Internal energy distributions of tungsten hexacarbonyl ions after neutralization—Reionization

The internal energy distributions P(ε) transferred to W(CO)^+√₆ during the kiloelectronvolt collisions that occur upon neutralization-reionization (NR) have been estimated based on the relative abundances of the W(CO)^+√₀₋₆ products present in NR spectra (thermochemical method). The average internal energy of the incipient W(CO)^+√₆ ^* ions arising after near thermoneutral neutralization with trimethylamine followed by reionization with O₂ is −9 eV for 8-keV precursor ions and is mainly deposited during reionization. For comparison, the mean internal energy of W(CO)^+√₆* after electron ionization (EI) or collisionally activated dissociation (CAD) is −6 eV. Making the neutralization step endothermic slightly increases the overall excitation gained; however, a large increase in endothermicity ( > 16 eV) causes only a modest rise of the average internal energy (< 2 eV). The P(ε) curve for NR increases exponentially up to 6 eV and levels off at higher energies, showing that the probability of imparting large internal energies (6–17 eV) is high. In sharp contrast, the most probable excitation on CAD is ≤ 6 eV, and the probability of deposition of larger energies declines exponentially. The mean internal energies after CAD and NR decrease steadily when the kinetic energy is lowered. The structure (minima-maxima) observed in the P(ε) distribution for EI, which most likely originates from Franck-Condon factors, is not reproduced in the distributions for NR or high energy CAD, despite the fact that all three methods involve electronic excitation. Because of the large internal energies transferred upon NR, NR mass spectrometry could be particularly useful in the differentiation of ionic isomers with high dissociation but low isomerization thresholds. (J Am Soc Mass Spectrom 1994, 5, 1093-1101)     

Structure and stability of ammonium-sulfate and guanidium-sulfate complex

Ab initio (HF/6-31G^** and B3LYP/6-31 + + G^**) methods have been used to study the stability and structure of complexes between CH₃SO⁻₃ and CH₃NH⁺₃ or C(NH₂)⁺₃. Results show that no hydrogen jump is involved in the complex formations, which is different from previous work studying complexes between CH₃COO⁻ and CH₃NH⁺₃. In addition, we have studied complexes between CH₃SO⁻₃ and HC(NH₂)⁺₃ or ⁺H₃NC(NH₂)₃, all of which have a cage structure.     

Theoretical determination of the ionization potentials of the 1σ and 2σ electrons of CO(Σ)

Large-basis-set calculations of near Hartree-Fock accuracy were performed on CO⁺(1σ-hole ²Σ⁺) and CO⁺)2σ-hole, ²Σ⁺); correlation energies for these systems and for CO were calculated using an atoms-in-molecule approach, relativistic energies and vibrational structure corrections were also considered. The results are: IP(CO, 1σ) = 542.4 (542.57) eV, IP(CO,2σ) = 297.0 (296.24) cV, D_c(CO, ¹Σ⁺) = 10.8 (11.1) Ev, D₃(CO⁺, 1σ, ²Σ⁺) = 11.9 eV, D_e(CO⁺, 2σ, ²Σ⁺) = 9.1 eV, where IP and D_e stand respectively for ionization potential and dissociation energy, and where the numbers in parentheses refer to the most recent experimental values. The electron transfers resulting from the ionization of inner-shell electrons are discussed. Finally a quantitative correlation is developed correlating absolute chemical shifts to charge densities. Agreement between the calculated values and those derived from the correlation is quite satisfactory.     

Two-centre model potential energy calculations for the Σ and Π states of Li2, Na2, K2, Rb2 and Cs2

Two-centre model potential calculations have been carried out for the ²Σ⁺_g,u and ²Π_g,u states of Li⁺₂, Na⁺₂, K⁺₂, Rb⁺₂ and Cs⁺₂. Comparison with other model potential calculations suggests that reliable potential curves have been obtained. The results indicate the usefulness of calculating diatomic energies by the method proposed.     

Mechanism of the Selective Catalytic Reduction of NOx with NH3 over W-Doped Fe/TiO2 Catalyst

The W-doped Fe/TiO2 catalyst prepared by an impregnation method exhibited a good NH3-selective cata- lytic reduction（SCR） activity and N2 selectivity with broad operation temperature window. The interaction between Fe and W could increase the amount of surface chemisorbed oxygen, and thus enhances the low temperature SCR activity by facilitating the fast SCR of 2NH3＋NO＋NO2 →2N2＋3H2O. The NH3-SCR reaction mechanism over the W-Fe/TiO2 was fully investigated via in situ diffuse reflectance infrared Fourier transform spectroscopy（in situ DRIFTS）. In the low temperature range（〈250 ℃）, the reactive surface species were mainly coordinated NH3, ionic NH~ and adsorbed NO2 species, and the SCR mainly followed the Langmuir-Hinshelwood mechanism, during which the adsorbed NO2 species became the important active sites. In the high temperature range（〉250 ℃）, the reactive surface species were mainly NH2, and the SCR mainly followed the Eley-Rideal mechanism, during which the for- mation of NH2NO intermediate species after H-abstraction of NH3 was the rate-determining step.     

Quenching of the dissociative electron attachment resonances of O2 physisorbed on amorphous ice

Measurements of electron stimulated desorption (ESD) yields of O⁻, at incident electron energies below 20 eV, from 0.15 monolayers (ML) of O₂ physisorbed at 20 K on a variety of molecular solids have been performed. It is observed that for O₂ condensed on 4 ML of H₂O, the O⁻ signal from dissociative electron attachment (DEA) to O₂ is entirely absent. We attribute this to a complete quenching of the dissociative ²Π_u, ²Σ⁺_g, and ²Σ⁺_u, resonances of O⁻₂ by the adjacent water molecules.     

An ab initio molecular orbital study on the gas-phase rapid reaction of the silicon cation Si with ammonia

The gas-phase rapid ion-molecule reaction Si⁺ (²P) + NH₃→ SiNH₂⁺ + H is theoretically investigated by the ab initio molecular orbital methods. Several possible pathways (A, B, C) on its potential energy surface have been examined, discussed and compared. Theoretical calculations indicate that pathway A is favourable in energy and that the reaction begins by forming a collision complex of the ion-dipole molecule Si-NH⁺₃, which forms with no barrier into the first energy well of the reaction coordinate. Migration of an H atom from an N atom to a Si atom forms the intermediate HSi-NH⁺₂, which corresponds to the second energy well and can fragment to the observed product SiNH⁺₂ by losing an H atom from the Si atom. The barriers for migration and fragmentation are 52.5 and 38.6 kcal mol⁻¹ respectively. Pathway A has a negative activation energy of −42.1 kcal mol⁻¹.     

Mechanism for resonant O electron stimulated desorption from condensed O2. Non-adiabatic transitions and dynamics

Absolute kinetic energy distributions and yields associated with ground state ³P and excited state ¹D oxygen atoms have been obtained for O⁻ anion electron stimulated desorption from condensed O₂ in the electron energy range 6–15 eV. The observed yields are understood as resulting essentially from dissociative electron attachment reactions via the two lowest ²Σ⁺_g O⁻₂ resonance states through adiabatic and non-adiabatic transitions to the limits O⁻(²P) + O(³P) and O⁻(²P) + O(¹D). The kinetic energy distributions show the prominent role of electron multiple collision processes and post-dissociation interactions of the O⁻ anions in the condensed phase.     

Toward the infrared spectroscopic observation of SiH5: the silanium ion

Ab initio quantum mechanical methods, including the self-consistent field, single and double excitation configuration interaction, and single and double excitation coupled cluster, have been applied to six stationary points on the SiH⁺₅ potential energy hypersurface. Equilibrium geometries were determined using analytic energy first derivative techniques. Relative energies of stationary points have been obtained. Harmonic vibrational frequencies of the global minimum were obtained at all levels of theory. Basis sets used include double-zeta plus polarization and triple-zeta plus double polarization. SiH⁺₅ should be regarded as involving weakly bound H₂ and SiH⁺₃ subunits, with a dissociation energy of only about 10 kcal/mol. Pseudorotation was found to be unfavorable in the SiH⁺₅ ion.     

Classical trajectory calculation for ion-pair formation in K+O2 collisions

Three-dimensional trajectory surface hopping calculations were performed on two diabatic energy surfaces. The covalent surface describes the K(²S) + O₂(³Σ⁻_g) state and the ionic surface K⁺(¹S) + O⁻₂(²Π_g). Transitions from one surface to another were computed through the Landau—Zener model. At small deflection angles, the energy loss distribution exhibits two peaks, as observed, due to O⁻₂ in its electronic ground state and to vibrationally excited O⁻₂.     

Phosphorous disulfide and its ions in the electronic ground state: a gas phase theoretical study

Ab initio molecular orbital theory and density functional theory calculations were performed on the electronic ground states of the open-shell PS₂ molecule and its singly charged ions. A comparison of the optimized molecular structures indicates as the stepwise one-electron reduction of the PS₂⁺ ion, to yield PS₂ and PS₂⁻, provokes a symmetric elongation of both PS bonds along with a bending of its linear equilibrium geometry. The ionization potential (IP), adiabatic electron affinity (EA_ad), and atomization energy (AE) of the open-shell PS₂ molecule were calculated at different levels of theory. The following values were obtained at the more realistic UMP4SDTQ/6-311+G(3df)//UHF/6-311+G(3df) level of theory: IP=8.32 eV, EA_ad=3.03 eV and AE=12.40 eV. At the same level of theory, the calculated vertical detachment energy (VDE) of the PS₂⁻ anion is 3.22 eV. The donor–acceptor complexes formed in the gas-phase upon interaction of either one or two ammonia molecules with PS₂⁺ were also investigated. The calculated gas-phase binding energies indicate that the formation of the bis-adduct is favored over that of the mono-adduct by a binding energy gain of about 20 kcal/mol.     

Prompt and delayed photoejection of hydrated electrons in the UV photolysis of aqueous solutions of copper(I) complexes

The ejection of hydrated electrons from 266-nm laser-photoexcited solutions containing Cu(NH₃)⁺₃, CuCl²⁻₃, or CuBr²⁻₃ occurs through two pathways on the nanosecond time scale: a prompt ejection (ττ>laser pulsewidth) which follows a first-order rate law. This behavior is consistent with electron ejection from two excited states: the primary CTTS state, and longer-lived triplet species consisting of an exciplex and its precursor. The quantum yields for both prompt and delayed ejection are quite high, in the 0.15–0.4 range.     

Direct dynamics study of <Emphasis Type="Italic">N</Emphasis>-protonated diglycine surface-induced dissociation. Influence of collision energy

A quantum mechanical and molecular mechanical (QM + MM) direct dynamics classical trajectory simulation is used to study energy transfer and fragmentation in the surface-induced dissociation (SID) of N-protonated diglycine, (gly)₂H⁺. The peptide ion collides with the hydrogenated diamond {111} surface. The Austin Model 1 (AM1) semiempirical electronic structure theory is used for the (gly)₂H⁺ intramolecular potential and molecular mechanical functions are used for the diamond surface potential and peptide/surface intermolecular potential. The simulations are performed at collision energies E_i of 30, 50, 70, and 100 eV and collision angle of 0° (perpendicular to the surface). The percent energy transfer to the peptide ion is nearly independent of E_i, while energy transfer to the surface increases with increase in E_i. A smaller percent of the energy remains in peptide translation as E_i is increased. These trends in energy transfer are consistent with previous trajectory simulations of SID. At each E_i the most likely initial pathway leading to fragmentation is rupture of the ⁺H₃NCH₂---CONHCH₂COOH bond. Fragmentation occurs by two general mechanisms. One is the traditional Rice-Ramsperger-Kassel-Marcus (RRKM) model in which the peptide ion is activated by its collision with the surface, “bounces off”, and then dissociates after undergoing intramolecular vibrational energy redistribution (IVR). The other mechanism is shattering in which the ion fragments as it collides with the surface. Shattering is the origin of the large increase in number of product channels with increase in E_i, i.e., 6 at 30 eV, but 59 at 100 eV. Shattering becomes the dominant dissociation mechanism at high E_i.     

Role of the sulfhydryl group on the gas phase fragmentation reactions of protonated cysteine and cysteine containing peptides

The gas phase fragmentation reactions of protonated cysteine and cysteine-containing peptides have been studied using a combination of collisional activation in a tandem mass spectrometer and ab initio calculations [at the MP2(FC)/6-31G*//HF/6-31G* level of theory]. There are two major competing dissociation pathways for protonated cysteine involving: (i) loss of ammonia, and (ii) loss of the elements of [CH₂O₂]. MS/MS, MS/MS of selected ions formed by collisional activation in the electrospray ionization source as well as ab initio calculations have been carried out to determine the mechanisms of these reactions. The ab initio results reveal that the most stable [M + H − NH₃]⁺ isomer is an episulfonium ion (A), whereas the most stable [M + H − CH₂O₂]⁺ isomer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H]⁺ ions of all the possible simple dipeptide and tripeptide methyl esters containing one cysteine (where all other residues are glycine) has also been investigated. When cysteine is at the N-terminal position, NH₃ loss is observed, although the relative abundance of the resultant [M + H − NH₃]⁺ ion decreases with increasing peptide size. In contrast, when cysteine is at any other position, water loss is observed. The proposed mechanism for loss of H₂O is in competition with those channels leading to the formation of structurally relevant sequence ions.     

One-electron oxidation of the iron(II) complex of 1,10-phenanthroline-5,6-quinone A pulse radiolysis study

One-electron oxidation of the ferrous tris-PQ complex, a model for lipoxygenase, was attempted using oxidants such as •OH, N^•₃, Br^•-₂, Tl²⁺ and TlOH⁺. •OH was found to react with the complex with a bimolecular rate constant of (3.9±0.6)x10⁹dm³mol^-1s^-1, a rate which is not very dissimilar to that for the reaction with the ligand PQ. However the product of the reaction was found to be a OH-adduct rather than a cation radical. No reaction was found to occur with N^•₃ or Br^•-₂. Both Tl²⁺ and TlOH⁺ reacted with the complex to form its oxidised species with rate constants of (7.0±1)x10⁸dm³mol^-1s^-1 and (4.0±0.8)x10⁸dm³mol^-1s^-1, respectively. From a comparison of the rate constants and the transient spectra it was concluded that the centre of oxidation is the ligand rather than the metal.     

Gas-phase stability of cluster ions SF m + (SF6) n with m = 0–5 and n = 1–3

The gas-phase stabilities of cluster ions SF⁺_m (SF₆)_n with m = 0−5 were determined by using a high pressure mass spectrometer. The bond energies of SF⁺_m (SF₆)₁ were found to be less than 10 kcal/mol and to decrease with m = 0 → 5. There appear to be rather large gaps in the bond energies between n = 1 and 2 for the clusters SF⁺_m (SF₆)_n with m = 0−4. The structures of SF⁺₅, SF⁺ (SF₆)₁, SF⁺₃ (SF₆)₁, and SF⁺₅ (SF₆)₁ were investigated by ab initio molecular orbital calculations. For SF⁺₅, the D_3h geometry is found to be most stable andC_4v is a transition state of the Berry pseudorotation. For the ion-molecule complexes, the “on-top hat” models were found to be the most stable structures.     

Autoionization spectra of Li2 and the XΣg ground state of Li2: Experimental and theoretical investigations

Autoionizing Rydberg levels of Li₂ molecules in a supersonic molecular beam are populated by stepwise excitation with two tunable pulsed dye lasers. The observed autoionization spectra show severe perturbations. Based on calculations of quantum defects and a perturbation treatment of l-uncoupling a tentative assignment of Rydberg series up to n = 32 is proposed. The convergence limits of these series yield a value of IP = 41475 cm⁻¹ for the adiabatic ionization potential and a vibrational constant ω_e = 263 cm⁻¹ for the X²Σ⁺_g ground state of Li⁺₂. The experimental results are compared with ab initio calculations combined with a core polarization potential, which yield the potential curve. the dissociation energy, the quadrupole moment and the vibrational frequency for the X²Σ⁺_g ground state of Li⁺₂, in the excellent agreement with experimental findings.     

Theoretical Study on Ionic Liquid Based on 1-Ethyl-3-Methyl- Imidazolium Cation and Hexafluorophosphate or Tetrafluoroborate

The Hartree-Fock and DFT/B3LYP methods have been employed to investigate the electronic structures of 1-ethy1-3-methyl-imidazolium cation(EMIM~ ),BF_4~-,PF_6~-,EMIM~ -BF_4~-,and EMIM~ -PF_6~- using the Gaussian-94 soft-package at 6-31 G(d,p)basis set level for hydrogen,carbon,nitrogen,boron, phosphorus,and fluorine atoms.Comparison of the electronic structures of the lowest energy of EMIM~ - BF_4~- and EMIM~ -PF_6~- pairs,and single EMIM~ ,BF_4~- and PF_6~- showed that the optimized EMIM~ -BF_4~- and EMIM~ -PF_6~- pair conformers were BF_4~- and PF_6~- outside the 5-ring plane between the ethyl group and the methyl group.The cohesion of C—H…F hydrogen bond between cation and anion is reinforced by charge assistance.The interaction energy between EMIM~ and PF_6~- is 328.8 kJ/mol at the B3LYP level and 326.6 kJ/mol at the Hartree-Fock level,whereas that between EMIM~ and BF_4~- is 353.5 kJ/mol at the B3LYP level and 350.5 kJ/mol at the Hartree-Fock level.The low energy interactions caused by bulky asymmetric EMIM~ ,and charge dispersion of cation and anion give rise to the low melting point of ionic liquid EMIM~ -BF_4~- and EMIM~ -PF_6~-.The two hydrogen bonding models of single hydrogen bond formation,and the hydrogen transfer between C_2 in EMIM~ and F in BF_4~- or PF_6~- were principally depicted.     

A computational study of adducts between atomic chlorine and carbon dioxide, carbonyl sulfide and carbon disulfide

The geometries and vibrational frequencies of the adducts ClCO₂, ClCOS and ClCS₂ were derived at the Hartree-Fock (HF) 3-21G (^*) level. The C_a, structure of ClCO₂ corresponds to one C-O bond and one C=O bond. Similarly, C_a, ClCS₂ has one C-S and one C=S bond, and ClCOS has one C-S and one C-O bond. Single-point spin-projected fourth-order Møller-Plesset (MP4) 3-21G (^*) calculations at these geometries were used in bond-separation reactions to derive ΔH^o₀ for adduct formation, which is calculated to be about 39 kJ mol⁻¹ exothermic for ClCOS and ClCS₂, but about 39 kJ mol⁻¹ endothermic for ClCO₂. The C_2v structures for ClCO₂ and ClCS₂ were also characterized. The geometry of ClCS₂ has not been determined experimentally; comparison with an available measured entropy for ClCS₂ suggests that the C_2v structure is the one formed by addition of Cl to CS₂, although the energy relative to the C_a form is not reliably calculated because of instability in the HF wavefunction.     

Vibrational dependence of the NH3 (v2)+NO and NO(v)+NH3 charge transfer cross sections

A tandem quadrupole mass spectrometer is used to study the charge transfer reactions NH₃⁺ + NO and NO⁺ + NH₃ over a collision energy range 1.5–13 eV. The vibrational state of the reagent ions is selected by resonance-enhanced multiphoton ionization. For the 0.9 eV exothermic process NH₃⁺ + NO → NH₃ + NO⁺ excitation of the v₂ umbrella bending mode (v₂ = 0–12) causes no marked change in the charge transfer cross section, while in the reverse process NO⁺ + NH₃ → NO + NH₃⁺ excitation of the NO⁺ vibration (v = 0–6) strongly enhanced the charge transfer cross section.