On the model dependence of kinetic shifts in unimolecular reactions: the dissociation of the cations of benzene and n-butylbenzene

Statistical adiabatic channel model/classical trajectory (SACM/CT) calculations have been performed for transitional mode dynamics in the simple bond fission reactions of C(6)H(6)(+) --> C(6)H(5)(+) + H and n-C(6)H(5)C(4)H(9)(+) --> C(7)H(7)(+) + n-C(3)H(7). Reduced-dimensionality model potentials have been designed that take advantage of ab initio results as far as available. Average anisotropy amplitudes of the potentials were fitted by comparison of calculated specific rate constants k(E,J) with measured values. The kinetic shifts of the calculated k(E) curves and the corresponding bond energies E(0)(J=0), derived as 3.90 +/- 0.05 eV for C(6)H(6)(+) and 1.78 +/- 0.05 eV for n-C(6)H(5)C(4)H(9)(+), were in good agreement with literature values from thermochemical studies. Kinetic shifts from fixed tight activated complex Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which also reproduces the measured k(E), were larger than the present SACM/CT results as well as earlier results from variational transition state theory (for C(6)H(6)(+)). The approach using RRKM theory was found to underestimate E(0)(J=0) by about 0.2-0.3 eV. A simplified SACM/CT-based method is also proposed which circumvents the trajectory calculations and allows derivation of E(0)(J=0) on the basis of measured k(E) and which provides similar accuracy as the full SACM/CT treatment.     

An analytic hindered rotor model for calculating microcanonical variational unimolecular rate constants from reaction path properties

A model is proposed for performing microcanonical variational transition state theory calculations which incorporates ideas from vibrator and flexible variational transition state models. Vibrational frequencies, moments of inertia, and potential energy for the variational transition state are found by reaction path following as for the vibrator model. However, the transitional modes are treated as hindered rotors using an analytic potential and an analytic density of states, which are fit to barriers for hindered rotation determined from reaction path following. The model proposed here differs from the flexible transition state model in that the density of states for the transitional modes is analytic and transitional modes and external rotational angular momenta are uncoupled. For the H + CH₃ ? CH₄ system, rate constants calculated with this new model are only 6–23% smaller on average from those of the flexible transition state model for values of total angular momentum which correspond to average rotational temperatures of 0–2000 K. Harmonic frequencies calculated for the transitional modes from the hindered rotor Hamiltonian are in good agreement with the exact values found by a reaction path analysis. © 1994 John Wiley & Sons, Inc.     

Hydrogen atom loss from the benzene cation. Why is the kinetic energy release so large?

The kinetic energy release distributions (KERDs) associated with the hydrogen loss from the benzene cation and the deuterium loss from the perdeuteriobenzene cation have been remeasured on the metastable time scale and analyzed by the maximum entropy method. The experimental kinetic energy releases are larger than expected statistically, in contradistinction to what has been observed for the C-X fragmentations of the halogenobenzene cations. H(D) loss from C(6)H(6)(+) (C(6)D(6)(+)) occurs via a conical intersection connecting the (2)A(2) and (2)A(1) electronic states. Two models are proposed to account for the experimental data: (i) a modified orbiting transition state theory (OTST) approach incorporating electronic nonadiabaticity; (ii) an electronically nonadiabatic version of the statistical adiabatic channel model (SACM) of Quack and Troe. The latter approach is found to be preferable. It leads to the conclusion that the larger the energy stored in the transitional modes, which partly convert to the relative interfragment motion, the shorter the value of the reaction coordinate at which the adiabatic channels cross, and the larger the probability of undergoing the (2)A(2) --> (2)A(1) transition required for hydrogen loss.     

ICP-AES中元素间干扰的逐步逼近校正方法

本文基于干扰系数校正法,提出一种新的校正元素间相互干扰的方法——逐步逼近校正法,以及相应的计算机软件系统。对复杂体系中元素间相互光谱干扰和元素“自身”非光谱干扰进行校正,获得了良好的结果。     

Ab initio treatment of the chemical reaction precursor complex Cl(2P)-HF. 2. Bound states and infrared spectrum

Bound energy levels and properties of the Cl(2P)-HF complex were obtained from full three-dimensional (3D) calculations, with the use of the ab initio computed diabatic potential surfaces from the preceding paper and the inclusion of spin-orbit coupling. For a better understanding of the dynamics of this complex we also computed a 2D model in which the HF bond length r was frozen at the vibrationally averaged values r0 and r1 and a 2 + 1D model in which the 3D potentials were averaged over the v(HF) = 0 and v(HF) = 1 vibrational wave functions of free HF. Also 1D calculations were made in which both r and the Cl-HF distance R were frozen. The complex is found to have the linear hydrogen bonded Cl-HF structure, with ground-state quantum numbers J = 3/2 for the overall angular momentum and /omega/ = 3/2 for its projection on the intermolecular axis R. The binding energy is D0 = 432.25 cm(-1) for v(HF) = 0 and D0 = 497.21 cm(-1) for v(HF) = 1. Bending modes with /omega/ = 1/2 and /omega/ = 5/2 are split by the Renner-Teller effect, since the electronic ground state is a degenerate 2pi state. A series of intermolecular (R) stretch modes was identified. Rotational constants and e-f parity splittings were extracted from the levels computed for J = 1/2 to 7/2. The computed red shift of the HF stretch frequency of 64.96 cm(-1) and the 35Cl-37Cl isotope shift of 0.033 cm(-1) are in good agreement with the values of 68.77 and 0.035 cm(-1) obtained from the recent experiment of Merritt et al. (Phys. Chem. Chem. Phys. 2005, 7, 67), after correction for the effect of the He nanodroplet matrix in which they were measured.     

SACM/CT Study of the dissociation/recombination dynamics of hydrogen peroxide on an ab initio potential energy surface. Part II. Specific rate constants k(E,J), thermal rate constants k infinity(T), and lifetime distributions

Statistical adiabatic channel model/classical trajectory (SACM/CT) calculations of the dissociation/recombination dynamics of hydrogen peroxide, H(2)O(2) <--> 2HO, have been performed on an ab initio potential energy surface by Kuhn, Rizzo, Luckhaus, Quack, and Suhm (J. Chem. Phys. 1999, 111, 2565). Specific rate constants k(E,J), thermal rate constants k(infinity)(T), and lifetime distributions are determined. After averaging over J, the derived k(E,J) are in quantitative agreement with non-exponential time-profiles of HO formation recorded after overtone excitation of H(2)O(2) near the dissociation threshold by Scherer and Zewail (J. Chem. Phys. 1987, 87, 97). The thermal high pressure rate constants for HO recombination agree with experimental data as well and can be represented by k(rec,infinity)/10(-10) cm(3) molecule(-1) s(-1) approximately [0.376 (298 K/T)(0.47) + 0.013 (T/298 K)(0.74)] over the range 60-1500 K. Non-statistical lifetime distributions are suggested not to have been of major relevance for the available experiment.     

用四氨基铝酞菁共振光散射技术测定纳克级核酸

痕量核酸对四氨基铝酞菁的共振散射光产生增强作用,且增强程度与核酸浓度之间有良好的线性关系.据此建立了测定核酸的高灵敏共振散射光增强分析方法.在pH=6.0、最大散射波长400nm处,测定小牛胸腺DNA(CTDNA)、鲑鱼精子DNA(SMDNA)和酵母RNA(YeastRNA)的线性范围分别是0～250ng/mL,0～200ng/mL和0～400ng/mL,检测限分别为1.4ng/mL,1.4ng/mL和2.7ng/mL.该法简单,灵敏度高,用于实际样品中核酸含量的测定,所得结果与紫外吸收法一致.     

[Rh(μ-Cl)(COD)]2 supported on activated carbons for the hydroformylation of 1-octene: effects of support surface chemistry and solvent

The [Rh(μ-Cl)(COD)]₂ complex has been heterogeneised on activated carbon and used as catalysts for the hydroformylation of 1-octene. The objective is to achieve the effective anchorage of the complex on the carbon surface, keeping or improving the catalytic properties of the complex in the homogeneous process. The effects of surface chemistry of the activated carbon and the solvent used as reaction media (hexane, acetone or methanol) on the activity and selectivity to linear products (alcohols and aldehydes) and on the lixiviation of the complex from support, using different methods of catalyst recovering, are investigated. Gas adsorption, temperature-programmed desorption (TPD), immersion calorimetry and XPS techniques were used for characterisation. Catalytic activity at 353 K and a total pressure of 5 MPa (H₂:CO ratio 1:1) was carried out in a stainless steel stirred tank reactor. Obtained results show that the heterogeneised complex shows a conversion level higher or similar to that of the homogeneous complex and with a higher selectivity to the linear products. Complex lixiviation depends on the support, the solvent and the method used to recover the catalyst from the reactor. The catalyst prepared with the functionalised activated carbon, where the anchorage takes place by ion-exchange, is more stable and active in further catalytic runs using the proper recovering method.     

An efficient solution of weak-collision master equations in thermal unimolecular reaction rate theory

A new finite-basis-set method of solving the weak-collision master equation of thermal unimolecular reactions in the general pressure regime is presented. Consecutive sections of the equilibrium probability density are used as basis functions. Significant advantages in terms of efficiency and applicability are obtained. Representative calculations are performed to illustrate the method's convergence properties and storage requirements. Calculations of collisional efficiency factors in the low-pressure limit β₀ and weak-collision broadening factors F^WC(ω) are performed to offer a simple concise representation of weak-collision effects. It is noted that weak-collision effects can be incorporated into fall-off curves with an accuracy of within an order of magnitude by simple scaling of the strong-collision fall-off curves. However, for accurate representation of weak-collision effects the weak-collision broadening must be taken into account.     

6 Mcps photon-counting X-ray computed tomography system using a 25 mm/s-scan linear LSO–MPPC detector and its application to gadolinium imaging

6 Mcps photon counting was carried out using a detector consisting of a 1.0 mm-thick LSO [Lu₂(SiO₄)O] single-crystal scintillator and an MPPC (multipixel photon counter) module in an X-ray computed tomography (CT) system. The maximum count rate was 6 Mcps (mega counts per second) at a tube voltage of 100 kV and a tube current of 0.91 mA. Next, a photon-counting X-ray CT system consists of an X-ray generator, a turntable, a scan stage, a two-stage controller, the LSO–MPPC detector, a counter card (CC), and a personal computer (PC). Tomography is accomplished by repeated linear scans and rotations of an object, and projection curves of the object are obtained by the linear scan with a scan velocity of 25 mm/s. The pulses of the event signal from the module are counted by the CC in conjunction with the PC. The exposure time for obtaining a tomogram was 600 s at a scan step of 0.5 mm and a rotation step of 1.0°, and photon-counting CT was accomplished using gadolinium-based contrast media.     

Electronic structure and reactivity of high-spin iron--alkyl- and--pterinperoxo complexes

The spectroscopic properties and electronic structure of the four-coordinate high-spin [FeIII(L3)(OOtBu)]+ complex (1; L3 = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate; tBu = tert-butyl) are investigated and compared to the six-coordinated high-spin [Fe(6-Me3TPA)(OHx)(OOtBu)]x+ system (TPA = tris(2-pyridylmethyl)amine, x = 1 or 2) studied earlier [Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2001, 123, 12802-12816]. Complex 1 is characterized by Raman features at 889 and 830 cm-1 which are assigned to the O-O stretch (mixed with the symmetric C-C stretch) and a band at 625 cm-1 that corresponds to nu(Fe-O). The UV-vis spectrum shows a charge-transfer (CT) transition at 510 nm from the alkylperoxo pi v* (v = vertical to C-O-O plane) to a d orbital of Fe(III). A second CT is identified from MCD at 370 nm that is assigned to a transition from pi h* (h = horizontal to C-O-O plane) to an Fe(III) d orbital. For the TPA complex the pi v* CT is at 560 nm while the pi h* CT is to higher energy than 250 nm. These spectroscopic differences between four- and six-coordinate Fe(III)-OOR complexes are interpreted on the basis of their different ligand fields. In addition, the electronic structure of Fe-OOPtn complexes with the biologically relevant pterinperoxo ligand are investigated. Substitution of the tert-butyl group in 1 by pterin leads to the corresponding Fe(III)-OOPtn species (2), which shows a stronger electron donation from the peroxide to Fe(III) than 1. This is related to the lower ionization potential of pterin. Reduction of 2 by one electron leads to the Fe(II)-OOPtn complex (3), which is relevant as a model for potential intermediates in pterin-dependent hydroxylases. However, in the four-coordinate ligand field of 3, the additional electron is located in a nonbonding d orbital of iron. Hence, the pterinperoxo ligand is not activated for heterolytic cleavage of the O-O bond in this system. This is also evident from the calculated reaction energies that are endothermic by at least 20 kcal/mol.     

Kinetics of gas-phase decomposition of oxetan and oxetan-2,2-d2

The pressure dependence of the first-order rate coefficient of oxetan and oxetan-2,2-d₂ decomposition has been studied in the pressure range from about 7 kPa down to 0.01 kPa at various temperatures between 673 and 758 K. Experimental data were analyzed using RRKM theory. Interpretation of the fall-off curves lends support to the high-pressure Arrhenius parameters A = 10^15.42s^?1 and E_A = 259.5 kJ/mol derived from measurements made in the pressure-independent range. Decomposition of oxetan is found to occur via biradical intermediates. Data for the kinetic isotope effect were used to derive kinetic parameters for the ring-opening elementary steps in oxetan and oxetan-d₂ decomposition.     

Surface-enhanced Raman scattering and surface-enhanced resonance Raman scattering excitation profiles of Ag-2,2'-bipyridine surface complexes and of [Ru(bpy)3]2+ on Ag colloidal surfaces: manifestations of the charge-transfer resonance contributions to the overall surface enhancement of Raman scattering

Excitation profiles of SERS (surface-enhanced Raman scattering) and/or SERRS (surface-enhanced resonance Raman scattering) spectral bands of two forms of a Ag-bpy (bpy = 2,2'-bipyridine) surface complex and of [Ru(bpy)3]2+ on Ag nanoparticle (hydrosol) surfaces were determined from the spectra excited in the 458-600 nm region and are reported together with the FT-SERS spectra of the Ag-bpy surface complex and FT Raman spectra of [Ru(bpy)3] Cl2. Seven of the observed 11 fundamentals as well as their first overtones and combination bands are selectively enhanced in SERS of the Ag-bpy surface complex formed in the Ag colloid/HCl/bpy system. The profiles of these bands show a common maximum at approximately 540 nm. The selectively enhanced bands of the Ag-bpy surface complex have nearly the same wavenumbers as those enhanced in the SERRS and resonance Raman spectra of [Ru(bpy)3]2+ upon excitation close to the 453 nm maximum of its MLCT absorption band. Moreover, the intensity patterns of the bpy vibrations of the two species match both in resonance (541 nm excitation for Ag-bpy, 458 nm for [Ru(bpy)3]2+) and in off-resonance (458 and 1064 nm for Ag-bpy, 1064 nm for [Ru(bpy)3]2+). The distinct band shapes of the excitation profiles of the selectively enhanced vibrational modes of the Ag-bpy surface complex, as well as the observation of overtones and combination bands in the SERS spectra upon excitation into this "band", are interpreted in terms of a charge-transfer resonance contribution to the overall SERS enhancement. In view of the near-coincidence of the vibrational modes coupled to the resonant electronic transition of Ag-bpy with those coupled to the MLCT transition of [Ru(bpy)3]2+, the resonant electronic transition is tentatively assigned to a Ag metal to bpy (pi*) CT transition.     

Assessment of TD-DFT- and TD-HF-based approaches for the prediction of exciton coupling parameters, potential energy curves, and electronic characters of electronically excited aggregates

The reliability of linear response approaches such as time-dependent Hartree-Fock (TD-HF) and time-dependent density functional theory (TD-DFT) for the prediction of the excited state properties of 3,4;9,10-tetracarboxylic-perylene-bisimide (PBI) aggregates is investigated. A dimer model of PBI is investigated as a function of a torsional motion of the monomers, which was shown before to be an important intermolecular coordinate in these aggregates. The potential energy curves of the ground state and the two energetically lowest neutral excited and charge-transfer (CT) states were obtained with the spin-component scaling modification of the approximate coupled-cluster singles-and-doubles (SCS-CC2) method as a benchmark for dispersion corrected TD-HF and a range of TD-DFT approaches. The highly accurate SCS-CC2 results are used to assess the other, computationally less demanding methods. TD-HF predicts similar potential energy curves and transition dipole moments as SCS-CC2, as well as the correct order of neutral and CT states. This supports an exciton trapping mechanism, which was found on the basis of TD-HF data. However, the investigated TD-DFT methods provide generally the opposite character for the excited states. As a consequence, these TD-DFT results have unacceptably large errors for optical properties of these dye aggregates.     

活性炭纤维对气相低浓度三氯乙烯的动态吸附

研究了3种不同经表面积活性炭纤维（ACF）对气相低浓度三氯乙烯（TCE）的动态吸附。ACF的比表面积为600m^2/g、1400m^2／g和1600m^2/g，TCE的浓度范围为27mg/m^3-2725mg/m^3。结果表明，比表面积为1400m^2/g和1600m^2/g的ACF对TCE的吸附较好；10％穿透时间的对数与TCE匠对数呈线性关系；穿透时间随温度和湿度的增加而缩短；穿透曲线可以用经验公式拟合。拟合值和实测值吻合良好；ACF吸附是去除气相低浓度TCE的有效方法。     

预测[Si, C, S]＋和[Si, C, S]－体系的稳定异构体

采用DFT, QCISD及CCSD(T)方法分别对二重态的[Si, C, S]＋和[Si, C, S]－体系势能面进行理论计算, 用QCISD/6-311＋G(d)方法, 在[Si, C, S]＋和[Si, C, S]－体系中, 我们分别得到了2个过渡态连接的3个稳定体和2个过渡态连接的4个稳定体, 经热力学及动力学分析发现, [Si, C, S]＋体系只有二重态线性的离子[Si—C—S]＋可能稳定存在, 而[Si, C, S]－体系有二重态线性的离子[Si—C—S]－和三元环c-[SiCS]－可能稳定存在.     

Model for a three-center decomposition reaction. I. Perfluorodiazirine

A semiempirical approach has been used to evaluate rate parameters for a three-center decomposition reaction from the point of view of transition state theory, with perfluorodiazirine serving as the prototype molecule. Several activated complex models are considered in which the reaction coordinate is chosen as the ? NCN bending mode. The constraints imposed include the principle of concerted bond-order conservation in passing from the initial to the final state, and use is made of empirical bond order–bond length and bond order–force constant relationships. The geometric configuration of the transition state sought is one which conforms with the lowest energy path and is also consistent with the observed entropy of activation. The potential energy of activation is taken as the optimum difference in binding energies (based on the INDO method) between the transition and initial states, and the critical energy is obtained by applying a correction for the zero-point energy difference, derived from normal coordinate analysis. Satisfactory agreement is found in the case of the activated complex model for which the total bond order is conserved and bonds undergoing rupture are assigned a fractional bond order (FBO) of 2/3, derived from the postulate (FBO) = α/β whe re α(=2) is the number of bonds breaking, and β(=3) is the number of bonds undergoing change in the ring opening.     

A theoretical study of the dynamic behavior of alkane hydroxylation by a compound I model of cytochrome P450

Dynamic aspects of alkane hydroxylation mediated by Compound I of cytochrome P450 are discussed from classical trajectory calculations at the B3LYP level of density functional theory. The nuclei of the reacting system are propagated from a transition state to a reactant or product direction according to classical dynamics on a Born-Oppenheimer potential energy surface. Geometric and energetic changes in both low-spin doublet and high-spin quartet states are followed along the ethane to ethanol reaction pathway, which is partitioned into two chemical steps: the first is the H-atom abstraction from ethane by the iron-oxo species of Compound I and the second is the rebound step in which the resultant iron-hydroxo complex and the ethyl radical intermediate react to form the ethanol complex. Molecular vibrations of the C-H bond being dissociated and the O-H bond being formed are significantly activated before and after the transition state, respectively, in the H-atom abstraction. The principal reaction coordinate that can represent the first chemical step is the C-H distance or the O-H distance while other geometric parameters remain almost unchanged. The rebound process begins with the iron-hydroxo complex and the ethyl radical intermediate and ends with the formation of the ethanol complex, the essential process in this reaction being the formation of the C-O bond. The H-O-Fe-C dihedral angle corresponds to the principal reaction coordinate for the rebound step. When sufficient kinetic energy is supplied to this rotational mode, the rebound process should efficiently take place. Trajectory calculations suggest that about 200 fs is required for the rebound process under specific initial conditions, in which a small amount of kinetic energy (0.1 kcal/mol) is supplied to the transition state exactly along the reaction coordinate. An important issue about which normal mode of vibration is activated during the hydroxylation reaction is investigated in detail from trajectory calculations. A large part of the kinetic energy is distributed to the C-H and O-H stretching modes before and after the transition state for the H-atom abstraction, respectively, and a small part of the kinetic energy is distributed to the Fe-O and Fe-S stretching modes and some characteristic modes of the porphyrin ring. The porphyrin marker modes of nu(3) and nu(4) that explicitly involve Fe-N stretching motion are effectively enhanced in the hydroxylation reaction. These vibrational modes of the porphyrin ring can play an important role in the energy transfer during the enzymatic process.     

Theoretical prediction of the contact distance dependence of the electron transfer reactivity of the ClO/ClO− coupling system

On the basis of the structures and properties of the ClO/ClO^? system obtained at the density functional theory (DFT) (UB3LYP) level, employing the 6‐311+G(3df) standard basis set, the electron transfer reactivity of this system is investigated. The results indicate that there are five possible stable coupling complexes that correspond to the generous minima on the global potential energy surfaces (PES). The most stable coupling complex is planar EC4, in which there is a O? O linkage with two trans‐Cl atoms. Their stabilization energies are calculated to be 20.57 (EC1: C₁), 20.54 (EC2: C₂, ²B), 20.69 (EC3: C₁), 20.70 (EC4: C_s, ²A′), and 20.69 (EC.5: C_2h, ²B_u) kcal/mol at the B3LYP/6‐311+G(3df) level; with the correction of the basis set superposition error (BSSE), the stability order of these encounter complexes is EC4 > EC.5 > EC3 > EC1 > EC2. Based on the five encounter complexes, five coupling modes are designed for the study of the electron transfer reactivity of this system. The dissociation energy curves at the activated states and the corresponding activation energies of these five coupling modes are obtained and are compared at the B3LYP/6‐311+G(3df) and MP2/6‐311+G* levels. The inapplicability of DFT methods has also been discussed in this article in predicting the energy curves, especially with a long contact distance, in which DFT methods give the abnormal behavior for the dissociations of the complexes caused by the “inverse symmetry breaking” problem. On the basis of the golden rule of the time‐dependent perturbation theory, the electron transfer reactivity and the contact distance dependence of the various electron transfer kinetics parameters (e.g., activation energy, coupling matrix element) have been analyzed at the UMP2(full)/6‐311+G* level. The electron transfer can take place over a range of contact distances, but the most effective coupling distance corresponds to only a small range. The coupling orientation analyses also indicate that the most favorable coupling mode to the electron transfer does not always correspond to the most stable encounter complex mechanism. Some highly energetic coupling modes are more favorable for the electron transfer. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Experimental activation entropy for heterogeneous catalytic oxidation of hydrogen and carbon monoxide

The entropy of the activated complex S, calculated from experimental kinetic data, is approximately constant in the series of oxide catalysts used for oxidation of hydrogen, which shows that these contacts are of the same type. The low S values indicate low mobility in the activated complex. A similar result is obtained for the oxidation of carbon monoxide on the oxides. The high S values observed in the oxidation of hydrogen on platinum and palladium indicate surface mobility of the transitional state and also a difference in the nature of metallic and of oxide catalysts for the oxidation of hydrogen.