Solvation Dynamics of Li+ and Cl- Ions in Liquid Methanol

DLPNO-CCSD(T) and DFT study of the acetate-assisted C-H activation of benzaldimine at [RuCl2(p-cymene)]2: the relevance of ligand exchange processes at ruthenium(II) complexes in polar protic media

To gain mechanistic insights into the acetate-assisted cyclometallations of arylimines promoted by [RuCl2(p-cymene)]2 in polar protic media, DFT geometry optimizations (with M06 and ωB97X-D3 functionals and the cc-pVDZ-PP[Ru] basis set) followed by DLPNO-CCSD(T)/CBS energy evaluations were performed using benzaldimine as a model substrate and methanol as the solvent (with CPCM or SMD models). The calculation results show that coordination of the imine to an acetate ruthenium precursor is followed by anion (chloride or acetate) dissociation as the rate-determining step of the process. H-Bonding of two explicit MeOH to the anion reduces the calculated activation energy to ca. 23 kcal mol-1, in good agreement with the experimental half-life at room temperature. Subsequent AMLA/CMD C-H activation of the intermediate cationic complexes is a faster, reversible process. Alternative reaction pathways involving neutral diacetate ruthenium complexes offer AMLA/CMD transition state structures of lower energy but are precluded due to higher energy barriers for the initial ligand exchange processes at ruthenium. Solvent assistance accelerates the final chloride/acetate exchange processes on the cycloruthenate intermediates, particularly when compression in the condensed phase is taken into consideration. The performance of six DFT functionals (with the aug-pVTZ-PP[Ru] basis set) was assessed using the DLPNO-CCSD(T)/CBS reference energies. Neutral diacetate ruthenium complexes were incorrectly predicted as being kinetically relevant when using hybrid DFT methods (PBE0-D3(BJ), M06-2X or ωB97M-V). Good agreement between the calculated barrier heights and our benchmark energy results was obtained by using double-hybrid DFT methods. PWPB95 with D3(BJ) or D4 dispersion energy corrections was found to be the most accurate (ΔG≠ MUE of ca. 1 kcal mol-1). This study may aid our understanding of and help with further experimental investigations of synthetically useful carboxylate-assisted C-H bond functionalizations involving (N,C)-cyclometallated (p-cymene)Ru(II) intermediate complexes in sustainable polar protic solvents.

Structure and dynamics of the Li+ ion in water, methanol and acetonitrile solvents: ab initio molecular dynamics simulations

Fundamental understanding of the structure and dynamics of the Li+ ion in solution is of utmost importance in different fields of science and technology, especially in the field of ion batteries. In view of this, ab initio molecular dynamics (AIMD) simulations of the LiCl salt in water, methanol and acetonitrile were performed to elucidate structural parameters such as radial distribution function and coordination number, and dynamical properties like diffusion coefficient, limiting ion conductivity and hydrogen bond correlation function. In the present AIMD simulation, one LiCl in water is equivalent to 0.8 M, which is close to the concentration of the lithium salt used in the Li-ion battery. The first sphere of coordination number of the Li+ ion was reaffirmed to be 4. The radial distribution function for different pairs of atoms is seen to be in good agreement with the experimental results. The calculated potential of mean force indicates the stronger interaction of the Li+ ion with methanol over water followed by acetonitrile. The dynamical parameters convey quite high diffusion and limiting ionic conductivity of the Li+ ion in acetonitrile compared to that in water and methanol which has been attributed to the transport of the Li-Cl ion pair in a non-dissociated form in acetonitrile. The AIMD results were found to be in accordance with the experimental findings, i.e. the limiting ion conductivity was found to follow the order acetonitrile > methanol > water. This study shows the importance of atomistic level simulations in evaluating the structural and dynamical parameters and in implementing the results for predicting and synthesizing better next generation solvents for lithium ion batteries (LIBs).

Solvation Structure of Li+ in Methanol and 2-Propanol Solutions Studied by ATR-IR and Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods

Neutron diffraction measurements have been carried out on 10 mol % LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) solutions in methanol- d₄ and 2-propanol- d₈ to obtain information on the solvation structure of Li⁺. The detailed coordination structure of solvent molecules within the first solvation shell of Li⁺ was determined through the least-squares fitting analysis of the difference function between normalized scattering cross sections observed for ⁶Li/⁷Li isotopically substituted sample solutions. The nearest-neighbor Li⁺···O distance and coordination number determined for the 10 mol % LiTFSA-methanol- d₄ solution are r_LiO = 1.98 ± 0.02 Å and n_LiO = 3.8 ± 0.6, respectively. In the 2-propanol- d₈ solution, it has been revealed that 2-propanol- d₈ molecules within the first solvation shell of Li⁺ take at least two different coordination geometries with the intermolecular nearest-neighbor Li⁺···O distance of r_LiO = 1.93 ± 0.04 Å. The Li⁺···O coordination number, n_LiO = 3.3 ± 0.3, is determined. Ion-pair formation in the LiTFSA-methanol and LiTFSA-2-propanol solutions has been investigated by the attenuated total reflection infrared spectroscopic method. Mole fractions of free, Li⁺-bound, and aggregated TFSA^- are derived from the peak deconvolution analysis of vibrational bands observed for TFSA^-.

Ab Initio Molecular Dynamics Study of Mg(2+) and Ca(2+) Ions in Liquid Methanol

Ab initio Car-Parrinello molecular dynamics simulations have been performed in order to investigate the solvation properties of Mg(2+) and Ca(2+) in fully deuterated methanol solution to better understand polarization effects induced by the ions. Charge transfer and dipole moment calculations have been performed to give more detailed insight on the role of the electronic reorganization and its effect on the first solvation shell stability. The perturbation of the methanol H-bond network has been investigated.

Aqueous solutions: state of the art in ab initio molecular dynamics

The simulation of liquids by ab initio molecular dynamics (AIMD) has been a subject of intense activity over the last two decades. The significant increase in computational resources as well as the development of new and efficient algorithms has elevated this method to the status of a standard quantum mechanical tool that is used by both experimentalists and theoreticians. As AIMD computes the electronic structure from first principles, it is free of ad hoc parametrizations and has thus been applied to a large variety of physical and chemical problems. In particular, AIMD has provided microscopic insight into the structural and dynamical properties of aqueous solutions which are often challenging to probe experimentally. In this review, after a brief theoretical description of the Born-Oppenheimer and Car-Parrinello molecular dynamics formalisms, we show how AIMD has enhanced our understanding of the properties of liquid water and its constituent ions: the proton and the hydroxide ion. Thereafter, a broad overview of the application of AIMD to other aqueous systems, such as solvated organic molecules and inorganic ions, is presented. We also briefly describe the latest theoretical developments made in AIMD, such as methods for enhanced sampling and the inclusion of nuclear quantum effects.

On the coupling of solvent characteristics to the electronic structure of solute molecules

We present the results of a theoretical investigation focusing on the solvent structure surrounding the -1, 0 and +1 charged species of F, Cl, Br and I halogen atoms and F2, Cl2, Br2 and I2 di-halogen molecules in a methanol solvent and its influence on the electronic structure of the solute molecules. Our results show a large stabilizing effect arising from the solute-solvent interactions. Well-formed first solvation shells are observed for all species, the structure of which is strongly influenced by the charge of the solute species. Detailed analysis reveals that coordination number, CN, solvent orientation, θ, and solute-solvent distance, d, are important structural characteristics which are coupled to changes in the electronic structure of the solute. We propose that the fundamental chemistry of any solute species is generally regulated by these solvent degrees of freedom.

Wavelet Transform for Spectroscopic Analysis: Application to Diols in Water

Wavelet transform has been used to correlate spectroscopic and structural properties from trajectories obtained by ab initio molecular dynamics simulations. This method has been applied to hydrogen bond dynamics of glycols in heavy water solutions, showing how the stretching frequency of the intramolecular O-H bond changes with the intermolecular hydrogen-bond distance. The resulting wavelet spectrograms have been interpreted according to H-bond strength and stability.

Quantifying the Reactivity of a Remarkably Long-Lived Difluorinated Enol in Acidic Methanol via Solution Kinetics and Electronic Structure Calculations

A simple enol acetal underwent rapid cleavage in acidic solution to generate a difluorinated enol, which was sufficiently long-lived to be characterized by 2D NMR in a protic solvent at ambient temperature. Density functional theory calculations on a model reaction suggest that there are significant differences in protonation transition state timing between the fluorinated and nonfluorinated enols.

Structure and Dynamics of Br- Ion in Liquid Methanol

A Car-Parrinello molecular dynamics simulation has been performed on a solution of Br- in liquid methanol analyzing with particular attention charge transfer and polarization effects. The first solvation shell has been characterized in terms of H-bonds, and it has been found that the high polarization of the bromide gives rise to a stable solvent cage. The differences in the coordination number with the chloride can be ascribed to the ionic radius and to the stronger perturbations brought by the solvent to the bromide ion.

Water Exchange Dynamics of Lithium(I) Ion in Aqueous Solution

The water exchange dynamics of the fourfold coordinated first hydration shell of the lithium(I) ion was studied by both direct and umbrella sampling QM/MM-MD and classical MD simulations. The structural changes and energetics accompanying the activation process are discussed. The overall exchange rate constant was found to be k(ex) = 5.8 x 10(9) s(-1) from classical MD simulations. QM/MM-MD umbrella sampling simulations predict an exchange rate constant of k(TST) = 1.01 x 10(10) s(-1) as obtained from classical transition-state theory. First-shell ligands exchange preferably via an associatively activated mode.

Solute size effects on the solvation structure and diffusion of ions in liquid methanol under normal and cold conditions

We have performed a series of molecular dynamics simulations of alkali metal (Li+, Na+, K+, Rb+, and Cs+) and halide (F-, Cl-, Br-, and I-) ions in liquid methanol at two different temperatures to investigate the effects of ion size on the hydration structure and diffusion of ions in methanol under normal and cold conditions. Simulations are also carried out for some of the larger cations such as I+, (CH3)4N+, and (C2H5)4N+ and also neutral alkali metal atoms in methanol at both temperatures. With the increase of ion size, the diffusion coefficients of both positive and negative ions are found to show anomalous behavior. For cations, it is found that the maximum of the diffusion coefficient versus ion size curve occurs at the rather large cation of (CH3)4N+ unlike in water where the maximum occurs at the relatively smaller ion of Rb+. For halide ions, the anomalous behavior, i.e., the increase of diffusion with ion size, continues up to iodide ion and no maximum is observed. These results are in good agreement with experimental observations. The diffusion coefficients of neutral atoms are found to be greater in methanol than that in water and they decrease monotonically with solute size, whereas the diffusion coefficients of the corresponding ions are found to be smaller in methanol. Accordingly, an ion experiences a smaller Stokes friction and a higher dielectric friction in methanol than in water. These contrasting effects are believed to be responsible for the shift of the maximum of ion diffusion toward a larger ion size when compared with similar anomalous size dependence in liquid water.