Experimental and theoretical investigation of some pyrrolidine derivatives by vibrational spectroscopy

FT-IR and Raman spectra of 1-pyrrolidino-1-cyclopentene (1pycp) and 1-pyrrolidino-1-cyclohexen (1pych) have been experimentally reported in the region of 4000-10 cm-1 and 4000-50 cm-1, respectively. The optimized geometric parameters (bond distances, bond and dihedral angles), normal mode frequencies and corresponding vibrational assignments, vibrational spectra, potential energy distributions (PED) belonging to vibrational assignments, conformational analysis, some thermodynamics functions, the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) of 1pycp (C9H15N) and 1pych (C10H17N) were theoretically examined by means of B3LYP hybrid density functional theory (DFT) method together with 6-31++G(d,p) basis set. Conformational isomers for both molecules have been supported by analysis of the potential energy surface (PES). Comparison between the experimental and theoretical results indicates that B3LYP method provides satisfactory evidence for the prediction of vibrational wavenumbers and structural parameters and the mixture of envelope/ twist and envelope conformers are supposed to be the most stable form of 1pych and 1pycp respectively.