Photochemistry and proton transfer reaction chemistry of selected cinnamic acid derivatives in hydrogen bonded environments

Abstract

Proton transfer reactions between cinnamic acid derivatives (MH) and ammonia are studied using a time-of-flight mass spectrometer equipped with a supersonic nozzle to entrain neutral species formed by 337 nm laser desorption. The supersonic nozzle is used to form clusters of the type MH(NH3) n where n ranges to numbers greater than 20. Multimeric clusters of MH, e.g. MH2(NH3) n are not detected in this experiment or are of low abundance. Photoexcitation of MH(NH3) n clusters by using 355 nm photons yields ionic species that correspond to direct multiphoton ionization, e.g. MH +·(NH3) n , and proton transfer reactions, e.g. H +(NH3) n . Analogous product ions are formed by photoexcitation of the methylamine, MH(CH 3NH 2) n , and ammonia/methanol, MH(NH 3)(CH 3OH) n , clusters. Detailed analysis of energetics data suggests that proton transfer occurs through neutral excited stare species, and a mechanism analogous to one proposed previously is used to rationalize the data. The energetics of proton transfer via a radical cation form of the cinnarnic acid dimer is also consistent with the data. The relevance of this work to fundamental studies of matrix-assisted laser desorption ionization (MALDI) is discussed. In particular, the role of excited state proton transfer (ESPT) in MALDI is discussed.