CLUSTER-ASSISTED DECOMPOSITION REACTIONS OF THE MOLECULAR ANIONS OF SF6 AND C7F14

The cluster ions formed by the attachment of dimethylsulfoxide (DMSO) and methanol to the molecular negative ions of C7F14 and SF6 have been studied by a pulsed e-beam high pressure mass spectrometer (PHPMS) and by an atmospheric pressure ionization mass spectrometer (APIMS). The free energy change (ΔG°) for the clustering equilibria reaction, M-+S⇌M-S, at 35°C are found to be -7.7 and -7.2 kcal/mol for S = DMSO and M-=C7F14- and SF6- respectively, and -6.4 and -4.5 kcal/mol for S = methanol and M-=C7F14- and SF6- respectively. While the cluster ions formed by DMSO are found to be stable against side reactions, those formed by methanol undergo decomposition processes in which the central core ion is fragmented. At 35 °C, the rate law for the decomposition of the SF6-(CH30H)1 ion is second-order, involving the M-(CH30H)1 cluster ion and another methanol molecule. While the C7F14-(CH30H)1 ion also decomposes through this second-order process, a competing unimolecular mechanism is also operative at 35°C. With increases in the PHPMS ion source temperature to 150°C, the unimolecular decomposition process becomes progressively dominant for both of the M-(CH30H)1 cluster ions of C7F14 and SF6. Methanol cluster ions of the type M-S2 are not observed under any of the conditions examined here. When methanol or water partial pressures of a few torr or higher are present in the buffer gas of the APIMS ion source, the decomposition reactions are very fast and only the fragment ions produced by these reactions are observed in the electron-capture (EC)-APIMS spectra of C7F14 and SF6 . Also, in the methanol-containing APIMS ion source, the course of the SF6- decomposition reaction is altered so that fragment ions of the type F-(S)n dominate the EC-APIMS spectrum of SF6 at all ion source temperatures. For C7F14, fragment ions of the type F-(S)n become dominant at lower ion source temperatures. These previously unknown reactions are expected to be important in the analysis of perfluorinated compounds by mass spectrometric methods that utilize ionization by electron capture or negative chemical ionization. The nature of the fragment ions produced in these cluster-assisted reactions may also provide a new source of information concerning the structures of the molecular negative ions of SF6 and C7F14. © 1990 American Society for Mass Spectrometry.