Roberto Rivelino

Quantifying multiple-body interaction terms in H-bonded HCN chains with many-body perturbation/coupled-cluster theories

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...

Note on the free energy of transfer of fullerene C60 simulated by using classical potentials.

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.

Rayleigh light scattering of hydrogen bonded clusters investigated by means of ab initio calculations

Ab initio calculations of depolarization ratios and intensities of classically scattered light, in terms of dipole polarizabilities and polarizability anisotropies, are reported for different hydrogen bonded molecular clusters. Five different groups of organic heterodimers formed with water are considered: HCHO · · · H2O, CH3HO · · · H2O, HCOOH · · · H2O, CH3CN · · · H2O, and (CH3)2CO · · · H2O, together with the water dimer H2O · · · H2O. The geometries of all complexes have been optimized by means of the second-order Moller–Plesset many-body perturbation theory (MP2), using the augmented correlation-consistent basis set with polarized valence of double-zeta quality (aug-cc-pVDZ). The calculated average dipole polarizabilities of the isolated molecules are in good agreement with available experimental results. The calculations are then extended to the complexes and, from these, the Rayleigh scattering activities and depolarization ratio changes, upon hydrogen bond formation, are obtained and analysed. The differences in activity and depolarization for Rayleigh scattered radiation between two groups of isomers, (i) HCN · · · H2O and H2O · · · HCN and (ii) CH3HO · · · H2O and H3OH · · · OH2, have also been investigated.

An ab initio study of the hydrogen-bonded H2O:HCN and HCN:H2O isomers

Abstract Ab initio calculations are performed on the hydrogen bond interaction between HCN and water to analyze the structure, binding energy and change in vibrational frequencies of the HCN:H2O isomer. After geometry optimization, single-point calculations are made with many-body perturbation/coupled-cluster theories with different basis sets. At the highest level, CCSD(T), we find that the binding energy between HCN and water is 3.4 kcal/mol, after correcting for the basis-set-superposition error. Changes in intra-molecular vibrational frequencies are analyzed.

Conformational stability of furfural in aqueous solution: the role of hydrogen bonding

The importance of hydration on the equilibrium involving internal rotation around carbon-carbon single bond is investigated for the furfural molecule dissolved in water. To analyze the solvent effects in stabilizing any preferred conformation of furfural, we perform Monte Carlo (MC) NPT simulations corresponding to different rotation angles of the carbonyl group. The hydrogen bonds formed between solute and solvent are also analyzed along the conformational equilibrium. They are found to be equivalent, both in number and in binding energy, for all rotation angles. These results give a strong evidence that the conformational stability and the rotation barrier of the furfural molecule in water relate to the bulk properties rater than solute-solvent hydrogen bonds.

Origin of the Red Shift for the Lowest Singlet π → π* Charge-Transfer Absorption of p-Nitroaniline in Supercritical CO2

The origin of the unusual solvatochromic shift of p-nitroaniline (PNA) in supercritical carbon dioxide (SCCO2) is theoretically investigated on the basis of experimental data. Ab initio quantum chemistry calculations have been employed to unveil the interaction of CO2 with this archetypical molecule. It is demonstrated that the nitro group of PNA works as an electron-donating site binding to the electron-deficient carbon atom of CO2, most probably via a Lewis acid-base interaction. Moreover, a cooperative C-H···O hydrogen bond seems to act as an additional stabilizing source during the solvation process of PNA in SCCO2. To support the influence of solute-solvent specific interactions on the lowest singlet π → π* charge-transfer excitation, we perform a sequential Monte Carlo time-dependent density functional theory simulation to evaluate the excited states of PNA in SCCO2 (T = 315 K, ρ = 0.81 g/cm(3)). A critical assessment of this simulation, compared to calculations carried out within the polarized continuum model, gives strong evidence that our proposed complexes are important in describing the solvatochromic shift of PNA in SCCO2. The calculated red shift from the gas phase accounts for 66% to 80% (depending on the degree of complexation) of the experimental data. Finally, these results also alleviate possible failures commonly attributed to long-range corrected functionals in reproducing the solvatochromism of PNA.

A first principles approach to the electronic properties of liquid and supercritical CO2

The electronic absorption spectra of liquid and supercritical CO2 (scCO2) are investigated by coupling a many-body energy decomposition scheme to configurations generated by Born-Oppenheimer molecular dynamics. A Frenkel exciton Hamiltonian formalism was adopted and the excitation energies were calculated with time dependent density functional theory. A red-shift of ∼ 0.2 eV relative to the gas-phase monomer is observed for the first electronic absorption maximum in liquid and scCO2. The origin of this shift, which is not very dependent on deviations from the linearity of the CO2 molecule, is mainly related to polarization effects. However, the geometry changes of the CO2 monomer induced by thermal effects and intermolecular interactions in condensed phase lead to the appearance of an average monomeric electric dipole moment〈μ〉= 0.26 ± 0.04 D that is practically the same at liquid and supercritical conditions. The predicted average quadrupole moment for both liquid and scCO2 is〈Θ〉= - 5.5 D Å, which is increased by ∼ -0.9 D Å relative to its gas-phase value. The importance of investigating the electronic properties for a better understanding of the role played by CO2 in supercritical solvation is stressed.

A configuration interaction model to investigate many-electron systems in cavities

A configuration interaction (CI) model to treat confined many-electron systems is presented. Our model proposes a spatially confined linear combination of configuration interaction (LCCI) functions, built from basis functions that do not satisfy confinement boundary conditions. As an application we have calculated total energies for the He ground state, assuming that the atom is enclosed within a spherical cavity with infinite potential walls. Comparisons with other results in the literature are made in order to verify our model.

Theoretical studies of hydrogen bonding in water -cyanides and in the base pair Gu - Cy

Abstract Density-functional (DFT) and many-body-perturbation theories (MBPT/CC) are used to study the hydrogen bonding in the water–cyanide complexes H–Cue606N⋯H2O, H3C–Cue606N⋯H2O and (CH3)3C–Cue606N⋯H2O. Structures, binding energies and changes in vibrational frequencies are analyzed. The calculated Cue606N stretching frequency is found to shift to the blue upon complexation in H–Cue606N⋯H2O and H3C–Cue606N⋯H2O. To investigate electron correlation effects on the binding energies of these complexes, single-point calculations are performed at the MBPT/CC (MP2, MP3, MP4, CCSD and CCSD(T)) levels using the optimized MP2 geometries. Binding energies are also obtained at different levels of DFT (B3LYP and PW91) and compared with the MBPT/CC results. All calculations include corrections for basis set superposition error (BSSE) and zero-point vibrational energies. Additionally, the triple hydrogen-bonded guanine–cytosine (Gu–Cy) base pair is analyzed. The binding energy of the Watson–Crick model for Gu–Cy is calculated using the Hartree–Fock calculations and DFT (B3LYP and BP86) methods. The results for the hydrogen bonding distances and binding energies are in good agreement with experimental and recent theoretical values. The calculated dipole moment of the Gu–Cy complex is compared with the direct vector sum of the isolated bases. After taking into account the BSSE effects we find that the electron polarization due to the hydrogen binding leads to an increase of ∼20% of the calculated dipole moment of the complex.

Probing Lewis Acid-Base Interactions with Born-Oppenheimer Molecular Dynamics: The Electronic Absorption Spectrum of p-Nitroaniline in Supercritical CO2.

The structure and dynamics of p-nitroaniline (PNA) in supercritical CO2 (scCO2) at T = 315 K and ρ = 0.81 g cm(-3) are investigated by carrying out Born-Oppenheimer molecular dynamics, and the electronic absorption spectrum in scCO2 is determined by time dependent density functional theory. The structure of the PNA-scCO2 solution illustrates the role played by Lewis acid-base (LA-LB) interactions. In comparison with isolated PNA, the ν(N-O) symmetric and asymmetric stretching modes of PNA in scCO2 are red-shifted by -17 and -29 cm(-1), respectively. The maximum of the charge transfer (CT) absorption band of PNA in scSCO2 is at 3.9 eV, and the predicted red-shift of the π → π* electronic transition relative to the isolated gas-phase PNA molecule reproduces the experimental value of -0.35 eV. An analysis of the relationship between geometry distortions and excitation energies of PNA in scCO2 shows that the π → π* CT transition is very sensitive to changes of the N-O bond distance, strongly indicating a correlation between vibrational and electronic solvatochromism driven by LA-LB interactions. Despite the importance of LA-LB interactions to explain the solvation of PNA in scCO2, the red-shift of the CT band is mainly determined by electrostatic interactions.