Exploration of the full conformational landscapes of gaseous aromatic amino acid phenylalanine: An ab initio study

Systematic ab initio calculations have been employed to characterize the conformational topology of gaseous aromatic phenylalanine (Phe). A total of 37 local minima were located by geometry optimization of all possible trial structures at the B3LYP/6-311++G(d,p) level of theory. The relative energies, dipole moments, rotational constants, harmonic frequencies, and vertical as well as adiabatic ionization energies of all the conformers were determined. The comparison of the theoretical and experimental ionization energies supports the conformational assignments through the UV rotational band contour analysis of the resonantly enhanced two-photon ionization (R2PI) spectrum by [Y. Lee, J. Jung, B. Kim, P. Butz, L. C. Snoek, R. T. Kroemer, J. P. Simons, J. Phys. Chem. A 108 (2004) 69]. The transition states among the nine lowest energy conformers were searched at the MP2/6-311G(d) level of theory and used to explain the absence of conformers 4 and 8 in the R2PI spectrum of jet-cooled Phe experiment. The conformational distributions of gaseous Phe at various temperatures were calculated according to the principle of the statistical mechanics and correlated the experimental observation reasonably well. In addition to the discussion by the geometric criteria, the intramolecular hydrogen bonding interactions of the conformers were also analyzed by the atoms in molecules (AIM) theory based upon the B3LYP/6-311++G(d,p) electron density.