Structural isomerism and thermodynamic properties of the methylzinc alkoxide molecules (MeZnOR)n (R = Me, tBu) and cations [(MeZnOMe)n]+ (n = 3, 4) studied by B3LYP and PCM calculations.

Zinc oxide is a semiconductor as well as a catalyst in important industrial processes, and alkylzinc alkoxide species of the type (RZnOR)(n) are precursors for the preparation of nanoscaled zinc oxide particles. In this work, the structures, thermodynamic properties, dipole moments, and molecular vibrations of substituted zinc oxide molecules and cations have been studied by density functional calculations at the B3LYP/6-31+G(2df,p) level. The neutral tetramers (MeZnOMe)(4) and (MeZnO(t)Bu)(4) are cubane-like species of T(d) symmetry in the gas phase, whereas the cation [(MeZnOMe)(4)](+) is approximately of C(3v) symmetry due to a Jahn-Teller distortion. For the neutral trimers (MeZnOMe)(3) and (MeZnO(t)Bu)(3) two structural isomers of almost identical energy were located on the potential energy surfaces: A closo-cluster with eight ZnO bonds is most stable both as isolated molecule and in a polarizable continuum, but a roof-shaped structure with seven ZnO bonds differs in enthalpy by less than 8 kJ mol(-1). In the case of the analogous cation [(MeZnOMe)(3)](+), the isomer with the roof-shaped Zn(3)O(3) skeleton is 31.7 kJ mol(-1) more stable in the gas phase than the cluster isomer. A particularly long Zn-C bond in the most stable cationic tri- and tetramers explains the loss of a methyl group, observed mass spectrometrically after electron impact ionization. The dissociation of the tetramers (MeZnOMe)(4) and (MeZnO(t)Bu)(4) into 4/3 trimers is endothermic and endergonic, but less so for the tert-butyl derivative compared to the all-methyl cluster. This destabilizing effect of tert-butyl substituents on the tri- and tetrameric clusters follows also from the larger internuclear distances between the non-hydrogen atoms in the structures of (MeZnO(t)Bu)(n) (n = 3 and 4) compared to the analogous all-methyl derivatives. Remarkable differences of the atomic charges of the carbon atoms of methyl or tert-butyl groups attached to oxygen have been found.