Metastable Molecular Ions Research Articles

Possible evidence for the production of Ar2∗-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2^{*-}$$\\end{document} metastable negative molecular ions in gaseous argon of two-phase detectors for dark matter searches

Our recent studies of electroluminescence (EL) properties in two-phase argon detectors for dark matter searches have revealed the presence of unusual delayed pulses in the EL signal in the form of two slow components with time constants of about 5 and 50 μs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \ ext {s}$$\\end{document}. These components were shown to be present in the charge signal itself, which clearly indicates that drifting electrons are temporarily trapped on two states of metastable negative argon ions which have never been observed before. In this work, using the pressure dependence of the ratio of slow component contributions measured in experiment, it is deduced that these states are those of two types of metastable negative molecular ions, Ar2∗-(b4Σu-)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{Ar}}}_2^{*-}(b \\ ^4\\varSigma _u^-)$$\\end{document} and Ar2∗-(a4Σg+)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{Ar}}}_2^{*-}(a \\ ^4\\varSigma _g^+)$$\\end{document} for the higher and lower energy level respectively.

Quantitative Identification of Nonpolar Perfluoroalkyl Substances by Mass Spectrometry.

Identifying and quantifying mixtures of compounds with very similar fragmentation patterns in their mass spectra presents a unique and challenging problem. In particular, the mass spectra of most per- and poly-fluoroalkyl substances (PFAS) lack a molecular ion. This complicates their identification, especially when using the absence of chromatographic separation. Here, we focus on linear, nonpolar, short-chain PFAS, which have received less attention than amphipathic PFAS despite their longer environmental lifetimes and greater global warming potentials. We identify and quantify n-C5F12 and n-C6F14 in binary mixtures by analyzing small changes in abundances of the main fragment ions following femtosecond tunnel laser ionization, without the need of chromatographic separation. Time-resolved femtosecond ionization mass spectrometry reveals that the metastable cation of both compounds undergoes predissociation within 1-2 ps of ion formation, with yields of C3F7+ showing evidence of coherent vibrational dynamics. These coherent oscillations are compared to low-level ion-state calculations and supported the idea that the oscillations in the C3F7+ ion yield are due to vibrations in the C5F12+• and C6F14+• radical cations and are associated with the predissociation dynamics of the metastable molecular ion. Surprisingly, we find that the fragment ions used for quantifying the mixtures have similar fragmentation dynamics. Conversely, the odd-electron C2F4+• fragment shows different time dependence between the two compounds, yet has negligible difference in the relative ion yield between the two compounds. Our findings indicate that femtosecond laser ionization may be a useful tool for identifying and quantifying mixtures of PFAS without the need of chromatography or high-resolution mass spectrometry.

Heavy N+ ion transfer in doubly charged N2Ar van der Waals cluster

Van der Waals clusters are weakly bound atomic/molecular systems and are an important medium for understanding micro-environmental chemical phenomena in bio-systems. The presence of neighboring atoms may open channels otherwise forbidden in isolated atoms/molecules. In hydrogen-bond clusters, proton transfer plays a crucial role, which involves mass and charge migration over large distances within the cluster and results in its fragmentation. Here we report an exotic transfer channel involving a heavy N+ ion observed in a doubly charged cluster produced by 1 MeV Ne8+ ions: (N2Ar)2+→N++NAr+. The neighboring Ar atom decreases the {mathrm{N}}_2^{2 + } barrier height and width, resulting in significant shorter lifetimes of the metastable molecular ion state {mathrm{N}}_2^{2 + }({{mathrm{X}}^{1}}{Sigma _{{mathrm{g}}}^{+}}). Consequently, the breakup of the covalent N+−N+ bond, the tunneling out of the N+ ion from the {mathrm{N}}_2^{2 + } potential well, as well as the formation of an N−Ar+ bound system take place almost simultaneously, resulting in a Coulomb explosion of N+ and NAr+ ion pairs.

A unified statistical RRKM approach to the fragmentation and autoneutralization of metastable molecular negative ions of hexaazatrinaphthylenes.

For the compounds promising for use as n-type semiconductors in organic electronics and energy storage devices, hexaazatrinaphthylene (HATNA) and its derivative hexamethoxy-hexaazatrinaphthylene (HMHATA), the monomolecular processes occurring under the exposure of molecules to low-energy (0-15 eV) free electrons were studied by means of resonant electron capture negative ion mass spectrometry. Resonant electron attachment results in the formation of eminently long-lived molecular negative ions (MNIs) in an abnormally wide range of incident electron energy (Ee) from 0 to 5-7 eV. For both compounds, this observation serves as an indication of the strong electron-accepting properties and high stability of MNIs against electron autodetachment. A weak yield of the only fragment NIs, dehydrogenated anions, was detected for HATNA at Ee > 6 eV. MNIs of HMHATA are less stable to dissociative decay because of the presence of weakly bound terminal substituents. This is evidenced by the mass spectral observation of intense fragmentation occurring above Ee≈ 1 eV and leading to a loss of up to 3 methyl groups as the Ee increases. A series of metastable NI peaks observed in the mass spectra testify to the delayed and sequential nature of fragmentation. Based on the principles of statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory, the theoretical model of dissociative decay of NIs was developed and then adopted to quantify the rates of ground-state anion decay via electron autodetachment. The experimentally measured electron autodetachment lifetimes and fragmentation rates were best reproduced by the model at molecular adiabatic electron affinities preset to 2.15 eV for HATNA and 1.88 eV for HMHATA, in reasonable agreement with the quantum chemical DFT PBE/3ζ predictions.

The role of molecular ions in cryogenic dusty plasmas of dc discharge in neon

The diffusion-drift model of positive column of glow discharge in neon have been used to study the effect of molecular ions in the interaction of plasma with dust particles of micron size. Simulations have been carried out at a pressure of 20 – 100 Pa and gas temperature of 77 K. The electric field of the discharge, the concentrations of discharge components, including metastable atoms and molecular ions, and the charge of dust particles have been simulated.

Carbon dust particles in a beam-plasma discharge

This paper focuses on dynamics of micro-sized carbon dust grains in beam-plasma discharge (BPD) plasmas. It was demonstrated that injected dust particles can be captured and transported along the discharge. Longitudinal average velocity of the particles in the central area of the plasma column was 17 m/sec, and 2 m/sec in the periphery. Dust injection caused a decrease of emission intensity of metastable nitrogen molecular ion. This effect is suggested for a spectroscopy method for particles’ potential measurements. Five-micron radius carbon dust grains obtained potential above 500 V in the experiments on PR-2 installation, proving the feasibility of BPDs for the charging of fine dust particles up to high potential values, unattainable in similar plasma conditions.

Behavior of molecules and molecular ions near a field emitter**This work is a partial contribution of the US Government and therefore is not subject to copyright in the United States.

The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modeling of ion trajectories, and the application of density-functional theory simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions.

Exploring the metastability and the pathways for polyanionic isomerization in the dianions and trianions of doubly- and triply-deprotonated benzene

Polyanions of the aromatic origin such as dianion of benzene (C6H6 2−) are generally metastable having negative electron affinity (EA). However, the de-protonated species of these anions such as doubly-deprotonated dianions of benzene (C6H4 2−) can have higher proton affinity compared to the magnitude of the negative EA, which can trigger an intra-molecular proton transfer besides the electron auto-detachment. Such species though may exhibit resonances as in the temporary anions and cannot be treated using the conventional variational methods of the quantum mechanics. In this work, we discuss the metastability, through a nuclear-charge stabilization method, for the poly-deprotonated dianions and trianions of benzene, while exploring the intra-molecular proton-transfer reaction pathways using the conventional quantum mechanical methods, for the isomerization between the isomeric doubly-deprotonated benzene dianions, namely the ortho-, meta-, and para-C6H4 2−, as well as between the isomeric triply-deprotonated benzene trianions, namely the 1,2,3-, 1,2,4-, and 1,3,5-C6H3 3− in their singlet state. For this, the potential energy surfaces of the polyanionic species, in particular those of the doubly-deprotonated dianions, are successfully explored through an automated and systematic search performed using the global reaction route mapping method. The computations performed at the levels of the MP2, DFT with and without dispersion, and CCSD(T) theories, identified elusive reaction pathways for the isomerization between the anionic species of the poly-deprotonated benzene. Notably, a single-step isomerization between the ortho- and para-C6H4 2− dianions is observed to proceed via a direct 1, 3-proton shift in an unconventional transition state involving a puckered benzene ring in a nearly half-boat conformation. However, for the isomerization between C6H3 3− benzene trianions, the present work was able to reveal pathway only between 1,2,4-C6H3 3− and 1,3,5-C6H3 3− trianions, indicating the problems like variational collapse inherent with the conventional methods while dealing with the molecular polyanions. However, the lifetime of the metastable molecular ions investigated in this work is estimated to be of the order of only a few femtoseconds, an order of magnitude higher than the proton-transfer rate along the proposed isomerization pathways, but this study is able to provide valuable insights into the mechanism of proton transfer in the presence of a competing electron auto-detachment process.

Isomerization of Metastable Amine Radical Cations by Dissociation—Recombination

The metastable molecular ions of primary aliphatic amines branched at C2 can isomerize by cleavage-recombination, thereby facilitating fragmentation reactions that require less energy than simple cleavage of the initial molecular ion. This process complements the reactions described by Audier to account for the conspicuous absence of the conventional a-cleavage among the major fragmentation reactions of the metastable molecular ions of primary amines.

Unimolecular reactions of gaseous picoline radical cations. A new experimental and computational study

The fragmentation reactions of the molecular ion of the three isomeric methylpyridines (2-picoline 1, 3-picoline 2, and 4-picoline 3) were studied using methods of tandem mass spectrometry and quantum chemical calculations with Gaussian-3 (G3) theory using the B3LYP density functional method (G3//B3LYP). Additionally RRKM calculations of the microcanonical rate constants of selected reaction pathways were performed. Derivatives of 2- and 4-picoline which bear an isotopically labeled methyl substituent (CD3: 1-d3 and 3-d3, 13CH3: 1-13C and 3-13C) were also analyzed. Of the four primary dissociation reactions of the picoline radical cations 1+–3+, the loss of a hydrogen atom, the elimination of an ethyne molecule and the elimination of a hydrogen cyanide molecule are observed in the MIKE spectra, while the loss of a methyl radical is only found in the collisional activation (CA) mass spectra of the molecular ions. The CA mass spectra of the labeled picoline radical ions reveal that some positional hydrogen exchange and carbon exchange takes place before the loss of the methyl radical. According to the G3 and RRKM calculations, the direct cleavage of the methyl substituent competes effectively with increasing internal energy of the ion with the loss of a methyl radical after rearrangements. The other three pathways found for metastable molecular ions are accompanied by efficient or complete hydrogen scrambling. The theoretical calculations prove that this scrambling occurs mainly by initial H-transfer from the methyl group to the pyridine ring and a subsequent H-ring walk. The calculations show further, that loss of H may occur from many of the intermediate ions after the rearrangements but that the direct loss of H from the methyl substituent competes successfully even at low energies, contrary to the behavior of the toluene radical ion. The elimination of an ethyne molecule gives rise to a broad flat-topped signal in the mass-analyzed ion kinetic energy (MIKE) spectra, and the MIKE spectra of 1-13C and 3-13C exhibit a 1:1 sharing of the 13C-label between ionic and neutral products. These particular features are explained by the theoretical calculations by a first rearrangement of the picoline radical cations to a seven-membered aza-cycloheptatriene radical cation 4+ followed by a rearrangement to a more rigid bicyclo-[3.2.0] isomer before the elimination of ethyne. Finally, the elimination of a hydrogen cyanide molecule exhibits only minor loss of 13C-label in the case of 1-13C and 3-13C. This observation does not agree with any mechanism including a rearrangement to 4+. The theoretical calculations suggest that instead the loss of the hydrogen cyanide molecule proceeds by a cleavage of the pyridine ring after the initial H-transfer from the methyl substituent. Although this pathway needs more energy than a rearrangement to 4+, its rate coefficients are superior owing to its loose transition states.

Site-specific hydrogen exchange and hydrogen transfer processes preceding the fragmentation of long-lived radical cations of ethyl dihydrocinnamate and related arylalkanoates

An electron ionisation study on the fragmentation of metastable molecular radical cations of ethyl 3-phenylpropanoate (ethyl dihydrocinnamate) and related arylalkanoic acid esters was performed by mass-analysed ion kinetic energy (MIKE) spectrometry. Six deuterium-labelled isotopomers of ethyl dihydrocinnamate were synthesised and studied by MIKE spectrometry. The fragmentation leading to ions C7H7O+ (m/z 107) involving migration of the alkoxycarbonyl group was also observed in the 70-eV mass spectra of related alkyl dihydrocinnamates, but it was found to be a high-energy process that does not compete at low energies in metastable molecular ions. Instead, the metastable ions of ethyl dihydrocinnamate undergo competing losses of carbon monoxide, ethanol and the combined loss of these neutral fragments, giving ionised styrene, C8H8•+ (m/z 104). A highly specific H/D interchange involving the four hydrogen atoms at the benzylic α- and ortho-positions was found to precede the losses of ethanol and [ethanol+CO]. This represents another striking case of complete 4H – scrambling that enables the molecular ion to fully equilibrate the interchanging hydrogen atoms prior to fragmentation. A mechanism rationalising these observations and extending previously suggested mechanisms is proposed involving a series of distonic ions and the intermediacy of an ion/neutral complex. The metastable ions of the related esters exhibit in part similar fragmentation behaviour, but the McLafferty reaction turns out to be more favourable with higher alkyl dihydrocinnamates. For example, n-propyl 3-phenylpropanoate and isopropyl 3-phenylpropanoate react through highly distinct fragmentation channels.

Metastable dimethyl phthalate molecular ions: Does the loss of a methoxyl radical proceed with or without anchimeric assistance?

Using MS/MS/MS experiments and quantum chemical calculations, it is demonstrated that oxygen (carbonyl) methylated phthalic anhydride cations are produced when metastable dimethyl phthalate molecular ions expel a methoxyl radical. At higher internal energies, isomeric acyl ions are competitively generated.

Trends in the Periodic System: The Mass Spectrum of Dimethylphenyl Phosphane and a Comparison of the Gas Phase Reactivity of Dimethylphenyl Pnictogene Radical Cations C6H5E(CH3)2•+, (E = N, P, As)

The mass spectrometric reactions of dimethylphenyl phosphane, 1, under electron impact have been studied by methods of tandem mass spectrometry and by using labeling with deuterium. The results are compared to those for the previously investigated dimethylaniline, 2, and dimethylphenyl arsane, 3, to examine the effects of heavy main group heteroatoms on the reactions of radical cations of the pnictogen derivatives C(6)H(5)E(CH(3))(2). Decomposition of the radical cation 1(*+) gives rise to large peaks in the 70 eV electron impact (EI) mass spectrum for loss of a radical, *CH(3), which is followed by abundant loss of a molecule, H(2), and formation of ion C(7)H(7)(+), and the 70 eV EI mass spectrum of the deuterated derivative 1d(3) shows that excessive positional hydrogen/deuterium (H/D) exchange accompanies all fragmentation reactions. This is confirmed by the mass analyzed kinetic energy (MIKE) spectrum of the molecular ion 1d(6)(*+) which displays a group of signals for the loss of all isotopomers, *C(H/D)(3), and three signals for formation of ions C(7)H(5)D(2)(+), C(7)H(4)D(3)+ and C(7)H(3)D(4)(+). The intensity distribution within this latter group of ions corresponds to a statistical positional exchange ("scrambling") of all six D atoms of the methyl substituents with only two H atoms of the phenyl group. In contrast, the intensity distribution of the signals for loss of *C(H/D)(3) uncovers a bimodal reaction. About 39% of metastable molecular ions 1(*+) eliminate *CH(3) after scrambling of the six H atoms of the methyl substituents with two H atoms of the phenyl group, while the remaining 61% of metastable 1(*+) lose specifically a CH(3) substituent without positional H exchange. Further, the metastable ion [M-CH(3)](+) eliminates, almost exclusively, a molecule H(2), which is preceded by excessive positional H/D exchange in the case of metastable ion [M-CD(3)](+). The formation of ion C(7)H(7)(+) from metastable ion [M-CH(3)](+) is not observed and this is a minor process, even under the high energy condition of collision-induced dissociation (CID). The mechanisms of these fragmentation and exchange reactions have been modeled by theoretical calculations using the DFT functionals at the level UHBLY/6- 311+G(2d,p)//UHBLYP/6-31+G(d). The key feature is a rearrangement of molecular ion 1(*+) to an alpha-distonic isomer 1dist1(*+) by a 1,2-H shift from the CH(3) substituent to the P atom in competition with a direct loss of a CH(3) substituent . The distonic ion 1dist1(*+) performs positional H exchange between H atoms of both CH(3) substituents and H atoms at the ortho-positions of the phenyl group and rearranges readily to the (conventional) isomer benzylmethyl phosphane radical cation 1bzl(*+). The ion 1bzl(*+) undergoes further positional H exchange before decomposition to ion C(7)H(7)(+) and a radical CH(3)P*H or by loss of a radical *CH(3). Finally, ions [M-CH(3)](+) of methylphenyl phosphenium structure 1a(+) and benzyl phosphenium structure 1b(+) interconvert easily parallel to positional H exchange involving all H atoms of the ions. Eventually, a molecule H(2) is lost by a 1,1-elimination from the PH(2) group of the protomer 1b-H(+) of 1b(+). The trends observed in the gas-phase chemistry of the pnictogen radical cations dimethylaniline 2(*+), dimethylphenyl phosphane 1(*+) and dimethylphenyl arsane 3(*+) can be comprehended by considering the variation of the energetic requirements of three competing reaction: (i) alpha-cleavage by loss of *H from a methyl substituent, (ii) rearrangement of the molecular ion to an alpha-distonic isomer by a 1,2-H shift and (iii) loss of *CH(3) by cleavage of the C-heteroatom bond. 2(*+) exhibits a strong N-C bond and a high activation barrier for 1,2-H shift and fragments far more predominantly by alpha-cleavage. Both 1(*+) and 3(*+) eliminate *CH(3) by cleavage of the weak C-heteroatom bond. The barrier for a 1,2-H shift is also distinctly smaller than for 2(*+) and, for the P-derivative 1(*+), the generation of the alpha-distonic ion is able to compete with loss of *CH(3).

Unimolecular chemistry of metastable dimethyl isophthalate radical cations

The MS/MS spectrum of the metastable molecular ions of dimethyl isophthalate 1 differs from that of the isomeric dimethyl terephthalate 2 by the observation of, inter alia, a quite intense loss of C,H 2,O ascribed to formaldehyde. Results obtained using a combination of mass spectrometry techniques suggest that this process could consist of an isomerization reaction of the molecular ion into an ion–neutral complex (INC) linking a benzoyl radical and neutral formaldehyde to a proton [Ar C O⋯H⋯O CH 2] +. Within the complex, a proton transfer catalyzed by formaldehyde occurs resulting in the production of an ionized cyclohexadienylidene methanone (ketene) structure.

On the loss of a methyl radical from metastable dimethyl terephthalate molecular ions

The MS/MS spectrum of the metastable molecular ions of dimethyl terephthalate 1 features an unexpected, quite intense, loss of a methyl radical. Using a combination of mass spectrometry techniques, it is shown that this process consists of a pseudo simple cleavage reaction isomerizing, in the first step, one of the two methoxycarbonyl substituents into a distonic ArC +(–OH)–OCH 2 (Ar = CH 3O 2C–C 6H 4–) connectivity. The final product is established to be protonated 5-carboxyphthalide. The overall mechanism of the methyl loss includes a sequence of five steps as supported by quantum chemical (DFT) calculations.

Decarboxylation of Metastable Methyl Benzoate Molecular Ions

By using a combination of mass spectrometric methodologies and density functional theory calculations [DFT/B3LYP/6-311 ++ G(d, p)], it is proposed that the decarboxylation of metastable methyl benzoate molecular ions occurs via distonic and ion-neutral complex (INC) intermediates. The same INC involving a complex between the benzyl radical and protonated carbon dioxide is also generated upon decarboxylation of metastable phenylacetic acid molecular ions. Internal proton transfer within the INC produces in fine a mixture of toluene and isotoluene radical cations.

Simple Three-Parameter Model Potential for Diatomic Systems: From Weakly and Strongly Bound Molecules to Metastable Molecular Ions

Based on a simplest molecular-orbital theory of H(2)(+), a three-parameter model potential function is proposed to describe ground-state diatomic systems with closed-shell and/or S-type valence-shell constituents over a significantly wide range of internuclear distances. More than 200 weakly and strongly bound diatomics have been studied, including neutral and singly charged diatomics (e.g., H(2), Li(2), LiH, Cd(2), Na(2)(+), and RbH(-)), long-range bound diatomics (e.g., NaAr, CdNe, He(2), CaHe, SrHe, and BaHe), metastable molecular dications (e.g., BeH(++), AlH(++), Mg(2)(++), and LiBa(++)), and molecular trications (e.g., YHe(+++) and ScHe(+++)).

The Unimolecular Reactions of Ionized Methylacetamides

The gas-phase decarbonylation of the N-methyl- and N,N-dimethylacetamide radical cations gives rise to the α-distonic isomers of the dimethylamine and trimethylamine molecular ions by transfer of the acetyl methyl to nitrogen in two steps: H transfer and CH2 transfer. Complete exchange af all carbon-bonded hydrogen atoms precedes loss of acetyl radicals or of ketene from the metastable amide molecular ions. The reactions are examined with deuterium labeling and with composite ab initio methods of the G3 family.

On the double benzylic hydrogen migration in the molecular ion of N-(5-phenylvaleryl)-1-azacyclopentane-2-thione

It is shown on the basis of deuterium labeling that in the metastable molecular ions of N-(5-phenylvaleryl)-1-azacyclopentane-2-thione, decomposing by unimolecular fragmentation, both benzylic hydrogens migrate to the thiolactam ring to generate the protonated γ-thiobutyric lactam species with m/ z 102. A mechanism is given for the formation of these ions that is proposed to be mediated by intramolecular acid–base reactions and an ion/molecule complex.

A ring walk of methylene groups in toluene radical cations. An extension of the toluene-cycloheptatriene rearrangement of aromatic radical cations. Theory and experiment.

The minimum energy reaction pathway (MERP) of the toluene-cycloheptatriene radical cation rearrangement (TOL/CHT-rearrangement) has been calculated by the UHF and DFT model at the level UHF/6-311+G(3df,2p)//UHF/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYp/6-31G(d), respectively, including the ring walk of the substituent by a 1,2-shift around the aromatic ring. This ring walk corresponds to interconversion of distonic ions and norcaradiene radical cations (the two intermediates of the TOL/CHT-rearrangement) by making and breaking of the external C-C bonds of the cyclopropane moiety of the intermediate norcaradiene structure. For toluene radical cation 1, UHF calculations adequately reproduce earlier results(4) and show, that the ring walk of the CH(3)-substituents requires slightly more energy than formation of the cycloheptatriene radical cation. By the DFT model, the distonic ion, which is formed initially by a 1,2-H shift from CH(3) to the benzene ring, is not stable but the transition state of an interconversion of norcaradiene radical cations along a ring walk of the CH(3) substituent. The activation energy for this ring walk exceeds that for formation of the cycloheptatriene radical cation by c. 30 kJ mol(-1). Thus, isomerization of 1 by a ring walk of the CH(3)-substituent competes with the TOL/CHT-rearrangement likely only for excited 1. The calculation was repeated for the MERPs of a TOL/CHT-rearrangement of para-xylene radical cation 5 and ethylbenzene radical cation 2, yielding basically the same results as for 1. According to the calculation, polar substituents alter significantly the relative energies of the competing routes of isomerization. For benzylcyanide 3 (X = CN), the activation energy for a ring walk of the NC-CH(2)-substituent is distinctly below that of a ring enlargement. For benzyl methyl ether 4 (X = OCH(3)), the distonic intermediate along the UHF-MERP is unusually stable. Further, the 7-methoxy-norcaradiene radical ion is unstable and corresponds to a transition state between isomeric distonic intermediates differing by a 1,2-shift of the side chain. In contrast, the 7-methoxy-norcaradiene radical ion is the only intermediate of the DFT-MERP, and the distonic ion is the transition state for a 1,2-shift of the cyclopropane ring. A ring walk of the CH(3)OCH(2)-substituent is much more favorable than formation of a 7-methoxy-cycloheptatriene radical cation in both MERPs. The findings of the theoretical calculation are substantiated by the mass spectrometric fragmentations of meta- and para-methoxymethylated 1-phenylethanols 8 and 9 and of para-methoxymethyl substituted benzyl ethyl ether 10 and benzyl n-propyl ether 11. Important fragmentation routes of metastable molecular ions of these compounds correspond to elimination of alcohols. Use of deuterated derivatives shows that the elimination occurs by a "false" ortho-effect which requires migration of a ROCH(2)-substituent around the benzene ring. Results of particular interest are obtained for the asymmetric bis-ethers 10 and 11. Here, the MIKE spectra of the molecular ions of deuterated analogs reveal a selective ring walk of the C(2)H(5)OCH(2)- and n-C(3)H(7)OCH(2)-side chain, respectively.