Determination of Ion−Solvent Equilibria in the Gas Phase. Hydration of Diprotonated Diamines and Bis(trimethylammonium) Alkanes

Many ions of interest in condensed phase chemistry cannot be produced in the gas phase by conventional methods. This is particularly the case for multiply charged inorganic and bio-organic ions. However, these species can be produced by electrospray, which is a method with which electrolyte ions present in a solution can be transferred to the gas phase. An ion-source reaction chamber was developed with which the equilibrium constants of ion−molecule equilibria and their temperature dependence can be determined, where the ions involved are produced by electrospray. The hydration equilibria M z + (H 2 O) n - 1 + H 2 O = M z + (H 2 O) n were determined for M z + = Na +, K +, n -C 3 H 7 NH 3 +, n -C 6 H 13 NH 3 + . The Δ G ° n - 1, n, Δ H ° n - 1, n, and Δ S ° n - 1, n were found to be in good agreement with previous determinations where these ions were produced by conventional techniques. Equilibria determinations were also obtained for the diprotonated α,ω-alkyldiammonium ions: H 3 N(CH 2 ) p NH 3 2+ where p = 5, 6, 7, 8, 9, 10, 12. The Δ H ° values obtained for these ions show that the hydration proceeds by water molecules occupying sequentially first the α charge site and then the other site, ω. It is found that the Δ H ° 0,1 ≈ Δ H ° 1,2 and Δ H ° 2,3 ≈ Δ H ° 3,4 where the second pair of enthalpies are slightly lower. The equality of the paired enthalpies is due to the large distance between the two charged sites. The entropy Δ S ° n - 1, n values also support alternate occupation of the α and ω charge sites. Equilibria were determined also for the methylated analogues: (CH 3 ) 3 N(CH 2 ) p N(CH 3 ) 3 2+, p = 2, 3, 4, 6. The hydration interactions for these ions is much weaker, as expected from previous work on the hydration of (CH 3 ) 4 N + by Meot-Ner and Deakyne. The observed entropy changes suggest a possible scheme of consecutive charged sites occupation which is different from that observed for the H 3 N(CH 2 ) p NH 3 2+ compounds. This different scheme is consistent with structure and energy predictions for the (CH 3 ) 4 N + hydrates due to Meot-Ner and Deakyne.