PNNL

Abstract

This work shows elusive carbonic acid being effectively stabilized in the gas phase by interacting with halide anions X- (X = F, Cl, Br, and I). The formed H2CO3?X- complexes, characterized by negative ion photoelectron spectroscopy and ab initio calculations, all contain intact trans-trans carbonic acid binding onto the respective halide via two identical strong ionic O-H···X- hydrogen bonds. For X = Cl, Br, and I, the complex spectra exhibit the corresponding X? signature by simply shifting to higher binding energy side, while an extremely broader band of 2 eV range is observed for X = F. These spectroscopic evidences indicate that the excess electron is removed from each halide in the former case, while a proton is transferred from carbonic acid to fluoride upon electron detachment for the latter. The above H2CO3?X- structures as well as those of the previously studied H2SO4?X- along the homologous halogen series cannot be explained using the proton affinity (PA) argument. Instead, a perfect correlation is found between those structural motifs and the constituent acid pKa values, strongly suggesting that pKa is the determinant factor that can be used to convey correct acid-base chemistry between these diprotic oxyacids and halides. This work was supported by U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Science, Geosciences, and Biosciences, and performed using EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle Memorial Institute for the DOE. The theoretical calculations were conducted on the EMSL Cascade Supercomputer. The ?nancial support of the National Natural Science Foundation of China (No. 21873089) and the National Key Research and Development Program of China (No. 2016YFF0200502) are gratefully acknowledged too.