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 Si2H5Cl, Si2H5Br, Si2H5F, 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 Si2H5Cl, Si2H5Br, 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 Si2H5Cl 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 Si2H5Cl to both Si2H5Br and Si2H5F. The fundamental vibrational frequencies determined from the scaled derived-DFT force fields for Si2H5Cl, Si2H5Br, and their isotopomers are in good agreement with the experimental data. The predicted values and mode assignments of the vibrational frequencies of Si2H5F and its isotopomers are also reported.