bluuebunny/crossref_metadata_2025 huggingface dataset:
On the accuracy of density functionals and their basis set dependence: An extensive study on the main group homonuclear diatomic molecules Li2 to Br2
Authors: Hertwig Roland H. , Koch Wolfram
Published: January 2005

The equilibrium bond distances, harmonic frequencies, and bond dissociation energies of the 21 homonuclear diatomics Li2—F2, Na2—Cl2, and K2—Br2 have been determined using approximate density functional theory (DFT) employing various widely used functionals and basis sets ranging from single zeta to triple zeta plus polarization quality. The results are in general much less sensitive to the size of the basis set as in conventional ab initio molecular orbital (MO) theory, while the choice of the functional is of much more significance. For one basis set (6‐311G*), the performance of the DFT‐based calculations has been compared and found to be superior to Hartree‐Fock (HF) Møller Plesset second order perturbation theory (MP2), or configuration interaction with single and double excitations (CISD) calculations. Particularly, no pathological cases, such as the group 2 dimers (Be2, Mg2, Ca2), are observed. © 1995 by John Wiley & Sons, Inc.
Unusual conformational‐determining interactions in oxymethylpyridines: An ab initio study and an improved method for refining molecular mechanics parameters
Authors: Norrby Per‐Ola , Wärnmark Kenneth , Åkermark Björn , Moberg Christina
Published: January 2005

The conformational preferences of oxymethylpyridines have been investigated by ab initio calculations and compared to similar calculations for oxymethylbenzene. The CO bond in the pyridine compounds was found to prefer eclipsing with a CC bond in the ring, in agreement with previous observations but in disaccord with tentative MM2 calculations. The effect was most pronounced in the 2‐substituted pyridine. The benzene compound, on the other hand, showed good agreement between the energies from MM2, MM3, and ab initio calculations. The conformational preferences are discussed in terms of stereoelectronic interactions. New MM2 and MM3 parameters were determined from ab initio calculations on nonstationary points on the energy hypersurface. The parameterization method is discussed. © 1995 by John Wiley & Sons, Inc.
Accurate modeling of the intramolecular electrostatic energy of proteins
Authors: Dudek Michael J. , Ponder Jay W.
Published: January 2005

The (ϕ, ψ) energy surface of blocked alanine (N‐acetyl–N′‐methyl alanineamide) was calculated at the Hartree‐Fock (HF)/6‐31G* level using ab initio molecular orbital theory. A collection of six electrostatic models was constructed, and the term electrostatic model was used to refer to (1) a set of atomic charge densities, each unable to deform with conformation; and (2) a rule for estimating the electrostatic interaction energy between a pair of atomic charge densities. In addition to two partial charge and three multipole electrostatic models, this collection includes one extremely detailed model, which we refer to as nonspherical CPK. For each of these six electrostatic models, parameters—in the form of partial charges, atomic multipoles, or generalized atomic densities—were calculated from the HF/6‐31G* wave functions whose energies define the ab initio energy surface. This calculation of parameters was complicated by a problem that was found to originate from the locking in of a set of atomic charge densities, each of which contains a small polarization‐induced deformation from its idealized unpolarized state. It was observed that the collective contribution of these small polarization‐induced deformations to electrostatic energy differences between conformations can become large relative to ab initio energy differences between conformations. For each of the six electrostatic models, this contribution was reduced by an averaging of atomic charge densities (or electrostatic energy surfaces) over a large collection of conformations. The ab initio energy surface was used as a target with respect to which relative accuracies were determined for the six electrostatic models. A collection of 42 more complete molecular mechanics models was created by combining each of our six electrostatic models with a collection of seven models of repulsion + dispersion + intrinsic torsional energy, chosen to provide a representative sample of functional forms and parameter sets. A measure of distance was defined between model and ab initio energy surfaces; and distances were calculated for each of our 42 molecular mechanics models. For most of our 12 standard molecular mechanics models, the average error between model and ab initio energy surfaces is greater than 1.5 kcal/mol. This error is decreased by (1) careful treatment of the nonspherical nature of atomic charge densities, and (2) accurate representation of electrostatic interaction energies of types 1—2 and 1—3. This result suggests an electrostatic origin for at least part of the error between standard model and ab initio energy surfaces. Given the range of functional forms that is used by the current generation of protein potential functions, these errors cannot be corrected by compensating for errors in other energy components. © 1995 by John Wiley & Sons, Inc.
Mono‐ and poly‐ligated complexes of Zn2+: An ab initio analysis of the metal–ligand interaction energy
Authors: Gresh Nohad , Stevens Walter J. , Krauss Morris
Published: January 2005

Detailed investigation of the binding energetics of Zn2+ to biologically relevant model ligands has been performed by large basis set restricted Hartree‐Fock computations. This list includes neutral and anionic ligands that model the sidechains of the amino acid residues of proteins as well as those involved in binding to the metal during enzymatic activation: water, formaldehyde, formamide, imadazole, methylthiol, and the formate, hydroxyl, methoxy, methylthiolate anions. The decomposition of the intermolecular interaction energy into its components (Coulomb, exchange, polarization, and charge transfer) has been done within the frozen fragment reduced variational space procedure (RVS) developed by Stevens and Fink [W. J. Stevens and W. H. Fink, Chem. Phys. Lett., 139, 15 (1987)]. The use of the RVS procedure was dictated by the very large magnitudes of the second‐order interaction energy terms in the divalent cation complexes and the need to obtain polarization and charge‐transfer contributions in a variational sense. The behavior of the interaction energy with radial and angular variation of the approach of the metal to the ligand is explored. In addition, the nonadditive behavior of polyligated complexes is studied for water and formate. This will also provide the data for a subsequent fit to a molecular mechanics procedure that considers the second‐order interactions. © 1995 by John Wiley & Sons, Inc.
A new optimization technique for artificial neural networks applied to prediction of force constants of large molecules
Authors: Fischer Thomas H. , Petersen Wesley P. , Lüthi Hans Peter
Published: January 2005

An artificial neural network (ANN) method for the prediction of force constants of chemical bonds in large, polyatomic molecules was developed. The force constant information evaluated is to be used for generating accurate estimates of the Hessian used in Newton‐Raphson‐type ab initio molecular structure optimization schemes. Different network topologies as well as a training procedure based on simulated annealing are evaluated. The results show that an ANN can be designed and trained to provide force constant information within a 1.5 to 5% error band even if the range of the force constants evaluated is very large (from triple bonds to hydrogen bridges). © 1995 by John Wiley & Sons, Inc.
The molecular structures and the absorption maxima of the H‐chromophores of the indigoid dyes
Authors: Chen P. C.
Published: January 2005

The molecular structures of the H‐chromophore of the indigoid dyes and five other isomers are studied by ab initio MP2/6‐31 + G*//HF/6‐31 + G* method. The bond angles are affected by the π‐electron conjugation. The molecular structures of the H‐chromophores and indigoid dyes indicate that the benzene rings and the five‐membered rings are structurally important. The absorption maxima of the H‐chromophores are successfully calculated by CI‐singles‐MP2/6‐31 + G* theory for the first time and correspond to the HOMO, LUMO transition. All these transitions are the π‐π* transitions. Like the indigoid dyes, trans isomers have the bathochromic shifts of the absorption maxima, and the bathochromic shifts are found with the best donor group of NH. From these calculations, the absorption maxima of some indigoid dyes can be explained by their H‐chromophores qualitatively. © 1995 by John Wiley & Sons, Inc.
A comprehensive study of the rotational energy profiles of organic systems by ab initio MO theory, forming a basis for peptide torsional parameters
Authors: Maxwell David S. , Tirado‐Rives Julian , Jorgensen William L.
Published: January 2005

Ab initio molecular orbital calculations have been carried out on over 50 model organic molecules and ions to provide the data necessary in the determination of torsional parameters for a force field involving polypeptides. The rotational energy profiles were obtained at the HF/6‐31G*//HF/6‐31G* level. The results were supported, in many cases, by full geometry optimizations and with consideration of correlation corrections at the MP2 level. With the exception of the dihedral angle being studied, all of the molecules were fully optimized with C1 symmetry. © 1995 by John Wiley & Sons, Inc.
Toward similarity measures for macromolecular bodies: Medla test calculations for substituted benzene systems
Authors: Walker P. Duane , Mezey Paul G.
Published: January 2005

A new method is proposed for the evaluation of numerical similarity measures for large molecules, defined in terms of their electron density (ED) distributions. The technique is based on the Molecular Electron Density Lego Assembler (MEDLA) approach, proposed earlier for the generation of ab initio quality electron densities for proteins and other macromolecules. The reliability of the approach is tested using a family of 13 substituted aromatic systems for which both standard ab initio electron density computations and the MEDLA technique are applicable. These tests also provide additional examples for evaluating the accuracy of the MEDLA technique. Electron densities for a series of 13 substituted benzenes were calculated using the standard ab initio method with STO‐3G, 3‐21G, and 6‐31G** basis sets as well as the MEDLA approach with a 6‐31G** database of electron density fragments. For each type of calculation, pairwise similarity measures of these compounds were calculated using a point‐by‐point numerical comparison of the EDs. From these results, 2D similarity maps were constructed, serving as an aid for quick visual comparisons for the entire molecular family. The MEDLA approach is shown to give virtually equivalent numerical similarity measures and similarity maps as the standard ab initio method using a 6‐31G** basis set. By contrast, significant differences are found between the standard ab initio 6‐31G** results and the standard ab initio results obtained with smaller STO‐3G and 3‐21G basis sets. These tests indicate that the MEDLA‐based similarity measures faithfully mimic the actual, standard ab initio 6‐31G** similarity measures, suggesting the MEDLA method as a reliable technique to assess the shape similarities of proteins and other macromolecules. The speed of the MEDLA computations allows rapid, pairwise comparisons of the actual EDs for a series of molecules, requiring no more computer time than other simplified, less detailed representations of molecular shape. The MEDLA method also reduces the need to store large volumes of numerical density data on disk, as these densities can be quickly recomputed when needed. For these reasons, the proposed MEDLA similarity analysis technique is likely to become a useful tool in computational drug design. © 1995 John Wiley & Sons, Inc.
Ab initio and molecular mechanics (MM2 and MM3) calculations of nonconjugated positively charged nitrogen‐containing compounds
Authors: McGaughey Georgia B. , Stewart Eugene L. , Bowen J. Phillip
Published: January 2005

High‐level ab initio calculations have been performed on N‐methyl‐N‐methyleneammonium and related compounds to obtain accurate rotational barriers, structures, and vibrational frequencies. The 6‐31G** basis set has been utilized at the Hartree‐Fock level of theory for these calculations because little experimental data are available. The MM2(91) and MM3(94) force fields have been parameterized to include these nonconjugated charged nitrogen‐containing compounds. Molecular mechanics geometries and vibrational frequencies compare well with the ab initio results. © 1995 John Wiley & Sons, Inc.
Localization and quantitative evaluation of potent local binding sites on the accessible Lennard–jones surface
Authors: Cohen Alexander A. , Shatzmiller Shimon E.
Published: January 2005

This article presents a new method for topological analysis of molecular surfaces. Explicit representation of the van der Waals interaction according to the Lennard‐Jones potential enabled determination of the function of the maximum radius of a hypothetical atomic probe in any location, r, inside the host's domain. The size of the spatial gradient of the maximal probe's volume (named the ξ value) at that location was found to be a good descriptor of the local shape of the host. Consequently, mapping of the host domain according to the ξ value could be used as a quantitative tool for localization of potent local binding sites. The proposed method is illustrated by mapping an organic host (calix[4]arene) as well as an enzyme (HIV‐aspartic protease). Analysis of the calix[4]arene derivative revealed that the proposed method reproduces immediately the known binding site of conic calix[4]arenes. The second test case demonstrated how the catalytic site of the enzyme could be disassembled into many local binding sites. Some of these sites, located according to the proposed method, were found to follow the shape of a known inhibitor of the enzyme in a complementary manner. © 1995 John Wiley & Sons, Inc.
Calculation of molecular geometries, relative conformational energies, dipole moments, and molecular electrostatic potential fitted charges of small organic molecules of biochemical interest by density functional theory
Authors: St.‐Amant Alain , Cornell Wendy D. , Kollman Peter A. , Halgren Thomas A.
Published: January 2005

Density functional theory is tested on a large ensemble of model compounds containing a wide variety of functional groups to understand better its ability to reproduce experimental molecular geometries, relative conformational energies, and dipole moments. We find that gradient‐corrected density functional methods with triple‐ζ plus polarization basis sets reproduce geometries well. Most bonds tend to be approximately 0.015 Å longer than the experimental results. Bond angles are very well reproduced and most often fall within a degree of experiment. Torsions are, on average, within 4 degrees of the experimental values. For relative conformational energies, comparisons with Hartree‐Fock calculations and correlated conventional ab initio methods indicate that gradient‐corrected density functionals easily surpass the Hartree‐Fock approximation and give results which are nearly as accurate as MP2 calculations. For the 35 comparisons of conformational energies for which experimental data was available, the root mean square (rms) deviation for gradient‐corrected functionals was approximately 0.5 kcal mol−1. Without gradient corrections, the rms deviation is 0.8 kcal mol−1, which is even less accurate than the Hartree‐Fock calculations. Calculations with extended basis sets and with gradient corrections incorporated into the self‐consistent procedure generate dipole moments with an rms deviation of 5%. Dipole moments from local density functional calculations, with more modest basis sets, can be scaled down to achieve roughly the same accuracy. In this study, all density functional geometries were generated by local density functional self‐consistent calculations with gradient corrections added in a perturbative fashion. Such an approach generates results that are almost identical to the self‐consistent gradient‐corrected calculations, which require significantly more computer time. Timings on scalar and vector architectures indicate that, for moderately sized systems, our density functional implementation requires only slightly less computer resources than established Hartree‐Fock programs. However, our density functional calculations scale much better and are significantly faster than their MP2 counterparts, whose results they approach. © 1995 John Wiley & Sons, Inc.
Numerical evaluation of three‐ and four‐center bielectronic integrals using exponential‐type atomic orbitals
Authors: Cesco J. C. , Denner C. C. , Rosso A. E. , Perez J. E. , Ortiz F. S. , Contreras R. H. , Giribet C. G. , De Azúa M. C. Ruiz
Published: January 2005

Three‐ and four‐center Slater orbital bielectronic integrals are evaluated by means of a complete function set. The method provides a series to approximate the bielectronic integrals. Their corresponding partial sums are analyzed in detail for 1s orbitals. The comparison with the Fourier transform–based method brings forth encouraging perspectives for the present approach. © 1995 John Wiley & Sons, Inc.
Calculation of relative free energy differences for the covalent hydration of organic compounds: A combined quantum mechanical and free energy perturbation study
Authors: Erion Mark D. , Reddy M. Rami
Published: January 2005

The relative free energy difference (ΔΔGhyd) for the reversible addition of water to two unsaturated molecules is accurately computed using a combination of ab initio quantum mechanical calculations and free energy perturbation methods. Initial attempts to calculate the absolute hydration free energy difference (ΔGhyd) for formaldehyde and trichloroacetaldehyde gave values that differed substantially from experimental results even after inclusion of electron correlation energy contributions using third‐order (MP3) and fourth‐order (MP4) Møller‐Plesset perturbation theory and QCISD(T) correlation methods at the 6‐31G** basis set level. Inaccuracies in ΔGhyd were attributed to errors in the calculation of both ΔGgas and ΔΔGsol. Gas phase quantum mechanical free energies (ΔGgas) varied significantly (2–3 kcal/mol) depending on the level of theory. Errors in ΔΔGsol were attributed to slow convergence of the calculations using the thermodynamic cycle perturbation (TCP) method with explicit solvent. These errors were minimized or canceled, however, when relative hydration free energy differences (ΔΔGhyd) were calculated using a combination of ab initio quantum mechanical calculations and free energy perturbation methods. Calculated values for a variety of aldehydes and ketones were consistent with experimental data. © 1995 John Wiley & Sons, Inc.
Kinetics of the iodine atom catalyzed gas phase geometrical isomerization of diiodoethylene. Rotation about a single bond
Authors: Golden David M. , Furuyama Shozo , Benson Sidney W.
Published: January 2005

The spectrophotometric determination of the rate of iodine atom catalyzed geometrical isomerization of diiodoethylene in the gas phase from 502.8 to 609.1°K leads to a rate constant for the bimolecular reaction between I and trans‐diiodoethylene of log kt−c(M−1 sec−1) = 8.85 ± 0.12 − (11.01 ± 0.30)/θ. Estimates of the entropy and enthalpy change for the addition of I atoms to trans‐diiodoethylene (process a.b) lead to log Ka.b(M−1) = −2.99 − 4.0/θ, and thus to log kc (sec−1) = log kt−c – log Kab = 11.8 −7.0/θ for the rate constant for rotation about the single bond in the adduct radical. The theory for calculation of the rotation rate constant is presented and it is shown that while the exact value depends on the barrier height, a value of 6.8 kcal/mole for this quantity leads to log k (sec−1) = 11.8 −6.7/θ. The activation energy points to a better value of the group contribution to heat of formation of the group C‐(I)2(H)(C) than one based on bond additivity.
The rate of addition of hydrogen atoms to olefins: Isobutane and ethylene
Authors: Knox J. H. , Dalgleish D. G.
Published: January 2005

The title reactions have been investigated in a fast flow system at pressures of about 2 torr and temperatures between 12 and 132°C. The following Arrhenius equations are derived for reaction (2)
equation image
where the units of k2 are l/mole sec and of E2, cal/mole, and the limits are the 95% confidence limits assuming random errors.

These equations are in good agreement with those which can be derived from previous investigations.
Arrhenius constants for the reactions of ozone with ethylene and acetylene
Authors: DeMore W. B.
Published: January 2005

The kinetics of ozonation of C2H4 and C2H2 have been studied in the gas phase from −40 to −95°C (C2H4) and +10 to −30°C (C2H2). The O3 concentrations were near 10−4 M, and the hydrocarbons were present in 2‐ to 25‐fold excess. A few experiments with propylene were also carried out. The reactions were followed by observing the rate of decay of O3 absorption at 2537 Å. Reaction stoichiometries and effects of added O2 were investigated. The second‐order rate constant for C2H4 was log k(M−1 sec−1) = (6.3 ± 0.2) – (4.7 ± 0.2)/θ (θ = 2.3RT). The rate was independent of the presence of excess O2. Rate measurements for C3H6 were less accurate because of aerosol interference. Combined with room temperature measurements of other workers, the C3H6 rate constant was log k(M−1 sec−1) = (6.0 ± 0.4) – (3.2 ± 0.6)/θ. The C2H2 rate constant was log k(M−1 sec−1) = (9.5 ± 0.4) – (10.8 ± 0.4)/θ. In the case of C3H6 the major product was propylene ozonide. Ethylene did not yield the ozonide, and the products of the O3–C2H4 and O3–C2H2 reactions were not identified. Pre‐exponential factors for the olefin reactions are consistent with a five‐membered ring transition state formed by 1,3 dipolar cycloaddition of O3. For C2H2, however, the much higher observed A factor suggests a different mechanism. Possible transition states for the O3–C2H2 reaction are discussed.
Kinetic studies of the reactions CH2I2 + HI⇄CH3I + I2 and 2 CH3I⇄CH4 + CH2I2 the heat of formation of the iodomethyl radical
Authors: Furuyama Shozo , Golden David M. , Benson Sidney W.
Published: January 2005

The rate of the reaction CH2I2 + HI ⇆ CH3I + I2 has been followed spectrophotometrically from 201.0 to 311.2°. The rate constant for the reaction fits the equation, log (k1/M−1 sec−1) = 11.45 ± 0.18 ‐ (15.11 ± 0.44)/θ. This value, combined with the assumption that E2 = 0 ± 1 kcal/mole, leads to ΔH (CH2I, g) = 55.0 ± 1.6 kcal/mole and DH (HCH2I) = 103.8 ± 1.6 kcal/mole.

The kinetics of the disproportionation, 2 CH3I ⇆ CH4 + CH2I2 were studied at 331° and are compatible with the above values.
Reactivities in olefin‐difluorocarbene reactions
Authors: Mitsch Ronald A. , Rodgers Alan S.
Published: January 2005

The relative rates of addition of difluorocarbene to a series of methyl‐substituted olefins have been determined and correlated with similar data for dichlorocarbene, chlorofluorocarbene and ground‐state oxygen atoms. The electrophilic nature and stabilization of difluorocarbene by the fluorine substituents is discussed. Relative activation energies for the difluorocyclopropane‐forming reaction have been estimated and correlated with properties of the olefins as derived from molecular orbital theory.
Some hydrogen atom abstraction reactions of CF2H and CFH2 radicals, and the CH bond dissociation energy in CF2H2
Authors: Pritchard G. O. , Perona M. J.
Published: January 2005

By photolyzing (CF2H)2CO and (CFH2)2CO the hydrogen atom abstraction reactions of CF2H radicals with (CF2H)2CO, H2, D2, CH4, C2H6, nC4H10 and isoC4H10, and the reactions of CFH2 radicals with (CFH2)2CO and nC4H10, have been studied. Arrhenius parameters for these reactions are compared with related systems. From a knowledge of the activation energies for the forward and reverse reactions a value of the bond dissociation energy, D(CF2HH) = 97.4 ± 1.3 kcal mole−1 at a mean temperature of 543°K is
equation image
obtained. This value is subject to much uncertainty due to possible compensation effects in the Arrhenius parameters. These effects are discussed for this and the other reactions, and the data suggest that D(CF2HH) is approximately 100 kcal mole−1, and that D(CFH2H) is very similar. Other literature data tend to confirm these approximate values.
An evaluation of rate data for the reactions of atomic Oxygen (O3P) with methane and ethane
Authors: Herron John T.
Published: January 2005

This review presents in tabular and graphical form rate data on the reactions of atomic oxygen (O3P) with methane and ethane. The reliability of these data is discussed and suggested values of the rate constants are given over specified temperature intervals. Specific values are given for 298 and 1000°K.
Thermal decomposition of benzotrifluoride. The CC bond strength and the heat of formation of the phenyl radical
Authors: Szilágyi I. , Bérces T.
Published: January 2005

The kinetics of the decomposition of benzotrifluoride was studied from 720°c to 859°c in a flow system with and without carrier gas. Consideration of the product distribution made possible the study of the decomposition into CF3 and C6H5 radicals, which appeared to be truly homogeneous in character. The first‐order rate constant of the CC bond fission, log k (sec−1) = (17.9 ± 0.5) (99.7 ± 2.5)/θ, did not change with change of initial concentration, pressure of the carrier gas, or contact time. The Arrhenius parameters have been related to the appropriate thermodynamic data. Assumption of 0 kcal/mole for the activation energy of the reverse combination reaction yielded DH(C6H5CF3) = 103.6 ± 2.5 kcal/mole and ΔH(C6H5) = 77.1 ± 3.0 kcal/mole.

Applicability of the simple first‐order formula to calculation of the rate constant has been also dealt with.
The reaction of trifluoromethyl radicals with ammonia
Authors: Morris E. R. , Thynne J. C. J.
Published: January 2005

The reaction of trifluoromethyl radicals with ammonia in the gas phase has been studied in the temperature range 30‐352°. Product formation is not explicable simply in terms of the reactions:

and curvature of the Arrhenius plot at high and low temperatures suggests that there are additional sources of fluoroform.
The elimination of HF from vibrationally excited fluoroethanes. The decomposition of 1,1,1‐trifluoroethane‐d0 and d3
Authors: Pritchard G. O. , Perona M. J.
Published: January 2005

The vibrationally excited molecules CF3CH and CF3CD have been synthesized by radical combination (produced by ketone photolysis), and HF and DF elimination from them studied as a function of temperature and pressure. Using RRK theory many calculations have recently been made of critical energies for the decomposition of "hot" fluoroethane molecules. Taking CF3CH as an example, it is concluded that the empiricism involved in such calculations renders results of doubtful significance. The non‐equilibrium kinetic isotope effect is kH/kD = 3.1 at 470°K. Arrhenius parameters are also presented for radical abstraction reactions from the ketone source molecules.
Some particular aspects of the low‐temperature oxidation of methane initiated by nitric acid
Authors: Iosif Ileana , Lazar D. P. , Schneider I. A.
Published: January 2005

During methane oxidation initiated by nitric acid or nitrogen dioxide, it is possible to observe, under certain conditions of temperature, initiator content, and flow rates, the formation of an adiabatic “temperature peak.” Because the maximum temperature of the peak does not exceed the initial temperature of the reactor by more than 220°C, the recorded temperature‐increase curves were used for a kinetic interpretation. The obtained kinetic parameters (overall n and E) agree with the values reported in the literature for isothermal methane oxidation and suggest that the overall mechanism does not change in the conditions of “temperature peak.”
Chemically activated 1,1‐dimethylcyclopropane from photolysis of isobutene–neopentane–diazomethane–oxygen mixtures
Authors: Taylor G. W. , Simons J. W.
Published: January 2005

A study of the structural isomerization rate of chemically activated 1,1‐dimethylcyclopropane from singlet methylene addition to the double bond of isobutene is reported. Singlet methylenes were produced from the 4358‐ and 3660‐Å photolysis of diazomethane in the presence of added oxygen. Theoretical rates calculated via RRKM theory are in excellent agreement with experiment for calculations utilizing activated complex structures and critical energies consistent with known thermal Arrhenius parameters, and excitation energies consistent with previous determinations of ΔH(CH2) + E*(CH2) = 116.1 and 112.6 kcal/mole for diazomethane photolyses at 3660 and 4358 Å, respectively.
Rates and mechanism of alkyne ozonation
Authors: B. Demore W.
Published: January 2005

The rates and products of the reactions of ozone with acetylene, methylacetylene, dimethylacetylene, and ethylacetylene have been studied in a long‐path infrared cell at 21 ± 1°C. The gas phase reaction gives products formed by cleavage of the carbon–carbon triple bond. A mechanism is proposed that involves formation of a short‐lived acid anhydride intermediate, which is energized by virtue of the reaction exothermicity and undergoes unimolecular decomposition. Formation of an α‐dicarbonyl was observed in every case, but there is evidence that a side reaction on the walls accounted for that product. The general relationship between alkyne and alkene ozonation is discussed. The rate measurements showed that, unlike the alkenes, the rate of alkyne ozonation is not greatly affected by substitution with simple alkyl groups. The rate constant for C2H2 agrees with earlier work and thus provides additional support for the previously derived high A‐factor for acetylene ozonation relative to alkenes.
The vibrational excitation of hydrogen fluoride behind incident shock waves
Authors: Solomon W. C. , Blauer J. A. , Jaye F. C. , Hnat J. G.
Published: January 2005

The vibrational excitation of HF occurring behind incident shock waves has been studied in the temperature range of 1400°K to 4100°K. The extent of excitation was determined as a function of time by continuously monitoring the emission intensity from the 1–0 band of HF centered at 2.5 μ. The data were interpreted in terms of the process

and gave a value of

for M = HF. The corresponding result for (τp)−1Ar was found to be insignificant in comparison to this result. Data were also obtained for the effect of F atoms upon the relaxation rate, i.e., it was found that
The reactions between SO2 and C2H2 (and C2H4) at elevated temperatures. A single‐pulse shock tube investigation
Authors: Fifer R. , Moreau R. , Bauer S. H.
Published: January 2005

The high‐temperature reaction between sulfur dioxide and acetylene in an excess of argon was studied in a 1−in. i.d. single‐pulse shock tube. Mixtures ranging from 1.81% to 5.40% SO2 and 1.60% to 4.90% C2H2 were heated to reflected shock temperatures of 1550°–2150°K, for dwell times of about 0.6 msec and gas dynamically quenched. Total reaction densities were 0.89 to 5.4 × 10−2 moles/1. The reaction products were analyzed by gas chromatography. A technique was developed for separating Ar, C2H4, C2H2, SO2, CO, CO2, H2S, COS, and CS2. The major products of the reaction are CO, H2, CS2, and sulfur. The products observed were compared with those predicted on the assumption that equilibrium was attained. Several preliminary experiments were carried out with ethylene‐sulfur dioxide mixtures, and the results indicated that for this combination the sulfur dioxide probably reacted with the acetylene generated from the decomposition of the ethylene, rather than directly with the ethylene. The rate of decline in the sulfur dioxide content in C2H2‐SO2 mixtures was found to be approximately second order (total) and can be empirically represented by

A mechanism is proposed to account for the overall reaction kinetics.
The reactions of O(3P) with ozone and carbonyl sulfide
Authors: Krezenski Daniel C. , Simonaitis R. , Heicklen Julian
Published: January 2005

The competitive reactions between 2‐trifluoromethylpropene (TMP) and OCS for O(3P) atoms were studied between 300° and 523°K, using the mercury‐senstitized photolysis of N2O as a source of O(3P). From the known value for the rate constant of the O(3P) + TMP reaction, k3 was found to be 1.6 × 10−11 exp (−4500/RT) cm3/particle‐sec, where reaction (3) is

Mixtures of O3 and OCS were photolyzed at 197°, 228°, 273°, and 299°K with radiation above 4300 Å to produce O(3P) from the photolysis of O3, and thus study the competition between reaction (3) and

From the above value of k3, k1 could be computed. When combined with all the previous data, the best espression for k1 is k1 = 1.2 × 10−11 exp (−4300/RT) cm3/particle‐sec.
Disproportionation‐combination ratios of small alkyl radicals formed by the mercury‐photosensitized hydrogenation of olefins in the gas phase
Authors: Falconer W. E. , Sunder W. A.
Published: January 2005

Disproportionation‐combination ratios have been obtained for small alkyl radicals produced in the mercury‐photosensitized hydrogenation of ethylene, propylene, butene‐1, butene‐2, and isobutene. For conditions favoring thermalized radicals, kd/kc (ethyl, ethyl) = 0.14, kd/kc (isopropyl, isopropyl) = 0.69, kd/kc (isopropyl, n‐propyl) = 0.41, kd/kc (2‐butyl, 2‐butyl) = 0.77, kd/kc (2‐butyl, n‐butyl) = 0.45, and kd/kc (t‐butyl, t‐butyl) = 3.1. The results are consistent with kd/kc (n‐propyl, n‐propyl) = 0.15 and kd/kc (n‐butyl, n‐butyl) = 0.14. The general agreement between these disproportionation‐combination ratios and those obtained where photolytic radical sources have been used indicates that under appropriate conditions, H atom addition to olefins is a suitable source for the study of thermalized alkyl radicals and may be especially useful for larger alkyl radicals for which clean photolytic precursors are not available.
Bromination of dichloromethane, chlorofluoromethane, and dichlorofluoromethane.
Authors: Copp D. E. , Tedder J. M.
Published: January 2005

Dichloromethane, chlorofluoromethane, and dichlorofluoromethane have been brominated competitively with each other and with chloroform and methyl chloride. Using the literature values for the rates of bromination of these latter two compounds, the rate constants for the three former compounds have been determined:

where R is taken to be 1.987 cal deg−1 mol−1 (1 cal = 4.186 J).
Kinetics of the thermal reaction of 1,1,2‐trimethylcyclopropane
Authors: O'Neal H. E. , Henfling D.
Published: January 2005

The gas‐phase reaction kinetics of 1,1,2‐trimethylcyclopropane have been studied in the temperature range of 700–755°K. First‐order unimolecular rate constants for over‐all reactant disappearance fit the Arrhenius equation

Reaction products and the relative rates of their formation are found to be reasonably consistent with the biradical mechanism.
Emission studies of the mechanism of gaseous biacetyl photolysis at 3450, 3650, 3880, and 4358 a and 28°C
Authors: Horowitz Abraham , Calvert Jack G.
Published: January 2005

The quantum yields of phosphorescence (Φp) of biacetyl have been determined in pure biacetyl, biacetyl‐SO2, and biacetyl‐c‐C6H12 mixtures in experiments using bands of radiation centered at 3450, 3650, 3880, and 4348 Å. It has been shown that the unexpected effect of gas concentration on the quantum yields of the sulfur dioxide triplet‐sensitized phosphorescence of biacetyl resulted largely from the significant destruction of biacetyl triplets at the wall of the cell. The kinetics of the variation of Φp with [Ac2], wavelength of the absorbed light, and added gases provide new estimates of the energy relations and the rate constants for the decomposition reaction of vibrationally rich biacetyl molecules in the first excited singlet state (1Ac2‡): 1Ac2‡ → products (1), 1Ac2‡ + Ac2 → 1Ac2 + Ac2 (2); the minimum energy necessary in 1Ac2‡ for reaction (1) to occur is estimated to be about 72.8 kcal/mole above the ground state of biacetyl: k1/k2 = (4.3 ± 0.1) × 10−3M at 3450 Å, (4.07 ± 0.04) × 10−4M at 3650 Å, and (5.6 ± 0.4) × 10−5M at about 3800 Å. The variation of the rate constant ratio is shown to be consistent with the expectations of the simple theory of excited molecule decomposition. Biacetyl triplet (3Ac2) rate constants were determined by measurements of Φp in O2 and NO‐containing mixtures: 3Ac2 + S → (Ac2–S, products) (8); for O2 = S, k8 = (5.76 ± 0.40) × 108 (3650 Å experiments), (5.76 ± 0.27) × 108 (4358 Å); for NO = S, k8 = (3.34 ± 0.20) × 109 (3650 Å), (3.33 ± 0.18) × 109 1./mole‐sec (4358 Å). A comparison between these and previous findings of the SO2 triplet (3SO2)‐sensitized excitation of biacetyl [5,6] show that the decomposition of the initial 3Ac2 product of the exothermic energy transfer reaction 3SO2 + Ac2 → SO2 + 3Ac2 is unimportant.
The Reactions of H Atoms with H2S and COS
Authors: Rommel H. , Schiff H. I.
Published: January 2005

The kinetics of the reactions of H atoms with H2S and with COS were measured at 298deg;K in a flow system using mass‐spectrometric detection. The rate constants were found to be 3.8 × 10−13and 2.2 × 10−14cm3part−1sec−1, respectively, with an estimated accuracy of 25%. At high flow rates of H2S, 0.5 molecules of H2S are consumed per H atom originally present. At all flow rates of COS, H2S is a major product, CO production equals COS consumption, and 0.5 molecules of COS are consumed per H atom. The results are consistent with the reaction HS + HS → H2S + S being the dominant process for thiyl radicals, and evidence is presented to indicate that its reaction rate is close to collisional frequency.
Kinetics of the reactions of cyclopropane derivatives, part II. The gas‐phase pyrolysis of cyclopropylamine
Authors: Parry K. A. W. , Robinson P. J.
Published: January 2005

At temperatures of 356‐425°C and pressures of 15–60 Torr, cyclopropylamine reacts to give an equimolar mixture of ammonia and N‐propylidenecyclopropylamine as the initial product. The reaction is first order, homogeneous, and unaffected by the presence of radical inhibitors, and thus proceeds by an initial rate‐determining unimolecular isomerization to give a reactive intermediate, which then reacts with a further molecule of cyclopropylamine to give the observed products. Reaction in the presence of added aliphatic amines gives other imines in addition, and the nature of these indicates that the intermediate is propenylamine or its tautomer propylideneamine:
equation image
A flash photolysis resonance fluorescence kinetics study of ground‐state sulfur atoms. III. Rate parameters for reaction of S(3P) with ethylene episulfide
Authors: Klemm R. B. , Davis D. D.
Published: January 2005

Absolute rate constants for the reaction of S(3P) with ethylene episulfide were measured over a C2H4S concentration range of 5, a total pressure of 20–200 tort, and a flash intensity range of ˜4. Over this range of variables, the bimolecular rate constant was found to be invariant. Because of limitations imposed by the physical properties of the reactant C2H4S, temperature variations were necessarily held to the range 298–355°K. The bimolecular rate constant was found to be invariant over this limited temperature range, having a value of (4.47 ± 0.26) × 10−11 cm3 molec.−1 sec−1. The possible influence of this reaction in studies of the S(3P)–ethylene system are discussed.
The kinetics of biacetyl phosphorescence sensitized in 2875 Å‐irradiated SO2–biacetyl–added gas (CO2, CO, N2) mixtures. The nature of the SO2 excited states formed at high added gas pressures
Authors: Horowitz Abraham , Calvert Jack G.
Published: January 2005

The quantum yields of sensitized biacetyl phosphorescence emission have been determined for SO2–biacetyl–CO2, SO2–biacetyl–N2, and SO2–biacetyl–CO mixtures irradiated at 2875 Å. Series of experiments were made at various SO2, biacetyl, and added‐gas concentrations (up to 4 × 10‐2 M). The kinetic data show an “excess” biacetyl emission which requires the participation of some undefined excited state or isomer of SO2 in addition to the emitting singlet (1SO2) and triplet (3SO2) states of SO2. These results confirm the experimental observations of Cehelnik et al. [12] and Wampler et al. [9] who reported that an “excess” quantum yield of CO2 occurred in 3130 Å‐irradiated SO2‐CO mixtures at high CO pressures. The kinetic analysis of the data is most consistent with the hypothesis of Wampler et al.; the nonreactive, undefined excited state or high‐energy isomer of SO2, designated here as X, is formed in part as 1SO2 is quenched: 1SO2 + M → (SO2M) (1); 1SO2 + M → 3SO2 + M (2); 1SO2 + M → X + M (3). X itself does not transfer energy to biacetyl. It decays largely by a nonradiative first‐order process at low pressures, X → SO2 (5), but it generates 3SO2 by collisional perturbation at high pressures, X + M → 1SO2 + M (4). In terms of this mechanism, the following rate constant ratios are derived from the present study: k2/(k1 + k2 + k3) = 0.020 ± 0.010 (M = CO2), 0.021 ± 0.010 (N2), 0.019 ± 0.010 (CO); k3/(k1 + k2 + k3) = 0.14 ± 0.02 (M = CO2), 0.085 ± 0.012 (CO), 0.11 ± 0.02 (N2); and k5/k4 (mole/l) = 0.020 ± 0.004 (M = CO2), 0.016 ± 0.003 (CO), 0.021 ± 0.005 (N2).
Kinetics of the reactions of cyclopropane derivatives. IV. Kinetic isotope effect in the isomerisation of cyclopropylamine
Authors: Luckraft D. A. , Robinson P. J.
Published: January 2005

The high‐pressure isotopic rate ratio kH/kD for isomerisation of cyclopropylamine and cyclopropylamine‐N‐d2 is 1.06 at 649–678 K, supporting a mechanism which does not involve migration of hydrogen from the amine group in the rate‐determining step.
A flash photolysis–resonance fluorescence kinetics study of ground‐state sulfur atoms. IV. Rate parameters for reaction of S(3P) with propene and 1‐butene
Authors: Klemm R. B. , Davis D. D.
Published: January 2005

Using the technique of flash photolysis‐resonance fluorescence, absolute rate constants have been measured for the reaction of S(3P) with propene and 1‐butene. Variations in experimental conditions included the following: temperature (215–500°K); total pressure a factor of 10; olefin concentration, a factor of 6; flash intensity (S atom concentration), a factor of 10. It was found that over these variations in the experimental conditions only the temperature had a measureable effect on the bimolecular rate constant. The derived Arrhenius rate expressions for the reactions (2) and (3) were as follows:

temperature range 214–500°K

Units are cm3 molec−1 s−1.
Relative rate constants for the reactions of trichloropropenyl radical. Application of the two‐parameter Taft equation
Authors: Afanas'ev I. B. , Baranova N. G. , Samokhvalov G. I.
Published: January 2005

The relative rate constants for the hydrogen atom abstraction by CCl3CHCH· radical from CH2Cl2, CHCl3, CH3COCH3, CH3CN, C6H5CH3, C6H5OCH3, CH3CHO, and CH3OH in the liquid phase at 20°C have been measured. It was shown that these reaction rate constants are correlated by the two‐parameter Taft equation with ρ* = 0.726 ± 0.096, r* = 1.22 ± 0.16. A relationship between r* and bond dissociation energy D(RH) has been found for the abstraction reactions of different free radicals.
Laser‐powered homogeneous pyrolysis
Authors: Shaub W. M. , Bauer S. H.
Published: January 2005

The large absorption cross sections for infrared radiation possessed by a few rather stable molecular species, their relatively fast vibration–translation relaxation times, and the availability of large laser fluxes at their characteristic frequencies have been effectively exploited to generate high bath temperatures for gas‐phase reactions under strictly homogeneous conditions. Controlled temperatures from 500 to 1500°K have been developed with a modest CO2 laser. Rise times for heating are estimated to be in the millisecond range, with reaction times ≈ 10 seconds. Operating pressures [reagents, radiation absorber (generally SF6), and diluent] range from 10 to 100 torr. Thus laser‐powered homogeneous pyrolysis (LPHP) is complementary to the single‐pulse shock‐tube technique [rise time ∼10−8 sec; reaction time ∼10−3 sec] for obtaining product distributions in gas‐phase pyrolyses, and for measuring relative rates in the absence of hot walls. The latter may (and generally do) introduce catalytic effects. Further, LPHP is very simple and ideally suited for exploratory investigations of small amounts of reagent (≈10−6 mole); also, sample cells may be heated externally to augment the vapor pressure of slightly volatile reagents. Conventional analytical procedures (gas chromatography, mass spectrometry) were used to follow the course of pyrolysis.

The primary chemical kinetic parameters, the temperature and density of the reactants, have both spatial and temporal distributions. Computer programs were developed for estimating T(r,x;t), given the radial intensity distributon in the incident beam, the absorption coefficient of the SF6 (as a function of density and temperature) for the irradiating line, as well as the mean heat capacity and thermal conductivity of the gas mixture. However, the computed profiles did not prove useful since the effects of convection, which could not be included in the calculations, perturb the initial distribution significantly within a few milliseconds. Indeed, it proved practical to enhance convection so as to homogenize the cell composition. When the incident radiation was chopped, frequencies ranging from 100 Hz to 1 Hz gave the same product distributions. A practical procedure is to use a “chemical thermometer” to establish a mean effective temperature for a given experimental configuration. Examples showing how to measure relative rate constants for molecular conversions, and the use of LPHP for qualitative determinations of product distributions as a function of temperature are presented. Finally, a number of high‐temperature conversions are described to illustrate the utility of LPHP for flash thermolysis.
The oxidation of hydrocarbons by water vapor behind high‐temperature shock‐waves
Authors: Shaviv A. , Bar‐Nun A.
Published: January 2005

The oxidation of acetylene by water vapor was studied behind the reflected shock in a single‐pulse shock tube. Computer simulation experiments reproduced the experimental results in the temperature range of 1500 to 2000°K. The kinetic scheme suggested here involves three major processes, (1) production of hydrogen atoms by the sequence of reactions which lead from acetylene to carbon; (2) production of OH radicals, mainly by the reaction H + H2O → H2 + OH, and (3) fast oxidation of the acetylene and other C/H species by the available oxidants in the system. The experimental results of methane oxidation suggest that methane is converted to acetylene prior to its oxidation. The implication of the experimental results to processes occurring in planetary atmospheres as a result of thunder shock waves is briefly discussed.
Kinetic and product distribution analysis for competitive recombination reactions
Authors: Ehrenson S.
Published: January 2005

Exact solution methods are developed for the complex kinetic and product distribution equations encountered in a particular type of competitive recombination or crossover reaction involving “cages.” Pertinent reaction systems are those where scavenging of the intermediates escaping the cages is restricted to species similar to these intermediates, thereby limiting potentially undesired side effects of the scavenger. The trustworthiness of the reaction constants which may be obtained, particularly the fraction of recombination occurring within the cage, are analyzed in terms of sources of possible error. Several extensions of the method are briefly discussed, such as in distinctions between inter‐ and intramolecular rearrangements. Full detailing of the solution methods should facilitate their use in the analysis of similar reaction schemes.