The geometries and IR frequencies of uranyl complexes were calculated by B3LYP method in density functional theory (DFT) using the relative effective core potential (RECP) on uranium and 6-31+G(d) basis set on other elements. Both gaseous and aqueous phases were considered and conductor-like polarized continuum model(CPCM) was used to consider the solvation effect of water. Ligands investigated in the present paper were F-,CO230-,and NO3-. A linear correlation between the frequency of the O=U=O symmetrical stretching vibration and the number (n) of ligands was established for the above-mentioned ligands according to the following two equations:νs=-Agasn+983 and νs=-Aaqn+ 821,where Agas and Aaq are characteristic coefficients that represent the shift in vibrational frequency for the addition of each ligand to the uranyl center. Results obtained for F-fit the equations with Agas=53 cm-1 and Aaq=11 cm-1; CO230-with Agas=85 cm-1 and Aaq=19 cm-1; NO3-with Agas=48 cm-1 and Aaq=-10 cm-1. The value of Aaq was found to correspond to the experimental results.
The π−cation−π interaction between a cation or a cationic group and several aromatic residues, although rather prevalent in biological systems, has not been studied theoretically. The ab initio MP2 calculations were carried out on the systems composed of TMA with two aromatic rings, viz. benzene, pyrrole, or indole, to explore how a cation or a cationic group interacts simultaneously with two aromatic residues in proteins or nucleic acids. The calculated results on π−TMA−π complexes revealed additivities of both the geometries and the binding energies relative to cation−π complexes. The preferred structure of such a complex can be constructed by superimposing the corresponding TMA−π complexes via the cation. The binding energies of the π−TMA−π sandwiches are the sums of the two corresponding TMA−π systems. The contribution of electron correlation to the overall binding energy is estimated to be at least 50%, with dispersion serving as the main component of the electron correlation interaction. Similar to geometrical and energetic additivity, the additivities in BSSE and ΔZPE were also found. Therefore, our finding provides a convenient and effective way to construct π−TMA−π sandwiches and to estimate their binding energies. Morokuma decomposition analysis on the binding energy indicated that the electrostatic, charge transfer, and polarity interactions drive the binding of TMA with aromatics, whereas the exchange repulsion and high order coupling always obstruct the TMA approaching aromatics. Charge-transfer happens to some extent during the complexation of TMA with aromatics, and the transferred NPA atomic charges and charge-transfer energies are almost same in different complexes of TMA−π or π−TMA−π. The interaction between the 2 aromatics in the sandwich π−TMA−π complexes is negligible because of their long interaction distances. All this information should be helpful in studying such interactions in biological systems.
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