Mass spectral study of meso-alkyl and meso-cycloalkyl calix(4)pyrroles under electron impact conditions.

A series of meso-cycloalkyl calix(4)pyrroles (I) and meso-dialkyl calix(4)pyrroles (II) has been studied under electron ionization (EI) mass spectral conditions. All the calix(4)pyrroles showed prominent molecular ions. The cleavage of the C--C bond linked at position 2 of the pyrrole ring (beta-cleavage) is the foremost and dominant fragmentation process. The beta-cleavage process, either through ring opening or directly, results in the loss of an alkyl radical from the molecular ion. The collision-induced dissociation (CID) spectra of I showed specific sequential expulsion of pyrrole and/or cycloalkyl rings from the molecular ion with or without hydrogen migrations, revealing more information on the structure of individual compounds than was available from the EI spectra. The isomeric cycloalkyl calix(4)pyrroles showed distinguishable CID spectra, indicating structure specificity in initial ring opening whereas, in the case of II, the EI mass spectrum contains all the structure-indicative fragment ions. The CID spectra of II resulted in a dominant [M-R]+ ion, with other characteristic ions being less abundant.