Two-dimensional (2D) binary XBi compounds, where X belongs to group III elements (B, Al, Ga, and In), in a buckled honeycomb structure may originate sizable gap Z2 topological insulators (TIs). These are characterized by exhibiting single band inversion at the Γ point as well as nontrivial edge states in their corresponding nanoribbons. By using first-principles calculations, we demonstrate that hydrogenation of XBi single layers leads to distinct and stable crystal structures, which can preserve their topological insulating properties. Moreover, hydrogenation opens a band gap in this new class of 2D Z2 TIs, with distinct intensities, exhibiting an interesting electronic behavior for viable room-temperature applications of these 2D materials. The nature of the global band gap (direct or indirect) and topological insulating properties depend on the X element type and spatial configuration of the sheet, as well as the applied strain. Our results indicate that the geometric configuration can be crucial for preserving totally the topological characteristics of the hydrogenated sheets. We identify sizable band inversions in the band structure for the relaxed hydrogenated GaBi and InBi in their chairlike configurations and for hydrogenated BBi and AlBi under strain. Based on these findings, hydrogenation gives rise to a flexible chemical tunability and can preserve the band topology of the pristine XBi phases

We have investigated the structure, adsorption, electronic states, and charge transfer of small water aggregates on the surface of a graphene layer using density functional theory. Our calculations were focused on water adsorbates containing up to five water molecules interacting with one and both sides of a perfect freestanding sheet. Different orientations of the aggregates with respect to the graphene sites were considered. The results show that the adsorption energy of one water molecule is primarily determined by its orientation, although it is also strongly dependent on the implemented functional scheme. Despite its intrinsic difficulties with dispersion interactions, the Perdew and Wang's exchange-correlation functional may be a viable alternative to investigate the adsorption of large molecular aggregates on a graphene surface. Although water physisorption is expected to occur in the regime of droplets, we found no induced impurity states close to the Fermi level of graphene interacting with small water clusters. In order to investigate the donor/acceptor tendency of the water clusters on graphene, we have performed a Bader charge analysis. Considering the charge transfer mechanism, we have noticed that it should preferentially occur from water to graphene only when the oxygen atom is pointing toward the surface. Otherwise, and in the case of larger adsorbed clusters, charge transfers systematically occur from graphene to water.

A combined Monte Carlo simulation with semiempirical quantum mechanics calculations has been performed to investigate the structure of hydrated fullerene (C 60 HyFn) and the influence of hydration on its UV-vis spectra. The statistical information of the C 60 fullerene aqueous solution (C 60 FAS) is obtained from NPT ensemble including one C 60 fullerene immerses in 898 water molecules. To obtain an efficient ensemble average, the auto-correlation function of the energy has been calculated. The analyzed center-of-mass pair-wise radial distribution function indicates that, on average, there are 65 and 151 water molecules around the first and second hydration shells, respectively, of a single C 60 molecule. To calculate the average UV-vis transition energies of C 60 HyFn, only the statistically uncorrelated configurations are used in the quantum mechanical calculations (INDO/CIS). These involve hundreds of supramolecular structures containing one C 60 fullerene surrounded by the first hydration shell. The calculated average transitions at 268 and 350 nm are in very good agreement with the experimental prediction.

Many-body perturbation/coupled-cluster calculations have been carried out to investigate the multiple-body energy terms and their contribution to the interaction energy of linear (HCN)N chains. All minimum energy geometries of the clusters (N=2–7) are obtained at the second-order many-body perturbation (MP2) levels of theory. Electron correlation and cooperative effects in the C–H⋯N hydrogen bonds are also quantitatively characterized during the aggregation process. It is found that the two- and three-body terms account for nearly all of the total interaction energy, but all high-body terms increase with the size of the cluster. Detailed numerical values are given for all the many-body contributions of the (HCN)N chains. Electron correlation effects are found to be important for the two- and three-body terms but have decreased importance for the higher-body terms. Cooperative effects are also investigated for the binding energy and dipole moment. The dipole moments of the HCN oligomers are larger than the sum of the individual monomers with differences ranging between 12% (N=2) and 28% (N=7). The limiting values for the binding energy and dipole moment of (HCN)N, per monomer, corresponding to very large N values, are estimated to be 22.9 kJ/mol and 3.87 D, per monomer, respectively. These results correspond to cooperative contributions of 5.8 kJ/mol to the energy, and 1.0 D to the dipole moment.

We investigate the impact of hydroxyl groups on the properties of C(60)(OH)(n) systems, with n = 1, 2, 3, 4, 8, 10, 16, 18, 24, 32 and 36 by means of first-principles density functional theory calculations. A detailed analysis from the local density of states has shown that adsorbed OH groups can induce dangling bonds in specific carbon atoms around the adsorption site. This increases the tendency to form polyhydroxylated fullerenes (fullerenols). The structural stability is analyzed in terms of the calculated formation enthalpy of each species. Also, a careful examination of the electron density of states for different fullerenols shows the possibility of synthesizing single molecules with tunable optical properties.

Ab initio quantum chemistry calculations reveal that HCN and mainly FCN can form Lewis acid-base complexes with formaldehyde associated with cooperative H bonds, as first noticed by Wallen et al. (Blatchford, M. A.; Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 14818-14819) for CO2-philic materials under supercritical conditions. The present results, obtained with MP2(Full)/aug-cc-pVDZ calculations, show that the degeneracy of the nu(2) mode in free HCN or FCN is removed upon complexation in the same fashion as that of CO2. The splitting of these bands along with the electron structure analysis provides substantial evidence of the interaction of electron lone pairs of the carbonyl oxygen with the electron-deficient carbon atom of the cyanides. Also, this work investigates the role of H bonds acting as additional stabilizing interactions in the complexes by performing the energetic and geometric characterization.

Diverse atomistic parameters of C60 have been developed and utilized to simulate fullerene solutions in biological environments. However, no thermodynamic assessment and validation of these parameters have been so far realized. Here, we employ extensive molecular dynamics simulations with the thermodynamic integration method in the isothermal-isobaric ensemble to investigate the transfer of a single fullerene C60 between different solvent environments using different potential models. A detailed analysis is performed on the structure and standard Gibbs free energy of transfer of C60 from benzene to ethanol. All of the interactions concerned in the transfer process are included via atomistic models. We notice that having only structural and dynamical properties is not decisive to validate reliable atomic parameters capable of describing a more realistic thermodynamic process. Thus, we employ the calculated free energy of transfer to validate more accurate atomic parameters for the solvation thermodynamics of fullerenes by direct comparison with the solubility experimental data.

Mixed hydrogen-bonded clusters H 2 O‚‚‚HCN, HCN‚‚‚H 2 O, H 2 O‚‚‚HCN‚‚‚H 2 O, and H 2 O‚‚‚HCN‚‚‚HCN‚‚‚ H 2 O are studied by using ab initio calculations. The optimized structures and harmonic vibrational frequencies are obtained at the DFT/B3LYP and MBPT/MP2 levels with the 6-311++G(d,p) basis set. To investigate electron correlation effects on the binding energies, single-point calculations are also performed using the CCSD(T) method with the optimized MP2 geometries. The complexation energies are obtained for these systems including correction for basis set superposition error. In addition, the cooperative effects in the properties of the complexes are investigated quantitatively. We found a cooperativity contribution of around 10% relative to the total interaction energy of the complex H 2 O‚‚‚HCN‚‚‚H 2 O. In the case of H 2 O‚‚‚HCN‚‚‚ HCN‚‚‚H 2 O, the binding energy of the HCN‚‚‚HCN is ca. 8 kJ/mol stronger in the mixed tetramer than in the corresponding isolated dimer. The effects of higher-order electron correlation are found to be mild, with MP2 giving a well-balanced result. Cooperative effects are predicted either by MP2 or by B3LYP in hydrogen bond distances and dipole moments of the clusters. In contrast, the B3LYP functional fails to account for the out-of-plane bend angle in H 2 O‚‚‚HCN, which is well-described by the MP2 method.