Chemical bonding properties in substituted disilynes

Abstract

The molecular structures of the Si₂HX, Si₂Li₂, SiGeHLi and C₂H₂ species (where X= H, Li, F and Cl) were studied. All of these species have more than one isomeric form. The critical points on the potential energy surfaces of the Si₂HX, Si₂Li₂ and C₂H₂ species and the minima on the SiGeHLi surface were located. The full six-dimensional potential energy surface (PES) of the Si₂Li₂ molecule was calculated (for the first time) using the CCSD(T)-F12a/cc-pVTZ-F12 level of theory. The core-valence, zero-point energy and relativistic corrections for the Si₂HLi and Si₂Li₂ species were calculated. Additionally, the electron affinity and Li⁺/H⁺ binding energies for the Si₂HLi and Si₂Li₂ structures were investigated. Furthermore, the anharmonic vibrational-rotational properties for the Si2HLi and Si₂Li₂ structures were calculated using second-order perturbation theory. The recently developed CCSD(T)-F12a method with the cc-pVTZ-F12 basis set was employed to obtain geometries and relative energies (for the Si₂HLi, Si₂HF, Si₂HCl and Si₂Li₂ structures) and vibrational frequencies (for the Si₂H₂ and Si₂Li₂ structures). The CCSD(T) method with the cc-pVXZ, aug-cc-pVXZ and aug-cc-pV(X+d)Z basis sets, CCSD(T)-F12a/cc-pVXZ (where X=2-4) and the B3LYP/6-311+G(d) levels of theory were also used. Comparison was made of the geometric properties and vibrational frequencies calculated at the different levels of theory. The calculated geometric properties for all the studied species and vibrational frequencies (for the Si₂H₂ structures) show good agreement with the experimental and theoretical literature. The PES of Si₂Li₂ was used to perform large scale variational vibrational calculations using the WAVR4 program. The first 2400 totally symmetric energy levels were calculated. The low-lying energy levels were qualitatively correct. Conclusive assignments of the vibrational modes of the Si₂Li₂ structures were made for the eleven lowest lying energy levels.