Abstract:

Ni₄Ti₂, [Ni₄Ti₂]²⁺ and [Ni₄Ti₂]^2-, Ni₄Ti₄, [Ni₄Ti₄]²⁺ and [Ni₄Ti₄]^2- were studied at the B3PW91/6-311+G(d) basis level with density functional theory(DFT). Introduction of Ti atoms to both sides of Ni₄ ring brings about stable conformations Ni₄Ti₂(D_4h), [Ni₄Ti₂]²⁺(D_4h), [Ni₄Ti₂]^2-(D_4h) and Ni₄Ti₄(D_2h) with square-planar ring of Ni₄. Nucleus-independent chemical shifts(NICS) calculations show positive NICS value in Ni₄Ti₂(D_4h) and Ni₄Ti₄(D_2h), negative NICS value in [Ni₄Ti₂]²⁺(D_4h), and more negative NICS value in [Ni₄Ti₂]^2-(D_4h). Analysis results show that the formation of diamagnetic circulation involving s and d orbitals is the main cause for the relatively strong aromaticity of [Ni₄Ti₂]²⁺(D_4h) and [Ni₄Ti₂]^2-(D_4h). In such case, Ti atoms provide d_z² and s orbitals, and Ni atoms form square with their d_z² and d_x²-y² orbitals, with which a spherical electron current forms in [Ni₄Ti₂]²⁺(D_4h) and [Ni₄Ti₂]^2-(D_4h) with conspicuous π electron current.

Key words: Density functional theory, Aromaticity, Nucleus-independent chemical shift, NiTi shape memory alloy