In this article, the new and exciting techniques of infrared consequence spectroscopy (sometimes called action spectroscopy) of gaseous ions are reviewed. These techniques include vibrational predissociation spectroscopy and infrared multiple photon dissociation spectroscopy and they typically complement one another in the systems studied and the information gained. In recent years infrared consequence spectroscopy has provided long-awaited direct evidence into the structures of gaseous ions from organometallic species to strong ionic hydrogen bonded structures to large biomolecules. Much is being learned with respect to the structures of ions without their stabilizing solvent which can be used to better understand the effect of solvent on their structures. This review mainly covers the topics with which the author has been directly involved in research: structures of proton-bound dimers, protonated amino acids and DNA bases, amino acid and DNA bases bound to metal ions and, more recently, solvated ionic complexes. It is hoped that this review reveals the impact that infrared consequence spectroscopy has had on the field of gaseous ion chemistry.

Infrared consequence spectroscopy of gaseous protonated and metal ion cationized complexes

Ammonia (NH3) is an irritant gas with a unique pungent odor; sub-parts per million-level breath ammonia is a medical biomarker for kidney disorders and Helicobacter pylori bacteria-induced stomach ...

Flexible Room-Temperature NH3 Sensor for Ultrasensitive, Selective, and Humidity-Independent Gas Detection.

Although messenger mediated spectroscopy is a widely-used technique to study gas phase ionic species, effects of messengers themselves are not necessarily clear. In this study, we report infrared photodissociation spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4) in the OH stretch region to investigate messenger(M)-dependent cluster structures of the H+(H2O)6 moiety. The H+(H2O)6, the protonated water hexamer, is the smallest system in which both the H3O+ (Eigen) and H5O2+ (Zundel) hydrated proton motifs coexist. All the spectra show narrower band widths reflecting reduced internal energy (lower vibrational temperature) in comparison with bare H+(H2O)6. The Xe-, CH4-, and N2-mediated spectra show additional band features due to the relatively strong perturbation of the messenger. The observed band patterns in the Ar-, Kr-, Xe-, N2-, and CH4-mediated spectra are attributed mainly to the “Zundel” type isomer, which is more stable. On the other hand, the Ne- and H2-mediated spectra are accounted for by a mixture of the “Eigen” and “Zundel” types, like that of bare H+(H2O)6. These results suggest that a messenger sometimes imposes unexpected isomer-selectivity even though it has been thought to be inert. Plausible origins of the isomer-selectivity are also discussed.

Infrared photodissociation spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4): messenger-dependent balance between H3O+ and H5O2+ core isomers

Ensemble infrared photodissociation (IRPD) spectra in the hydrogen stretch region (∼2800–3800 cm–1) are reported for aqueous nanodrops containing ∼250 water molecules and either SO42–, I–, Na+, Ca2+, or La3+ at 133 K. Each spectrum has a broad feature in the bonded-OH region (∼2800–3500 cm–1) and a sharp feature near 3700 cm–1, corresponding to the free-OH stretch of surface water molecules that accept two hydrogen bonds and donate one hydrogen bond (AAD water molecules). A much weaker band corresponding to AD surface water molecules is observed for all ions except SO42–. The frequencies of the AAD free-OH stretch red-shift with increasingly positive charge, consistent with a Stark effect as a result of the ion’s electric field at the droplet surface, and from which the corresponding frequency for water molecules at the surface of neutral nanodrops of this size is estimated to be 3699.3–3700.1 cm–1. The intensity of the AAD band increases with increasing positive charge, consistent with a greater populati...

Effects of Ions on Hydrogen-Bonding Water Networks in Large Aqueous Nanodrops

Ionophore-doped sensing membranes exhibit greater selectivities and wider measuring ranges if their membrane matrixes are noncoordinating and solvate interfering ions poorly. This is particularly true for fluorous phases, which are the least polar and polarizable condensed phases known. In this work, fluorous membrane matrixes were used to prepare silver ion-selective electrodes (ISEs). Sensing membranes composed of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and one of four fluorophilic Ag+-selective ionophores with one or two thioether groups were investigated. All electrodes exhibited Nernstian responses to Ag+ in a wide range of concentrations. Their selectivities for Ag+ over interfering ions were found to depend on host preorganization and the length of the −(CH2)n− spacers separating the coordinating thioether group from the strongly electron withdrawing perfluoroalkyl groups. ISEs based on the most selective of the four ionophores, that is, 1,3-bis(perfluor...

Highly Selective Detection of Silver in the Low ppt Range with Ion-Selective Electrodes Based on Ionophore-Doped Fluorous Membranes

M(+)(H(2)O)(n) and M(+)(H(2)O)(n)Ar ions (M=Cu and Ag) are studied for exploring coordination and solvation structures of noble-metal ions These species are produced in a laser-vaporization cluster source and probed with infrared (IR) photodissociation spectroscopy in the OH-stretch region using a triple quadrupole mass spectrometer Density functional theory calculations are also carried out for analyzing the experimental IR spectra Partially resolved rotational structure observed in the spectrum of Ag(+)(H(2)O)(1) x Ar indicates that the complex is quasilinear in an Ar-Ag(+)-O configuration with the H atoms symmetrically displaced off axis The spectra of the Ar-tagged M(+)(H(2)O)(2) are consistent with twofold coordination with a linear O-M(+)-O arrangement for these ions, which is stabilized by the s-d hybridization in M(+) Hydrogen bonding between H(2)O molecules is absent in Ag(+)(H(2)O)(3) x Ar but detected in Cu(+)(H(2)O)(3) x Ar through characteristic changes in the position and intensity of the OH-stretch transitions The third H(2)O attaches directly to Ag(+) in a tricoordinated form, while it occupies a hydrogen-bonding site in the second shell of the dicoordinated Cu(+) The preference of the tricoordination is attributable to the inefficient 5s-4d hybridization in Ag(+), in contrast to the extensive 4s-3d hybridization in Cu(+) which retains the dicoordination This is most likely because the s-d energy gap of Ag(+) is much larger than that of Cu(+) The fourth H(2)O occupies the second shells of the tricoordinated Ag(+) and the dicoordinated Cu(+), as extensive hydrogen bonding is observed in M(+)(H(2)O)(4) x Ar Interestingly, the Ag(+)(H(2)O)(4) x Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag(+)(H(2)O)(3) x Ar but revived at n=4 Size dependent variations in the spectra of Cu(+)(H(2)O)(n) for n=5-7 provide evidence for the completion of the second shell at n=6, where the dicoordinated Cu(+)(H(2)O)(2) subunit is surrounded by four H(2)O molecules The gas-phase coordination number of Cu(+) is 2 and the resulting linearly coordinated structure acts as the core of further solvation processes

Infrared spectroscopy of Cu+(H2O)(n) and Ag+(H2O)(n): coordination and solvation of noble-metal ions.

Coordination and solvation structures of the Cu+(NH3)n ions with n = 3–8 are studied by infrared photodissociation spectroscopy in the NH-stretch region with the aid of density functional theory calculations. Hydrogen bonding between NH3 molecules is absent for n = 3, indicating that all NH3 molecules are bonded directly to Cu+ in a tri-coordinated form. The first sign of hydrogen bonding is detected at n = 4 through frequency reduction and intensity enhancement of the infrared transitions, implying that at least one NH3 molecule is placed in the second solvation shell. The spectra of n = 4 and 5 suggest the coexistence of multiple isomers, which have different coordination numbers (2, 3, and 4) or different types of hydrogen-bonding configurations. With increasing n, however, the di-coordinated isomer is of growing importance until becoming predominant at n = 8. These results signify a strong tendency of Cu+ to adopt the twofold linear coordination, as in the case of Cu+(H2O)n.

Coordination and solvation of copper ion: infrared photodissociation spectroscopy of Cu+(NH3)n (n = 3–8)

Infrared photodissociation spectroscopy of V+(H2O)n (n = 2-8): Coordinative saturation of V+ with four H2O molecules

Coordination structures of the Co(+)(NH(3))(n) and Ni(+)(NH(3))(n) ions are probed by infrared (IR) photodissociation spectroscopy with the aid of density functional theory (DFT) calculations. The IR spectra of N(2)-tagged Co(+)(NH(3))(n) (n = 1-4) exhibit two distinct bands assignable to the symmetric and antisymmetric NH stretches of the NH(3) molecules binding directly to Co(+). Size-dependent changes in the spectra of Co(+)(NH(3))(n) (n = 4-8) indicate that the first shell of Co(+) is filled with four NH(3) molecules and the resulting 4-coordinated structure forms the central core of further solvation. The spectra of Ni(+)(NH(3))(n) (n = 3-8) suggest that the coordination number of Ni(+) is also four, although a minor 3-coordinated isomer is identified for Ni(+)(NH(3))(4). Despite the same coordination number, the DFT calculations predict a distorted square-planar coordination for Ni(+)(NH(3))(4) and a distorted tetrahedral coordination for Co(+)(NH(3))(4). The coordination of Ni(+)(NH(3))(4) is explainable by using a simple model based on the geometry of a half-filled 3d orbital in Ni(+). This suggests that the Ni(+) ion gives priority to the minimization of the metal-ligand repulsion in accommodating four ligands in the first shell. On the other hand, the same model fails to explain the coordination of Co(+)(NH(3))(4). An interpretation for this is that the Co(+) ion gives priority to the minimization of the ligand-ligand repulsion.

Infrared photodissociation spectroscopy of Co+(NH3)n and Ni+(NH3)n: preference for tetrahedral or square-planar coordination

Infrared (IR) spectra are measured for Ag+(NH3)n with n = 3–8 in the NH-stretch region using photodissociation spectroscopy. The spectra of n = 3 and 4 exhibit absorption features only near the frequencies of the isolated NH3, indicating that every NH3 molecule is coordinated individually to Ag+. For n ≥ 5, the occupation of the second shell is evidenced by lower-frequency features characteristic of hydrogen bonding between NH3 molecules. Density functional theory and MP2 calculations are carried out in support of the experiments. A detailed comparison of the experimental and theoretical IR spectra reveals the preference for a tetrahedral coordination in the n = 5 and 6 ions. Likewise, most of the features observed in the spectra of n = 7 and 8 can be assigned to isomers containing a tetrahedrally coordinated inner shell as the basic structural motif. These results signify that the ammonia-solvated Ag+ ion has a propensity toward a coordination number of four and the resulting tetrahedral Ag+(NH3)4 complex forms the central core of further solvation process.

Kazuya Inoue

Papers

Infrared spectroscopy of Cu+(H2O)(n) and Ag+(H2O)(n): coordination and solvation of noble-metal ions.

Coordination and solvation of copper ion: infrared photodissociation spectroscopy of Cu+(NH3)n (n = 3–8)

Infrared photodissociation spectroscopy of V+(H2O)n (n = 2-8): Coordinative saturation of V+ with four H2O molecules

Infrared photodissociation spectroscopy of Co+(NH3)n and Ni+(NH3)n: preference for tetrahedral or square-planar coordination

Coordination structures of the silver ion: infrared photodissociation spectroscopy of Ag+(NH3)n (n = 3–8)