Effect of intramolecular hydrogen bonding of α, ω–diamines on the structure and exchange affinity of 18–crown–6-amine host-guest complexes in gas phase: A collision cross section measurements by ion mobility spectrometry

Abstract

The effect of intramolecular hydrogen bonding on the thermodynamics of the host–guest interactions have been investigated through the gas phase exchange studies of protonated normal primary amines and α,ω–diamines with ammonium ion in 18–crown–6 (18C6) cavity using ion mobility spectrometry (IMS). The cross sectional areas were calculated for α, ω–diamines and normal primary amines in free and complex form. The results showed that protonated α, ω–diamines have more compact structure than that of the protonated normal primary amines in the free forms. This is attributed to the formation of intramolecular hydrogen bridges in protonated α, ω–diamines. In addition, we found that 18C6 attachment prevents from the formation of intramolecular hydrogen bond in α, ω–diamines. The exchange reactions were also investigated for different amines and diamines. The values of the standard molar enthalpy change for exchange reactions were obtained from temperature dependence of exchange constants (Kex) of complexes using the Van’t Hoff plots. The results show that the tendency of diamines to interaction with 18C6 is lower than the normal amines due to the intramolecular hydrogen bond formation. The Kex of diamines to interaction with 18C6 follows the order 1,2–diaminoethane (1,2–DAE)>1,3–diaminopropane (1,3–DAP)>1,5–diaminopentane (1,5–DAP)>1,4–diaminobutane (1,4–DAB). This trend indicates that the most stable intramolecular hydrogen bond is formed in 1, 4–DAB through the formation six-member cycles.