Affinity of Ions to Hydrophobic Interfaces

Radial and angular distribution functions from these simulations are shown in Fig. 5.4. Comparison of RDFs between the central carbon on TEA (Ctba) and water oxygen, 1 , and F clearly shows that fluoride ions are repelled from the first hydration shell of TEA, which is determined by the minimum in TBA-water RDF at 6.5 A (the first minimum at 4.0 A corresponds to the waters 

Distribution of ions around a solute can be deduced from the frequency shift in characteristic vibrational modes of the solute upon increasing the salt concentration in solution. Such measurements with Raman-MCR (multi-curve resolution) technique were performed by Een-Amotz and co-workers for low-concentration TEA aqueous solutions at various concentrations of Nal and NaF [37]. These measurements show no frequency shift in CH stretch region of TEA in a presence of NaF, in agreement with EFP simulations showing no fluoride ions in proximity of TEA. On the other hand, there is a small reproducible red shift in CH stretch vibration in Nal solutions, with a magnitude of 1 cm /M. That is, a total shift of 3 cm in a 3M Nal solution was observed. The QM/EFP scheme, in which TEA and iodide ions were described by MP2/6-311-I—l-G and water molecules were represented by effective fragments, was used to reproduce this salt concentration dependent frequency shift. For that, 100 snapshots from the EFP 

MD TBA-Nal-water trajectories were gathered and QM/EFP partial Hessian frequency calculations were performed. These frequencies were compared with analogues QM/EFP averaged frequencies of salt-free TBA-water solution. The average shift of Raman active modes of TBA in 2.7 M aqueous Nal solution was calculated as 4 1 cm , in a very good agreement with experiment. 

These results suggest that EFP accurately predicts affinity of ions to molecular interfaces, which is very promising for future applications of EFP to studying interfacial phenomena. 

Many chemical and biological processes call for a quantum treatment, due to a chemical reaction, electronic excitation or necessity of a more accurate description. Hybrid quantum mechanics/molecular mechanics (QM/MM) approach, pioneered by Warshel and coworkers [50], provides means to represent a part of the system with a rigorous ab initio method while modeling the rest of the S5 em with a classical force field. The QM/MM Hamiltonian of the combined system can be represented as  

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