Theoretical investigation of geometry and stability of small lithium-iodide Li_nI (n= 2-6) clusters

Abstract

We present theoretical investigation of the structural characteristics and stabilities of neutral and positively charged LinI (n = 2‐6) species. The structural isomers were found by using a randomized algorithm to search for minima structures, followed by B3LYP optimizations; the single‐point RCCSD(T)/cc‐pwCVTZ(‐PP) calculations were performed in order to compute relative energies, binding energies per atom, adiabatic and vertical ionization energies, and dissociation energies. Stability was compared to the pure lithium clusters; there is a typical odd‐even alternation; iodine doped clusters are more stable than pure lithium clusters. Lithium “cage” transfers its valence electron to the iodine atom to form neutral I−−Lin+ and cationic I−−Lin2+ clusters. An electron departures the lithium cage upon ionization. An important reason for the larger stability of closed‐shell species is the existence of the HOMO 3c/2e natural bond orbitals. © 2013 Wiley Periodicals, Inc.

Theoretical and experimental investigation of geometry and stability of small potassium‐iodide KnI (n = 2–6) clusters

“…Anyway, we can find some structural similarities with previously reported structures of pure potassium clusters . Finally, we came to the same conclusion as it was for lithium‐iodide clusters—the best way to describe the growth of these heterogeneous clusters is by adding one (or two, in special cases mentioned above) potassium atom to K n −1 I (0,+1) to obtain K n I (0,+1) clusters.…”

“…The most stable neutral cluster species up to the n = 5 favor planar structures. We obtained some similar structures to those published for Li n I ( n = 2–6) and Li n Cl ( n = 2–6), however, the potential energy surface seems even shallower, and since we included higher spin structures, more isomers are obtained. The change from planar to the 3D most stable structures takes place at six K atom, as in the case of Li clusters.…”

“…Comparing homogeneous and heterogeneous clusters, one can see that doping of potassium clusters with iodide significantly contribute to their stability. However, heterogeneous K n I clusters are less stable than corresponding Li n I clusters . Taking into account that the measured binding (cohesive) energy for bulk potassium ( n → ∞) is of 0.94 eV, one can conclude that BE for K n I clusters decrease to a minimum value for some n and then increase to the bulk limit, where iodide atom could be considered as impurity.…”

“…These clusters have an even number of electrons—they are closed‐shell species; on the contrary, the open‐shell species that have an odd number of electrons are relatively less stable: K 2 I, K 4 I, K 6 I, K 3 I + , and K 5 I + . We have already shown by means of the natural bond orbital (NBO) analysis, that in the case of Li n I clusters, the important reason for higher stability of closed‐shell species is the delocalization of the 3center/2electron type, which is more energetically favored than the typical 2center/2electron bonds. Although we have not used the NBO analysis in this work, we can assume that electronic structures of presented potassium iodide clusters are very similar to lithium iodide clusters, and that the reason for higher stability of the closed‐shell species is the same.…”

“…The lowest‐lying isomers of potassium‐iodide K n I ( n = 2–6) clusters were found by using a random‐kick procedure, which represents a variant of the stochastic search method, first reported by Saunders, and later modified by Tai and Nguyen . This randomized algorithm was already used for obtaining the lowest isomers of heterogeneous lithium clusters, Li n I ( n = 2–6) and Li n Cl m ( n > m, n = 1–6, m = 1–3). It consists of two parts.…”