Estudo da reatividade de ions e moleculas por colisões dissociativas e reações ion-molecula em fase gasosa via espectrometria de massas pentaquadeupolar

AUTOR(ES)
DATA DE PUBLICAÇÃO

1998

RESUMO

To study the gas phase chemistry of ions and molecules, some enaminones, two heterocumulene C2NO2 isomers and several isomers of pyridinium, pyrimidinium, furanyl and thiophenyl ions were chosen as systems of interest. Enaminones, were chosen because they have been extensively studied in solution by our research group. Firstly, the eletrophilic addition of methyl cation in some enaminones was carried out because of the continuing interest in alkylation sites of polyfunctional compounds. The methylated adducts were obtained via chemical ionization (CI) with CH3I and then dissociated by collisional induced dissociation (CID). The feasible fragmentation pathways were determined by studing the CID spectra ( MS and MS) of the ions which help to determine the preferencial alkylation site. Isotopic labeling experiments as well as energy resolved mass spectrometry (ERMS) were used to prove the likelihood of proposed fragmentation pathways. The heterocumulene cations O=C=N=C=O 5 e O=C=C=N=O 6 were characterized via structurally diagnostic ion-molecule reactions and by low energy CID. G2(MP2) ab initio calculations were carried out to investigate the relative stabilities and dissociation thresholds for eight C2NO2isomers. Reactions with 2-methyl-1,3-dioxolane, pyridine, isoprene, 2-methoxyethanol and 1,3-dioxane were used to investigate the reactivity of 5 and 6 ions. High resolution mass spectra were acquired to confirm the composition of the ions as being pure O=C=N=C=O or a rnixture of this one with other isobars, depending on the employed precursor. Finally, the reactivity of pyridinium, pyrimidinium, furanyl and thiophenyl ions was also studied by dissociative collisions and ion-molecule reactions similar to those applied for ions 5 and 6 G2(MP2) ab initio calculations were also applied to these ions. The results show it is possible to locate the charge site of heteroaromatic cations, and therefore to determine the position of the substituent in the neutral precursors.

ASSUNTO(S)

espectrometria

ACESSO AO ARTIGO

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