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.

Automated summary

The gas-phase structures of [Pb·Pro-H](+), [Ba·Pro-H](+), and [Pt·Pro-H](+) complexes were characterized using M06-L and X3LYP density functional theory calculations. Different isomeric structure types were investigated for each complex, and vibrational bands were assigned based on harmonic frequency analysis to aid in isomer assignments. The study provides a more direct approach for structure elucidation of the [M·Pro-H](+) complexes compared to previous methods.