Spectroscopic Signature of Proton Location in Proton Bound HSO₄^–·H⁺·X^– (X = F, Cl, Br, and I) Clusters

Abstract

Proton transfer plays a vital role in a variety of chemical and biological processes. The proton location in proton bound clusters, specifically, in the anions of HSO4 –·H+·X– (X = F, Cl, Br, and I), has been studied by negative ion photoelectron spectroscopy and ab initio theoretical calculations. The measured photoelectron spectra of HSO4 –·H+·X– (X = Cl, Br, and I) closely resemble those of X– by simply shifting to higher electron binding energies, suggesting that X– is the chromophore of the electron photodetachment, a fact clearly violating gas-phase acidity predictions. However, in the X = F case, the spectrum of HSO4 –·H+·F– is more similar to that of HSO4 –, indicating that H+ stays with F– and that the HSO4 – moiety carries the extra electron. Accompanying theoretical analyses are in excellent agreement with the experimental measurements and observations. This work provides direct spectroscopic evidence of the proton locations, clearly showing cases in which proton affinities of the constituent bases cannot correctly predict the right chemistry involving proton transfer processes.

Stabilizing the Exotic Carbonic Acid by Bisulfate Ion

“…While isomers 1, 3, and 4 may be regarded as HSO4 -•H 2 CO 3 , isomer 2 is better recognized as H 2 SO4•HCO 3 − , which is only about 2 kJ/mol higher in energy than isomer 1. Isomer 2 is unexpected solely from gas-phase proton affinity prediction and its stability can be attributed to the formation of two strong hydrogen bonds and highly delocalized extra electrons according to previous findings by Hou et al [24,27,[36][37][38]. Such electron delocalization can be partly reflected by the highest occupied molecular orbitals (HOMOs) as presented in Figure S2.…”

“…Hydrogen bonding is an effective way to stabilize otherwise unstable and uncommon structures [24][25][26][27][28][29][30][31][32][33][34]. For example, Hou et al have performed a series of studies on bisulfate ion-containing complexes, showing that hydrogen bonding interactions can alter the protonation pattern in the complex, which violates the gas-phase proton affinity prediction [24,27,[35][36][37][38]. This motivated us to wonder whether an appropriate partner such as bisulfate ion could also stabilize the exotic conformers of H 2 CO 3 molecule.…”

Molecular Specificity and Proton Transfer Mechanisms in Aerosol Prenucleation Clusters Relevant to New Particle Formation

“…The dotted and gray curves represent the original and shifted spectra of HSO 4 – and X – , respectively. Figure modified with permission from ref . Copyright 2019 American Chemical Society.…”

Observation and Exploitation of Spin–Orbit Excited Dipole-Bound States in Ion–Molecule Clusters

“…We chose an amino acid-iodide complex so that we could employ the recently developed “iodide-tagging” negative ion PES technique , to interpret electronic and structural information on amino acid-iodide clusters using iodide as a messenger. This approach is based on the fact that the PE spectra of such clusters are expected to be dominated by atomic iodide transitions that exhibit two distinct bands separated by ∼0.9 eV arising from the 2 P 3/2 and 2 P 1/2 spin–orbit states of the iodine atom, − and different isomers can then be identified from resolved peaks based on their slightly different EBEs. The Arg·I – anion was used in this work, since arginine possesses the largest proton affinity among the α-amino acids, a key aspect affecting zwitterion stability .…”