Cina Foroutan-Nejad

All-Metal Aromaticity: Revisiting the Ring Current Model among Transition Metal Clusters

We present new insight into the nature of aromaticity in metal clusters. We give computational arguments in favor of using the ring-current model over local indices, such as nucleus independent chemical shifts, for the determination of the magnetic aromaticity. Two approaches for estimating magnetically induced ring currents are employed for this purpose, one based on the quantum theory of atoms in molecules (QTAIM) and the other where magnetically induced current densities (MICD) are explicitly calculated. We show that the two-zone aromaticity/antiaromaticity of a number of 3d metallic clusters (Sc3(-), Cu3(+), and Cu4(2-)) can be explained using the QTAIM-based magnetizabilities. The reliability of the calculated atomic and bond magnetizabilities of the metallic clusters are verified by comparison with MICD computed at the multiconfiguration self-consistent field (MCSCF) and density functional levels of theory. Integrated MCSCF current strength susceptibilities as well as a visual analysis of the calculated current densities confirm the interpretations based on the QTAIM magnetizabilities. In view of the new findings, we suggest a simple explanation based on classical electromagnetic theory to explain the anomalous magnetic shielding in different transition metal clusters. Our results suggest that the nature of magnetic aromaticity/antiaromaticity in transition-metal clusters should be assessed more carefully based on global indices.

A Dissected Ring Current Model for Assessing Magnetic Aromaticity: A General Approach for both Organic and Inorganic Rings

A model based on classical electrodynamics is used to measure the strength of ring currents of different molecular orbitals, i.e., σ‐ and π‐orbitals, and characteristics of ring current loops, i.e., ring current radii and height of current loops above/below the ring planes, among a number of organic as well as inorganic molecules. For the π‐current, the present model represents an improvement of previous approaches to determine ring current intensity. It is proven that the present model is more precise than previous models as they could not explain presence of the minimum in the plot of NICSπzz versus distance close to the ring plane. Variations in the charge of molecules and the types of constituent atoms of each species affect the ring current radii of both σ‐ and π‐current loops as well as the height of π‐current loops above/below the ring plane. It is suggested that variation in the distribution of the one‐electron density in different systems is the main source of differences of the ring current characteristics.

Understanding the electronic factors responsible for ligand spin-orbit NMR shielding in transition-metal complexes

The significant role of relativistic effects in altering the NMR chemical shifts of light nuclei in heavy-element compounds has been recognized for a long time; however, full understanding of this phenomenon in relation to the electronic structure has not been achieved. In this study, the recently observed qualitative differences between the platinum and gold compounds in the magnitude and the sign of spin-orbit-induced (SO) nuclear magnetic shielding at the vicinal light atom ((13)C, (15)N), σ(SO)(LA), are explained by the contractions of 6s and 6p atomic orbitals in Au complexes, originating in the larger Au nuclear charge and stronger scalar relativistic effects in gold complexes. This leads to the chemical activation of metal 6s and 6p atomic orbitals in Au complexes and their larger participation in bonding with the ligand, which modulates the propagation of metal-induced SO effects on the NMR signal of the LA via the Spin-Orbit/Fermi Contact (SO/FC) mechanism. The magnitude of the σ(SO)(LA) in these square-planar complexes can be understood on the basis of a balance between various metal-based 5d → 5d* and 6p → 6p* orbital magnetic couplings. The large and positive σ(SO)(LA) in platinum complexes is dominated by the shielding platinum-based 5d → 5d* magnetic couplings, whereas small or negative σ(SO)(LA) in gold complexes is related to the deshielding contribution of the gold-based 6p → 6p* magnetic couplings. Further, it is demonstrated that σ(SO)(LA) correlates quantitatively with the extent of M-LA electron sharing that is the covalence of the M-LA bond (characterized by the QTAIM delocalization index, DI). The present findings will contribute to further understanding of the origin and propagation of the relativistic effects influencing the experimental NMR parameters in heavy-element systems.

Is NICS a reliable aromaticity index for transition metal clusters

Abstract In the present account the nature of aromaticity/antiaromaticity of fourteen metallic complexes/clusters are reexamined. These species were classified as aromatic by means of different nucleus independent chemical shift (NICS) based approaches, previously. Visualization of the current density and magnetizability of atomic basins reveals that none of the studied systems are magnetic aromatic, i.e. sustain diamagnetic ring current. It is demonstrated that negative NICS values near the ring plane of the studied molecules originates from remarkably strong local paramagnetic current around their transition metal atom nuclei. This phenomenon has been observed only for Sc3− all-metal cluster but current study demonstrates that the influence of the local paramagnetic currents around transition metal atoms on NICS is a general phenomenon that must be carefully considered prior to classification of the metallic systems as aromatic. Furthermore, this study suggests that NICS is not a reliable aromaticity index for transition-metal clusters/molecules.

The Laplacian of Electron Density versus NICSzz Scan: Measuring Magnetic Aromaticity among Molecules with Different Atom Types

The electron density versus NICS(zz) (the out-of-plane component of nucleus-independent chemical shifts (NICS)) scan for assessing magnetic aromaticity among similar molecules with different ring sizes is improved by scanning the Laplacian of electron density versus NICS(zz) to include molecules containing different types of atoms. It is demonstrated that the new approach not only reproduces the results of the previous method but also surpasses that in the case of species with different atom types. The relative positions of curves in the plots of the Laplacian of electron density versus NICS(zz) correlate well with the ring current intensities of these molecules both near and far from the ring planes of the considered molecules. Accordingly, relative magnetic aromaticity of a number of planar hydrocarbons and a group of double aromatic metallic/semimetallic species are studied and discussed.

Reply to the ‘Comment on “The electron density vs. NICS scan: a new approach to assess aromaticity in molecules with different ring sizes”’ by A. Stanger, Phys. Chem. Chem. Phys., 2011, 13, DOI: 10.1039/c0cp02407d

Accordingly, describing our proposed methodology as a ‘‘two-dimensional’’ NICS scan he has criticized this approach basedon the following question: ‘‘if a two dimensional description ofaromaticity (and antiaromaticity) is wished, the r at and abovethe (Ring Critical Point) RCP (which is the parameter that FSR(Foroutan-Nejad, Shahbazian, Rashidi-Ranjbar) chose) shouldalso show a different behaviour for aromatic vs. antiaromaticsystems. Is this really the case (in the r vs. NICS approach)?’’(Italics are added by the present authors.)Throughouthiscomment, based on this hypothesis, the author tries to demon-strate that electron density cannot be used as a measure ofaromaticity.Furthermore,hehas tried toshowthat the amountof electron density in a point of space is mainly influenced bythe distance from nuclei and is not directly related to thenumber of p-electrons. Though Stanger rightly insists on somepoints regarding the relation of electron density and aromati-city, they are irrelevant to our proposed methodology andparticularly his primary question clearly reveals that he deeplymisinterpreted our motivations for introducing NICS

Supramolecular Covalence in Bifurcated Chalcogen Bonding

Supramolecular interactions are generally classified as noncovalent. However, recent studies have demonstrated that many of these interactions are stabilized by a significant covalent component. Herein, for systems of the general structure [MX6 ]2- :YX2 (M=Se or Pt; Y=S, Se, or Te; X=F, Cl, Br, I), featuring bifurcated chalcogen bonding, it is shown that, although electrostatic parameters are useful for estimating the long-range electrostatic component of the interaction, they fail to predict the correct order of binding energies in a series of compounds. Instead, the Lewis basicity of the individual substituents X on the chalcogen atom governs the trends in the binding energies through fine-tuning the covalent character of the chalcogen bond. The effects of substituents on the binding energy and supramolecular electron sharing are consistently identified by an arsenal of theoretical methods, ranging from approaches based on the quantum chemical topology to analytical tools based on the localized molecular orbitals. The chalcogen bonding investigated herein is driven by orbital interactions with significant electron sharing; this can be designated as supramolecular covalence.

Modulating Electron Sharing in Ion-π-Receptors via Substitution and External Electric Field: A Route toward Bond Strengthening.

Substituted coronenes, a family of ion-π receptors whose ion-affinities can be explained exclusively neither via ion-quadrupole nor induction/polarization mechanisms, are studied. The best descriptors of ion-affinity among these species are those characterizing charge-transfer between ions and the π-systems, e.g. vertical ionization potential, electron affinity, and the relative energies of charge-transfer excited-states (CTESs). The variation of the electric multipole moments, polarizability, binding energy, and relative energy of CTESs in the presence of an external electric field (EEF) is evaluated. The results indicate that the EEF has a negligible effect on the polarizability and quadrupole moment of the systems. However, it significantly affects the binding energies, CTES energies, and the dipole moments of the receptors. Contrary to the changes in the dipole moment, the variation pattern of the binding energy is more consistent with the pattern observed for the CTES energy changes. Finally, by analyzing the exchange-correlation component of the binding energy we demonstrate that the increased binding energy, i.e. bond strengthening, originates from enhanced electron sharing and multicenter covalency between the ions and the π-systems as a result of the state-mixing between the ground-state and the CTESs. According to our findings, we hypothesize that the electron sharing and in extreme cases the multicenter covalency are the main driving forces for complexation of ions with extended π-receptors such as carbon nanostructures.

Method/basis set dependence of NICS values among metallic nano-clusters and hydrocarbons

The influence of various all-electron basis sets and effective core potentials employed along with several DFT functionals (B3LYP, B3PW91, BLYP, BP86 and M06) on the magnitude of nucleus independent chemical shift (NICS) values in different metallic nano-clusters and hydrocarbons is studied. In general, it is demonstrated that the NICS values are very sensitive to the applied method/basis set; however, the method/basis set dependence is more prominent for computed NICS values in transition metal clusters. In hydrocarbons, medium-size basis sets perform roughly similar to large basis sets in most cases. It is also found that NICS(0) values are more sensitive to the method/basis set variation compared to the NICS values computed at 1 or 2 Å above the ring plane. However, in many cases, no broad-spectrum regulation is found for the effect of basis set/method on the magnitude of NICS values. A detailed study showed that bond length alternation in a molecule has an insignificant effect on the magnitude of NICS values so the influence of method/basis sets on the magnitude of NICS values mostly arises from the different predicted ring current intensities at various computational levels.

Dipolar molecules inside C70: an electric field-driven room-temperature single-molecule switch

We propose a two-state electric field-driven room-temperature single-molecule switch based on a dipolar molecule enclosed inside ellipsoidal fullerene C70. We show that the two low-energy minima of the molecular dipole inside the C70 cage provide distinguishable molecular states of the system that can be switched by application of an external electric field.