Ring Currents in Polycyclic Sodium Clusters

Abstract

In the recent work by Khatua et al. [Khatua, S.; Roy, D. R.; Bultinck, P.; Bhattacharjee, M.; Chattaraj, P. K. Phys. Chem. Chem. Phys. 2008, 10, 2461-2474 the synthesis and structure of a fac-trioxo molybdenum metalloligand and its sodium complex containing 1D hexagonal chains of sodium ions was reported. In the same paper, the aromaticity of hexagonal Na-clusters was quantified by means of the nucleus-independent chemical shift and electronic multicentre indices. It was shown that the aromaticity of hexagonal Na-clusters is of the same order as the aromaticity of analogous benzenoid hydrocarbons. In the present study current density maps are used to rationalize the aromaticity of polycyclic Na-clusters. It is shown that although polycyclic Na-systems sustain a diatropic ring current, the induced current density is several times weaker than in analogous benzenoid hydrocarbons. A detailed analysis indicates that the current density in hexagonal Na-systems is almost completely determined by four HOMO σ-electrons. 1,2 the concept of aromaticity was extended from its usual organic realm to the field of all-metal inorganic clusters. Compared to the aromaticity encountered in organic molecules, primarily being of the traditional π-type, all-metal systems can exhibit a multifold aromaticity [3][4][5] and conflicting aromaticity, 2,3,[6][7][8][9] arising from their σ-, π-, δ- The problem with claims that a certain species is aromatic is that this conclusion may depend rather significantly on the property used to characterize aromaticity. As an example: a ring that exhibits bond length equalization as in benzene but does not sustain a ring current could be classified by some as aromatic but by others as not aromatic depending on the relative importance they attach to the property of bond lengths equalization or ring currents respectively. The same is true for e.g., electron delocalization and ring currents. Although one of us has previously shown that electron delocalization and ring currents in carbohydrates go hand in hand, 20-23 the more general and subtle interrelationship is that a delocalized system is a necessary but not sufficient requirement for a ring current. In previous work on Na 6 systems, electron delocalization was gauged by the multicentre index and information on a ring current was obtained indirectly from NICS values. The problem with NICS, however, is that although usually a negative (or aromatic) NICS value reflects the 3 existence of a ring current, there is no direct way to extract and thus beyond doubt prove the existence of an underlying ring current. INTRODUCTION COMPUTATIONAL METHODSThe structures of the Na 4n+2 (n = 1-5) entities (Figure1) were taken from the experimental structural data given by Khatua et al. 13,14 The molecular structures of the linear polyacenes C 4n+2 H 2n+4 (n = 1-5) were optimized at the B3LYP/6-311+G* level.Computed Hessian matrices showed the optimized structures to correspond to minima on the potential energy surface.The current density maps presented in this pap...

Heterotrimetallic compounds containing Mo–M–Li [M = K, Rb and Cs] clusters: synthesis, structure, bonding, aromaticity and theoretical investigations of Li2M2 [M = K and Rb] and Cs4 rings

“…It has been shown that by using basis sets with an effective core potential, one is able to reduce the impact of the core electrons thereby allowing a more efficient analysis of global current densities in Na-clusters. 76 In addition, NICS values calculated using an effective core potential are in good agreement with all electron NICS values. 76 In all calculations a unit magnetic field perpendicular to the molecular plane was used and the calculated ring currents were plotted on a grid in a set of planes parallel to the molecular plane with a diatropic current represented by a counterclockwise circulation.…”

Aromaticity in heterocyclic analogues of benzene: Dissected NICS and current density analysis

“…This index became popular rapidly because of its easy computation in practically all quantum chemical packages. However, its indiscriminate use has often been criticized . These criticisms have pointed out to the validity of NICS being limited by spurious contributions from the in‐plane tensor components, which are not related with the aromaticity phenomenon.…”

“…However, its indiscriminate use has often been criticized. [18][19][20][21][22][23][24][25][26] These criticisms have pointed out to the validity of NICS being limited by spurious contributions from the in-plane tensor components, which are not related with the aromaticity phenomenon. To avoid these issues, different ways to analyze NICS have been suggested.…”

The intriguing class of altan‐molecules

“…A growing community of researchers is involved in the computation and visualization of ring currents . Yet, at odds with what happened long ago with the chemical shift scale, international committees have not suggested any convention either for their graphical representation or for their quantitative measures, and therefore, care is always needed when looking or comparing maps or tables to understand the connection between direction of arrows, numerical values and tropicities.…”