bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ab initio calculations of the ultraviolet resonance Raman spectra of uracil
Authors: Peticolas Warner L. , Rush Thomas 
Published: January 2005

AbstractAn equation been derived to calculate, ab initio, the frequencies and intensities of a resonant Raman spectrum from the transform theory of resonance Raman scattering. This equation has been used to calculate the intensities of the ultraviolet resonance Raman spectra from the first π‐π* excited state of uracil and 1,3‐dideuterouracil. The protocol for this calculation is as follows: (1) The force constant matrix elements in Cartesian coordinate space, the vibrational frequencies, and the minimum energy ground and excited state geometries of the molecule are calculated ab initio using the molecular orbital program Gaussian 92, (2) the force constants in Cartesian coordinates are transformed into force constants in the space of a set of 3N – 6 nonredundant symmetrized internal coordinates, (3) the G matrix is constructed from the energy minimized ground state Cartesian coordinates and the GFL = LΛ eigenvalue equation is solved in internal coordinate space, (4) the elements of the L and L−1 matrices are calculated, (5) the changes in all of the internal coordinates in going from the ground to the excited state are calculated, and (6) these results are used in combination with the transform theory of resonance Raman scattering to calculate the relative intensities of each of the 3N – 6 vibrations as a function of the exciting laser frequency. There are no adjustable parameters in this calculation, which reproduces the experimental frequencies and intensities with remarkable fidelity. This indicates that the Dushinsky rotation of the modes in the excited state of these molecules is not important and that the simplest form of the transform theory is adequate. © 1995 John Wiley & Sons, Inc.
Reduced variable molecular dynamics
Authors: Turner James , Weiner Paul , Robson Barry , Venugopal Ravi , Schubele Harry , Singh Ramen 
Published: January 2005

AbstractThis article describes an extension to previously developed constraint techniques. These enhanced constraint methods will enable the study of large computational chemistry problems that cannot be easily handled with current constrained molecular dynamics (MD) methods. These methods are based on an O(N) solution to the constrained equations of motion. The benefits of this approach are that (1) the system constraints are solved exactly at each time step, (2) the solution algorithm is noniterative, (3) the algorithm is recursive and scales as O(N), (4) the algorithm is numerically stable, (5) the algorithm is highly amenable to parallel processing, and (6) potentially greater integration step sizes are possible. It is anticipated that application of this methodology will provide a 10‐ to 100‐improvement in the speed of a large molecular trajectory as compared with the time required to run a conventional atomistic unconstrained simulation. It is, therefore, anticipated that this methodology will provide an enabling capacity for pursuing the drug discovery process for large molecular systems. © 1995 John Wiley & Sons, Inc.
Incorporating the protein – dipole Langevin – dipole model into Tanford‐Kirkwood theory
Authors: Zhexin Xiang , Fuhua Huang , Yunyu Shi , Yinwu Xu 
Published: January 2005

AbstractA new technique incorporating the protein–dipole Langevin–dipole (PDLD) model into the Tanford‐Kirkwood (TK) formula has been proposed which provides a rather detailed description of solvent and ionic strength effects on the electrostatic energies. Applications of this method to realistic problems have been performed and concern the solvation energies of four residues of bovine pancreatic trypsin inhibitor (BPTI) and the pK shift of His‐64 of mutant subtilisin BPN′. We focus our calculation on the back‐field effects of bulk solvent.The determination of protein‐induced dipoles is cumbersome due to the solvent screening effects. The protein‐induced dipoles are dependent on their local electric fields, which come from the protein net charges, the surrounding water molecules, and bulk solvent. The bulk solvent will usually screen the electric fields from the protein net charges and dipoles, which are defined here as the back‐field effects of bulk solvent on protein net charges and dipoles, respectively.
Our calculations indicate that the back‐field effects of bulk solvent on protein dipoles can simply be ignored, introducing a relative error less than 3%; whereas such back field‐effects on protein net charges are relatively important and cannot simply be ignored, especially when considering a system of highly charged species. © 1995 John Wiley & Sons, Inc.
Iodine monochloride as a thermal source of chlorine atoms: The reaction of chlorine atoms with hydrogen
Authors: Benson S. W. , Cruickshank F. R. , Shaw R. 
Published: January 2005

AbstractThe use of iodine monochloride (ICl) as a thermal source of chlorine atoms in known concentration is discussed with particular reference to the suppression, by large excesses of iodine, of the chain processes normally associated with chlorine atom reactions. The kinetics and mechanism of the reaction of ICl with hydrogen are presented in a study covering the temperature range 205–337°C, and the pressure ranges: ICl, 6–20 torr; I2, 3–13 torr; and H2, 9–520 torr. The reaction, followed spectrophotometrically in a static system, is shown to be homogeneous, first order in ICl and in H2, and inverse half‐order in I2, over several half‐lifetimes of the ICl, yielding HCl as the sole product. The rate data obtained in this work for the reaction
equation image
are combined with the critically evaluated results of other workers in an Arrhenius plot covering the temperature range 286–730°C, and three orders‐of‐magnitude in the rate constant, yielding the results, log k1/(1/mole sec) = 10.68–5.26/θ, where θ = 2.303RT in kcal/mole. This value of k1 is lower by a factor of about two than that proposed in a recent review by Fettis and Knox, and is clearly at variance by a factor of two or more with the most recent data of Clyne and Stedman.
Cleavage of vinylic mercuric halides by aqueous acids
Authors: Kreevoy Maurice M. , Landholm Richard A. 
Published: January 2005

AbstractIsotope effects, general acid catalysis, and relative reactivities show that proton transfer to one of the unsaturated carbon atoms is rate determining for the acidolysis of unsaturated alkylmercuric halides. For compounds, R1R2CCHHgX, substitution of CH3 for H at R1 or R2 leads to an acceleration of a factor of ∼ 30. This relatively small acceleration, the relative facility of the reactions, and the magnitude of the Br− catalytic terms, suggests an olefin–mercuric halide complex as the product of the rate‐determining step, rather than a simple carbonium ion.The Brøonsted catalysis law is obeyed with a variety of carboxylic acids, giving an ∝ of 0.69 ± 0.04, but acids of other structures give substantially deviant catalytic coefficients, in a pattern similar to that generated by other A‐SE2 reactions. The acetic acid catalytic coefficient is larger by a factor of 102 than that predicted if it were due to specific hydronium ion–general base catalysis instead of true general acid catalysis.The overall solvent isotope effect, kH/kD, is 2.55 ± 0.10. The competitive isotope effect, κH/κD, is 6.84 ± 0.06. Taken with a model in which the proton is transferred directly from the H3O+ unit of the aquated proton to the substrate, these are sufficient to successfully predict the rate at all intermediate isotopic compositions.
Entropies and heat capacities of free radicals
Authors: O'Neal H. E. , Benson S. W. 
Published: January 2005

AbstractMethods are presented for rapidly estimating the entropies and heat capacities of free radicals from the known S0 and C of structurally similar compounds. The methods consist of estimating the differences due to changes in mass, vibration frequencies, spin, symmetry, and changes in rotational barriers. Tables of contributions to S0 and C by different frequencies over the temperature range 300–1500°K are presented to facilitate the tabulation of the above differences. Conjugated radicals, such as benzyl and allyl, are included. It is shown that the greatest uncertainties in the estimates arise from uncertainties in the barriers to rotation in the radicals.The results are applied to kinetic data on the pyrolysis of branched hydrocarbons and the reverse reactions of radical recombination. Major discrepancies exist in these data which can be nearly reconciled by postulating improbably high rotational barriers of 8 kcal for CH3 rotation in isopropyl and t‐butyl radicals.It is shown that radical thermochemistry can be fitted into group schemes and tables of groups values are given for the rapid estimation of ΔH, S0, and C for different organic radicals, including those containing sulfur, oxygen, and nitrogen.
The addition of methyl radicals to tetrafluoroethylene
Authors: Sangster J. M. , Thynne J. C. J. 
Published: January 2005

AbstractThe addition of methyl radicals to tetrafluoroethylene in the gas phase has been studied over the temperature range 80–180°C, using a material balance method.

Arrhenius parameters of 1011.95±0.23 (mole−1 cm3 sec−1) and 5.7 ± 0.4 (kcal/mole) have been measured for the addition reaction.Electrophilic reagents such as O or CF3 appear to react almost equally readily with ethylene and tetrafluoroethylene but methyl radicals add much more rapidly to tetrafluoroethylene than to ethylene, the difference in reactivity being principally due to an activation energy difference of ∼2 kcal/mole.
Kinetic study of the gas‐phase reaction c‐C5H10 + I2 ⇄ c‐C5H8 + 2HI the heat of formation of cyclopentyl radical
Authors: Furuyama Shozo , Golden David M. , Benson Sidney W. 
Published: January 2005

AbstractThe kinetics of the gas‐phase dehydrogenation of cyclopentane to cyclopentene is found to be consistent with a slow attack by an I atom (step 4, text) on cyclopentane in the range 282–382°C. The measured rate constants fit the Arrhenius equation, log k4 = 11.95 ± 0.08 – (24.9 ± 0.23)/θ 1 mole−1 sec−1, where θ = 2.303RT in kcal/mole. This leads to a value of ΔH magnified image = 24.3 ± 1 kcal/mole and a bond dissociation energy DHmagnified image = 94.9 ± 1 kcal/mole. The latter value is identical with DH0(i‐Pr‐H) = 95 ± 1 kcal/mole and signifies that cyclopentane and the cyclopentyl radical have the same strain energy. Arrhenius parameters are deduced for all six steps in the reaction mechanism. Surface reactions are shown to be unimportant.Cyclopentyl iodide is an unstable intermediate in the reaction and the rate constant for its bimolecular formation from HI + cyclopentene is found to be log k6 = 8.40 ± 0.29 ‐ (26.9 ± 0.8)/θ 1 mole−1 sec−1. Together with the equilibrium constant, this yields for the unimolecular elimination of HI from cyclopentyl iodide, the rate constant, log k5 = 13.3 ± 0.3 – (42.8 ± 1.2)/θ sec−1.
Pyrolysis of 1,2‐dichloropropane
Authors: Martens G. J. , Godfroid M. , Ramoisy L. 
Published: January 2005

AbstractThe thermal decomposition of 1,2‐dichloropropane at atmospheric pressure has been studied in the temperature range 227–590°C, in a flow system. Above 450°C, the reaction is homogenous and unimolecular with a rate constant:

Below 450°C, a low activation energy, probably heterogenous process competes with the gas phase reactionThe primary reaction products are HCl and the monochloropropene isomers; the relative amounts of each isomer depend on the temperature in the low but not in the high temperature region. The direction of the HCl elimination is discussed in terms of substituent effects at the α‐ and β‐carbon positions and compared with literature data on similar reactionsSecondary products are formed principally by further pyrolysis of allyl chloride. The first‐order rate constant of this reaction is given by:

.
A simple, self‐consistent electrostatic model for quantitative prediction of the activation energies of four‐center reactions. II.*
Authors: Haugen G. R. , Benson S. W. 
Published: January 2005

AbstractA simple electrostatic model of point dipoles is used which permits direct calculation of the activation energies for the addition of the molecules H2O, H2S, H3N, and H3P to olefins. These calculated values agree with the known experimental data to within ±2 kcal/mole on the average. It was found that the best fit could be obtained with a polar transition state that corresponded to a reduction in bond order from 1 to ½ for the bond‐breaking coordinates and an increase in bond order from 0 to 0.18 for the bond‐forming coordinates. The replacement of a hydrogen atom of the species H2O, H2S, H3N, or H3P by a polarizable methyl group is expected to stabilize the charge on the central atoms. The following stabilization energies for the pairs H2OCH3OH, H2SCH3SH, H3NCH3NH2, H3PCH3PH2 were calculated: −4.8 kcal/mole, −0.7 kcal/mole, −1.9 kcal/mole, −0.8 kcal/mole, respectively.
A thermochemical and kinetic analysis of the mechanisms of pyrolysis of polycyclic molecules
Authors: O'Neal H. E. , Benson Sidney W. 
Published: January 2005

AbstractThe quantitative kinetics of pyrolysis of some cyclic and polycyclic compounds are examined from the point of view of biradical intermediates. Transition state methods for estimating the Arrhenius parameters of the lowest free energy biradical pathway to products in cyclic and polycyclic compound reactions are described and illustrated. A large number of the polycyclic reactions are found to have Arrhenius parameters consistent with the biradical mechanism estimates. Other reactions are found to have much faster experimental rates and must therefore be concerted. The value of activation energy and activation entropy estimates as discriminatory tests of mechanism, i.e., single step (concertedness) or consecutive step processes, is discussed.
The reaction of the hydroxyl radical with acetylene
Authors: Breen J. E. , Glass G. P. 
Published: January 2005

AbstractThe reaction of OH with acetylene was studied in a discharge flow system at room temperature. OH was generated by the reaction of atomic hydrogen with NO2and was monitored throughout the reaction using ESR spectroscopy. Mass‐spectrometric analysis of the reaction products yielded the following results: (1) less than 3 molecules of OH were consumed, and less than 2 molecules of H2O were formed for every molecule of acetylene that reacted; (2) CO was identified as the major carbon‐containing product; (3) NO, formed in the generation of OH, reacted with a reaction intermediate to give among other products N2O. These observations placed severe limitations on the choice of a reaction mechanism. A mechanism containing the reaction OH + C2H2→ HC2O + H2better accounted for the experimental results than one involving the abstraction reaction OH + C2H2→ C2H + H2O. The rate constant for the initial reaction was measured as 1.9 ± 0.6 × 10−13cm3molecule−1sec−1.
Orientation of addition of hydrogen atoms to propylene, butene‐1, and isobutene
Authors: Falconer W. E. , Sunder W. A. 
Published: January 2005

AbstractThe orientation of addition of H atoms to the asymmetric olefins propylene, butene‐1, and isobutene has been determined as a function of atom concentration, olefin concentration, hydrogen pressure, total pressure, and olefin conversion. Conditions have been determined for which complicating secondary processes are believed to be unimportant. The percentage of nonterminal addition is 5.7, 5.7, and 0.48 for propylene, butene‐1, and isobutene, respectively. From these data activation energy differences between nonterminal and terminal addition of 1.7 and 3.2 kcal/mole for the linear olefins and isobutene, respectively, may be calculated. For D atom addition to propylene, a small isotope effect was observed, nonterminal addition being 5.4%. These observations are consistent with a predominantly free radical or electroneutral character for H atoms.
A kinetic study of the thermal elimination of hydrogen fluoride from 1,2‐difluoroethane. Determination of the bond dissociation energies D(CH2FCH2F) and D(CH2FH).
Authors: Kerr J. A. , Timlin D. M. 
Published: January 2005

AbstractThe kinetics of the thermal elimination of HF from 1,2‐difluoroethane

have been studied in a static system over the temperature range 734–820°K. The reaction was shown to be first order and homogeneous, with a rate constant of

where θ = 2.303RT in kcal/mole. The A‐factor falls within the normal range for such reactions and is in line with transition state theory; the activation energy is similarly consistent with an estimate based on data for the analogous reactions of ethyl fluoride and other alkyl halides. The above activation energy has been compared with values of the critical energy calculated from data on the decomposition of chemically activated 1,2‐difluoroethane by the RRKM theory and the bond dissociation energy, D(CH2FCH2F) = 88 ± 2 kcal/mole, derived. It follows from thermochemistry that ΔHf0(CH2F) = ‐7.8 and D(CH2FH) = 101 ± 2 kcal/mole. Bond dissociation energies in fluoromethanes and fluoroethanes are discussed.
Iron (II) and iron (III) isotope exchange in water–dimethyl sulfoxide mixtures
Authors: Henry Marian S. , Reynolds Warren L. 
Published: January 2005

AbstractThe rate of the isotope exchange reaction between iron(II) and iron(III) perchlorates has been measured in a solvent mixture containing a 3:2 mole ratio of water to dimethyl sulfoxide over the temperature range from 25° to −98°C. In this temperature range, the reactants can diffuse together faster than they can undergo isotope exchange. The activation enthalpy and entropy for the acid‐independent reaction were 6.0 ± 1.2 kcal/mole and −38 ± 17 cal/deg mole, respectively. Below −22°C, the acid‐dependent exchange reaction did not contribute significantly to the exchange. In liquid media at −112° and −117°C and in a solid glass at −136°C, no isotope exchange was observed over the period of a calculated half‐life for the reaction. At these temperatures, the rate at which the reactants diffuse together is slower than the calculated rate of isotope exchange. In a solid glass at −196°C, no isotope exchange was observed over the period of one week.
A study of the intersystem crossing reaction induced in gaseous sulfur dioxide molecules by collisions with nitrogen and cyclohexane at 27°C
Authors: Horowitz Abraham , Calvert Jack G. 
Published: January 2005

AbstractThe quantum yields of the sulfur dioxide triplet (3SO2)‐sensitized phosphorescence of biacetyl (Φsens) were determined in experiments with N2–SO2–Ac2 and c‐C6H12–SO2–Ac2 mixtures excited at 2875 Å at 27°C. The fraction of the biacetyl triplets which reacts homogeneously by radiative or nonradiative decay reactions was determined in a series of runs at constant [SO2]/[M] and [SO2]/[Ac2] ratios but at varied total pressure. A kinetic treatment of the Φsens results and singlet sulfur dioxide (1SO2) quenching rate constant data gave the following new kinetic estimates: 1SO2 + M → (SO2–M) (1b) 1SO2 + M → 3SO2 + M (2b); for 1SO2–N2 collisions, k2b/(k1b + k2b) = 0.033 ± 0.008; for 1SO2–c‐C6H12 collisions, k2b/(k1b ± k2b) = 0.073 ± 0.024; previous studies have shown this ratio to be 0.095 ± 0.005 for 1SO2–SO2 collisions. It was concluded that the inter‐system crossing ratio in 1SO2 induced by collision is relatively insensitive to the nature of the collision partner M. However, the individual rate constants for the collision‐induced spin inversion of 1SO2 (k2b) and the total 1SO2‐quenching constants (k1b + k2b) are quite sensitive to the nature of M: k2b/k2a varies from 0.10 ± 0.03 for M = N2 to 1.11 ± 0.37 for M = c‐C6H12, and (k1b + k2b)/(k1a + k2a) varies from 0.29 for M = N2 to 1.44 for M = c‐C6H12; k1a and k1b are the rate constants for the reactions 1SO2 ‐ SO2 → (2SO2) (1a) and 1SO2 + SO2 → 3SO2 + SO2 (2a), respectively.
A comparative kinetic study of the N‐Nitrosation of sulfanilamide by cyclohexyl nitrite and by nitrous acid
Authors: Shenton A. J. , Johnson R. M. 
Published: January 2005

AbstractA comparative study of the kinetics of the N‐nitrosation of sulfanilamide by cyclohexyl nitrite and by nitrous acid has been made, using an automated technique. The results indicate that a common nitrosating species is effective in analogous reactions involving cyclohexyl nitrite or nitrous acid. The N‐nitrosation reaction has been followed spectrophotometrically by coupling the diazotized product with 1‐naphthol. Under certain conditions a competitive reaction takes place in which coupling occurs with unreacted sulfanilamide.
The reaction of C2F5 radicals with H2S
Authors: Bassett J. E. , Whittle E. 
Published: January 2005

AbstractThe reaction of C2F5 radicals with H2S

was studied over the range 1°−123°C using C2F5 radicals generated by photolysis of perfluoropropionic anhydride. The rate constant kH for reaction (2) is given by

where θ = 2.303RT/cal mole−1. The relevance of this result to conflicting published data on the analogous reaction between CF3 radicals and H2S is discussed. It is concluded that there is little difference in the Arrhenius parameters for reaction of CF3 and C2F5 radicals with H2S.
The radiolysis of mixtures of carbon dioxide and hydrogen
Authors: Beattie W. H. 
Published: January 2005

AbstractThe self‐radiolysis of CO2 in excess tritium (3H2) has been studied at pressures of 0.1 to 1.0 atm, temperatures of −80° to +100°C, and in the presence of added H2O, He, or Ar. The primary products of decomposition are CO and 3H2O. Secondary products are C3H4, C23H4, and a white polymer. The rates of disappearance of CO2 and formation of products and G‐;values were measured. The disappearance of CO2 initially obeys first‐order kinetics, then slows down with time at a rate depending upon the initial pressure of 3H2. The initial rates are proportional to pressures of CO2 and 3H2. They are independent of temperature, decreased by addition of H2O vapor, and increased by addition of He or Ar. The proposed mechanism of decomposition of CO2 and formation of products involves ionization of CO2 followed by dissociative recombination forming CO and O. Then the O reacts with a hydrogen‐containing species forming OH and H2O, and a back reaction forms CO2 from CO and OH.
The reaction of OH with NO2 and the deactivation of O(1D) by CO
Authors: Simonaitis R. , Heicklen Julian 
Published: January 2005

AbstractNO2 was photolyzed with 2288 Å radiation at 300° and 423°K in the presence of H2O, CO, and in some cases excess He. The photolysis produces O(1D) atoms which react with H2O to give HO radicals

or are deactivated by CO to O(3P) atoms

The ratio k5/k3 is temperature dependent, being 0.33 at 300°K and 0.60 at 423°K. From these two points, the Arrhenius expression is estimated to be k5/k3 = 2.6 exp(−1200/RT) where R is in cal/mole – °K. The OH radical is either removed by NO2

or reacts with CO

The ratio k2/kα is 0.019 at 300°K and 0.027 at 423°K, and the ratio k2/k0 is 1.65 × 10−5M at 300°K and 2.84 × 10−5M at 423°K, with H2O as the chaperone gas, where kα = k1 in the high‐pressure limit and k0[M] = k1 in the low‐pressure limit. When combined with the value of k2 = 4.2 × 108 exp(−1100/RT) M−1sec−1, kα = 6.3 × 109 exp (−340/RT)M−1sec−1 and k0 = 4.0 × 1012M−2sec−1, independent of temperature for H2O as the chaperone gas. He is about 1/8 as efficient as H2O.