Theoretical Study of Isoprene Dissociative Photoionization

Theoretical calculations have been carried out to investigate the possible dissociation channels of isoprene. We focus on the major fragment ions of C₅H₇⁺,C₅H₅⁺,C₄H₅⁺,C₃H₆⁺,C₃H₅⁺,C₃H₄⁺,C₃H₃⁺ and C₂H₃⁺, which were observed experimentally from the isoprene dissociative photoionization. The energy calculations were performed with the CBS-QB3 model. All the geometries and energies of the fragments, intermediates and transition states involved in the dissociations channels were determined. Finally, the mechanisms of the dissociation pathways were discussed on the comparison of theoretical and experimental results.     

Dissociative Photoionization of 1,4-Dioxane with Tunable VUV Synchrotron Radiation

The photoionization and photodissociation of 1,4-dioxane have been investigated with a reflectron time-of-flight photoionization mass spectrometry and a tunable vacuum ultraviolet synchrotron radiation in the energy region of 8.0-15.5 eV. Parent ion and fragment ions at m/z 88, 87, 58, 57, 45, 44, 43, 41, 31, 30, 29, 28 and 15 are detected under supersonic conditions. The ionization energy of DX as well as the appearance energies of its fragment ions C₄H₇O₂⁺, C₃H₆O⁺, C₃H₅O⁺, C₂H₅O⁺, C₂H₄O⁺, C₂H₃O⁺, C₃H₅⁺, CH₃O⁺, C₂H₆⁺, C₂H₅⁺/CHO⁺, C₂H₄⁺ and CH₃⁺ was determined from their photoionization efficiency curves. The optimized structures for the neutrals, cations, transition states and intermediates related to photodissociation of DX are characterized at the B3LYP/6-31+G(d,p) level and their energies are obtained by G3B3 method. Possible dissociative channels of the DX are proposed based on comparison of experimental AE values and theoretical predicted ones. Intramolecular hydrogen migrations are found to be the dominant processes in most of the fragmentation pathways of 1,4-dioxane.     

Dissociative Photoionization of m-Xylene

The photoionization and dissociative photoionization of m-xylene (C₈H₁₀) were researched by using synchrotron radiation vacuum ultraviolet (SR-VUV) and supersonic expanding molecular beam reflectron time-of-flight mass spectrometer (RFTOF-MS) system. The photoionization efficiency spectra (PIEs) of parent ion C₈H₁₀⁺ and main fragment ions C₈H₉⁺ and C₇H₇⁺ were observed, and the ionization energy (IE) of m-xylene and appearance energies (AEs) of main fragment ions C₈H₉⁺ and C₇H₇⁺ were determined to be 8.60 ± 0.03 eV, 11.76 ± 0.04 eV and 11.85 ± 0.05 eV, respectively. Structures of reactant, transition states (TSs), intermediates (INTs), and products involved in two dominant dissociation channels were optimized at the B3LYP/6-311++G(d,p) level, and the relative energies were calculated at the G3 level. Based on the results, two major dissociative photoionization channels, C₇H₇⁺+CH₃ and C₈H₉⁺+H were calculated at the B3LYP/6-311++G(d,p) level. On the basis of theoretical and experimental results, the dissociative photoionization mechanisms of m-xylene were proposed. The C–H or C–C bond dissociation and hydrogen migration are the main processes in the dissociation channels of m-xylene cation.

     

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.     

Photoionization and Dissociation Study of 2-Methyl-2-propen-1-ol: Experimental and Theoretical Insights

The photoionization and dissociation of 2-methyl-2-propen-1-ol (MPO) have been investigated by using molecular beam experimental apparatus with tunable vacuum ultravioletsynchrotron radiation in the photon energy region of 8.0-15.5 eV. The photoionization efficiency (PIE) curves for molecule ion and fragment ions: C₄H₈O⁺、C₄H₇O⁺、C₃H₅O⁺、C₄H₇⁺、C₄H₆⁺、C₄H₅⁺、C₂H₄O⁺、C₂H₃O⁺、C₃H₆⁺、C₃H₅⁺、C₃H₃⁺、CH₃O⁺、CHO⁺ have been measured, and the ionization energy (IE) and the appearance energies (AEs) of the fragment ions have been obtained. The stable species and the first order saddle points have been calculated on the CCSD(T)/cc-pvTZ//B3LYP/6-31+G(d,p) level. With combination of theoretical and experimental results, the dissociative photoionization pathways of 2-methyl-2-propen-1-ol are proposed. Hydrogen migrations within the molecule are the dominant processes in most of the fragmentation pathways of MPO.     

Dissociative photoionization of isoprene: experiments and calculations

Vacuum ultraviolet (VUV) dissociative photoionization of isoprene in the energy region 8.5–18 eV was investigated with photoionization mass spectroscopy (PIMS) using synchrotron radiation (SR). The ionization energy (IE) of isoprene as well as the appearance energies (AEs) of its fragment ions C₅H₇⁺, C₅H₅⁺, C₄H₅⁺, C₃H₆⁺, C₃H₅⁺, C₃H₄⁺, C₃H₃⁺ and C₂H₃⁺ were determined with photoionization efficiency (PIE) curves. The dissociation energies of some possible dissociation channels to produce those fragment ions were also determined experimentally. The total energies of C₅H₈ and its main fragments were calculated using the Gaussian 03 program and the Gaussian‐2 method. The IE of C₅H₈, the AEs for its fragment ions, and the dissociation energies to produce them were predicted using the high‐accuracy energy model. According to our results, the experimental dissociation energies were in reasonable agreement with the calculated values of the proposed photodissociation channels of C₅H₈. Copyright © 2009 John Wiley & Sons, Ltd.     

Photofragmentation of Isoleucine by Vacuum Ultraviolet Photoionization

Vacuum ultraviolet photon-induced ionization and dissociation of isoleucine are investi-gated with synchrotron radiation photoionization mass spectroscopy and theoretical cal-culations. The main fragment ions at m/z=86, 75, 74, 69, 57, 46, 45, 44, 41, 30, 28, and 18 from isoleucine are observed in the mass spectrum at the photon energy of 13 eV. From the photoionization e±ciency curves, appearance energies for the principal fragment ions C₅H₁₂N⁺ (m/z=86)、C₂H₅NO₄⁺ (m/z=75)、C₅H₉⁺ (m/z=69)、C₄H₉⁺(m/z=57), and CH₄N⁺(m/z=30) are determined to be 8.84±0.07, 9.25±0.06, 10.20±0.12, 9.25±0.10, and 11.05±0.07 eV, respectively, and possible formation pathways are established in detail by the calculations at the B3LYP/6-31++G(d, p) levels. These proposed channels include simple bond cleavage reactions as well as reactions involving intermediates and transition structures. The experimental and computational appearance energies or barriers are in good agreement.     

Vacuum ultraviolet photoionization mass spectrometric study of cyclohexene

In this work, photoionization and dissociation of cyclohexene have been studied by means of coupling a reflectron time‐of‐flight mass spectrometer with the tunable vacuum ultraviolet (VUV) synchrotron radiation. The adiabatic ionization energy of cyclohexene as well as the appearance energies of its fragment ions C₆H₉⁺, C₆H₇⁺, C₅H₇⁺, C₅H₅⁺, C₄H₆⁺, C₄H₅⁺, C₃H₅⁺ and C₃H₃⁺ were derived from the onset of the photoionization efficiency (PIE) curves. The optimized structures for the transition states and intermediates on the ground state potential energy surfaces related to photodissociation of cyclohexene were characterized at the ωB97X‐D/6‐31+g(d,p) level. The coupled cluster method, CCSD(T)/cc‐pVTZ, was employed to calculate the corresponding energies with the zero‐point energy corrections by the ωB97X‐D/6‐31+g(d,p) approach. Combining experimental and theoretical results, possible formation pathways of the fragment ions were proposed and discussed in detail. The retro‐Cope rearrangement was found to play a crucial role in the formation of C₄H₆⁺, C₄H₅⁺ and C₃H₅⁺. Intramolecular hydrogen migrations were observed as dominant processes in most of the fragmentation pathways of cyclohexene. The present research provides a clear picture of the photoionization and dissociation processes of cyclohexene in the 8‐ to 15.5‐eV photon energy region. Copyright © 2016 John Wiley & Sons, Ltd.     

Vacuum Ultraviolet Photoionization and Dissociative Photoionization of Capecitabine, 50-Deoxy-5-'uorocytidine,and 50-Deoxy-5-'uorouridine

Vacuum ultraviolet (VUV) photoionization and dissociative photoionization of capecitabine and its metabolites, 5'-deoxy-5-fluorocytidine (5'-DFCR) and 50-deoxy-5- fluorouridine (5'-DFUR), were investigated with infrared laser desorption/tunable synchrotron VUV pho-toionization mass spectrometry. Molecular ions (M+) with small amounts of fragments can be found for these compounds at relatively low photon energies, while more fragment ions would be produced by increasing the photon energies. (M-H₂O)⁺, (base+H)⁺, (base+2H)⁺,(base+30)⁺, (base+60)⁺, and sugar moiety were proposed for these nucleoside drugs with similar backbones. Decomposition channels for the major fragments were discussed in de-tail. Moreover, ab initio calculations were introduced to study the dehydration pathways of three fluoro-nucleosides. Corresponding appearance energies for the (M-H₂O)⁺ ions were computed.     

A photoionization study of [C4H7]+ ions in the gas phase

The ionization and [C₄H₇]⁺ appearance energies for a series of C₄H₇CI and C₄H₇Br isomers have been measured by dissociative photoionization mass spectrometry. Cationic heats of formation, based on the stationary electron convention, are derived. No threshold ion is observed with a heat of formation corresponding to the trans-1-methylallyl cation, although there is evidence for formation of the less stable cis isomer. A 298 K heat of formation of 871 kJ mol^?1 is obtained for the cyclopropylcarbinyl cation, with the cyclobutyl cation having a higher value of 886 kJ mol^?1. At the HF/6-31G^** level, ab initio molecular orbital calculations show the 2-butenyl, isobutenyl and homoallyl cations to be stable forms of [C₄H₇]⁺, being less stable than the trans-1-methylallyl cation by 101 kJ mol^?1, 159 kJ mol^?1 and 164 kJ mol^?1, respectively. However, threshold formation is not observed for any of these ions, the fragmentation of appropriate precursor molecules producing [C₄H₇]⁺ ions with lower energy structures.     

VUV Photoionization and Dissociation of Tyramine and Dopamine: the Joint Experimental and Theoretical Studies

Photon induced dissociation investigations of neutral tyramine and dopamine are carried out with synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. At low photon energy, only molecular ions are measured by virtue of near-threshold photoionization. While increasing photon energy to 11.7 eV or more, four distinct fragment ions are obtained for tyramine and dopamine, respectively. Besides, the ioniza-tion energies of tyramine and dopamine are determined to be 7.98±0.05 and 7.67±0.05 eV by measuring the photoionization efficiency curves of corresponding molecular ions. With help of density function theory calculations, the detailed fragmentation pathways are es-tablished as well. These two molecular cations have similar aminoethyl group elimination pathways, C₇H₈O₂^+·(m/z=124) and C₇H₈O^+·(m/z=108) are supposed to be generated by the McLafferty rearrangement via γ-hydrogen (γ-H) shift inducing β-fission. And CH₂NH₂⁺ is proposed to derive from the direct fission of C7-C8 bond. Besides, the McLafferty rear-rangement and the C7-C8 bond fission are validated to be dominant dissociation pathways for tyramine and dopamine cations.     

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 Photoionization study of some benzoyl compounds—Thermochemistry of [C7H5O]+ formation

Appearance energies for [C₇H₅O]⁺ fragment ions, formed from a variety of benzoyl compounds, have been measured by photon impact. The mean heat of formation calculated for [C₇H₅O]⁺ is 705 ± 6 kJ mol^?1, which can be correlated with either a single threshold structure or a mixture of structures with similar heats of formation. Previously observed variations of ΔH_f([C₇H₅O]⁺) with precursor molecule are shown to be consistent with a structure dependent kinetic shift, rather than the presence of isolated electronic states.     

Molecular ionization-dissociation of fluoranthene at 266 nm: Energetic and dissociative pathways

Experimental studies have been carried out for nanosecond 266-nm laser-induced photoionization and dissociation of fluoranthene, C₁₆H₁₀ with pulse energies from 0.5 to 20 mJ using a time of flight mass spectrometer. The fragmentation patterns have been characterized and discussed with respect to the number of absorbed photons. They fall into three regimes. The first regime involves low energy processes, where the molecular parent ion promptly dissociates, resulting in the formation of Cm⁺Hn(m=11−15) by a process where up to two photons are absorbed. The second regime involves intermediate energy, where dissociative processes are activated by up to three-photon absorption and produce a second group of daughter ions: C10⁺Hn, C₉⁺Hn, and C₈⁺Hn. Finally, there is a third dissociative process, characterized by the absorption of up to four photons, producing C₇⁺Hn, C₆⁺Hn, C₅⁺Hn, C₄⁺Hn, and C₃⁺Hn. Most of the detected ions are of the form Cm⁺Hn with m < n. Total deprotonation has also been observed. The mechanism proposed involves the dissociation of the parent ion, which then dissociates by different competitive channels. Helium, neon and argon were used as carrier gases (CG). A detailed discussion is presented regarding the use of He as the CG. The laser pulse intensity allows the absorption of up to nine photons, observed through the formation of multiply charged ions of some of the CG atoms.     

Photoionization studies of substituted trimethylamines

Ionization energies of substituted trimethylamines (CH₃)₂NCH₂X, together with appearance energies of [M ? X]⁺ ions, have been measured by the photoionization technique. The heats of formation for the following radicals have been derived (in kJ mol^?1): C₂H₃^˙ (+ 314), Ph^. (+ 347), p-CH₃C₆H₄^˙ (+ 326), HC₂^˙ (+ 494), (CH₃)₂N?H₂ (+ 134), NC^˙ (+ 452), CF₃^. (? 435), (CH₃)₃Si^˙ (+ 54.5). The heats of formation obtained for [(CH₃)₂NCH₂]⁺ (+ 649) and (CH₃)₃Si⁺ (+ 610) lead to ionization energies of 5.35 eV for (CH₃)₂N?H₂ and 5.75 eV for (CH₃)₃Si^˙.     

Heat of formation for the propanoyl cation by photoionization mass spectrometry

The ionization energies and [C₃H₅O]⁺ appearance energies for a series of oxygenated organic compounds have been measured by dissociative photoionization mass spectrometry. The adiabatic ionization energy for cyclopentanol is observed to be 9.72 eV. A 298 K heat of formation of 591.2±2.3kJ mol^?1, based on the stationary electron convention, is derived for the propanoyl cation in the gas phase. A heat of formation of –86±6 kJ mol^?1 is obtained for methylketene, which leads to an absolute proton affinity of 853±8 kJ mol^?1.     

Photoionization and Pyrolysis of a 1,4‐Azaborinine: Retro‐Hydroboration in the Cation and Identification of Novel Organoboron Ring Systems

The photoionization and dissociative photoionization of 1,4‐di‐tert‐butyl‐1,4‐azaborinine by means of synchrotron radiation and threshold photoelectron photoion coincidence spectroscopy is reported. The ionization energy of the compound was determined to be 7.89 eV. Several low‐lying electronically excited states in the cation were identified. The various pathways for dissociative photoionization were modeled by statistical theory, and appearance energies AE_0K were obtained. The loss of isobutene in a retro‐hydroboration reaction is the dominant pathway, which proceeds with a reverse barrier. Pyrolysis of the parent compound in a chemical reactor leads to the generation of several yet unobserved boron compounds. The ionization energies of the C₄H₆BN isomers 1,2‐ and 1,4‐dihydro‐1,4‐azaborinine and the C₃H₆BN isomer 1,2‐dihydro‐1,3‐azaborole were determined from threshold photoelectron spectra.     

Molecular fragmentations: Part VII. Structures and energies of mass spectral fragments derived from benzene

Molecular structures and energies have been calculated, using MINDO/3, of the mass spectral ions arising from benzene: (C₆H₆)⁺ (three non-valence isomers); (C₆H₅⁺); (C₅H₃⁺) (four isomers); (C₄H₄)⁺ (three isomers); (C₄H₃)⁺ (two isomers); (C₄H₂)⁺ (four isomers); (C₃H₃)⁺; and (C₂H₂)⁺. Calculations have been made for the conjugate neutral fragments, allowing calculation of appearance potentials, and also for the ion (C₆H₇)⁺.     

A reinvestigation of the collisional activation mass spectra of [C7H7]+ ion mixtures

The collisional activation (CA) mass spectra of the two isomeric [C₇H₇]⁺ ions, benzyl and tropyl, have been reassessed. The structure-characteristic feature of their CA mass spectra, the m/z 77:74 abundance ratio, has been confirmed as 3.15 ± 0.2 for benzyl cations and lowered to 035 ± 03 for tropyl ions. Benzyl–tropyl cation mixture analyses were made and were in general agreement with earlier CA results, but still disagree with the results of ion cyclotron resonance experiments. The behavior of toluene molecular ions close to their dissociation threshold to [C₇H₇]⁺ + H˙ was examined; for metastable [C₇H₈]⁺˙ ions an approximately 55:45 benzyl:tropyl ratio was found. Observations are discussed in relation to photoionization and photoelecrron-photoionization coincidence studies, both of which predict high tropyl ion contents at low energies. However, at the lowest energies attainable in this study the benzyl content failed to fall below 50% and it is concluded that toluene molecular ions do not generate tropyl cations at their dissociation limit.