Density functional theory computational study of [M•Pro-H]+ (M = Pb, Ba, or Pt) complexes in the gas phase

We performed M06-L and X3LYP density functional theory calculations to characterize gas-phase structures of [Pb center dot Pro-H](+), [Ba center dot Pro-H](+), and [Pt center dot Pro-H](+) complex isomers. Two distinct isomeric structure types, namely the A- and H-type are investigated for each complex for multiplicities of 1, 3, and 5; the lowest-energy isomers of [Pb center dot Pro-H](+) and [Ba center dot Pro-H](+) are singlet A-types, whereas the lowest-energy isomer of [Pt center dot Pro-H](+) is a singlet H-type. Vibrational bands of each isomer are assigned based on harmonic frequency analysis over the 800-4000 cm(-1) range, and signature modes predicted in the spectral region below 3200 cm(-1) are suggested for comparison with vibrational spectroscopy results so that isomer assignments can be made. Our study provides a more direct approach for structure elucidations of the [M center dot Pro-H](+) complexes than a previous study, which reports Boltzmann population analysis based on systematic calculations to predict the most abundant isomer of the [Pb center dot Pro-H](+) complex.