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·Pro‐H] + , [Ba·Pro‐H] + , and [Pt·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·Pro‐H] + and [Ba·Pro‐H] + are singlet A‐types, whereas the lowest‐energy isomer of [Pt·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 ·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·Pro‐H] + complex.