Charge-Transfer Spectroscopy of Ag+(Benzene) and Ag+(Toluene).

Gas-phase ion-molecule complexes of silver cation with benzene or toluene are produced via laser vaporization in a pulsed supersonic expansion. These ions are mass-selected and photodissociated with tunable UV-visible lasers. In both cases, photodissociation produces the organic cation as the only fragment via a metal-to-ligand charge-transfer process. The wavelength dependence of the photodissociation produces electronic spectra of the charge-transfer process. Broad structureless spectra result from excitation to the repulsive wall of the charge-transfer excited states. Additional transitions are detected correlating to the forbidden 1S → 1D silver cation-based atomic resonance and to the HOMO-LUMO excitation on the benzene or toluene ligand. Transitions to these states produce the same molecular cation photofragments produced in the charge-transfer transitions, indicating an unanticipated excited-state curve-crossing mechanism. Spectra measured for these ions are compared to those for ions tagged with argon atoms. The presence of argon causes a significant shift on the energetic positions of these electronic transitions for both Ag+(benzene) and Ag+(toluene).

Pt+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy.

Platinum cation complexes with multiple acetylene molecules are studied with mass spectrometry and infrared laser spectroscopy. Complexes of the form Pt+(C2H2)n are produced in a molecular beam by laser vaporization, analyzed with a time-of-flight mass spectrometer, and selected by mass for studies of their vibrational spectroscopy. Photodissociation action spectra in the C-H stretching region are compared to the spectra predicted for different structural isomers using density functional theory. The comparison between experiment and theory demonstrates that platinum forms cation-π complexes with up to three acetylene molecules, producing an unanticipated asymmetric structure for the three-ligand complex. Additional acetylenes form solvation structures around this three-ligand core. Reacted structures that couple acetylene molecules (e.g., to form benzene) are found by theory to be energetically favorable, but their formation is inhibited under the conditions of these experiments by large activation barriers.

Photodissociation Spectroscopy and Photofragment Imaging to Probe Fe+(Benzene)1,2 Dissociation Energies.

Tunable laser photodissociation spectroscopy measurements and photofragment imaging experiments are employed to investigate the dissociation energy of the Fe+(benzene) ion-molecule complex. Additional spectroscopy measurements determine the dissociation energy of Fe+(benzene)2. The dissociation energies for Fe+(benzene) determined from the threshold for the appearance of the Fe+ fragment (48.4 ± 0.2 kcal/mol) and photofragment imaging (≤49.3 ± 3.2 kcal/mol) agree nicely with each other and with the value determined previously by collision-induced dissociation (49.5 ± 2.9 kcal/mol), but they are lower than the values produced by computational chemistry at the density functional theory level using different functionals recommended for transition-metal chemistry. The threshold measurement for Fe+(benzene)2 (43.0 ± 0.2 kcal/mol) likewise agrees with the value (44.7 ± 3.8 kcal/mol) from previous collision-induced dissociation measurements.

Photodissociation Spectroscopy and Photofragment Imaging of the Fe+(Acetylene) Complex.

Tunable laser photodissociation spectroscopy in the 700-400 nm region and photofragment imaging experiments are employed to investigate the Fe+(acetylene) ion-molecule complex. At energies above a threshold at 679 nm, continuous dissociation is detected throughout the visible wavelength region, with regions of broad structure. Comparison to the spectrum predicted by time-dependent density functional theory (TD-DFT) indicates that the complex has a quartet ground state. The dissociation threshold for Fe+(acetylene) at 679 nm provides the dissociation energy on the quartet potential energy surface. Correction for the atomic quartet-sextet spin state energy difference provides an adiabatic dissociation energy of 36.8 ± 0.2 kcal/mol. Photofragment imaging of the Fe+ photoproduct produced at 603.5 nm produces significant kinetic energy release (KER). The photon energy and the maximum value of the KER provide an upper limit on the dissociation energy of D0 ≤ 34.6 ± 3.2 kcal/mol. The dissociation energies determined from the spectroscopy and photofragment imaging experiments agree nicely with the value determined previously by collision-induced dissociation (38.0 ± 2.6 kcal/mol). However, both values are significantly lower than those produced by computational chemistry at the DFT level using different functionals recommended for transition-metal chemistry.

Cation Complexes of Uranium and Thorium with Cyclooctatetraene: Photochemistry and Decomposition Products.

Ion-molecule complexes of uranium or thorium singly-charged positive ions bound to cyclooctatetraene (COT), i.e., M+(COT)1,2, are produced by laser ablation and studied with UV laser photodissociation. The ions are selected by mass and excited at 355 or 532 nm, and the ionized dissociation products are detected using a reflectron time-of-flight mass spectrometer. The abundant fragments M+(C6H6), M+(C4H4), and M+(C2H2) occur for complexes of both metals, whereas the M+(C4H2), M+(C3H3), and M+(C5H5) fragments are prominent for uranium complexes but not for thorium. Additional experiments investigate the dissociation of M+(benzene)1,2 ions which may be intermediates in the fragmentation of the COT ions. The experiments are complemented by computational quantum chemistry to investigate the structures and energetics of fragment ions. Various cation-π and metallacycle structures are indicated for different fragment ions. The metal ion-ligand bond energies for corresponding complex ions are systematically greater for the thorium species. The computed thermochemistry makes it possible to explain the mechanistic details of the photochemical fragmentation processes and to reveal new actinide organometallic structures.

Photochemical Synthesis and Spectroscopy of Covalent PAH Dimers.

Laser photochemistry of pressed-pellet samples of polycyclic aromatic hydrocarbons (PAHs) produces covalently bonded dimers and some higher polymers. This chemistry was discovered initially via laser desorption time-of-flight mass spectrometry experiments, which produced masses (m/z) of 2M-2 and 2M-4 (where M is the monomer parent mass). Dimers are believed to be formed from photochemical dehydrogenation and radical polymerization chemistry in the desorption plume. Replication of these ablation conditions at higher throughput allowed PAH dimers of pyrene, perylene, and coronene to be produced and collected in milligram quantities. Differential sublimation provided purification of the dimers and elimination of residual monomers. The purified dimers were investigated with UV-visible, IR, and Raman spectroscopy, complemented by computational studies using density functional theory at the CAM-B3LYP/def2-TZV level. Calculations and predicted spectra were calibrated by comparison with the corresponding monomers and used to determine the lowest energy dimer structures. Infrared and Raman spectroscopy provided few distinctive signatures, but UV-visible spectra detected new transitions for each dimer. The comparison of simulated and experimental spectra allows determination of the most prevalent structures for the PAH dimers. The work presented here provides interesting insights into the spectroscopy of extended aromatic systems and a new strategy for the photochemical synthesis of large PAH dimers.

Coordination and Spin States in Fe+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy.

Fe+(acetylene)n ion-molecule complexes are produced in a supersonic molecular beam with pulsed laser vaporization. These ions are mass selected and studied with infrared photodissociation spectroscopy in the C-H stretching region, complemented by computational chemistry calculations. All C-H stretch vibrations are shifted to frequencies lower than the vibrations of isolated acetylene because of the charge transfer that occurs between the metal ion and the molecules. Complexes in the size range of n = 1-4 are found to have structures with individual acetylene molecules bound to the core metal ion via cation-π interactions. The coordination is completed with four ligands in a structure close to a distorted tetrahedron. Larger complexes in the range of n = 5-8 have external acetylene molecules solvating this n = 4 core ion via CH-π bonding to inner-shell ligands. DFT computations predict that quartet spin states are more stable for all complex sizes, but infrared spectra for quartet and doublet spin states are quite similar, precluding definitive determination of the spin states. There is no evidence for any of these complexes having acetylenes coupled into reacted structures. This is consistent with computed thermochemistry, which finds significant activation barriers to such reactions.

Communication: Electronic transition of the l-C6 + cation at 417 nm.

A new electronic transition is reported for the linear C6 + cation with an origin at 416.8 nm. This spectrum can be compared to the matrix isolation spectra at lower energies reported previously by Fulara et al. [J. Chem. Phys. 123, 044305 (2005)], which assigned linear and cyclic isomers, and to the gas phase spectrum reported previously by Campbell and Dunk [Rev. Sci. Instrum. 90, 103101 (2019)], which detected the same cyclic-isomer spectrum reported by Fulara. Comparisons to electronically excited states and vibrations predicted by various forms of theory allow assignment of the spectrum to a new electronic state of linear C6 +. The spectrum consists of a strong origin band, two vibronic progression members at higher energy and four hot bands at lower energies. The hot bands provide the first gas phase information on ground state vibrational frequencies. The vibrational and electronic structure of C6 + provide a severe challenge to computational chemistry.

Photoinduced charge transfer in the Zn-methanol cation studied with selected-ion photofragment imaging.

The Zn+(methanol) ion molecule complex produced by laser vaporization is studied with photofragment imaging at 280 and 266 nm. Photodissociation produces the methanol cation CH3OH+ via excitation of a charge-transfer excited state. Surprisingly, excitation of bound excited states produces the same fragment via a curve crossing prior to separation of products. Significant kinetic energy release is detected at both wavelengths with isotropic angular distributions. Similar experiments are conducted on the perdeuterated methanol complex. The Zn+ cation is a minor product channel that also exhibits significant kinetic energy release. An energetic cycle using the ionization energies of zinc and methanol together with the kinetic energy release produces an upper limit on the Zn+-methanol bond energy of 33.7 ± 4.2 kcal/mol (1.46 ± 0.18 eV).

Infrared spectroscopy of RG-Co+(H2O) complexes (RG = Ar, Ne, He): The role of rare gas "tag" atoms.

RGn-Co+(H2O) cation complexes (RG = Ar, Ne, He) are generated in a supersonic expansion by pulsed laser vaporization. Complexes are mass-selected using a time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy, measuring the respective mass channels corresponding to the elimination of the rare gas "tag" atom. Spectral patterns and theory indicate that the structures of the ions with a single rare gas atom have this bound to the cobalt cation opposite the water moiety in a near-C2v arrangement. The O-H stretch vibrations of the complex are shifted compared to those of water because of the metal cation charge-transfer interaction; these frequencies also vary systematically with the rare gas atom attached. The efficiencies of photodissociation also vary with the rare gas atoms because of their widely different binding energies to the cobalt cation. The spectrum of the argon complex could only be measured when at least three argon atoms were attached. In the case of the helium complex, the low binding energy allows the spectra to be measured for the low-frequency H-O-H scissors bending mode and for the O-D stretches of the deuterated analog. The partially resolved rotational structure for the antisymmetric O-H and O-D stretches reveals the temperature of these complexes (6 K) and establishes the electronic ground state. The helium complex has the same 3B1 ground state as the tag-free complex studied previously by Metz and co-workers ["Dissociation energy and electronic and vibrational spectroscopy of Co+(H2O) and its isotopomers," J. Phys. Chem. A 117, 1254 (2013)], but the A rotational constant is contaminated by vibrational averaging from the bending motion of the helium.

Infrared spectroscopy of the protonated HCl dimer and trimer.

The protonated HCl dimer and trimer complexes were prepared by pulsed discharges in supersonic expansions of helium or argon doped with HCl and hydrogen. The ions were mass selected in a reflectron time-of-flight spectrometer and investigated with photodissociation spectroscopy in the IR and near-IR regions. Anharmonic vibrational frequencies were computed with VPT2 at the MP2/cc-pVTZ level of theory. The Cl-H stretching fundamentals and overtones were measured in addition to stretch-torsion combinations. VPT2 theory at this level confirms the proton-bound structure of the dimer complex and provides a reasonably good description of the anharmonic vibrations in this system. The trimer has a HCl-HClH+-ClH structure in which a central chloronium ion is solvated by two HCl molecules via hydrogen bonding. VPT2 reproduces anharmonic frequencies for this system, including several combinations involving core ion Cl-H stretches, but fails to describe the relative band intensities.

Coordination and Solvation in Gas-Phase Ag+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy.

Silver-acetylene cation complexes of the form Ag+(C2H2)n (n = 1-9) were produced via laser ablation in a supersonic expansion of acetylene/argon. The ions were mass selected and studied via infrared laser photodissociation spectroscopy in the C-H stretching region (3000-3500 cm-1). Fragmentation patterns indicate that four ligands are strongly coordinated to the metal cation. Density functional theory calculations were performed in support of the experimental data. Together, infrared spectroscopy and theory provide insight into the structure and bonding of these complexes. The Ag+(C2H2)n (n = 1-4) species are shown to be η2-bonded, cation-π complexes with red-shifted C-H stretches on the acetylene ligands. Unlike Cu+(C2H2)n and Au+(C2H2)n complexes, which have a maximum coordination of three, silver cation is tetrahedrally coordinated to four acetylene ligands. Larger complexes (n = 5-9) are formed by solvation of the Ag+(C2H2)4 core with acetylene. Similar to Cu+(C2H2)n and Au+(C2H2)n complexes, acetylene solvation leads to new and interesting infrared band patterns that are quite distinctive from those of the smaller complexes.

Infrared Spectroscopy of Zn(Acetylene)n+ Complexes: Ligand Activation and Nascent Polymerization.

Zinc-acetylene ion-molecule complexes were produced by laser vaporization in a supersonic expansion. These complexes were mass selected and studied with infrared laser photodissociation spectroscopy complemented by computational chemistry. The combined approach of infrared spectroscopy and theory provides information on the structures and bonding of these complexes, as well as evidence for intracluster reactions. Fragmentation patterns demonstrate that the coordination number of strongly bonded ligands is three. Infrared spectra compared to those predicted by theory allow identification of different isomers at each cluster size. The coordination in these complexes varies between η2 and η1 metal-acetylene connections. Structures based on η2 bonding form a symmetric D3h configuration for the n = 3 complex. This unreactive core ion forms larger clusters with only weakly bonded acetylene in solvation structures. Structures based on three η1-bonded acetylenes form a near-C3v core ion which is the doorway configuration for subsequent reactions. Electron transfer to the next (fourth) acetylene produces a metal-carbon bond and a trans-bent metal-vinyl structure with a terminal radical site. This radical site attaches a fifth acetylene to produce a vinyl-dimer structure. Evidence for continued reactions in the larger clusters is obscured by solvating acetylenes with more intense IR bands. The asymmetric coordination of zinc cations and the critical configuration with three-fold coordination that leads to reactivity are new features of intracluster metal-molecular reactions.

Cation-π Complexes of Silver Studied with Photodissociation and Velocity-Map Imaging.

Ag+(aromatic) ion-molecule complexes of benzene, toluene, or furan are generated in the gas phase by laser vaporization in a supersonic expansion. These ions are mass selected in a time-of-flight spectrometer and studied with ultraviolet laser photodissociation and photofragment imaging. UV laser excitation results in dissociative charge transfer (DCT) for these ions, producing neutral silver atom and the respective aromatic cation as the photofragments. Velocity-map imaging and slice imaging techniques are employed to investigate the kinetic energy release in these photodissociation processes. In each case, DCT produces significant kinetic energy, and evidence is also found for excitation of the internal rovibrational degrees of freedom for the molecular cations. Analysis of the kinetic energy release together with the known ionization energies of silver and the molecular ligands provides new information on the cation-π bond energies.

Photodissociation and Theory to Investigate Uranium Oxide Cluster Cations.

Uranium oxide cluster cations of the form UnOm+ are produced by laser vaporization of a depleted uranium rod in a pulsed supersonic expansion. Ions are mass-analyzed and mass-selected with a time-of-flight spectrometer and studied with UV laser multiphoton dissociation. Cations of the stoichiometry UO2(UO3)n+ were observed as photofragments from all photodissociated cluster cations. (UO3)n+ clusters were also observed to result from dissociation of larger (UO3)n+ clusters, with UO3 neutral as a common leaving group. Electronic structure calculations were used to investigate the stability of the prominent uranium oxide cluster cations using density functional theory (DFT) with the hybrid B3LYP exchange-correlation functional and at the CCSD(T) level with cc-pVnZ-PP basis sets (n = D,T), including diffuse orbitals as computational expense and availability permitted. Clustering energies, relative energies and dissociation energies of the cations are reported. The lowest energy neutral (UO3)n clusters up to n = 3 are rings, n = 4 and 5 are chains with very low energy rings, and n = 6 is 3D. The lowest energy structures for UO2(UO3)n+ are composed of uranyl-like UO2+ units bound by bridging oxygens to other UO22+ units for n = 2 and 3, and for n = 4 a more complex 3D structure is predicted.

Microsolvation in V+(H2O)n Clusters Studied with Selected-Ion Infrared Spectroscopy.

Gas-phase ion-molecule clusters of the form V+(H2O)n (n = 1-30) are produced by laser vaporization in a supersonic expansion. These ions are analyzed and mass-selected with a time-of-flight mass spectrometer and investigated with infrared laser photodissociation spectroscopy. The small clusters (n ≤ 7) are studied with argon tagging, while the larger clusters are studied via the elimination of water molecules. The vibrational spectra for the small clusters include only free O-H stretching vibrations, while larger clusters exhibit redshifted hydrogen bonding vibrations. The spectral patterns reveal that the coordination around V+ ions is completed with four water molecules. A symmetric square-planar structure forms for the n = 4 ion, and this becomes the core ion in larger structures. Clusters up to n = 8 have mostly two-dimensional structures, but hydrogen bonding networks evolve to three-dimensional structures in larger clusters. The free O-H vibration of acceptor-acceptor-donor (AAD)-coordinated surface molecules converges to a frequency near that of bulk water by the cluster size of n = 30. However, the splitting of this vibration for AAD- versus AD-coordinated molecules is still different compared to other singly charged or doubly charged cation-water clusters. This indicates that cation identity and charge-site location in the cluster can produce discernable spectral differences for clusters in this size range.

The Role of Tunneling in the Spectra of H5+ and D5+ up to 7300 cm-1.

The spectra for H5+ and D5+ are extended to cover the region between 4830 and 7300 cm-1. These spectra are obtained using mass-selected photodissociation spectroscopy. To understand the nature of the states that are accessed by the transitions in this and prior studies, we develop a four-dimensional model Hamiltonian. This Hamiltonian is expressed in terms of the two outer H2 stretches, the displacement of the shared proton from the center of mass of these two H2 groups, and the distance between the H2 groups. This choice is motivated by the large oscillator strength associated with the shared proton stretch and the fact that the spectral regions that have been probed correspond to zero, one, and two quanta of excitation in the H2 stretches. This model is analyzed using an adiabatic separation of the H2 stretches from the other two vibrations and includes the non-adiabatic couplings between H2 stretch states with the same total number of quanta of excitation in the H2 stretches. Based on the analysis of the energies and wave functions obtained from this model, we find that when there are one or more quanta of excitation in the H2 stretches the states come in pairs that reflect tunneling doublets. The states accessed by the transitions in the spectrum with the largest intensity are assigned to the members of the doublets with requisite symmetry that are localized on the lowest-energy adiabat for a given level of H2 excitation.

Infrared photodissociation spectroscopy and anharmonic vibrational study of the HO4 + molecular ion.

Molecular cations of HO4 + and DO4 + are produced in a supersonic expansion. They are mass-selected, and infrared photodissociation spectra of these species are measured with the aid of argon-tagging. Although previous theoretical studies have modeled these systems as proton-bound dimers of molecular oxygen, infrared spectra have free OH stretching bands, suggesting other isomeric structures. As a consequence, we undertook extensive computational studies. Our conformer search used a composite method based on an economical combination of single- and multi-reference theories. Several conformers were located on the quintet, triplet, and singlet surfaces, spanning in energy of only a few thousand wavenumbers. Most of the singlet and triplet conformers have pronounced multiconfigurational character. Previously unidentified covalent-like structures (H-O-O-O-O) on the singlet and triplet surfaces likely represent the global minima. In our experiments, HO4 + is formed in a relatively hot environment, and similar experiments have been shown capable of producing multiple conformers in low-lying electronic states. None of the predicted HO4 + isomers can be ruled out a priori based on energetic arguments. We interpret our argon-tagged spectra with Second-Order Vibrational Perturbation Theory with Resonances (VPT2+K). The presence of one or more covalent-like isomers is the only reasonable explanation for the spectral features observed.

Infrared spectroscopy of H+(CO)2 in the gas phase and in para-hydrogen matrices.

The H+(CO)2 and D+(CO)2 molecular ions were investigated by infrared spectroscopy in the gas phase and in para-hydrogen matrices. In the gas phase, ions were generated in a supersonic molecular beam by a pulsed electrical discharge. After extraction into a time-of-flight mass spectrometer, the ions were mass selected and probed by infrared laser photodissociation spectroscopy in the 700 cm-1-3500 cm-1 region. Spectra were measured using either argon or neon tagging, as well as tagging with an excess CO molecule. In solid para-hydrogen, ions were generated by electron bombardment of a mixture of CO and hydrogen, and absorption spectra were recorded in the 400 cm-1-4000 cm-1 region with a Fourier-transform infrared spectrometer. A comparison of the measured spectra with the predictions of anharmonic theory at the CCSD(T)/ANO1 level suggests that the predominant isomers formed by either argon tagging or para-hydrogen isolation are higher lying (+7.8 kcal mol-1), less symmetric isomers, and not the global minimum proton-bound dimer. Changing the formation environment or tagging strategy produces other non-centrosymmetric structures, but there is no spectroscopic evidence for the centrosymmetric proton-bound dimer. The formation of higher energy isomers may be caused by a kinetic effect, such as the binding of X (=Ar, Ne, or H2) to H+(CO) prior to the formation of X H+(CO)2. Regardless, there is a strong tendency to produce non-centrosymmetric structures in which HCO+ remains an intact core ion.

The Lung Mucosa Environment in the Elderly Increases Host Susceptibility to Mycobacterium tuberculosis Infection.

As we age, there is an increased risk for the development of tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) infection. Few studies consider that age-associated changes in the alveolar lining fluid (ALF) may increase susceptibility by altering soluble mediators of innate immunity. We assessed the impact of adult or elderly human ALF during Mtb infection in vitro and in vivo. We identified amplification of pro-oxidative and proinflammatory pathways in elderly ALF and decreased binding capability of surfactant-associated surfactant protein A (SP-A) and surfactant protein D (SP-D) to Mtb. Human macrophages infected with elderly ALF-exposed Mtb had reduced control and fewer phagosome-lysosome fusion events, which was reversed when elderly ALF was replenished with functional SP-A/SP-D. In vivo, exposure to elderly ALF exacerbated Mtb infection in young mice. Our studies demonstrate how the pulmonary environment changes as we age and suggest that Mtb may benefit from declining host defenses in the lung mucosa of the elderly.