A predictive model for unimolecular reactions of organic ions

The slow unimolecular dissociations of six members of the [C_nH_2n-3]⁺ (n = 3-8) series of unsaturated carbonium ions are explained in terms of a potential surface approach together with some concepts of mechanistic organic chemistry. The occurrence of some dissociations is shown to be precluded because either the reacting configurations or product combinations are inaccessible at energies appropriate to metastable transitions. The approach permits correct predictions to be made concerning the shapes of metastable peaks for dissociations which occur without σ-bond formation in the final step. In particular, the observation of a composite peak, thus indicating two channels for reaction, for C₂H₄ loss from [C₇H₁₁]⁺ is naturally accommodated.     

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.     

Reactions of the dications C7H6, C7H7, and C7H8 with methane: Predominance of doubly charged intermediates

The reactions of methane with the dications C₇H₆²⁺, C₇H₇²⁺, and C₇H₈²⁺ generated by electron ionization of toluene are studied using mass-spectrometry tools. It is shown that the reactivity is dominated by the formation of doubly charged intermediates, which can either eliminate molecular hydrogen to yield doubly charged products or undergo charge-separation reactions leading to the formation of a methyl cation and the corresponding C₇H_n+1⁺ monocation. Typical processes observed for dications, like electron transfer or proton transfer, are largely suppressed. The theoretically derived mechanism of the reaction between C₇H₆²⁺ and CH₄ indicates that the formation of the doubly charged intermediate is kinetically preferred at low internal energies of the reactants. In agreement, the experimental results show a pronounced hydrogen scrambling and dominant formation of the doubly charged products at low collision energies, whereas direct hydride transfer prevails at larger collision energies.     

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.     

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.     

Structural determination of [C7H7O]+ ions in the gas phase by ion cyclotron resonance spectrometry

The [C₇H₇O]⁺ ions from a series of compounds have been studied using ion cyclotron resonance spectrometry. The techniques employed in this gas phase ion structure determination of [C₇H₇O]⁺ were photodissociation, ion-molecule reactions, and collisionally activated dissociation using a Fourier transform mass spectrometer (FTMS-CAD). In addition to the low energy FTMS-CAD results, high energy CAD data obtained with a sector mass spectrometer is also provided. Evidence was found for five unique [C₇H₇O]⁺ structures, including the hydroxybenzyl ion, the hydroxytropylium ion, the protonated benzaldehyde ion, the methylaryloxy ion and the phenyl methylene ether ion. Ion-molecule reactions, invovling both proton transfer and methylene transfer, provided the most unambiguous results and yielded qualitative and quantitative evidence for the five structures. However, a combined approach using the three techniques was necessary to identify all of the structures. The tropylium form of [C₇H₇O]⁺ was found to absorb strongly at 305 nm, while the protonated benzaldehyde ion was found to have a strong absorption band at 305 nm and a weak band at 370 nm. The proton affinity of 2,4,6-cycloheptatrienone was determined to be 918±8 kJ mol^?1, which is considerably lower than a previously reported value. In addition, deprotonation reactions of the methylaryloxy ion yielded a proton affinity of 871±14 kJ mol^?1 for 4-methylenecyclohexa-2,6-diene-1-one.     

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.     

SiH+5 and the proton affinity of monosilane

Tandem mass spectrometric studies show that SiH⁺₅ is formed in bimolecular reactions of SiH₄ and NH⁺₂, C₂H⁺₃, C₂H⁺₆ and C₃H⁺₈ ions. The dependence of the reaction cross sections on ion energy indicates the formation of SiH⁺₅ from NH⁺₂, C₂H⁺₃, and C₂H⁺₆ to be exothermic reactions, while formation from C₃H⁺₈ is endothermic. Using known thermochemical data, these facts permit the assignment of 150 and 156 kcal/mole to the lower and upper limits of the proton affinity of monosilane.     

Mono-reduced Corannulene: To Couple and Not to Couple in One Crystal

One-electron reduction of corannulene, C₂₀H₁₀, with Li metal in diglyme resulted in crystallization of [{Li⁺(diglyme)₂}₄(C₂₀H₁₀^.−)₂(C₂₀H₁₀-C₂₀H₁₀)²⁻] ( 1 ), as revealed by single-crystal X-ray diffraction. This hybrid product contains two corannulene monoanion-radicals along with a dianionic dimer, crystallized with four Li⁺ ions wrapped by diglyme molecules. The dimeric (C₂₀H₁₀-C₂₀H₁₀)²⁻ anion provides the first crystallographically confirmed example of spontaneous radical dimerization for C₂₀H₁₀^.−. The C−C bond length between the two C₂₀H₁₀^.− bowls of 1.588(5) Å is consistent with the single σ-bond character of the linker. The trans-disposition of two bowls in the centrosymmetric (C₂₀H₁₀-C₂₀H₁₀)²⁻ dimer is observed with the torsion angle around the central C−C bond of 180°. Comprehensive theoretical analysis of formation/decomposition processes of the dimeric dianion has been carried out in order to evaluate the nature of bonding and energetics of the C₂₀H₁₀^.− coupling. It is found that such σ-bonded dimers are thermodynamically unstable due to large preparation energy and repulsive Pauli component of the bonding, but kinetically persistent due to a high energy barrier provided by the existing spin-crossing point.     

Neutral and ion chemistry in a C2H4-SiH4 discharge

This paper reports on a mass spectrometric study of the neutral and ionic species in a low-pressure rf discharge sustained in a C₂H₄-SiH₄ mixture diluted in helium. It is shown that C₂H₄ is readily decomposed into C₂H ₂ ^* and C₂H₃. The formation of secondary products such as C₄H₂, C₄H₄, and C₄H₆ is observed and confirms the presence of C₂H₂ in the discharge. Methylsilane (CH₃SiH₃) and ethylsilane (C₂H₅SiH₃) are also synthesized in this discharge. It is also observed that the major ions C₂H ₄ ⁺ , C₃H ₅ ⁺ , SiH ₃ ⁺ , Si₂H ₄ ⁺ , SiCH ₃ ⁺ , SiC₂H ₃ ⁺ , and SiC₂H ₇ ⁺ are not representative of the direct ionization of neutral species. Their formation is thus interpreted on the basis of ion-molecule reactions.     

A tandem mass spectrometric investigation of (C2H5)2X+ ions (X = Cl,Br, I): The involvement of classical and nonclassical ethyl structures therein

The formation of diethyl halonium ions (C₂H₅)₂X⁺ (X = Cl, Br, I) by a variety of ion-molecule reactions is described. The dissociation characteristics (metastable and collision-induced dissociation mass spectra) of these ions and their isomers were studied in detail. Some of the neutral fragmentation products were examined by their collision-induced dissociative ionization mass spectra. The participation of classical (1, CH₃CH₂X⁺CH₂CH₃) and nonclassical forms of the ions was considered. Dissociation reactions for which loss of positional identity of H-D atoms took place, for example C₂H₄ loss (a common fragmentation of metastable ions) and C₂H₅ ⁺ formation, were interpreted as involving nonclassical ions, 2. It was concluded that the ion-molecule reactions produced both ion structures, but in different halogen-dependent proportions. For (C₂H₅)₂C1⁺ ions, 2 is the major species, for (C₂H₅)₂Br⁺ both 1- and 2-type ions are generated, whereas for (C₂H₅)₂I⁺ the classical form 1 must be the predominant structure.     

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.     

Photofragmentation of 2‐Deoxy‐D‐Ribose Molecules in the Gas Phase

We have measured the synchrotron‐induced photofragmentation of isolated 2‐deoxy‐D ‐ribose molecules (C₅H₁₀O₄) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecule′s ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH₃⁺, OH⁺, H₃O⁺, C₂H₃⁺, C₂H₄⁺, CH_xO⁺ (x=1,2,3), C₂H_xO⁺ (x=1–5), C₃H_xO⁺ (x=3–5), C₂H₄O₂⁺, C₃H_xO₂⁺ (x=1,2,4–6), C₄H₅O₂⁺, C₄H_xO₃⁺ (x=6,7), C₅H₇O₃⁺, and C₅H₈O₃⁺. The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C₃H₅O⁺), whereas a general intensity decrease is observed for all other fragments— relative to the m/q=57 fragment—with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH₃⁺, H₃O⁺, C₂H₄⁺, CH₃O⁺, and C₂H₅O⁺, involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar–phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules.     

Study of ion structures produced by the reaction of 2-propyl cations with water and methanol; covalently bound v. Hydrogen-bridged adducts

In contrast to an earlier report,¹ the collisonally induced dissociation of protonated 2-propanol and t-butyl alcohol yields spectra that are indistinguishable from those of the corresponding [C₃H₇/H₂O]⁺ and [C₄H₉/H₂O]⁺ ions generated by the (formal) gas phase addition reactions in a high pressure ion source of [s-C₃H₇]⁺ and [t-C₄H₉]⁺ ions with the n-donor H₂O. Similarly, [s-C₃H₇/CH₃OH]⁺ ions generated by both gas phase protonation of n- and s-propyl methyl ethers and addition reactions of [C₃H₇]⁺ to CH₃OH display mode-of-generation-independent collisionally induced dissociation characteristics. However, analysis of the unimolecular dissociation (loss of propene) of the [C₃H₇/CH₃OH]⁺ system, including a number of its deuterium, ¹³C- and ¹⁸O-labelled isotopomers, supports the idea that prior to unimolecular dissociation, covalently bound [C₃H₇- O(H)CH₃]⁺ ions intercovert with hydrogen-bridged adduct ions, analogous to the behaviour of the distonic ethene-, propene- and ketene-H₂O radical cations.     

Degenerate skeletal rearrangement and energy partitioning in the fragmentation of the norbornyl cation

The electron-impact-induced fragmentation of a ¹³C labelled exo-2-norbornyl chloride has been studied. When the norbornyl cation, [C₇H₁₁]⁺, [M – Cl], dissociates by elimination of C₂H₄, carbon atoms are randomly lost showing that extensive skeletal rearrangement (in addition to the complete hydrogen atom scrambling reported earlier) has taken place in the norbornyl cation prior to dissociation. The metastable ion peak associated with the above fragmentation consists of superimposed gaussian and flat-top components; it is proposed that these correspond to the formation of isomeric [C₅H₇]₊ daughter ions whose heats of formation differ by ～0.4 eV.     

[C6H6, C3H7 +]and [C6H7 +, C3H6] ion-neutral complexes intermediate in the reactions of protonated propylbenzenes

From the mass-analysed ion kinetic energy spectra of labelled ions, kinetic energy releases and thermodynamic data, it is proved that protonated n-propylbenzene (1) isomerizes into protonated isopropyl benzene (2). It is also shown that the dissociation of the less energetic metastable ions of (2), leading to [iso-C₃H₇]⁺ and [C₆H₇]⁺ product ions, is preceded by H exchange. This H exchange involves two interconverting ion-neutral complexes [C₆H₆, iso-C₃H₇⁺] (2π) and [C₆H₇⁺, C₃H₆] (2α).     

An investigation of the decomposition of the common intermediate ions produced by electron impact. IV—[C6H5CO]+ ions from several alkyl benzoates

The decomposition of the [C₆H₅CO]⁺ ions produced from eight alkyl benzoates by electron impact has been studied. By calculating the heat of formation of [C₆H₅CO]⁺ ions from the appearance potential value, it is shown that the ions from C₆H₅COOR when R?H, CH₃, C₂H₅ have some excess energy, and those where R = n-C₃H₇, iso-C₃H₇, n-C₄H₉, iso-C₄H₉, iso-C₅H₁₁ are produced with more excess energy. It is also shown that by taking this excess energy into account, there is a linear relationship between the heat of formation of the activated complex produced in the reaction [C₆H₅CO]⁺→[C₆H₅]⁺ + CO and the vibrational degree of freedom of the neutral fragment ? OR.     

Gas-phase electrophilic addition reactions of chlorinated alkyl ions

Chemical ionization was used to study gas-phase electrophilic addition reactions of chloromethyl ions ([CH_xCl_3-x]⁺, x = 0, 1, 2) with a number of substituted benzenes (C₆H₅Y, Y = NH₂, OH, CHO, CN, NO₂). Mass-analyzed ion kinetic energy spectrometry was used to characterize the reaction products with respect to the site of electrophilic addition (ring v. substituent). In some cases, examination of secondary reaction products (ion–molecule adduct which has undergone an elimination reaction in the ion source) aided in establishing the original site of electrophilic addition. Aniline, benzonitrile and nitrobenzene exhibited preferential substituent interaction, while phenol and benzaldehyde gave a mixture of ring and substituent reaction products. These gas-phase results differ considerably from solution-phase Friedel–Crafts alkylation; however, they are consistent with the notion of preferential σ-bond formation at polarizable centers of negative charge.     

Benzene as a selective chemical ionization reagent gas

Dilute mixtures of C₆H₆ or C₆D₆ in He provide abundant [C₆H₆]^+· or [C₆D₆]^+· ions and small amounts of [C₆H₇]⁺ or [C₆D₇]⁺ ions as chemical ionization (CI) reagent ions. The C₆H₆ or C₆D₆ CI spectra of alkylbenzenes and alkylanilines contain predominantly M^+· ions from reactions of [C₆H₆]^+· or [C₆D₆]^+· and small amounts of MH⁺ or MD⁺ ions from reactions of [C₆H₇]⁺ or [C₆D₇]⁺. Benzene CI spectra of aliphatic amines contain M^+·, fragment ions and sample-size-dependent MH⁺ ions from sample ion-sample molecules reactions. The C₆D₆ CI spectra of substituted pyridines contain M^+· and MD⁺ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample-size-dependent MH⁺ ions from sample ion-sample molecule reactions. Two mechanisms are observed for the formation of MD⁺ ions: proton transfer from [C₆D₆]^+· or charge transfer from [C₆D₆]^+· to give M^+·, followed by deuteron transfer from C₆D₆ to M^+·. The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments. Proton transfer from [C₆H₆]^+· or [C₆D₆]^+· is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic. For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.