Infrared Multiple Photon Dissociation Spectra Research Articles

Cluster Size Dependent Interaction of Free Manganese Oxide Clusters with Acetic Acid and Methyl Acetate

We have employed infrared multiple-photon dissociation (IR-MPD) spectroscopy together with density functional theory (DFT) calculations to study the interaction of series of subnanometer sized manganese oxide clusters, MnxOy+ (x = 1-6, y = 0-9) with acetic acid (HOAc) and methyl acetate (MeOAc). Reaction with HOAc leads to strongly cluster size and composition dependent IR-MPD spectra, indicating molecular adsorption on MnOx+ clusters and thermodynamically favorable but kinetically hampered HOAc dissociation (deprotonation) on Mn2O4+ and Mn3O5+. Other cluster sizes exhibit the preferred formation of a dissociative bidentate chelating structure. In contrast to HOAc, all clusters bind MeOAc via the carbonyl group as an intact molecule, and dissociation appears to be kinetically hindered under the given experimental conditions.

Mid-infrared spectroscopic signatures of dibenzopyrene cations – The effect of symmetry on PAH IR spectroscopy

In this work, we characterize – for the first time – in the gas phase infrared spectra of three isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations of C24H14 composition that belong to distinctly different symmetry groups (C2h, Cs and C1). Mid-infrared (Mid-IR) spectra are recorded by means of infrared multiple photon dissociation (IRMPD) spectroscopy at the free electron laser for infrared experiments (FELIX) laboratory. The measured infrared (IR) band positions compare reasonably well with density functional theory (DFT) calculated values. The number of IR active bands increases as the symmetry of the molecule lowers. The IRMPD spectra of irregular PAHs are found to be dense and do not resemble the sharp signatures typical of astronomical IR bands, but rather look like the broad plateau on which these are perched. This lends credit to the GrandPAH hypothesis that suggests that small and irregular PAHs are weeded out by the strong interstellar radiation field and only large regular PAHs remain.

Structural characterization of [M,C,2H]+ products formed by reaction of 5d metal cations Pt+ and Ir+ with ethylene oxide and Ta+ with methane using messenger spectroscopy

Structural characterization of gas-phase [M,C,2H]+ (M = Ta, Ir, Pt), formed by reacting laser ablation formed M+ with ethylene oxide (c-C2H4O) or methane under multiple collision conditions, is achieved using infrared multiple-photon dissociation (IR-MPD) spectroscopy with the intracavity free-electron laser FELICE. After product formation, part of the product distribution is complexed with Ar, allowing for simultaneous recording of IR-MPD spectra of both bare [M,C,2H]+, which dissociates via dehydrogenation, and [M,C,2H]+∙Ar, which loses Ar. Comparison of the spectra with density functional theory (DFT) calculations allows for an internally consistent assignment of the spectra to the Ta+CH2 (3A′′) distorted carbene, Pt+CH2 (2A1) carbene, and to the HIr+CH (1A′) carbyne-hydride. Evidence for a symmetric Ta+CH2∙Ar (3B2) complex is also obtained. For Pt and Ir, these structures match those found in previous work when these species were formed by reaction of M+ with methane, CH4 and CD4. Under the current conditions, no clear signs of the previously observed Ir+CH2 (3A2) carbene product were found, consistent with its higher energy, especially after Ar complexation. Potential energy surfaces for the reactions of Pt+ and Ir+ with c-C2H4O are also computed.

Infrared multiple photon dissociation action spectroscopy of protonated unsymmetrical dimethylhydrazine and proton-bound dimers of hydrazine and unsymmetrical dimethylhydrazine.

The gas-phase structures of protonated unsymmetrical 1,1-dimethylhydrazine (UDMH) and the proton-bound dimers of UDMH and hydrazine are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser and an optical parametric oscillator laser system. To identify the structures present in the experimental studies, the measured IRMPD spectra are compared to spectra calculated at the B3LYP-GD3BJ/6-311+G(d,p) level of theory. These comparisons show that protonated UDMH binds the proton at the methylated nitrogen atom (α) with two low-lying α conformers probably being populated. For (UDMH)2H+, the proton is shared between the methylated nitrogen atoms with several low-lying α conformers likely to be populated. Higher-lying conformers of (UDMH)2H+ in which the proton is shared between α and β (unmethylated) nitrogen atoms cannot be ruled out on the basis of the IRPMD spectrum. For (N2H4)2H+, there are four low-lying conformers that all reproduce the IRMPD spectrum reasonably well. As hydrazine and UDMH see usage as fuels for rocket engines, such spectra are potentially useful as a means of remotely monitoring rocket launches, especially in cases of unsuccessful launches where environmental hazards need to be assessed.

Infrared Study of OCS Binding and Size-Selective Reactivity with Gold Clusters, Aun+ (n = 1-10).

OCS binding to and reactivity with isolated gold cluster cations, Aun+ (n = 1-10), has been studied by infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with quantum chemical calculations. The distribution of complexes AunSx(OCS)m+ formed reflects the relative reactivity of different cluster sizes with OCS, under the multiple collision conditions of our ablation source. The IR-MPD spectra of Aun(OCS)+ (n = 3-10) clusters are interpreted in terms of either μ1 or μ2 S binding motifs. Analysis of the fragmentation products following infrared excitation of parent Aun(OCS)+ clusters reveals strongly size-selective (odd-even) branching ratios for OCS and CO loss, respectively. CO loss signifies infrared-driven OCS decomposition on the cluster surface and is observed to occur predominantly on even n clusters (i.e., those with odd electron counts). The experimental data, including fragmentation branching ratios, are consistent with calculated potential energy landscapes, in which the initial species trapped are molecularly bound entrance channel complexes, rather than global minimum inserted structures. Attempts to generate Rhn(OCS)+ and Ptn(OCS)+ equivalents failed; only sulfide reaction products were observed in the mass spectrum, even after cooling the cluster source to -100 °C.

Energetic Stabilization of Carboxylic Acid Conformers by Manganese Atoms and Clusters.

Free cationic manganese atoms and clusters Mnx+ (x = 1-3) have been reacted with small carboxylic acids (formic, acetic, and propionic acids) and methyl acetate in a flow tube reactor held at room temperature. The geometry of the thus formed complexes has subsequently been studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density-functional theory (DFT) calculations. The IR-MPD spectra of the acid complexes show two signals in the C═O stretch region indicating the coexistence of two conformers. In agreement, the DFT calculations reveal that the-intrinsically less stable-cis-conformer of the carboxylic acids binds more strongly to Mn+ than the trans-conformer, which leads to the energetic stabilization of the former. This stronger binding is attributed to a stronger electrostatic interaction with the manganese cation. A similar stabilization is also predicted for the cis-conformer of methyl acetate; however, the resulting change of the C═O stretch eigenfrequency is too small to be resolved in the experiment. This finding can open up completely new routes for the future room-temperature preparation of the cis-conformers of carboxylic acids and their derivatives.

A Combined Infrared Ion Spectroscopy and Computational Chemistry Study of Hydroxyproline Isomers.

Hydroxyproline is a common variation of proline, with diverse biological roles. The hydroxylation of proline gives rise to several (natural and/or synthetic) isomeric forms, including both positional isomers and stereoisomers. While mass spectrometry is widely touted as a very selective analytical technique, the identification of closely related isomers often poses a challenge. In these cases, allied technologies become helpful in providing full characterization. Here, infrared multiple photon dissociation (IRMPD) spectroscopy is used to differentiate between three isomers, namely cis-3-hydroxyproline, cis-4-hydroxyproline, and trans-4-hydroxyproline. In contrast to the protonated species which show only minor variations in their IRMPD spectra, lithiated species were found to display significant spectral differences, making their differentiation more straightforward. The conformational origin of these spectral differences was investigated by complementary quantum-chemical calculations.

IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions

Protonation at the formyl oxygen atom of benzaldehydes leading to the formation of carboxonium ions yields two distinct isomers, depending on the relative orientation of the proton either cis or trans with respect to the hydrogen atom on the adjacent carbon. In this context, the IR multiple photon dissociation (IRMPD) spectra of protonated ortho, meta, and para-hydroxybenzaldehydes (OH-BZH+ ), delivered into the gas phase by electrospray ionization of hydro-alcoholic solutions, are reported in the 3200-3700 cm-1 spectral range. This range is characteristic of O-H stretching modes and thus able to differentiate cis and trans carboxonium isomers. Comparison between IRMPD spectra and DFT calculations at the B3LYP/6-311++G(2df2p) level suggests that for both p-OH-BZH+ and m-OH-BZH+ only cis conformers are present in the ion population analyzed. For o-OH-BZH+ , IRMPD spectroscopy points to a mixture comprising one trans and more than one cis conformers. The energy barrier for cis-trans isomerization calculated for each OH-BZH+ isomer is a measure of the degree of π-electron delocalization. Furthermore, IRMPD spectra of p-OH-BZH+ , m-OH-BZH+ and protonated phenol (this last used as reference) were recorded also in the fingerprint range. Both the observed C-O and O-H stretching vibrations appear to be a measure of π-electron delocalization in the ions.

Development of a photoinduced fragmentation ion trap for infrared multiple photon dissociation spectroscopy.

Methods for isomer discrimination by mass spectroscopy are of increasing interest. Here we describe the development of a three-dimensional ion trap for infrared multiple photon dissociation (IRMPD) spectroscopy that enables the acquisition of the infrared spectrum of selected ions in the gas phase. This system is suitable for the study of a myriad of chemical systems, including isomer mixtures. A modified three-dimensional ion trap was coupled to a CO2 laser and an optical parametric oscillator/optical parametric amplifier (OPO/OPA) system operating in the range 2300 to 4000 cm-1 . Density functional theory vibrational frequency calculations were carried out to support spectral assignments. Detailed descriptions of the interface between the laser and the mass spectrometer, the hardware to control the laser systems, the automated system for IRMPD spectrum acquisition and data management are presented. The optimization of the crystal position of the OPO/OPA system to maximize the spectroscopic response under low-power laser radiation is also discussed. OPO/OPA and CO2 laser-assisted dissociation of gas-phase ions was successfully achieved. The system was validated by acquiring the IRMPD spectra of model species and comparing with literature data. Two isomeric alkaloids of high economic importance were characterized to demonstrate the potential of this technique, which is now available as an open IRMPD spectroscopy facility in Brazil.

Influence of the local environment on the intrinsic structures of gas-phase cytidine-5′-monophosphates

Gas-phase conformations of the deprotonated disodium cationized 2′-deoxycytidine-5′-monophosphate and cytidine-5′-monophosphate nucleotides, [pdCyd−H+2Na]+ and [pCyd−H+2Na]+, are studied by infrared multiple photon dissociation (IRMPD) action spectroscopy. Analysis of the experimental results is assisted by complimentary electronic structure calculations of low-energy conformers at the B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) level of theory. These calculations provide relative energetics and predicted IR spectra of the calculated conformers for comparison to the measured IRMPD action spectra in the IR fingerprint and hydrogen-stretching regions. Comparisons between the predicted IR and measured IRMPD spectra provide insight into the conformations accessed by [pdCyd−H+2Na]+ and [pCyd−H+2Na]+ during the experiments. Comparison of these calculations and spectroscopic analysis with those performed in previous studies for cytidine nucleotides representing those present in different local environments allows for elucidation of the impact of the local environment on the intrinsic structure of these nucleotides. Comparison of these results with similar studies of the cytidine nucleosides also helps reveal the impact of the phosphate moiety on structure. Although several conformers of both [pdCyd−H+2Na]+ and [pCyd−H+2Na]+ are observed experimentally, a common sodium cation binding mode is observed, highlighting the importance of the stabilization it provides to the cytidine nucleotides.

Characterization of protonated AcAlaDab and AcDabAla by IRMPD spectroscopy and molecular modeling

Alanine (Ala) based oligopeptides containing a residue of 2,4-diaminobutyric acid (Dab) serve as a unique model to examine the effect of the side-chain ionization on the conformational changes of the peptide backbone structure. In this work, two isomeric dipeptide ions, AcAlaDabH+ and AcDabAlaH+, are studied by infrared multiple photon dissociation (IRMPD) spectroscopy and quantum chemical modeling. The IRMPD spectra were recorded in a Fourier transform-ion cyclotron resonance mass spectrometer coupled to an infrared free electron laser. For each peptide ion, seven theoretical infrared (IR) spectra were obtained from a set of unique low-energy conformations calculated at the ωB97X-D/6-311 + G(d,p) level of theory. A manually weighted IR spectrum containing two conformations gives the best prediction for each IRMPD spectrum. An interesting spectral feature is that the frequency of the amide I band of AcAlaDabH+ is about 30 cm−1 lower compared to that of AcDabAlaH+. This redshift of the amide I band implies a stronger hydrogen-bonding interaction between the charged NH3+ group on the side-chain of Dab and the CO group of the backbone in the former ion. The stronger hydrogen-bonding interaction is likely due to a combination of a shorter H⋯O distance and a more favorable alignment of the N–H–O moiety in a unique conformation adopted by AcAlaDabH+, which results in an increased stability of the ion. The results explain an earlier observation that AcAlaDab has a higher proton affinity than AcDabAla.

Ion spectroscopy and guided ion beam studies of protonated asparaginyl-threonine decomposition: Influence of a hydroxyl containing C-Terminal residue on deamidation processes

The spontaneous deamidation of asparagine residues plays a significant role in various biological functions and degenerative, aging diseases. Here, we present a full description of the deamidation reaction (as well as other key dissociation processes) of protonated asparaginyl-threonine, [AsnThr + H]+, via complementary infrared multiple photon dissociation (IRMPD) ion spectroscopy, threshold collision-induced dissociation (TCID), and theoretical studies. IRMPD spectra allow for the clear identification of precursor and product ion structural conformations when compared to theoretically calculated spectra for likely structures. Analysis of kinetic energy dependent cross sections measured via TCID with xenon using a guided ion beam tandem mass spectrometer allows for characterization of the energies involved in the decomposition processes of interest. Threshold energies are compared to relative single point energies of major reaction species calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels of theory, thereby determining important rate-limiting steps involved in [AsnThr + H]+ decomposition. Our studies confirm the formation of a succinimide intermediate via deamidation of [AsnThr + H]+, an observation consistent with condensed-phase deamidation analyses. Interestingly, our spectroscopic results suggest deamidation does not produce furanone isomers even though theoretical results indicate this pathway (exhibited in gas-phase analyses of similar dipeptide systems) should be competitive. Dehydration of [AsnThr + H]+ is also observed, where theory suggests that oxazolone and oxazoline formation are competitive at threshold energies, but IRMPD analyses conclusively confirm the formation of the oxazoline structure. The comprehensive results presented (in addition to complementary studies discussed herein) allow for a valuable analysis of C-terminal residue side-chain effects on the deamidation process.

Structures of [GPGG + H – H2O]+ and [GPGG + H – H2O – NH[dbnd]CH2]+ ions; evidence of rearrangement prior to dissociation

Infrared multiple photon dissociation (IRMPD) spectroscopy shows the [GPGG + H – H2O]+ ion to have an imidazolone structure. Collision-induced dissociation of this [b4]+ ion results in the loss of HNCH2 from the first residue; the IRMPD spectrum of this MS3 product ion is very similar to that of the [b4]+ ion itself, strongly indicating that the [b4 – HNCH2]+ ion also has an imidazolone structure. Losses of CO and glycine are the dominant dissociation pathways for the [b4 – HNCH2]+ ion. The latter loss requires tautomerism of the keto-form of the imidazolone ring to become the lower-energy enol-form, prior to dissociation. Isotopic labelling showed that loss of CO occurs from the ring of the keto-form. Density functional theory calculations were performed at both the B3LYP/6–311++G (d,p) and M06–2X/6–311++G (d,p) levels and the results are in good agreement.

The Sequence of Coronene Hydrogenation Revealed by Gas-phase IR Spectroscopy

Abstract Gas-phase coronene cations ( ) can be sequentially hydrogenated with up to 24 additional H atoms, inducing a gradual transition from a planar, aromatic molecule toward a corrugated, aliphatic species. The mass spectra of hydrogenated coronene cations show that molecules with odd numbers of additional hydrogen atoms (n H) are dominant, with particularly high relative intensity for “magic numbers” n H = 5, 11, and 17, for which hydrogen atoms have the highest binding energies. Reaction barriers and binding energies strongly affect the hydrogenation sequence and its site specificity. In this contribution, we monitor this sequence experimentally by the evolution of infrared multiple-photon dissociation (IRMPD) spectra of gaseous with n H = 3–11, obtained using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer. For weakly hydrogenated systems (n H = 3, 5) multiple-photon absorption mainly leads to loss of H atoms (and/or H2). With increasing n H, C2H2 loss becomes more relevant. For n H = 9, 11, the carbon skeleton is substantially weakened and fragmentation is distributed over a large number of channels. A comparison of our IRMPD spectra with density functional theory calculations clearly shows that only one or two hydrogenation isomers contribute to each n H. This confirms the concept of hydrogenation occurring along very specific sequences. Moreover, the atomic sites participating in the first 11 steps of this hydrogenation sequence are clearly identified.

IRMPD action spectroscopy, ER-CID experiments, and theoretical approaches investigate intrinsic L-thymidine properties compared to D-thymidine: Findings support robust methodology

L-Thymidine (L-dThd) is the enantiomer of D-thymidine (dThd), a naturally-occurring pyrimidine nucleoside found within DNA nucleic acids. L-dThd, also known as Telbivudine, does not occur naturally, but in the last decade has found successful application as an antiviral medication for hepatitis B virus infection. In this work, the gas-phase conformers of the protonated and sodium cationized forms of L-dThd, [L-dThd+H]+ and [L-dThd + Na]+, are investigated using infrared multiple photon dissociation (IRMPD) action spectroscopy complemented by electronic structure calculations performed at the B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) level of theory. Comparisons between the experimental IRMPD spectra and theoretical linear IR spectra elucidate the stable low-energy conformations adopted by these L-dThd complexes generated by electrospray ionization. Minor 2,4-dihydroxy tautomers (T) and O2 protonated conformers contribute to the experimental [L-dThd+H]+ population, whereas conformers involving tridentate binding of Na+ to the O2, O4′, and O5′ atoms primarily contribute to the experimental [L-dThd + Na]+ population. Theory predicts a tautomer as the protonated ground conformer of [L-dThd+H]+ with thymine in an anti orientation and a tridentate (O2O4′O5′) sodium cationized ground conformer with a syn thymine orientation, consistent with theoretical predictions for [dThd+H]+ and [dThd + Na]+, respectively. Both protonated and sodium cationized L-dThd and dThd illustrate highly parallel IRMPD spectral features as expected. Survival yield analyses of data from energy-resolved collision-induced dissociation experiments elucidate the relative stabilities of [L-dThd+H]+ and [L-dThd + Na]+ as compared to the corresponding enantiomeric systems. Identical results are exhibited in the survival yield analyses as anticipated for enantiomeric complexes to simple cations. This work employs the same robust methodology that has provided structural characterization and energetic insight for similar systems preceding it to validate the parallel theoretical and experimental behaviors expected for enantiomers.

The structure of proton-bound Triethylammonia (X = F, Cl) Clusters

ABSTRACTIon pair complexes of triethylammonium, [(CH2CH3)3NH]+, and (X = F, Cl) are studied with a combination of infrared multiple photon dissociation (IRMPD) action spectroscopy and density functional theory calculations. IRMPD spectra show that the charge-carrying proton is oriented towards a trigonal face of the halogenated closo-dodecaborate in both derivatives. Resonant IR absorption via or [(CH2CH3)3NH]+ bands in the 800–1600 cm−1 region drives dissociation via the lowest energy proton-transfer fragmentation channel and the charge transfer (i.e. redox) threshold, which is second lowest in energy. By tuning the power of the interrogating laser or the wavenumber (to select specific vibrational transitions), one can control the degree of fragmentation via the two lowest energy dissociation thresholds.

Impact of the 2'- and 3'-Sugar Hydroxyl Moieties on Gas-Phase Nucleoside Structure.

Modified nucleosides have been an important target for pharmaceutical development for the treatment of cancer, herpes simplex virus, and the human immunodeficiency virus (HIV). Amongst these nucleoside analogues, those based on 2',3'-dideoxyribose sugars are quite common, particularly in anti-HIV applications. The gas-phase structures of several protonated 2',3'-dideoxyribose nucleosides are examined in this work and compared with those of the analogous protonated DNA, RNA, and arabinose nucleosides to elucidate the influence of the 2'- and combined 2',3'-hydroxyl groups on intrinsic structure. Infrared multiple photon dissociation (IRMPD) action spectra are collected for the protonated 2',3'-dideoxy forms of adenosine, guanosine, cytidine, thymidine and uridine, [ddAdo+H]+, [ddGuo+H]+, [ddCyd+H]+, [ddThd+H]+, and [ddUrd+H]+, in the IR fingerprint and hydrogen-stretching regions. Molecular mechanics conformational searching followed by electronic structure calculations generates low-energy conformers of the protonated 2',3'-dideoxynucleosides and corresponding predicted linear IR spectra to facilitate interpretation of the measured IRMPD action spectra. These experimental IRMPD spectra and theoretical calculations indicate that the absence of the 2'- and 3'-hydroxyls largely preserves the protonation preferences of the canonical forms. The spectra and calculated structures indicate a slight preference for C3'-endo sugar puckering. The presence of the 3'- and further 2'-hydroxyl increases the available intramolecular hydrogen-bonding opportunities and shifts the sugar puckering modes for all nucleosides but the guanosine analogues to a slight preference for C2'-endo over C3'-endo. Graphical Abstract.

Structures of Rhodium Oxide Cluster Cations Rh7Om+ (m = 4–7, 12, 14) Revealed by Infrared Multiple Photon Dissociation Spectroscopy

Vibrational spectra of Rh7Om+ (m = 4–7, 12, 14) were measured in the 300–1300 cm–1 range via infrared multiple photon dissociation (IRMPD) spectroscopy. For the oxygen poor cluster sizes, Rh7Om+ (m = 4–7), IRMPD spectra were recorded through photodissociation of Rh7Om+–Ar complexes. IR spectra for Rh7Om+ (m = 12, 14) were recorded via the release of an O2 molecule from Rh7Om+ producing Rh7Om–2+; no O2 loss was observed from Rh7Om+ (m = 4–7, 8, 10). By comparison with the calculated vibrational spectra of several stable isomers obtained using density functional theory, these IR spectra are assigned to geometrical structures. For m = 4–7, all O atoms are bound to Rh atoms only, with a transition of the Rh core from capped octahedral to capped prismatic between m = 5 and m = 6. For the oxygen-rich Rh7O12+ and Rh7O14+, molecular O2 is adsorbed on a bridge site between two Rh atoms. The frequencies of the bands observed bands signal that the O2 molecules are activated, indicating that rhodium oxide clusters with Rh2O3 composition are still capable to donate electrons to activate O–O bonds in adsorbed O2.

Vibrational spectroscopy of the hexahydrated sulfate dianion revisited: role of isomers and anharmonicities.

We report on the gas phase vibrational spectroscopy of the hexahydrated sulfate dianion, SO42-(H2O)6, and its fully deuterated isotopologue, SO42-(D2O)6, using infrared photodissociation (IRPD) spectroscopy of the D2-tagged dianions in combination with density-functional-theory calculations on minimum-energy structures as well as finite temperature ab initio molecular dynamics (AIMD) simulations. The IRPD spectra were recorded at an ion trap temperature of 12 K and in the spectral range from 650 to 3800 cm-1, covering the intramolecular modes of the solvent (OH/OD stretches and H2O/D2O bends) at higher energies, those of the solute (sulfate stretches) at intermediate energies and the intermolecular solute librational modes at the lowest energies. Isomer-specific double resonance in combination with messenger-tag dependent IRPD spectra show that only a single isomer is contributing significantly and that this isomer is not the highly symmetric Td but rather the lower symmetry C3 isomer. Temperature-dependent IR multiple photon dissociation spectra of bare SO42-(H2O)6 suggest that the C3 isomer remains the most stable one up to 200 K. The AIMD simulations reveal that the IRPD spectra can only be fully understood when anharmonic effects as well as entropy-driven hydrogen bond network fluctuations are considered.

Infrared photodissociation spectroscopy of di-manganese oxide cluster cations.

Infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations have been employed to elucidate the geometric structure of a series of di-manganese oxide clusters Mn2Ox+ (x = 4-7). The theoretical exploration predicts that all investigated clusters contain a rhombus-like Mn2O2 core with up to four, terminally bound, oxygen atoms. The short Mn-O bond length of the terminal oxygen atoms of ≤1.58 Å indicates triple bond character instead of oxyl radical formation. However, the IR-MPD spectra reveal that higher energy isomers with up to two O2 molecules η2-coordinated to the cluster core can be kinetically trapped under the given experimental conditions. In these complexes, all O2 units are activated to superoxide species. In addition, the sequential increase of the oxygen content in the cluster allows for a controlled increase of the positive charge localized on the Mn atoms reaching a maximum for Mn2O7+.