Pyrrolidine,a five membered heterocyclic molecule,is widely existing in organism.Herein,infrared spectra of pyrrolidine monomer in neutral and cationic states were obtained by vacuum ultraviolet ionization,infrared photodissociation and time of flight mass spectrometry.Both in neutral and cationic states,it is found that their CH stretching vibration bands are red shifted.In the IR spectrum of neutral pyrrolidine,because the electric dipole moment of NH is small,we have not observed the NH stretching vibration bands.However,the NH stretching vibration band of pyrrolidine is greatly enhanced after ionization,and this band red-shifts compared with the previous experiment.The red shifts of CH stretching vibrations in neutral and cationic states are caused by the negative and positive hypercon-jugation,respectively.The enhancement and red shift of the NH stretching band are owing to the ejection of the electrons on the N atom after ionization.Through the calculations,it is found that the acidity of the CH bond is a little stronger than that of NH bond.These kinds of studies would be helpful to understand the intrinsic properties of biomolecules in neutral and cationic states,and to provide reference for the further study of living organic macromolecules.