N-Aromatic Complexation in Tetraphenyl Porphyrin Iron (III)-Pyridine: Evidence of Spin-Flip via Gas-Phase Electronic Spectroscopy

There is growing interest in the electronic properties of metalloporphyrins especially in relation to their interactions with other molecular species in their local environment. Here, UV-VIS laser photodissociation spectroscopy in vacuo has been applied to an iron-centred metalloporphyrin (FeTPP+) and its N-aromatic adduct with pyridine (py) to determine the electronic effect of complexation. Both the metalloporphyrin (FeTPP+) and pyridine adduct (FeTPP+⋅py) absorb strongly across the spectral region studied (652-302 nm: 1.91-4.10 eV). Notably, a large blue shift was observed for the dominant Soret band (41 nm) upon complexation (0.47±0.02 eV), indicative of strong pyridine binding. Significant differences in the profiles (i. e. number and position of bands) of the electronic spectra are evident comparing FeTPP+ and FeTPP+⋅py. Time-dependent density functional theory calculations were used to assign the spectra, revealing that the FeIII spin-state flips from S=3/2 to S=5/2 upon complexation with pyridine. For FeTPP+, all bright spectral transitions are found to be π-π* in character, with electron density variously distributed across the porphyrin and/or its phenyl substituents. Similar electronic excitations are observed for FeTPP+⋅py, with an additional bright transition which involves charge transfer from the porphyrin to the pyridine moiety.

Spectroscopic investigation of size-dependent CO2 binding on cationic copper clusters: analysis of the CO2 asymmetric stretch

Photofragmentation spectroscopy, combined with quantum chemical computations, was employed to investigate the position of the asymmetric CO2 stretch in cold, He-tagged Cun[CO2]+ (n = 1-10) and Cun[CO2][H2O]+ (n = 1-7) complexes. A blue shift in the band position was observed compared to the free CO2 molecule for Cun[CO2]+ complexes. Furthermore, this shift was found to exhibit a notable dependence on cluster size, progressively redshifting with increasing cluster size. The computations revealed that the CO2 binding energy is the highest for Cu+ and continuously decreases with increasing cluster size. This dependency could be explained by highlighting the role of polarization in electronic structure, according to energy decomposition analysis. The introduction of water to this complex amplified the redshift of the asymmetric stretch, showing a similar dependency on the cluster size as observed for Cun[CO2]+ complexes.

Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation.

Observation of inserted oxocarbonyl species in the tantalum cation-mediated activation of carbon dioxide dictated by two-state reactivity

Reductive activation of carbon dioxide (CO2) has drawn increasing attention as an effective and convenient method to unlock this stable molecule, especially via transition metal-catalyzed reactions. Taking the [TaC4O8]+ ion-molecule complex formed in the laser ablation source as a representative, the reactivity of the tantalum metal cation towards CO2 molecules is explored using infrared photodissociation spectroscopy combined with quantum chemical calculations. The strong absorption in the carbonyl stretching region provides solid evidence for the insertion reactions into CO bonds by the tantalum cation. Two inserted oxocarbonyl products are identified based on the great agreement between the experimental results and simulated infrared spectra of energetically low-lying structures in the singlet and triplet states. The pivotal role of two-state reactivity in driving CO2 activation among three different spin states is rationalized by potential energy surface analysis. Our conclusion provides valuable insight into the intrinsic mechanisms of CO2 activation by the tantalum metal cation, highlighting the affinity of tantalum for CO bond insertion in addition to typical "end-on" binding configurations.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Size-Specific Infrared Spectroscopic Study of the Reactions between Water Molecules and Neutral Vanadium Dimer: Evidence for Water Splitting

Investigation of the reactions between water molecules and neutral metal clusters is important in water splitting but is very challenging due to the inherent difficulty of size selection. Here, we report a size-specific infrared-vacuum ultraviolet spectroscopic study on the reactions of water with neutral vanadium dimer. The V₂O₃H₄ and V₂O₄H₆ products were characterized to have unexpected V₂(μ₂-OH)(μ₂-H)(η¹-OH)₂ and V₂(μ₂-OH)₂(η¹-H)₂(η¹-OH)₂ structures, indicative of a water decomposition. A combination of theory and experiment reveals that the water splitting by V₂ is both thermodynamically exothermic and kinetically facile in the gas phase. The present system serves as a model for clarifying the pivotal roles played by neutral metal clusters in water decomposition and also opens new avenues toward systematic understanding of water splitting by a large variety of single-cluster catalysts.

Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H3O+-Ar and H+(H2O)2-Ar

Infrared photodissociation of protonated water clusters with an Ar atom, namely H₃O⁺-Ar and H⁺(H₂O)₂-Ar, was investigated by an imaging technique for mass-selected ions, to reveal the intra- and intermolecular vibrational dynamics. The presented system has the advantage of achieving fragment ion images with the cluster size- and mode-selective photoexcitation of each OH stretching vibration. Translational energy distributions of photofragments were obtained from the images upon the excitation of the bound (ν_b) and free (ν_f) OH stretching vibrations. The energy fractions in the translational motion were compared between ν_b^I and ν_f^I in H₃O⁺-Ar or between ν_b^II and ν_f^II in H⁺(H₂O)₂-Ar, where the labels "I" and "II" represent H₃O⁺-Ar and H⁺(H₂O)₂-Ar, respectively. In H₃O⁺-Ar, the ν_f^I excitation exhibited a smaller translational energy than ν_b^I. This result can be explained by the higher vibrational energy of ν_f^I, which enabled it to produce bending (ν₄) excited H₃O⁺ fragments that should be favored according to the energy-gap model. In contrast to H₃O⁺-Ar, the ν_b^II excitation of an Ar-tagged H₂O subunit and the ν_f^II excitation of an untagged H₂O subunit resulted in very similar translational energy distributions in H⁺(H₂O)₂-Ar. The similar energy fractions independent of the excited H₂O subunits suggested that the ν_b^II and ν_f^II excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H₂O-H⁺-H₂O moiety. However, the translational energy distributions for H⁺(H₂O)₂-Ar did not agree with a statistical dissociation model, which implied another aspect of the process, that is, Ar dissociation via incomplete energy randomization in the whole H⁺(H₂O)₂-Ar cluster.

An infrared study of CO2 activation by holmium ions, Ho+ and HoO. Phys Chem Chem Phys 2022;24:22716-22723. [PMID: 36106954 DOI: 10.1039/d2cp02862j]

We report a combined experimental and computational study of carbon dioxide activation at gas-phase Ho⁺ and HoO⁺ centres. Infrared action spectra of Ho(CO₂)_n⁺ and [HoO(CO₂)_n]⁺ ion-molecule complexes have been recorded in the spectral region 1700-2400 cm^-1 and assigned by comparison with simulated spectra of energetically low-lying structures determined by density functional theory. Little by way of activation is observed in Ho(CO₂)_n⁺ complexes with CO₂ binding end-on to the Ho⁺ ion. By contrast, all [HoO(CO₂)_n]⁺ complexes n ≥ 3 show unambiguous evidence for formation of a carbonate radical anion moiety, . The signature of this structure, a new vibrational band observed around 1840 cm^-1 for n = 3, continues to red-shift monotonically with each successive CO₂ ligand binding with net charge transfer from the ligand rather than the metal centre.

Ultraviolet photodissociation of Mg+-NO complex: Ion imaging of a reaction branching in the excited states

Ultraviolet photodissociation processes of gas phase Mg⁺-NO complex were studied by photofragment ion imaging experiments and theoretical calculations for excited electronic states. At 355 nm excitation, both Mg⁺ and NO⁺ photofragment ions were observed with positive anisotropy parameters, and theoretical calculations revealed that the two dissociation channels originate from an electronic transition from a bonding orbital consisting of Mg⁺ 3s and NO π* orbitals to an antibonding counterpart. For the NO⁺ channel, the photofragment image exhibited a high anisotropy (β = 1.53 ± 0.07), and a relatively large fraction (∼40%) of the available energy was partitioned into translational energy. These observations are rationalized by proposing a rapid dissociation process on a repulsive potential energy surface correlated to the Mg(¹S) + NO⁺(¹Σ) dissociation limit. In contrast, for the Mg⁺ channel, the angular distribution was more isotropic (β = 0.48 ± 0.03) and only ∼25% of the available energy was released into translational energy. The differences in the recoil distribution for these competing channels imply a reaction branching on the excited state surface. On the theoretical potential surface of the excited state, we found a deep well facilitating an isomerization from bent geometry in the Franck-Condon region to linear and/or T-shaped isomer. As a result, the Mg⁺ fragment was formed via the structural change followed by further relaxation to lower electronic states correlated to the Mg⁺(²S) + NO(²Π) exit channel.

Cryo spectroscopy of N2 on cationic iron clusters

Infrared photodissociation (IR-PD) spectra of iron cluster dinitrogen adsorbate complexes [Fe_n(N₂)_m]⁺ for n = 8-20 reveal slightly redshifted IR active bands in the region of 2200-2340 cm^-1. These bands mostly relate to stretching vibrations of end-on coordinated N₂ chromophores, a μ_1,end end-on binding motif. Density Functional Theory (DFT) modeling and detailed analysis of n = 13 complexes are consistent with an icosahedral Fe₁₃ ⁺ core structure. The first adsorbate shell closure at (n,m) = (13,12)-as recognized by the accompanying paper on the kinetics of N₂ uptake by cationic iron clusters-comes with extensive IR-PD band broadening resulting from enhanced couplings among adjacent N₂ adsorbates. DFT modeling predicts spin quenching by N₂ adsorption as evidenced by the shift of the computed spin minima among possible spin states (spin valleys). The IR-PD spectrum of (17,1) surprisingly reveals an absence of any structure but efficient non-resonant fragmentation, which might indicate some weakly bound (roaming) N₂ adsorbate. The multiple and broad bands of (17,m) for all other cases than (17,1) and (17,7) indicate a high degree of variation in N₂ binding motifs and couplings. In contrast, the (17,7) spectrum of six sharp bands suggests pairwise equivalent N₂ adsorbates. The IR-PD spectra of (18,m) reveal additional features in the 2120-2200 cm^-1 region, which we associate with a μ_1,side side-on motif. Some additional features in the (18,m) spectra at high N₂ loads indicate a μ_1,tilt tilted end-on adsorption motif.

Spectroscopy and Infrared Photofragmentation Dynamics of Mixed Ligand Ion-Molecule Complexes: Au(CO)x(N2O)y. J Phys Chem A 2021;125:7266-7277. [PMID: 34433267 DOI: 10.1021/acs.jpca.1c05800]

We report a combined experimental and computational study of the structure and fragmentation dynamics of mixed ligand gas-phase ion-molecule complexes. Specifically, we have studied the infrared spectroscopy and vibrationally induced photofragmentation dynamics of mass-selected Au(CO)_x(N₂O)_y⁺ complexes. The structures can be understood on the basis of local CO and N₂O chromophores in different solvation shells with CO found preferentially in the core. Rich fragmentation dynamics are observed as a function of complex composition and the vibrational mode excited. The dynamics are characterized in terms of branching ratios for different ligand loss channels in light of calculated internal energy distributions. Intramolecular vibrational redistribution appears to be rapid, and dissociation is observed into all energetically accessible channels with little or no evidence for preferential breaking of the weakest intermolecular interactions.

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

RG_n-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-C_2v 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.

Spectroscopic Identification of Transition-Metal M[η2-(O,O)C] Species for Highly-Efficient CO2 Activation

The CO₂ activation by transition metals is important in CO₂ utilization but has proven to be challenging for experimental targets. Here we report first synthesis and spectroscopic characterization of transition-metal M[η²-(O,O)C] species with bidentate double oxygen metal-CO₂ coordination in the [ZrO(CO₂)_n≥4]⁺ complexes. The Zr[η²-(O,O)C] species yields a CO₂^- radical ligand, showing a high efficiency in CO₂ activation. We find that two important prerequisites are demanded for certain metals to form this intriguing M[η²-(O,O)C] species. One is that the metal center has high reduction capability, and the other is that the oxidation state of the metal center is lower than its highest one by 1. This study highlights the pivotal roles played by the M[η²-(O,O)C] species in CO₂ activation and also open new avenues toward the development of related single-atom catalysts with isolated transition-metal atoms dispersed on supports.

Infrared photodissociation spectroscopic investigation on VO+ and NbO+ hydrolysis catalyzed by water molecules

We investigate the hydrolysis of vanadium/niobium monoxide cation (VO⁺/NbO⁺) with water molecules in the gas phase.

Sodium cationization can disrupt the intramolecular hydrogen bond that mediates the sunscreen activity of oxybenzone

A key decay pathway by which organic sunscreen molecules dissipate harmful UV energy involves excited-state hydrogen atom transfer between proximal enol and keto functional groups. Structural modifications of this molecular architecture have the potential to block ultrafast decay processes, and hence promote direct excited-state molecular dissociation, profoundly affecting the efficiency of an organic sunscreen. Herein, we investigate the binding of alkali metal cations to a prototype organic sunscreen molecule, oxybenzone, using IR characterization. Mass-selective IR action spectroscopy was conducted at the free electron laser for infrared experiments, FELIX (600-1800 cm-1), on complexes of Na+, K+ and Rb+ bound to oxybenzone. The IR spectra reveal that K+ and Rb+ adopt binding positions away from the key OH intermolecular hydrogen bond, while the smaller Na+ cation binds directly between the keto and enol oxygens, thus breaking the intramolecular hydrogen bond. UV laser photodissociation spectroscopy was also performed on the series of complexes, with the Na+ complex displaying a distinctive electronic spectrum compared to those of K+ and Rb+, in line with the IR spectroscopy results. TD-DFT calculations reveal that the origin of the changes in the electronic spectra can be linked to rupture of the intramolecular bond in the sodium cationized complex. The implications of our results for the performance of sunscreens in mixtures and environments with high concentrations of metal cations are discussed.

Structures and Infrared Spectra of [M(CO2)7]+ (M = V, Cr, and Mn) Complexes

Gas-phase infrared photodissociation spectra of [V(CO₂) _n]⁺ complexes revealed three new vibrational bands at 1140, 1800, and 3008 cm^-1 at n = 7, the features of which are retained in the larger clusters (Ricks, A. M.; Brathwaite, A. D.; Duncan, M. A. J. Phys. Chem. A 2013, 117, 11490-11498). However, structural assignment of this intriguing feature remains open. Herein, quantum chemical calculations on [V(CO₂)₇]⁺ were carried out to identify the structure of the low-lying isomers and to assign the observed spectral features. The comparison of calculated infrared spectra of [V(CO₂)₇]⁺ with experimental infrared spectra identified the formation of a bent CO₂^- species, suggesting the ligand-induced activation of CO₂ by the vanadium cation. The structures and infrared spectra of [Cr(CO₂)₇]⁺ and [Mn(CO₂)₇]⁺ were also predicted and discussed.

IR Signature of Size-Selective CO2 Activation on Small Platinum Cluster Anions, Ptn - (n=4-7)

Infrared multiple photon dissociation spectroscopy (IR-MPD) has been employed to determine the nature of CO₂ binding to size-selected platinum cluster anions, Pt_n ^- (n=4-7). Interpreted in conjunction with density functional theory simulations, the results illustrate that the degree of CO₂ activation can be controlled by the size of the metal cluster, with dissociative activation observed on all clusters n≥5. Of potential practical significance, in terms of the use of CO₂ as a useful C1 feedstock, CO₂ is observed molecularly-bound, but highly activated, on the Pt₄ ^- cluster. It is trapped behind a barrier on the reactive potential energy surface which prevents dissociation.

Coordination-induced CO2 fixation into carbonate by metal oxides

Infrared spectroscopic studies reveal how the coordination induces CO₂ fixation into carbonate by a cationic yttrium oxide model catalyst.

Infrared spectroscopy of O˙- and OH- in water clusters: evidence for fast interconversion between O˙- and OH˙OH. Phys Chem Chem Phys 2017;19:25346-25351. [PMID: 28891582 PMCID: PMC7100789 DOI: 10.1039/c7cp04577h]

We present infrared multiple photon dissociation (IRMPD) spectra of (H2O)nO˙- and (H2O)nOH- cluster ensembles for n[combining macron] ≈ 8 and 47 in the range of 2400-4000 cm-1. Both hydrated ions exhibit the same spectral features, in good agreement with theoretical calculations. Decomposition of the calculated spectra shows that bands originating from H2OO˙- and H2OOH- interactions span almost the whole spectral region of interest. Experimentally, evaporation of OH˙ is observed to a small extent, which requires interconversion of (H2O)nO˙- into (H2O)n-1OH˙OH-, with subsequent H2O evaporation preferred over OH˙ evaporation. The modeling shows that (H2O)nO˙- and (H2O)n-1OH˙OH- cannot be distinguished by IRMPD spectroscopy.

A Surprisingly Simple Electrostatic Model Explains Bent Versus Linear Structures in M(+)-RG2 Species (M = Group 1 Metal, Li-Fr; RG = Rare Gas, He-Rn)

It is found that a simple electrostatic model involving competition between the attractive dispersive interaction and induced-dipole repulsion between the two RG atoms performs extremely well in rationalizing the M(+)-RG2 geometries, where M = group 1 metal and RG = rare gas. The Li(+)-RG2 and Na(+)-RG2 complexes have previously been found to exhibit quasilinear or linear minimum-energy geometries, with the Na(+)-RG2 complexes having an additional bent local minimum [A. Andrejeva, A. M. Gardner, J. B. Graneek, R. J. Plowright, W. H. Breckenridge, T. G. Wright, J. Phys. Chem. A, 2013, 117, 13578]. In the present work, the geometries for M = K-Fr are found to be bent. A simple electrostatic model explains these conclusions and is able to account almost quantitatively for the binding energy of the second RG atom, as well as the form of the angular potential, for all 36 titular species. Additionally, results of population analyses are presented together with orbital contour plots; combined with the success of the electrostatic model, the expectation that these complexes are all physically bound is confirmed.

Influence of hydration on ion-biomolecule interactions: M(+)(indole)(H2O)(n) (M = Na, K; n = 3-6)

The indole functional group can be found in many biologically relevant molecules, such as neurotransmitters, pineal hormones and medicines. Indole has been used as a tractable model to study the hydration structures of biomolecules as well as the interplay of non-covalent interactions within ion-biomolecule-water complexes, which largely determine their structure and dynamics. With three potential binding sites: above the six- or five-member ring, and the N-H group, the competition between π and hydrogen bond interactions involves multiple locations. Electrostatic interactions from monovalent cations are in direct competition with hydrogen bonding interactions, as structural configurations involving both direct cation-indole interactions and cation-water-indole bridging interactions were observed. The different charge densities of Na(+) and K(+) give rise to different structural conformers at the same level of hydration. Infrared spectra with parallel hybrid functional-based calculations and Gibbs free energy calculations revealed rich structural insights into the Na(+)/K(+)(indole)(H2O)3-6 cluster ion complexes. Isotopic (H/D) analyses were applied to decouple the spectral features originating from the OH and NH stretches. Results showed no evidence of direct interaction between water and the NH group of indole (via a σ-hydrogen bond) at current levels of hydration with the incorporation of cations. Hydrogen bonding to a π-system, however, was ubiquitous at hydration levels between two and five.

Photoelectron imaging and theoretical study on nascent hydrogen bond network in microsolvated clusters of Au- (CH3OH)n (n = 1-5)

We first demonstrate the photoelectron spectroscopic evidence of the transition of two competitive solvation patterns in the Au(-)(CH3OH)n (n = 1-5) clusters. Quantum chemical calculations have been carried out to characterize the geometric structures, energy properties and hydrogen-bonded patterns, and to aid the spectral assignment. It has been found that the nonconventional hydrogen bonds dominate the small clusters (n = 1 and 2), whereas the conventional hydrogen bonds play more and more important role from n = 2 to n = 5. This finding provides concrete hydrogen bond network evolution of Au(-) surrounded by methanol molecules.

Investigation by two-color IR dissociation spectroscopy of Hoogsteen-type binding in a metalated nucleobase pair mimic

Two Ag(I) ions, deprotonated 1-methyl-thymine (1MT-H)(-) and 1,3-dideaza-adenine (DDA), self-assemble in methanolic solution to a cationic [(Ag)2(1MT-H)(DDA)](+) complex as identified by electrospray ionization mass spectrometry. Assignment of vibrational bands and identification of the silver coordination pattern arise from comparison of one- and two-color Infrared Multiple Photon Dissociation (IRMPD) spectra (1000-4000 cm(-1)) of isolated and trapped complexes to calculated spectra from density functional theory. This approach reveals two structurally and energetically close isomers that resemble a metalated Hoogsteen-like binding motif. The two color IR/IR double resonance scheme proved in particular useful to observe weakly absorbing or weakly fragmenting vibrational modes.

Probing the competition among different coordination motifs in metal-ciprofloxacin complexes through IRMPD spectroscopy and DFT calculations

The vibrational spectra of ciprofloxacin complexes with monovalent (Li(+), Na(+), K(+), Ag(+)) and polyvalent (Mg(2+), Al(3+)) metal ions are recorded in the range 1000-1900 cm(-1) by means of infrared multiple-photon dissociation (IRMPD) spectroscopy. The IRMPD spectra are analyzed and interpreted in the light of density functional theory (DFT)-based quantum chemical calculations in order to identify the possible structures present under our experimental conditions. For each metal-ciprofloxacin complex, four isomers are predicted, considering different chelation patterns. A good agreement is found between the measured IRMPD spectrum and the calculated absorption spectrum of the most stable isomer for each complex. Metal ion size and charge are found to drive the competition among the different coordination motifs: small size and high charge density metal ions prefer to coordinate the quinolone between the two carbonyl oxygen atoms, whereas large-size metal ions prefer the carboxylate group as a coordination site. In the latter case, an intramolecular hydrogen bond compensates the weaker interaction established by these cations. The role of the metal cation on the stabilization of ionic and nonionic structures of ciprofloxacin is also investigated. It is found that large-size metal ions preferentially stabilize charge separated motifs and that the increase of metal ion charge has a stabilizing effect on the zwitterionic form of ciprofloxacin.

Attaching molecular hydrogen to metal cations: perspectives from gas-phase infrared spectroscopy

In this perspective article we describe recent infrared spectroscopic investigations of mass-selected M(+)-H(2) and M(+)-D(2) complexes in the gas-phase, with targets that include Li(+)-H(2), B(+)-H(2), Na(+)-H(2), Mg(+)-H(2), Al(+)-H(2), Cr(+)-D(2), Mn(+)-H(2), Zn(+)-D(2) and Ag(+)-H(2). Interactions between molecular hydrogen and metal cations play a key role in several contexts, including in the storage of molecular hydrogen in zeolites, metal-organic frameworks, and doped carbon nanostructures. Arguably, the clearest view of the interaction between dihydrogen and a metal cation can be obtained by probing M(+)-H(2) complexes in the gas phase, free from the complicating influences of solvents or substrates. Infrared spectra of the complexes in the H-H and D-D stretch regions are obtained by monitoring M(+) photofragments as the excitation wavelength is scanned. The spectra, which feature full rotational resolution, confirm that the M(+)-H(2) complexes share a common T-shaped equilibrium structure, consisting essentially of a perturbed H(2) molecule attached to the metal cation, but that the structural and vibrational parameters vary over a considerable range, depending on the size and electronic structure of the metal cation. Correlations are established between intermolecular bond lengths, dissociation energies, and frequency shifts of the H-H stretch vibrational mode. Ultimately, the M(+)-H(2) and M(+)-D(2) infrared spectra provide a comprehensive set of benchmarks for modelling and understanding the M(+)···H(2) interaction.

UNUSUAL LASER-INDUCED ABSORPTIONS OF Ca+-FORMALDEHYDE: MOLECULAR ORBITAL INTERACTION

We have theoretically studied the photodissociation spectroscopy of Ca +-formaldehyde complex using the TD-B2-PLYP method. The SDD pseudopotential and basis sets for Ca and 6-31++G (2df, 2pd) basis sets for C , H , and O atoms were employed in all calculations. In this way, we have reassigned the photodissociation spectroscopy of this complex. All experimentally observed spectral features can be well explained by our calculation. Besides the charge–dipole interaction, a strong molecule–orbital interaction also exists in the excited states and plays an important role in photoexcitation of the Ca+–CH2O complex.

Invited review article: laser vaporization cluster sources

The laser vaporization cluster source has been used for the production of gas phase atomic clusters and metal-molecular complexes for 30 years. Numerous experiments in the chemistry and physics of clusters have employed this source. Its operation is simple in principle, but there are many subtle design features that influence the number and size of clusters produced, as well as their composition, charge state, and temperature. This article examines all aspects of the design of these cluster sources, discussing the relevant chemistry, physics, and mechanical aspects of experimental configurations employed by different labs. The principles detailed here provide a framework for the design and implementation of this source for new applications.