Selected ion flow tube study of NH4+ association and of product switching reactions with a series of organic molecules

Abstract

A study has been made of the association reactions of NH4+ with a series of organic molecules, CH3OH, C2H5OH, (CH3)2O, CH3CHO, CH3CO2H and (CH3)2CO, at 300K using a Selected Ion Flow Tube (SIFT). Ternary association rate coefficients have been determined and the product ions of several of the reactions have been probed by their further reactivities. The reaction with CH3NH2 was also studied, but this proceeds by proton transfer, being the only case in the study where this is energetically possible. The association reactions are generally rapid, and are pressure saturated in some cases, indicating strong bonding. Probing of the reactivity of the product ions of the (CH3)2O, CH3CHO, CH3CO2H and (CH3)2CO associations with the same set of molecules has shown that the reactions occur exclusively by switching, where one associated molecule is replaced by another. These switching reactions, when exothermic, can be very efficient, but the efficiency is strongly dependent on energetics. The study enabled both forward and reverse rate coefficients to be determined, and hence equilibrium constants, K, at 300K; from these K’s relative bond energies were estimated. Relative bond energies for NH4+ bound to the neutral are (CH3)2CO>CH3CO2H>CH3CHO>(CH3)2O, with the bond to (CH3)2CO being substantially stronger than the others. The reaction mechanism is suggestive of a strong cluster bond, although a chemically reactive alternative is possible and is discussed. Interstellar implications are mentioned.