Reactions of doubly deprotonated 2,6-naphthalenedicarboxylic acid with alcohols: Proton transfer versus solvation

• Published in: International journal of mass spectrometry 2017-07, Vol.418, p.180-187
• Main Authors: Khairallah, George N., Meyer, Matthew M., O’Hair, Richard A.J., Fattahi, Alireza, Schmidt, Jacob, Kass, Steven R.
• Format: Article
• Language: English
• Subjects: Cluster anions; Dianion; Gas-phase ion-molecule reactions; IRMPD; Microsolvation
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/j.ijms.2016.09.005

[Display omitted] •A dicarboxylate dianion clusters with alcohols.•Sequential solvation is observed.•Surprisingly, the weakest base undergoes proton transfer.•Solvation of the conjugate acid results in a kinetically enhanced process.•Coulomb barriers can be mitigated by cluster formation. Electrospray ionization of 2,6-naphthalenedicarboxylic acid readily affords its doubly deprotonated dicarboxylate dianion (12−). This species clusters with background water and added alcohols in an ion trap at ∼10−3Torr. Sequential solvation is observed to afford mono and dicoordinated ions. Surprisingly, the latter cluster (12−• 2TFE) is protonated by 2,2,2-trifluoroethanol (TFE) whereas 12−and 12−• TFE are not even though ΔH°acid(TFE)=361.7±2.5kcalmol−1 (as given in the NIST website at http://webbook.nist.gov) and the B3LYP/6-31+G(d,p) proton affinities are 384.7 (12−), 377.6 (12−• TFE), and 362.7 (12−• 2TFE) kcal mol−1. That is, only the weakest base in this series, and the dianion with an equal number of solvent molecules and charged sites, undergoes proton transfer. In a FTMS instrument at lower pressures (∼10−8Torr) inefficient proton abstraction is observed with the monosolvated dianion. This difference, and the observed reactivities of 12−, 12−• TFE and 12−• 2TFE are rationalized with the aid of computed potential energy surfaces. The chemical structures of these cluster ions were also probed via collision-induced dissociations, infrared photodissociation from 2700 to 3200cm−1, and extensive calculations. All of the TFE species are found to be solvated dianions, but incipient proton transfer to afford electrostatically defying anion-anion clusters is noted in two cases. In proton transfer reactions, formation of the conjugate acid as a solvated ion lowers the energy of the system and reduces the Coulomb repulsion barrier facilitating the overall process.