This study concentrates on two polymers: linear polyethylenimine, L-PEI, and linear polypropylenimine, L-PPI. Detailed atomistic molecular models have been developed with the help of ab initio and quantum-mechanical calculations. The vibrational spectra of ethylenediamine, EDA, and 1,3-diaminopropane, DAP, which are the monomers leading to L-PEI and L-PPI, respectively, have been computed using different ab initio basis sets, from 3-21G to 6-31+G*. The results obtained with the different basis sets are compared to determine which basis set is the most efficient to compute data about oligomers from n=1–5, where n is the number of repeat units used. The trends in the computed vibrational frequencies and intensities are monitored as a function of the chain length. The data are analysed in conjunction with the trends in computed equilibrium geometries.