Experimentally determined and calculated values of the energies required to form doubly charged ions of the bromomethanes CH3Br,CH2Br2 and CHBr3

Two experimental techniques were used to determine the double ionization energies of CH₃Br, CH₂Br₂ and CHBr₃. In one, these energies were measured directly by double-charge-transfer spectroscopy. In the other, charge stripping of [CH₃Br]⁺, [CH₂Br₂]⁺ and [CHBr₃]⁺ ions was investigated and the ionization energies of the singly charged ions were measured. The double ionization energies of the molecules obtained by adding known single ionization energies of the molecules to the single ionization energies of the ions were in good agreement with those determined by double-charge-transfer spectroscopy. The relevant mean values from the two techniques were 28.9 ± 0.5, 27.5 ± 0.5 and 29.1 ± 0.5 eV for the double ionization energy of CH₃Br, CH₂Br₂ and CHBr₃, respectively. The results of ab initio calculations using second-order Møller-Plesset perturbation theory were in good agreement with the observed double ionization energies; they were consistently slightly lower than the experimental values.     

Imaging the [1+1] Two-Photon Dissociation Dynamics of Br2+ in a Cold Ion Beam

The [1+1] two-photon dissociation dynamics of mass-selected ⁷⁹Br₂⁺ has been studied in acold ion beam using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The quartet 1⁴Σ^-_u,3/2 state of ⁷⁹Br₂⁺ is employed as an intermediate state to initiate resonance enhanced two-photon excitation to high-lying dissociative states in the 4.0-5.0 eV energy region above the ground rovibronic state. Total kinetic energy release (TKER) and the twodimensional recoiling velocity distributions of fragmented ⁷⁹Br⁺ ions are measured using the technique of DC-slice velocity map imaging. Branching ratios for individual state-resolved product channels are determined from the TKER spectra. The measured photofragment angular distributions indicate that the dissociation of ⁷⁹Br₂⁺ occurs in dissociative Ω=3/2 state via ΔΩ=0 parallel transition from the 1⁴Σ^-_u,3/2 intermediate state. Due to the considerable spin-orbit coupling effects in the excited states of ⁷⁹Br₂⁺, higher-lying dissociative quartet states are likely responsible for the observed photodissociation processes.     

The dissociation energies of gaseous Br2 and I2

New rotational analyses have been made of the B³Π_0⁺u—X¹Σ⁺_g systems of ⁷⁹Br₂, ⁸¹Br₂ and ¹²⁷I₂. The density of vibrational states near the dissociation limit in the upper states follows the LeRoy—Bernstein predictions for n = 5. From short extrapolations, the ground state dissociation energies are found to be: D₀(⁷⁹Br⁸¹Br) = 15895.6 cm^?1, D₀(¹²⁷I₂) = 12440.1 cm^?1.     

Theoretical study of the metastable peaks corresponding to the rotational predissociation of the CH4+ and CD4+ ions

Semiclassical calculations on diatomic type models are presented in order to account for the kinetic energy released in the metastable dissociation of the CH₄⁺ and CD₄⁺ ions; the discrete structure of the metastable peaks is considered.     

The vinyloxonium cation (CH2CHOH2+)

Ab initio Molecular orbital calculations with large basis sets and incorporating correlation are used to examine the structures and relative energies of the vinyloxonium (CH₂CHOH₂⁺) and 1-hydroxyethyl (CH₃CHOH⁺) cations. The best structure of the vinyloxonium ion has the OH₂ plane perpendicular to the CCO plane. The energy difference between the vinyloxonium and 1-hydroxyethyl cations is predicted to be 92 kJ mol^?1, substantially greater than a recent experimental estimate of 41 ± 12 kJ mol^?1     

Coincidence studies of fluorescence and dissociation processes in electronic excited states of I2+, Br2+, IBr+ and ICl+

The PIFCO technique in which mass-selected photoion—fluorescence photon coincidences are counted, was used to investigate whether I₂⁺, IBr⁺ and ICl⁺ fluoresce. Measurements were made of lifetimes and fluorescence quantum yields of electronic excited states of these ions. Emission was discovered for I₂⁺ and IBr⁺, but ICl⁺ apparently does not fluoresce. Information on the radiative properties of Br₂⁺ was obtained as a by-product of the work on IBr⁺. Fragment ion kinetic energy releases were determined and provide information on dissociative ionization processes in the halogen and interhalogen ions studied.     

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.     

Kinetics of Surface-Induced Dissociation of N(CH3) 4 + and N(CD3) 4 + Using Silicon Nanoparticle Assisted Laser Desorption/Ionization and Laser Desorption/Ionization

The implementation of surface-induced dissociation (SID) to study the fast dissociation kinetics (sub-microsecond dissociation) of peptides in a MALDI TOF instrument has been reported previously. Silicon nanoparticle assisted laser desorption/ionization (SPALDI) now allows the study of small molecule dissociation kinetics for ions formed with low initial source internal energy and without MALDI matrix interference. The dissociation kinetics of N(CH₃)₄⁺ and N(CD₃)₄⁺ were chosen for investigation because the dissociation mechanisms of N(CH₃)₄⁺ have been studied extensively, providing well-characterized systems to investigate by collision with a surface. With changes in laboratory collision energy, changes in fragmentation timescale and dominant fragment ions were observed, verifying that these ions dissociate via unimolecular decay. At lower collision energies, methyl radical (CH₃) loss with a sub-microsecond dissociation rate is dominant, but consecutive H loss after CH₃ loss becomes dominant at higher collision energies. These observations are consistent with the known dissociation pathways. The dissociation rate of CH₃ loss from N(CH₃)₄⁺ formed by SPALDI and dissociated by an SID lab collision energy of 15 eV corresponds to log k = 8.1, a value achieved by laser desorption ionization (LDI) and SID at 5 eV. The results obtained with SPALDI SID and LDI SID confirm that (1) the dissociation follows unimolecular decay as predicted by RRKM calculations; (2) the SPALDI process deposits less initial energy than LDI, which has advantages for kinetics studies; and (3) fluorinated self-assembled monolayers convert about 18% of laboratory collision energy into internal energy. SID TOF experiments combined with SPALDI and peak shape analysis enable the measurement of dissociation rates for fast dissociation of small molecules.     

Investigation on the mechanism and applications of the reaction Cl2+2HBr=2HCl+Br2

The mechanism of reaction Cl₂+2HBr=2HCl+Br₂ has been carefully investigated with density functional theory (DFT) at B3LYP/6-311G^** level. A series of three-centred and four-centred transition states have been obtained. The activation energy (138.96 and 147.24 kJ/mol, respectively) of two bimolecular elementary reactions Cl₂+HBr→HCl+BrCl and BrCl+HBr→HCl+Br₂ is smaller than the dissociation energy of Cl₂, HBr and BrCl, indicating that it is favorable for the title reaction occurring in the bimolecular form. The reaction has been applied to the chemical engineering process of recycling Br₂ from HBr. Gaseous Cl₂ directly reacts with HBr gas, which produces gaseous mixtures containing Br₂, and liquid Br₂ and HCl are obtained by cooling the mixtures and further separated by absorption with CCl₄. The recovery percentage of Br₂ is more than 96%, and the Cl₂ remaining in liquid Br₂ is less than 3.0%. The paper provides a good example of solving the difficult problem in chemical engineering with basic theory.     

Photodissociation of CH3Cl+ and CH3Br+ in a tandem quadrupole mass spectrometer

A new tandem quadrupole photodissociation mass spectrometer was used to measure photodissociation cross sections for the reactions, CH₃Cl⁺ → CH₃⁺ + Cl and CH₃Br⁺ → CH₃⁺ + Br in the gas phase using wavelength-selected light. The results on CH₃Cl⁺ are compared with the earlier work of Dunbar. For both reactions we are able to observe photodissociations occurring with small cross sections (≈ 2 × 10^?20 cm²) in the visible region near the thermochemical thresholds.     

Experiments and Calculations on Photoionization and Dissociative Photoionization of CH2CO

The dissociative photoionization of molecular‐beam cooled CH₂CO in a region of ?10–20 eV was investigated with photoionization mass spectrometry using a synchrotron radiation as the light source. Photoionization efficiency curves of CH₂CO⁺ and of observed fragment ions CH₂⁺, CHCO⁺, HCO⁺, C₂O⁺, CO⁺, and C₂H₂⁺ were measured to determine their appearance energies. Relative branching ratios as a function of photon energy were determined. Energies for formation of these observed fragment ions and their neutral counterparts upon ionization of CH₂CO are computed with the Gaussian‐3 method. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. The principal dissociative processes are direct breaking of C=C and C‐H bonds to form CH₂⁺ + CO and CHCO⁺ + H, respectively; at greater energies, dissociation involving H migration takes place.     

Endothermic reactions of Ni+ with H2, HD and D2

An ion-beam apparatus is employed to study the reaction of Ni⁺ with H₂, HD, and D₂ as a function of kinetic energy. These reactions lead to the endothermic formation of NiH⁺, NiH⁺ and NiD⁺, and NiD⁺, respectively. Interpretation of the threshold for these processes yields the average bond energies, D⁰(Ni⁺H) = 1.86 ± 0.09 eV and D⁰(Ni⁺D) = 1.90 ± 0.14 eV. The total reaction cross sections for all three systems are similar; however, a striking isotope effect is observed for Ni⁺ reacting with HD. The dependence of the cross sections on relative kinetic energy is discussed in terms of simple models for reaction.     

Electron impact ionization of unstable enols: H2CCHOH,H2CC(OH)?CH3 and H2CC(OH)?C2H5

Ethenol, 2-hydroxypropene and 2-hydroxybutene-1 were prepared by low-pressure pyrolysis of cyclobutanol, 1-methylcyclobutanol and 1-ethylcyclobutanol, respectively. Mass spectra, ionization energies, appearance energies of metastable ions and kinetic energy releases were determined on a reverse Nier-Johnson double-focusing mass spectrometer. Mercury and CH₃ radicals from the pyrolysis of dimethylmercury were employed for calibration of the energy scale. The ionization energy of 2-hydroxybutene-1 is 8.55 ± 0.1 eV and the appearance energies of [C₂H₅CO]⁺ and [CH₃CO]⁺ from that molecule are 10.25 ± 0.1 and 10.40 ± 0.1 eV, respectively. Changes observed in metastable peak shapes for certain fragmentation reactions upon pyrolysis are discussed.     

The proton formation from methane by dissociative ionization in the electron energy range of 25–40 eV

The proton formation by dissociative electroionization of methane has been investigated in the energy range of 25–40 eV. The kinetic energy-versus-appearance energy shows five different H⁺ producing processes respectively at 26.3 ± 0.2 eV, 26.9 ± 0.2 eV, 29.4 ± 0.3 eV, 32.7 ± 0.2 eV and 35.7 ± 0.5 eV. These critical energies are discussed in terms of different dissociation channels probably opened through predissociation of doubly excited states of CH⁺₄. On the high energy side of the electron energy range investigated in the present work, the proton would appear through the dissociation of the CH⁺ ion as an intermediate.     

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.     

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^?]

     

Unimolecular dissociations of excited C3H6O+: A comparative photoelectron—photoion coincidence study of 1,2-epoxypropane and acetone

The unimolecular fragmentation of internal energy selected 1,2-epoxypropane cations has been studied by fixed-wavelength photoelectron—photoion coincidence spectroscopy. Branching ratios for the prominent fragment ions are reported up to an ionization energy of I = 14 eV. It is shown that 1,2-epoxypropane cations initially formed with none or only little vibrational excitation in the electronic ground state do not dissociate, though their excess energy with respect to the lowest energetic fragmentation pathway is 1.25 eV. As the internal energy is increased, slow fragmentation into several dissociation channels is observed. This is used to explain a comparably slow dissociation process observed in the case of acetone molecular ions initially excited to their electronic à state. CH₂C(OH)CH₃⁺ and/or CH₃CHCHOH⁺ are proposed as precursors for these low-rate unimolecular reactions.     

Completing dissociation channels of nitromethane and methyl nitrite ions and the role of electronic and internal modes of excitation

Unimolecular dissociation reactions of the isomeric ions of nitromethane and methyl nitrite have been investigated for various electronic states of the ions up to an ionization energy of 18 eV using a photoelectron-photoion coincidence technique. The principal dissociation channels of the nitromethane ion lead to CH₃⁺, NO₂⁺ and NO⁺. Of these only NO⁺ is formed from methyl nitrite ion. A comparison of the NO⁺ dissociation data from both precursors indicates that rearrangement of the nitromethane to the more stable methyl nitrite ion structure preceding NO⁺ formation occurs in competition with direct CN bond cleavage dissociation of the nitromethane ion.Although the product kinetic energy distributions, measured as a function of the internal energy of a given parent-ion electronic state, were statistical in nature, evidence of a more specific dependence upon the initial electronic state was found. The formation of excited stated of the neutral NO₂ fragment which accompanies the CH₃⁺ product from the higher electronic states of the nitromethane ion was identified from the product kinetic energy data. The secondary dissociation of the NO₂⁺ product to NO⁺ + O was also recognized from the kinetic energy data by a comparison of the reduced, or scaled, forms of the energy distribution functions above and below the thermodynamic limit for the secondary reaction.     

Comparison of collisional activation spectra of some positive ions produced both by charge reversal of negative ions and by decomposition of positive ions

The charge reversal collision induced decomposition mass analyzed ion kinetic energy spectrum of allyl anion has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of allyl cation and found to be identical except for the presence of +2 ions formed by charge stripping in the spectrum of the [C₃H₅]⁺ ion. Likewise, the collision induced dissociation mass analyzed ion kinetic energy charge reversal spectrum of [CH₃Se]^? has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of [CH₃Se]⁺ and found to be identical. A study of the pressure dependence of the collision induced dissociation mass analyzed ion kinetic energy spectrum of [C₃H₅]⁺ and [C₃H₅]^? showed increasing fragmentation with increasing collision gas pressure, and suggests that a greater mean number of collisions converts more energy to internal modes in the collision induced dissociation mass analyzed ion kinetic energy experiment even at low pressures.     

Energy disposal in the rotational pre-dissociation of ND3+

Measurements of the fragmentation ND₃⁺ → ND₂⁺ + D and of the resulting kinetic energy distributions were made near the dissociation threshold and confirm that rotational energy promotes the reaction. Interpretation of the data by a statistical-dynamical theory indicates that high angular momentum states tend to release a greater proportion of their energy into translation of the products.