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

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 Mn, 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(CH3NH2)(n), and ammonia/methanol, MH(NH3)(CH3OH)(n), clusters. Detailed analysis of energetics data suggests that proton transfer occurs through neutral excited state 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 cinnamic 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. (C) 1998 Elsevier Science B.V. All rights reserved.