Ganga Periyasamy

Computational studies on magnetism and the optical properties of transition metal embedded graphitic carbon nitride sheets

Using density functional theory (DFT), we have explored the structural, electronic, magnetic and optical properties of two-dimensional 3d-transition metal (TM)-embedded graphitic carbon nitride (g-C3N4) sheets. g-C3N4 sheets are structurally modified in different ways depending upon the radius of embedded-TM atoms and the crystal field stabilization energy gained by the corresponding geometry. Bare g-C3N4, which is a wide-gap semiconductor, becomes metallic upon TM inclusion. The d-orbitals of TMs hybridize with the pπ-orbitals of the g-C3N4 framework and close the band gap in TM-embedded g-C3N4 (TM-g-C3N4). Interestingly, for V, Cr and Fe embedded g-C3N4, the TM atoms interact ferromagnetically to each other and result in a ferromagnetic ground state. However, Mn couples antiferromagnetically and Cu and Zn are nonmagnetic in the ground state of their corresponding TM-g-C3N4 sheets. Because of structural distortion, Co- and Ni-g-C3N4 do not have a well-ordered magnetic orientation. Performing Heisenberg-model-based Monte Carlo simulations, we predict that V-, Cr- and Fe-g-C3N4 would possess Curie temperatures (Tc) of 205 K, 170.5 K, and 115 K, respectively. Furthermore, these modified g-C3N4 sheets also show prominent absorption at low energy, which evidently confirms their efficient photoabsorption capacity. The present study demonstrates the multifunctional behavior of TM-g-C3N4, which shows significant promise for application in various fields such as in memory devices or for photocatalysis.

On the ultrafast charge migration and subsequent charge directed reactivity in Cl⋯N halogen-bonded clusters following vertical ionization

In this article, we have presented ultrafast charge transfer dynamics through halogen bonds following vertical ionization of representative halogen bonded clusters. Subsequent hole directed reactivity of the radical cations of halogen bonded clusters is also discussed. Furthermore, we have examined effect of the halogen bond strength on the electron-electron correlation- and relaxation-driven charge migration in halogen bonded complexes. For this study, we have selected A-Cl (A represents F, OH, CN, NH2, CF3, and COOH substituents) molecules paired with NH3 (referred as ACl:NH3 complex): these complexes exhibit halogen bonds. To the best of our knowledge, this is the first report on purely electron correlation- and relaxation-driven ultrafast (attosecond) charge migration dynamics through halogen bonds. Both density functional theory and complete active space self-consistent field theory with 6-31 + G(d, p) basis set are employed for this work. Upon vertical ionization of NCCl⋯NH3 complex, the hole is predicted to migrate from the NH3-end to the ClCN-end of the NCCl⋯NH3 complex in approximately 0.5 fs on the D0 cationic surface. This hole migration leads to structural rearrangement of the halogen bonded complex, yielding hydrogen bonding interaction stronger than the halogen bonding interaction on the same cationic surface. Other halogen bonded complexes, such as H2NCl:NH3, F3CCl:NH3, and HOOCCl:NH3, exhibit similar charge migration following vertical ionization. On the contrary, FCl:NH3 and HOCl:NH3 complexes do not exhibit any charge migration following vertical ionization to the D0 cation state, pointing to interesting halogen bond strength-dependent charge migration.

Structure models for the hydrated and dehydrated nitrate-intercalated layered double hydroxide of Li and Al

Imbibition of LiNO3 into gibbsite results in the formation of a single phase layered double hydroxide of the composition LiAl2(OH)6(NO3)·1.2H2O. This phase undergoes reversible dehydration along with the compression of the basal spacing accompanied by the reorientation of the nitrate in the interlayer gallery. The hydrated phase is a solid solution of two lattices: (i) a hexagonal lattice defining the ordering of atoms within the metal hydroxide layer, and (ii) a lattice of orthorhombic symmetry defining the ordering of atoms within the interlayer. DFT calculations of the hydration behaviour show that there is no registry between the two sublattices. In the dehydrated phase, the nitrate ion is intercalated with its molecular plane parallel to the metal hydroxide layer and the crystal adopts a structure of hexagonal symmetry.

DFT studies on the influence of ligation on optical and redox properties of bimetallic [Au4M2] clusters

Density functional theory based calculations have been employed to understand the lowest energy conformers of bare [Au4M2] and ligated [Au4M2(SCH3)6] and [Au4M2(PH3)6]2+ (where M = Au, Cu, Ag, Ni, Pd, Pt) clusters in the gas phase and in various implicit solvent media (water, DMSO and DCM). Computations predict that [Au4M2] clusters in all three charge states exist with a planar 2D-geometry, with distortion introduced by the hetero atoms. And the studies show that the ligation promotes a 2D to 3D geometrical conversion either through bridging coordination or single bond formation. The sulfur atom in the thiol ligand becomes a part of the cluster skeleton, while the PH3 forms a passivation layer around the cluster. Moreover, the presence of sulfur in the cluster skeleton increases the chemical stability of coinage metal containing clusters, while stability decreases for d8 metal containing clusters. And the PH3 passivation layer decreases the chemical stability of both coinage metal and d8 metal atom containing clusters. The computed redox behaviors show that the addition of an electron requires less energy compared to that needed for removal, and both occur with negligible geometrical reorganisation. The calculated blue shift in excitation energy values show a ligand to metal charge transfer in the –SCH3 ligated cluster. However, the red shift in wavelength is observed for the –PH3 passivated cluster, which corresponds to excitation from the HOMO to LUMO, where the orbitals have an equal contribution from the metals and ligands.

Density functional theoretical studies on electronic structural, optical and oxidation properties of thioxylated peptides

Thioxylated peptide bond in simple dipeptides and tripeptides is found to amend the structural, optical and redox properties of the same relying on the position of substitution (N- and C-terminals). The electronic structural investigations show that the geometrical parameters are found to be highly influenced by the thioxo substitution at N-terminus. However, the absorbance properties are influenced by thioxo substitution at C-terminal, where blue shifts in π–π* transitions are observed compared to unsubstituted one. Similarly, the cationization reveals that the substitution at C-terminal promotes the conformational shift. Further, the computational studies clearly indicate that the N-terminal substitution effects the ground state geometry, while substitution at other position influences the excited and redox properties. The observed conformation switches in C-terminal conformers have been proposed as a candidate for redox switch, where the states (neutral and cation) were probed using computed vibrational data and chemical shift values.