Experimental and theoretical elastic cross sections for electron collisions with the C3H6 isomers

In the present work we report cross sections for electron collisions with the isomers propene (C3H6) and cyclopropane (c-C3H6). Electron-scattering differential cross sections (DCS) are reported for measurements carried out for energies 1.5-100 eV and the angular range of 20 degrees-120 degrees. Elastic integral cross sections (ECS), DCS, and momentum-transfer cross sections (MTCS) are reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range of 2.0-40 eV and angular range of 0 degrees-180 degrees. The resemblance of the pi* shape resonance in the cross sections, observed at 1.5-2.0 eV for propene, to those in C2H4 and C2F4 clearly points to the effect of the double bond in the molecular structures for these molecules. Below 60 eV, we observed clear differences in peak positions and magnitudes between the DCS, ECS, and MTCS for C3H6 and c-C3H6, which we view as the isomer effect.     

Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M

The rate constant of the reaction NCN + O has been directly measured for the first time. According to the revised Fenimore mechanism, which is initiated by the NCN forming reaction CH + N(2)→ NCN + H, this reaction plays a key role for prompt NO(x) formation in flames. NCN radicals and O atoms have been quantitatively generated by the pyrolysis of NCN(3) and N(2)O, respectively. NCN concentration-time profiles have been monitored behind shock waves using narrow-bandwidth laser absorption at a wavelength of λ = 329.1302 nm. Whereas no pressure dependence was discernible at pressures between 709 mbar < p < 1861 mbar, a barely significant temperature dependence corresponding to an activation energy of 5.8 ± 6.0 kJ mol(-1) was found. Overall, at temperatures of 1826 K < T < 2783 K, the rate constant can be expressed as k(NCN + O) = 9.6 × 10(13)× exp(-5.8 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (±40%). As a requirement for accurate high temperature rate constant measurements, a consistent NCN background mechanism has been derived from pyrolysis experiments of pure NCN(3)/Ar gas mixtures, beforehand. Presumably, the bimolecular secondary reaction NCN + NCN yields CN radicals hence triggering a chain reaction cycle that efficiently removes NCN. A temperature independent value of k(NCN + NCN) = (3.7 ± 1.5) × 10(12) cm(3) mol(-1) s(-1) has been determined from measurements at pressures ranging from 143 mbar to 1884 mbar and temperatures ranging from 966 K to 1900 K. At higher temperatures, the unimolecular decomposition of NCN, NCN + M → C + N(2) + M, prevails. Measurements at temperatures of 2012 K < T < 3248 K and at total pressures of 703 mbar < p < 2204 mbar reveal a unimolecular decomposition close to its low pressure limit. The corresponding rate constants can be expressed as k(NCN + M) = 8.9 × 10(14)× exp(-260 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(±20%).     

A comparative study of some oxygen and sulphur substituted alkyl radicals

Several sulphur containing alkyl radicals have been generated and their ESR spectra and hyperfine coupling constants compared with those of the oxygen analogues. Deviations from planarity are smaller when a tervalent carbon atom is bonded to sulphur than when bonded to oxygen. Moreover greater delocalization of the unpaired electron is apparent in the sulphur radicals and this leads to higher barriers to internal rotation in radicals of the form H₂SR compared with H₂OR.     

Evaluation of the viscosity cross sections for elastic electron-atom collisions in krypton and xenon at low electron energies

The results of viscosity cross sections evaluations for elastic electron-atom scattering from rare gases krypton and xenon, carried out in the energy interval from 0.1 to 54.4 eV are presented and discussed. The evaluations are based on four independent sets of partialwave phaseshifts, one theoretical and three experimental, taken from literature. An analytical estimate for the contribution of all the terms not included in the summation, for truncation at any chosen value of the electron angular momentum, is derived. The validity of assumption of isotropic angular scattering is investigated in the energy range considered. The present results invariably show significant deviations from the values calculated with this assumption.     

Close-coupling calculations of low-energy inelastic and elastic processes in 4He collisions with H2: A comparative study of two potential energy surfaces

The two most recently published potential energy surfaces (PESs) for the HeH2 complex, the so-called MR (Muchnick and Russek) and BMP (Boothroyd, Martin, and Peterson) surfaces, are quantitatively evaluated and compared through the investigation of atom-diatom collision processes. The BMP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the PES compared to that of the MR surface. We found significant differences in inelastic rovibrational cross sections computed on the two surfaces for processes dominated by large changes in target rotational angular momentum. In particular, the H2(nu=1,j=0) total quenching cross section computed using the BMP potential was found to be a factor of 1000 larger than that obtained with the MR surface. A lesser discrepancy persists over a large range of energies from the ultracold to thermal and occurs for other low-lying initial rovibrational levels. The MR surface was used in previous calculations of the H2(nu=1,j=0) quenching rate coefficient and gave results in close agreement with the experimental data of Audibert et al. which were obtained for temperatures between 50 and 300 K. Examination of the rovibronic coupling matrix elements, which are obtained following a Legendre expansion of the PES, suggests that the magnitude of the anisotropy of the BMP potential is too large in the interaction region. However, cross sections for elastic and pure rotational processes obtained from the two PESs differ typically by less than a factor of 2. The small differences may be ascribed to the long-range and anharmonic components of the PESs. Exceptions occur for (nu=10,j=0) and (nu=11,j=1) where significant enhancements have been found for the low-energy quenching and elastic cross sections due to zero-energy resonances in the BMP PES which are not present in the MR potential.     

A consistent model for the thermal decomposition of NCN3 and the singlet– triplet relaxation of NCN

The thermal decomposition of cyanogen azide (NCN₃) and the subsequent collision‐induced intersystem crossing (CIISC) process of cyanonitrene (NCN) have been investigated by monitoring excited electronic state ¹NCN and ground state ³NCN radicals. NCN was generated by the pyrolysis of NCN₃ behind shock waves and by the photolysis of NCN₃ at room temperature. Falloff rate constants of the thermal unimolecular decomposition of NCN₃ in argon have been extracted from ¹NCN concentration–time profiles in the temperature range 617 K <T< 927 K and at two different total densities: k(ρ ≈ 3 × 10^?6 mol/cm³)/s^?1=4.9 × 10⁹ × exp (?71±14 kJ mol^?1/RT) (± 30%); k(ρ ≈ 6 × 10^?6 mol/cm³)/s^?1=7.5 × 10⁹ × exp (‐71±14 kJ mol^?1/RT) (± 30%). In addition, high‐temperature ¹NCN absorption cross sections have been determined in the temperature range 618 K <T< 1231 K and can be expressed by σ /(cm²/mol)= 1.0 × 10⁸ ?6.3 × 10⁴ K^?1 × T (± 50%). Rate constants for the CIISC process have been measured by monitoring ³NCN in the temperature range 701 K <T< 1256 K resulting in k_CIISC (ρ ≈ 1.8 ×10^?6 mol/cm³)/ s^?1=2.6 × 10⁶× exp (‐36±10 kJ mol^?1/RT) (± 20%), k_CIISC (ρ ≈ 3.5×10^?6 mol/cm³)/ s^?1 = 2.0 × 10⁶ × exp (?31±10 kJ mol^?1/RT) (± 20%), k_CIISC (ρ ≈ 7.0×10^?6 mol/cm³)/ s^?1=1.4 × 10⁶ × exp (?25±10 kJ mol^?1/RT) (± 20%). These values are in good agreement with CIISC rate constants extracted from corresponding ¹NCN measurements. The observed nonlinear pressure dependences reveal a pressure saturation effect of the CIISC process. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 30–40, 2013     

Relocalization in floppy free radicals: the OCNO and OCCHO isoelectronic series.

A survey of the XCNY and XCCHY radicals with X and Y = CH(2), NH, and O has been carried out by ab initio QCISD/6-311G(d,p) calculations to assess the impact of low-lying excited electronic states on the molecular dynamics. Multiple canonical structures may be drawn for each of these structural formulas, with the principal competition for most stable configuration between a (2)A' form with four electrons in a" orbitals and a (2)A" form with five a" electrons. Other low-lying configurations may include a 5a" state with nominally pentavalent nitrogen and a 6a" state. Optimized geometries and harmonic frequencies were evaluated for the lowest-energy minima on the potential energy surfaces. Localized unpaired electron density causes the 4a" state to be the most stable for (NH)CCHO and OCCHY, whereas allylic resonance stabilization favors the 5a" state for all other radicals in the set. For five of the 18 molecules studied, secondary minima (excluding conformers) are found within 30 kJ mol(-)(1) of the most stable state at the QCISD/6-311G(d,p) level, suggesting that photolysis or pyrolysis of parent compounds may result in multiple isomers of the resulting reactive intermediates. Predicted equilibrium geometries, approximate thermochemical quantities for dissociation of the central bond, and selected spectroscopic parameters are presented for all 18 structural formulas. Convergence tests were also performed for the glyoxallyl radical (OCCHO) to resolve discrepancies between single- and multireference post-SCF results. These tests find that extension of the MCSCF methods to include sigma bonding orbitals or virtual-orbital CI brings MCSCF relative energies into agreement with results from standard single-reference CI and CC methods. Relative configurational energies evaluated at Hartree-Fock levels routinely differ from post-SCF values by 30 kJ mol(-1) or more.     

Differential cross sections for elastic scattering and electron detachment in collisions of halogen anions with noble gas atoms

Electron detachment in collision of halogen anions and noble gas atoms was investigated in the collision energy range of 500-3000 eV. Differential cross sections were determined both for elastic scattering and for the electron detachment process in the angular range of 0-3°. It was found that direct electron detachment, due to the interaction of the discrete ion-atom state with a continuum of states increases with increasing collisional energy. Furthermore, the experimental evidence indicates that direct detachment primarily occurs in a limited range of impact parameters. A model, based on angular coupling, is described which qualitatively explains these two experimentally observed features.     

Reactivities of hydroxyl and perhydroxyl radicals toward cytosine and thymine: A comparative study

As the hydroxyl (OH) and perhydroxyl (OOH) radicals are known to play important roles in biological systems, their reactions with cytosine and thymine were studied. Addition reactions of these radicals at different sites of cytosine and thymine, and hydrogen abstraction reactions by each of the two radicals from the different sites of the two molecules were studied at the B3LYP/6‐31G(d,p), B3LYP/AUG‐cc‐pVDZ and BHandHLYP/AUG‐cc‐pVDZ levels of density functional theory. Effect of solvation in aqueous media on the reactions was studied at all these levels of theory using single point energy calculations using the polarizable continuum model. The present study shows that whereas the OH radical would abstract H atoms from the various sites of cytosine and thymine efficiently, the OOH radical would have poor reactivity in this regard. The OH radical is also predicted to be much more reactive than the OOH radical with regard to addition reactions at the C5 and C6 sites of both thymine and cytosine, though the OOH radical is also predicted to have significant reactivity in this respect. © 2012 Wiley Periodicals, Inc.     

Cidep in radicals undergoing electron transfer.A time-resolved flash photolysis ESR study

Unexpected effects in the time-resolved ESR signals from radicals exhibiting chemically induced dynamic electron polarization (CIDEP) are explained quantitatively by considering an electron-transfer reaction.     

G-tensors of the flavin adenine dinucleotide radicals in glucose oxidase: a comparative multifrequency electron paramagnetic resonance and electron-nuclear double resonance study

The flavin adenine dinucleotide (FAD) cofactor of Aspergillus niger glucose oxidase (GO) in its anionic (FAD*-) and neutral (FADH*) radical form was investigated by electron paramagnetic resonance (EPR) at high microwave frequencies (93.9 and 360 GHz) and correspondingly high magnetic fields and by pulsed electron-nuclear double resonance (ENDOR) spectroscopy at 9.7 GHz. Because of the high spectral resolution of the frozen-solution continuous-wave EPR spectrum recorded at 360 GHz, the anisotropy of the g-tensor of FAD*- could be fully resolved. By least-squares fittings of spectral simulations to experimental data, the principal values of g have been established with high precision: gX=2.00429(3), gY=2.00389(3), gZ=2.00216(3) (X, Y, and Z are the principal axes of g) yielding giso=2.00345(3). The gY-component of FAD*- from GO is moderately shifted upon deprotonation of FADH*, rendering the g-tensor of FAD*- slightly more axially symmetric as compared to that of FADH*. In contrast, significantly altered proton hyperfine couplings were observed by ENDOR upon transforming the neutral FADH* radical into the anionic FAD*- radical by pH titration of GO. That the g-principal values of both protonation forms remain largely identical demonstrates the robustness of g against local changes in the electron-spin density distribution of flavins. Thus, in flavins, the g-tensor reflects more global changes in the electronic structure and, therefore, appears to be ideally suited to identify chemically different flavin radicals.     

Electron and positron elastic scattering in gaseous and liquid water: A comparative study

In the present work, we present both theoretical differential and total cross sections for the elastic scattering process of positrons and electrons in liquid and vapour water for energies ranging from 10 eV to 10 keV. The calculations are performed in the partial-wave formalism by means of a complex interaction potential taking into account static potential as well as fine effects like exchange and polarization contributions. The theoretical results obtained in this free-parameter quantum-mechanical treatment are compared to available experimental data and good agreement is generally observed. Moreover, quantitative differences are reported between the positron and electron scattering, in vapour as well as in liquid water.     

A comparative study on the experimentally derived electron densities of three protease inhibitor model compounds

In order to contribute to a rational design of optimised protease inhibitors which can covalently block the nucleophilic amino acids of the proteases' active sites, we have chosen three model compounds (aziridine , oxirane and acceptor-substituted olefin ) for the examination of their electron-density distribution. Therefore, high-resolution low temperature (9, 27 and 100 K) X-ray diffraction experiments on single-crystals were carried out with synchrotron and conventional X-radiation. It could be shown by the analysis of the electron density using mainly Bader's Theory of Atoms in Molecules, Volkov's EPMM method for interaction energies, electrostatic potentials and Gatti's Source Function that aziridine is most suitable for drug design in this field. A regioselective nucleophilic attack at carbon atom C1 could be predicted and even hints about the reaction's stereoselectivity could be obtained. Moreover, the comparison between two data sets of aziridine (conventional X-ray source vs. synchrotron radiation) gave an estimate concerning the reproducibility of the quantitative results.     

A simple approach to treat anisotropic elastic collisions in Monte Carlo calculations of the electron energy distribution function in cold plasmas

A simplified method allowing one to treat anisotropic electron heavy species elastic scattering in MonteCarlo models of gas discharges with the proper value for collision frequency is proposed The method is applied to an electric discharge in a Ne · Xe/HCl mixture, and the results are compared with the solution of the two-term expansion of the Boltzmann equation under the same conditions. Methods for reduction of computational time in Monte Carlo codes and the use of the Monte Carlo flux method are also discussed.     

A comparative study of the structure and bonding of HOO,HOS, HSO,and HSS radicals by CNDO/2 and INDO methods

Equilibrium geometries, force constants, barriers to linearity, charge distributions, dipole moments, and electron spin density of HOO, HOS, HSO, and HSS radicals are calculated by CNDO/2 and INDO methods using respectively the original and some recently introduced scheme of parametrization. Three sets of calculations, namely, CNDO/2(sp), CNDO/2(spd), and INDO, are performed, and the results are compared with the ab initio and experimental values, wherever available. A good agreement is obtained for geometry in the case of CNDO/2 (sp) and INDO calculations. The performance of CNDO/2 (spd) calculations in this regard is quite unreliable. The stretching force constants are considerably overestimated by all the methods, while the bending force constants are in reasonable agreement with the ab initio values. With respect to dipole moments, the CNDO/2 values are in better agreement with the ab initio results than the INDO values. In all the cases, the dipole moment vector directions are in complete disagreement with the ab initio predictions.     

Assessing alkyl-, silyl-, and halo-substituent effects on the electron affinities of silyl radicals

Neutral anion energy differences for a large class of alpha-substituted silyl radicals have been computed to determine the effect of alkyl, silyl, and halo substituents on their electron affinities. In particular, we report theoretical predictions of the adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) for a series of methyl-, silyl-, and halo-substituted silyl radical compounds. This work utilizes the carefully calibrated DZP++ basis set, in conjunction with the pure BLYP and OLYP functionals, as well as with the hybrid B3LYP, BHLYP, PBE1PBE, MPW1K, and O3LYP functionals. Bromine has the largest effect in stabilizing the anions, and the BLYP/DZP++ AEA for SiBr(3) is 3.29 eV. The other predicted electron affinities are for SiH(3) (1.37 eV), SiH(2)CH(3) (1.09 eV), SiH(2)F (1.54 eV), SiH(2)Cl (1.94 eV), SiH(2)Br (2.05 eV), SiH(2)(SiH(3)) (1.77 eV), SiH(CH(3))(2) (0.92 eV), SiHF(2) (1.86 eV), SiHCl(2) (2.53 eV), SiHBr(2) (2.67 eV), Si(CH(3))(3) (0.86 eV), SiF(3) (2.66 eV), SiCl(3) (3.21 eV), Si(SiH(3))(3) (2.25 eV), and SiFClBr (3.13 eV). For the five silyl radicals where experimental data are available, the BLYP functional gives the most accurate determination of AEAs; the average absolute error is 0.04(1) eV, whereas the corresponding errors for the O3LYP, MPW1K, PBE1PBE, B3LYP, OLYP, and BHLYP functionals are 0.05(8), 0.06(0), 0.06(3), 0.08(5), 0.11(5), and 0.15(3) eV, respectively.     

Theoretical study on the isomerization behavior between alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals

[reaction: see text] Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis set, with (QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional methods (BHandHLYP, B3LYP) predict that alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals exist as isomers. At the CCSD(T)/cc-pVDZ//BHandHLY/cc-pVDZ level of theory, energy barriers of 15.1 and 17.7-21.7 kJ mol(-)(1) are calculated for the isomerization of s-trans-propenoyl and s-trans-crotonoyl radical to ketenylmethyl and 1-ketenylethyl radical, respectively. Similar results are obtained for the reactions of s-trans isomers involving silyl, germyl, and stannyl groups with energy barriers (DeltaE++) of 12.2-12.4, 13.1-13.9, and 12.9-18.2 kJ mol(-)(1) at the CCSD(T)/DZP//BHandHLYP/DZP calculation, respectively. These results suggest that alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals are not canonical forms but are isomeric species that can rapidly interconvert.     

Substituent effect on electron affinity,gas‐phase basicity,and structure of monosubstituted propynyl radicals and their anions: A theoretical study

The substituent effect of electron‐withdrawing groups on electron affinity and gas‐phase basicity has been investigated for substituted propynl radicals and their corresponding anions. It is shown that when a hydrogen of the α‐CH₃ group in the propynyl system is substituted by an electron‐withdrawing substituent, electron affinity increases, whereas gas‐phase basicity decreases. These results can be explained in terms of the natural atomic charge of the terminal acetylene carbon of the systems. The calculated electron affinities are 3.28 eV (?C?C? CH₂F), 3.59 eV (?C?C? CH₂Cl) and 3.73 eV (?C?C? CH₂Br), and the gas‐phase basicities of their anions are 359.5 kcal/mol (^?:C?C? CH₂F), 354.8 kcal/mol (:C?C? CH₂Cl) and 351.3 kcal/mol (^?:C?C? CH₂Br). It is concluded that the larger the magnitude of electron‐withdrawing, the greater is the electron affinity of radical and the smaller is the gas‐phase basicity of its anion. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009