Heinz Heimbach

Gaseous odd- and even-electron ions

Abstract The principal collision-induced fragmentations of simple protonated ketones, aldehydes, ethers, amines, sulphides, alcohols, acids, nitriles and halides are discussed. These protonated molecules decompose mainly by loss of alkane, alkene and RX (R = alkyl, H; X = OH, SH, NH 2 , Br, I). Substantial radical losses are only observed for small protonated molecules. Deuterium-labelling demonstrates that the XH bond is particularly strong. The fragmentation of (MH) + ions is compared with that of the corresponding (M) +− ions. The spectra of the (M) +− ions are dominated by direct bond cleavages, in particular α-cleavages, as a result of both the stability of the ionic fragment and the loose transition state. In (MH) + ions direct bond cleavages lead to energetically less favourable products. Thus rearrangement reactions play a more important role in the decomposition of these ions. (MH) + ions are more stable relative to fragmentation than (M) +− ions.

A study of gaseous 3-membered ring oxonium and sulphonium ions

The 1-methyl-1-thionia cyclopropane 3 and 1-phenyl-1-thionia cyclopropane 4 ions are stable, with lifetimes greater than 10−5 sec, and can be identified from their collisional activation spectra. Their metastable counterparts (lifetime window 10−6–10−5 sec) have undergone ring opening to the isomeric structures CH3S+CHCH3 9 and C6H5S+CHCH3 11 prior to decomposition. The 1-methyl-1-oxonia cyclopropane 1 and 1-phenyl-1-oxonia cyclopropane 2 ions could not be generated: instead acyclic structures CH3O+CHCH3 5 and C6H5O+CHCH3 7 were found for both metastable and long living species. Loss of a phenoxy radical from C6H5OCH2CH2OCH3 is shown to be preceded by a reciprocal hydrogen transfer and is not due to a SNi-type reaction.

Massenspektrometrische Untersuchungen zur Genese von [M—Me] [M-Et] [C7H7]+ und [C7H8]+ aus n-Butyl- und n-Pentylbenzol: Zeitabhängige Nachbargruppeneffekte, Isomerisierungen und kompetitive Wasserstoff-Umlagerungen / Mass Spectrometric Investigations on the Formation of [M-Me] [M-Et] [C7H7]+ und [C7H8]+ from w-Butyl and w-Pentyl Benzenes: Time Dependent Neighbouring Group Participation, Isomerization and Competitive Hydrogen Rearrangements

Abstract The details of the elimination of CH3· and C2H5· from the molecular ions of n-butyl and n-pentyl benzenes as well as the formation of C7H7+ and C7H7+ -have been established using a combination of different mass spectrometric techniques. Among these techniques are field ionization kinetics (FIK), collisional activation (CA), unimolecular decomposi-tion of metastable ions (MI), kinetic energy release determinations (Tkin)> appearance potential measurements (AP) as well as high resolution mass spectrometry. The appli-cation of these methods and the investigation of nine 13 carbon and eleven deuterium labelled n-butyl and n-pentyl benzenes clearly demonstrate that 1) the loss of methyl and ethyl from terminal positions of the alkyl chains are accompanied by an interaction with the phenyl ring and 2) the regioselectivity of these processes is time dependent. Contrary to previous conclusions it is shown that hydrogen transfer in the formation of C7H8+ -occurs via five-, six-and seven-membered transition states. The time dependence of the hydrogen exchange reaction preceding the alkene eliminations is discussed in detail.

HX-Eliminierung (X = OH, NH2) aus primären ω-phenylsubstituierten Alkoholen und Aminen: Indiz für interne Solvatisierung von Radikalkationen in der Gasphase oder Resultat einer selektiven Aktivierung von CH-Bindungen? / HX-Elimination (X = OH, NH2) from Primary ω-Phenylsubstituted Alcohols and Amines: a Probe for Internal Solvation of Gaseous Radical Cations or the Result of a Selective Activation of CH-Bonds ?

Using collisional activation mass spectrometry the structures of the [M–HX]+• ions (X = OH, NH2) from 5-phenylpentanol-1 (1), 5-phenylpentylamine (2) and 6-phenylhexanol-1 (3) have been established. Contrary to previous results it is shown for the first time that in addition to ionized 1-phenyl-alkenes-1 some other double bound isomers as well as phenylcyclopentane and phenylcyclohexane are formed. The results strongly suggest that the HX eliminations are induced by a selective activation of CH bonds.

Isomeric gaseous C2H4Cl+ ions

Gaseous ethylenechloronium ions and their methylchlorocarbenium isomers are stable and can be identified from their collisition activation spectra.