Organic reactions at interfaces: charge minimization in desorption ionization of Zwitterionic compounds

Abstract

Zwitterionic compounds of the general formula R(CH 3) 2N + (CH 2) n X −1 (R = CH 3, n-C 12H 25, n-C 16H 33 or n-C 22H 45; X = CO 2, SO 3, OSO 3 or P(CH 3)O 2) and the cytidine-5′-di-phosphocholine monosodium salt, upon examination by secondary ion mass spectrometry, undergo beam-induced alkyl transfer reactions. These processes are indicated by (M + CH 3) + and (M + R) + ions in the positive ion SIMS spectra of the solid samples, while the absence of (M − CH 3) − and (M − R) − ions in the negative ion spectra suggests that the reaction does not occur by simple intermolecular alkyl transfer. The extent of alkyl transfer decreases when the sample is examined in solid NH 4Cl or p-toluenesulfonic acid ( p-TSA) or liquid triethanolamine (TEOA) matrices, supporting an intermolecular mechanism. When the negative ion spectra are recorded for the compounds in TEOA solution, each displays (M − CH 3) − or (M − R) − ions, which is interpreted as evidence for intermolecular alkyl transfer from the analyte to the nucleophilic solvent. Charge site remote fragmentation, methyl-for-sodium and hydrogen-for-sodium substitution reactions and multiple alkyl transfers are also observed. The relative abundances of the ions in the spectra of the neat samples and those diluted in solid NH 4Cl are time dependent. The extent of fragmentation increases with irradiation time until approximately a monolayer of the surface has been modified, after which fragmentation is constant. These observations are consistent with a transalkylation mechanism in which an alkyl group is abstracted from a protonated molecule by an adjacent zwitterion. This mechanism, which is thermochemically favored, suggests that minimization of the number of charges desorbed into the gas phase is a significant driving force in interfacial chemistry.