Intramolecular hydrogen transfer in the ionization process of α-alanine

Abstract

Photoionization and dissociative photoionization of alpha-alanine have been studied with infrared laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (IR LD/VUV PIMS) and theoretical calculations. By scanning photoionization efficiency (PIE) spectra, appearance energies (AEs) of the fragments CH(3)CHNH(3)(+) (m/z = 45, doublet state) and CH(3)CH = NH(2)(+) (m/z = 44, singlet state) are measured to be 9.53 and 9.21 +/- 0.05 eV, respectively, which are consistent with the theoretical values of 9.61 and 9.37 eV with ab initio G3B3 calculations. Formation of the CH(3)CHNH(3)(+) ion produced via intramolecular hydrogen transfer and subsequent decarboxylation processes is competitive to that of the CH(3)CH[double bond, length as m-dash]NH(2)(+) ion derived from a direct C-C(O) bond cleavage. The photoionization mass spectrum obtained at the photon energy of 10.0 eV shows that the intensity of CH(3)CH[double bond, length as m-dash]NH(2)(+) is much stronger than that of the CH(3)CHNH(3)(+) fragment. The formation of the CH(3)CHNH(3)(+) and CH(3)CH[double bond, length as m-dash]NH(2)(+) ions is thought to be thermodynamically and kinetically favorable, respectively. Detailed formation pathways for the two cations have been proposed by using ab initio calculations. The calculated results provide a clear picture of the photoionization and dissociative photoionization processes of the alpha-alanine cation.