in producing enols, especially in the combustion of propanol, are also provided. The obtained rate constants for these reactions over a wide range of temperatures and pressures are helpful in developing combustion mechanisms for ethanol and propanol.

Thermochemical and kinetic studies of hydrogen abstraction reaction from C16H10 isomers by H atoms

Ab initio calculations were used to derive thermochemical and kinetic properties of the hydrogen abstraction reaction from C16H10 isomers (pyrene, fluoranthene, aceanthrylene, and acephenanthrylene) by H atoms. The study of these reactions is very important to better understand the soot formation processes. The structural properties of all studied species were optimized using the B97D3/6-311++G(d,p) level of theory. The reaction energies and barrier heights were evaluated using several density functional theory methods (B97D3, B3LYP, and M06-2X) and compared to highly-correlated ab initio wave functions within the UCCSD(T)-F12 approach. It was shown that the UCCSD(T)-F12 computational protocol can be used with such large systems to provide more accurate results compared to DFT calculations. Reliable rate constants were obtained using the Transition State Theory with Eckart's tunnelling correction over the temperature range of 250–2500 K. The influence of the structure and the position of the hydrogen abstraction CH sites of C16H10 isomers on the energetics and the rate constant were discussed and compared with available literature data.

Kinetics of hydrogen abstraction from CH3SH by OH radicals: An ab initio RRKM-based master equation study

This work presents the mechanism and chemical kinetics for the CH3SH + OH reaction in a broad condition range (200–2000 K & 0.76–760,000 Torr) using the CCSD(T)/aug-cc-pV(T,Q)Z//M06-2X/aug-cc-pV(T+d)Z and Rice–Ramsperger–Kassel–Marcus based master equation (RRKM-ME) calculations. The treatments of the hindered internal rotor (HIR) and tunneling effects were taken into account. The good agreement between our calculated rate constants and experimental values resolves the previous discrepancy between experiment and theory. The reaction mechanism is revealed in detail as: (i) the H-abstraction channel from the thiol (-SH) group is more thermodynamically and kinetically favorable than from the methyl (-CH3) group; (ii) U-shaped temperature-dependent behaviors and weakly positive pressure-dependence at low temperature (T ≤ 500 K) of the rate constants are observed; and (iii) both HIR and tunneling corrections play a role in obtaining reliable rate constants. Besides, the performance of selected chemical quantum methods (i.e., M06-2X, BH&HLYP, B3LYP, and MP2) on the calculated rate constants was examined thoroughly. Furthermore, it is predicted that CH3SH is not a persistent organic pollutant due to its short atmospheric lifetime (~5 h) due to OH radicals and its products (CH2SH and CH3S) can photolyze under the sunlight.

A DFT study on recombination of alkyl radicals to C2-C17 normal alkanes & branched C8 alkanes and corresponding C-C bond pyrolysis reaction

Alkane C–C dissociation is one of the key elementary reactions in combustion and pyrolysis. Corresponding rate constants are important in kinetic models. However, reliable rate constants for long and branched alkanes are still limited, and accurate rate constant calculation for large alkanes is time-consuming. In this work we have systematically investigated the high-pressure-limit rate constants of alkyl recombination to normal CnH2n+2 (n=2–17) and some branched alkanes with UM06-2X/cc-pvdz//VRC-TST method.Corresponding dissociation rate constants have been converted using calculated free energies at G4MP2 level. This combined method is found to reproduce dissociation rate constants well (error < 101) for small alkanes at 700–2000 K. Our results suggested that it’s a good approximation to use short alkane (C3 for example) dissociation rate constants for longer alkanes above 800 K. Dissociation reactions leading to CH3 radical are much harder than those leading to other alkyls.It's a good approximation to use rate constant of dissociation leading to C2H5 radical for other alkyl dissociations other than CH3. Both modified and a four-parameter Arrhenius equations give good fitting to calculated rate constants. Our results give further insight into C–C bond dissociation of long and branched alkanes, and provide alternative kinetic data for relevant combustion/pyrolysis kinetic models.

On-the-fly kinetics of hydrogen abstraction from polycyclic aromatic hydrocarbons by methyl/ethyl radicals.

This work provides a rigorous procedure, within the framework of the Reaction Class Transition State Theory (RC-TST) and the Structure-Activity Relationship (SAR), for predicting reliable thermal rate constants on-the-fly for hydrogen abstraction reactions by methyl/ethyl radicals from Polycyclic Aromatic Hydrocarbons (PAHs) in a temperature range of 300-3000 K. All necessary RC-TST parameters were derived from ab initio calculations for a representative set of 36 reactions on which different error analyses and comparisons with available literature data were carried out. In addition to the good agreement between the RC-TST rate constants and the literature data, the detailed error analyses show that RC-TST/SAR, utilizing either the Linear Energy Relationship (LER) where only the reaction energy is needed or Barrier Height Grouping (BHG) where no additional data is needed, can predict the thermal rate constants for any reaction in the title reaction class with an average systematic error of less than 50% when compared to the explicit rate calculations. Therefore, the constructed RC-TST procedure can be confidently used to obtain reliable rate constants on the fly in an attempt to effectively construct detailed kinetic mechanisms for PAH-related fuels.

Atmospheric oxidation of halogenated aromatics: comparative analysis of reaction mechanisms and reaction kinetics.

Atmospheric transport is the major route for global distribution of semi-volatile compounds such as halogenated aromatics as well as their major exposure route for humans. Their major atmospheric removal process is oxidation by hydroxyl radicals. There is very little information on the reaction mechanism or reaction-path dynamics of atmospheric degradation of halogenated benzenes. Furthermore, the measured reaction rate constants are missing for the range of environmentally relevant temperatures, i.e. 230-330 K. A series of recent theoretical studies have provided those valuable missing information for fluorobenzene, chlorobenzene, hexafluorobenzene and hexachlorobenzene. Their comparative analysis has provided additional and more general insight into the mechanism of those important tropospheric degradation processes as well as into the mobility, transport and atmospheric fate of halogenated aromatic systems. It was demonstrated for the first time that the addition of hydroxyl radicals to monohalogenated as well as to perhalogenated benzenes proceeds indirectly, via a prereaction complex and its formation and dynamics have been characterized including the respective transition-state. However, in fluorobenzene and chlorobenzene reactions hydroxyl radical hydrogen is pointing approximately to the center of the aromatic ring while in the case of hexafluorobenzene and hexachlorobenzene, unexpectedly, the oxygen is directed towards the center of the aromatic ring. The reliable rate constants are now available for all environmentally relevant temperatures for the tropospheric oxidation of fluorobenzene, chlorobenzene, hexafluorobenzene and hexachlorobenzene while pentachlorophenol, a well-known organic micropollutant, seems to be a major stable product of tropospheric oxidation of hexachlorobenzene. Their calculated tropospheric lifetimes show that fluorobenzene and chlorobenzene are easily removed from the atmosphere and do not have long-range transport potential while hexafluorobenzene seems to be a potential POP chemical and hexachlorobenzene is clearly a typical persistent organic pollutant.

Rate constant for the H˙ + H2O → ˙OH + H2 reaction at elevated temperatures measured by pulse radiolysis.

Maintaining the structural integrity of materials in nuclear power plants is an essential issue associated with safe operation. Hydrogen (H2) addition or injection to coolants is a powerful technique that has been widely applied such that the reducing conditions in the coolant water avoid corrosion and stress corrosion cracking (SCC). Because the radiation-induced reaction of ˙OH + H2 → H˙ + H2O plays a crucial role in these systems, the rate constant has been measured at operation temperatures of the reactors (285-300 °C) by pulse radiolysis, generating sufficient data for analysis. The reverse reaction H˙ + H2O → ˙OH + H2 is negligibly slow at ambient temperature; however, it accelerates considerably quickly at elevated temperatures. Although the reverse reaction reduces the effectiveness of H2 addition, reliable rate constants have not yet been measured. In this study, the rate constants have been determined in a temperature range of 250-350 °C by pulse radiolysis in an aqueous I- solution.

Atmospheric oxidation of hexachlorobenzene: New global source of pentachlorophenol

Hexachlorobenzene is highly persistent, bioaccumulative, toxic and globally distributed, a model persistent organic pollutant. The major atmospheric removal process for hexachlorobenzene is its oxidation by hydroxyl radicals. Unfortunately, there is no information on the reaction mechanism of this important atmospheric process and the respective degradation rates were measured in a narrow temperature range not of environmental relevance. Thus, the geometries and energies of all stationary points significant for the atmospheric oxidation of hexachlorobenzene are optimized using MP2/6-311G(d,p) method. Furthermore, the single point energies were calculated with G3 method on the optimized minima and transition-states. It was demonstrated for the first time that the addition of hydroxyl radicals to hexachlorobenzene proceeds indirectly, via a prereaction complex. In the prereaction complex the hydroxyl radical is almost perpendicular to the aromatic ring while oxygen is pointing to its center. In contrast, in the transition state it is nearly parallel with the aromatic ring. The reliable rate constants are calculated for the first time for the atmospheric oxidation of hexachlorobenzene for all environmentally relevant temperatures. It was also demonstrated for the first time that pentachlorophenol is the major stable product in the addition of hydroxyl radicals to hexachlorobenzene and that atmosphere seems to be a new global secondary source of pentachlorophenol.

Ultrasonic Absorption Measurements of Propionic Acid Derivatives of NSAID and β‐Cyclodextrin in Aqueous Solution

Ultrasonic absorption measurements in the frequency range from 0.2 to 8.2 MHz with emphasis on the low‐frequency range below 1 MHz were carried out for β‐cyclodextrin (β‐CD) solutions with ibuprofen and naproxen at 25°C. A single relaxational absorption was observed and the cause of the relaxation was attributed to a perturbation of a chemical equilibrium associated with a complexation reaction between β‐CD and ibuprofen or naproxen. We determined reliable rate and equilibrium constants and standard volume changes of the reaction.

The importance of ensemble averaging in enzyme kinetics.

ConspectusThe active site of an enzyme is surrounded by a fluctuating environment of protein and solvent conformational states, and a realistic calculation of chemical reaction rates and kinetic isotope effects of enzyme-catalyzed reactions must take account of this environmental diversity. Ensemble-averaged variational transition state theory with multidimensional tunneling (EA-VTST/MT) was developed as a way to carry out such calculations. This theory incorporates ensemble averaging, quantized vibrational energies, energy, tunneling, and recrossing of transition state dividing surfaces in a systematic way. It has been applied successfully to a number of hydrogen-, proton-, and hydride-transfer reactions. The theory also exposes the set of effects that should be considered in reliable rate constants calculations.We first review the basic theory and the steps in the calculation. A key role is played by the generalized free energy of activation profile, which is obtained by quantizing the classical potential of mean force as a function of a reaction coordinate because the one-way flux through the transition state dividing surface can be written in terms of the generalized free energy of activation. A recrossing transmission coefficient accounts for the difference between the one-way flux through the chosen transition state dividing surface and the net flux, and a tunneling transmission coefficient converts classical motion along the reaction coordinate to quantum mechanical motion. The tunneling calculation is multidimensional, accounting for the change in vibrational frequencies along the tunneling path and shortening of the tunneling path with respect to the minimum energy path (MEP), as promoted by reaction-path curvature. The generalized free energy of activation and the transmission coefficients both involve averaging over an ensemble of reaction paths and conformations, and this includes the coupling of protein motions to the rearrangement of chemical bonds in a statistical mechanically correct way. The standard deviations of the transmissions coefficients provide information on the diversity of the distribution of reaction paths, barriers, and protein conformations along the members of an ensemble of reaction paths passing through the transition state.We first illustrate the theory by discussing the application to both wild-type and mutant Escherichia coli dihydrofolate reductase and hyperthermophilic Thermotoga maritima dihydrofolate reductase (DHFR); DHFR is of special interest because the protein conformational changes have been widely studied. Then we present shorter discussions of several other applications of EA-VTST/MT to transfer of protons, hydrogen atoms, and hydride ions and their deuterated analogs. Systems discussed include hydride transfer in alcohol dehydrogenase, xylose isomerase, and thymidylate synthase, proton transfer in methylamine dehydrogenase, hydrogen atom transfer in methylmalonyl-CoA mutase, and nucleophilic substitution in haloalkane dehalogenase and two-dimensional potentials of mean force for potentially coupled proton and hydride transfer in the β-oxidation of butyryl-coenzyme A catalyzed by short-chain acyl-CoA dehydrogenase and in the pyruvate to lactate transformation catalyzed by lactate dehydrogenase.

Insights into accelerated liposomal release of topotecan in plasma monitored by a non-invasive fluorescence spectroscopic method

A non-invasive fluorescence method was developed to monitor liposomal release kinetics of the anticancer agent topotecan (TPT) in physiological fluids and subsequently used to explore the cause of accelerated release in plasma. Analyses of fluorescence excitation spectra confirmed that unencapsulated TPT exhibits a red shift in its spectrum as pH is increased. This property was used to monitor TPT release from actively loaded liposomal formulations having a low intravesicular pH. Mathematical release models were developed to extract reliable rate constants for TPT release in aqueous solutions monitored by fluorescence and release kinetics obtained by HPLC. Using the fluorescence method, accelerated TPT release was observed in plasma as previously reported in the literature. Simulations to estimate the intravesicular pH were conducted to demonstrate that accelerated release correlated with alterations in the low intravesicular pH. This was attributed to the presence of ammonia in plasma samples rather than proteins and other plasma components generally believed to alter release kinetics in physiological samples. These findings shed light on the critical role that ammonia may play in contributing to the preclinical/clinical variability and performance seen with actively-loaded liposomal formulations of TPT and other weakly-basic anticancer agents.

Analysis of Marching-modulus Rubber Rheograms

Rheometry is widely used throughout the rubber industry to characterise the cure behaviour of rubber mixes. Industrial rheometers produce continuous traces or rheograms in which torque is plotted as a function of time as a rubber progressively cures. Rheograms in which a distinct maximum or steady-state value is obtained within a reasonable time present few problems when the main objective is to determine optimum vulcanisation times. Nevertheless, some rubber mixes display cure profiles that do not exhibit steady-state values within acceptable time scales; taking extremely long periods of time to do so. Running rheometers over prolonged periods in order to attain flat or steady state cures is neither productive nor economical. In such situations, it is desirable to develop a methodology whereby steady-state values can be estimated from short running times. Assuming that cure advances by way of first-order kinetics a graphical technique is introduced enabling the steady-state torque value to be estimated from short running times. The steady-state torque value obtained from the graphical procedure was compared with experimental values; poor agreement was found being within about 11%. Notwithstanding the poor agreement between the steady-state torque value observed experimentally and that predicted by the graphical method we nevertheless continue to recommend the use of the graphical technique. Our justification for the use of the graphical method is that the procedure provides reliable rate constants for the incipient stages of vulcanisation without having to determine full extents of cure. The rate constant determined by the graphical procedure was in good agreement with typical values reported in the literature7 thus affording credibility to our graphical methodology. As further confirmation as to the reliability of our technique, an analytical method introduced by Rigby2 was employed to verify the authenticity of our results. Comparison of our results with those ascertained from the analytical method showed reasonable agreement. In order to account for the significantly large difference between the graphically-estimated and experimental steady-state torque value, the ideal and simplistic assumption that curing proceeds by first order kinetics throughout the entire curing period was abandoned. Instead, it was assumed that the crosslinking process comprises two concomitant first-order curing reactions; notably normal and slow crosslinking reactions. The kinetic expression describing the two simultaneous vulcanisation reactions in terms of rheometry parameters is: [Formula: see text] where Tt is the torque at time t; Ta is the minimum torque; ti is the induction time; T¥,c and T¥,sc are the steady-state torque values for the normal and slow crosslinking processes respectively; and, kc and ksc represent the first-order rate constants for the normal and slow crosslinking processes respectively. Excellent agreement was found between the experimental steady-state torque value and that predicted by the composite kinetic expression.

Unimolecular HCl and HF Elimination Reactions of 1,2-Dichloroethane, 1,2-Difluoroethane, and 1,2-Chlorofluoroethane: Assignment of Threshold Energies

The recombination of CH(2)Cl and CH(2)F radicals generates vibrationally excited CH(2)ClCH(2)Cl, CH(2)FCH(2)F, and CH(2)ClCH(2)F molecules with about 90 kcal mol(-1) of energy in a room temperature bath gas. New experimental data for CH(2)ClCH(2)F have been obtained that are combined with previously published studies for C(2)H(4)Cl(2) and C(2)H(4)F(2) to define reliable rate constants of 3.0 x 10(8) (C(2)H(4)F(2)), 2.4 x 10(8) (C(2)H(4)Cl(2)), and 1.9 x 10(8) (CH(2)ClCH(2)F) s(-1) for HCl and HF elimination. The product branching ratio for CH(2)ClCH(2)F is approximately 1. These experimental rate constants are compared to calculated statistical rate constants (RRKM) to assign threshold energies for HF and HCl elimination. The calculated rate constants are based on transition-state models obtained from calculations of electronic structures; the energy levels of the asymmetric, hindered, internal rotation were directly included in the state counting to obtain a more realistic measure for the density of internal states for the molecules. The assigned threshold energies for C(2)H(4)F(2) and C(2)H(4)Cl(2) are both 63 +/- 2 kcal mol(-1). The threshold energies for CH(2)ClCH(2)F are 65 +/- 2 (HCl) and 63 +/- 2 (HF) kcal mol(-1). These threshold energies are 5-7 kcal mol(-1) higher than the corresponding values for C(2)H(5)Cl or C(2)H(5)F, and beta-substitution of F or Cl atoms raises threshold energies for HF or HCl elimination reactions. The treatment presented here for obtaining the densities of states and the entropy of activation from models with asymmetric internal rotations with high barriers can be used to judge the validity of using a symmetric internal-rotor approximation for other cases. Finally, threshold energies for the 1,2-fluorochloroethanes are compared to those of the 1,1-fluorochloroethanes to illustrate substituent effects on the relative energies of the isomeric transition states.

Low-Frequency Ultrasonic Relaxation of β-Cyclodextrin and Adenosine 5'-Monophosphate in Aqueous Solution

Nucleotides are the building blocks of nucleic acids and essential for many cellular functions. In this study, ultrasonic absorption spectra of β-cyclodextrin (β-CD) and adenosine 5'-monophosphate (AMP) in aqueous solution were measured over the broad frequency range 0.1-40 MHz with emphasis on the low-frequency range below 1 MHz. Here we show that the interaction of β-CD and AMP complies with a typical spectrum of a single relaxation process. We determined reliable rate (kb) and equilibrium (K) constants and a standard volume change ( V) of the reaction. They are kb = 2.3×10 -6 s -1 , K = 89 M -1 , and  V = 13.8 (10 -6 m 3 mol -1 ), respectively.