To study the influence of the position of the double bond and ring size on the stability of hydrogen bonded complexes, the 1:1 complexes formed between 2,2,2-trifluoroethanol (TFE) and three heterocyclic compounds including 2,3-dihydrofuran (2,3-DHF), 2,5-dihydrofuran (2,5-DHF) and 3,4-dihydropyran (3,4-DHP) were investigated systematically. The formation of hydrogen bonded TFE−2,3-DHF, TFE−2,5-DHF and TFE−3,4-DHP complexes were identified by gas phase FTIR spectroscopy at room temperature, and the OH-stretching fundamental transition of TFE was red shifted upon complexation. The competition between the O atom and π-electrons bonding sites within the complexes was studied, and the O−H···π type hydrogen bond was found to be less stable than the O−H···O in all three cases. The observed red shifts of the OH-stretching fundamental transitions in the complexes were attributed to the formation of O−H···O hydrogen bond. Equilibrium constants of the complexation reactions were determined from measured and calculated OH-stretching fundamental intensities. Both theoretical calculations and experimental results reveal that the hydrogen bond strengths in the complexes follow the sequence: TFE−2,5-DHF > TFE−2,3-DHF ≈ TFE−3,4-DHP, thus the position of the double bond exerts significantly larger influence than ring size on the stability of the selected hydrogen bonded complexes.Atmospheric nanometer sized clusters play an important role in the formation and growth of aerosol particles. These clusters are usually stabilized by hydrogen bonding interactions [1][2][3][4][5] . Investigation of hydrogen bonded complexes formed between atmospheric relevant molecules is important in understanding the characteristics of particles at the molecular level. According to previous researches, hydrogen bonded complexes can be formed among a number of molecules, such as alcohols, amines and aromatic compounds [5][6][7][8] . Alcohols have the ability to form hydrogen bonded complexes either as hydrogen bond donor or hydrogen bond acceptor through the H atom or the O atom of the hydroxyl group, respectively. A particular case of atmospherically relevant alcohol in complex formation is the 2,2,2-trifluoroethanol (TFE), one of the widely used fluorinated alcohols that is available on a large commercial scale. It has been used in many reactions as solvent, cosolvent, catalyst, and it is also important for the preparation of many pharmaceuticals and other chemicals [9][10][11][12] . Increased production and use of TFE will lead to an increase of its emission into the atmosphere. The electron withdrawing CF 3 group present in TFE enhances its acidity and makes it a strong proton donor 13,14 . Considering the close correlation between hydrogen bonded clusters and new particles formation, it is necessary to study the interactions of TFE with compounds present in the atmosphere.Hydrogen bonding interactions between TFE and various atmospheric components have been explored by many studies, both theoretically and experimentally. Hydrogen bonded complexes...