On the relation between the activation energy for electron attachment reactions and the size of their thermal rate coefficients

Rate coefficients k(T) for dissociative electron attachment (DEA) to molecules in many cases exhibit a more or less strong rise with increasing temperature T (the electron temperature T(e) and the molecular temperature T(G) are assumed to be in thermal equilibrium, i.e., T = T(e) = T(G)). This rise is frequently modeled by the Arrhenius equation k(T) = k(A) exp[-E(a)∕(k(B)T)], and an activation energy E(a) is deduced from fits to the experimental data k(T). This behavior reflects the presence of an energy barrier for the anion on its path to the dissociated products. In a recent paper [J. Kopyra, J. Wnorowska, M. Forys?, and I. Szamrej, Int. J. Mass Spectrom. 268, 60 (2007)] it was suggested that the size of the rate coefficients for DEA reactions at room temperature exhibits an exponential dependence on the activation energy, i.e., k(E(a); T ≈ 300 K) = k(1) exp[-E(a)∕E(0)]. More recent experimental data for molecules with high barriers [T. M. Miller, J. F. Friedman, L. C. Schaffer, and A. A. Viggiano, J. Chem. Phys. 131, 084302 (2009)] are compatible with such a correlation. We investigate the validity and the possible origin of this dependence by analyzing the results of R-matrix calculations for temperature-dependent rate coefficients of exothermic DEA processes with intermediate barrier toward dissociation. These include results for model systems with systematically varied barrier height as well as results of molecule-specific calculations for CH(3)Cl, CH(3)Br, CF(3)Cl, and CH(2)Cl(2) (activation energies above 0.2 eV) involving appropriate molecular parameters. A comparison of the experimental and theoretical results for the considered class of molecules (halogenated alkanes) supports the idea that the exponential dependence of k(T = 300 K) on the activation energy reflects a general phenomenon associated with Franck-Condon factors for getting from the initial neutral vibrational levels to the dissociating final anion state in a direct DEA process. Cases are discussed for which the proposed relation does not apply.     

Dependence of the energies of fusion on the intermembrane separation: optimal and constrained

We calculate the characteristic energies of fusion between planar bilayers as a function of the distance between them, measured from the hydrophobic/hydrophilic interface of one of the two nearest, cis, leaves to the other. The two leaves of each bilayer are of equal composition: 0.6 volume fraction of a lamellar-forming amphiphile, such as dioleoylphosphatidylcholine, and 0.4 volume fraction of a hexagonal-forming amphiphile, such as dioleoylphosphatidylethanolamine. Self-consistent field theory is employed to solve the model. We find that the largest barrier to fusion is that to create the metastable stalk. This barrier is the smallest, about 14.6k(B)T, when the bilayers are at a distance about 20% greater than the thickness of a single leaf, a distance which would correspond to between 2 and 3 nm for typical bilayers. The very size of the protein machinery which brings the membranes together can prevent them from reaching this optimum separation. For even modestly larger separations, we find a linear rate of increase of the free energy with distance between bilayers for the metastable stalk itself and for the barrier to the creation of this stalk. We estimate these rates for biological membranes to be about 7.1k(B)Tnm and 16.7 k(B)Tnm, respectively. The major contribution to this rate comes from the increased packing energy associated with the hydrophobic tails. From this we estimate, for the case of hemagglutinin, a free energy of 38k(B)T for the metastable stalk itself and a barrier to create it of 73 k(B)T. Such a large barrier would require that more than a single hemagglutinin molecule be involved in the fusion process, as is observed.     

Theory of multichannel thermal unimolecular reactions. 2. Application to the thermal dissociation of formaldehyde

The thermal dissociation of formaldehyde proceeds on three channels, the molecular-elimination channel H2CO --> H2 + CO (1), the radical-forming bond-fission channel H2CO --> H + HCO (2), and the bond-fission-initiated, intramolecular-hydrogen-abstraction channel H2CO --> H...HCO --> H2 + CO (3) which also forms molecular products. The kinetics of this system in the low-pressure range of the unimolecular reaction is shown to be governed by a subtle superposition of collisional channel coupling to be treated by solving a master equation, of rotational channel switching accessible through ab initio calculations of the potential as well as spectroscopic and photophysical determinations of the threshold energies and channel branching above the threshold energy for radical formation which can be characterized through formaldehyde photolysis quantum yields as well as classical trajectory calculations. On the basis of the available information, the rate coefficients for the formation of molecular and radical fragments are analyzed and extrapolated over wide ranges of conditions. The modeled rate coefficients in the low-pressure range of the reaction (neglecting tunneling) over the range 1400-3200 K in the bath-gas Ar in this way are represented by k0,Mol/[Ar] approximately 9.4 x 10(-9) exp(-33,140 K/T) cm3 molecule(-1) s(-1) and k0,Rad/[Ar] approximately 6.2 x 10(-9) exp(-36,980 K/T) cm3 molecule(-1) s(-1). The corresponding values for the bath-gas Kr, on which the analysis relies in particular, are k0,Mol/[Kr] approximately 7.7 x 10(-9) exp(-33,110 K/T) and k0,Rad/[Kr] approximately 4.1 x 10(-9) exp(-36 910 K/T) cm3 molecule(-1) s(-1). While the threshold energy E0,2 for channels 2 and 3 is taken from spectroscopic measurements, the threshold energy E0,1 for channel 1 is fitted on the basis of experimental ratios k0,Rad/k0,Mol in combination with photolysis quantum yields. The derived value of E0,1(1) = 81.2 (+/-0.9) kcal mol(-1) is in good agreement with results from recent ab initio calculations, 81.9 (+/-0.3) kcal mol(-1), but is higher than earlier results derived from photophysical experiments, 79.2 (+/-0.8) kcal mol(-1). Rate coefficients for the high-pressure limit of the reaction are also modeled. The results of the present work markedly depend on the branching ratio between channels 2 and 3. Expressions of this branching ratio from classical trajectory calculations and from photolysis quantum yield measurements were tested. At the same time, a modeling of the photolysis quantum yields was performed. The formaldehyde system so far presents the best characterized multichannel dissociation reaction. It may serve as a prototype for other multichannel dissociation reactions.     

Anisotropic interfacial free energies of the hard-sphere crystal-melt interfaces

We present a reliable method to define the interfacial particles for determining the crystal-melt interface position, which is the key step for the crystal-melt interfacial free energy calculations using capillary wave approach. Using this method, we have calculated the free energies gamma of the fcc crystal-melt interfaces for the hard-sphere system as a function of crystal orientations by examining the height fluctuations of the interface using Monte Carlo simulations. We find that the average interfacial free energy gamma(0) = 0.62 +/- 0.02k(B)T/sigma(2) and the anisotropy of the interfacial free energies are weak, gamma(100) = 0.64 +/- 0.02, gamma(110) = 0.62 +/- 0.02, gamma(111) = 0.61 +/- 0.02k(B)T/sigma(2). The results are in good agreement with previous simulation results based on the calculations of the reversible work required to create the interfaces (Davidchack and Laird, Phys. Rev. Lett. 2000, 85, 4571). In addition, our results indicate gamma(100) > gamma(110) > gamma(111) for the hard-sphere system, similar to the results of the Lennard-Jones system.     

Pair correlation functions and the self-diffusion coefficient of Lennard-Jones liquid in the modified free volume theory of diffusion

In this paper, we apply the Matteoli-Mansoori empirical formula for the pair correlation function of simple fluids obeying the Lennard-Jones potential to calculate reduced self-diffusion coefficients on the basis of the modified free volume theory. The self-diffusion coefficient thus computed as functions of temperature and density is compared with the molecular dynamics simulation data and the self-diffusion coefficient obtained by the modified free volume theory implemented with the Monte Carlo simulation method for the pair correlation function. We show that the Matteoli-Mansoori empirical formula yields sufficiently accurate self-diffusion coefficients in the supercritical regime, provided that the minimum free volume activating diffusion is estimated with the classical turning point of binary collision at the mean relative kinetic energy 3k(B)T/2, where k(B) is the Boltzmann constant and T is the temperature. In the subcritical regime, the empirical formula yields qualitatively correct, but lower values for the self-diffusion coefficients compared with computer simulation values and those from the modified free volume theory implemented with the Monte Carlo simulations for the pair correlation function. However, with a slightly modified critical free volume, the results can be made quite acceptable.     

Stability of supershort single-walled carbon nanotubes

How short can single-walled carbon nanotubes (SWNTs) be? How stable are such supershort SWNTs (ss-SWNTs)? This work is the first to address these questions. On the basis of binding energy (E(B)), standard heats of formation , and strain energy (E(S)), we found that SWNTs with only one benzene ring in the axial direction, which we refer to as supershort SWNTs (ss-SWNTs), can be thermodynamically stable. On the basis of the data of E(B), , and E(S), the relative stabilities of ss-SWNTs, fullerenes, polycyclic aromatic hydrocarbons, and butadiyne are discussed. This study has laid a theoretical foundation for the possible synthesis of ss-SWNTs.     

Quantum tunneling and quantum phase interference in a [Mn(II)2Mn(III)2] single-molecule magnet

[Mn4(hmp)6(H2O)2(NO3)2](NO3)2.2.5H2O (1) has been synthesized from the reaction of 2-hydroxymethylpyridine (Hhmp) with Mn(NO3)2.4H2O in the presence of tetraethylammonium hydroxide. 1 crystallizes in the triclinic P space group with two crystallographically independent centrosymmetrical [Mn4(hmp)6(H2O)2(NO3)2]2+ complexes in the packing structure. Four Mn ions are arranged in a double-cuboidal fashion where outer Mn2+ are heptacoordinated and inner Mn3+ are hexacoordinated. dc magnetic measurements show that both Mn2+...Mn3+ and Mn3+...Mn3+ interactions are ferromagnetic with J(wb)/k(B) = +0.80(5) K, and J(bb)/k(B) = +7.1(1) K, respectively, leading to an S(T) = 9 ground state. Combined ac and dc measurements reveal the single-molecule magnet (SMM) behavior of 1 with both thermally activated and ground-state tunneling regimes, including quantum phase interference. In the thermally activated regime, the characteristic relaxation time (tau) of the system follows an Arrhenius law with tau0 = 6.7 x 10(-)(9) s and delta(eff)/k(B) = 20.9 K. Below 0.34 K, tau saturates indicating that the quantum tunneling of the magnetization becomes the dominant relaxation process as expected for SMMs. Down to 0.04 K, field dependence of the magnetization measured using the mu-SQUID technique shows the presence of very weak inter-SMM interactions (zJ'/k(B) approximately -1.5 x 10(-3) K) and allows an estimation of D/k(B) at -0.35 K. Quantum phase interference has been used to confirm the D value and to estimate the transverse anisotropic parameter to E/k(B) = +0.083 K and the ground-state tunnel splitting delta(LZ) = 3 x 10(-7) K at H(trans) = 0 Oe. These results rationalize the observed tunneling time (tau(QTM)) and the effective energy barrier (delta(eff)).     

Ground- and triplet excited-state properties correlation: a computational CASSCF/CASPT2 approach based on the photodissociation of allylsilanes

Excited-state properties, although extremely useful, are hardly accessible. One indirect way would be to derive them from relationships to ground-state properties which are usually more readily available. Herewith, we present quantitative correlations between triplet excited-state (T?) properties (bond dissociation energy, D?(T?), homolytic activation energy, E(a)(T?), and rate constant, k(r)) and the ground-state bond dissociation energy (D?), taking as an example the photodissociation of the C-Si bond of simple substituted allylsilanes CH?=CHC(R1R2)-SiH? (R1 and R2 = H, Me, and Et). By applying the complete-active-space self-consistent field CASSCF(6,6) and CASPT2(6,6) quantum chemical methodologies, we have found that the consecutive introduction of Me/Et groups has little effect on the geometry and energy of the T? state; however, it reduces the magnitudes of D?, D?(T?) and E(a)(T?). Moreover, these energetic parameters have been plotted giving good linear correlations: D?(T?) = α? + β? · D?, E(a)(T?) = α? + β? · D?(T?), and E(a)(T?) = α? + β? · D? (α and β being constants), while k(r) correlates very well to E(a)(T?). The key factor behind these useful correlations is the validity of the Evans-Polanyi-Semenov relation (second equation) and its extended form (third equation) applied for excited systems. Additionally, the unexpectedly high values obtained for E(a)(T?) demonstrate a new application of the principle of nonperfect synchronization (PNS) in excited-state chemistry issues.     

Experimental and modeling studies of the pressure and temperature dependences of the kinetics and the OH yields in the acetyl + O2 reaction

The acetyl + O(2) reaction has been studied by observing the time dependence of OH by laser-induced fluorescence (LIF) and by electronic structure/master equation analysis. The experimental OH time profiles were analyzed to obtain the kinetics of the acetyl + O(2) reaction and the relative OH yields over the temperature range of 213-500 K in helium at pressures in the range of 5-600 Torr. More limited measurements were made in N(2) and for CD(3)CO + O(2). The relative OH yields were converted into absolute yields by assuming that the OH yield at zero pressure is unity. Electronic structure calculations of the stationary points of the potential energy surface were used with a master equation analysis to fit the experimental data in He using the high-pressure limiting rate coefficient for the reaction, k(∞)(T), and the energy transfer parameter, (ΔE(d)), as variable parameters. The best-fit parameters obtained are k(∞) = 6.2 × 10(-12) cm(-3) molecule(-1) s(-1), independent of temperature over the experimental range, and (ΔE(d))(He) = 160(T/298?K) cm(-1). The fits in N(2), using the same k(∞)(T), gave (ΔE(d))(N(2)) = 270(T/298?K) cm(-1). The rate coefficients for formation of OH and CH(3)C(O)O(2) are provided in parametrized form, based on modified Troe expressions, from the best-fit master equation calculations, over the pressure and temperature ranges of 1 ≤ p/Torr ≤ 1.5 × 10(5) and 200 ≤ T/K ≤ 1000 for He and N(2) as the bath gas. The minor channels, leading to HO(2) + CH(2)CO and CH(2)C(O)OOH, generally have yields <1% over this range.     

Variational analysis of the phenyl + O2 and phenoxy + O reactions

Variational transition state analysis was performed on the barrierless phenyl + O2 and phenoxy + O association reactions. In addition, we also calculated rate constants for the related vinyl radical (C2H3) + O2 and vinoxy radical (C2H3O) + O reactions and provided rate constant estimates for analogous reactions in substituted aromatic systems. Potential energy scans along the dissociating C-OO and CO-O bonds (with consideration of C-OO internal rotation) were obtained at the O3LYP/6-31G(d) density functional theory level. The CO-O and C-OO bond scission reactions were observed to be barrierless, in both phenyl and vinyl systems. Potential energy wells were scaled by G3B3 reaction enthalpies to obtain accurate activation enthalpies. Frequency calculations were performed for all reactants and products and at points along the potential energy surfaces, allowing us to evaluate thermochemical properties as a function of temperature according to the principles of statistical mechanics and the rigid rotor harmonic oscillator (RRHO) approximation. The low-frequency vibrational modes corresponding to R-OO internal rotation were omitted from the RRHO analysis and replaced with a hindered internal rotor analysis using O3LYP/6-31G(d) rotor potentials. Rate constants were calculated as a function of temperature (300-2000 K) and position from activation entropies and enthalpies, according to canonical transition state theory; these rate constants were minimized with respect to position to obtain variational rate constants as a function of temperature. For the phenyl + O2 reaction, we identified the transition state to be located at a C-OO bond length of between 2.56 and 2.16 A (300-2000 K), while for the phenoxy + O reaction, the transition state was located at a CO-O bond length of 2.00-1.90 A. Variational rate constants were fit to a three-parameter form of the Arrhenius equation, and for the phenyl + O2 association reaction, we found k(T) = 1.860 x 1013T-0.217 exp(0.358/T) (with k in cm3 mol-1 s-1 and T in K); this rate equation provides good agreement with low-temperature experimental measurements of the phenyl + O2 rate constant. Preliminary results were presented for a correlation between activation energy (or reaction enthalpy) and pre-exponential factor for heterolytic O-O bond scission reactions.     

The reaction OH + C2H4: an example of rotational channel switching

The low-temperature data for the reaction between OH and C(2)H(4) is treated canonically as either a two-well or one-well problem using the "Multiwell" suite of codes, in which a "well" refers to a minimum in the potential energy surface. The former is analogous to the two transition state model of Greenwald et al. [Greenwald, E. E.; North, S. W.; Georgievskii, Y.; Klippenstein, S. J. J. Phys. Chem. A2005, 109, 6031], while the latter reflects the dominance of the so-called "inner transition state". External rotations are treated adiabatically, causing changes in the magnitude of effective barriers as a function of temperature. Extant data are well-described with either model using only the average energy transferred in a downward direction, upon collision, ΔE(d)(T), as a fitting parameter. The best value for the parameters describing the rate coefficient as a function of temperature (200 < T/K < 400) (Data at lower temperature is too sparse to yield a recommendation.) and pressure in the form used in the NASA/JPL format [Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 17, Jet Propulsion Laboratory, 2011] are k(0) = 1.0 × 10(-28)(T/300)(-3.5) cm(6) molecule(-2) s(-1) and k(∞) to 8.0 × 10(-12)(T/300)(-2.3) cm(3) molecule(-1) s(-1).     

State-resolved collisional quenching of vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) by D35Cl(v = 0)

Supercollision relaxation of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) with D35Cl is investigated using high-resolution transient IR diode laser absorption spectroscopy at 4.4 microm. Highly excited pyrazine is prepared by pulsed UV excitation at 266 nm, followed by rapid radiationless decay to the ground electronic state. The rotational energy distribution of the scattered DCl (v = 0,J) molecules with J = 15-21 is characterized by T(rot) = 755+/-90 K. The relative translational energy increases as a function of rotational quantum number for DCl with T(rel) = 710+/-190 K for J = 15 and T(rel) = 1270+/-240 K for J = 21. The average change in recoil velocity correlates with the change in rotational angular momentum quantum number and highlights the role of angular momentum in energy gain partitioning. The integrated energy-transfer rate for appearance of DCl (v = 0,J = 15-21) is k(2)(int) = 7.1x10(-11) cm3 molecule(-1) s(-1), approximately one-eighth the Lennard-Jones collision rate. The results are compared to earlier energy gain measurements of CO2 and H2O.     

Lamellar gels and spontaneous vesicles in catanionic surfactant mixtures

Caillé analysis of the small-angle X-ray line shape of the lamellar phase of 7:3 wt/wt cetyltrimethylammonium tosylate (CTAT)/sodium dodecylbenzene sulfonate (SDBS) bilayers shows that the bending elastic constant is kappa = (0.62 +/- 0.09)k(B)T. From this and previous results, the Gaussian curvature constant is kappa = (-0.9 +/- 0.2)k(B)T. For 13:7 wt/wt CTAT/SDBS bilayers, the measured bending elasticity decreases with increasing water dilution, in good agreement with predictions based on renormalization theory, giving kappa(o) = 0.28k(B)T. These results show that surfactant mixing is sufficient to make kappa approximately k(B)T, which promotes strong, Helfrich-type repulsion between bilayers that can dominate the van der Waals attraction. These are necessary conditions for spontaneous vesicles to be equilibrium structures. The measurements of the bending elasticity are confirmed by the transition of the lamellar phase of CTAT/SDBS from a turbid, viscoelastic gel to a translucent fluid as the water fraction is decreased below 40 wt %. Freeze-fracture electron microscopy shows that the gel is characterized by spherulite defects made possible by spontaneous bilayer curvature and low bending elasticity. This lamellar gel phase is common to a number of catanionic surfactant mixtures, suggesting that low bending elasticity and spontaneous curvature are typical of these mixtures that form spontaneous vesicles.     

A molecular dynamics study of the lateral free energy profile of a pair of cholesterol molecules as a function of their distance in phospholipid bilayers

Free energy profile of a pair of cholesterol molecules in a leaflet of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers in the liquid-crystalline phase has been calculated as a function of their lateral distance using a combination of NPT-constant atomistic molecular dynamics calculations (P = 1 atm and T = 310.15 K) and the thermodynamic integration method. The calculated free energy clearly shows that the two cholesterol molecules form a dimer separated by a distance of 1.0-1.5 nm in POPC bilayers. Well depth of the free energy profile is about 3.5 kJ/mol, which is comparable to the thermal energy k(B)T at 310.15 K. This indicates that the aggregation of cholesterol molecules in the bilayers depends on the temperature as well as the concentration of the system. The free energy function obtained here may be used as a reference when coarse grained potential model is investigated for this two-component system. Local structure of POPC molecules around two cholesterol molecules has also been investigated.     

Behavior of rate coefficients for ion-ion mutual neutralization, 300-550 K

Rate coefficients k(MN) have been measured for a number of anion neutralization reactions with Ar(+) and Kr(+) over the temperature range 300-550 K. For the first time, the data set includes anions of radicals and other short-lived species. In the present paper, we review these results and make note of correlations with reduced mass, electron binding energy of the anion (equivalent to the electron affinity of the corresponding neutral), and temperature, and compare with expectations from absorbing sphere models. An intriguing result is that the data for diatomic anions neutralized by Ar(+) and Kr(+) have k(MN) values close to 3 × 10(-8) cm(3) s(-1) at 300 K, a figure which is lower than those for all of the polyatomic anions at 300 K except for SF(5)(-) + Kr(+). For the polyatomic anions studied here, neutralized by Ar(+) and Kr(+), the reduced mass dependence agrees with theory, on average, but we find a stronger temperature dependence of T(-0.9) than expected from the theoretical E(-0.5) energy dependence of the rate coefficient at thermal energies. The k(MN) show a weak dependence on the electron binding energy of the anion for the polyatomic species studied.     

Methods for studying reaction kinetics in gas chromatography, exemplified by using the 1-chloro-2,2-dimethylaziridine interconversion reaction

In this paper, methods are described that are used for studying first-order reaction kinetics by gas chromatography. Basic theory is summarized and illustrated using the interconversion of 1-chloro-2,2-dimethylaziridine enantiomers as a representative example. For the determination of the kinetic and thermodynamic activation data of interconversion the following methods are reviewed: (i) classical kinetic methods where samples of batch-wise kinetic studies are analyzed by enantioselective gas chromatography, (ii) stopped-flow methods performed on one chiral column, (iii) stopped-flow methods performed on an achiral column or empty capillary coupled in series with two chiral columns, (iv) on-flow method performed on an achiral column coupled in series with two chiral columns, and (v) reaction gas chromatography, known as a dynamic gas chromatography, where the interconversion is performed on chiral column during the separation process. The determination of kinetic and thermodynamic activation data by methods (i) through (iv) is straightforward as the experimental data needed for the evaluation (particularly the concentration of reaction constituents) are accessible from the chromatograms. The evaluation of experiments from reaction chromatography method (v) is complex as the concentration bands of reaction constituents are overlapped. The following procedures have been developed to determination peak areas of reaction constituents in such complex chromatograms: (i) methods based on computer-assisted simulations of chromatograms where the kinetic activation parameters for the interconversion of enantiomers are obtained by iterative comparison of experimental and simulated chromatograms, (ii) stochastic methods based on the simulation of Gaussian distribution functions and using a time-dependent probability density function, (iii) approximation function and unified equation, (iv) computer-assisted peak deconvolution methods. Evaluation of the experimental data permits the calculation of apparent rate constants for both the interconversion of the first eluted (k (A-->B)(app)) as well as the second eluted (k(B-->A)(app)) enantiomer. The mean value for all the rate constants (from all the reviewed methods) was found for 1-chloro-2,2-dimethylaziridine A-->B enantiomer interconversion at 100 degrees C: k (A-->B)(app)=21.2 x 10(-4)s(-1) with a standard deviation sigma=10.7 x 10(-4). Evaluating data for reaction chromatography at 100 degrees C {k (app)=k(A-->B)(app)=k(B-->A)(app)=13.9 x 10(-4)s(-1), sigma=3.0 x 10(-4)s(-1)} shows that differences between k(A-->B)(app) and k(B-->A)(app) are the same within experimental error. It was shown both theoretically and experimentally that the Arrhenius activation energy (E(a)) calculated from Arrhenius plots (lnk(app) versus 1/T) is proportional to the enthalpy of activation {E(a)=DeltaH+RT}. Statistical treatment of Gibbs activation energy values gave: DeltaG (app)=110.5kJmol(-1), sigma=2.4kJmol(-1), DeltaG (A-->B)(app)=110.5kJmol(-1), sigma=2.2kJmol(-1), DeltaG (B-->A)(app)=110.3kJmol(-1), sigma=2.8kJmol(-1). This shows that the apparent Gibbs energy barriers for the interconversion of 1-chloro-2,2-dimethylaziridine enantiomers are equal DeltaG (app)=DeltaG(A-->B)(app)=DeltaG(B-->A)(app) and within the given precision of measurement independent of the experimental method used.     

Theoretical study on tertiary structural elements of beta-peptides: nanotubes formed from parallel-sheet-derived assemblies of beta-peptides

Parallel or polar strands of beta-peptides spontaneously form nanotubes of different sizes in a vacuum as determined by ab initio calculations. Stability and conformational features of [CH3CO-(beta-Ala)k-NHCH3]l (1 < or = k < or = 4, 2 < or = l < or = 4) models were computed at different levels of theory (e.g., B3LYP/6-311++G(d,p)// B3LYP/6-31G(d), with consideration of BSSE). For the first time, calculations demonstrate that sheets of beta-peptides display nanotubular characteristics rather than two-dimensional extended beta-layers, as is the case of alpha-peptides. Of the configurations studied, k = l = 4 gave the most stable nanotubular structure, but larger assemblies are expected to produce even more stable nanotubes. As with other nanosystems such as cyclodextrane, these nanotubes can also incorporate small molecules, creating a diverse range of applications for these flexible, biocompatible, and highly stable molecules. The various side chains of beta-peptides can make these nanosystems rather versatile. Energetic and structural features of these tubular model systems are detailed in this paper. It is hoped that the results presented in this paper will stimulate experimental research in the field of nanostructure technology involving beta-peptides.     

Accurate potential energy surface and quantum reaction rate calculations for the H+CH4-->H2+CH3 reaction

Calculations for the cumulative reaction probability N(E) (for J=0) and the thermal rate constant k(T) of the H+CH(4)-->H(2)+CH(3) reaction are presented. Accurate electronic structure calculations and a converged Shepard-interpolation approach are used to construct a potential energy surface which is specifically designed to allow the precise calculation of k(T) and N(E). Accurate quantum dynamics calculations employing flux correlation functions and multiconfigurational time-dependent Hartree wave packet propagation compute N(E) and k(T) based on this potential energy surface. The present work describes in detail the various convergence test performed to investigate the accuracy of the calculations at each step. These tests demonstrate the predictive power of the present calculations. In addition, approximate approaches for reaction rate calculations are discussed. A quite accurate approximation can be obtained from a potential energy surface which includes only interpolation points on the minimum energy path.     

Dynamics of the D+ + H2 and H+ + D2 reactions: a detailed comparison between theory and experiment

An extensive set of experimental measurements on the dynamics of the H(+) + D(2) and D(+) + H(2) ion-molecule reactions is compared with the results of quantum mechanical (QM), quasiclassical trajectory (QCT), and statistical quasiclassical trajectory (SQCT) calculations. The dynamical observables considered include specific rate coefficients as a function of the translational energy, E(T), thermal rate coefficients in the 100-500 K temperature range. In addition, kinetic energy spectra (KES) of the D(+) ions reactively scattered in H(+) + D(2) collisions are also presented for translational energies between 0.4 eV and 2.0 eV. For the two reactions, the best global agreement between experiment and theory over the whole energy range corresponds to the QCT calculations using a gaussian binning (GB) procedure, which gives more weight to trajectories whose product vibrational action is closer to the actual integer QM values. The QM calculations also perform well, although somewhat worse over the more limited range of translational energies where they are available (E(T) < 0.6 eV and E(T) < 0.2 eV for the H(+) + D(2) and D(+) + H(2) reactions, respectively). The worst agreement is obtained with the SQCT method, which is only adequate for low translational energies. The comparison between theory and experiment also suggests that the most reliable rate coefficient measurements are those obtained with the merged beams technique. It is worth noting that none of the theoretical approaches can account satisfactorily for the experimental specific rate coefficients of H(+) + D(2) for E(T)≤ 0.2 eV although there is a considerable scatter in the existing measurements. On the whole, the best agreement with the experimental laboratory KES is obtained with the simulations carried out using the state resolved differential cross sections (DCSs) calculated with the QCT-GB method, which seems to account for most of the observed features. In contrast, the simulations with the SQCT data predict kinetic energy spectra (KES) considerably cooler than those experimentally determined.     

A simple method relating specific rate constants k(E,J) and Thermally averaged rate constants k(infinity)(T) of unimolecular bond fission and the reverse barrierless association reactions

This work describes a simple method linking specific rate constants k(E,J) of bond fission reactions AB --> A + B with thermally averaged capture rate constants k(cap)(T) of the reverse barrierless combination reactions A + B --> AB (or the corresponding high-pressure dissociation or recombination rate constants k(infinity)(T)). Practical applications are given for ionic and neutral reaction systems. The method, in the first stage, requires a phase-space theoretical treatment with the most realistic minimum energy path potential available, either from reduced dimensionality ab initio or from model calculations of the potential, providing the centrifugal barriers E(0)(J). The effects of the anisotropy of the potential afterward are expressed in terms of specific and thermal rigidity factors f(rigid)(E,J) and f(rigid)(T), respectively. Simple relationships provide a link between f(rigid)(E,J) and f(rigid)(T) where J is an average value of J related to J(max)(E), i.e., the maximum J value compatible with E > or = E0(J), and f(rigid)(E,J) applies to the transitional modes. Methods for constructing f(rigid)(E,J) from f(rigid)(E,J) are also described. The derived relationships are adaptable and can be used on that level of information which is available either from more detailed theoretical calculations or from limited experimental information on specific or thermally averaged rate constants. The examples used for illustration are the systems C6H6+ <==> C6H5+ + H, C8H10+ --> C7H7+ + CH3, n-C9H12+ <==> C7H7+ + C2H5, n-C10H14+ <==> C7H7+ + C3H7, HO2 <==> H + O2, HO2 <==> HO + O, and H2O2 <==> 2HO.