Alkali-Metal Trihalides: M+X3– Ion Pair or MX–X2 Complex?

The alkali-metal trihalides MX3 (M = Li, Na, K, Rb, and Cs; X = Cl, Br, and I) are systematically studied using coupled-cluster methods. Benchmarks using CCSD(T) against diatomic experimental results suggest satisfactory performance for the weighted core-valence basis sets (new basis sets for K, Rb, and Cs) selected for predicting reliable structures and harmonic vibrational frequencies. An isomer search using the B3LYP functional yields a planar, yet asymmetric T-shaped Cs structure as the global minimum for all MX3 species. Much higher level CCSD(T) computations show a moderate to strong distortion of the X3– anion by the M+ cation in the respective equilibrium geometries. Most obviously, for LiCl3, the two Cl–Cl distances are separated by 0.786 A. Even for CsI3, the structure least distorted from the M+X3– model, the two I–I distances differ by 0.243 A. It does not take much energy to distort the parent anions along an antisymmetric stretch, so this is no surprise. The normal modes of vibration of the ...