The multiradical character of one- and two-dimensional graphene nanoribbons.

When is an acene stable? The pronounced multiradical character of graphene nanoribbons of different size and shape was investigated with high-level multireference methods. Quantitative information based on the number of effectively unpaired electrons leads to specific estimates of the chemical stability of graphene nanostructures.

The stability of the acetic acid dimer in microhydrated environments and in aqueous solution.

The thermodynamic stability of the acetic acid dimer conformers in microhydrated environments and in aqueous solution was studied by means of molecular dynamics simulations using the density functional based tight binding (DFTB) method. To confirm the reliability of this method for the system studied, density functional theory (DFT) and second order Møller-Plesset perturbation theory (MP2) calculations were performed for comparison. Classical optimized potentials for liquid simulations (OPLS) force field dynamics was used as well. One focus of this work was laid on the study of the capabilities of water molecules to break the hydrogen bonds of the acetic acid dimer. The barrier for insertion of one water molecule into the most stable cyclic dimer is found to lie between 3.25 and 4.8 kcal mol(-1) for the quantum mechanical methods, but only at 1.2 kcal mol(-1) for OPLS. Starting from different acetic acid dimer structures optimized in gas phase, DFTB dynamics simulations give a different picture of the stability in the microhydrated environment (4 to 12 water molecules) as compared to aqueous solution. In the former case all conformers are converted to the hydrated cyclic dimer, which remains stable over the entire simulation time of 1 ns. These results demonstrate that the considered microhydrated environment is not sufficient to dissociate the acetic acid dimer. In aqueous solution, however, the DFTB dynamics shows dissociation of all dimer structures (or processes leading thereto) starting after about 50 ps, demonstrating the capability of the water environment to break up the relatively strong hydrogen bridges. The OPLS dynamics in the aqueous environment shows--in contrast to the DFTB results--immediate dissociation, but a similar long-term behavior.

Molecular dynamics simulations of water molecule-bridges in polar domains of humic acids.

The stabilizing effect of water molecule bridges on polar regions in humic substances (HSs) has been investigated by means of molecular dynamics (MD) simulations. The purpose of these investigations was to show the effect of water molecular bridges (WAMB) for cross-linking distant locations of hydrophilic groups. For this purpose, a tetramer of undecanoid fatty acids connected to a network of water molecules has been constructed, which serve as a model for spatially fixed aliphatic chains in HSs terminated by a polar (carboxyl) group. The effect of environmental polarity has been investigated by using solvents of low and medium polarity in force-field MD. A nonpolar environment simulated by n-hexane was chosen to mimic the stability of WAMB in a hydrophilic hotspot surrounded by a nonpolar environment, while the more polar acetonitrile environment was chosen to simulate a more even distribution of polarity around the carboxylic groups and the water molecules. The dynamics simulations show that the rigidity of the oligomer chains is significantly enhanced as soon as the water cluster is large enough to comprise all four carboxyl groups. Increasing the temperature leads to evaporization processes which destabilize the rigidity of the tetramer-water cluster. Embedding it into the nonpolar environment introduces a pronounced cage effect which significantly impedes removal of water molecules from the cluster region. On the other hand, a polar environment facilitates their diffusion from the polar region. One important consequence of these simulations is that although the local water network is the stabilizing factor for the organic matter matrix, the degree of stabilization is additionally affected by the presence of nonpolar surroundings.

Thermodynamic stability of hydrogen-bonded systems in polar and nonpolar environments.

The thermodynamic properties of a selected set of benchmark hydrogen-bonded systems (acetic acid dimer and the complexes of acetic acid with acetamide and methanol) was studied with the goal of obtaining detailed information on solvent effects on the hydrogen-bonded interactions using water, chloroform, and n-heptane as representatives for a wide range in the dielectric constant. Solvent effects were investigated using both explicit and implicit solvation models. For the explicit description of the solvent, molecular dynamics and Monte Carlo simulations in the isothermal-isobaric (NpT) ensemble combined with the free energy perturbation technique were performed to determine solvation free energies. Within the implicit solvation approach, the polarizable continuum model and the conductor-like screening model were applied. Combination of gas phase results with the results obtained from the different solvation models through an appropriate thermodynamic cycle allows estimation of complexation free energies, enthalpies, and the respective entropic contributions in solution. Owing to the strong solvation effects of water the cyclic acetic acid dimer is not stable in aqueous solution. In less polar solvents the double hydrogen bond structure of the acetic acid dimer remains stable. This finding is in agreement with previous theoretical and experimental results. A similar trend as for the acetic acid dimer is also observed for the acetamide complex. The methanol complex was found to be thermodynamically unstable in gas phase as well as in any of the three solvents.

Cation-π interactions in competition with cation microhydration: a theoretical study of alkali metal cation-pyrene complexes.

Cation-π interactions were systematically investigated for the adsorption of H+ and alkali metal cations M+ to pyrene by means of Møller-Plesset perturbation theory (MP2) and density functional theory (DFT). The main aims were to determine the preferred adsorption sites and how the microhydration shell influences the adsorption process. The preferred adsorption sites were characterized in terms of structural parameters and energetic stability. Stability analysis of the M+-pyrene complexes revealed that the binding strength and the barrier to transitions between neighboring sites generally decreased with increasing cation size from Li+ to Cs+. Such transitions were practically barrierless (<<1 kcal/mol) for the large Rb+ and Cs+ ions. Further, the influence of the first hydration shell on the adsorption behavior was investigated for Li+ and K+ as representatives of small and large (alkali metal) cations, respectively. While the isolated complexes possessed only one minimum, two minima-corresponding to an inner and an outer complex-were observed for microhydrated complexes. The small Li+ ion formed a stable hydration shell and preferentially interacted with water rather than pyrene. In contrast, K+ favored cation-π over cation-water interactions. It was found that the mechanism for complex formation depends on the balance between cation-π interactions, cation-water complexation, and the hydrogen bonding of water to the π-system.

Methyl and pentyl chloride in a microhydrated environment and at the liquid water-vapor interface: a theoretical study.

The solvation properties of methyl and pentyl chloride were studied in a microhydrated environment with up to 10 explicit water molecules and at the liquid water-vapor interface. Geometry optimizations were performed in the former case using the density functional based tight binding (DFTB), DFTB-D, and Møller-Plesset perturbation theory (MP2) levels of theory. The microhydrated alkyl chloride complexes were characterized in terms of hydrogen bonding and energetic stability. The DFTB and DFTB-D results were verified by comparison with those obtained by MP2. Good agreement between the MP2 and DFTB-D results is found. Complexes where the alkyl chloride molecule is attached to an edge of the water cluster are found to be most stable. Pronounced stability is also observed for cubic arrangements of the alkyl chloride-water complexes. Molecular dynamics simulations based on the DFTB and DFTB-D methods were used to study the adsorption process of the alkyl chloride molecules to a water surface. The dynamics simulations show that the methyl chloride molecule is located at the water surface preferentially with the methyl group oriented toward the water surface, while for pentyl chloride, owing to the longer nonpolar hydrocarbon chain, a parallel alignment at the water surface is found with the hydrocarbon chain pointing slightly to the gas phase. Despite some quantitative differences, the present work provides confirmation of the somewhat surprising preferential orientation of the methyl chloride molecule at the water-vapor interface predicted in a recent study using molecular dynamics simulations based on an empirical force field (Harper et al., J. Phys. Chem. A 2009, 113, 2015-2024). The observed difference in preferred alignment at the aqueous surface between the methyl chloride and the longer-chain alkyl chloride is likely to have consequences for the chemistry of alkyl halides adsorbed on the surface of aqueous and ice particles in the atmosphere.