Calculations of the isotopic shifts of fundamental frequencies for dihaloid silanes

Abstract

Fundamental frequencies of SiH 2F 2 and their isotopic shifts were calculated using ab initio MP2 and QCISD(T) and DFT (B3LYP) methods. The accuracy of calculations was tested by comparison with experimental SiH 2F 2 and SiD 2F 2 frequencies. While most of the fundamental frequencies are predicted very well, the accuracy of calculations of the deuterium isotopic shift was found to be 20–40 cm −1. The most noticeable shift on silicone isotope substitution was found to be ∼7 cm −1 for the Si–F stretch mode of B 2 symmetry. Some doubts upon the accuracy of the quantum chemical calculations of the silicone isotopic shifts induced us to find a more accurate method to calculate isotopic shifts. Therefore, the method of analytical calculation of isotopic shifts for the B 1 and B 2 vibration modes of dihaloid silanes was proposed and tested. The results of analytical calculations of the isotopic shifts of the Si–F and Si–Cl stretching vibrations of SiH 2F 2 and SiH 2Cl 2 molecules under isotopic substitution of silicone and chlorine atoms are reported.