Spin-Crossing in an Organometallic Pt–Benzene Complex

PtOx Cly (OH)z (H2 O)n Complexes under Oxidative and Reductive Conditions: Impact of the Level of Theory on Thermodynamic Stabilities

Platinum-based catalysts with Cl^- , OH^- , O^2- and H₂ O ligands, are involved in many industrial processes. Their final chemical properties are impacted by calcination and reduction applied during the preparation and activation steps. We investigate their stability under these reactive conditions with density functional theory (DFT). We benchmark various functionals (PBE-dDsC, optPBE, B3LYP, HSE06, PBE0, TPSS, RTPSS and SCAN) against ACFDT-RPA. PBE-dDsC is well adapted, although hybrid functionals are more accurate for redox reactions. Thermodynamic phase diagrams are determined by computing the chemical potential of the species as a function of temperature and partial pressures of H₂ O, HCl, O₂ and H₂ . The stability and nature of the Pt species are highly sensitive to the activation conditions. Under O₂ , high temperatures favour PtO₂ while under H₂ , platinum is easily reduced to Pt(0). Chlorine modifies the coordination sphere of platinum during calcination by stabilizing PtCl₄ and shifts the reduction of platinum to higher temperatures under H₂ .

Spin-State Splittings in 3d Transition-Metal Complexes Revisited: Benchmarking Approximate Methods for Adiabatic Spin-State Energy Differences in Fe(II) Complexes

The CASPT2+δMRCI composite approach reported in a companion paper has been extended and used to provide high-quality reference data for a series of adiabatic spin gaps (defined as ΔE = E_quintet - E_singlet) of [Fe^IIL₆]²⁺ complexes (L = CNH, CO, NCH, NH₃, H₂O), either at nonrelativistic level or including scalar relativistic effects. These highly accurate data have been used to evaluate the performance of various more approximate methods. Coupled-cluster theory with singles, doubles, and perturbative triples, CCSD(T), is found to agree well with the new reference data for Werner-type complexes but exhibits larger underestimates by up to 70 kJ/mol for the π-acceptor ligands, due to appreciable static correlation in the low-spin states of these systems. Widely used domain-based local CCSD(T) calculations, DLPNO-CCSD(T), are shown to depend very sensitively on the cutoff values used to construct the localized domains, and standard values are not sufficient. A large number of density functional approximations have been evaluated against the new reference data. The B2PLYP double hybrid gives the smallest deviations, but several functionals from different rungs of the usual ladder hierarchy give mean absolute deviations below 20 kJ/mol. This includes the B97-D semilocal functional, the PBE0* global hybrid with 15% exact-exchange admixture, as well as the local hybrids LH07s-SVWN and LH07t-SVWN. Several further functionals achieve mean absolute errors below 30 kJ/mol (M06L-D4, SSB-D, B97-1-D4, LC-ωPBE-D4, LH12ct-SsirPW92-D4, LH12ct-SsifPW92-D4, LH14t-calPBE-D4, LHJ-HFcal-D4, and several further double hybrids) and thereby also still overall outperform CCSD(T) or uncorrected CASPT2. While exact-exchange admixture is a crucial factor in favoring high-spin states, the present evaluations confirm that other aspects can be important as well. A number of the better-performing functionals underestimate the spin gaps for the π-acceptor ligands but overestimate them for L = NH₃, H₂O. In contrast to a previous suggestion, non-self-consistent density functional theory (DFT) computations on top of Hartree-Fock orbitals are not a promising path to produce accurate spin gaps in such complexes.

Adsorption of a single Pt atom on graphene: spin crossing between physisorbed triplet and chemisorbed singlet states

Diffusion Monte Carlo (DMC) calculations have been performed to study the adsorption of a single Pt atom on pristine graphene. We obtain the adsorption energy curves of a single Pt atom adsorbed at three different adsorption sites (bridge, on-top, hollow) as functions of the vertical distance from a graphene surface for both spin singlet and triplet states. The bridge-site adsorption in a singlet spin state is found to be energetically most stable, which is consistent with previous theoretical predictions. As the Pt atom moves away from a graphene surface, spin triplet states are favored over spin singlet states for all three adsorption sites, reflecting that the ground state of an isolated Pt atom is in a spin triplet state. Furthermore, our DMC calculations reveal local-minimum features in the triplet region which is understood to be due to van der Waals interaction between the Pt atom and graphene. This provides a comprehensive understanding for a spin crossing from a physisorbed triplet state to a chemisorbed singlet state in the adsorption process of a single Pt atom on graphene.

Extrapolation of Atomic Natural Orbitals of basis set to complete basis set limit

Relativistic Atomic Natural Orbitals (ANO-RCC) are extrapolated to the complete basis set limit. ANO-RCC-VXZP (X = D, T, Q) basis sets were extrapolated using standard extrapolation techniques. Five noncovalent complexes, characterized by hydrogen, dispersion and halogen interactions, were chosen. Accurate description of the studied complexes is allowed only after the inclusion of electron correlation and large basis sets which have to include polarization and diffuse functions. Results are in quantitative agreement with the benchmark data obtained by standard aug-cc-pVXZ-DK (X = D, T, Q) basis sets considering chemical accuracy of ±1 kcal/mol.

Diffusion Monte Carlo Perspective on the Spin-State Energetics of [Fe(NCH)6](2.)

The energy difference between the high spin and the low spin state of the model compound [Fe(NCH)6](2+) is investigated by means of Diffusion Monte Carlo (DMC), where special attention is dedicated to analyzing the effect of the fix node approximation on the accuracy of the results. For this purpose, we compare several Slater-Jastrow and multireference Slater-Jastrow trial wave functions. We found that a Slater-Jastrow trial wave function constructed with the generalized Kohn-Sham orbitals from hybrid DFT represents the optimal choice. This is understood by observing that hybrid functionals account for the subtle balance between exchange and correlation effects and the respective orbitals accurately describe the ligand-metal hybridization as well as the charge reorganization accompanying the spin transition. Finally the DMC results are compared with those obtained by Hartree-Fock, DFT, CASSCF, and CASPT2. While there is no clear reference value for the high spin-low spin energy difference, DMC and high level CCSD(T) calculations agree within around 0.3 eV.

Modelling of graphene functionalization

This perspective describes the available theoretical methods and models for simulating graphene functionalization based on quantum and classical mechanics.

Resolving the origin of the multimode Jahn–Teller effect in metallophthalocyanines

What is causing a distortion in phthalocyanines?

Spin and structural features of oxygen dissociation on tetrahedral Ag20 and Ag19Au clusters

The spin-crossing mechanism of oxygen dissociation on Ag₂₀ and monodoped Ag₁₉Au clusters was investigated using spin-polarized scalar-relativistic DFT. The activation energy decreases by 11–29 kJ mol⁻¹ relative to that for the spin-conserving pathway.

Structural and electronic properties of the Pt(n)-PAH complex (n = 1, 2) from density functional calculations

A detailed density functional study of the Pt atom and the Pt dimer adsorption on a polyaromatic hydrocarbon (PAH) is presented. The preferred adsorption site for a Pt atom is confirmed to be the bridge site. Upon adsorption of a single Pt atom, however, it is found here that the electronic ground state changes from the triplet state (5d(9)6s(1) configuration) to the closed-shell singlet state (5d(10)6s(0) configuration), which consequently will affect the catalytic activity of Pt when single Pt atoms bind to a carbon surface. The preferred adsorption site for the Pt dimer in the upright configuration is the hollow site. In contrast to the adsorption of a single Pt atom, the formation of a Pt-C bond in the adsorption of a Pt dimer is not accompanied by a change in the spin state, so the most stable electronic state is still the triplet state. While the atomic charge on the Pt atoms and dimers (in parallel configuration) in the Ptn-PAH complex is positive, a negative charge is found on the upper Pt atom for the upright configuration, indicating that single layers of Pt atoms will have a different catalytic activity as compared to Pt clusters on a carbon surface. Comparing the Pt-C bond length and the charge transfer on different sites, the magnitude of the charge transfer decreases with bond elongation, indicating that the catalytic activity of the Pt atom and dimer can be changed by modifying its chemical surroundings. The adsorption energy for the Pt dimer on a PAH surface is larger than that for two individual Pt atoms on the surface indicating that aggregation of Pt atoms on the PAH surface is favorable.

The nature of bonding and electronic properties of graphene and benzene with iridium adatoms

The strong correlation effects contribute to the opening of the band gap in graphene covered with the Ir adatoms.

Structural dynamics of possible late-stage intermediates in folding of quadruplex DNA studied by molecular simulations

Explicit solvent molecular dynamics simulations have been used to complement preceding experimental and computational studies of folding of guanine quadruplexes (G-DNA). We initiate early stages of unfolding of several G-DNAs by simulating them under no-salt conditions and then try to fold them back using standard excess salt simulations. There is a significant difference between G-DNAs with all-anti parallel stranded stems and those with stems containing mixtures of syn and anti guanosines. The most natural rearrangement for all-anti stems is a vertical mutual slippage of the strands. This leads to stems with reduced numbers of tetrads during unfolding and a reduction of strand slippage during refolding. The presence of syn nucleotides prevents mutual strand slippage; therefore, the antiparallel and hybrid quadruplexes initiate unfolding via separation of the individual strands. The simulations confirm the capability of G-DNA molecules to adopt numerous stable locally and globally misfolded structures. The key point for a proper individual folding attempt appears to be correct prior distribution of syn and anti nucleotides in all four G-strands. The results suggest that at the level of individual molecules, G-DNA folding is an extremely multi-pathway process that is slowed by numerous misfolding arrangements stabilized on highly variable timescales.