Search

The results of high-pressure variable-temperature and variable ionizing electron energy studies of gas-phase ion-molecule reactions of dimethyl ether in krypton are presented. Near the ionization threshold a series of peaks corresponding to (CH 3 OCH 3 ) n H + (n=1−4) clusters are observed. At higher ionizing electron energies, two new series of peaks appear, corresponding to [CH 3 OCH 2 ] + (CH 3 OCH 3 ) n and [(CH 3 ) 3 O] + (CH 3 OCH 3 ) n clusters. The onium ion, [(CH 3 ) 3 O] + , has been previously reported at elevated temperatures under methane chemical ionization conditions. It was suggested that the onium ion is formed by reaction of (CH 3 ) 2 OH + with CH 3 OCH 3 with subsequent elimination of methanol, i.e. by fragmentation of an adduct ion. The present results strongly suggest that, under our conditions, [CH 3 OCH 2 ] + rather than thermal (CH 3 ) 2 OH + , is the precursor to [(CH 3 ) 3 O] +

The ionisation potentials and characteristic fragmentations of a series of anilinium oxides and their isomeric ethers have been used to assign structures for the gaseous species from the zwitterions. Transmethylation and direct evaporation of the zwitterions are competing processes. A bulky substituent ortho to the oxygen or a strongly electron attracting group in the 2-isomer will prevent or reduce the transmethylation. If the volatility is much reduced, however, the temperature increase for evaporation results in transmethylation. The 4-isomer was transmethylated even with two tert-butyl groups ortho to the oxygen. The appearance potentials for an isomeric pair showed the energy difference between the zwitterion and the ether in the gas phase to be 11.5 kcal/mol.

Carnitine and its O-acyl derivatives undergo two major pyrolytic reactions in the mass spectrometer. Firstly elimination of water from carnitine or acid from acylcarnitine takes place followed by intramolecular displacement and formation of crotonyl lactone and trimethylamine. Secondly intramolecular displacement occurs with formation of a substituted γ-lactone and trimethylamine. For the lower acid derivatives only the elimination pathway is important. For carnitine and higher derivatives both processes are important. The electron induced fragmentations of the major pyrolysis products are relatively simple. The most important pathways for the major components are discussed. The pyrolytic composition and mode of fragmentation are characteristic and can be used for identification purposes.

o-N,N,N-Trimethylanilinium oxide is evaporated structurally unchanged, admixed with the corresponding transalkylated ether. The ionization potential for the anilinium oxide was determined as 6·8 eV which was 0·8 eV lower than for the corresponding ether. An explanation for the unusually low IP value is advanced. In IP measurements in binary mixtures the semi-log plot method showed parallel curves for different compositions in the gas phase, but no linear relationship between the recorded IP and composition was found. Appearance potential measurements show that the energy of the zwitterion in the gas phase is at least 10 kcal/mol higher than in the corresponding ether.

Comparision of the fragmentation patterns and ionisation potentials for 3-hydroxy-2-phenylpyridine and N-arylpyridinium-3-oxides shows that the latter are evaporated as mesoins. The 2,4-dinitrophenyl derivative constitutes an exception in that transarylation to the corresponding ether takes place before evaporation.

The zwitterions of diazotized ortho and para aminophenol are very volatile and give strong peaks in the mass spectra corresponding to the molecular ion. The m-isomer, which cannot be stabilized through a simple quinonoid structure, gives rise to a complex spectrum through decomposition and coupling reactions. The zwitterions of diazotized anthranilic and sulphanilic acids did not give the molecular ion peak.

Ionization potential measurements of the volatile species from the three isomers of the tautomeric hydroxyl and mercapto pyridines show that these are in the hydroxy and mercapto forms rather than appearing as lactams and betaines. The corresponding N-methyl lactams and S- and O-methyl ethers were also studied for comparative purposes.

Comparative studies of the ionisation potentials of some sydnones indicate that these molecules are largely present in the gaseous phase as their valence isomeric N-nitroso ketenes.

The ionisation potentials for a series of thiazolo[3,2-a]pyridinium-3-oxides are reduced by about 1.7 to 1.8eV on introduction of an 8-OH group. The gaseous species from the former compounds are ascribed a mesoion, structure while the hydroxy-derivatives are rearranged to non charged molecules.