0022-4766/15/5606-1223 © 2015 by Pleiades Publishing, Ltd. 1223 Journal of Structural Chemistry. Vol. 56, No. 6, pp. 1223-1225, 2015. Original Russian Text © 2015 E. B. Bartashevich, S. E. Nasibullina, V. G. Tsirelson. ELECTRON DELOCALIZATION INDICES AS CRITERIA FOR THE IDENTIFICATION OF STRONG HALOGEN BONDS OF IODINE E. B. Bartashevich 1 , S. E. Nasibullina 1 , and V. G. Tsirelson 2 UDC 541.57:541.27 It is found that electron delocalization indices can be efficiently used as a sorting tool for the classification of bonds between iodine atoms in a wide range of various covalent and noncovalent I…I/I–I interactions. The characteristics of bonds in a symmetrical triiodide anion can serve as benchmarks allowing to distinguish the I…I halogen bond from the I–I covalent bond. DOI: 10.1134/S0022476615060360 Keywords: electron delocalization indices, halogen bonds, triiodide. The interaction of halogens resulting from the predominant electrostatic attraction between the lone pairs of one halogen and the region with higher electrostatic potentials along the covalent bond of another (R–Hal + …Hal – –R) was called the halogen bonding [1]. These bonds play an important structure-forming role in many halogen-containing molecular crystals. Under that logic, one can assume that a bond strengthened by a charge in a triiodide anion should be considered as the strongest I…I halogen bond. In such a case, electron delocalization is observed over the whole subunit, and all the three iodine atoms contribute to the frontier molecular orbitals [2]. The experimental mass spectrometric determination of the dissociation energy of the triiodide anion ( 3 I - → I 2 + I – ) yields a value D e = –126.4 kJ/mol [3]. It should be noted that a similar benchmark system for a hydrogen bond is the [F…H…F] – anion (D e = –155 kJ/mol [4]). Quantum chemical calculations of the interaction energy of iodine subunits (I 2 and I – ) in the triiodide anion indicate a wide range of values: from –113.9 kJ/mol (CCSD(T) ECP-TZ(2df )) up to –174.0 kJ/mol (B3LYP/LACVP*) [5]. The spread of these values is due to the usage of different methods of calculations. Moreover, there is a problem of attributing bonds to specific classes, which is directly related to the fact that the characteristics of the [I…I] – bond can vary with the geometry optimization of the complex or cluster removed from the continuum of its environment. Thus, for instance, the equilibrium in the gas phase configuration of the complex of the triiodide anion with a heterocyclic cation becomes more similar to the С–I…[I] – …I–I structure rather than to С–I…[I–I–I] – . Therefore, it is needed to determine the upper limit of stability, which makes it possible to distinguish a strong halogen bond from a covalent one. In the present work we have studied the behavior of the electron delocalization indices δ(I, I) for a wide series of I–I/I…I interactions and the possibility to use them as benchmarks allowing to distinguish a halogen bond strengthened by the I…I – charge from a covalent I–I bond. Experimental. The delocalization index [6, 7] corresponds to the number of electrons delocalized or shared between atoms А and В. In the one-electron approximation the delocalization indices are determined by calculating the overlap matrix 1 South Ural State University (National Research University), Chelyabinsk, Russia; kbartash@yandex.ru. 2 Mendeleev University of Chemical Technology, Moscow, Russia. Translated from Zhurnal Strukturnoi Khimii, Vol. 56, No. 6, pp. 1279-1281, November-December, 2015. Original article submitted April 14, 2015.