Open-shell State Research Articles

Photoionization and core resonances from range-separated time-dependent density-functional theory for open-shell states: Example of the lithium atom.

We consider the calculations of photoionization spectra and core resonances of open-shell systems using range-separated time-dependent density-functional theory. Specifically, we use the time-dependent range-separated hybrid (TDRSH) scheme, combining a long-range Hartree-Fock exchange potential and kernel with a short-range potential and kernel from a local density-functional approximation, and the time-dependent locally range-separated hybrid (TDLRSH) scheme, which uses a local range-separation parameter. To efficiently perform the calculations, we formulate a spin-unrestricted linear-response Sternheimer approach in a non-orthogonal B-spline basis set using appropriate frequency-dependent boundary conditions. We illustrate this approach on the Li atom, which suggests that TDRSH and TDLRSH are adequate simple methods for estimating the single-electron photoionization spectra of open-shell systems.

Cofactor-Free Dioxygenases-Catalyzed Reaction Pathway via Proton-Coupled Electron Transfer.

Understanding the general mechanism of the metal-free and cofactor-free oxidases and oxygenases catalyzed activation of triplet O2 is one of the most challenging questions in the field of enzymatic catalysis. Herein, we have performed Quantum Mechanics/Molecular Mechanics (QM/MM) multiscale simulations to reveal the detailed mechanism of the HOD catalyzed (i.e., 1-H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase from Arthrobacter nitroguajacolicus Rü61a) decomposition of N-heteroaromatic compounds. The complete catalytic mechanism includes four steps: (1) proton transfer from 1-H-3-hydroxy-4-oxoquinaldine (QND) substrate to His251 residue coupled with an electron transfer from QND to triplet O2 (i.e., PCET), (2) formation of C-O bond via an open-shell singlet diradical recombination pathway, (3) ring-closure to form a bicyclic ring, and (4) dissociation of CO. The dissociation of CO is determined as the rate-limiting step, and its calculated energy barrier of 14.9 kcal/mol is consistent with the 15.5 kcal/mol barrier derived from experimental kinetic data. The mechanistic profile is not only valuable for understanding the fundamental pathway of cofactor-free oxidases and oxygenases-catalyzed reactions involving the triplet O2 activation but also discloses a new pathway that undergoes the processes of PCET and open-shell singlet transition state.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand.

The reaction of TaMe3 Cl2 with the rigid acridane-derived trisamine H3 NNN yields the tantalum(V) complex [TaCl2 (NNNcat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2 (NNNsq )}2 O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex.

Monometallic Endohedral Azafullerene.

Azafullerenes derived from nitrogen substitution of carbon cage atoms render direct modifications of the cage skeleton, electronic, and physicochemical properties of fullerene. Gas-phase ionized monometallic endohedral azafullerene (MEAF) [La@C81N]+ formed via fragmentation of a La@C82 monoadduct was detected in 1999, but the pristine MEAF has never been synthesized. Here, we report the synthesis, isolation, and characterization of the first pristine MEAF La@C81N, tackling the two-decade challenge. Single-crystal X-ray diffraction study reveals that La@C81N has an 82-atom cage with a pseudo C3v(8) symmetry. According to DFT computations, the nitrogen substitution site within the C82 cage is proposed to locate at a hexagon/hexagon/pentagon junction far away from the encapsulated La atom. La@C81N exists in stable monomer form with a closed-shell electronic state, which is drastically different from the open-shell electronic state of the original La@C82. Our breakthrough in synthesizing a new type of azafullerene offers a new insight into the skeletal modification of fullerenes.

Highly Strained Arene-Fused 1,2-Diborete Biradicaloid.

The stepwise reduction of a doubly cyclic alkyl(amino)carbene (CAAC)-stabilized 2,3-bis(dibromoboryl)naphthalene enables the isolation of the corresponding mono- and bis(boryl) radicals (one- and two-electron reduction), a 2π-aromatic 1,2-diborete (four-electron reduction), which shows biradical character in the solid-state EPR spectrum, and its cyclic bis(alkylidene)diboron dianion (six-electron reduction). The X-ray crystallographic analysis of the diborete shows a highly strained and twisted four-membered ring with a formal cis-diborene motif featuring a very elongated B-B double bond. Calculations based on DFT and multireference approaches reveal that the diborete possesses an open-shell singlet biradicaloid ground state, which is slightly energetically preferred to its EPR-active triplet-state congener. The addition of CO to the diborete resulted in B-B bond splitting and the formation of the corresponding closed-shell singlet, doubly Lewis base-stabilized bis(borylene), whereas a twofold γ insertion of phenyl azide generates a 1,3-bis(diazenyl)-1,3,2,4-diazadiboretidine.

Computational Exploration of the Intersystem Crossing from the X̃3A2 to the ã1A1 State in Boryl Nitrenes upon Photoexcitation.

The boryl nitrene CatBN (Cat = catecholato) turns highly reactive toward small inert molecules upon irradiation of its triplet ground state X̃3A2 with light of wavelength λ > 550 nm. A computational study of a model boryl nitrene using complete active space self-consistent field (CASSCF) theory provides evidence for the population of the highly reactive electronic state ã1A1 upon irradiation. Potential energy scans connecting different critical points (minima, minimum energy crossing points, and conical intersections) reveal two possible pathways that could relax photoexcited boryl nitrene from the Franck-Condon region of Ã3B1 to the ã1A1 state minimum. Considering the energy barriers to relaxation from one electronic state to another and the magnitude of spin-orbit couplings, the energetically most favorable pathway involves photoexcitation to Ã3B2, followed by intersystem crossing to the open-shell singlet state (b̃1A2) and internal conversion to ã1A1. The relevant minimum energy crossing point is about 7-8 kcal mol-1 higher in energy than the Franck-Condon region.

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

Difficulty of the evaluation of the barrier height of an open-shell transition state between closed shell minima: The case of small C4n rings.

C4n cyclacenes exhibit strong bond-alternation in their equilibrium geometry. In the two equivalent geometries, the system keeps an essentially closed-shell character. The two energy minima are separated by a transition state suppressing the bond-alternation, where the wave function is strongly diradical. This paper discusses the physical factors involved in this energy difference and possible evaluations of the barrier height. The barrier given as the energy difference between the restricted density functional theory (DFT)/B3LYP for the equilibrium and the broken symmetry DFT/B3LYP of the transition state is either negative or small, in contradiction with the most reliable Wave Function Theory calculations. The minimal (two electrons in two molecular orbitals) Complete Active Space self-consistent field (CASSCF) overestimates the barrier, and the subsequent second-order perturbation cancels it. Due to the collective character of the spin-polarization effect, it is necessary to perform a full π CASSCF + second-order perturbation to reach a reasonable value of the barrier, but this type of treatment cannot be applied to large molecules. DFT procedures treating on an equal foot the closed-shell and open-shell geometries have been explored, such as Mixed-Reference Spin-Flip Time-dependent-DFT and a new spin-decontamination proposal, namely, DFT-dressed configuration interaction, but the results still depend on the density functional. M06-2X without or with spin-decontamination gives the best agreement with the accurate wave function results.

The intermediate state approach for doubly excited dark states in EOM-coupled-cluster theory.

Solution of dark, doubly excited states using equation-of-motion coupled-cluster (EOM-CC) usually equires at least triple excitations or even quadruples beyond the standard singles and doubles (EOM-CCSD) for an appropriate treatment. A new route to obtain these doubly excited states using EOM-CCSD is demonstrated. Traditionally, EOM-CC is performed on a closed shell reference state that has a well-described single reference CC wavefunction. In this Communication, we attempt to use low spin open-shell states such as the MS = 0 triplet and open-shell singlet as a reference state. Using this intermediate excited state as a reference state provides us with the benefit of obtaining a doubly excited state, as a single excitation at the cost of EOM-CCSD.

An All-Organic Photochemical Magnetic Switch with Bistable Spin States.

Controlling the electronic spin state in single molecules through an external stimulus is of interest in developing devices for information technology, such as data storage and quantum computing. We report the synthesis and operation mode of two all-organic molecular spin-state switches that can be photochemically switched from a diamagnetic [electron paramagnetic resonance (EPR)-silent] to a paramagnetic (EPR-active) form at cryogenic temperatures due to a reversible electrocyclic reaction of its carbon skeleton. Facile synthetic substitution of a configurationally stable 1,14-dimethyl-[5]helicene with radical stabilizing groups at the 4,11-positions afforded two spin-state switches as 4,11-dioxo or 4,11-bis(dicyanomethylidenyl) derivatives in a closed diamagnetic form. After irradiation with an LED light source at cryogenic temperatures, a stable paramagnetic state is readily obtained, making this system a bistable magnetic switch that can reversibly react back to its diamagnetic form through a thermal stimulus. The switching can be monitored with UV/vis spectroscopy and EPR spectroscopy or induced by electrochemical reduction and reoxidation. Variable-temperature EPR spectroscopy of the paramagnetic species revealed an open-shell triplet ground state with an experimentally determined triplet-singlet energy gap of ΔET-S < 0.1 kcal mol-1. The inherent chirality and the ability to separate the enantiomers turns this helical motif into a potential chiroptical spin-state switch. The herein developed 4,11-substitution pattern on the dimethyl[5]helicene introduces a platform for designing future generations of organic molecular photomagnetic switches that might find applications in spintronics and related fields.

Aza-Triangulene: On-Surface Synthesis and Electronic and Magnetic Properties.

Nitrogen heteroatom doping into a triangulene molecule allows tuning its magnetic state. However, the synthesis of the nitrogen-doped triangulene (aza-triangulene) has been challenging. Herein, we report the successful synthesis of aza-triangulene on the Au(111) and Ag(111) surfaces, along with their characterizations by scanning tunneling microscopy and spectroscopy in combination with density functional theory (DFT) calculations. Aza-triangulenes were obtained by reducing ketone-substituted precursors. Exposure to atomic hydrogen followed by thermal annealing and, when necessary, manipulations with the scanning probe afforded the target product. We demonstrate that on Au(111), aza-triangulene donates an electron to the substrate and exhibits an open-shell triplet ground state. This is derived from the different Kondo resonances of the final aza-triangulene product and a series of intermediates on Au(111). Experimentally mapped molecular orbitals match with DFT-calculated counterparts for a positively charged aza-triangulene. In contrast, aza-triangulene on Ag(111) receives an extra electron from the substrate and displays a closed-shell character. Our study reveals the electronic properties of aza-triangulene on different metal surfaces and offers an approach for the fabrication of new hydrocarbon structures, including reactive open-shell molecules.

Potassium Trimethylsilanolate-Promoted, Anhydrous Suzuki-Miyaura Cross-Coupling Reaction Proceeds via the "Boronate Mechanism": Evidence for the Alternative Fork in the Trail.

Previous studies have shown that the critical transmetalation step in the Suzuki-Miyaura cross-coupling proceeds through a mechanism wherein an arylpalladium hydroxide complex reacts with an aryl boronic acid, termed the oxo-palladium pathway. Moreover, these same studies have established that the reaction between an aryl boronate and an arylpalladium halide complex (the boronate pathway) is prohibitively slow. Herein, studies on isolated intermediates, along with kinetic analysis, have demonstrated that the Suzuki-Miyaura reaction promoted by potassium trimethylsilanolate (TMSOK) proceeds through the boronate pathway, in contrast with other, established systems. Furthermore, an unprecedented, binuclear palladium(I) complex containing a μ-phenyl bridging ligand was characterized by NMR spectroscopy, mass spectrometry, and computational methods. Density functional theory (DFT) calculations suggest that the binuclear complex exhibits an open-shell ground electronic state, and reaction kinetics implicate the complex in the catalytic cycle. These results expand the breadth of potential mechanisms by which the Suzuki-Miyaura reaction can occur, and the novel binuclear palladium complex discovered has broad implications for palladium-mediated cross-coupling reactions of aryl halides.

Nonbenzenoid High-Spin Polycyclic Hydrocarbons Generated by Atom Manipulation.

We report the on-surface synthesis of a nonbenzenoid triradical through dehydrogenation of truxene (C27H18) on coinage metal and insulator surfaces. Voltage pulses applied via the tip of a combined scanning tunneling microscope/atomic force microscope were used to cleave individual C-H bonds in truxene. The resultant final product truxene-5,10,15-triyl (1) was characterized at the single-molecule scale using a combination of atomic force microscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Our analyses show that 1 retains its open-shell quartet ground state, predicted by density functional theory, on a two monolayer-thick NaCl layer on a Cu(111) surface. We image the frontier orbital densities of 1 and confirm that they correspond to spin-split singly occupied molecular orbitals. Through our synthetic strategy, we also isolate two reactive intermediates toward the synthesis of 1, derivatives of fluorenyl radical and indeno[1,2-a]fluorene, with predicted open-shell doublet and triplet ground states, respectively. Our results should have bearings on the synthesis of nonbenzenoid high-spin polycyclic frameworks with magnetism beyond Lieb's theorem.

A correctly scaling rigorously spin-adapted and spin-complete open-shell CCSD implementation for arbitrary high-spin states.

In this paper, we report on a correctly scaling novel coupled cluster singles and doubles (CCSD) implementation for arbitrary high-spin open-shell states. The chosen cluster operator is completely spin-free, i.e., employs spatial substitutions only. It is composed of our recently developed Löwdin-type operators [N. Herrmann and M. Hanrath, J. Chem. Phys. 153, 164114 (2020)], which ensure (1) spin completeness and (2) spin adaption, i.e., spin purity of the CC wave function. In contrast to the proof-of-concept matrix-representation-based implementation presented there, the present implementation relies on second quantization and factorized tensor contractions. The generated singles and doubles operators are embedded in an equation generation engine. In the latter, Wick's theorem is used to derive prefactors arising from spin integration directly from the spin-free full contraction patterns. The obtained Wick terms composed of products of Kronecker deltas are represented by special non-antisymmetrized Goldstone diagrams. Identical (redundant) diagrams are identified by solving the underlying graph isomorphism problem. All non-redundant graphs are then automatically translated to locally-one term at a time-factorized tensor contractions. Finally, the spin-adapted and spin-complete (SASC) CCS and CCSD variants are applied to a set of small molecular test systems. Both correlation energies and amplitude norms hint toward a reasonable convergence of the SASC-CCSD method for a Baker-Campbell-Hausdorff series truncation of order four. In comparison to spin orbital CCSD, SASC-CCSD leads to slightly improved correlation energies with differences of up to 1.292mEH (1.10% with respect to full configuration identification) for quintet CH2 in the cc-pVDZ basis set.

Computational Study of Electronic Properties in Oligoacenes

Calculations were performed for closed shell singlet states, open shell singlet states and open-shell triplet states using DFT and B3LYP functional in oligoacenes. 6-31+G(d,p) basis set was used to calculate energies for polyacenes in Hartrees at various levels of spin-state configuration. For systems beyond pentacene, open shell sing let state is more stablilized for hexacene, heptacene, octacene and nonacene and singlet triplet gap decreases as size of polyacene increases. This work is used to describe unrestricted DFT calculations, spin contamination and quantum mechanical applications..

Indeno[2,1-a]fluorene-11,12-dione Radical Anions: Synthesis, Characterization, and Properties.

The one-electron reduction of indeno[2,1-a]fluorene-11,12-dione (IF) with various alkali metals prepare the radical anion salts. The data about different structures, properties, and characterization was obtained by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) measurements, and physical property measurement system (PPMS). Compound IF.- K+ (18-c-6) is regarded as a one-dimensional magnetic chain through C-H⋅⋅⋅C interaction. Theoretical calculations and magnetic results showed that [IF.- K+ (15-c-5)]2 is a dimer with an open-shell ground state. Compounds IF.- Na+ (15-c-5) and IF.- K+ (cryptand) are monoradical anion salts: IF2 .- Li+ possesses unique π-stack structure with an interplanar separation less than 3.46 Å, making it a semiconductor (δRT =1.9×10-4 S ⋅ cm-1 ). This work gives insights into multifunctional radical anions, and describes the design and development of different functional radicals.

Defect-Induced π-Magnetism into Non-Benzenoid Nanographenes.

The synthesis of nanographenes (NGs) with open-shell ground states have recently attained increasing attention in view of their interesting physicochemical properties and great prospects in manifold applications as suitable materials within the rising field of carbon-based magnetism. A potential route to induce magnetism in NGs is the introduction of structural defects, for instance non-benzenoid rings, in their honeycomb lattice. Here, we report the on-surface synthesis of three open-shell non-benzenoid NGs (A1, A2 and A3) on the Au(111) surface. A1 and A2 contain two five- and one seven-membered rings within their benzenoid backbone, while A3 incorporates one five-membered ring. Their structures and electronic properties have been investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy and scanning tunneling spectroscopy complemented with theoretical calculations. Our results provide access to open-shell NGs with a combination of non-benzenoid topologies previously precluded by conventional synthetic procedures.

Cofactor-free ActVA-Orf6 monooxygenase catalysis via proton-coupled electron transfer: a QM/MM study.

Uncovering the comprehensive catalytic mechanism for the activation of triplet O2 through metal-free and cofactor-free oxidases and oxygenases remains one of the most challenging problems in the area of enzymatic catalysis. Herein, we performed multiscale simulation with molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) techniques to reveal the detailed mechanism of ActVA-Orf6 monooxygenase catalyzed oxygenation of phenols to quinones from Streptomyces coelicolor, such as the oxidation of 6-deoxydihydrocarafungin (DDHK) to dihydrocarafungin (DHK). The entire catalytic mechanism consists of three steps: (1) proton-coupled electron transfer (PCET) from the substrate DDHK to triplet O2 with the aid of an explicit water molecule, (2) the formation of a C-O bond via an open-shell singlet diradical complexation pathway, and (3) dehydration via a six-membered ring mode assisted by one water molecule. The complete energetic profiles show that the rate-determining step is the dehydration with an energy barrier of 20.7 kcal mol-1, which is close to that of 19.7 kcal mol-1 derived from experimental kinetic data. Our mechanistic study not only helps to deeply understand the fundamental mechanism of metal-free and cofactor-free oxidase and oxygenase catalyzed different reactions, but also discloses a new route that proceeds through the processes of PCET and the open-shell singlet transition state.

A Computational Journey across Nitroxide Radicals: From Structure to Spectroscopic Properties and Beyond.

Nitroxide radicals are characterized by a long-lived open-shell electronic ground state and are strongly sensitive to the chemical environment, thus representing ideal spin probes and spin labels for paramagnetic biomolecules and materials. However, the interpretation of spectroscopic parameters in structural and dynamic terms requires the aid of accurate quantum chemical computations. In this paper we validate a computational model rooted into double-hybrid functionals and second order vibrational perturbation theory. Then, we provide reference quantum chemical results for the structures, vibrational frequencies and other spectroscopic features of a large panel of nitroxides of current biological and/or technological interest.

Three-State Switching of an Anthracene Extended Bis-thiaxanthylidene with a Highly Stable Diradical State.

A multistable molecular switching system based on an anthracene-extended bis-thiaxanthylidene with three individually addressable states that can be interconverted by electrochemical, thermal, and photochemical reactions is reported. Besides reversible switching between an open-shell diradical- and a closed-shell electronic configuration, our findings include a third dicationic state and control by multiple actuators. This dicationic state with an orthogonal conformation can be switched electrochemically with the neutral open-shell triplet state with orthogonal conformation, which was characterized by EPR. The remarkably stable diradical shows kinetic stability as a result of a significant activation barrier for isomerization to a more stable neutral closed-shell folded geometry. We ascribe this activation barrier of ΔG⧧(293 K) = 25.7 kcal mol–1 to steric hindrance in the fjord region of the overcrowded alkene structure. The folded closed-shell state can be converted back to the diradical state by irradiation with 385 nm. The folded state can also be oxidized to the dicationic state. These types of molecules with multiple switchable states and in particular stable diradicals show great potential in the design of new functional materials such as memory devices, logic gates, and OFETs.