Jude A. Kelley

A pulsed supersonic entrainment reactor for the rational preparation of cold ionic complexes

We describe an ion source for the efficient preparation of cold ion-molecule complexes, X−⋅M. The method relies on condensation of solvent molecules, M, onto argon-solvated ions, X−⋅Arm, where the X−⋅Arm species are formed in a primary expansion and the molecular partner, M, is interfaced to this flow in the hydrodynamic region by supersonic entrainment. This hybrid “supersonic afterglow” reactor provides a clean synthetic approach for both bare and argon-solvated complexes, where the latter are particularly useful since their structures can be characterized by “nanomatrix” infrared predissociation spectroscopy.

Hydration of a structured excess charge distribution: Infrared spectroscopy of the O2−⋅(H2O)n, (1≤n≤5) clusters

To explore how a structured excess charge distribution affects the hydration of an anion, we report mid-IR, argon predissociation spectra for the hydrated superoxide cluster anions, O2−⋅(H2O)n, 1⩽n⩽5. This size range was chosen to establish the evolution of the structures through the putative shell closing [Weber et al., Science 287, 2461 (2000)] for superoxide hydration at the tetrahydrate. Whereas the observed bonding motifs for n⩽4 are those of single water molecules and dimeric subclusters bound to the ion, the pentahydrate spectrum displays strong bands in the region typically associated with ring modes of the water trimer. The present results reinforce the conclusion that the tetrahydrate adopts an especially robust structure in which each water molecule forms a single ionic H bond to one of the lobes of the π* highest occupied molecular orbital in superoxide.

Infrared spectroscopic observation of the argon isomer distribution in evaporative ensembles of I−⋅ROH⋅Arm (R=methyl, ethyl, isopropyl) clusters

Vibrational predissociation spectra of argon-solvated iodide–alcohol clusters (I−⋅ROH⋅Arm, ROH=MeOH, EtOH, i-PrOH) are reported in the OH stretching region (3200–3400 cm−1). The spectra display multiple peaks associated with the ionic H-bonded OH stretching fundamental, which vary according to the extent of argon solvation. At small m, the number of peaks reflects the total number of attached argon atoms, such that peaks associated with fewer argons persist (with a small blue shift) in the spectra of the larger clusters, while new peaks appear red shifted by about 12 cm−1 with each additional argon. The effect saturates in a manner that depends on the particular alcohol (mmax=6 for MeOH, 5 for EtOH, and 4 for i-PrOH). We interpret these observations to indicate the presence of multiple isomers in the evaporative ensemble, which are distinguishable according to the different arrangements of argon atoms among two effective binding sites.

The infrared predissociation spectra of Cl−·H2O·Arn (n=1–5): experimental determination of the influence of Ar solvent atoms

Abstract We establish the argon solvent size dependence of the Cl − ·H 2 O·Ar n predissociation spectra, and discuss the discrepancies between previously reported predissociation spectra of the Cl − ·H 2 O and Cl − ·H 2 O·Ar 3 complexes [Choi et al., J. Phys. Chem. A 102 (1998) 503; Ayotte et al., J. Am. Chem. Soc. 120 (1998) 12361]. The argon-induced shift in the ∼3130 cm −1 ionic H-bonded OH stretching band, calculated to be large (>30 cm −1 /Ar red-shift) by Satoh and Iwata [Chem. Phys. Lett. 312 (1999) 522], is found to be quite small (IHB band center=3128±3 cm −1 for 1⩽ n ⩽5). We compare this result with similar behavior displayed by the bare versus argon-solvated bromide monohydrate.

Linking the photoelectron and infrared spectroscopies of the (H2O)6− isomers

We report a novel photoelectron spectroscopy variation of population labeling spectroscopy and apply it to assign the isomeric carrier of the strong autodetaching OH stretching vibrational resonances reported previously [J. Phys. Chem. 100, 16782 (1996) and J. Chem. Phys. 108, 444 (1998)] for a mixed ensemble of (H2O)6− isomers. The vibrational bands are traced to the isomer with the higher vertical electron detachment energy (VDE). This result indicates that resonances are most readily observed for vibrational bands which lie below the VDE of the parent species.

Double-contact ion-molecule binding: Infrared characterization of the ionic H bonds to formic acid in the I−⋅HCOOH complex

We report mid-IR predissociation spectra of the I−⋅HCOOH⋅Arm(m=1–4) ion-acid complexes. The spectra are consistent with a planar structure where both hydrogens are engaged in ionic H bonds. Upon binding to the ion, the OH stretching fundamental displays a much more dramatic redshift (792 cm−1) than that of the CH stretch (99 cm−1), giving rise to a complex series of bands in the 2750–2950 cm−1 region. The contributions of the CH and OH stretches to the spectrum are isolated by recording spectra of the I−⋅DCOOH and I−⋅HCOOD species, which reveal that the OH stretching vibration is accompanied by combination bands involving soft modes while the CH stretch spectrum is dominated by a single feature. Some of the complexity in the I−⋅HCOOH spectrum arises from a strong Fermi resonance interaction between the v=1 level of the OH stretch and an overtone or combination band involving CH motion. We compare this behavior to that of the previously reported I−⋅CH3OH and I−⋅H2O complexes.

Preparation and photoelectron spectrum of the CH3I− anion: rare gas cluster mediated synthesis of an ion–radical complex

We report the preparation of the bare and argon-solvated anion of CH3I, and characterize this species using negative ion photoelectron spectroscopy at 3.495 eV. The photoelectron spectrum consists of a narrow band appearing 0.11±0.02 eV above the binding energy of isolated iodide. Such behavior is similar to that displayed by iodide-(closed shell) solvent molecule complexes, indicating that photodetachment does not access the bound region of the CH3I potential. These observations suggest that CH3I− rearranges (after electron capture) to an ion-radical complex. We advance the hypothesis that this complex adopts a C2v structure where the ion is hydrogen bonded to the methyl radical.

Infrared spectra of hydrogen-bonded ion–radical complexes: I−⋅HCH2 and Br−⋅HCHBr

We report the preparation and infrared spectra of the CH3I− and CH2Br2− anions formed by argon cluster-mediated electron attachment to the neutral molecular precursors. Infrared predissociation spectra were acquired for both the bare and argon-solvated species in the C–H stretching region. Partial rotational structure was recovered in the CH3I− system, consistent with the hydrogen-bonded, C2v structure suggested in an earlier analysis of its photoelectron spectrum [J. Kim et al., J. Am. Soc. Mass Spectrom. 10, 810 (1999)]. The spectrum and photofragmentation pattern confirm that this species is trapped in a very weakly bound ion–methyl radical form (I−⋅HCH2) involving a single ionic H bond. The CH2Br2− anion displays a similar spectrum, where one CH stretch is significantly redshifted, again signaling the single H-bonding motif.

Observation of sharp vibronic bands in the O4− `core ion' by mid infrared predissociation spectroscopy of O4−·Arn clusters

Abstract We report the first observation of an infrared (v 0,0 ∼4150 cm −1 ) electronic band system arising from excitation of the ground state O 4 − ion, which we discuss in the context of the expected transition (J. Chem. Phys. 114 (2001) 3010) between the two low lying isomeric forms of this species. Surprisingly, the band displays sharp vibrational fine structure, opening the way for a detailed spectroscopic characterization of a charge–resonance stabilized dimer ion.