STRUCTURES AND BINDING-ENERGIES OF EM(6) CLUSTERS (E=C, SI, GE, SN, PB, M=LI, NA, K, RB, CS) - PBNA6 IS NOT UNIQUE

A computational study of twenty-four EM(6) clusters is described (E = C, Si, Ge, Sn or Pb; M = Li, Na, K, Rb or Cs). Almost all are octahedral. Total binding energies, obtained at the DZP/QCISD(T) level of theory using pseudopotential methods, range from 446 to 1108 kJ/mol and decrease as both E and M increase in size. Several EM(4) and EM(5) systems were also studied, together with PbNa7 and PbNa8. Loss of one M atom from EM(6) requires more energy than from EM(5). PbNa7 and PbNa8 are relatively weakly bound. Trends in structures and binding energies are analysed. PbNa6, detected by mass spectrometry [Chem. Phys. Letters 237 (1995) 334], is not unique, either geometrically, thermodynamically, or for its electronic structure; possible relationships with CLi6 are discussed. The detection of many more EM(6) clusters can be anticipated.