Photodissociation Spectra Research Articles

Optical Spectroscopy of the Au4+ Cluster: The Resolved Vibronic Structure Indicates an Unexpected Isomer

AbstractKnowledge of the geometric and electronic structure of gold clusters and nanoparticles is vital for understanding their catalytic and photochemical properties at the molecular level. In this study, we report the vibronic optical photodissociation spectrum of cold and mass‐selected Au4+ clusters measured at a resolution high enough to allow for comparison with Franck–Condon simulations of the excited state transitions based on time‐dependent density functional theory calculations. The three vibrational frequencies identified for the lowest‐lying optically accessible excited state at 2.17 eV stem from the Y‐shaped isomer (C2v) and not from the rhombic isomer (D2h) considered to be the ground state structure of Au4+. This study demonstrates that an analysis of low‐resolution electronic spectra by calculations of vertical transitions alone is not sufficient for a reliable isomer assignment of such metal clusters.

Infrared photodissociation of cold CH3+–He2 complexes

ABSTRACTA cryogenic 22-pole ion trap apparatus is used to probe weakly bound cold CH–He complexes by IR action spectroscopy at 4 K. The infrared photodissociation spectrum of the antisymmetric C–H stretching vibration is recorded in the range of 3102–3139 cm−1. A comparison between the experimental spectrum and a symmetric rotor Hamiltonian indicates that this band is strongly perturbed, but we report here a preliminary set of spectroscopic parameters.

Electronic spectroscopy and nanocalorimetry of hydrated magnesium ions [Mg(H2O)n]+, n = 20-70: spontaneous formation of a hydrated electron?

Hydrated singly charged magnesium ions [Mg(H2O)n]+ are thought to consist of an Mg2+ ion and a hydrated electron for n > 15. This idea is based on mass spectra, which exhibit a transition from [MgOH(H2O)n-1]+ to [Mg(H2O)n]+ around n = 15-22, black-body infrared radiative dissociation, and quantum chemical calculations. Here, we present photodissociation spectra of size-selected [Mg(H2O)n]+ in the range of n = 20-70 measured for photon energies of 1.0-5.0 eV. The spectra exhibit a broad absorption from 1.4 to 3.2 eV, with two local maxima around 1.7-1.8 eV and 2.1-2.5 eV, depending on cluster size. The spectra shift slowly from n = 20 to n = 50, but no significant change is observed for n = 50-70. Quantum chemical modeling of the spectra yields several candidates for the observed absorptions, including five- and six-fold coordinated Mg2+ with a hydrated electron in its immediate vicinity, as well as a solvent-separated Mg2+/e- pair. The photochemical behavior resembles that of the hydrated electron, with barrierless interconversion into the ground state following the excitation.

Switching of binding site from nonpolar to polar ligands toward cationic benzonitrile revealed by infrared spectroscopy.

Noncovalent interactions of aromatic molecules in their various charge states with their surrounding environment are of fundamental importance in chemistry and biology. Herein, we analyze the infrared photodissociation spectra of mass-selected cationic clusters of benzonitrile (BN, cyanobenzene, C6H5CN) with L = Ar, N2, and H2O (W), in the CH and OH stretch range (2950-3800 cm-1) with the aid of density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level to probe the interaction of this fundamental aromatic cation in its 2B1 ground electronic state with nonpolar, quadrupolar, and dipolar solvent molecules. While Ar and N2 prefer π-stacking to the aromatic ring of BN+ strongly supported by dispersion forces, W forms a bifurcated CH⋯O ionic hydrogen bond to two adjacent CH groups stabilized by electrostatic forces. Comparison of the BN+-L dimers with related aromatic clusters reveals the effect of ionization, protonation, and substitution of functional groups on the type and strengths of the competing ligand binding motifs.

Unusually sharp FTIR ν(OH) bands and very weak O[sbnd]H⋯F hydrogen bonds in M2(H2O)1,2B12F12 hydrates (M[dbnd]Na[sbnd]Cs)

We report attenuated total reflection FTIR spectra of microcrystalline samples of M2(H2O)n(B12F12) (MNa, K, Rb, Cs) and Na2(H2O)6(B12Cl12). The ν(OH) bands are unusually narrow compared to the spectra of other alkali metal salt hydrates, with most full widths at half-max less than 20 cm−1 and some less than 10 cm−1. In addition, these bands are not as redshifted compared to the spectra of other alkali metal salt hydrates. These unusual effects are attributed to extremely weak OH⋯F and OH⋯Cl hydrogen bonding between the coordinated H2O molecules and the B12F122− and B12Cl122− superweak anions. The ν(OH) bands in these compounds are as narrow as the bands in the FTIR spectrum of monomeric H2O absorbed in poly(vinylidene fluoride), in which the only hydrogen bonding is OH⋯FC, or as the bands in the IR photodissociation spectrum of the gas-phase species [(Ar)Na(H2O)2]+. These unusual spectra are compared to the literature spectrum of [NEt4]4[Hg(CB11F11)2]2∙H2O, which also exhibits sharp ν(OH) bands with relatively small redshifts from the ν(OH) values for H2O(g).

IR photodissociation spectra of SixH4x-4+ (x = 4–8): Evidence for Si[sbnd]H[sbnd]Si proton bridges

Although silane-type cations of the form SixHy+ are thought to play a significant role in plasma chemistry and astrochemistry, their structural and energetic properties are largely unexplored in the size range x ≥ 3. Herein, infrared photodissociation (IRPD) spectra of mass-selected SixHy+ ions (specifically SixH4x-4+ with x = 4–8, Si4H12+-Si8H28+) are recorded in the SiH stretch range and analyzed by dispersion-corrected density functional calculations at the B3LYP-D3/aug-cc-pVDZ level. The SixH4x-4+ ions are produced in a SiH4/H2/He plasma molecular beam expansion. The IRPD process leads to the loss of SiH4 ligands, which corresponds to the lowest-energy fragment channel. Spectral analysis of the IRPD spectra reveals that all SixH4x-4+ ions have at least one SiHSi bridge. The characteristic fingerprint of these three-center two-electron (3c-2e) bonds is the highly IR active antisymmetric stretch fundamental of the SiHSi bridge (σ SiHSi) occurring in the 1600–2100 cm−1 range, whose frequency strongly depends on the structural and energetic details of the SiHSi bridge. Although the investigated SixH4x-4+ ions can formally be described by the formula Si2H4+(SiH4)x-2, the cluster growth is more complex. The appearance of the σSiHSi bands confirms that all considered SixH4x-4+ ions with x ≤ 5 are formed by polymerization reactions. Larger clusters (x ≥ 6) show evidence for the additional presence of weakly-bonded SiH4 ligands attached to smaller chemically-bonded core ions. Correlations of the properties of the SiHSi bridges (bond distances, bond angles, binding energies, SiH stretch frequencies), which vary between strong symmetric 3c-2e chemical bonds and weak hydrogen bonds, are discussed.

Infrared Signatures of Protonated Benzonitrile

Abstract Aromatic hydrocarbons and their protonated ions are important constituents of the interstellar medium (ISM). The recent discovery of benzonitrile (BN; cyanobenzene, C6H5CN) in the ISM suggests that its protonated ion (H+BN) is also present. Herein, we present vibrational signatures of H+BN obtained via infrared photodissociation (IRPD) spectra of its clusters with up to four nonpolar ligands (L = Ar/N2) recorded in the NH (ν NH) and CH (ν CH) stretch range. Protonation of BN occurs at the N atom of the nitrile group. Systematic complexation shifts (Δν NH) observed in the IRPD spectra of H+BN-L n are assigned to cluster structures by comparison to quantum chemical calculations. In the most stable H+BN-L n structures, the first ligand (n = 1) forms a NH+… L ionic hydrogen bond (H-bond), while additional ligands (n = 2–4) are attached to the aromatic ring via π stacking. For L = Ar, a less stable π-bonded H+BN-Ar isomer is also detected, and its IR spectrum provides an accurate experimental estimate of ν NH = 3555 ± 3 cm−1 for bare H+BN, an intense characteristic fingerprint of this ion in the 3 μm range. Comparison of C6H5CNH+ with HCNH+ and CH3CNH+ reveals that the acidity of the NH proton in RCNH+ ions increases in the order R = C6H5 < CH3 < H.

Temperature-Dependent Infrared Photodissociation Spectroscopy of (CO2)3+ Cation.

Infrared photodissociation spectra of He-buffer-gas-cooled (CO2)3+ were measured at ion trap temperatures of 15, 50, 150, and 280 K. Electronic structure calculations at the mPW2PLYPD/aug-cc-pVDZ level were performed to identify the structures of the low-lying isomers and to assign the observed spectral features. The experimental and calculated infrared spectra show that the (CO2)3+ cations formed in the source are primarily dominated by the charge partially delocalized C2O4+ motif, in which the positive charge is partially delocalized over the two CO2 molecules. Thermal heating at elevated internal temperature supplies sufficient energy to overcome the isomerization barriers and gives access to the charge completely delocalized (CO2) n+ ( n = 3) motif, in which the positive charge is almost completely delocalized over all of the constituent CO2 molecules.

Probing the Structures of Solvent-Complexed Ions Formed in Electrospray Ionization Using Cryogenic Infrared Photodissociation Spectroscopy.

The gas-phase infrared photodissociation (IRPD) spectra of solvent-tagged small biomolecules are studied in a cryogenic ion trap at 17 K. In this study para-aminobenzoic acid (PABA) and tyramine molecules are noncovalently tagged with water or acetonitrile in the electrospray ionization (ESI) source. The complexes are then cooled in the cryogenic trap prior to spectroscopic measurements. These molecules provide two putative sites for solvent attachment: the protonated amine (NH3+) and the OH groups. Comparisons of the experimental IR spectra to theoretical spectra obtained with density functional theory show that the NH3+ site is mainly favored. Evidence for the formation of both NH3-bound and OH-bound conformers is found only in tyramine, despite having similar solution- and gas-phase energetics to that of PABA. Since the structures cannot interconvert in the gas phase, this suggests an isomerization during the electrospray process.

Photodissociation of Sodium Iodide Clusters Doped with Small Hydrocarbons.

Marine aerosols consist of a variety of compounds and play an important role in many atmospheric processes. In the present study, sodium iodide clusters with their simple isotope pattern serve as model systems for laboratory studies to investigate the role of iodide in the photochemical processing of sea‐salt aerosols. Salt clusters doped with camphor, formate and pyruvate are studied in a Fourier transform ion cyclotron resonance mass spectrometer (FT‐ICR MS) coupled to a tunable laser system in both UV and IR range. The analysis is supported by ab initio calculations of absorption spectra and energetics of dissociative channels. We provide quantitative analysis of IRMPD measurements by reconstructing one‐photon spectra and comparing them with the calculated ones. While neutral camphor is adsorbed on the cluster surface, the formate and pyruvate ions replace an iodide ion. The photodissociation spectra revealed several wavelength‐specific fragmentation pathways, including the carbon dioxide radical anion formed by photolysis of pyruvate. Camphor and pyruvate doped clusters absorb in the spectral region above 290 nm, which is relevant for tropospheric photochemistry, leading to internal conversion followed by intramolecular vibrational redistribution, which leads to decomposition of the cluster. Potential photodissociation products of pyruvate in the actinic region may be formed with a cross section of <2×10−20 cm2, determined by the experimental noise level.

Characterization of Respiration in Paraburkholderia tropica Ppe8T under Static Culture Conditions

Objective: To investigate the respiratory chain of Paraburkholderia tropica under limited oxygen conditions would allow us to better understand their functions in plant tissue. Methods: We first optimized the medium for P. tropica biomass production in static culture. Oxidase and Oxidoreductase activities were determined in absence and presence de inhibitors. Spectroscopic analysis of cytochromes was obtained at room and liquid nitrogen temperature. Findings: Isolated membranes showed respiratory oxidase activities with succinate, glucose, NADH and ascorbate-N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD). Succinate and glucose respiration were inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide, antimycin A and myxothiazol, suggesting the presence of a bc1 complex.KCN inhibition curves in the presence of ascorbate-TMPD, succinate or glucose were biphasic, with KCN-sensitive and -resistant oxidases. Spectral analysis with physiological or artificial substrates revealed the presence of only cytochromes b and c, with remarkable cytochrome reduction by ascorbate alone. The CO difference and photodissociation spectra of glucosereduced membranes revealed one component with the spectral features of a cytochrome o´-like CO complex. Air-oxidation spectra of the cytochromes with KCN showed that cytochrome c is an obligate participant in the KCN-sensitive pathway and revealed the presence of a cyanide-resistant oxidase cytochrome bo. Novelty: The characterization of the electron transport system of P. tropica allowed us to propose the following components: 1. Dehydrogenases for succinate, glucose and NADH, 2. A ubiquinone, 3. A bc complex and 4. Two oxidases, cbb-type cytochrome c oxidase and bo-type quinol oxidase. Keywords: bc1 complex, cbb oxidase, Cytochromes, Paraburkholderia tropica, Quinol Oxidase, Respiration

Photofragment imaging and electronic spectroscopy of Al2.

A combination of photodissociation spectroscopy, ion imaging, and high-level theory is employed to refine the bond strength of the aluminum dimer cation (Al2+) and elucidate the electronic structure and photodissociation dynamics between 38 500 and 42 000 cm-1. Above 40 400 cm-1, structured photodissociation is observed from an extremely anharmonic excited state, which calculations identify as the double minimum G 2Σ+u state. The photodissociation spectrum of the G 2Σ+u ← X 2Σ+g transition in Al2+ gives an average vibrational spacing of 170 cm-1 for the G 2Σ+u state and ν0 = 172 cm-1 for the ground state. Photofragment images of G 2Σ+u ← X 2Σ+g transitions indicate that once the Al (4P) + Al+ (1S) product channel is energetically accessible, it dominates the lower energy, spin-allowed pathways despite being spin-forbidden. This is explained by a proposed competition between radiative and non-radiative decay pathways from the G 2Σ+u state. The photofragment images also yield D0 (Al+-Al) = 136.6 ± 1.8 kJ/mol, the most precise measurement to date, highlighting the improved resolution achieved from imaging at near-threshold energies. Additionally, combining D0 (Al+-Al) with IE (Al) and IE (Al2) gives an improved neutral D0 (Al-Al) = 136.9 ± 1.8 kJ/mol.

Experimental Identification of the Active Site in the Heteronuclear Redox Couples [AlVOx ]+. /CO/N2 O (x=3, 4) by Gas-Phase IR Spectroscopy.

Cryogenic ion vibrational spectroscopy was used in combination with electronic structure calculations to identify the active site in the oxygen atom transfer reaction [AlVO4 ]+. +CO→[AlVO3 ]+. +CO2 . Infrared photodissociation spectra of messenger-tagged heteronuclear clusters demonstrate that in contrast to [AlVO4 ]+. , [AlVO3 ]+. is devoid of a terminal Al-Ot unit while the terminal V=Ot group remains intact. Thus it is the Al-Ot moiety that forms the active site in the [AlVOx ]+. /CO/N2 O (x=3, 4) redox couples, which is in line with theoretical predictions.

Helium-induced electronic transitions in photo-excited Ba+-Hen exciplexes.

The possibility for helium-induced electronic transitions in a photo-excited atom is investigated using Ba+ excited to the 6p 2P state as a prototypical example. A diabatization scheme has been designed to obtain the necessary potential energy surfaces and couplings for complexes of Ba+ with an arbitrary number of helium atoms. It involves computing new He-Ba+ electronic wave functions and expanding them in determinants of the non-interacting complex. The 6p 2P ← 6s 2S photodissociation spectrum of He⋯Ba+ calculated with this model shows very weak coupling for a single He atom. However, several electronic relaxation mechanisms are identified, which could potentially explain the expulsion of barium ions from helium nanodroplets observed experimentally upon Ba+ photoexcitation. For instance, an avoided crossing in the ring-shaped He7Ba+ structure is shown to provide an efficient pathway for fine structure relaxation. Symmetry breaking by either helium density fluctuations or vibrations can also induce efficient relaxation in these systems, e.g., bending vibrations in the linear He2Ba+ excimer. The identified relaxation mechanisms can provide insight into helium-induced non-adiabatic transitions observed in other systems.

Microhydration of Dibenzo-18-Crown-6 Complexes with K+, Rb+, and Cs+ Investigated by Cold UV and IR Spectroscopy in the Gas Phase.

In this Article, we examine the hydration structure of dibenzo-18-crown-6 (DB18C6) complexes with K+, Rb+, and Cs+ ion in the gas phase. We measure well-resolved UV photodissociation (UVPD) spectra of K+·DB18C6·(H2O) n, Rb+·DB18C6·(H2O) n, and Cs+·DB18C6·(H2O) n ( n = 1-8) complexes in a cold, 22-pole ion trap. We also measure IR-UV double-resonance spectra of the Rb+·DB18C6·(H2O)1-5 and the Cs+·DB18C6·(H2O)3 complexes. The structure of the hydrated complexes is determined or tentatively proposed on the basis of the UV and IR spectra with the aid of quantum chemical calculations. Bare complexes (K+·DB18C6, Rb+·DB18C6, and Cs+·DB18C6) have a similar boat-type conformation, but the distance between the metal ions and the DB18C6 cavity increases with increasing ion size from K+ to Cs+. Although the structural difference of the bare complexes is small, it highly affects the manner in which each is hydrated. For the hydrated K+·DB18C6 complexes, water molecules bind on both sides (top and bottom) of the boat-type K+·DB18C6 conformer, while hydration occurs only on top of the Rb+·DB18C6 and Cs+·DB18C6 complexes. On the basis of our analysis of the hydration manner of the gas-phase complexes, we propose that, for Rb+·DB18C6 and Cs+·DB18C6 complexes in aqueous solution, water molecules will preferentially bind on top of the boat conformers because of the displaced position of the metal ions relative to DB18C6. In contrast, the K+·DB18C6 complex can accept H2O molecules on both sides of the boat conformation. We also propose that the characteristic solvation manner of the K+·DB18C6 complex will contribute entropically to its high stability and thus to preferential capture of K+ ion by DB18C6 in solution.

Cation-Size-Dependent Conformational Locking of Glutamic Acid by Alkali Ions: Infrared Photodissociation Spectroscopy of Cryogenic Ions.

Consolidated knowledge of conformation and stability of amino acids and their clusters is required to understand their biochemical recognition. Often, alkali ions interact with amino acids and proteins. Herein, infrared photodissociation (IRPD) spectra of cryogenic metalated glutamic acid ions (GluM+, M = Li-Cs) are systematically analyzed in the isomer-specific fingerprint and XH stretch ranges (1100-1900, 2600-3600 cm-1) to provide a direct measure for cation-size-dependent conformational locking. GluM+ ions are generated by electrospray ionization and cooled down to 15 K in a cryogenic quadrupole ion trap. The assignment of the IRPD spectra is supported by density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level. In the global minimum of GluM+, the flexibility of Glu is strongly reduced by the formation of rigid ionic CO···M+···OC metal bridges, corresponding to charge solvation. The M+ binding energy decreases monotonically with increasing cation size from D0 = 314 to 119 kJ/mol for Li-Cs. Whereas for Li and Na only the global minimum of GluM+ is observed, for K-Cs at least three isomers exist at cryogenic temperature. The IRPD spectra of cold GluM+ ions are compared to IR multiple-photon dissociation spectra measured at room temperature. Furthermore, we elucidate the differences of the impact of protonation and metalation on the structure and conformational locking of Glu.

IR Spectrum and Structure of Protonated Monosilanol: Dative Bonding between Water and the Silylium Ion.

We report the spectroscopic characterization of protonated monosilanol (SiH3 OH2+ ) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH3 OH2+ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH3 OH2+ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H3 Si+ -OH2 bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence pz orbital of SiH3+ and CH3+ .

IR Spectrum and Structure of Protonated Monosilanol: Dative Bonding between Water and the Silylium Ion

AbstractWe report the spectroscopic characterization of protonated monosilanol (SiH3OH2+) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH3OH2+ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH3OH2+ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H3Si+‐OH2 bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence pz orbital of SiH3+ and CH3+.

Deconstructing Prominent Bands in the Terahertz Spectra of H7O3+ and H9O4+: Intermolecular Modes in Eigen Clusters.

We report experimental vibrational action spectra (210-2200 cm-1) and calculated IR spectra, using recent ab initio potential energy and dipole moment surfaces, of H7O3+ and H9O4+. We focus on prominent far-IR bands, which postharmonic analyses show, arise from two types of intermolecular motions, i.e., hydrogen bond stretching and terminal water wagging modes, that are similar in both clusters. The good agreement between experiment and theory further validates the accuracy of the potential and dipole moment surfaces, which was used in a recent theoretical study that concluded that infrared photodissociation spectra of the cold clusters correspond to the Eigen isomer. The comparison between theory and experiment adds further confirmation of the need of postharmonic analysis for these clusters.

Strategy for Modeling the Infrared Spectra of Ion-Containing Water Drops.

Hydrated ions are ubiquitous in environmental and biological media. Understanding the perturbation exerted by an ion on the water hydrogen bond network is possible in the nanodrop regime by recording vibrational spectra in the O-H bond stretching region. This has been achieved experimentally in recent years by forming gaseous ions containing tens to hundreds of water molecules and recording their infrared photodissociation spectra. In this paper, we demonstrate the capabilities of a modeling strategy based on an extension of the AMOEBA polarizable force field to implement water atomic charge fluctuations along with those of intramolecular structure along the dynamics. This supplementary flexibility of nonbonded interactions improves the description of the hydrogen bond network and, therefore, the spectroscopic response. Finite temperature IR spectra are obtained from molecular dynamics simulations by computing the Fourier transform of the dipole moment autocorrelation function. Simulations of 1-2 ns are required for extensive sampling in order to reproduce the experimental spectra. Furthermore, bands are assigned with the driven molecular dynamics approach. This method package is shown to compare successfully with experimental spectra for 11 ions in water drops containing 36-100 water molecules. In particular, band frequency shifts of the free O-H stretching modes at the cluster surface are well reproduced as a function of both ion charge and drop size.