Molecular clusters Cs3X3 and Cs4X4 (X = Br, I): Quantum chemical study of structure and thermodynamic properties

Abstract: The properties of trimer Cs3X3 and tetramer Cs4X4 (X = Br, I) molecules

have been studied using DFT with B3LYP5 functional and MP2 and MP4 methods.

Two equilibrium geometrical structures of trimers, hexagonal (D3 h) and “butterflyshaped”

(Cs), and one for tetramers, distorted cubic (Td), are confirmed to exist;

geometrical parameters and vibrational spectra are determined. The relative

concentration of Cs3X3 isomers has been evaluated; the butterfly-shaped isomer

dominates over hexagonal in saturated vapour in a broad temperature range. The

dissociation reactions through different channels have been considered and enthalpies

of formation ΔfH°(0) of clusters determined:−858 ± 20 kJ mol−1 (Cs3Br3),

−698 ± 20 kJ mol−1 (Cs3I3), −1270 ± 30 kJ mol−1 (Cs4Br4) and −1045 ± 30 kJ mol−1

(Cs4I4). The Gibbs free energies ΔrG°(T) calculated for the dissociation reactions of

trimer and tetramer molecules have indicated that these molecules are resistive in

narrow temperature range only and decompose spontaneously with temperature

increase with elimination of dimer molecules.

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