Infrared Multiple Photon Dissociation Research Articles

Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert.

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η6-cym)RuX(pympyr)]+. In these complexes, the central ruthenium(+II) ion is coordinated to an η6-bound p-cymene (η6-cym), a bidentate 2-R-4-(2-pyridinyl)pyrimidine ligand (pympyr) with R = NH2 or N(CH3)2, and an anion X = I-, Br-, Cl-, or CF3SO3-. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C-H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise "dark" bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η6-cym)RuX(pympyr)]+ (R = N(CH3)2) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D2 under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H+ and H- from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before.

N3 and O2 Protonated Conformers of the Cytosine Mononucleotides Coexist in the Gas Phase.

The gas-phase conformations of the protonated forms of the DNA and RNA cytosine mononucleotides, [pdCyd+H]+ and [pCyd+H]+, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy over the IR fingerprint and hydrogen-stretching regions complemented by electronic structure calculations. The low-energy conformations of [pdCyd+H]+ and [pCyd+H]+ and their relative stabilities are computed at the B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) and MP2(full)/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) levels of theory. Comparisons of the measured IRMPD action spectra and B3LYP/6-311+G(d,p) linear IR spectra computed for the low-energy conformers allow the conformers present in the experiments to be determined. Similar to that found in previous IRMPD action spectroscopy studies of the protonated forms of the cytosine nucleosides, [dCyd+H]+ and [Cyd+H]+, both N3 and O2 protonated cytosine mononucleotides exhibiting an anti orientation of cytosine are found to coexist in the experimental population. The 2'-hydroxyl substituent does not significantly influence the most stable conformations of [pCyd+H]+ versus those of [pdCyd+H]+, as the IRMPD spectral profiles of [pdCyd+H]+ and [pCyd+H]+ are similar. However, the presence of the 2'-hydroxyl substituent does influence the relative intensities of the measured IRMPD bands. Comparisons to IRMPD spectroscopy studies of the deprotonated forms of the cytosine mononucleotides, [pdCyd-H]- and [pCyd-H]-, provide insight into the effects of protonation versus deprotonation on the conformational features of the nucleobase and sugar moieties. Likewise, comparisons to results of IRMPD spectroscopy studies of the protonated cytosine nucleosides provide insight into the influence of the phosphate moiety on structure. Comparison with previous ion mobility results shows the superiority of IRMPD spectroscopy for distinguishing various protonation sites. Graphical Abstract ᅟ.

Improved Infrared Spectra Prediction by DFT from a New Experimental Database.

This work aims to improve the computation of infrared spectra of gas-phase cations using DFT methods. Experimental infrared multiple photon dissociation (IRMPD) spectra for ten Zn and Ru organometallic complexes have been used to provide reference data for 64 vibrational modes in the 900-2000 cm-1 range. The accuracy of the IR vibrational frequencies predicted for these bands has been assessed over five DFT functionals and three basis sets. The functionals include the popular B3LYP and M06-2X hybrids and the range-separated hybrids (RSH) CAM-B3LYP, LC-BLYP, and ωB97X-D. B3LYP gives the best mean absolute error (MAE) and root-mean-square error (RMSE) values of 7.1 and 9.6 cm-1 , whilst the best RSH functional, ωB97X-D, gives 12.8 and 16.6 cm-1 , respectively. Using linear correlations instead of scaling factors improves the prediction accuracy significantly for all functionals. Experimental and computed spectra for a single complex can show significant differences even when the molecular structure is calculated correctly, and a means of defining confidence limits for any given computed structure is also provided.

Conformations and N-glycosidic bond stabilities of sodium cationized 2′-deoxycytidine and cytidine: Solution conformation of [Cyd + Na]+ is preserved upon ESI

The local structures of DNA and RNA are influenced by protonation, deprotonation and noncovalent interactions with metal cations. In order to determine the effects of sodium cationization on the structures of 2′-deoxycytidine and cytidine, infrared multiple photon dissociation (IRMPD) action spectra of these sodium cationized nucleosides, [dCyd+Na]+ and [Cyd+Na]+, are measured using the FELIX free electron laser and an OPO/OPA laser system. Energy-resolved collision-induced dissociation (ER-CID) experiments for the protonated and sodium cationized forms of the cytosine nucleosides are performed using a Bruker amaZon ETD quadrupole ion trap mass spectrometer (QIT MS) to evaluate the relative propensities of protons and sodium cations for activating the glycosidic bonds of the cytosine nucleosides. Complementary electronic structure calculations are performed to determine the stable low-energy conformations of [dCyd+Na]+ and [Cyd+Na]+. For both cytosine nucleosides, theory suggests that tridentate binding of Na+ to the O2, O4′ and O5′ atoms of the cytosine nucleobase and sugar moiety is the most stable binding mode. However, comparison of the measured IRMPD action spectrum and computed linear IR spectra suggests that anti oriented bidentate conformers of [Cyd+Na]+ are predominantly populated in the experiments. The 2′-hydroxyl substituent of Cyd stabilizes the anti oriented bidentate conformers of [Cyd+Na]+, and enables formation of a 2′OH⋯3′OH hydrogen-bonding interaction. The 2′-hydroxyl substituent is found to stabilize the glycosidic bond of Cyd vs. that of dCyd for both the protonated and sodium cationized cytosine nucleosides. Compared to protonation, sodium cationization activates the N-glycosidic bond less effectively.

Gas-Phase Conformations and N-Glycosidic Bond Stabilities of Sodium Cationized 2'-Deoxyguanosine and Guanosine: Sodium Cations Preferentially Bind to the Guanine Residue.

2'-Deoxyguanosine (dGuo) and guanosine (Guo) are fundamental building blocks of DNA and RNA nucleic acids. In order to understand the effects of sodium cationization on the gas-phase conformations and stabilities of dGuo and Guo, infrared multiple photon dissociation (IRMPD) action spectroscopy experiments and complementary electronic structure calculations are performed. The measured IRMPD spectra of [dGuo+Na]+ and [Guo+Na]+ are compared to calculated IR spectra predicted for the stable low-energy structures computed for these species to determine the most favorable sodium cation binding sites, identify the structures populated in the experiments, and elucidate the influence of the 2'-hydroxyl substituent on the structures and IRMPD spectral features. These results are compared with those from a previous IRMPD study of the protonated guanine nucleosides to elucidate the differences between sodium cationization and protonation on structure. Energy-resolved collision-induced dissociation (ER-CID) experiments and survival yield analyses of protonated and sodium cationized dGuo and Guo are performed to compare the effects of these cations toward activating the N-glycosidic bonds of these nucleosides. For both [dGuo+Na]+ and [Guo+Na]+, the gas-phase structures populated in the experiments are found to involve bidentate binding of the sodium cation to the O6 and N7 atoms of guanine, forming a 5-membered chelation ring, with guanine found in both anti and syn orientations and C2'-endo (2T3 or 3T2) puckering of the sugar. The ER-CID results, IRMPD yields and the computed C1'-N9 bond lengths indicate that sodium cationization activates the N-glycosidic bond less effectively than protonation for both dGuo and Guo. The 2'-hydroxyl substituent of Guo is found to impact the preferred structures very little except that it enables a 2'OH···3'OH hydrogen bond to be formed, and stabilizes the N-glycosidic bond relative to that of dGuo in both the sodium cationized and protonated complexes.

Cytosine Radical Cations: A Gas-Phase Study Combining IRMPD Spectroscopy, UVPD Spectroscopy, Ion-Molecule Reactions, and Theoretical Calculations.

The radical cation of cytosine (Cyt.+ ) is generated by dissociative oxidation from a ternary CuII complex in the gas phase. The radical cation is characterized by infrared multiple photon dissociation (IRMPD) spectroscopy in the fingerprint region, UV/Vis photodissociation (UVPD) spectroscopy, ion-molecule reactions, and theoretical calculations (density functional theory and ab initio). The experimental IRMPD spectrum features diagnostic bands for two enol-amino and two keto-amino tautomers of Cyt.+ that are calculated to be among the lowest energy isomers, in agreement with a previous study. Although the UVPD action spectrum can also be matched to a combination of the four lowest energy tautomers, the presence of a nonclassical distonic radical cation cannot be ruled out. Its formation is, however, unlikely due to the high energy of this isomer and the respective ternary CuII complex. Gas-phase ion-molecule reactions showed that Cyt.+ undergoes hydrogen-atom abstraction from 1-propanethiol, radical recombination reactions with nitric oxide, and electron transfer from dimethyl disulfide.

Infrared Multiple-Photon Dissociation Action Spectroscopy of the b2+ Ion from PPG: Evidence of Third Residue Affecting b2+ Fragment Structure.

Infrared multiple-photon dissociation (IRMPD) action spectroscopy was performed on the b2+ fragment ion from the protonated PPG tripeptide. Comparison of the experimental infrared spectrum with computed spectra for both oxazolone and diketopiperazine structures indicates that the majority of the fragment ion population has an oxazolone structure with the remainder having a diketopiperazine structure. This result is in contrast with a recent study of the IRMPD action spectrum of the PP b2+ fragment ion from PPP, which was found to be nearly 100% diketopiperazine (Martens et al. Int. J. Mass Spectrom. 2015, 377, 179). The diketopiperazine b2+ ion is thermodynamically more stable than the oxazolone but normally requires a trans/cis peptide bond isomerization in the dissociating peptide. Martens et al. showed through IRMPD action spectroscopy that the PPP precursor ion was in a conformation in which the first peptide bond is already in the cis conformation and thus it was energetically favorable to form the thermodynamically-favored diketopiperazine b2+ ion. In the present case, solution-phase NMR spectroscopy and gas-phase IRMPD action spectroscopy show that the PPG precursor ion has its first amide bond in a trans configuration suggesting that the third residue is playing an important role in both the structure of the peptide and the associated ring-closure barriers for oxazolone and diketopiperazine formation. Graphical Abstract ᅟ.

One-Electron Oxidation of Methionine-Containing Dipeptides of Reverse Sequence: Sulfur versus Sulfoxide Characterized by IRMPD Spectroscopy and Static and Dynamics DFT Simulations.

Gas-phase structural modifications induced by the oxidation of methionine of the two peptides of reverse sequence, methionine-valine (Met-Val) and valine-methionine (Val-Met), have been studied by mass-selected IR multiple photon dissociation (IRMPD) spectroscopy in the 800-2000 cm-1 fingerprint range at the Centre Laser Infrarouge d'Orsay free-electron laser facility. The oxidation has been achieved by •OH radicals generated by γ radiolysis. IRMPD spectra were interpreted by static and harmonic DFT calculations and Born-Oppenheimer molecular dynamics simulations, which are employed to take into account all anharmonic and finite-temperature effects. The diagnostic signature of the sulfoxide group in the final products of Met-Val and Val-Met oxidations, which is missing in the spectra of native peptides, has been recorded. Evidence has also been gathered that a mixture of R and S isomers of close energies is formed. An interconversion between different isomers has been unveiled in the case of the oxidized Met-Val dipeptide.

Vibrational Signatures of Isomeric Lithiated N-acetyl-D-hexosamines by Gas-Phase Infrared Multiple-Photon Dissociation (IRMPD) Spectroscopy.

Three lithiated N-acetyl-D-hexosamine (HexNAc) isomers, N-acetyl-D-glucosamine (GlcNAc), N-acetyl-D-galactosamine (GalNAc), and N-acetyl-D-mannosamine (ManNAc) are investigated as model monosaccharide derivatives by gas-phase infrared multiple-photon dissociation (IRMPD) spectroscopy. The hydrogen stretching region, which is attributed to OH and NH stretching modes, reveals some distinguishing spectral features of the lithium-adducted complexes that are useful in terms of differentiating these isomers. In order to understand the effect of lithium coordination on saccharide structure, and therefore anomericity, chair configuration, and hydrogen bonding networks, the conformational preferences of lithiated GlcNAc, GalNAc, and ManNAc are studied by comparing the experimental measurements with density functional theory (DFT) calculations. The experimental results of lithiated GlcNAc and GalNAc show a good match to the theoretical spectra of low-energy structures adopting a 4 C 1 chair conformation, consistent with this motif being the dominant conformation in condensed-phase monosaccharides. The epimerization effect upon going to lithiated ManNAc is significant, as in this case the 1 C 4 chair conformers give a more compelling match with the experimental results, consistent with their lower calculated energies. A contrasting computational study of these monosaccharides in their neutral form suggests that the lithium cation coordination with Lewis base oxygens can play a key role in favoring particular structural motifs (e.g., a 4 C 1 versus 1 C 4 ) and disrupting hydrogen bond networks, thus exhibiting specific IR spectral features between these closely relatedlithium-chelated complexes. Graphical Abstract ᅟ.

The intrinsic basicity of the phosphate backbone exceeds that of uracil and thymine residues: protonation of the phosphate moiety is preferred over the nucleobase for pdThd and pUrd.

The gas-phase conformations of the protonated forms of thymidine-5'-monophosphate and uridine-5'-monophosphate, [pdThd+H]+ and [pUrd+H]+, are investigated by infrared multiple photon dissociation (IRMPD) action spectroscopy and electronic structure calculations. The IRMPD action spectra of [pdThd+H]+ and [pUrd+H]+ are measured over the IR fingerprint and hydrogen-stretching regions using the FELIX free electron laser and an OPO/OPA laser system. Low-energy conformations of [pdThd+H]+ and [pUrd+H]+ and their relative stabilities are computed at the MP2(full)/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) and B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) levels of theory. Comparisons of the measured IRMPD action spectra and B3LYP/6-311+G(d,p) linear IR spectra computed for the low-energy conformers indicate that the dominant conformers of [pdThd+H]+ and [pUrd+H]+ populated in the experiments are protonated at the phosphate oxo oxygen atom, with a syn nucleobase orientation that is stabilized by strong P[double bond, length as m-dash]OH+O2 and P-OHO4' hydrogen-bonding interactions, and C2'-endo sugar puckering. Minor abundance of conformers protonated at the O2 carbonyl of the nucleobase residue may also contribute for [pdThd+H]+, but do not appear to be important for [pUrd+H]+. Comparisons to previous IRMPD spectroscopy investigations of the protonated forms of thymidine and uridine, [dThd+H]+ and [Urd+H]+, and the deprotonated forms of pdThd and pUrd, [pdThd-H]- and [pUrd-H]-, provide insight into the effects of the phosphate moiety and protonation on the conformational features of the nucleobase and sugar moieties. Most interestingly, the thymine and uracil nucleobases remain in their canonical forms for [pdThd+H]+ and [pUrd+H]+, unlike [dThd+H]+ and [Urd+H]+, where protonation occurs on the nucleobases and induces tautomerization of the thymine and uracil residues.

Conformation of protonated glutamic acid at room and cryogenic temperatures.

Recognition properties of biologically relevant molecules depend on their conformation. Herein, the conformation of protonated glutamic acid (H+Glu) isolated in quadruple ion traps is characterized by vibrational spectroscopy at room and cryogenic temperatures and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. The infrared multiple photon dissociation (IRMPD) spectrum recorded in the fingerprint range at room temperature using an IR free electron laser is attributed to the two most stable and nearly isoenergetic conformations (1-cc and 2-cc) with roughly equal population (ΔG298 = 0.0 kJ mol-1). Both have bridging C[double bond, length as m-dash]O(HNH)+O[double bond, length as m-dash]C ionic H-bonds of rather different strengths but cannot be distinguished by their similar IRMPD spectra. In contrast, the higher-resolution single-photon IRPD spectrum of H2-tagged H+Glu recorded in the conformation-sensitive X-H stretch range in a trap held at 10 K distinguishes both conformers. At low temperature, 1-cc is roughly twice more abundant than 2-cc, in line with its slightly lower calculated energy (ΔE0 = 0.5 kJ mol-1). This example illustrates the importance of cryogenic cooling, single-photon absorption conditions, and the consideration of the X-H stretch range for the identification of biomolecular conformations involving hydrogen bonds.

Gas-phase vibrational spectroscopy of triphenylamine: the effect of charge on structure and spectra.

The effect of ionization by oxidation and protonation on the structure and IR spectrum of isolated, gas-phase triphenylamine (TPA) has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy in the fingerprint range from 600 cm-1 to 1800 cm-1 using an infrared free electron laser. IR spectra calculated using density functional theory (DFT) convincingly reproduce the experimental data. Spectral and structural differences are identified among neutral TPA, TPA˙+ and protonated TPA and qualitatively related to effects of resonance delocalization. As a consequence of electron delocalization, computed structural parameters for TPA remain virtually unchanged upon removal of an electron. Nonetheless, CC and CN stretching vibrations in the IR spectra of TPA˙+ undergo a red shift of up to 52 cm-1 as compared to those in TPA. Since ionization also strongly influences the relative band intensities, a vibrational projection analysis was used to correlate vibrational modes of TPA with those of TPA˙+. The experimental IR spectrum of gas-phase protonated TPA indicates that protonation occurs on the nitrogen atom, despite delocalization of the lone electron pair. Upon protonation, the structure changes from the nearly planar geometry to a near-tetrahedral configuration.

Identification of the protonation site of gaseous triglycine: the cis-peptide bond conformation as the global minimum.

Extensive ab initio investigations have been performed to characterize stable conformers of protonated triglycine (GGGH) in the gas phase. Calculations using the composite CBS-QB3 method confirmed that the most favorable site of protonation on triglycine at 298 K is still the traditional amino nitrogen, rather than the more-recently reported amide oxygen. Furthermore, a non-proline cis-peptide bond conformer is identified for the first time as the global minimum of GGGH. Further transition state calculations considering the temperature effects explained why the previous experimental infrared multiple photon dissociation (IRMPD) spectrum contains a combination of two local minima, rather than a global one. First-principles simulations have been performed for near-edge X-ray absorption fine-structure (NEXAFS) spectra and X-ray photoelectron spectra (XPS) at the C, N and O K-edges to identify the notable spectral differences that enable the unambiguous identification of different protonated forms. The calculated proton affinity (PA) and gas basicity (GB) of triglycine are in excellent agreement with the experimental values. Our study thus provides valuable insights into the protonation of short peptides and illustrates the competition between cis and trans peptide bonds.

Intramolecular cation-π interactions in protonated phenylalanine derivatives.

The structures and properties of a series of phenylalanine (Phe) derivatives have been investigated in a joint computational and experimental infrared multiple photon dissociation (IRMPD) study. IRMPD spectra in the 1000-2000 cm-1 region show that protonation is localized on the amine group in all cases. Intramolecular cation-π interactions between the ammonium group and the phenyl ring heavily influence molecular geometries and properties such as gas phase basicity and proton affinity. By varying substituents on the phenyl ring, one can sensitively tune the cation-π interaction and, therefore, the molecular structure and properties. Variations in molecular structures and properties as a function of phenyl ring substitution are shown to correlate with substituent Hammett parameters.

Infrared spectroscopy of O˙- and OH- in water clusters: evidence for fast interconversion between O˙- and OH˙OH.

We present infrared multiple photon dissociation (IRMPD) spectra of (H2O)nO•− and (H2O)nOH− cluster ensembles for 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 H2O⋅⋅⋅O•− and H2O⋅⋅⋅OH− 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.

Facile pentagon formation in the dissociation of polyaromatics.

Energetic processing of gaseous polycyclic aromatic hydrocarbons (PAHs) plays a pivotal role in the chemistries of inter- and circumstellar environments, certain planetary atmospheres, and also in the chemistry of combustion and soot formation. Although the precursor PAH species have been extensively characterized, the products from these gaseous breakdown reactions have received far less attention. It has been particularly challenging to accurately determine their molecular structure in gas-phase experiments, where comparisons against theoretical modeling are best made. Here we report on a combined experimental and theoretical study of the dissociative ionization of two nitrogen containing polycyclic aromatic hydrocarbons of C13H9N composition, acridine and phenanthridine. The structures of HCN-loss fragments are resolved by infrared multiple-photon dissociation (IRMPD) spectroscopy of the mass-isolated products in an ion trap mass spectrometer. Quantum-chemical computations as well as reference IRMPD spectra are employed to unambiguously identify the molecular structures. Furthermore, computations at the density functional level of theory provide insight into chemical pathways leading to the observed products. Acenaphthylene˙+ and benzopentalene˙+ - two aromatic species containing pentagons - are identified as the main products, suggesting that such species are easily formed and may be abundant in regions where thermal or photoprocessing of polyaromatics occurs.

Cisplatin Primary Complex with l-Histidine Target Revealed by IR Multiple Photon Dissociation (IRMPD) Spectroscopy.

The primary complex obtained from cisplatin and l-histidine in water has been detected and isolated by electrospray ionization. The so-obtained cis-[PtCl(NH3 )2 (histidine)]+ complex has been characterized in detail by high-resolution mass spectrometry (MS), tandem MS, IR multiple photon dissociation (IRMPD) spectroscopy, and by quantum chemical calculations. The structural features revealed by IRMPD spectroscopy indicate that platinum binds to the imidazole group, which presents tautomeric forms. Thus, depending on the position of the amino acid pendant on the imidazole ring, isomeric complexes are formed that are remarkably different with respect to the ease with which they undergo fragmentation when activated either by energetic collisions or by multiple IR photon absorption. It is shown here how IRMPD kinetics can allow their relative proportions to be estimated.

Ammoniated Complexes of Uracil and Transition Metal Ions: Structures of [M(Ura-H)(Ura)(NH3)]+ by IRMPD Spectroscopy and Computational Methods (M = Fe, Co, Ni, Cu, Zn, Cd).

The structures of deprotonated d-block metal dication bound uracil dimers, solvated by a single ammonia molecule, were explored in the gas phase using infrared multiple photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance-mass spectrometer. The IRMPD spectra were then compared with computed IR spectra for various isomers. Calculations were performed using B3LYP with the 6-31+G(d,p) basis set for all atoms, with the exception of Cd, for which the LANL2DZ basis set with relativistic core potentials was used. The calculations were then repeated using the def2-TZVPP basis set on all atoms and were compared to the first set of calculations. The lowest-energy structures are those in which one uracil is deprotonated at the N3 position and, aside from the Cu complex, the intact uracil is a tautomer in which the N3 hydrogen is at the O4 carbonyl oxygen. The metal displays a tetradentate interaction to the uracil moieties, with the exception of Cu, which is tridentate, and the ammonia molecule is bound directly to the metal center. In the Cu complex, a square planar geometry is observed about the metal center, consistent with Jahn-Teller distortions commonly observed in Cu(II) complexes, and the intact uracil assumes its canonical tautomer. All other metal cation complexes are five-coordinate, square pyramidal complexes, with the intact uracil adopting a tautomer in which the N3 hydrogen is on O4. The IRMPD spectroscopic data are consistent with the computed infrared spectra for the lowest-energy structures in all cases.

Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Asparagine Complexes with Zn2+ and Cd2+ and Their Deamidation Processes.

Complexes of asparagine (Asn) cationized withZn2+ and Cd2+ were examined by infrared multiple photondissociation (IRMPD) action spectroscopy using lightgenerated from a free electron laser. Electrospray ionizationyielded complexes of deprotonated Asn with Zn2+, [Zn(Asn-H)]+, and intact Asn with CdCl+, CdCl+(Asn). Series of low energy conformers for each complex were found using quantum chemical calculations in order to identify the structures formed experimentally. The experimentally obtained spectra were compared to those calculated from optimizedstructures at the B3LYP/6-311+G(d,p) level for [Zn(Asn-H)]+ and the B3LYP/def2-TZVP level with an SDD effective core potential on cadmium for the CdCl+(Asn) system. The main binding motif observed for the CdCl+ complex is a charge solvated, tridentate [N, CO, COs] structure where the metal binds to the backbone amino group and carbonyl oxygens of the carboxylic acid and side-chain amide groups. The Zn2+ system deprotonates at the backbone carboxylic acid and prefers a [N, CO-, COs] binding motif, where binding was observed at the carboxylate site along with the backbone amino group and side-chain carbonyl groups. In both cases, the theoretically determined lowest-energy conformers explain the experimental [Zn(Asn-H)]+ and CdCl+(Asn) spectra well. Additionally, complete mechanistic pathways were found for each of the major dissociation reactions of [Zn(Asn-H)]+ (primary loss of CO2, followed by the sequential loss of NH3) and CdCl+(Asn) (concomitant loss of NH3 + CO).

Vibrational fingerprints of a tetranuclear cobalt carbonyl cluster within a cryo tandem ion trap

We present well resolved (Δν∼=3cm−1) Infrared Multiple Photon Dissociation (IR-MPD) spectra of [Co4(CO)n(dppa-H+)]− (n=3–10, dppa=NH(PPh2)2) in the carbonyl stretching range when isolating these species at low temperatures (26K and below). We utilize IR-MPD in conjunction with DFT calculations to investigate the influence of CO ligands on structure and spin multiplicity of the Co4 core. The recorded spectra reveal multiple bands of CO stretching vibrations which shift by reduction of CO coverage. This indicates a corresponding change of coordination motifs, and we find a conclusive interpretation of all recorded IR features in the case of the saturated n=10 compound, likely in a singlet state as in solution. We tentatively postulate a singlet/triplet to quintet spin-flip upon removal of the first one or two CO ligands (from n=10 to n=9, 8) as well as a conceivable isomerization at a particular CO coverage (n=6).