Cryogenic ion trap vibrational spectroscopy of the microhydrated sulfate dianions SO42-(H2O)3-8.

Harald Knorke,Knut R Asmis,Jonas Warneke,Zhi-Feng Liu,Huiyan Li

doi:10.1039/d0cp04386a

Abstract

Infrared photodissociation spectra of the D2-tagged microhydrated sulfate dianions with three to eight water molecules are presented over a broad spectral range that covers the OH stretching and H2O bending modes of the solvent molecules at higher energies, the sulfate stretching modes of the solute at intermediate energies and the intermolecular solute librational modes at the lowest energies. A low ion temperature combined with messenger-tagging ensures well-resolved vibrational spectra that allow for structure assignments based on a comparison to harmonic and anharmonic IR spectra from density functional theory (DFT) calculations. DFT ab initio molecular dynamics simulations are required to disentangle the broad and complex spectral signatures of microhydrated sulfate dianions in the OH stretching region and to identify systematic trends in the correlation of the strength and evolution of the solute-solvent and solvent-solvent interactions with cluster size. The onset for the formation of the second solvation shell is observed for n = 8.

Highlights

The sulfate dianion, SO42À, is among the best studied anions, since it plays an important role in diverse areas such as in atmospheric processes, e.g. sulfate aerosol formation and chemistry in the upper troposphere, in the regulation of metabolic and cellular processes, or in astrogeochemistry.[1,2,3] Due to its compact, doubly charged nature, the interaction of sulfate with water is strong and it is classified as one of the most cosmotropic anions in the Hofmeister series.[4]
The OH stretching region is further divided into regions covering the free and quasi-free§ OH stretching modes (A, 43615 cmÀ1) as well as OH stretching modes involved in very weak (B, 3615 to 3500 cmÀ1), weak (C, 3500 to 3300 cmÀ1) and medium strength (D, 3300 to 3000 cmÀ1) hydrogen bonds (HBs).[54]
D2-tagging microhydrated sulfate dianions introduces a new level of spectral quality in the IRPD spectra of SO42À(H2O)[3,4,5,6,7,8] compared to previous Infrared multiple photon photodissociation (IRMPD) measurements, which enables a more reliable determination of band positions and improved structural assignments

Summary

Introduction

The sulfate dianion, SO42À, is among the best studied anions, since it plays an important role in diverse areas such as in atmospheric processes, e.g. sulfate aerosol formation and chemistry in the upper troposphere, in the regulation of metabolic and cellular processes, or in astrogeochemistry.[1,2,3] Due to its compact, doubly charged nature, the interaction of sulfate with water is strong and it is classified as one of the most cosmotropic anions in the Hofmeister series.[4]. Dianion, while smaller clusters are prone to either electron autodetachment or proton transfer and formation of HSO4À and OHÀ.[5] Wang and co-workers used anion photoelectron spectroscopy to provide an estimate of the repulsive Coulomb barrier and hydration energies for up to n = 40.7,10–12 Williams and co-workers studied dissociation pathways and energetics up to n = 17 by mass spectrometric means.[13] Infrared multiple photon photodissociation (IRMPD) spectra of cryogenically-cooled SO42À(H2O)[3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24] ions at B20 K measured in the mid-IR range (550 to 1800 cmÀ1) provided a more detailed insight into the evolution of the cluster structures with size.[14] Photoelectron spectra of cold anions[15] as well as IRMPD spectra in the O–H stretching region[16] yielded new insights, initially for n r 7 and later for clusters up to n = 80.9,17,18

Methods

Results

Conclusion