Dynamics of Rydberg electron transfer to CH3CN: velocity dependent studies

The dynamics of free-ion production through electron transfer in K(np)/CH3CN collisions are examined through measurements using velocity-selected Rydberg atoms. The data show that Rydberg electron transfer leads to the creation of two groups of dipole-bound CH3CN- ions, one long lived (tau>85 micros), the other short lived (tau<1 micros). The velocity dependences associated with the production of both groups of ions are similar, the ion formation rate decreasing markedly with decreasing Rydberg atom velocity, principally as a consequence of postattachment electrostatic interactions between the product ions. The results are in reasonable accord with the predictions of a Monte Carlo collision model that considers the effect of crossings between the diabatic potential curves for the covalent K(np)/CH3CN system and the K+/CH3CN- ion pair. This model also accounts for the relatively small reaction rate constants, approximately 0.5-1.0 x 1.0(-8) cm(3) s(-1), associated with the formation of long-lived CH3CN- ions. No velocity dependence in the lifetime of the CH3CN- ions is observed.     

Low-energy electron capture by 6-Aza-2-thiothymine: investigations by electron attachment and electron transmission spectroscopies

The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out.     

Theoretical prediction of new dipole-bound singlet states for anions of interstellar interest

Anions that exhibit dipole-bound singlet states have been proposed as a potential class of molecules that may be identified in the interstellar medium. Using high-level coupled cluster theory, we have computed the dipole moments, electron binding energies, and excited states of 14 neutral radicals and their corresponding closed-shell anions. We have calibrated our methods against experimental data for CH(2)CN(-) and CH(2)CHO(-) and demonstrated that coupled cluster theory can closely reproduce experimental dipole moments, electron binding energies, and excitation energies. Using these same methods, we predict the existence of dipole-bound excited states for six of the 14 previously unknown anions, including CH(2)SiN(-), SiH(2)CN(-), CH(2)SiHO(-), SiN(-), CCOH(-), and HCCO(-). In addition, we predict the existence of a valence-bound excited state of CH(2)SiN(-) with an excitation wavelength near 589 nm.     

Low-temperature branching ratios for the reaction of state-prepared N2(+) with acetonitrile

In this work, the primary product branching ratio (BR) for the reaction of state-prepared nitrogen cation (N(2)(+)) with acetonitrile (CH(3)CN), a possible minor constituent of Titan's upper atmosphere, is reported. The ion-molecule reaction occurs in the collision region of the supersonic nozzle expansion that is characterized by a rotational temperature of 45 ± 5 K. A BR of 0.86 ± 0.01/0.14 ± 0.01 is obtained for the formation CH(2)CN(+) and the CH(3)CN(+) product ions, respectively. The reported BR overwhelmingly favors the formation of CH(2)CN(+) product channel and is consistent with a simple capture process that is accompanied by a nonresonant dissociative charge transfer reaction. The BRs are independent of the N(2) rotational levels excited. Apart from providing insights onto the dynamics of the title ion-molecule reaction, the reported BR represents the most accurate available low-temperature experimental measurement for the reaction useful to aid in the accurate modeling of Titan's nitrile chemistry.     

Temperature dependence of negative ion lifetimes

The autodetachment lifetimes of SF6-* and C6F6-* ions formed by charge transfer in K(np)/SF6, C6F6 collisions are measured as a function of target temperature over the range of approximately 300-600 K with the aid of time-of-flight techniques and a Penning ion trap. At room temperature only formation of long-lived SF6 -* ions with lifetimes tau >or similar to 1 ms is seen. As the temperature is increased the lifetime of these long-lived ions is reduced, some having lifetimes as short as approximately 0.4 ms. The appearance of a short-lived, tau 相似文献   

Singlet-triplet splittings and ground- and excited-state electron affinities of selected cyanosilylenes, XSiCN (X = H, F, Cl, CH3, SiH3, CN)

Several cyanosilylenes, XSiCN, (X = H, F, Cl, CH3, SiH3, CN) have been investigated using the RHF-ACPF and CAS(2,2)-ACPF methods in conjunction with the aug-cc-pVTZ basis sets. All silylenes are found to have singlet ground states. The ground-state electron affinities are found to be rather high, i.e., 1.832, 1.497, 1.896, 1.492, 2.235, and 2.631 eV for HSiCN, FSiCN, ClSiCN, H3CSiCN, H3SiSiCN, and Si(CN)2, respectively. The existence of bound excited negative ion states has been discovered for the first time within these silylenes. All these bound excited anion states belong to the totally symmetric irreducible representations and can be characterized as dipole-bound negative ion states. All triplet excited states have even larger dipole moments than the singlet states and are, therefore, "dressed" by dipole-bound negative ion states, which correspond to Feshbach resonances.     

Primary branching ratios for the low-temperature reaction of state-prepared N2+ with CH4, C2H2, and C2H4

Product branching ratios (BRs) are reported for ion-molecule reactions of state-prepared nitrogen cation (N(2)(+)) with methane (CH(4)), acetylene (C(2)H(2)). and ethylene (C(2)H(4)) at low temperature using a modified ion imaging apparatus. These reactions are performed in a supersonic nozzle expansion characterized by a rotational temperature of 40 ± 5K. For the N(2)(+) + CH(4) reaction, a BR of 0.83:0.17 is obtained for the dissociative charge-transfer (CT) reaction that gives rise to the formation of CH(3)(+) and CH(2)(+) product ions, respectively. The N(2)(+) + C(2)H(2) ion-molecule reaction proceeds through a nondissociative CT process that results in the sole formation of C(2)H(2)(+) product ions. The reaction of N(2)(+) with C(2)H(4) leads to the formation of C(2)H(3)(+) and C(2)H(2)(+) product ions with a BR of 0.74:0.26, respectively. The reported BR for the N(2)(+) + C(2)H(4) reaction is supportive of a nonresonant dissociative CT mechanism similar to the one that accompanies the N(2)(+) + CH(4) reaction. No dependence of the branching ratios on N(2)(+) rotational level was observed. In addition to providing direct insight into the dynamics of the state-prepared N(2)(+) ion-molecule reactions with the target neutral hydrocarbon molecules, the reported low-temperature BRs are also important for accurate modeling of the nitrogen-dominated upper atmosphere of Saturn's moon, Titan.     

State-resolved dynamics of the CN(B2Sigma+) and CH(A2Delta) excited products resulting from the VUV photodissociation of CH3CN

Fourier transform visible spectroscopy, in conjunction with VUV photons produced by a synchrotron, is employed to investigate the photodissociation of CH3CN. Emission is observed from both the CN(B2Sigma+-X2Sigma+) and CH(A2Delta-X2Pi) transitions; only the former is observed in spectra recorded at 10.2 and 11.5 eV, whereas both are detected in the 16 eV spectrum. The rotational and vibrational temperatures of both the CN(B2Sigma+) and CH(A2Delta) radical products are derived using a combination of spectral simulations and Boltzmann plots. The CN(B2Sigma+) fragment displays a bimodal rotational distribution in all cases. Trot(CN(B2Sigma+)) ranges from 375 to 600 K at lower K' and from 1840 to 7700 K at higher K' depending on the photon energy used. Surprisal analyses indicate clear bimodal rotational distributions, suggesting CN(B2Sigma+) is formed via either linear or bent transition states, respectively, depending on the extent of rotational excitation in this fragment. CH(A2Delta) has a single rotational distribution when produced at 16 eV, which results in Trot(CH(A2Delta))=4895+/-140 K in v'=0 and 2590+/-110 K in v'=1. From thermodynamic calculations, it is evident that CH(A2Delta) is produced along with CN(X2Sigma+)+H2. These products can be formed by a two step mechanism (via excited CH3* and ground state CN(X2Sigma+)) or a process similar to the "roaming" atom mechanism; the data obtained here are insufficient to definitively conclude whether either pathway occurs. A comparison of the CH(A2Delta) and CN(B2Sigma+) rotational distributions produced by 16 eV photons allows the ratio between the two excited fragments at this energy to be determined. An expression that considers the rovibrational populations of both band systems results in a CH(A2Delta):CN(B2Sigma+) ratio of (1.2+/-0.1):1 at 16 eV, thereby indicating that production of CH(A2Delta) is significant at 16 eV.     

Rotationally correlated reactivity in the CH (v = 0, J, F(i)) + O2 → OH (A) + CO reaction

The rotational-state-selected CH (v = 0, J, F(i)) beam has been prepared by using an electric hexapole and applied to the crossed beam reaction of CH (v = 0, J, F(i)) + O(2) → OH (A) + CO at different O(2) beam conditions. The rotational state selected reactive cross sections of CH (RSSRCS-CH) turn out to depend remarkably on the rotational state distribution of O(2) molecules at a collision energy of ～?0.19 eV. The reactivity of CH molecules in the N = 1 rotational states (namely ∣J = 1∕2, F(2)> and ∣J = 3∕2, F(1)> states, N designates the angular momentum excluding spin) becomes strongly enhanced upon a lowering of the rotational temperature of the O(2) beam. The RSSRCS-CH in these two rotational states correlate linearly with the population of O(2) molecule in the specific K(O(2)) frame rotation number states: CH(|J = 1/2,F(2)>) with O(2)(|K(O(2)) = 1>);CH(|J = 3/2,F(1)>) with O(2)(|K(O(2)) = 3>). These linear correlations mean that the rotational-state-selected CH molecules are selectively reactive upon the incoming O(2) molecules in a specific rotational state; here, we use the term "rotationally correlated reactivity" to such specific reactivity depending on the combination of the rotational states between two molecular reactants. In addition, the steric asymmetry in the oriented CH (∣J = 1∕2,?F(2),?M = 1∕2>) + O(2) (|K(O(2)) = 1>) reaction turns out to be negligible (< ±1%). This observation supports the reaction mechanism as theoretically predicted by Huang et al. [J. Phys. Chem. A 106, 5490 (2002)] that the first step is an intermediate formation with no energy barrier in which C-atom of CH molecule attacks on one O-atom of O(2) molecule at a sideways configuration.     

Characterization of reactive intermediates generated during photolysis of 4-acetoxy-4-aryl-2,5-cyclohexadienones: oxenium ions and aryloxy radicals

Aryloxenium ions 1 are reactive intermediates that are isoelectronic with the better known arylcarbenium and arylnitrenium ions. They are proposed to be involved in synthetically and industrially useful oxidation reactions of phenols. However, mechanistic studies of these intermediates are limited. Until recently, the lifetimes of these intermediates in solution and their reactivity patterns were unknown. Previously, the quinol esters 2 have been used to generate 1, which were indirectly detected by azide ion trapping to generate azide adducts 4 at the expense of quinols 3, during hydrolysis reactions in the dark. Laser flash photolysis (LFP) of 2b in the presence of O(2) in aqueous solution leads to two reactive intermediates with lambda(max) 360 and 460 nm, respectively, while in pure CH(3)CN only one species with lambda(max) 350 nm is produced. The intermediate with lambda(max) 460 nm was previously identified as 1b based on direct observation of its decomposition kinetics in the presence of N(3)(-), comparison to azide ion trapping results from the hydrolysis reactions, and photolysis reaction products (3b). The agreement between the calculated (B3LYP/6-31G(d)) and observed time-resolved resonance Raman (TR(3)) spectra of 1b further confirms its identity. The second intermediate with lambda(max) 360 nm (350 nm in CH(3)CN) has been characterized as the radical 5b, based on its photolytic generation in the less polar CH(3)CN and on isolated photolysis reaction products (6b and 7b). Only the radical intermediate 5b is generated by photolysis in CH(3)CN, so its UV-vis spectrum, reaction products, and decay kinetics can be investigated in this solvent without interference from 1b. In addition, the radical 5a was generated by LFP of 2a and was identified by comparison to a published UV-vis spectrum of authentic 5a obtained under similar conditions. The similarity of the UV-vis spectra of 5a and 5b, their reaction products, and the kinetics of their decay confirm the assigned structures. The lifetime of 1b in aqueous solution at room temperature is 170 ns. This intermediate decays with first-order kinetics. The radical intermediate 5b decomposes in a biphasic manner, with lifetimes of 12 and 75 mus. The decay processes of 5a and 5b were successfully modeled with a kinetic scheme that included reversible formation of a dimer. The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals.     

Dynamical calculations of charge-transfer-to-solvent excited states of small I- (CH3CN)n clusters

Relaxation dynamics of photoexcited charge-transfer-to-solvent (CTTS) states for the I(-)(CH(3)CN)(n) (n = 2 and 3) clusters has been theoretically studied using electronic structure methods. First, we have calculated several lowest singlet and triplet potential energy surfaces using the multireference configuration interaction method. It was found that the character of the singlet CTTS excited-state potential surfaces is very similar to that of the triplet CTTS states. Due to a small singlet-triplet splitting, the lowest triplet potential energy surface was used as a good model to understand the dynamics of the photoexcited singlet CTTS states. We have carried out direct molecular dynamics simulations on the lowest triplet surface at the B3LYP level. When an I(-) anion is exteriorly solvated by CH(3)CN molecules, we found that the (CH(3)CN)(n)(-) anion cluster is effectively produced. In addition, when the I(-) anion is placed in the interior in I(-)(CH(3)CN)(n) clusters, photoexcitation gives an acetonitrile monomer anion plus neutral monomers. However, if the initial geometric configuration is distorted from the minimum structure, we also found that the (CH(3)CN)(2)(-) anion cluster, where an excess electron is internally trapped, is formed via I(-)(CH(3)CN)(2) + hnu --> I + (CH(3)CN)(2)(-) process.     

Intra- and intermolecular vibrational energy transfer in tungsten carbonyl complexes W(CO)5(X) (X=CO, CS, CH3CN, and CD3CN)

Vibrational energy relaxation of degenerate CO stretches of four tungsten carbonyl complexes, W(CO)6, W(CO)5(CS), W(CO)5(CH3CN), and W(CO)5(CD3CN), is observed in nine alkane solutions by subpicosecond time-resolved infrared (IR) pump-probe spectroscopy. Between 0 and 10 ps after the vibrational excitation, the bleaching signal of the ground-state IR absorption band shows anisotropy. Decay of the anisotropic component corresponds either to the rotational diffusion of the molecule or to the intramolecular vibrational energy transfer among the degenerate CO stretch modes. The time constant of the anisotropy decay, tauaniso, shows distinct solvent dependence. By comparing the results for the T1u CO stretch of W(CO)6 and the A1 CO stretch of W(CO)5(CS), the time constant of the rotational diffusion, taur, and the time constant of the intramolecular energy transfer among the three degenerate vibrational modes, taue, are determined as 12 and 8 ps, respectively. The tauaniso value increases as the number of carbon atoms in the alkane solvent increases. After 10 ps, the recovery of the bleaching becomes isotropic. The isotropic decay represents the vibrational population relaxation, from v=1 to v=0. In heptane, the time constant for the isotropic decay, tau1, for W(CO)5(CS) and W(CO)6 was 140 ps. The tau1 for the two acetonitrile-substituted complexes, however, shows a smaller value of 80 ps. The vibrational energy relaxation of W(CO)5(CH3CN) and W(CO)5(CD3CN) is accelerated by the intramolecular energy redistribution from the CO ligand to the acetonitrile ligand. In the nine alkane solutions, the tau1 value of W(CO)6 ranges between 124 and 158 ps, showing the apparent V-shaped solvent dependence with its minimum in decane, while the tau1 value shows little solvent dependence for W(CO)5(CH3CN) and W(CO)5(CD3CN).     

Synthesis, structure, and magnetic properties of [(S)-[PhCH(CH3)N(CH3)3]][Mn(CH3CN)2/3Cr(ox)3] x (CH3CN)_(solvate), a 2D chiral magnet containing a quaternary ammonium chiral cation

The synthesis, structure, and magnetic properties of a novel oxalate-based bimetallic magnet obtained by using the chiral (S)-trimethyl-(1-phenyl-ethyl)-ammonium, ((S)-[PhCH(CH3)N(CH3)3](+)), cation as template is reported. This compound can be formulated as [(S)-[PhCH(CH3)N(CH3)3]][Mn(CH3CN)2/3Cr(ox)3] x (CH3CN)_(solvate), and it crystallizes in the chiral trigonal space group P3. It shows a distorted two-dimensional honeycomb structure formed by Mn(II) and Cr(III) ions connected through oxalate anions with [(S)-[PhCH(CH3)N(CH3)3](+) cations and solvent molecules intercalated between the oxalate layers. Two-thirds of the Mn(II) ions of the honeycomb anionic network are heptacoordinated. This compound behaves as a soft ferromagnet with an ordering temperature of 5.6 K.     

Electron transfer from sodium to oriented nitromethane, CH3NO2: probing the spatial extent of unoccupied orbitals

Beams of sodium atoms with energies of a few eV are crossed with a beam of oriented CH3NO2 molecules to study the effect of collision energy and orientation on electron transfer. The electron transfer produces Na+ ions and free electrons, parent negative ions (CH)NO2-), and fragmentation ions NO2- and O- in proportions that depend on the collision energy. The steric asymmetry is very small or zero and suggests that production of all of the ions is favored by sideways attack with respect to the permanent dipole along the C-N axis. In these experiments, the electron appears to be transferred into the 2B1 state of the anion comprising mainly the pi*NO LUMO, producing a valence-bound state rather than a dipole-bound state.     

Electronic state dependence of the ion-molecule reaction CH(3)CN(+) + CH(3)CN → CH(4)CN(+) + CH(2)CN: threshold electron-secondary ion coincidence (TESICO) and direct ab initio molecular dynamics study

The ion-molecule reaction, CH(3)CN(+) + CH(3)CN → CH(3)CNH(+) + CH(2)CN, has been investigated using the threshold electron-secondary ion coincidence (TESICO) technique. Relative reaction cross sections for two microscopic reaction mechanisms, i.e., proton transfer (PT) from the acetonitrile ion CH(3)CN(+) to neutral acetonitrile CH(3)CN and hydrogen atom abstraction (HA) by CH(3)CN(+) from CH(3)CN, have been determined for two low-lying electronic states, (2)E and (2)A(1) of the CH(3)CN(+) primary ion. The cross section for PT of the (2)A(1) state was smaller than that of the (2)E state, whereas that of HA are almost the same in the two states. Ab initio calculations showed that the dissociation of the C-H(+) bond of CH(3)CN(+) is easier in the (2)E state than that in the (2)A(1) state. The direct ab initio molecular dynamics (MD) calculations showed that two mechanisms, direct proton transfer and complex formation, contribute the reaction dynamics.     

Molecular elimination in photolysis of fluorobenzene at 193 nm: internal energy of HF determined with time-resolved Fourier-transform spectroscopy

Following photodissociation of fluorobenzene (C(6)H(5)F) at 193 nm, rotationally resolved emission spectra of HF(1相似文献   

亚稳态He(23S)、Ar(3P0.2)原子与C2H3分子传能反应

自由基CN、CH、H在燃烧化学、大气化学、天体发光、环境污染等方面占有极为重要的地位，对于这些自由基发光及形成动力学机理的探讨，无疑是重要的．近年来，人们利用亚稳态惰性原子与膨化物碰撞传能，探讨了CN（AB－＋X）的化学发光［‘一、发现亚稳态的Ar（‘几，。）原子与H     

·OH自由基与CH3CN反应机理及动力学

在CBS-QB3水平上研究了CH3CN 和·OH反应的势能面, 其中包括两个中间体和9个反应过渡态. 分别给出了各主要物质的稳定构型、相对能量及各反应路径的能垒. 根据计算的CBS-QB3势能面, 探讨了CH3CN+·OH反应机理. 计算结果表明, 生成产物P1(·CH2CN+H2O)的反应路径在整个反应体系中占主要地位. 运用过渡态理论对产物通道P1(·CH2CN+H2O)的速率常数k1(cm3·molecule-1·s-1)进行了计算. 预测了k1(cm3·molecule-1·s-1)在250-3000 K温度范围内的速率常数表达式为k1(250-3000 K)=2.06×10^-20T^3.045exp(-780.00/T). 通过与已有的实验值进行对比得出, 在实验所测定的250-320 K 范围内, 计算得到的k1的数值与已有的实验值比较吻合. 由初始反应物生成产物P1 (·CH2CN+H2O)只需要克服一个14.2 kJ·mol-1的能垒. 而产物·CH2CN+H2O生成后要重新回到初始反应物CH3CN+·OH, 则需要克服一个高达111.2 kJ·mol-1的能垒,这就表明一旦产物P1生成后就很难再回到初始反应物.     

NMR and electron paramagnetic resonance studies of [Gd(CH3CN)9]3+ and [Eu(CH3CN)9]2+: solvation and solvent exchange dynamics in anhydrous acetonitrile

Homoleptic acetonitrile complexes [Gd(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](3) and [Eu(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](2) have been studied in anhydrous acetonitrile by (14)N- and (1)H NMR relaxation as well as by X- and Q-band EPR. For each compound a combined analysis of all experimental data allowed to get microscopic information on the dynamics in solution. The second order rotational correlation times for [Gd(CH(3)CN)(9)](3+) and [Eu(CH(3)CN)(9)](2+) are 14.5 ± 1.8 ps and 11.8 ± 1.1 ps, respectively. Solvent exchange rate constants determined are (55 ± 15) × 10(6) s(-1) for the trivalent Gd(3+) and (1530 ± 200) × 10(6) s(-1) for the divalent Eu(2+). Surprisingly, for both solvate complexes CH(3)CN exchange is much slower for the less strongly N-binding acetonitrile than for the more strongly coordinated O-binding H(2)O. It is concluded that this exceptional behavior is due to the extremely fast water exchange, whereas the exchange behavior of CH(3)CN is more regular. Electron spin relaxation on the isoelectronic ions is much slower than on the O-binding water analogues. This allowed a precise determination of the hyperfine coupling constants for each of the two stable isotopes of Gd(3+) and Eu(2+) having a nuclear spin.     

Carbazolyl nitrenium ion: electron configuration and antiaromaticity assessed by laser flash photolysis, trapping rate constants, product analysis, and computational studies

Laser flash photolysis of 1-(carbazol-9-yl)-2,4,6-trimethylpyridinium tetrafluoroborate generates the carbazolyl nitrenium ion (tau = 333 ns, kobs = 3.0 x 106 M-1s-1) having absorption bands at 570 and 620 nm in CH3CN. The nitrenium ion is found to have reactivity comparable to structurally similar closed-shell diarylnitrenium ions, but spectroscopic evidence favors an open-shell singlet diradical assignment for the observed nitrenium ion. The carbazolyl nitrenium ion is also more reactive than diarylnitrenium ions as a likely result of antiaromatic character. Ab initio and hybrid DFT calculations were performed to address the degree of antiaromaticity in this and similar nitrenium ions through analysis of optimized geometries, nucleus independent chemical shifts, and isodesmic reactions.