Cu2+ in liquid ammonia-The impact of solvent flexibility and electron correlation in ab initio quantum mechanical charge field molecular dynamics.

The impact of solvent flexibility and electron correlation on the simulation results of Cu2+ in liquid ammonia has been investigated via an ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulation approach. To achieve this, three different simulation systems were considered in this study, namely Cu2+ in rigid and flexible ammonia at Hartree-Fock (HF) level of theory, as well as resolution of identity second order Møller-Plesset (MP2) perturbation theory in the rigid body case. In all cases, a stable octahedral [Cu(NH3 )6 ]2+ complex subject to dynamic Jahn-Teller distortions without the occurrence of ligand exchange was observed. The Cu2+ - NH3 distance in the first shell agrees well with the experimental and other theoretical data. In all three cases, the structural data shows that the rigid-body ammonia model in conjunction with the HF level of theory provides accurate data for the first solvation shell, while at the same time, the computational demand and thus the achievable simulation time are much more beneficial. The vibrational analysis of the Cu2+ - NH3 interaction yields similar force constants in the three investigated systems indicating that there is no distinct difference on the dynamical properties of the first solvation shell. In addition to the QMCF MD simulations, a number of natural bond orbital (NBO) analyses were carried out, confirming the strong electrostatic character of the Cu2+ - NH3 interaction.

Solvation effects on wavenumbers and absorption intensities of the OH-stretch vibration in phenolic compounds - electrical- and mechanical anharmonicity via a combined DFT/Numerov approach.

Previous measurements of fundamental, first-, second- and third overtones of the OH-stretching vibration of phenol and 2,6-difluoro-phenol by use of visible (Vis), near-infrared (NIR) and infrared (IR) spectroscopy revealed an oscillating pattern in the intensity quotient between the two kinds of solvents, carbon tetrachloride and n-hexane, upon increase of the vibrational quantum number, which could not be reproduced utilizing quantum mechanical calculations in implicit solvation. In the present study this phenomenon was successfully explained for the first time, employing an explicit consideration of solute-solvent interactions in combination with modern grid-based methods to solve the time-independent Schrödinger equation. The capabilities of this framework of (i) not requiring any assumptions on the form of the resulting wave function, (ii) focusing the description on the vibrational mode of interest and (iii) taking solute-solvent interactions explicitly into account are a particularly lucid example of the advantages in applying state-of-the-art approaches in investigations of challenging vibrational quantum problems. The property of grid-based methods being directly applied onto any given potential energy grid together with point (i) enable to analyse the impact of mechanical- and electrical anharmonicity independently. Especially the detailed investigation of the latter contribution when moving from a harmonic to an anharmonic potential in conjunction with the explicit consideration of solvent effects at the example of an actual chemical system (i.e. not discussing these effects employing mere model potentials) demonstrate the manifold benefit achieved using the applied DFT/Numerov strategy.

The coupling of localised, vibrational modes - Probing OH-bands of organic molecules via a two dimensional Numerov approach.

In this article the extension of the grid-based Numerov approach to probe two coupled, localised vibrational modes is assessed. The theoretically obtained wave numbers are compared to experimental results for five increasingly complex organic molecules carrying two OH groups measured in gas-phase as well as carbon tetrachloride. By using an appropriate spacing of the associated potential energy grid a deviation of the predicted wave numbers with experiment of ≤1% is achieved for both the fundamental and the first overtone bands. In particular the calculated wave numbers of aliphatic species in vacuum underline the versatility of this approach. In addition, it is demonstrated that bicubic interpolation is a viable strategy to greatly reduce the required data points and thus, the computational effort. Comparison of predicted wave numbers obtained for different conformers with experimental data enables the identification of the most relevant conformer present in solution. Since especially the accurate calculation of overtone vibrations is known to be challenging in case of strongly anharmonic potentials such as OH bonds, the presented approach provides a particularly efficient route to study the properties of the associated overtone contribution under the influence of inter-mode coupling. This is due to the fact that the Numerov approach requires no assumption about form and composition of the vibrational wave functions. In addition, the presented method also provides one of the simplest routes to access combined excitations of the considered vibrational modes.

Assessing the predictability of anharmonic vibrational modes at the example of hydroxyl groups - ad hoc construction of localised modes and the influence of structural solute-solvent motifs.

The performance of the grid-based Numerov procedure for the prediction of fundamental as well as the first vibrational overtone has been systematically probed for harmonic and anharmonic model systems. In addition to monitor the prediction with respect to the spacing of the potential grid the influence of higher order approximations to the second derivative (i.e. stencils) in Schrödingers equation is evaluated. The latter enable a significant increase of the grid-spacing to achieve results of similar accuracy obtained with smaller stencil sizes. Application to the hydroxyl vibrational mode of methanol, phenol and the natural product thymol in vacuum and carbon tetrachloride predicted wavenumbers within less than 1% of experiment. Due to the highly localised character of the OH-vibration the ad hoc construction of the associated normal mode yields results of similar accuracy than those obtained using the analytical normal modes, effectively eliminating the requirement of an analytical normal mode evaluation of the entire system. This property was shown to be of particular advantage when considering explicit solute-solvent contacts, which have been demonstrated to be superior compared to an implicit representation of solvent effects. The combination of the observed findings (i.e. enlarged grid-spacing due to the application of higher order stencils, construction of localised normal modes) is envisaged to be of particular benefit when investigating localised modes in large systems, such as OH or NH groups in large (bio)macromolecules or solid-state surfaces.