Ligand field design enables quantum manipulation of spins in Ni2+ complexes.

Creating the next generation of quantum systems requires control and tunability, which are key features of molecules. To design these systems, one must consider the ground-state and excited-state manifolds. One class of systems with promise for quantum sensing applications, which require water solubility, are d8 Ni2+ ions in octahedral symmetry. Yet, most Ni2+ complexes feature large zero-field splitting, precluding manipulation by commercial microwave sources due to the relatively large spin-orbit coupling constant of Ni2+ (630 cm-1). Since low lying excited states also influence axial zero-field splitting, D, a combination of strong field ligands and rigidly held octahedral symmetry can ameliorate these challenges. Towards these ends, we performed a theoretical and computational analysis of the electronic and magnetic structure of a molecular qubit, focusing on the impact of ligand field strength on D. Based on those results, we synthesized 1, [Ni(ttcn)2](BF4)2 (ttcn = 1,4,7-trithiacyclononane), which we computationally predict will have a small D (Dcalc = +1.15 cm-1). High-field high-frequency electron paramagnetic resonance (EPR) data yield spin Hamiltonian parameters: gx = 2.1018(15), gx = 2.1079(15), gx = 2.0964(14), D = +0.555(8) cm-1 and E = +0.072(5) cm-1, which confirm the expected weak zero-field splitting. Dilution of 1 in the diamagnetic Zn analogue, [Ni0.01Zn0.99(ttcn)2](BF4)2 (1') led to a slight increase in D to ∼0.9 cm-1. The design criteria in minimizing D in 1via combined computational and experimental methods demonstrates a path forward for EPR and optical addressability of a general class of S = 1 spins.

Tunable Cr4+ Molecular Color Centers.

The inherent atomistic precision of synthetic chemistry enables bottom-up structural control over quantum bits, or qubits, for quantum technologies. Tuning paramagnetic molecular qubits that feature optical-spin initialization and readout is a crucial step toward designing bespoke qubits for applications in quantum sensing, networking, and computing. Here, we demonstrate that the electronic structure that enables optical-spin initialization and readout for S = 1, Cr(aryl)4, where aryl = 2,4-dimethylphenyl (1), o-tolyl (2), and 2,3-dimethylphenyl (3), is readily translated into Cr(alkyl)4 compounds, where alkyl = 2,2,2-triphenylethyl (4), (trimethylsilyl)methyl (5), and cyclohexyl (6). The small ground state zero field splitting values (<5 GHz) for 1-6 allowed for coherent spin manipulation at X-band microwave frequency, enabling temperature-, concentration-, and orientation-dependent investigations of the spin dynamics. Electronic absorption and emission spectroscopy confirmed the desired electronic structures for 4-6, which exhibit photoluminescence from 897 to 923 nm, while theoretical calculations elucidated the varied bonding interactions of the aryl and alkyl Cr4+ compounds. The combined experimental and theoretical comparison of Cr(aryl)4 and Cr(alkyl)4 systems illustrates the impact of the ligand field on both the ground state spin structure and excited state manifold, laying the groundwork for the design of structurally precise optically addressable molecular qubits.

Analysis of the Contributions to the Kinetic and Potential Energies of an H Atom in the Presence of a Point Charge: The Molecular Virial Theorem Revisited.

The molecular virial theorem states that for a diatomic molecule or for an atom in the presence of a point charge, the changes in the average kinetic energy and average potential energy are equal to [Formula: see text] and [Formula: see text], respectively, where U is the interaction energy and R is the internuclear separation or the atom-point charge separation. In this paper we directly evaluate the ⟨T⟩ and ⟨V⟩ expectation values of an H atom in the presence of a distant point charge, obtaining exact analytical expressions by use of Dalgarno-Lewis perturbation theory.

Prediction of a Non-Valence Temporary Anion State of (NaCl)2.

The equation-of-motion coupled cluster method is used to characterize the low-lying anion states of (NaCl)2 in its rhombic structure. This species is known to possess a non-valence bound anion of Ag symmetry. Our calculations also demonstrate that it has a non-valence temporary anion of B2u symmetry, about 14 meV above threshold. The potential energy curves of the two anion states and of the ground state of the neutral molecule are reported as a function of distortion along the symmetric stretch normal coordinate. Implications for experimental detection of the temporary anion state are discussed. The sensitivity of the results to the inclusion of high-order correlation effects and of core correlation is examined.

Prediction of a Nonvalence Temporary Anion Shape Resonance for a Model (H2O)4 System.

Ab initio calculations are used to demonstrate the existence of a nonvalence temporary anion shape resonance for a model (H2O)4 cluster system with no net dipole moment. The cluster is composed of two water dimers, the distance between which is varied. Each dimer possesses a weakly bound nonvalence anion state. For large separations of the dimer subunits, there are two bound nonvalence anion states (of Ag and B2u symmetry) corresponding to the symmetric and asymmetric combinations of the nonvalence anion states of the two dimer subunits. As the separation between the dimer subunits is decreased, the B2u anion increases in energy and becomes a temporary anion shape resonance. The real part of the resonance energy is determined as a function of the distance between the dimers and is found to increase monotonically from just above threshold to 28 meV for the range of geometries considered. Over this same range of geometries, the resonance half-width varies from 0 to 21 meV. The B2u anion, both when bound and when temporary, has a very diffuse charge distribution. The effective radial potential for the interaction of the excess electron with the cluster has a barrier at large distance arising from the electron-quadrupole interaction in combination with the repulsive angular momentum ( l = 1) contribution. This barrier impacts both the resonance energy and its lifetime.

Theoretical approaches for treating non-valence correlation-bound anions.

In this work, we use a model (H2O)4 cluster, the bent CO2 molecule, and tetracyanoethylene as systems to explore the applicability of various electronic structure methods for characterizing non-valence correlation-bound anion states. The methods examined include the algebraic diagrammatic construction, various equation-of-motion coupled cluster methods, orbital-optimized MP2, and Brueckner coupled cluster doubles with perturbative triples. We demonstrate that the key to treating this challenging class of anions is the use of methods that include adequate orbital relaxation in response to long-range dispersion-like correlation effects.