EXCITED-STATE PROTON-TRANSFER IN LASER MASS-SPECTROMETRY

Molecular ion formation was studied for several bifunctional carboxylic acids to elucidate the role of excited-state proton tranfer (EPST) in UV laser mass spectrometry (LMS). Formation of (M - 2H + 3Li)+ in the laser mass spectra of both 1- and 4-fluorencarboxylic acids constitutes direct evidence for ESPT. Molecular ion formation for several aromatic and aliphatic hydroxy carboxylic acids reveals that the extent of (M - 2H + 3Li)+ ion formation by aromatic acids is controlled by the relative abundance and/or lifetime of the zwitterionic tautomer in the excited singlet state. Spectra of 4-fluorenecarboxylic acid-9-d2 mixed with LiCl failed to show any tricationized ions (M - H - D + 3 Li)+ because of kinetic isotope effects. However, laser mass spectra of 4-fluorenecarboxylic acid-d showed significant abundances of (M + H)+ ions, providing evidence for excited-state self-protonation.