Inmaculada García Cuesta

Understanding the ring current effects on magnetic shielding of hydrogen and carbon nuclei in naphthalene and anthracene

The local response to an external magnetic field normal to the molecular plane of naphthalene and anthracene was investigated via current density and magnetic shielding density maps. The Biot‐Savart law shows that the deshielding caused by π‐ring currents in naphthalene is stronger for α‐ than for β‐protons due to geometrical factors. The shielding tensor of the carbon nuclei in both molecules is strongly anisotropic and its out‐of‐plane component determines the up‐field chemical shift of 13C in nuclear magnetic resonance spectra. The π‐ring currents flowing beyond the C‐skeleton in front of a probe carbon nucleus, and on remote parts of the molecular perimeter, yield positive contributions to the out‐of‐plane component of carbon shielding as big as ≈10–15% of the total values. Near Hartree‐Fock estimates of magnetizability and magnetic shielding at the nuclei fully consistent with the current model are reported.

Multi-scale theoretical investigation of molecular hydrogen adsorption over graphene: coronene as a case study

The physisorption of molecular hydrogen onto coronene is studied using a multi-scale theoretical approach with Density Functional Theory (DFT) calculations and Molecular Dynamics (MD) simulations. We consider two different kinds of model conformation for the approach of hydrogen towards the coronene i.e., systematic and random. For the systematic attack of hydrogen over coronene, the resulting potential energy profiles from DFT analysis are further found to resemble the Morse potential, and even the highly flexible Murrell–Sorbie (M–S) potential. The resulting M–S fitting also shows a zero-point energy correction of ∼16–17%. On the other hand, the potential energies from the random approach have been implemented into the Improved Lennard-Jones (ILJ) force field of the DL_POLY package following a prior statistical treatment. The MD simulations have been performed at different temperatures from 10 to 390 K. For the interaction of seven hydrogen molecules with coronene, the DFT method shows an average interaction energy of −3.85 kJ mol−1 per H2, which is slightly smaller than the Coupled Cluster value (CCSD(T)) of −4.71 kJ mol−1 that was calculated for a single molecule in the most favorable situation. Moreover, the MD calculations reveal a mean interaction energy of −3.69 kJ mol−1 per H2 (a gross mean Ecfg of −25.98 kJ mol−1 at T = 299.97 K), which is again in good agreement with the aforementioned DFT results, proving the quality of the approach used for the study of van der Waals interactions between hydrogen and graphene.

Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. IV. Dihydrophospholophosphole isomers

The geometry of the heteropentalenes formed by two phosphole units has been determined at the DFT level. The magnetic susceptibility and the nuclear magnetic shielding at the nuclei of these systems have also been calculated using gauge‐including atomic orbitals and a large Gaussian basis set to achieve near Hartree–Fock estimates. A comparative study of the various isomers, of their flattened analogs, and of the parent phosphole molecule, shows that the [3,4‐c] isomer is the most aromatic system in the set considered, assuming diatropicity and degree of planarity as indicators, even if it is the less stable in terms of total molecular energy. Plots of magnetic field‐induced current densities confirm diatropicity of P‐containing bis‐heteropentalenenes, showing, however, significant differences from the analogous systems with distinct heteroatoms. The maps give evidence of spiral flow nearby CC bonds, compatible with prevalent distortive behavior of π electrons exalted by pyramidalization at P, and competing against the σ electron compression, which would favor planar structure.

A coupled cluster calculation of the spectrum of urea

Several coupled cluster methods have been used to compute the vertical excitation energies and oscillator strengths of the lowest singlet states of urea. Except for one excitation, the results are in good agreement with experiment, but previously non-detected transitions have been found.

Calculation of excitation energies from the CC2 linear response theory using Cholesky decomposition.

A new implementation of the approximate coupled cluster singles and doubles CC2 linear response model is reported. It employs a Cholesky decomposition of the two-electron integrals that significantly reduces the computational cost and the storage requirements of the method compared to standard implementations. Our algorithm also exploits a partitioning form of the CC2 equations which reduces the dimension of the problem and avoids the storage of doubles amplitudes. We present calculation of excitation energies of benzene using a hierarchy of basis sets and compare the results with conventional CC2 calculations. The reduction of the scaling is evaluated as well as the effect of the Cholesky decomposition parameter on the quality of the results. The new algorithm is used to perform an extrapolation to complete basis set investigation on the spectroscopically interesting benzylallene conformers. A set of calculations on medium-sized molecules is carried out to check the dependence of the accuracy of the results on the decomposition thresholds. Moreover, CC2 singlet excitation energies of the free base porphin are also presented.

MP2 Study of Physisorption of Molecular Hydrogen onto Defective Nanotubes: Cooperative Effect in Stone-Wales Defects.

We use large-scale MP2 calculations to investigate the physisorption of molecular hydrogen on (9,0) defective carbon nanotubes (CNTs) of C72H18. These large (supra)molecular systems are typically studied using conventional DFT methods, which do not describe well the van der Waals interactions responsible for this process. Here we use CCSD(T)-calibrated MP2 calculations to estimate binding energies by considering four defective structures (hydrogenated divacancy, octagon-pentagon, and two Stone-Wales defects). The largest physisorption energies for the nondefective CNT are for configurations in which H2 points toward the center of one ring. The computed interaction energies for defect-free CNT are in the range 5.7 to 5.9 kJ/mol, in good agreement with the experimental value of 5.98 kJ/mol. The defects introduced in the (9,0)-CNT increase the surface area of the nanotube, such that the largest surface in found in the 55-77 Stone-Wales defective CNT that furthermore is the most aromatic. Only that defect enlarges the physisorption binding energy, which can become >25% larger. Moreover, a cooperative effect in the adsorption of H2 not appearing in the regular structure is found.

How nitrogen modifies the nuclear magnetic shielding in tetraazanaphthalenes

Although for planar conjugated hydrocarbons the out-of-plane component of proton magnetic shielding is an unquestionable quantitative aromaticity indicator, the same is not true for tetraazanaphthalenes. As in these compounds the (core + σ)-currents associated to the nitrogen nuclei diminish the perpendicular component of shielding, abnormal values of 1H NMR σzz are obtained. Therefore, a consistent aromaticity measure must be based only on the π-contribution to the out-of-plane component of proton magnetic shielding. Otherwise, the behavior of these compounds in presence of an external magnetic field parallels that of naphthalene, with the nitrogen nuclei contributing to the ring current in a comparable amount to carbon nuclei. The π-current contribution to magnetic shielding represents 6–8% of the out-of-plane shielding for nitrogen and 9–12% for carbon.

Modeling the Interaction of Carbon Monoxide with Flexible Graphene: From Coupled Cluster Calculations to Molecular-Dynamics Simulations

The interaction of CO with graphene was studied at different theoretical levels. Quantum-mechanical calculations on finite graphene models with the use of coronene for coupled cluster calculations and circumcoronene for B97D calculations showed that there was no preferential site for adsorption and that the most important factor was the orientation of CO relative to graphene. The parallel orientation was preferred, with binding energies around 9 kJ mol-1 at the CCSD(T) and B97D levels, which was in good agreement with experimental findings. From a large number of CO-circumcoronene and CO-CO interactions, computed at different distances and randomly generated orientations, parameters were fit to the improved Lennard-Jones potential. Such potentials, together with others describing the intramolecular dynamics of graphene, were subsequently employed in classical molecular-dynamics simulations of the adsorption of CO on graphene by using the canonical ensemble. The obtained results showed that the introduction of flexibility in graphene, which simulated the effects associated to curvature of the surface, diminished the adsorption level and that, as expected, adsorption also diminished with temperature.

Modelization of the \(\hbox {H}_{2}\) adsorption on graphene and molecular dynamics simulation

In the search for efficient molecular dynamics simulation models both simplicity and acceptable accuracy matter. In the present study, a model of the graphene-H2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}