Low-energy resonances in photodissociation of CH+

Close coupling calculations for the photodissociation cross sections of CH⁺+hv → C⁺(²P) + H(²S) exhibit near threshold peaks not ascribable only to quasi-bound levels in the A ¹Π potential. Our calculations demonstrate the importance of non-adiabatic transitions and thereby suggest the necessity to revise earlier predictions of the rate of radiative association of C⁺ with H.     

Photoexcitation processes of CH3OH: Rydberg states and photofragment fluorescence

Photoabsorption and fluorescence cross sections of methanol vapor were mearured using synchrotron radiation. Weak structures observed in the 110–140 nm region are classified into three Rydberg series. Quasidiatomic repulsive potential curves for the states dissociating into CH₃ + OH(A²Σ⁺) are obtained from the measured fluorescence cross section. The photodissociation processes are discussed in accord with the fluorescence observed. The fluorescence quantum yield (< 0.8%) for photodissociation of CH₃OH is one order of magnitude smaller than that of H₂O, indicating a correlation that the fluorescence quantum yield decreases with increasing number of molecular orbitals.     

Modified electron transfer model for excitation functions of the type M + RX → MX + R and M+ + RX− (M = alkali,R = alkyl group,X = halogen) and M′ + N2O → MO* + N2 (M′= Ba OR Sm)

A one-dimensional model is described for the excitation functions of reactions that are initiated by an electron transfer at close range. The process is governed by a barrier in the entrance channel, the abortive reflection of trajectories at higher energies and by the competition of an adiabatic and a diabatic channel for the reactive flux. The model is fitted to measured cross sections for the (K.Rb)+CH₃I, K+C₂H₅Br and (Ba.Sm)÷ N₂O reactions and the electron transfer cross section for K+CH₃I → K⁺ + CH₃I^- is successfully predicted from the fitting parameters of the reactive channel.     

Photodissociation of CH4+ via ion cyclotron resonance

The molecular methane ion CH₄⁺ was observed to dissociate to the methyl cation and a hydrogen atom upon irradiation with while light from a 2.5 kW xenon are lamp. The maximum photodissociation cross section was estimated as 3×10^?19 cm² at 3350 Å and a spectrum was obtained.     

1,2-Eliminations from (CH3)2NH+CH2CH3 and (CH3)2NH 2+ : Guided dissociations

1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations, elimination of CH₂=CH₂ and CH₃CH₃ from (CH₃)₂NH⁺CH₂CH₃ (1) and of CH₄ from (CH₃)₂NH₂⁺ are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from methyl to the bridging position in ethyl and then to N to reach (CH₃)₂NH₂⁺ + CH₂=CH₂ from 1. CH₃CH₃ elimination by H-transfer to C₂H₅⁺ to form CH₃NH⁺=CH₂ + CH₃CH₃ also takes place. (CH₃)₂NH₂⁺ eliminates methane by CN bond extension followed by β-H-transfer to give CH₂=NH⁺ + CH₄. Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type of reaction. As in many complex-mediated reactions, these reactions transfer H between incipient fragments. They are distinguished from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described lower energy reactions, have 1,1-like transition states, or utilize highly asynchronous 1,2 transition states. All of these avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry.     

Photodissociation of the dimer ions Ar+2, Ne+2, and (CO2)+2

The tandem quadrupole photodissociation mass spectrometer has been used to study photodissociation reactions of Ar⁺₂, Ne⁺₂, and (CO₂)⁺₂. The cross sections for photodissociation of Ar⁺₂ exhibited a strong dependence on ion source pressure, varying from 2 × 10 ^?18cm² at 0.1 torr to 6 × 10^?19cm² at 0.5 torr. A large photodissociation cross section (2 × 10^?17cm² for the reaction (CO₂)⁺₂ → CO⁺₂+ CO₂ was observed at the red end of the visible spectrum (580–620 nm) suggesting that this may be an important reaction in CO₂ rich planetary ionspheres such as that of Mars.     

Chemiluminescent ion-molecule reactions in the system C+ + H2

The optical emission resulting from collisions between C⁺ ions and H₂ gas was measured in the energy range 2 to 20 eV_c.m.. The observed spectrum consists mainly of the CH⁺ A ¹Π → X ¹Σ⁺ band system; CH⁺ (A ^fΠ) is shown to be formed in the chemiluminescent reactio: C⁺(²P⁰) + H₂ → CH⁺(A ¹Π) + H(²S). The energy dependence of the emission cross section was measured. The occurrence of this reaction is discussed in terms of a electronic state correlation diagram for the system.     

Photodissociation spectrum of CH3I+ prepared by multiphoton ionization

Two-photon resonant multiphoton ionization has been used to prepare rotationally cooled and selected CH₃I⁺ ions with controlled vibrational and electronic distributions. These ions are photodissociated by a second laser. The highly simplified photodissociation spectrum displays a clear dissociation threshold as well as bands hidden in earlier spectra. The data suggests corrections to previously determined vibrational constants.     

Vibrational state analysis of unrelaxed BaI from the reactions Ba + CH3I and Ba + CH2I2

The reactions Ba + CH₃I → BaI + CH₃ and Ba + CH₂I₂ → BaI + CH₂I have been investigated by the method of laser-induced fluorescence. Excitation spectra are reported for BaI products formed under single-collision conditions in a “beam-gas” arrangement. The production of BaI for Ba + CH₂I₂ is found to be a major reaction pathway with a cross section about twice that for Ba + CH₃I. The relative vibrational populations show for both reactions bell-shaped distributions peaking close to υ = 21 for Ba + CH₃I and υ = 39 for Ba + CH₂I₂. The corresponding average fraction of the total reaction exoergicity that appears as BaI vibration is $\text{f}$_υ = 0.18 for Ba + CH₃I and $\text{f}$_υ = 0.29 for Ba + CH₂I₂. In the case of Ba + CH₃I, an estimate for the average relative translational energy of the products, obtained from the primitive angular distribution measurements of Lin, Mims and Herm, can be combined with the average vibrational excitation of BaI to provide evidence that the internal excitation of the methyl radical exceeds that of BaI. A model is discussed which postulates an electron jump in the exit valley of the Ba + CH₃I reaction to account for this feature of the reaction dynamics.     

Collisional Dissociation of CH3O2+

The collision-induced breakup of CH₃O₂⁺ ions, produced in various binary ion—neutral reactions, was investigated in a drift experiment in the energy range from 0.2 to 1.2 eV. The products observed were HCO⁺ (50%) and H₃O⁺ (50%), independent of the collision energy inducing the breakup. The energy barrier for the breakup is 22 ± 6 kcal/mole.     

A kinetic study of proton transfer reactions of NH+4, CH3NH+3, and PH+4 with calcium atoms

Cross section measurements for the proton transfer reactions of NH⁺₄, CH₃NH⁺₃, and PH⁺₄ with Ca(g) have been obtained over a range of low ion kinetic energies. For all reactions studied the cross sections drop sharply with increase in ion kinetic energy, indicating exothermic behavior. The results show that Ca(g) is an unusually strong base with a proton affinity in excess of 9.2 eV. Cross sections for the PH⁺₄Ca reaction are an order to magnitude higher than those for the NH⁺₄Ca reaction for ion energies between one and three eV. This effect is not explained by simple theories of ion-induced dipole interactions. It is suggested that the enhanced rate of the PH⁺₄Ca reaction may be due to d-orbital participation.     

Energy dependence of the total reaction cross section for Br− + CH4

The energy dependence of the total cross section for the reaction Br^? + CH₄ → CH₂Br^? + H₂ was measured in a beam experiment. From the threshold energy it can be assumed that the above reaction proceeds via the same transition state as the nucleophilic substitution leading to the also observed H^?. Thus we propose Br^? + CH₄ → [CH₄Br^?]

     

On the structure and photodissociation of cluster ions in the gas phase. (N2) (O2+) and (NO)2+

Results are presented for two experiments on N₂O₂⁺ cluster ions formed via the reactions O₂⁺ + N₂ + M → (N₂) (O₂⁺) + M (i), and NO⁺ + NO + M → (NO)₂⁺ + M (ii). In the first experiment the N₂O₂⁺ clusters are collisionally dissociated. The resulting collision-induced dissociation (CID) spectra show almost exclusively O₂⁺ and N₂⁺ products from N₂ O₂⁺ formed via the first reaction, and almost exclusively NO⁺ products from N₂O₂⁺ formed via the second reaction. In the second experiment, single-photon photodissociation of N₂O₂⁺ ions produced by both reactions (i) and (ii) was investigate using 514.5 and 634 nm radiation. The results indicate that the N₂O₂⁺ cluster from reaction (i) cannot be photodissociated while the N₂O₂⁺ cluster from reaction (ii) undergoes photodissociation at both wavelengths. These experiments indicate that two distinct N₂O₂⁺ cluster ions exist and that reactions (i) and (ii) selectively produce the two ions.     

Close-coupling elastic and inelastic integral and differential cross sections for Li+ + H2 collisions

The quantum mechanical close-coupling formalism is applied to the study of elastic and rotationally inelastic Li⁺ + H₂ collisions making use of the Kutzelnigg-Staemmler-Hoheisel potential energy surface. Integral and differential cross sections for j = 0 → 0 and j = 0 → 2 are obtained in the collision energy range 0.2 to 0.9 eV and for j = 1 → 1 and j = 1 → 3 at 0.6 eV. A rainbow structure is observed in both the elastic and inelastic angular distributions and a quenching of the fast oscillations is found in the cross sections for j = 1 initially compared to the case j = 0 initially. At 0.6 eV. the calculated quantum mechanical angular distributions are compared to those from a classical trajectory calculation using the same surface and to the experimental ones. The dynamics of rotational excitation in the Li⁺ + H₂ system is contrasted to rotational excitation in systems for which the atom-diatom interaction is predominantly repulsive.     

The photodissociation of CH+ through absorption into shape resonances of the A 1Π state

The locations and widths of shape resonances of the A ¹Π state of CH⁺ are calculated together with the absorption oscilla the strengths for absorption into those resonances which predissociate. The photodissociation cross sections through the two broadest resonances are calculated.     

The scale of reagent migration in the reaction CH3 + CH3CO2− in crystalline sodium acetate trihydrate

The ESR method has been applied to studies on the reaction CH₃ + CH₃CO₂^? → CH₄ + CH₂CO₂^? in a NaAc3H₂O lattice consisting of a CH₃CO₂Na/CD₃CO₂Na mixture. It has been shown that: (i) the CH₃ radical is at a lattice point in the position occupied by a methyl group in an undamage molecule; (ii) an arbitrary point near a radical is occupied equiprobably by either an H- or D-molecule; (iii) when a transferred H atom is substituted by a D, the isotropic effect is no less than 40. The scale of reagent migration during the reaction has been estimated.     

The photodissociation of CH+ through absorption into the A1Π state

Cross sections for the photodissociation of ground state CH⁺ ions through absorption into the A¹Π state are calculated for the lowest three vibrational levels. At threshold the cross sections for υ″ = 0, 1 and 2 are respectively 5.0 × 10^?20 cm², 8.1 × 10^?19 cm² and 2.6 × 10^?18 cm².     

Recent Chemistry Based on the [RuCp(CH3CN)3]+ Cation: Reappraisalof an Old Precursor

 This article gives an overview of recent chemistry based on the tris-acetonitrile complex [RuCp(CH₃CN)₃]⁺. Due to the labile nature of the CH₃CN ligands, substitution reactions are a dominant feature of this complex. Important derivatives are the highly reactive complexes [RuCp(PR ₃)(CH₃CN)₂]⁺ which are a source of the 14e⁻ fragment [RuCp(PR ₃)]⁺. These species are catalytically active in the redox isomerization of allyl alcohols to give aldehydes and ketones. Furthermore, the cationic complex [RuCp(κ¹(P),η²-PPh₂CH₂CH₂CH*CH₂)(CH₃CN)]PF₆ derived from the reaction of [RuCp(CH₃CN)₃]⁺ with PPh₂CH₂CH₂CH*CH₂ is a model compound for studying coupling reactions of olefins and acetylenes. In addition, [RuCp(CH₃CN)₃]⁺ is a valuable precursor for the synthesis of configurationally stable chiral three-legged piano-stool ruthenium complexes. These are currently being intensively investigated as Lewis acid catalysts in asymmetric synthesis.     

Reactions of C2(a 3πu) with CH4, C2H2, C2H4, C2H6, and O2 using multiphoton UV excimer laser photolysis

C₂(a ³π_u) disappearance rate constants of 1.44, 0.96, 0.0296, 0.0130 and < 10^?6(x10^?10cm³s^?1) are reported for reactions with C₂H₄, C₂H₂, O₂, C₂H₆, and CH₄, respectively at 298 K. C₂(a ³π_u) fragments are generated by multiphoton ArF excimer laser photodissociation at C₂H₂, and monitored by dye laser induced fluorescence. Arguments are presented which favor chemical reactions over the C₂(a ³π_u) → (X ¹σ⁺_g) quenching channel. C₂ + C₂H₂ represents the one possible exception to the reactive channel.     

On the mechanism of the ion-molecule reaction CH3+ + CH4 = C2H5+ + H2

Experimental evidence supporting the “direct” reaction model and the “intermediate complex” model for the reaction CH₃⁺(CH₄, H₂)C₂H₅⁺ are analysed. It is shown that the evidence for the former can equally well be interpreted in terms of a proposed model of persistent complex formation and decay. The plausibility of a “direct” mechanism is discussed and is found to be poor.