Evidence for the co-existence of isomers of water dimer radical cations and their inter-conversion in a linear ion trap

Clarifying the Identity of the m/z 36 Ion in Water Microdroplet Mass Spectra

Several lines of evidence are presented that when ultrapure water is sprayed into air and the resulting water microdroplets are mass analyzed a peak at m/z 36 appears, which is identified as the water dimer cation. The water dimer cation, (H2O)2+•, and the hydrated ammonium cation, NH4+·H2O, have essentially indistinguishable mass differences in a low-resolution mass spectrum. By comparing the behavior of (H2O)2+• and NH4+·H2O with temperature and salt concentration, we can rule out the formation of NH4+·H2O in the spraying of ultrapure water into laboratory air.

Observation of the hemibond formation in (H2O-Arn)+ radical cation clusters by electronic spectroscopy and ion imaging technique

The hemibond is a non-classical covalent bond formed by the overlap of non-bonding orbitals of a radical (cation) and a closed-shell molecule. For (H2O-Arn)+ radical cation clusters, competition between the hemibonded type and hydrogen-bonded (H-bonded) type isomers has been discussed on the basis of infrared spectroscopy and theoretical computations. It has been commonly recognized that the H-bonded type is predominant, while the coexistence of the hemibonded type remains a topic of debate. Hemibonded species are known to exhibit very strong electronic transitions in the ultraviolet and/or visible (UV-vis) region, which are marker bands for hemibond formation. In this study, we performed electronic spectroscopy and photofragment ion imaging experiments on (H2O-Arn)+ to observe the hemibond formation between H2O+ and Ar. The observed spectra of (H2O-Arn)+ (n = 1-3) exhibit absorption in the UV and visible regions. A comparison with quantum chemical calculations suggests the coexistence of the hemibonded type in (H2O-Arn)+ (n = 1 and 2). In addition, the photofragment ion imaging experiment on (H2O-Ar)+ showed an angular distribution attributed to the absorption of the hemibonded type, providing firm experimental evidence of the coexistence of the hemibonded type.

Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent

Exploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.