Predicting Magnetic Barriers in Lanthanide Complexes with Electrostatic Potential Charges.

Single-molecule magnets (SMMs) with slow relaxation of magnetization and blocking temperatures above that of liquid nitrogen are essential for practical applications in high-density data storage devices and quantum computers. A rapid and accurate prediction of the effective magnetic relaxation barrier () is needed to accelerate the discovery of high-performance SMMs. Using density functional theory and multireference calculations, we explored correlations between , partial atomic charges, and the anisotropic barrier for a series of sandwich-type lanthanide complexes containing cyclooctatetraene, substituted cyclopentadiene, phospholyl, boratabenzene, or borane ligands.

Predicting kinetics of spin-dependent reactions in an external magnetic field with nonadiabatic statistical theory.

We introduce a theoretical framework to study the kinetics of the chemical reactions involving transitions between electronic states with different spin quantum numbers in an external magnetic field. The new equations for calculating transition probabilities and rate constants are used to generalize the nonadiabatic statistical theory, which now accounts for both the spin-orbit and Zeeman couplings between electronic states. Focusing on the singlet-triplet transitions, we define two dimensionless parameters to characterize (1) the magnetic field strength relative to the strength of spin-orbit coupling and (2) the relative magnitudes of the spin-orbit coupling matrix elements that couple the singlet state to different components of the triplet state.

Effect of Extreme Variations of Fundamental Constants on the Structure of Atoms and Molecules.

The fundamental constants (FCs) of physics are promoted to dynamic quantities in modern theories. So far most of the literature focused on small fractional variations in the values of FCs. In this paper, we investigate the novel regime of extreme but transient variations of FCs.

Analytical nonadiabatic coupling and state-specific energy gradient for the crystal field Hamiltonian describing lanthanide single-ion magnets.

Paramagnetic molecules with a metal ion as an electron spin center are promising building blocks for molecular qubits and high-density memory arrays. However, fast spin relaxation and decoherence in these molecules lead to a rapid loss of magnetization and quantum information. Nonadiabatic coupling (NAC), closely related to spin-vibrational coupling, is the main source of spin relaxation and decoherence in paramagnetic molecules at higher temperatures.

Tuning symmetry and magnetic blocking of an exchange-coupled lanthanide ion in isomeric, tetrametallic complexes: [LnCl(TiCp)].

The synthesis and magnetic properties of two pairs of isomeric, exchange-coupled complexes, [LnCl(TiCp)] (Ln = Gd, Tb), are reported. In each isomeric pair, the central lanthanide ion adopts either a pseudo-octahedral (O-Ln) or trigonal prismatic geometry (TP-Ln) yielding complexes with or molecular symmetry, respectively. Ferromagnetic exchange coupling is observed in TP-Ln as indicated by the increases in below 30 K.

Predicting Kinetics and Dynamics of Spin-Dependent Processes.

ConspectusPredicting mechanisms and rates of nonadiabatic spin-dependent processes including photoinduced intersystem crossings, thermally activated spin-forbidden reactions, and spin crossovers in metal centers is a very active field of research. These processes play critical roles in transition-metal-based and metalloenzymatic catalysis, molecular magnets, light-harvesting materials, organic light-emitting diodes, photosensitizers for photodynamic therapy, and many other applications. Therefore, accurate modeling of spin-dependent processes in complex systems and on different time scales is important for many problems in chemistry, biochemistry, and materials sciences.

The role of the intermediate triplet state in iron-catalyzed multi-state C-H activation.

Efficient activation and functionalization of the C-H bond under mild conditions are of a great interest in chemical synthesis. We investigate the previously proposed spin-accelerated activation of the C(sp)-H bond by a Fe(II)-based catalyst to clarify the role of the intermediate triplet state in the reaction mechanism. High-level electronic structure calculations on a small model of a catalytic system utilizing the coupled cluster with the single, double, and perturbative triple excitations [CCSD(T)] are used to select the density functional for the full-size model.

Strong Relativistic Effects in Lanthanide-Based Single-Molecule Magnets.

Lanthanide-based single-molecule magnets (SMMs) are promising building blocks for quantum memory and spintronic devices. Designing lanthanide-based SMMs with long spin relaxation time requires a detailed understanding of their electronic structure, including the crucial role of the spin-orbit coupling (SOC). While traditional calculations of SOC using the perturbation theory applied to a solution of the nonrelativistic Schrödinger equation are valid for light atoms, this approach is questionable for systems containing heavy elements such as lanthanides.

NAST: Nonadiabatic Statistical Theory Package for Predicting Kinetics of Spin-Dependent Processes.

We present a nonadiabatic statistical theory (NAST) package for predicting kinetics of spin-dependent processes, such as intersystem crossings, spin-forbidden unimolecular reactions, and spin crossovers. The NAST package can calculate the probabilities and rates of transitions between the electronic states of different spin multiplicities. Both the microcanonical (energy-dependent) and canonical (temperature-dependent) rate constants can be obtained.

Intersystem crossing and internal conversion dynamics with GAIMS-TeraChem: Excited state relaxation in 2-cyclopentenone.

Excited states relaxation in complex molecules often involves two types of nonradiative transitions, internal conversion (IC) and intersystem crossing (ISC). In the situations when the timescales of IC and ISC are comparable, an interplay between these two types of transitions can lead to complex nonadiabatic dynamics on multiple electronic states of different characters and spin multiplicities. We demonstrate that the generalized ab initio multiple spawning (GAIMS) method interfaced with the fast graphics processing unit-based TeraChem electronic structure code can be used to model such nonadiabatic dynamics involving both the IC and ISC transitions in molecules of moderate size.

Modeling Spin-Crossover Dynamics.

In this article, we review nonadiabatic molecular dynamics (NAMD) methods for modeling spin-crossover transitions. First, we discuss different representations of electronic states employed in the grid-based and direct NAMD simulations. The nature of interstate couplings in different representations is highlighted, with the main focus on nonadiabatic and spin-orbit couplings.

Full Visible Spectrum and White Light Emission with a Single, Input-Tunable Organic Fluorophore.

The blue emission of M2biQ can be tuned to specific wavelengths throughout the visible region by changing the identity of the cation it interacts with. These optical properties are observed in MeCN solution and the solid state. White light is obtained in MeCN by using either the proper ratio of zinc ions or acid.

Spin controlled surface chemistry: alkyl desorption from Si(100)-2×1 by nonadiabatic hydrogen elimination.

An understanding of the role that spin states play in semiconductor surface chemical reactions is currently limited. Herein, we provide evidence of a nonadiabatic reaction involving a localized singlet to triplet thermal excitation of the Si(100) surface dimer dangling bond. By comparing the β-hydrogen elimination kinetics of ethyl adsorbates probed by thermal desorption experiments to electronic structure calculation results, we determined that a coverage-dependent change in mechanism occurs.

Intersystem crossing in tunneling regime: T → S relaxation in thiophosgene.

The T1 excited state relaxation in thiophosgene has attracted much attention as a relatively simple model for the intersystem crossing (ISC) transitions in polyatomic molecules. The very short (20-40 ps) T1 lifetime predicted in several theoretical studies strongly disagrees with the experimental values (∼20 ns) indicating that the kinetics of T1 → S0 ISC is not well understood. We use the nonadiabatic transition state theory (NA-TST) with the Zhu-Nakamura transition probability and the multireference perturbation theory (CASPT2) to show that the T1 → S0 ISC occurs in the quantum tunneling regime.

In Silico Molecular Engineering of Dysprosocenium-Based Complexes to Decouple Spin Energy Levels from Molecular Vibrations.

Molecular nanomagnets hold great promise for spintronics and quantum technologies, provided that their spin memory can be preserved above liquid-nitrogen temperatures. In the past few years, the magnetic hysteresis records observed for two related dysprosocenium-type complexes have highlighted the potential of molecular engineering to decouple vibrational excitations from spin states and thereby enhance magnetic memory. Herein, we study the spin-vibrational coupling in [(Cp)Dy(Cp*)] (Cp = pentaisopropylcyclopentadienyl, Cp* = pentamethylcyclopentadienyl), which currently holds the hysteresis record (80 K), by means of a computationally affordable methodology that combines first-principles electronic structure calculations with a phenomenological ligand field model.

Locating Minimum Energy Crossings of Different Spin States Using the Fragment Molecular Orbital Method.

Spin-dependent processes involving nonadiabatic transitions between electronic states with different spin multiplicities play important roles in the chemistry of complex systems. The rates of these processes can be predicted based on the molecular properties at the minimum energy crossing point (MECP) between electronic states. We present the development of the MECP search technique within the fragment molecular orbital (FMO) method applicable to large complex systems.

Electronic Transitions Responsible for C Diffuse Interstellar Bands.

Diffuse interstellar bands (DIBs) are puzzling absorption features believed to contain critical information about molecular evolution in space. Despite the fact that C recently became the first confirmed carrier of several DIBs, the nature of the corresponding transitions is not understood. Using electronic structure methods, we show that the two strong C DIBs cannot be explained by electronic transitions to the two different excited E states or the two spin-orbit components of the lowest E state, as suggested before.

Predicting Intersystem Crossing Rates with AIMS-DFT Molecular Dynamics.

Accurate prediction of the intersystem crossing rates is important for many different applications in chemistry, physics, and biology. Recently, we implemented the ab initio multiple spawning (AIMS) molecular dynamics method to describe the intersystem crossing processes, where nonradiative transitions between electronic states with different spin multiplicities are mediated by spin-orbit coupling. Our original implementation of the direct AIMS dynamics used the complete active space self-consistent field (CASSCF) method to describe multiple coupled electronic states on which multidimensional Gaussian wave packets were propagated.

Lactate Racemase Nickel-Pincer Cofactor Operates by a Proton-Coupled Hydride Transfer Mechanism.

Lactate racemase (LarA) of Lactobacillus plantarum contains a novel organometallic cofactor with nickel coordinated to a covalently tethered pincer ligand, pyridinium-3-thioamide-5-thiocarboxylic acid mononucleotide, but its function in the enzyme mechanism has not been elucidated. This study presents direct evidence that the nickel-pincer cofactor facilitates a proton-coupled hydride transfer (PCHT) mechanism during LarA-catalyzed lactate racemization. No signal was detected by electron paramagnetic resonance spectroscopy for LarA in the absence or presence of substrate, consistent with a +2 metal oxidation state and inconsistent with a previously proposed proton-coupled electron transfer mechanism.

Ab initio calculations of spectroscopic constants and vibrational state lifetimes of diatomic alkali-alkaline-earth cations.

We investigate the lifetimes of vibrational states of diatomic alkali-alkaline-earth cations to determine their suitability for ultracold experiments where long decoherence time and controllability by an external electric field are desirable. The potential energy and permanent dipole moment curves for the ground electronic states of LiBe, LiMg, NaBe, and NaMg are obtained using the coupled cluster with singles doubles and triples and multireference configuration interaction methods in combination with large all-electron cc-pCVQZ and aug-cc-pCV5Z basis sets. The energies and wave functions of all vibrational states are obtained by solving the Schrödinger equation for nuclei with the B-spline basis set method.

Chiral Peropyrene: Synthesis, Structure, and Properties.

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using p-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis.

Spin-Forbidden Transitions between Electronic States in the Active Site of Rubredoxin.

Rubredoxin is a small iron-sulfur protein involved in biological electron transfer, which is accomplished by changing the oxidation state of the iron atom in the active site. We investigate the possibility of spin-forbidden transitions between the lowest energy electronic states with different spin multiplicities in the rubredoxin active site models [Fe(SCH)] (n = 2-, 1-, 0) using nonadiabatic transition state theory (NA-TST). The equilibrium structures, minimum energy crossing point structures and Hessians were obtained with density functional theory.

Ab Initio Multiple Spawning Method for Intersystem Crossing Dynamics: Spin-Forbidden Transitions between (3)B1 and (1)A1 States of GeH2.

Dynamics at intersystem crossings are fundamental to many processes in chemistry, physics, and biology. The ab initio multiple spawning (AIMS) method was originally developed to describe internal conversion dynamics at conical intersections where derivative coupling is responsible for nonadiabatic transitions between electronic states with the same spin multiplicity. Here, the applicability of the AIMS method is extended to intersystem crossing dynamics in which transitions between electronic states with different spin multiplicities are mediated by relativistic spin-orbit coupling.

Nonadiabatic transition state theory and trajectory surface hopping dynamics: intersystem crossing between (3)B1 and (1)A1 states of SiH2.

The ability of time-independent nonadiabatic transition state theory (NA-TST) to reproduce intersystem crossing dynamics obtained from the more computationally demanding Tully fewest switches trajectory surface hopping method is investigated. The two approaches are applied to the intersystem crossing between the ground (1)A1 state and lowest energy (3)B1 state of SiH2, coupled through the spin-orbit interaction. For NA-TST, the transition probabilities are calculated using the Landau-Zener formula and the Delos formula which accounts for tunneling.