Re-Evaluation of the Proton Affinity of 18-Crown-6 Using Competitive Threshold Collision-Induced Dissociation Techniques

The proton affinity (PA) of 18-crown-6 (18C6) is determined using competitive threshold collision-induced dissociation (TCID) techniques. The PA of 18C6 is derived from four thermochemical cycles involving the relative thresholds for production of the protonated bases, H+(B), and protonated crown, H+(18C6), from the collision-induced dissociation (CID) of four proton bound heterodimers, (B)H+(18C6). The bases examined include glycine (Gly), alanine (Ala), imidazole (Imid), and 4-methylimidazole (4MeImid). In all cases, CID pathways for the loss of intact B and 18C6 are observed in competition. Loss of intact 18C6 is observed as the lowest-energy CID pathway for the (Imid)H+(18C6) and (4MeImid)H+(18C6) complexes. In contrast, loss of intact Gly and Ala is observed as the lowest-energy CID pathway for the (Gly)H+(18C6) and (Ala)H+(18C6) complexes, respectively. Excellent agreement between the measured and calculated (B)H+-18C6 and (18C6)H+-B bond dissociation energies (BDEs) is found with M06 theory, whereas B3LYP theory systematically underestimates these BDEs. On the basis of the relative TCID thresholds for the primary and competitive CID pathways, as well as the literature PM of the bases, the PA of 18C6 is evaluated. The PA determined here for 18C6 exhibits excellent agreement with M06 and B3LYP theories, and very good agreement with the value reported by Meot-Ner determined using high pressure mass spectrometry (HPMS) techniques, suggesting that the PA of 18C6 reported in the NIST Webbook based on HPMS measurements by Kebarle and co-workers is overestimated.