Fluxionality Modulating the Magnetic Anisotropy in Lanthanoarene [(ηCnRn)2Ln(II/III)] (n = 4-8) Single-Ion Magnets.

Lanthanoarenes have emerged as the best bet for the futuristic application of single-ion magnets in information storage devices. While dysprosocenium molecules with various substituents at the arene ring exhibit a very large blocking temperature, the corresponding Er(III) analogues do not, and this is reversed if the size of the arene ring is eight. Using a combination of ab initio CASSCF and DFT-based molecular dynamics (MD) study, we have explored 25 Dy(III)/Er(III)/Ho(II)/Tb(II)/Dy(II) arene complexes with the ring size varying from 4 to 8 to understand the differences observed and decipher the correlation of structure to the spin dynamics behavior. Among the oxidation state of +2 complexes studied, Tb(II) exhibits the highest barrier, with the Cp-Tb-Cp angle being linear. Further, one of the four-membered arene model studied exhibits a very large barrier of 1442 cm-1, suggesting a potential high-blocking SIM. While bulky substituents at the arene ring help increase the axiality and the CR-Ln-CR angle, this also fetches several agostic C-H···Ln interactions, which injects transverse anisotropy. Furthermore, MD coupled with the CASSCF study reveals that the fluxional behavior of the arene ring generates several rotational conformers that are even accessible at lower temperatures offering a shortcut to the magnetization relaxation process. The importance of structural fluctuations in controlling the magnetic anisotropy by choosing apt metal-ion/ring partners and the corresponding substituents has been highlighted to offer clues to the futuristic SIM design.

In silico design to enhance the barrier height for magnetization reversal in Dy(iii) sandwich complexes by stitching them under the umbrella of corannulene.

Lanthanide based single molecular magnets (SMMs), particularly dysprocenium based SIMs, are well known for their high energy barrier for spin reversal (U eff) and blocking temperatures (T B). Enhancing these two parameters and at the same time obtaining ambient stability is key to realising end-user applications such as compact storage or as qubits in quantum computing. In this work, by employing an array of theoretical tools (DFT, ab initio CASSCF and molecular dynamics), we have modelled six complexes [(η5-corannulene)Dy(Cp)] (1), [(η5-corannulene)Dy(C6H6)] (2), [(η6-corannulene)Dy(Cp)] (3), [(η6-corannulene)Dy(C6H6)] (4), [(exo-η5-corannulene)Dy(endo-η5-corannulene)] (5), and [(endo-η5-corannulene)Dy(endo-η5-corannulene)] (6) containing corannulene as a capping ligand to stabilise Dy(iii) half-sandwich complexes. Our calculations predict a strong axiality exerted by the Dy-C interactions in all complexes. Ab initio calculations predict a very large barrier height for all six molecules in the order 1 (919 cm-1) ≈ 3 (913 cm-1) > 2 (847 cm-1) > 4 (608 cm-1) ≈ 5 (603 cm-1) ≈ 6 (599 cm-1), suggesting larger barrier heights for Cp ring systems, followed by six-membered arene systems and then corannulene. DFT based molecular dynamics calculations were performed on complexes 3, 5 and 6. For complexes 3 and 5, the geometries that are dynamically accessible are far fewer. The range of U eff computed for molecular dynamics snapshots is high, indicating a possibility of translating the large U eff obtained into attractive blocking temperatures in these complexes, but the converse is found for 6. Furthermore, an in-depth C-H bond vibrational analysis performed on complex 3 suggests that the vibration responsible for reducing the blocking temperature in dysprocenium SIMs is absent here as the C-H bonds are stronger and corannulene steric strain prevents the C(Cp)-Dy-C(Cor) bending. As [(η6-corannulene)TM(X)]+ (TM = Ru, Zr, Os, Rh, Ir and X = C5Me5, C6Me6) are known, the predictions made here have a higher prospect of yielding stability under ambient conditions, a very large U eff value and a high blocking temperature - a life-giving combination to new generation SMMs.

A Design Criteria to Achieve Giant Ising-Type Anisotropy in CoII -Encapsulated Metallofullerenes.

Discovery of permanent magnetisation in molecules just like in hard magnets decades ago led to the proposal of utilising these molecules for information storage devices and also as Q-bits in quantum computing. A significant breakthrough with a blocking temperature as high as 80 K has been recently reported for lanthanocene complexes. While enhancing the blocking temperature further remains one of the primary challenges, obtaining molecules that are suitable for the fabrication of the devices sets the bar very high in this area. Encouraged by the fact that our earlier predictions of potential single-molecule magnets (SMMs) in lanthanide-containing endohedral fullerenes have been verified, here we set out to undertake a comprehensive study on CoII -ion-encapsulated fullerene as potential SMMs. To study this class of molecules, we have utilised an array of theoretical methods ranging from density functional to ab initio CASSCF/NEVPT2 methods for obtaining reliable estimate of zero-field splitting parameters D and E. Additionally, we have also employed, for the first time a combination of molecular dynamics based on DFT methods coupled with CASSCF/NEVPT2 methods to seek the role of conformational isomers in the relaxation of magnetisation. Particularly, we have studied, Co@C28 , Co@C38 and Co@C48 cages and their isomers as potential target molecules that could yield substantial magnetic anisotropy. Our calculations categorically reveal a very large Ising anisotropy in this class of molecules, with Co@C48 cages predicted to yield D values as high as -127 cm-1 . Our calculations on the smaller cages reveal the free movement of CoII ion inside the cage, leading to the likely scenario of faster relaxation of magnetisation. However, larger fullerene cages were found to solve this issue. Further models with incorporating units such as {CoOZn}, {CoScZnN} inside larger fullerenes yield axial zero-field splitting values as high as -200 cm-1 with negligible E/D values. As these units represent a strong axiality coupled with a viable way to obtain air-stable low-coordinate CoII complexes, this opens up a new paradigm in the search of SMMs in this class of molecules.

Confinement induced binding of noble gas atoms.

The stability of Ngn@B12N12 and Ngn@B16N16 systems is assessed through a density functional study and ab initio simulation. Although they are found to be thermodynamically unstable with respect to the dissociation of individual Ng atoms and parent cages, ab initio simulation reveals that except Ne2@B12N12 they are kinetically stable to retain their structures intact throughout the simulation time (500 fs) at 298 K. The Ne2@B12N12 cage dissociates and the Ne atoms get separated as the simulation proceeds at this temperature but at a lower temperature (77 K) it is also found to be kinetically stable. He-He unit undergoes translation, rotation and vibration inside the cavity of B12N12 and B16N16 cages. Electron density analysis shows that the He-He interaction in He2@B16N16 is of closed-shell type whereas for the same in He2@B12N12 there may have some degree of covalent character. In few cases, especially for the heavier Ng atoms, the Ng-N/B bonds are also found to have some degree of covalent character. But the Wiberg bond indices show zero bond order in He-He bond and very low bond order in cases of Ng-N/B bonds. The energy decomposition analysis further shows that the ΔEorb term contributes 40.9% and 37.3% towards the total attraction in the He2 dimers having the same distances as in He2@B12N12 and He2@B16N16, respectively. Therefore, confinement causes some type of orbital interaction between two He atoms, which akins to some degree of covalent character.

Bohmian trajectory from the "classical" Schrödinger equation.

The quantum-classical correspondence is studied for a periodically driven quartic oscillator exhibiting integrable and chaotic dynamics, by studying the Bohmian trajectory of the corresponding "classical" Schrödinger equation. Phase plots and the Kolmogorov-Sinai entropy are computed and compared with the classical trajectory as well as the Bohmian trajectory obtained from the time dependent Schrödinger equation. Bohmian mechanics at the classical limit appears to mimick the behavior of a dissipative dynamical system.

Molecular reactivity dynamics in a confined environment.

Time evolution of various reactivity parameters viz. hardness, electrophilicity, chemical potential, polarizability, etc. in a confined environment has been studied through quantum fluid density functional theory formalism during time dependent processes such as proton-molecule collisions and molecule-field interaction. A Dirichlet type boundary condition has been incorporated to confine the systems. Responses in the reactivity parameters of the diatomic molecules, in the dynamical context, in ground state as well as in excited state, have been reported. Harmonic spectra are generated in the cases of the external laser field interacting with H2 and N2 molecules.

A tug-of-war between electronic excitation and confinement in a dynamical context.

Quantum fluid density functional theory has been used to study the time evolution of various reactivity parameters such as hardness, electrophilicity, entropy, chemical potential, polarizability, electronegativity etc. in a confined environment during time dependent processes like atom-ion collision and atom-field interaction. Responses in the reactivity parameters of the helium atom, in the dynamical context, for ground state as well as in excited state, have been reported. The confinement is incorporated through a Dirichlet type boundary condition. With a decrease in the size of the cylindrical box, the system gets harder and less polarizable. Simultaneous excitation and confinement may bring back the ground state behavior.