Lewis acid-base interactions of SiF4 with molecular anions formed by electron capture reactions

Lewis acid-base interactions between SiF4 and a wide range of molecular neg ative ions are reported here for the first time. The molecular anions inclu de those formed by simple electron attachment to p-benzoquinone, benzopheno ne, nitrobenzene, and 21 substituted nitrobenzenes and also include the o- and p-nitrophenoxy anions. From measurements performed by pulsed electron-b eam high pressure mass spectrometry, equilibrium constants and free energie s for the association reactions, M- + SiF4 reversible arrow M- (SiF4), at 1 50 degreesC are reported for each of the molecular anions, M-. It is shown that the strengths of these Lewis acid-base interactions of SiF4 are much g reater than ion-dipole interactions previously reported between these molec ular anions and several common solvent molecules of relatively high dipole moment, including methanol, acetonitrile, dimethylformamide, and dimethlysu lfoxide. The strengths of the Lewis acid-base interactions of SIF, with mol ecular anions show a strong inverse dependence on the electron affinity of the parent molecule and on the availability of a specific Lewis base site o n the molecular anion that can be closely approached by the central Si atom of SiF4. It is also shown that strong interactions between SiF4 and the mo lecular anions derived from compounds of very low electron affinity can be gainfully used for the trace detection of such compounds by electron captur e mass spectrometry. (Int J Mass Spectrom 206 (2001) 53-61) (C) 2001 Elsevi er Science B.V.