High kinetic energy-ion mobility spectrometry-mass spectrometry investigations of several volatiles and their fully deuterated analogues

The first High Kinetic Energy-Ion Mobility Spectrometry-Mass Spectrometry (HiKE-IMS-MS) studies involving six volatiles (acetone, acetonitrile, methanol, ethanol, 2-propanol, and 1-butanol) and their fully deuterated analogues are reported. The goal is to further our understanding of the ion-molecule chemistry occurring in the HiKE-IMS. This is needed for its full analytical potential to be reached. Product ions are identified as a function of the reduced electric field (30-115 Td) and the influence of sample air humidity in the reaction region on deuterium/hydrogen (D/H) exchange reactions is discussed. Reagent ions include H3O+(H2O)(m), (n = 0, 1, 2 or 3), NO+(H2O)(n) (m = 0 or 1) and O-2(+center dot). Reactions with H3O+(H2O)(m), lead to protonated monomers (through either proton transfer or ligand switching). Reactions with NO+ involve association with acetone and acetonitrile, hydride anion abstraction from ethanol, 2-propanol, and 1-butanol, and hydroxide abstraction from 2-propanol and 1-butanol. With the exception of acetonitrile, O-2(+center dot) predominantly reacts with the volatiles via dissociative charge transfer. A number of sequential secondary ion-volatile processes occur leading to the formation of dimer and trimer-containing ion species, whose intensities depend on a volatile's concentration and the reduced electric field in the reaction region. Deuterium/hydrogen (D/H) exchange does not occur for product ions from acetone-d(6) and acetonitrile-d(3), owing to their inert methyl functional groups. For the deuterated alcohols, rapid D/H-exchange reaction at the hydroxy group is observed, the amount of which increased with the increasing humidity of the sample air and/or lowering of the reduced electric field.Peer reviewe

Ultracold water cluster anions

Attachment of free electrons to water clusters embedded in helium droplets leads to watercluster anions (H2O)(n)(-) and (D2O)(n)(-) of size n \u3e= 2. Small water-cluster anions bind to up to 10 helium atoms, providing compelling evidence for the low temperature of these complexes, but the most abundant species are bare cluster anions. In contrast to previous experiments on bare water clusters, which showed very pronounced magic and anti-magic anion sizes below n = 12, the presently observed size distributions vary much more smoothly, and all sizes are easily observed. Noticeable differences are also observed in the stoichiometry of fragment anions formed upon dissociative electron attachment and the energy dependence of their yield. Spectroscopic characterization of these ultracold water-cluster anions promises to unravel the relevance of metastable configurations in experiments and the nature of the still controversial bonding sites for the excess electron in small water-cluster anions

Argon clusters embedded in helium nanodroplets

Electron impact ionization of argon clusters embedded in helium droplets is investigated. Superior mass resolution makes it possible to distinguish between nominally isobaric cluster ions. An abundance maximum for ArHe(12)(+) is unambiguously confirmed; the spectra also prove the formation of Ar(2)He(n)(+) complexes that had been claimed to fragment into pure Ar(2)(+). Distributions of larger argon cluster ions containing up to 60 atoms closely resemble distributions observed upon electron impact or photoionization of bare argon clusters; caging and evaporative cooling provided by the helium matrix do not suffice to quench fragmentation of the nascent argon cluster ions. Intriguing abundance anomalies are observed in distributions of argon cluster ions that contain water, nitrogen or oxygen impurities. The strong abundance of Ar(55)H(2)O(+), Ar(54)O(2)(+) and Ar(54)N(2)(+) contrasts with the virtual absence of slightly larger cluster ions containing the corresponding impurities. The features are probably related to enhanced cluster ion stability upon closure of the second icosahedral shell but the difference in magic numbers (54 versus 55) and the well-known reactivity of charged argon-nitrogen complexes sugges

Experimental evidence for the existence of an electronically excited state of the proposed dihydrogen radical cation He-H-H-He

In a recent report, Uggerud and co-workers (A. Krapp et al., Chem. Eur. J. 2008. 14, 4028) proposed the existence of, a flew class of, radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H(2) molecules we observe various He(x)H(y)(+) cluster ions, including He(2)H(2)(+), which would belong to the proposed class of radical cations. Mass-analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field-free region of the mass spectrometer. One reaction is into HeH(2)(+) + He with a low kinetic energy release of 15 4 meV. Surprisingly, another unimolecular reaction is Observed. into HeH(+) + HeH (or He + H). The probability of this reaction is ail order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited stated they arc consistent with the proposed linear, centrosymmetric ion structure of He-H-H-He(+)

On the size of ions solvated in helium clusters

Helium nanodroplets are doped with SF(6), C(4)F(8), CCl(4), C(6)H(5)Br, CH(3)I, and I(2). Upon interaction with free electrons a variety of positively and negatively charged cluster ions X(+/-) He(n) are observed where X(+/-) = F(+/-), Cl(+/-), Br(+/-), I(+), I(2)(+), or CH(3)I(+). The yield of these ions versus cluster size n drops at characteristic sizes n(s) that range from n(s)=10.2 +/- 0.6 for F(+) to n(s) = 22.2 +/- 0.2 for Br(-). n(s) values for halide anions are about 70% larger than for the corresponding cations. The steps in the ion yield suggest closure of the first solvation shell. We propose a simple classical model to estimate ionic radii from n(s). Assuming the helium density in the first solvation shell equals the helium bulk density one finds that radii of halide anions in helium are nearly twice as large as in alkali halide crystals, indicating the formation of an anion bubble due to the repulsive forces that derive from the exchange interaction. In spite of the simplicity of our model, anion radii derived from it agree within approximately 10% with values derived from the mobility of halide anions in superfluid bulk helium, and with values computed by quantum Monte Carlo methods for X(-)He(n) cluster anions

The submersion of sodium clusters in helium nanodroplets: Identification of the surface -> interior transition

The submersion of sodium clusters beyond a critical size in helium nanodroplets, which has recently been predicted on theoretical grounds, is demonstrated for the first time. Confirmation of a clear transition from a surface location, which occurs for alkali atoms and small clusters, to full immersion for larger clusters, is provided by identifying the threshold electron energy required to initiate Nan cluster ionization. On the basis of these measurements, a lower limit for the cluster size required for submersion, n ≥ 21, has been determined. This finding is consistent with the recent theoretical prediction