Michael Mormann

Gas-phase basicities of the isomeric dihydroxybenzoic acids and gas-phase acidities of their radical cations

The thermochemical acid/base properties of the six dihydroxybenzoic acids (x,y-DHB) as prototypical matrices used in matrix-assisted laser desorption/ionization (MALDI) have been investigated. The ground-state gas-phase basicities (GB) of the six DHB isomers and the gas-phase acidities (ΔGacid) of the corresponding radical cations ([x,y-DHB].+) have been determined by Fourier-transform ion cyclotron resonance mass spectrometry employing the thermokinetic method. The gas-phase basicities vary from 814 kJ mol−1 for the least basic isomer, 3,5-DHB, to 831 kJ mol−1 for the most basic isomer, 2,4-DHB. The obtained gas-phase acidities of the corresponding radical cations vary from 815 kJ mol−1 for the most acidic species, 3,4-DHB, to 858 kJ mol−1 for the least acidic one, 2,5-DHB. The results indicate that ground-state proton transfer from the matrix radical cations to the analyte may play a role in the ionization process of MALDI, whereas proton transfer from protonated matrix molecules can be excluded.

Protonated 1,3,5-cycloheptatriene and 7-alkyl-1,3,5-cycloheptatrienes in the gas phase: Ring contraction to the isomeric alkylbenzenium ions

1,3,5-Cycloheptatriene (1) and various 7-alkyl-1,3,5-cycloheptatrienes (3, 6, 9, 13, and 16-19) were subjected to gas-phase protonation under Cl(CH4) and Cl(iC(4)H(10)) conditions and the MIKE spectra of their [M + H](+) ions were measured Loss of CH, from the parent ion [1 + H](+) and almost exclusive loss of C2H4 from the methyl derivative [3 + H](+) indicate ring contraction of the dihydrotopylium ions to protonated toluene (toluenium ions) and protonated ethylbenzene (ethylbenzenium ions), respectively, prior to fragmentation. With increased exothermicity of protonation, ions [3 + H](+) also isomerize to xylenium ions. Similarly, higher protonated n-alkylcycloheptatrienes undergo skeletal isomerization to the corresponding chain-elongated (n + 1)-alkylbenzenium and to the corresponding n-alkyltoluennium ions. Starting with ethyldihydrotropylium ions, a competing isomerization channel is opened giving rise to expulsion of C2H4 from the constituents of the seven-membered ring, as evidenced bg deuterium labelling and an unusually high kinetic energy release, Isoalkyl analogues behave in a similar manner with increased hydrogen exchange between the alpha position of the side chain and the ring. Copyright (C) 1999 John Wiley & Sons, Ltd.

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics, structure and interconversion of dihydrotropylium ions.

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C7H9+ ions, [CHT + H]+. The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 ± 4 kJ mol−1, was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C7H9+ population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 ± 4 kJ mol−1, and the heat of formation of the cyclo-C7H9+ ion, ΔHf0([CHT + H]+) = 884 ± 4 kJ mol−1. Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H(1)]+, generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 ± 2 kJ mol−1 and ΔHf0300([CHT + H(1)]+) = 892 ± 2 kJ mol−1. However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C–C bond formation, generating protonated norcaradiene [NCD + H]+, a valence tautomer being 19 kJ mol−1 more stable than [CHT + H(1)]+. The 1,4-dihydrotropylium ion, [CHT + H(3)]+, generated by protonation of CHT at C-3, is 17 kJ mol−1 less stable than [CHT + H(1)]+. The bicyclic isomer [NCD + H]+ is separated by relatively high barriers, 70 and 66 kJ mol−1 from the monocyclic isomers, [CHT + H(1)]+ and [CHT + H(3)]+, respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H(1)]+ does not rearrange to the bicyclic isomer [NCD + H]+ under mild protonation conditions.

Loss of methane and ethene from long-lived gaseous xylenium ions (protonated xylene) after “composite” scrambling

Abstract The unimolecular fragmentation of long-lived gaseous xylenium ions, CH3ue5f8C6H5+ue5f8CH3, has been studied in detail using 13 C -labeling, in addition to deuterium labeling, in combination with mass-analyzed ion kinetic energy (MIKE) spectrometry. Metastable xylenium ions generated from the EI-induced loss of COOR (R=H, CH3) from 1,4-dimethyl-1,4-dihydrobenzoic acid or its methyl ester or by protonation of para-xylene under CI(CH4) conditions eliminate H2, CH4 and, most remarkably, C2H4. The kinetic energy release characteristics of these fragmentation processes are reported. 2 H -labeling reveals that the loss of methane occurs by protonolytic cleavage of the Cαue5f8Cipso bonds, excluding intra-complex benzylic hydride abstraction by CH3+ ions. 13 C -labeling confirms the composite scrambling behavior preceding both methane and ethene losses. CH4 loss of a major fraction (92%) of xylenium ions occurs without C scrambling but with some concomitant hydrogen exchange (Hα/Hring) via nonclassical tolylmethonium ions, whereas a minor fraction (8%) undergoes complete C and H scrambling. C2H4 loss is preceded by different sequences of reversible ring expansion and ring contraction reactions involving methyldihydrotropylium ions (protonated methylcycloheptatriene) which, by irreversible ring contraction, eventually form ethylbenzenium ions (protonated ethylbenzene). A major fraction (66–75%) of the ions exhibits “specific” behavior, with the one of the methyl carbons being incorporated specifically into the ethene fragment and the other only after randomization with the carbons of the tolyl unit. A minor fraction (34–25%) of the ions undergoes complete C and H scrambling prior to ethene loss, involving both methyl groups. 1,2-CH3 shifts are invoked to occur in the xylenium and/or dihydrotropylium ions.

The gas-phase basicities of 6-methylfulvene and 6,6-dimethylfulvene as determined by the thermokinetic method

The gas-phase basicities (GB) of 6-methylfulvene and 6,6-dimethylfulvene have been determined by Fourier transform ion cyclotron resonance spectrometry using the thermokinetic method. Proton affinities (PA) and heats of formation derived from the GB values reveal the relatively high thermochemical stability of fulvenium ions which, in line with the previous studies on the parent C6H6 species, is only c. 40–50 kJ mol−1 less than those of the most stable protonated forms of the isomeric alkylbenzenes.

Interconversion of gaseous bicyclo[3.2.1]oct-2-en-4-yl cations and protonated 7-alkylcycloheptatrienes: [5 + 2] cycloreversion in competition with fragmentation by way of alkylbenzenium ions

Abstract Several bicyclo[3.2.1]oct-2-en-3-yl cations, as isomers of protonated 7-methyl- and 7-ethylcycloheptatriene and of the protonated C 8 H 10 and C 9 H 12 alkylbenzenes, respectively, have been studied by deuterium labeling and mass-analyzed ion kinetic energy (MIKE) and collision-induced dissociation/MIKE spectrometry. Labeling reveals that the bicyclic framework undergoes fast and apparently complete hydrogen equilibration prior to fragmentation, involving a series of skeletal and hydrogen rearrangements (1,2-C and 1,2-H shifts). Fragmentation of the bicyclic ions C 8 H 11 + and C 9 H 13 + is manyfold: It occurs in part by way of the isomeric alkylbenzenium ions, e.g. CH 3 CH 2 C 6 H 6 + and CH 3 C 6 H 5 CH 3 + , and C 2 H 5 C 6 H 5 CH 3 + and CH 3 CH 2 CH 2 C 6 H 6 + , respectively, with the corresponding 7-alkyldihydrotropylium ions as intermediates. Another fraction of the bicyclic ions does not fragment by way of alkylbenzenium ions but apparently by [5 + 2] cycloreversion of bicyclo[3.2.1]octenyl framework itself. This process is indicated by ethene expulsion associated with an unusually large kinetic energy release ( T ∗ ≈ 300 meV). The characteristic high-KER ethene loss was also found for protonated 7-ethylcycloheptatriene but not for protonated 7-methylcycloheptatriene, suggesting a delicate balance of the activation energies and confirming, in turn, that bicyclo[3.2.1]oct-2-en-yl cations are intermediates during the fragmentation of higher alkyldihydrotropylium ions.