On the stability of [Pb(Proline)]2+ complexes. Reconciling theory with experiment

Gas‐Phase Interactions between Lead(II) Ions and Cytosine: Tandem Mass Spectrometry and Infrared Multiple‐Photon Dissociation Spectroscopy Study

“…For simplicity in nomenclature, the basis sets used for Pb in conjunction with 6-31G(d,p) and 6-311+G(3df,2p) for the remaining atoms, will now be referred to as 6-31G(d,p) and 6-311 + G(3df,2p) basis sets, respectively. It is important to note that very recently a new theoretical assessment of lead-containing compounds was published, [28] and the B3LYP functional was found to predict reasonably well the experimental infrared data. This could be considered an additional argument of the level of theory chosen in the present work.…”

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

“…Due to the uncertainty in the calculation results, the authors stated that experiments are necessary for structure identifications for [Pb·Pro‐H] + , and assigned the isomer based on spectroscopy results as stated above. The suggestion for the necessity of experiments was, however, contested in another study [14], where the authors argued that structures can be identified using the Boltzmann statistics based on the converged energy value from a series of calculations. Specifically, the authors claimed that the converged Gibbs energy difference of 7.6 kJ·mol −1 (79 meV) between the A‐ and B‐types obtained using the M06 method gives a Boltzmann distribution of 98% A‐type, which is consistent with the experimental result that only the spectral feature of the A‐type isomer is observed [14] in the infrared spectrum.…”

“…The suggestion for the necessity of experiments was, however, contested in another study [14], where the authors argued that structures can be identified using the Boltzmann statistics based on the converged energy value from a series of calculations. Specifically, the authors claimed that the converged Gibbs energy difference of 7.6 kJ·mol −1 (79 meV) between the A‐ and B‐types obtained using the M06 method gives a Boltzmann distribution of 98% A‐type, which is consistent with the experimental result that only the spectral feature of the A‐type isomer is observed [14] in the infrared spectrum. It should be noted that the Boltzmann statistics is applicable only to systems where the molecular species are in thermal equilibrium with their surroundings, meaning that ion complexes and clusters in vacuum do not necessarily follow the isomer distribution predicted by the statistics [27].…”

“…The gas‐phase complexes serve as models for studying inherent ion‐ligand interactions without complications arising from solvent molecules typically found in a biological environment, and stepwise hydration of the complexes allows one to systematically investigate solvent‐mediated structural evolution of the gas‐phase species. Complexes of a metal ion and a common amino acid represent the model system, and their structures and dissociation reactions have been studied [1–14] extensively through experiments and computations. In these complexes, the bond between the ion and the ligand is formed through the interaction of the metal with carboxyl, amino, or aromatic groups [5,6,10,13], and the observed isomeric variations depend on factors such as the conformations of the amino acid, interaction between the ion and the amino acid side chain, and the hydrogen affinity of the metal ion.…”

“…Given the computational uncertainty, the isomer assignment was made based on a comparison with spectroscopy data, and the authors of the work emphasized the importance of experiment for gaining an insight into the structures of the complex. However, a later study [14] cast a skeptical view on the necessity of experiment, claiming that structure identification is possible based on calculations alone if they produce converging energy values, which in turn can be used for the Boltzmann statistics to determine the most abundant isomer.…”