ABSTRACT
Magnetoresponsive three-membered rings of d- and f-block elements have been thoroughly investigated with the help of electronic structure calculation methods. The magnetic response of the clusters was evaluated by the Nucleus Independent Chemical Shifts (NICS)(zz)-scan curves, which in conjunction with symmetry-based selection rules for the most significant translationally and rotationally allowed transitions helped rationalize and predict the orbital-type of aromaticity/antiaromaticity of the clusters. The magnetoresponsive early (Groups 3, 4, and 5) transition metal M(3) rings exhibit successive aromatic and antiaromatic zones separated by a nodal plane. The magnetoresponsive late (Groups 11 and 12) transition metal M(3) rings exhibit long-range aromatic zone with the NICS(zz)(R) values decaying rapidly and monotonically with respect to R. The magnetic response of Group 10 transition metal M(3) rings is similar to that of the early transition metal M(3) rings, but it is long-range antiaromatic only for the [c-Ni(3)] cluster. The NICS(zz)-scan curve of the [(H(t)La)(3)(mu(2)-H)(6)] cluster is indicative of weak pure sigma-aromaticity due to the induced diatropic ring current from the translationally allowed a(1)(') --> e' and e' --> a(1)(') transitions. The aromatic-antiaromatic behavior of the [(H(t)Ce)(3)(mu(2)-H)(6)](+) and [(H(t)Tm)(3)(mu(2)-H)(6)](2-) clusters is similar to that of the early d-block elements. The magnetic response of [(H(t)Yb)(3)(mu(2)-H)(6)](3-) is similar to that of [c-Hg(3)](2-). The [(H(t)Lu)(3)(mu(2)-H)(6)] cluster can be considered as a doubly (sigma + pi) aromatic system, with the sigma-aromatic component being much stronger than the pi-aromatic one. Finally, the [(X(t)Re)(3)(mu(2)-X)(6)] and [(X(t)Ru)(3)(mu(2)-X)(6)](+) (X = Cl, Br, I) clusters exhibit significant aromatic character with the greatest contribution to the induced diatropic ring currents coming from pi-type transitions.
ABSTRACT
Bimetallic Cu(3)Au(3) clusters have been investigated using electronic structure calculation techniques (DFT) to understand their electronic, magnetic, and optical properties as well as the geometrical structures. The most stable homotop is the planar cyclo-[Cu(3)(micro-Au)(3)] form consisting of a triangular positively charged Cu(3) structural core with negatively charged Au atoms occupying exposed positions. This structure is characterized by the maximum number of heterobonds and peripheral positions of Au atoms. Possible growth formats of the cyclo-[Cu(3)(micro-Au)(3)] homotops have been explored following both the edge-capping and the stepwise metal atom substitution mechanism. The bonding pattern along with the density of states (DOS) plots of the cyclo-[Cu(3)(micro-Au)(3)] homotop are thoroughly analyzed and compared with those of the pure cyclo-[Cu(3)(micro-Cu)(3)] and cyclo-[Au(3)(micro-Au)(3)] clusters. Particular attention was paid on the stability of these bimetallic clusters in relation with the ring-shaped electron density distribution (aromaticity). It was found that all 3-membered metal rings exhibit significant aromatic character, which was verified by a number of established criteria of aromaticity, such as structural, energetic, magnetic (NICS profiles), and out-of-plane ring deformability criteria. The NICS (1) values correlate well with the out-of-plane ring deformation energy. Finally, a comprehensive analysis of the optical spectra of the CuAu, Cu(2), and Au(2) diatomics and the cyclo-[Cu(3)(micro-Au)(3)], cyclo-[Cu(3)(micro-Cu)(3)], and cyclo-[Au(3)(micro-Au)(3)] clusters placed the electronic assignments of the optical transitions on a firm footing.
