Cornelis E. C. A. Hop

Electrospray mass spectrometry of borane salts: The electrospray needle as an electrochemical cell

Two borane salts ([(Me)4N][B3H8] and Cs[B3H8]) were examined by electrospray mass spectrometry in the positive ion mode. Acetonitrile solutions provided the most informative spectra; the salts exhibited a remarkable degree of clustering under electrospray conditions, and virtually all signals corresponded to cationic cluster ions of the general formula {[cationm+]x[anionn−]y}(mx − ny)+. In contrast, methanol solutions of these salts produced only B(OCH3)4− cluster ions under otherwise identical conditions. 11B NMR analyses corroborate the identities of the methanol solution species that enter the electrospray source and the reaction product generated during the electrospray process.

Protonation of isomeric methoxyhexenes. Effects of double bond⋯H+⋯OCH3 interaction

Abstract Protonated methoxyhexenes exist as hydrogen-bonded structures in the gas phase. The hydrogen bonding facilitates exchange between the proton of the ether group and the hydrogen atoms of the hexenyl chain that precedes dissociation by loss of methanol. The position of the double bond has an effect on the formation of cyclic intermediates and the mechanisms for hydrogen transfer in metastable ions and following collisional activation. Collisional neutralization of protonated methoxyhexenes results in complete dissociation of the intermediate radicals by cleavage of the CH 2 -O and O-H bonds, but not the O-CH 3 bond. A mechanism for intramolecular hydrogen atom trapping by the double bond in the radicals is suggested. Collisionally activated dissociation and neutralization-reionization mass spectra of isomeric dimethylsilyloxyhexene cations provide little structural information as to the position of the double bond.

Protonation of ferrocene in the gas phase

Hydrogen-deuterium exchange, proton and deuteron transfer, and collision-induced dissociation experiments involving protonated ferrocene, [Fe(cC5H5)2]H+, and isotopically labeled analogues have been carried out using a Fourier transform ion cyclotron resonance (FTICR) spectrometer and a double-focusing mass spectrometer of reversed geometry. These experiments reveal that the structure in which the added proton is bound to one of the cyclopentadienyl rings, possibly via agostic interaction with the iron atom, plays an important role in the gas-phase behavior of protonated ferrocene. It is demonstrated that extensive hydrogen atom scrambling occurs in the cyclopentadiene ring and that the extra hydrogen can also switch from one ring to the other, probably via the iron atom. An interpretation is presented which implicates slow thermal unimolecular rearrangement on the FKR time scale from a metal-protonated form to a ring-protonated form which is higher in energy. This interpretation successfully rationalizes the current data as well as previous gas-phase measurements and is found to be in good agreement with solution and matrix isolation studies.

Metastable and collision-induced fragmentation studies of all C4H12Si+· isomers; a systematic study of structure-reactivity relations

Metastable ion (MI) and collision-induced dissociation (CID) mass spectra have been recorded and compared for all nine C4H12Si+. isomers. The (Me)4Si+., t-BuSiH3+., s-BuSiH3+, and (Me)2EtSiH+. isomers have unique MI and CID mass spectra. The MI mass spectra, including the kinetic energy release values, of (Me)(i-Pr)SiH2+. and (Me)(n-Pr)SiH2+. are identical, which implies isomerization. MI data also suggest that a fraction of the n-BuSiH3+. ions rearrange into branched (Me)2EtSiH+. ions and a fraction of the n-BuSiH3+. ions rearrange into branched s-BuSiH3+. ions. A comparison with the isomeric C5H12+. pentanes reveals a crucial difference: H2 loss occurs for n-BuSiH3+., i-BuSiH3+., s-BuSiH3+., (Me)(n-Pr)SiH2+., (Me)(i-Pr)SiH2+., and Et2SiH2+., but not for any of the C5Hi12+. isomers. Generation of four- or five-membered silicon containing rings is suggested for H2 loss from the C4H12Si+. silanes.

Metastable and collision-induced fragmentation studies and thermochemistry of isomeric C4H11Si+ ions and their adducts with C4H12Si silanes

Metastable Ion (MI) and collision-induced dissociation (CID) mass spectra for all isomeric even-electron [C(4)H(12)Si - H](+) ions were recorded and compared. Deuterium labeling experiments indicated that most precursors give rise to silylium ions. Silylium ions with two or more methyl groups are found to lose C(2)H(4) after isomerization via a straightforward hydrogen transfer to the appropriate ethylsilylium ion. Similarly, all isomeric propyl- and butyl-containing silylium ions are found to lose C(2)H(4) by rearrangement preceding dissociation. In the CI source of the mass spectrometer many of the silylium ions are found to cluster with the parent neutral silane present in the source to give stable [M - H](+)+M adduct ions. The MI and CID spectra of these adduct ions were also obtained. In the MI spectra of all adducts, except i-BuSiH(3), only the starting silylium ion is observed. Under CID conditions generation of silylium ions dominates. Deuterium labeling studies show that this dissociation may be accompanied by some rearrangement, in particular for the adducts from i-BuSiH(3). High-pressure mass spectrometric clustering equilibrium measurements were also carried out to determine the enthalpies and entropies of binding of the silylium ions to the neutral silanes. These measurements yield insight into the effects of various alkyl group substitutions on the association thermochemistry in these adducts. Copyright 2000 John Wiley & Sons, Ltd.