Theoretical studies of bonding properties and vibrational spectra of chosen ladder-like silsesquioxane clusters

•Ladder-like silsesquioxane clusters were studied theoretically.•DFT calculations of IR spectra using Gaussian09 software were carried out.•Results for clusters with different lengths and side groups were compared.•Bonding properties were studied by QTAiM and BVM formalisms approach.

In this work the DFT calculations were carried out by means of Gaussian 09 program using B3LYP XC functional and 6-311G(d) basis set for chosen ladder-like silsesquioxane model clusters. Vibrational frequencies computations and infrared spectra were obtained subsequently for above mentioned structures. The results of topological analysis of total electron density obtained in SCF calculations (Quantum Theory of Atoms in Molecules approach) and structural analysis based on Bond Valence Method are presented in order to analyze length and ramification of ladder-like structures and various functional group influence.The calculated infrared spectra show that peak derived from vibrations of Si–O “chains” is shifting towards lower frequencies with increasing length of the ladder while at the same time, the overall strains (BVM) are diminishing.