Zero kinetic energy photoelectron spectroscopy of tryptamine and the dissociation pathway of the singly hydrated cation cluster

The relative ionization energies of tryptamine conformations are determined by zero kinetic energy photoelectron spectroscopy and photoionization efficiency measurements. The relative cationic conformational stabilities are compared to the published results for the neutral molecule. In the cation, the interaction strength changes significantly between amino group and either the phenyl or the pyrrole moiety of the indole chromophore where most of the positive charge is located, leading to different conformational structures and relative conformer energies in the cation. In particular, the measured adiabatic ionization potential of isomer B is 60 928 +/- 5 cm(-1), at least 400 cm(-1) higher than any of the 6 other tryptamine isomers which all have ionization potentials within 200 cm(-1) of each other. In addition to the monomer, measurements were made on the A conformer of the tryptamine(+)-H2O complex including the ionization threshold and cation dissociation energy measured using a threshold photoionization fragmentation method. The water cluster exhibits an unexpectedly high ionization potential of 60 307 +/- 100 cm(-1), close to the conformer A monomer of 60 320 +/- 100 cm(-1). It also exhibits surprisingly low dissociation energy of 1750 +/- 150 cm(-1) compared to other H-bonding involved cation-H2O complexes which are typically several thousands of wavenumbers higher. Quantum chemical calculations indicate that upon ionization the structure of the parent molecule in the water complex remains mostly unchanged due to the rigid intermolecular double hydrogen bonded water molecule bridging the monomer backbone and its side chain thus leading to the high ionization potential in the water cluster. The surprisingly low dissociation energy measured in the cationic water complex is attributed to the formation of a much more stable structural isomer H+ in the exit channel. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752080]