Ion molecule reactions at high pressure: recent proton affinities, gas phase acidities and hydrocarbon clustering results

Abstract

After a brief historic survey of the development of high pressure ion molecule reaction measurements, research in three different areas is used to illustrate the possibilities of high pressure ion molecule equilibria studies. The proton transfer equilibria: B 1H + + B 2 = B 1 + B 2H + involving ∼30 bases B with proton affinities between water and ammonia were measured. A complete ladder of equilibria linking water to ammonia and often involving multiple cycles was obtained. Using the external standard PA(iso-butene) = 193 kcal/mole (Lossing, Semeluk, Tsang) one obtained absolute values for the proton affinities of the compounds. These include: water 169, hydrogen sulfide 171.9, CH 3OH 182.3, benzene 183.4, acetaldehyde 185.4, ethanol 186.8, dimethyl ether 190.1, acetone 194.6, diethyl ether 198.1 and ammonia 200.7 kcal/mole. Measurements of equilibria: A 1 − + A 2H = A 1H + A 2 − provide relative gas phase acidities of acids AH. Use of external standards leads to values for the bond dissociation energy-electron affinity difference, D(A-H) - EA(A). Such values were obtained for several carbon and nitrogen acids. The expected greater acidity of nitrogen acids RNH relative to carbon acids RNCH is found only in saturated compounds. Most strong nitrogen and carbon acids involve conjugated double bonds. For these systems the analogous carbon and nitrogen acids are of very similar acidity. It is suggested that this is due to resonance stabilization of the neutral (free) nitrogen acids. Some of the D(A-H) - EA(H) values obtained are: pyrolle 41.9, cyclopentadiene 39.1, acetanilide 35.3, phenyl acetone 36.2, acetylacetone 28, diacetamide 30.1, barbituric acid 16.3 kcal/mole. Studies of clustering reactions in hydrocarbon positive ions are interpreted in terms of three centre bonds. The systems whose energetics are considered are CH 5 + (CH 4) 2 and isomers of protonated ethane, propane and butane.