We present a description of the optical properties of a water-soluble BODIPY derivative in the gas phase and in water. Comparison with a hydrophobic BODIPY derivative with very similar structure but deprived of the hydrophilic groups, clarifies the effects of functionalization. The changes in solution are studied at different levels of modeling. In particular, we make use of Car–Parrinello molecular dynamics to take thermal motion into account, by generating a set of molecular configurations at about room temperature and embedding them in the polarizable continuum model (PCM) for the solvent. The theoretical scheme is that of time-dependent density functional theory which we apply using different approximations for the exchange-correlation functionals. Changes in the low-lying excitation spectrum (≈2.5–4.5 eV) of the solvated molecule in water relative to gas phase are shown to be due not only to the electrostatic interaction with the solvent but also to the thermal motion that induces a downward energy shift and broadening of the absorption lines. Addition of explicit water molecules to the PCM only affects spectral features at higher energies.

We present Car–Parrinello molecular dynamics simulations—combined with metadynamics—of the reactions accompanying the capture of \(\hbox {CO}_2\) in an aqueous amine solution. The selected amine is 2-amino-2-methyl-1,3-propanediol, which has been investigated experimentally in the search for optimum absorbents. In analogy with the empirical search, we use as reference aqueous monoethanolamine (MEA) at 30 wt%, namely the chemical absorbent most frequently used for the removal of \(\hbox {CO}_2\) from combustion gases. In particular, we refer to our own results which have led to a detailed characterization of several reaction paths for the absorption of \(\hbox {CO}_2\) and the amine regeneration (Ma et al. in J Chem Theory Comput 11:3189, 2015). Our simulations refer to the zwitterion mechanism leading to the formation of a carbamate or carbamic acid and to the release of the molecule in solution. The scenario established for the reactions in MEA is confirmed, and in particular the role of the zwitterion mechanism also for \(\hbox {CO}_2\) release. We find that the difference in the structure of the two molecules does not influence the dynamics of either the formation or the dissociation of the zwitterion. However, it affects its interaction with water in a significant way, thus reducing the probability of carbamate formation and its stability in solution.

Atom vacancies are intrinsic defects of carbon nanotubes. Using a zigzag nanotube as reference, this paper focuses on the comparison of calculations performed within density functional theory and a number of classical force fields widely used for carbon systems. The results refer to single and double vacancies and, in particular, to the induced structural changes, the formation energies, and the energy barriers relative to elementary processes such as reconstruction, migration, and coalescence. Characterization of these processes is remarkably different in the different approaches. These findings are meant to contribute to the construction of DFT-based classical schemes for carbon nanostructures.

Aqueous monoethanolamine (MEA) solution is commonly used for post-combustion carbon capture via chemical absorption. Extensive research has been carried out to characterize both uptake and release of carbon dioxide (CO2), with the aim of improving process performance. However, an intensive research is still needed on fundamental aspects of the key chemical reactions, to achieve a comprehensive understanding of the cyclic process at the microscopic level and a quantitative assessment. We present several ab initio simulations of MEA solutions at a concentration of 30 wt %-the current standard in the industry-and study the dynamics of key multistep chemical reactions, using the metadynamics technique. Pathways for the entire cycle are investigated and characterized in terms of related free-energy and enthalpy barriers, and of the accompanying variations in both structural and electronic properties. The results of this study lead us to propose, among competing processes, an unforeseen scenario in which the zwitterion acts as sn intermediate not only of CO2 uptake, in the form of carbamate, but also of its release. Rate-limiting steps are the formation of the zwitterion for the former and MEAH(+) deprotonation for the latter. Water is shown to play a multifaceted role, which is crucial in determining the development and the energetics of each step of the reactions. The level of comprehension here achieved for MEA should help defining a strategy for solvent optimization.

We introduce a new ab initio derived reactive potential for the simulation of CdTe within density functional theory (DFT) and apply it to calculate both static and dynamical properties of a number of systems (bulk solid, defective structures, liquid, surfaces) at finite temperature. In particular, we also consider cases with low sulfur concentration (CdTe:S). The analysis of DFT and classical molecular dynamics (MD) simulations performed with the same protocol leads to stringent performance tests and to a detailed comparison of the two schemes. Metadynamics techniques are used to empower both Car-Parrinello and classical molecular dynamics for the simulation of activated processes. For the latter, we consider surface reconstruction and sulfur diffusion in the bulk. The same procedures are applied using previously proposed force fields for CdTe and CdTeS materials, thus allowing for a detailed comparison of the various schemes.

Chemical absorption in amine aqueous solutions is a widespread technology for postcombustion carbon capture, and a large effort is ongoing to improve their performance. Characterization of the "reactant" and "product" solutions at the microscopic level is highly desirable for process optimization. Recently X-ray scattering experiments and "in situ" infrared spectroscopy have been applied to this aim, but a complete and convincing interpretation is missing. We present large-scale ab initio molecular dynamics simulations of monoethanolamine solutions at experimental concentration and temperature and analyze how structural and vibrational properties change after carbamate formation. An exhaustive account of the experimental data is obtained. Fingerprints of the reaction products and specific interactions are unravelled. Hydration effects are specific to each component of the solution and are essential for a correct assignment of the experimental data.

The study of oxygen chemisorption on single-walled carbon nanotubes generally relies on simple atomistic models and hence hampers the possibility to understand whether nanotube size or adduct concentration have a role in determining the surface-adsorbate interaction. Our large-scale DFT-based simulations show that structural and electronic properties as well as diffusion barriers strongly depend on both nanotube diameter and adsorbate concentration. Our atomistic models cover nanotube of different chirality with diameters from 0.6 to 1.5 nm and oxygen concentration from 0.1 to 1%. In particular, the tendency to cluster increases with concentration and stabilizes ether (ET) groups but affects hopping barriers only to a minor extent. Significant differences with graphene are found, also for 1.5 nm diameter nanotubes. Extension to species isoelectronic to oxygen reveals dissimilarities, and especially for sulfur that tends to form epoxides (EP), to diffuse more easily and to rapidly close the energy gap for increasing concentration. The relative ET-EP stability can be described in terms of the bare-bond curvature, a concentration-dependent chemical descriptor here introduced. Comparison of these DFT calculations-using different exchange-correlation functionals-and our additional investigation with a reactive force-field (ReaxFF) clarifies several similarities but also discrepancies between the predictions of the two schemes.

The top-down formation of a fullerene from a graphene flake is investigated via extensive ab initio molecular dynamics simulations in the range 300-3000 K, accelerated by metadynamics. Topological (SPRINT) coordinates are used to ensure a prejudice-free exploration of the free-energy surface and path collective variables to provide reliable free-energy barriers. The low-barrier zipping of the 2D nanoflake into a 3D nanocone is revealed as the early key transformation, mediated by a four-membered ring. Multiple-step pathways lead it toward different but always fully tricoordinated 0D closed cages. This scenario comprises several key chemical reactions characteristic of carbon at the nanoscale, as known from diverse experiments.

Density-functional-theory based calculations of two single-walled carbon nanotubes of different chirality settle open issues on the sidewall chemisorption of atomic oxygen at low concentrations. Ether groups are the thermodynamically favored configurations. If kinetically trapped in epoxide groups, oxygen introduces characteristic new levels in the gap of the nanotube that are detected with scanning tunneling spectroscopy experiments. Discrepancies with previous predictions are shown to originate from the inadequacy of previous models to describe low-concentration oxygen adsorbated on nanotubes.

Density functional theory-based calculations of the (10,0) zigzag single-walled carbon nanotube with hydrogen chemisorbed exohedrally show that electron pairing and strain minimization lead hydrogen atoms to cluster and preferentially sit in axial configurations. This tendency to confine in highly ordered configurations contrasts with the results we obtain when we employ the widely used force field AIREBO that predicts a preference for a sparse hydrogen distribution. The nature of the frontier orbitals is significantly dependent on the specific configuration of the adsorbate, being either unperturbed delocalized states of the bare nanotube or localized “impurity” states. The infrared absorption spectrum calculated for a model with hydrogen bound both on the surface and at the edges of the nanotube allows an unambiguous assignment of the characteristic features observed for hydrogenated single-walled carbon nanotubes.