Molecular structure of silsesquioxanes determined by matrix-assisted laserdesorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometr y was used to deduce the three-dimensional structure of a complex silsesqui oxane polymer. Four distinct: levels of structure were observed in the mass spectrum. The overall shape of the peak distribution was typical of polyme rs formed by condensation reactions. The mass separation between major clus ters of peaks, each major cluster corresponding to an oligomer with a uniqu e number of repeat units, confirmed that the synthesis proceeded as expecte d with no side reactions: The mass separation between peaks within a major cluster showed that intramolecular reactions during synthesis resulted in t he elimination of water. The loss of water was ascribed to the formation of closed loops in the polymer structure. A simple arithmetic algorithm is pr esented for identifying these peaks. Autocorrelation techniques were used t o determine the number and distribution of intramolecular closed loops per oligomer. This knowledge was used to deduce whether a particular oligomer i s branched-linear, ladder, polyhedral, or some combination of these. The si ngle-oligomer isotopic distribution was used to determine that cationizatio n was present from both sodium and potassium ions. (C) 1999 American Societ y for Mass Spectrometry.