Ion-Pair Photodissociation of Trichloromonofluoromethane

Anionic fragments, F^- and Cl^- including two isotope species ³⁵Cl^- and ³⁷Cl^-, are ob-served in the photoexcitations of CFCl₃. The ion-pair anion e±ciency spectra of ³⁵Cl^- and ³⁷Cl^- are recorded in the photon energy range of 7.75～22.00 eV. The threshold of ion-pair dissociation CFCl₃→CFCl₂⁺+Cl^- is experimentally determined to be 7.94±0.04 eV. With the references of the high-resolution photoabsorption spectra reported in the literatures, we make tentative assignments of the electron valence-to-Rydberg transitions. Furthermore, the multibody ion-pair fragmentation processes to Cl^- are discussed by comparison between the calculated thermochemical thresholds and the experimental efficiency spectrum.     

Sequential two photon dissociation of cyanobenzene cation

In continuation of our studies in the new area of multiple photon laser dissociation of ions, we report evidence for the sequential two photon dissociation of cyanobenzene radical cation, C₆H₅CN⁺, to produce predominantly C₆H⁺₄ and HCN. The complete excitation function for this process, as well as for a single photon process occurring at higher energies, are compared to the photoelectron spectrum of cyanobenzene to elucidate the nature of the transitions involved. An exact kinetic expression is derived and used to obtain information about the absorption spectra of C₆H₅CN⁺ in its ground vibronic state and with internal energy between 2.0 and 2.8 eV. Finally, data from our earlier study of benzene radical cation is reanalyzed and discussed.     

Kinetic energy dependence of the reactions of N+ and N2+ with molybdenum

Mass-selected beams of N⁺ and N₂⁺ in the energy range 5–50 eV react with molybdenum to produce a surface nitride. The relative reaction cross section for N⁺ reaction is higher than that of N₂⁺ in the range 5–25 eV and N₂⁺ exhibits a reaction threshold near 7 eV. The N₂⁺ threshold suggests collisional dissociation prior to reaction.     

Electron impact dissociation of cyanobenzene radical cations by ion cyclotron resonance spectroscopy

Trapped ion cyclotron resonance spectroscopy has been used for the first time to study the electron impact dissociation of ions. Fragmentation of C₆H₅CH⁺ to produce C₆H⁺₄ and HCN is observed to occur at low electron energies (3–9 eV). The extent of dissociation is observed to be linear in emission current, rising from a threshold at 3.0 ± 0.5 eV to a maximum cross section estimated to be 6A² at 7.5 ± 0.5 eV. The implications of these results are discussed.     

Hartree-Fock energy curves for XΠ and Σ states of HF

Hartree-Fock energy curves have been calculated for the X²Π and ²Σ⁺ states of HF⁺ and applied to an analysis of the photoelectron spectra of HF. The ²Σ⁺ energy curve is found to have a barrier about 0.07 eV high due primarily to a repulsive ion-quadrupole interaction, and a depth of 0.37 eV. This curve will support two bound states and one shape resonance with a half-width of 0.015 eV. The energy curves are probably accurate to 0.1 eV but the analysis shows that results accurate to within 0.03 eV are required to resolve the experimental questions on the dissociation energy for the ground state of HF. The most recent experimental photoelectron results of Berkowitz (following article) encouraged a model calculation of the vibrational states of the ²Σ⁺ state. Assuming a dissociation energy of 0.45 eV and retaining the barrier three bound and one shape resonance vibrational levels are calculated for HF⁺ in agreement with the results reported by Berkowitz.     

Differential cross section for CN− formation from dissociative electron attachment to the cyanogen molecule C2N2

The differential cross section for CN^? formaition from dissoiiativc attachment on C₂N₂ has been obtained in a crossed-beam experiment. Below 10 eV incident electron energy thc CN^? cross section shows two broad overlapping peaks with maxima at 5.4 and 7.3 eV corresponding to the formation of CN^? in its ground electronic state ¹σ⁺ plus the CN radical in the first excited state²π and the ground stale ²Σ+ respcctively     

Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na⁺(H₂O)₃₀ results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na⁺(H₂O)₃₀, are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.     

Collisional dissociation of CH2Br+2 in selected internal energy states

The collisionally induced dissociation of CH₂Br⁺₂ to yield CH₂Br⁺ + Br has been investigated by photoelectronphotoion coincedence spectroscopy in which nominally zero kinetic electrons were detected. The reactant CH₂Br⁺₂ ions were produced by photoionzation with intenal energies of 0.0, 0.20 and 0.60 eV. For all three internal energies, the kinetic energy threshold for dissociation is just equal to the energy defect.     

Theoretical study of [F−; e+] and [CN−; e+]

Ab initio calculations are presented for the [F^?; e⁺] and [CN^?; e⁺] complexes. Positron affinities of 4.99 and 3.79 eV are obtained for F^? and CN^?, respectively. The excitation energies to the low-lying excited states of the positron complexes are also calculated.     

Ion—molecule reactions of CH+3 with acetonitrile at thermal translational energies

The 0.1 eV translational energy reactions of CH⁺₃ and CD⁺₃ with acetonitrile were studied in a tandem Dempster-ion cyclotron resonance mass spectrometer. Channels leading to CH₃CNH⁺ (7%), H₂CN⁺ (58%), and C₂H⁺₅ (35%) were observed. For CD⁺₃ reactant ions the H₂CN⁺ and C₂H⁺₅ products show evidence of complete H, D isotopic scrambling, suggesting an intermediate complex.     

Investigation of lacidipine,a new 1,4-dihydropyridine antihypertensive agent and three chemically related compounds by means of chemical ionization,mass spectrometry and collision-induced dissociation

Chemical ionization of two 1,4-dihydropyridines, lacidipine and its Z-isomer, and their corresponding pyridines in three different reagent gases and the collision-induced dissociation (CID) of their respective mass-selected protonated molecular ions in the collision energy range 10–200 eV were performed on a multiple quadrupole instrument. The weakness of the Breasted acid NH₄⁺ as a protonating agent is clearly manifested in one of the ammonia positive-ion chemical ionization (CI⁺) mass spectra which displays the addition ion, [M + NH₄]⁺, as the favoured reaction channel. The stereochemistry of the precursor molecules, the exothermicity of the protonation process and the threshold of certain dissociation channels as a function of the collision energy are among the arguments invoked to explain some of the observed differences between the CI⁺ mass spectra and the CID data of the different isomers investigated. In an attempt to present a more comprehensive study, some high-performance liquid chromatographic retention times and resolutions are also given.     

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.     

Ab initio calculations and molecular dynamics simulation of H2 adsorption on CN3Be3+ cluster

Hydrogen adsorption properties of the CN₃Be₃⁺ cluster have been studied using density functional theory and MP2 method with a 6–31++G** basis set. Five hydrogen molecules get adsorbed on the CN₃Be₃⁺ cluster with a hydrogen storage capacity of 10.98 wt%. Adsorption of three H₂ molecules on one of the three Be atoms in a cluster is reported for the first time. It is due to the more positive charge on this Be atom than the remaining two. The average value for H₂ adsorption energy in CN₃Be₃⁺ (5H₂) complexes is 0.41 (0.43) eV/H₂ at MP2 (wB97XD) level, which fits well within the ideal range. Adsorption energy from electronic structure calculations plays an important role in retaining the number of H₂ molecules on a cluster during atom-centered density matrix propagation (ADMP) molecular dynamics (MD) simulations. According to ADMP-MD simulations, out of five H₂ adsorbed molecules on CN₃Be₃⁺, four and two H₂ molecules remain absorbed on CN₃Be₃⁺ cluster at 275 K and 350 K, respectively, during the simulation.

     

Theoretical investigation of the character of the electronic states of H2O along a linear dissociation path leading to OH + H

A series of MRD CI calculations is presented for the linear asymmetric dissociation of singlet states of the water molecule. The present calculations complement previous work on H₂O, providing further information on conical intersections in the higher excited states and on the nature and dissociation limits of the third ¹A' state in C_s symmetry, which is shown to correlate with the ion-pair limit (OH⁻ +H⁺). At large distances the ion-pair state is no longer the third ¹A' species as other states, corresponding to the OH(X ²Π) + H(2s,2p) dissociation limit, are then lower in energy.     

Translational spectroscopy of N+ fragments from 5 keV collision-induced dissociation of N2+ ions by helium

Collision-induced dissociation of a 5–10 keV N₂⁺ beam impinging on a helium target has been reinvestigated by translational spectroscopy. The laboratory kinetic energy distribution of N⁺ fragments exhibits height structures on a continuum. They correspond to N⁺ and N fragments ejected in the c.m. frame with kinetic energies W of 4.75, 6.4, 6.8 and 8.1 eV and they are interpreted as transitions into excited states of N₂⁺ lying at more than 30 eV above the ground state of N₂. The experimental W distribution extending over 12 eV is compared to distributions calculated using the model of vertical Franck—Condon electronic excitation with different assumptions for the initial and final states.     

Stereoelectronic effects on the retro-Diels-Alder fragmentation of ionized bicyclo[4.3.0]nona-3,7-dienes

The electron impact and collision-induced dissociation mass spectra of cis- and trans-annulated bicyclo[4.3.0]nona-3,7-dienes differ in their relative abundances of [C₅H₆]⁺˙ fragments formed by the retro-Diels-Alder decomposition. The formation of [C₅H₆]⁺˙ is not preceded by hydrogen migration in the short-lived and long-lived molecular ions. The appearance energy of [C₅H₆]⁺˙ from both annulation isomers is identical within experimental error: AE_cis([C₅H₆]⁺˙)=10.56±0.10 eV and AE_trans([C₅H₆]⁺˙)=10.54±0.15 eV. The barrier to the retro-Diels-Alder fragmentation lies 68–76 kJ mol^?1 above the thermo-chemical threshold corresponding to [C₅H₆]⁺˙ + C₄H₆. Investigation of the two-dimensional reaction coordinate by the Topological Molecular Orbital treatment shows that the lowest energy path for the retro-Diels-Alder reaction involves a two-step dissociation of the C(5)? C(6) and C(1)? C(2) bonds in the molecular ion, the latter step overcoming a barrier, calculated as 80 kJ mol^?1 above the thermochemical threshold. The stereochemical difference between the geometric isomers is due to stereoelectronic assistance of the π orbitals of the cis-annulated isomer in the cleavage of the C(5)? C(6) bond. Other mechanisms of the retro-Diels–Alder reaction are discussed.     

Experimental and theoretical study on the photoionization of styrene

This study employed a vacuum ultraviolet synchrotron radiation source and reflectron time-of-flight mass spectrometry (TOF-MS) to investigate the photoionization and dissociation of styrene. By analyzing the photoionization mass spectrum and efficiency curve alongside G3B3 theoretical calculations, we determined the ionization energy of the molecular ion, appearance energy of fragment ions, and relevant dissociation pathways. The major ion peaks observed in the photoionization mass spectra of styrene correspond to C₈H₈⁺, C₈H₇⁺ and C₆H₆⁺. The ionization energy of styrene is measured as 8.46 ± 0.03 eV, whereas the appearance energies of C₈H₇⁺ and C₆H₆⁺ are found to be 12.42 ± 0.03 and 12.22 ± 0.03 eV, respectively, in agreement with theoretical values. The main channel for the photodissociation of styrene molecular ions is the formation of benzene ions, whereas the dissociation channel that loses hydrogen atoms is the secondary channel. Based on the experimental results and empirical formulas, the required dissociation energies (E_d) of C₈H₇⁺, C₈H₆⁺ and C₆H₆⁺ are calculated to be (3.96 ± 0.06), (4.00 ± 0.06) and (3.76 ± 0.06) eV, respectively. Combined with related thermochemical parameters, the standard enthalpies of formations of C₈H₈⁺, C₈H₇⁺, C₈H₆⁺ and C₆H₆⁺ are determined to be 964.2, 1346.3, 1350.2 and 1327.0 kJ/mol, respectively. Based on the theoretical study, the kinetic factors controlling the styrene dissociation reaction process are determined by using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This provides a reference for further research on the atmospheric photooxidation reaction mechanism of styrene in atmospheric and interstellar environments.     

A comparative basis-set study of NeH+ using coupled-cluster techniques

Unrestricted Hartree-Fock, coupled-cluster calculations are reported for the ground state of NeH⁺ using atomic basis sets of increasing size and accuracy for both Ne and H. The goal is to determine the basis set and coupled-cluster level of calculation needed to obtain a NeH⁺ potential energy curve of known accuracy. Here, it is shown that calculations using a quintuple zeta basis at the coupled-cluster singles and doubles level with noniterative triples, CCSD(T) , predict a Ne—H bond dissociation energy that is within about 0.01 eV of the exact Born–Oppenheimer molecular electronic structure result. Spectroscopic constants determined using the Simons–Parr–Finlan procedure are found to be in very good agreement with the experimental results. Calculations at the augmented quadruple zeta level for the two lowest triplet excited states of the NeH⁺ species are presented. Both of these states separate into ground-state Ne⁺ and H(1s). The resulting potential curves predict stable minima at the SCF, CCSD, and CCSD(T) levels with dissociation energies of about 0.07 eV. Spectroscopic constants from the potential curves and dissociation constants are reported. © 1994 John Wiley & Sons, Inc.     

(Noble Gas)n-NC+ Molecular Ions in Noble Gas Matrices: Matrix Infrared Spectra and Electronic Structure Calculations

An investigation of pulsed-laser-ablated Zn, Cd and Hg metal atom reactions with HCN under excess argon during co-deposition with laser-ablated Hg atoms from a dental amalgam target also provided Hg emissions capable of photoionization of the CN photo-dissociation product. A new band at 1933.4 cm⁻¹ in the region of the CN and CN⁺ gas-phase fundamental absorptions that appeared upon annealing the matrix to 20 K after sample deposition, and disappeared upon UV photolysis is assigned to (Ar)_nCN⁺, our key finding. It is not possible to determine the n coefficient exactly, but structure calculations suggest that one, two, three or four argon atoms can solvate the CN⁺ cation in an argon matrix with C−N absorptions calculated (B3LYP) to be between 2317.2 and 2319.8 cm⁻¹. Similar bands were observed in solid krypton at 1920.5, in solid xenon at 1935.4 and in solid neon at 1947.8 cm⁻¹. H¹³CN reagent gave an 1892.3 absorption with shift instead, and a 12/13 isotopic frequency ratio–nearly the same as found for ¹³CN⁺ itself in the gas phase and in the argon matrix. The CN⁺ molecular ion serves as a useful infrared probe to examine Ng clusters. The following ion reactions are believed to occur here: the first step upon sample deposition is assisted by a focused pulsed YAG laser, and the second step occurs on sample annealing: (Ar)₂⁺+CN→Ar+CN⁺→(Ar)_nCN⁺.     

Dissociation of ethane by electron impact

The absolute total dissociation cross section for ethane is reported for electron energies between 10 and 600 eV. A maximum value of 7.6 × 10^?16 cm² occurs at 80 eV while the apparent threshold is ≈ 10 eV. Dissociative ionization is more probable than dissociation into neutral fragments at all energies except in the threshold region. The data indicates that fragmentation involving methane elimination (c^? + C₂ H₆ → e^? + CH₄ + CH₂) occurs in less than 2% of the dissociative events for 50 < E < 600 eV. Arguments are presented which suggest that some of the lower excited states of ethane are stable against dissociation.