Abstract The investigation of several 13carbon and deuterium labelled n-butyl and n-pentyl benzenes demonstrate that chemical ionization (reagent gas: methane) induces specific carbon-carbon bond cleavages of the alkyl group. The extent of competing reaction channels as for instance direct alkene elimination versus dealkylation/reprotonation is analyzed. Partial hydrogen exchange processes between reagent ions and substrate molecules are restricted to the phenyl ring. Intramolecular exchange reactions between the side chain and the aromatic ring which are typical for the open shell molecular ions of alkyl benzenes are not observed for analogous closed shell cations.

By means of chemical ionization 1,3-dioxolanylium as well as 1,3-dioxanylium ions are formed in proton catalysed ! 3$ assisted heterolysis in the gas phase. The effects of both constitution and configuration are discussed and compared with the results of analogous reactions in the condensed phase. It is shown that the unimolecular decompositions of [MH]' ions containing two vicinal substituents, e.g. Br or OAc, are not governed by the proton affinity of the departing neutrals HBr or HOAc, respectively. The findings partially contradict the results on HX loss (X: substituent) from protonated monosubstituted compounds.

The eight epimeric 3,4-dimethyl-l,2-cyclopentanediols have been investigated as bistrimethylsilyl (TMS) ethers and as diacetates by means of isobutane chemical ionization (CI). Correlations between stereochemistry and CI spectra appear to be limited in the case of the TMS ethers. All eight diacetates, however, can be unequivocally identified from their isobutane CI spectra.

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The helium charge exchange and the isobutane chemical ionization spectra of 4-methoxy cyclohexanecarboxylic acid ethyl esters (1c and 1t), 2,6-dimethyl-4-hydroxytetrahydropyranes (2c and 2t), 2,4,6-trimethyl-4-hydroxytetrahydropyranes (8c and 3t), and 2,7-dimethoxy-cis-decalins (4c and 4t) have been screened for correlations with the stereochemistry of these molecules. From these data it appears that chemical ionization is especially suited for configurational assignments in epimeric molecules.

Methane and isobutane chemical ionization mass spectrometry is sqerior to the classical electron impact technique for the analysis of aliphatic macrocyclic polyethers of the 4n-crown-n type. The latter reagent gas is particularly suited for molecular weight determinations.To our knowledge only a few reports'-' have discussed the electron impact (EI) behaviour of macrocyclic polyethers so far. In all but two instances'" these studies have been concerned with compounds containing at least one aromatic unit in the macrocycle. These exhibit readily detectable molecular ions which are very abundant when at least two such units are present. Obviously, the lack of data (and/or interest) in aliphatic crown ethers is due to the extremely low abundance' of their molecular ions (if detected at all8). We feel that chemical ionization (CI)' using methane or isobutane as a reagent gas is the appropriate ionization technique for the analysis of such EI labile molecules. First, proton transfer CI is a relatively soft ionization mode producing ions with substantially lower internal energy than under EI conditions.' Second, the energy deposited in the quasimolecular ions can be controlled through the selection of the reagent gas (methane or isobutane), as has been demonstrated'*'' in several other instances.The recent publication' of the isobutane CI spectrum of 20-crown-4 ether prompted us to disclose our own results on 4n-crown-n type ethers A,, B,, C, and D,. The relevant methane and isobutane CI data are collected in Tables 1 and 2. All compounds yield detectable quasimolecular ions [MH]+ in their methane CI spectra (Table 1). The minimal abundance of 0.9 Oh&, recorded for C3 corresponds to a relative intensity of (0.9 X 100)/13.1= 7.6% and still allows facile molecular weight determination. Note that the smaller compounds (n = 1) also undergo significant hydride abstraction to give the [M -H]+ species. Apparently, the protonation reaction becomes increasingly favoured with increasing ring size, presumbaly through a better trapping of the transferred proton by one of the central oxygen atoms of the coiled/folded molecule. Possibly a rapid proton transfer reaction from one oxygen atom to another also adds to the

Electron impact induced fragmentation of 4-(o-phenoxymethyl and -ethyl)tetrahydropyranes results in abundant [M -93]+ ions through loss of a phenoxy radical. These ions are also produced in high yields by isobutane chemical ionization through loss of phenol from the quasimolecular ions. Labelling data (aH, 13C) provide evidence for the l-oxoniabicyclo[2.2.l]heptane and l-oxoniabicyclo[2.2.2]octane structures of the electron impact produced decomposing (ion source and first field free region) ions [M -93]+ in the case of the methyl and ethyl compounds, respectively.