Density Functional Theory Studies on Vibrational Spectra of Si2H5X (X = F, Cl, Br) and Their Isotopomers

Density function theory calculations using the B3‘ exchange functional and three correlation functionals of PW91, LYP, and P86 at the 6-31G** basis set level have been carried out to study the molecular structures and fundamental vibrational frequencies of Si 2 H 5 Cl, Si 2 H 5 Br, Si 2 H 5 F, and their isotopomers. The ab initio restricted Hartee−Fock (RHF) approach with the 6-31G** basis set has also been used to calculate the fundamental vibrational frequencies of Si 2 H 5 Cl, Si 2 H 5 Br, and their isotopomers. Comparsion of the calculated and the experimental results shows that the three DFT methods of B3PW91, B3LYP, and B3P86 predict more accurate fundamentals than the scaled RHF method with a scale factor of 0.9. The calculated DFT force constants with respect to the five Si−H(D) stretch modes for Si 2 H 5 Cl and its isotopomers have been scaled to the experimental data with a factor of 0.936 determined by a least-squares fitting to the experimental vibrational frequencies. The scale factor of 0.936 is then transferred from Si 2 H 5 Cl to both Si 2 H 5 Br and Si 2 H 5 F. The fundamental vibrational frequencies determined from the scaled derived-DFT force fields for Si 2 H 5 Cl, Si 2 H 5 Br, and their isotopomers are in good agreement with the experimental data. The predicted values and mode assignments of the vibrational frequencies of Si 2 H 5 F and its isotopomers are also reported.