Fernando Ruipérez

A natural orbital functional for multiconfigurational states

An explicit formulation of the Piris cumulant λΔ,Π matrix is described herein, and used to reconstruct the two-particle reduced density matrix (2-RDM). Then, we have derived a natural orbital functional, the Piris Natural Orbital Functional 5, PNOF5, constrained to fulfill the D, Q, and G positivity necessary conditions of the N-representable 2-RDM. This functional yields a remarkable accurate description of systems bearing substantial (near)degeneracy of one-particle states. The theory is applied to the homolitic dissociation of selected diatomic molecules and to the rotation barrier of ethylene, both paradigmatic cases of near-degeneracy effects. It is found that the method describes correctly the dissociation limit yielding an integer number of electrons on the dissociated atoms. PNOF5 predicts a barrier of 65.6 kcal/mol for the ethylene torsion in an outstanding agreement with Complete Active Space Second-order Perturbation Theory (CASPT2). The obtained occupation numbers and pseudo one-particle energies at the ethylene transition state account for fully degenerate π orbitals. The calculated equilibrium distances, dipole moments, and binding energies of the considered molecules are presented. The values obtained are accurate comparing those obtained by the complete active space self-consistent field method and the experimental data.

Pro-oxidant activity of aluminum: Promoting the Fenton reaction by reducing Fe(III) to Fe(II)

The possibility for an Al-superoxide complex to reduce Fe(III) to Fe(II), promoting oxidative damage through the Fenton reaction, is investigated using highly accurate ab initio methods and density functional theory in conjunction with solvation continuum methods to simulate bulk solvent effects. It is found that the redox reaction between Al-superoxide and Fe(III) to produce Fe(II) is exothermic. Moreover, the loss of an electron from the superoxide radical ion in the Al-superoxide complex leads to a spontaneous dissociation of molecular oxygen from aluminum, recovering therefore an Al(3+) hexahydrated complex. As demonstrated in previous studies, this complex is again prone to stabilize another superoxide molecule, suggesting a catalytic cycle that augments the concentration of Fe(II) in the presence of Al(III). Similar results are found for Al(OH)(2+) and Al(OH)(2)(+) hydrolytic species. Our work reinforces the idea that the presence of aluminum in biological systems could lead to an important pro-oxidant activity through a superoxide formation mechanism.

Complete vs restricted active space perturbation theory calculation of the Cr2 potential energy surface

In this paper, we calculate the potential energy surface (PES) and the spectroscopic constants of the chromium dimer using the recently developed restricted active space second-order perturbation (RASPT2) method. This approach is benchmarked against available experimental measurements and the complete active space second-order perturbation theory (CASPT2), which is nowadays established as one of the most accurate theoretical models available. Dissociation energies, vibrational frequencies, and bond distances are computed at the RASPT2 level using several reference spaces. The major advantage of the RASPT2 method is that with a limited number of configuration state functions, it can reproduce well the equilibrium bond length and the vibrational frequency of the Cr dimer. On the other hand, the PES is well described only at short distances, while at large distances, it compares very poorly with the CASPT2. The dissociation energy is also ill-behaved, but its value can be largely improved using a simple workaround that we explain in the text. In the paper, we also address the effect of the Ionization Potential Electron Affinity (IPEA) shift (a parameter introduced in the zeroth-order Hamiltonian in the CASPT2 method to include the effect of two-electron terms) and show how its default value of 0.25 is not suitable for a proper description of the PES and of the spectroscopic parameters and must be changed to a more sound value of 0.45.

Pro-oxidant Activity of Aluminum: Stabilization of the Aluminum Superoxide Radical Ion

The pro-oxidant activity of aluminum, a nonredox metal, through superoxide formation is studied by theoretical methods, determining the ESR g-tensor values of O2(•–) with a variety of metals and the reaction energies for Al3+ superoxide affinity in solution. First, the intrinsic ability of aluminum to induce a splitting of the πg levels is compared to that of other significant biological metals, such as Na+, K+, Mg2+, and Ca2+. Additional properties such as bond lengths, ionization potentials, and electron affinities are also determined, and the coherency with the trends observed from ESR g-tensor values is analyzed. As it corresponds to the high charge and its small size, there is a strong interaction between Al3+ and the superoxide. We predict that this strong inherent interaction remains when aluminum is microsolvated. Finally, we analyze the possibility of Al3+ superoxide formation in solution, leading to the conclusion that substitution of the first coordination shell water molecules is plausible, but not of hydroxides. This points to the possibility of Al3+ superoxide formation in solution, which would be pH-dependent. Taking into account the earlier established linear relationship between metal–superoxide interactions and promoting effects in electron-transfer reactions, our work reinforces the idea that the presence of aluminum in biological systems could lead to an important pro-oxidant activity through a superoxide formation mechanism.

Communication: Chemical bonding in carbon dimer isovalent series from the natural orbital functional theory perspective

The natural orbital functional theory admits two unique representations in the orbital space. On the one hand, we have the natural orbitals themselves that minimize the energy functional, and which afford for a diagonal one-particle reduced density matrix but not for a diagonal Lagrangian orbital energy multipliers matrix. On the other hand, since it is possible to reverse the situation but only once the energy minimization has been achieved, we have the so-called canonical representation, where the Lagrangian orbital energy multipliers matrix is diagonal but the one-particle reduced density matrix is not. Here it is shown that the former representation, the natural orbital representation, accounts nicely for the quadrupole bond character of the ground states of C2, BN, CB(-), and CN(+), and for the double bond order character of the isovalent (1)Σg (+) state of Si2. Similarly, the canonical orbital representation accounts correctly for the ionization spectra of all these species.

Room temperature synthesis of non-isocyanate polyurethanes (NIPUs) using highly reactive N-substituted 8-membered cyclic carbonates

There is a growing interest to develop green synthetic pathways towards industrially relevant polymers such as polyurethanes without the use of toxic and dangerous isocyanate monomers. The most promising route towards non-isocyanate polyurethanes (NIPUs) is the aminolysis of dicyclic carbonates derived from renewable resources. Although, cyclic carbonates of 5- and 6-members have been successfully proposed, aminolysis of these compounds requires the use of high temperatures to obtain high conversions and subsequently high molecular weight NIPUs. Indeed, these cyclic carbonates do not allow the achievement of high molecular weight NIPUs using low reactive diamines analogous to two of the most industrially relevant aliphatic diisocyanates. Herein, we report a (bis) N-substituted 8-membered cyclic carbonate that could be prepared from naturally abundant epoxides, diamines and dimethyl carbonate using sustainable chemical routes. This N-substituted 8 membered cyclic carbonate appeared to be much more reactive than the smaller 5- and 6-membered cyclic carbonates. Due to this increased reactivity, we obtained high molecular weight NIPUs using a variety of diamines, including industrially relevant hindered aliphatic diamines, such as 5-amino-1,3,3-trimethylcyclohexanemethylamine (IPDA) and 4,4′-methylenebis(cyclohexylamine). The synthesis of NIPUs was demonstrated at room temperature without the need for any additional catalyst. Altogether, this paper shows that (bis) N-substituted 8-membered cyclic carbonates are ideal starting materials for the synthesis of sustainable non-isocyanate polyurethanes (NIPUs).

Performance of PNOF5 Natural Orbital Functional for Radical Formation Reactions: Hydrogen Atom Abstraction and C-C and O-O Homolytic Bond Cleavage in Selected Molecules.

Radical formation through hydrogen abstraction and C-C and O-O homolytic bond cleavage from selected molecules is investigated by use of natural orbital functional theory in its PNOF5 natural orbital functional implementation, and the results are compared to high-level ab initio complete active space self-consistent field (CASSCF) and complete active space with second-order perturbation theory (CASPT2) methods and experimental data. It is observed that PNOF5 is able to treat the strong electron correlation effects along the homolysis of X-H (X = C, N, O) and X-X (X = C, O) bonds, leading, in general, to the correct trends in the corresponding bond strengths and a good description of the resultant electronic structure for these radicals. In general, PNOF5 bond energies are lower than the experimental ones, because of partial lack of dynamical electron correlation. However, the part of dynamical electron correlation recovered by PNOF5 allows it to give more accurate results than CASSCF methods with a minimum window required to treat near-degeneracy effects. In addition, inspection of the natural orbital occupancies with respect to the CASSCF ones shows an outstanding performance of PNOF5 in treating degenerate and quasidegenerate states, giving a correct description of diradicals and diradicaloids formed upon C-C cleavage in cyclopropane and derivatives.

Transient mechanochromism in epoxy vitrimer composites containing aromatic disulfide crosslinks

4-Aminophenyl disulfide (4-AFD) has been recently reported as a hardener for the design of reprocessable, repairable and recyclable epoxy networks. Now we show that such a hardener is also responsible for a mechanochromic effect, due to an intermediate radical species formed upon damage, as confirmed by theoretical studies.

Quantum chemical study of 4f-->5d excitations of trivalent lanthanide ions doped in the cubic elpasolite Cs2NaYCl6. Ce3+ to Tb3+.

Wave-function-based ab initio calculations on the lowest states of the 4f(n),4f(n-1)5d(t2g)1, and 4f(n-1)5d(e(g))1 configurations of (LnCl6)3- clusters (Ln=Ce to Tb) embedded in the cubic elpasolite Cs2NaYCl6 have been performed, in an attempt to contribute to a comprehensive understanding of the 4f-->5d excitations of lanthanide ions in crystals. Reliable data are provided on the changes of bond lengths and breathing mode vibrational frequencies upon 4f-->5d(t2g) and 4f-->5d(e(g)) excitations, as well as on minimum-to-minimum and vertical absorption and emission transitions, and on the Stokes shifts. The available experimental data are discussed and predictions are made. The stabilization of the 4f-->5d(baricenter) excitation of the doped ions with respect to the 4f-->5d excitations of the free ions, which is a key variable for the understanding of these excitations in solid hosts, is analyzed and found to be due, in two-thirds, to dynamic ligand correlation effects and, in one-third, to orbital relaxation, charge transfer, and covalency effects present in a mean-field approximation.

Diradicals and Diradicaloids in Natural Orbital Functional Theory

Natural orbital functional (NOF) theory in its PNOF3 and PNOF4 implementations is used to investigate the electronic structure and energetics of selected diradicals and diradicaloids: the trimethylenmethane (TMM) diradical and the imino-allyl (IMA) and oxyallyl (OXA) diradicaloids. These compounds are taken as paradigmatic cases of molecules with full and partial diradical character, and are considered as a case study to illustrate the potentiality of Piris NOF (PNOF) to yield the correct treatment of near-degeneracy effects in diradicals. The degree of diradical character is determined through the inspection of the corresponding natural orbital occupation numbers. In addition, the energetics of these compounds with respect to their cyclic isomers (methylenecyclopropane, MCP, iminocyclopropane, ICP, and cyclopropanone, OCP) is investigated. We have found that PNOF4 shows a promising description of these species, and yield the correct trends in occupation numbers as compared with wave-function methods such as CASSCF. Therefore, PNOF4 can give a correct description of near-degeneracy effects, and hence, is a promising method to treat diradicals and diradicaloids in chemistry, a delicate type of molecular systems to simulate by theoretical methods.