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The interaction of organic molecules with oxonium cations within their solvation shell may lead to the emergence of dynamic supramolecular structures with recurrently changing host–guest chemical identity. We illustrate this phenomenon in benchmark proton-bonded complexes of water with polyether macrocyles. Despite the smaller proton affinity of water versus the ether group, water in fact retains the proton in the form of H3O+, with increasing stability as the coordination number increases. Hindrance in many-fold coordination induces dynamic reversible (ether)·H3O+ ⇌ (etherH+)·H2O interconversion. We perform infrared action ion spectroscopy over a broad spectral range to expose the vibrational signatures of the loose proton bonding in these systems. Remarkably, characteristic bands for the two limiting proton bonding configurations are observed in the experimental vibrational spectra, superimposed onto diffuse bands associated with proton delocalization. These features cannot be described by static equilibrium structures but are accurately modeled within the framework of ab initio molecular dynamics., Area of Physical Chemistry

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Experimental IR spectra in the 500-1850 cm-1 fingerprint frequency range are presented for the isolated, gaseous redox pair ions [Ru(bpy)3]2+, and [Ru(bpy)3]+, where bpy = 2,2'-bipyridine. Spectra are obtained using the FELIX free-electron laser and a quadrupole ion trap mass spectrometer. The 2+ complex is generated by electrospray ionization and the charge-reduced radical cation is produced by gas-phase one-electron reduction in an ion-ion reaction with the fluoranthene radical anion. Experimental spectra are compared against computed spectra predicted by density functional theory (DFT) using different levels of theory. For the closed-shell [Ru(bpy)3]2+ ion, the match between experimental and computed IR spectra is very good; however, this is not the case for the charge-reduced [Ru(bpy)3]+ ion, which demands additional theoretical investigation. When using the hybrid B3LYP functional, we observe that better agreement with experiment is obtained upon reduction of the Hartree-Fock exact-exchange contribution from 204 calculations using the M06 functional appear to be promising in terms of the prediction of IR spectra; however, it is unclear if the correct electronic structure is obtained. The M06 and B3LYP functionals indicate that the added electron in [Ru(bpy)3)]+ is delocalized over the three bpy ligands, while the long-range corrected LC-BLYP and the CAM-B3LYP functionals show it to be more localized on a single bpy ligand. Although these latter levels of theory fail to reproduce the experimentally observed IR frequencies, one may argue that the unusually large bandwidths observed in the spectrum are due to the fluxional character of a complex with the added electron not symmetrically distributed over the ligands. The experimental IR spectra presented here can serve as benchmark for further theoretical investigations.

We present the first experimental vibrational spectra of gaseous oxidized derivatives of C60 in protonated and radical cation forms, obtained through infrared multiple-photon dissociation spectroscopy using the FELIX free-electron laser. Neutral C60O has two nearly iso-energetic isomers: the epoxide isomer in which the O atom bridges a CC bond that connects two six-membered rings and the annulene isomer in which the O atom inserts into a CC bond connecting a five- and a six-membered ring. To determine the isomer formed for C60O+ in our experiment a question that cannot be confidently answered on the basis of the DFT-computed stabilities alone we compare our experimental IR spectra to vibrational spectra predicted by DFT calculations. We conclude that the annulene-like isomer is formed in our experiment. For C60OH+, a strong OH stretch vibration observed in the 3 μm range of the spectrum immediately reveals its structure as C60 with a hydroxyl group attached, which is further confirmed by the spectrum in the 400-1600 cm-1 range. We compare the experimental spectra of C60O+ and C60OH+ to the astronomical IR emission spectrum of a fullerene-rich planetary nebula and discuss their astrophysical relevance.

Microspheres containing radioactive 166holmium-acetylacetonate are employed in emerging radionuclide therapies for the treatment of malignancies. At the molecular level, details on the coordination geometries of the Ho complexes are however elusive. Infrared ion spectroscopy (IRIS) was used to characterize several 165Ho-acetylacetonate complexes derived from non-radioactive microspheres. The coordination geometry of four distinct ionic complexes were fully assigned by comparison of their measured IR spectra with spectra calculated at the density functional theory (DFT) level. The coordination of each acetylacetonate ligand is dependent on the presence of other ligands, revealing an asymmetric chelation motif in some of the complexes. A fifth, previously unknown constituent of the microspheres was identified as a coordination complex containing an acetic acid ligand. These results pave the way for IRIS-based identification of microsphere constituents upon neutron activation of the metal center.

Liquid chromatography-mass spectrometry (LC-MS) is, due to its high sensitivity and selectivity, currently the method of choice in (bio)analytical studies involving the (comprehensive) profiling of metabolites in body fluids. However, as closely related isomers are often hard to distinguish on the basis of LC-MS(MS) and identification is often dependent on the availability of reference standards, the identification of the chemical structures of detected mass spectral features remains the primary limitation. Infrared ion spectroscopy (IRIS) aids identification of MS-detected ions by providing an infrared (IR) spectrum containing structural information for a detected MS-feature. Moreover, IR spectra can be routinely and reliably predicted for many types of molecular structures using quantum-chemical calculations, potentially avoiding the need for reference standards. In this work, we demonstrate a workflow for reference-free metabolite identification that combines experiments based on high-pressure liquid chromatography (HPLC), MS and IRIS with quantum-chemical calculations that efficiently generate IR spectra and give the potential to enable reference-standard free metabolite identification. Additionally, a scoring procedure is employed which shows the potential for automated structure assignment of unknowns. Via a simple, illustrative example where we identify lysine in the plasma of a hyperlysinemia patient, we show that this approach allows the efficient assignment of a database-derived molecular structure to an unknown.

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.

Metallated gas-phase structures consisting of an intact and deprotonated histidine (His) ligand, M(His-H)(His)+, where M = Ca and Ni, were examined using infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). In parallel, ab initio quantum-chemical calculations identified several low-energy isomers for each complex. Experimental action spectra were compared to linear absorption spectra calculated at the B3LYP level of theory, using the 6-311+G(d,p) basis set. Single-point energies were calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. For Ca(His-H)(His)+, the dominant structure has the metal center coordinating with the π nitrogen of the imidazole ring (Nπ) and both oxygen atoms of the carboxylate group of the deprotonated His ligand while coordinating with the backbone amine (Nα), Nπ, and the carbonyl oxygen of the carboxylic acid of the intact His ligand. The Ni(His-H)(His)+ species coordinates the metal ion through Nα, Nπ, and the carbonyl oxygen for both the deprotonated and intact His ligands, but also shows evidence for a minor secondary structure where the deprotonated His coordinates the metal at Nα, Nπ, and the deprotonated carbonyl oxygen and the intact His ligand is zwitterionic, coordinating the metal with both carboxylate oxygens. Different levels of theory predict different ground structures, highlighting the need for utilizing multiple levels of theory to help identify the gas-phase structure actually observed experimentally.

Uranium trioxide, UO3, has a T-shaped structure with bent uranyl, UO22+, coordinated by an equatorial oxo, O2-. The structure of cation UO3+ is similar but with an equatorial oxyl, O center dot-. Neutral and cationic uranium trioxide coordinated by nitrates were characterized by collision induced dissociation (CID), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory. CID of uranyl nitrate, [UO2 (NO3)3]- (complex A1), eliminates NO2 to produce nitrate-coordinated UO3+, [UO2 (O. )(NO3)2]-(B1), which ejects NO3 to yield UO3 in [UO2 (O)(NO3)]- (C1). Finally, C1 associates with H2O to afford uranyl hydroxide in [UO2(OH)2 (NO3)]- (D1). IRMPD of B1, C1, and D1 confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: (B1) radical oxyl O.-; (C1) oxo O2-; and (D1) two hydroxyls, OH- . As the nitrates are bidentate, the equatorial coordination is six in A1, five in B1, four in D1, and three in C1. Ligand congestion in low-coordinate C1 suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1 epitomizes the inverse trans influence in UO3, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl v3 IR frequencies indicate the following donor ordering: O2- [best donor] >> O.- > OH-> NO3-.

Cisplatin, (NH3)2PtCl2, has been known as a successful metal-based anticancer drug for more than half a century. Its analogue, Argplatin, arginine-linked cisplatin, (Arg)PtCl2, is being investigated because it exhibits reactivity towards DNA and RNA that differs from that of cisplatin. In order to understand the basis for its altered reactivity, the deprotonated and sodium cationized forms of Argplatin, [(Arg-H)PtCl2]− and [(Arg)PtCl2 + Na]+, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy in the IR fingerprint and hydrogen-stretching regions. Complementary electronic structure calculations are performed using density functional theory approaches to characterize the stable structures of these complexes and to predict their infrared spectra. Comparison of the theoretical IR spectra predicted for various stable conformations of these Argplatin complexes to their measured IRMPD spectra enables determination of the binding mode(s) of Arg to the Pt metal center to be identified. Arginine is found to bind to Pt in a bidentate fashion to the backbone amino nitrogen and carboxylate oxygen atoms in both the [(Arg-H)PtCl2]− and [(Arg)PtCl2 + Na]+ complexes, the NO− binding mode. The neutral side chain of Arg also interacts with the Pt center to achieve additional stabilization in the [(Arg-H)PtCl2]− complex. In contrast, Na+ binds to both chlorido ligands in the [(Arg)PtCl2 + Na]+ complex and the protonated side chain of Arg is stabilized via hydrogen-bonding interactions with the carboxylate moiety. These findings are consistent with condensed-phase results, indicating that the NO− binding mode of arginine to Pt is preserved in the electrospray ionization process even under variable pH and ionic strength.

Infrared multiple photon dissociation (IRMPD) spectroscopy experiments in the 600-2000 cm-1 region and computational chemistry studies were combined with the aim of elucidating the structures of protonated imidacloprid (pIMI), and its unimolecular decomposition products. The computed IR spectra for the lowest energy structures for pIMI as well as for protonated desnitrosoimidacloprid, corresponding to the loss of NO radical (pIMI-NO), and protonated imidacloprid urea corresponding to the loss of N2O (pIMIU) were found to reproduce the experimental IRMPD spectrum quite well. The complex IRMPD spectrum for protonated desnitroimidaclpride (pDIMI), resulting from the loss of NO2 radical from pIMI, was explained as a contribution from several computed structures, including those involving simple loss of NO2 radical and some isomerization. However, based on a comparison of the computed IR spectrum for the lowest energy structure of pDIMI and the IRMPD spectrum, it was concluded that the lowest energy structure is a minor contributor to the experimental spectrum. This observation is rationalized as being due to the energy requirement for isomerization to the lowest energy structure, being substantially higher than that for simple loss of NO2 radical. Experimental mass spectrometry fragmentation results indicated that the loss of N, O2, H was the result of a loss of NO radical followed by loss of OH radical. A comparison of the experimental IRMPD and computed IR spectra revealed that following NO radical loss, the structure entailing a hydride shift from the methylene bridge to the guanidine moiety followed by OH radical elimination, generated the best match with the experimental IRMPD spectrum. This was consistent with the computed potential energy surfaces showing this structure as having the lowest energy requirement.

We report herein the first detailed study of the mechanism of redox reactions occurring during the gas-phase dissociative electron transfer of prototypical ternary [CuII(dien)M]˙2+ complexes (M, peptide). The two final products are (i) the oxidized non-zwitterionic π-centered [M]˙+ species with both the charge and spin densities delocalized over the indole ring of the tryptophan residue and with a C-terminal COOH group intact, and (ii) the complementary ion [CuI(dien)]+. Infrared multiple photon dissociation (IRMPD) action spectroscopy and low-energy collision-induced dissociation (CID) experiments, in conjunction with density functional theory (DFT) calculations, revealed the structural details of the mass-isolated precursor and product cations. Our experimental and theoretical results indicate that the doubly positively charged precursor [CuII(dien)M]˙2+ features electrostatic coordination through the anionic carboxylate end of the zwitterionic M moiety. An additional interaction exists between the indole ring of the tryptophan residue and one of the primary amino groups of the dien ligand; the DFT calculations provided the structures of the precursor ion, intermediates, and products, and enabled us to keep track of the locations of the charge and unpaired electron. The dissociative one-electron transfer reaction is initiated by a gradual transition of the M tripeptide from the zwitterionic form in [CuII(dien)M]˙2+ to the non-zwitterionic M intermediate, through a cascade of conformational changes and proton transfers. In the next step, the highest energy intermediate is formed; here, the copper center is 5-coordinate with coordination from both the carboxylic acid group and the indole ring. A subsequent switch back to 4-coordination to an intermediate IM1, where attachment to GGW occurs through the indole ring only, creates the structure that ultimately undergoes 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 HN CH2 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 – HN CH2]+ ion also has an imidazolone structure. Losses of CO and glycine are the dominant dissociation pathways for the [b4 – HN CH2]+ 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 molecular structures of six open-shell z3-ions resulting from electron transfer dissociation mass spectrometry (ETD MS) were investigated using infrared ion spectroscopy in the 800–1850 and 3200–3700 cm−1 spectral ranges in combination with density functional theory and molecular mechanics/molecular dynamics calculations. We assess in particular the question of whether the radical remains at the Cα-site of the backbone cleavage, or whether it migrates by H-atom transfer to another, energetically more favorable position. Calculations performed herein as well as by others show that radical migration to an amino acid side chain or to an α-carbon along the peptide backbone can lead to structures that are more stable, by up to 33 kJ mol−1 for the systems investigated here, by virtue of resonance stabilization of the radical in these alternative positions. Nonetheless, for four out of the six z3-ions considered here, our results quite clearly indicate that radical migration does not occur, suggesting that the radical is kinetically trapped at the site of ETD cleavage. For the two remaining systems, a structural assignment is less secure and we suggest that a mixture of migrated and unmigrated structures may be formed.

The molecular family of nucleotide triphosphates (NTPs), with adenosine 5′-triphosphate (ATP) as its best-known member, is of high biochemical importance as their phosphodiester bonds form Nature's main means to store and transport energy. Here, gas-phase IR spectroscopic studies and supporting theoretical studies have been performed on adenosine 5′-triphosphate, cytosine 5′-triphosphate and guanosine 5′-triphosphate to elucidate the intrinsic structural properties of NTPs, focusing on the influence of the nucleobase and the extent of deprotonation. Mass spectrometric studies involving collision induced dissociation showed similar fragmentation channels for the three studied NTPs within a selected charge state. The doubly charged anions exhibit fragmentation similar to the energy-releasing hydrolysis reaction in nature, while the singly charged anions show different dominant fragmentation channels, suggesting that the charge state plays a significant role in the favorability of the hydrolysis reaction. A combination of infrared ion spectroscopy and quantum-chemical computations indicates that the singly charged anions of all NTPs are preferentially deprotonated at their β-phosphates, while the doubly-charged anions are dominantly αβ-deprotonated. The assigned three-dimensional structure differs for ATP and CTP on the one hand and GTP on the other, in the sense that ATP and CTP show no interaction between nucleobase and phosphate tail, while in GTP they are hydrogen bonded. This can be rationalized by considering the structure and geometry of the NTPs where the final three dimensional structure depends on a subtle balance between hydrogen bond strength, flexibility and steric hindrance.

Synthesis and spectra of isocytosino-8-trifluoromethylquinolone (1), as well as the gas phase InfraRed Multiple Photon Dissociation (IRMPD) spectra in the fingerprint region of the corresponding deprotonated anion (3), its d3 analogue, the monodeprotonated homodimer (2), and its d7 analogue are reported here. The anions represent nucleobase analogues having the hydrogen bonding pattern ADAAD (where A stands for acceptor and D stands for donor), in which the site of negative charge is unambiguous (as opposed to guanine, which has more than one acidic nitrogen). The match between experimental vibrational spectra and calculation is good, except for the out-of-plane HNH bends of the undeuterated and deuterated monomer anions between 400 and 600 cm−1. The anionic homodimers form in a parallel orientation.

The role of specific amino acid side-chains in the fragmentation chemistry of gaseous protonated peptides resulting from collisional activation remains incompletely understood. For small peptides containing asparagine and glutamine, a dominant fragmentation channel induced by collisional activations is, in addition to deamidation, the loss of neutral water. Identifying the product ion structures from H2O-loss from four protonated dipeptides containing Asn or Gln using infrared ion spectroscopy, mechanistic details of the dissociation reactions are revealed. Several sequential dissociation reactions have also been investigated and provide additional insights into the fragmentation chemistry. While water loss can in principle occur from the C-terminus, the side chain or the amide bond carbonyl oxygen, in most cases the C-terminus was found to detach H2O, leading to a b2-sequence ion with an oxazolone structure for AlaGln, and bifurcating mechanisms leading to both oxazolone and diketopiperazine species for AlaAsn and AsnAla. In contrast, GlnAla expels water from the amide side chain leading to an imino-substituted prolinyl structure.

Complexes of divalent metal ions (Cu2+ and Ni2+) with histidine tripeptide complexes (HAA, AHA and AAH) are interesting gas-phase models for some of the most widely observed patterns of metal ion binding to peptides and proteins. Gas-phase structures were characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in ion-trapping mass spectrometers, along with density functional theory (DFT) computations. Ground states are square-planar with two deprotonated amide nitrogens bound to the metal ion via a double iminol rearrangement (IM binding mode), but contrary to expectations based on solution behavior, the histidine imidazole group is not bound to the metal, but instead is hydrogen bonded remote from the metal ion. The alternative “charge-solvated” (CS) binding mode (amide carbonyl oxygens binding the metal ion) lies higher in energy, but in many cases was observed to be present as conformationally unrelaxed ions with an abundance (relative to the IM conformation) that was dependent on instrument configuration and source and trap conditions. Taking advantage of the ability to form and trap both IM and CS conformations for a few of the complexes, the infrared spectroscopy of both conformations was explored in the fingerprint (1000–1800 cm−1) and hydrogen-stretching (3200–3800 cm−1) regions. For the fingerprint region, agreement is very good between the observed IRMPD spectra and the spectra predicted by DFT calculations at the B3LYP/6–311 + +g(d,p) level. Agreement in the H-stretching region is not perfect, but the characteristic IM and CS spectral patterns are evident.

International audience; Dissociation chemistry of the diproline anion [Pro2-H]− is studied using chemical dynamics simulations coupled with quantum-chemical calculations and RRKM analysis. Pro2– is chosen due to its reduced size and the small number of sites where deprotonation can take place. The mechanisms leading to the two dominant collision-induced dissociation (CID) product ions are elucidated. Trajectories from a variety of isomers of [Pro2-H]− were followed in order to sample a larger range of possible reactivity. While different mechanisms yielding y1– product ions are proposed, there is only one mechanism yielding the b2– ion. This mechanism leads to formation of a b2– fragment with a diketopiperazine structure. The sole formation of a diketopiperazine b2 sequence ion is experimentally confirmed by infrared ion spectroscopy of the fragment anion. Furthermore, collisional and internal energy activation simulations are used in parallel to identify the different dynamical aspects of the observed reactivity.

The following gas-phase uranyl/12-crown-4 (12C4) complexes were synthesized by electrospray ionization: [UO2(12C4)2]2+ and [UO2(12C4)2(OH)]+. Collision-induced dissociation (CID) of the dication resulted in [UO2(12C4-H)]+ (12C4-H is a 12C4 that has lost one H), which spontaneously adds water to yield [UO2(12C4-H)(H2O)]+. The latter has the same composition as complex [UO2(12C4)(OH)]+ produced by CID of [UO2(12C4)2(OH)]+ but exhibits different reactivity with water. The postulated structures as isomeric [UO2(12C4-H)(H2O)]+ and [UO2(12C4)(OH)]+ were confirmed by comparison of infrared multiphoton dissociation (IRMPD) spectra with computed spectra. The structure of [UO2(12C4-H)]+ corresponds to cleavage of a C–O bond in the 12C4 ring, with formation of a discrete U–Oeq bond and equatorial coordination by three intact ether moieties. Comparison of IRMPD and computed IR spectra furthermore enabled assignment of the structures of the other complexes. Theoretical studies of the chemical bonding features of the complexes provide an understanding of their stabilities and reactivities. The results reveal bonding and structures of the uranyl/12C4 complexes and demonstrate the synthesis and identification of two different isomers of gas-phase uranyl coordination complexes.

Infrared multiple photon dissociation spectroscopy was performed on protonated and cationized canavanine (Cav), a non-protein amino acid oxy-analog of arginine. Infrared spectra in the XH stretching region (3000–4000 cm−1) were obtained at the Centre Laser Infrarouge d’Orsay (CLIO) facility. Comparison of the experimental infrared spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory indicates that canavanine is in a canonical neutral form in CavH+, CavLi+, and CavNa+; therefore, these cations are charge-solvated structures. The infrared spectrum of CavK+ is consistent with a mixture of Cav in canonical and zwitterionic forms leading to both charge-solvated and salt-bridged cationic structures. The Cav moiety in CavCs+ is shown to be zwitterionic, forming a salt-bridged structure for the cation. Infrared spectra in the fingerprint region (1000–2000 cm−1) obtained at the FELIX Laboratory in Nijmegen, Netherlands support these assignments. These results show that a single oxygen atom substitution in the side chain reduces the stability of the zwitterion compared to that of the protein amino acid arginine (Arg), which has been shown previously to adopt a zwitterionic structure in ArgNa+ and ArgK+. This difference can be explained in part due to the decreased basicity of Cav (PA = 1001 kJ/mol) as compared to arginine (PA = 1051 kJ/mol), but not entirely, as lysine, which has nearly the same proton affinity as Cav, (∼993 kJ/mol) forms only canonical structures with Na+, K +, and Cs+. A major difference between the zwitterionic forms of ArgM+ and CavM+ is that the protonation site is on the side chain for Arg and on the N-terminus for Cav. This results in systematically weaker salt bridges in the Cav zwitterions. In addition, the presence of another hydrogen-bonding acceptor atom in the side chain contributes to the stability of the canonical structures for the smaller alkali cations.

Electron attachment dissociation (electron capture dissociation (ECD) and electron transfer dissociation (ETD)) applied to gaseous multiply protonated peptides leads predominantly to backbone N–Cα bond cleavages and the formation of c- and z-type fragment ions. The mechanisms involved in the formation of these ions have been the subject of much discussion. Here, we determine the molecular structures of an extensive set of c-type ions produced by ETD using infrared ion spectroscopy. Nine c3- and c4-ions are investigated to establish their C-terminal structure as either enol-imine or amide isomers by comparison of the experimental infrared spectra with quantum-chemically predicted spectra for both structural variants. The spectra suggest that all c-ions investigated possess an amide structure; the absence of the NH bending mode at approximately 1000–1200 cm–1 serves as an important diagnostic feature.

Contains fulltext : 182103.pdf (Publisher’s version ) (Open Access) Evidence is presented for cyclization to yield 2-bora-1,3-diazacycloalkanium cations in the gas phase. While the neutral compounds in solution and solid phase are known to possess an acyclic structure (as revealed by X-ray diffraction), the gaseous cations (from which borohydride BH4(-) ion has been expelled) have a cyclic structure, as revealed by InfraRed Multiple Photon Dissociation (IRMPD) spectroscopy and collisionally activated decomposition (CAD). The IRMPD decomposition of the monocyclic ions proceeds principally via H2 expulsion, although CAD experiments show additional pathways. Pyrolyses of solid monomeric salts and small oligomers produce higher polymers that are consistent with H2 expulsion as the major pathway. Deuterium labeling experiments show that scrambling occurs prior to IRMPD or CAD decomposition in the gas phase.

Ni(II) ions can be caged by surrounding peptide ligands in two basic binding patterns: the “iminol” (IM) binding pattern, where chelation occurs by deprotonated amide nitrogens, or the charge-solvated (CS) binding pattern, where chelation occurs by amide carbonyl oxygens. Gas-phase observation may clarify the factors affecting this choice in solution and in peptide and protein matrices. Infrared spectroscopic determination of gas-phase structures shows here how microsolvation by just one water molecule switches the balance of this choice from IM to CS for the Ni2+Gly3 complex, in contrast with the always-CS structure of the Ni2+Gly4 complex. Quantum-chemical calculations indicate that CS complexation is even more favored in the aqueous limit. Considering gas-phase conditions as comparable to low-pH solutions can reconcile this prediction with the common observation of IM-type binding in solutions at higher pH. This is likely the first gas-phase observation of solvation-induced IM-to-CS transition in oligopeptide complexes with doubly charged transition-metal ions.

In the complex formed between the calcium cation (Ca2+) and a deprotonated HisHis dipeptide, the complex adopts a charge solvation (CS) structure. Ca2+, a weak binding main group metal cation, interacts with the oxygens of the peptide carbonyl moiety and the deprotonated C-terminus. In contrast, the much stronger binding Ni2+ cation deprotonates the peptide nitrogen and induces an iminolate (Im) ligand structure in the [Ni(HisHis-H)]+ complex ion. The combination of infrared multiple-photon dissociation (IRMPD) spectroscopy and quantum chemistry evidence these two representative binding motifs. The iminolate coordination pattern identified and characterized in the [Ni(HisHis-H)]+ complex serves as a model case for nickel complexes of poly-histidyl-domains and is thereby also of interest to better understand the fundamentals of immobilized metal ion affinity chromatography as well as of Ni co-factor chemistry in enzymology.

Infrared multiple photon dissociation (IRMPD) spectroscopy has been used to probe the structures of the three protonated base-pair mismatches containing 9-ethylguanine (9eG) in the gas phase. Computational chemistry has been used to determine the relative energies and compute the infrared spectra of these complexes. By comparing the IRMPD spectra with the computed spectra, in all cases, it was possible to deduce that what was observed experimentally were the lowest energy computed structures. The protonated complex between 9eG and 1-methylthymine (1mT) is protonated at N7 of 9eG—the most basic site of all three bases in this study—and bound in a Hoogsteen type structure with two hydrogen bonds. The experimental IRMPD spectrum for the protonated complex between 9eG and 9-methyladenine (9mA) is described as arising from a combination of the two lowest energy structures, both which are protonated at N1 of adenine and each containing two hydrogen bonds with 9eG being the acceptor of both. The protonated dimer of 9eG is protonated at N7 with an N7–H+–N7 ionic hydrogen bond. It also contains an interaction between a C–H of protonated guanine and the O6 carbonyl of neutral guanine which is manifested in a slight red shift of that carbonyl stretch. The protonated 9eG/9mA structures have been previously identified by X-ray crystallography in RNA and are contained within the protein database.

Long-range electron transfer in proteins can be rationalized as a sequential short-distance electron-hopping processes via amino acid residues having low ionization energy as relay stations. Tyrosine residues can serve as such redox-active intermediates through one-electron oxidation to form a π-radical cation at its phenol side chain. An electron transfer from a vicinal functional group to this π-electron hole completes an elementary step of charge migration. However, transient oxidized/reduced intermediates formed at those relay stations during electron transfer processes have not been observed. In this study, formation of analog reactive intermediates via electron donor–acceptor coupling is observed by using IRMPD action spectroscopy. An elementary charge migration at the molecular level in model tyrosine-containing peptide radical cations [M]˙+ in the gas phase is revealed with its unusual Cα–Cβ bond cleavage at the side chain of the N-terminal residue. This reaction is induced by the radical character of the N-terminal amino group (–NH2˙+) resulting from an n → π+ interaction between the nonbonding electron pair of NH2 (n) and the π-electron hole at the Tyr side chain (π+). The formation of –NH2˙+ is supported by the IRMPD spectrum showing a characteristic NH2 scissor vibration coupled with Tyr side-chain stretches at 1577 cm−1. This n → π+ interaction facilitates a dissociative electron transfer with NH2 as the relay station. The occurrence of this side-chain cleavage may be an indicator of the formation of reactive conformers featuring the n → π+ interaction.

The structures and properties of metal cationized complexes of 9-ethylguanine (9eG) and 1-methylcytosine (1mC), (9eG:1mC)M+, where M+ = Li+, Na+, K+, Rb+, Cs+ as well as the protonated complex, (9eG:1mC)H+, have been studied using a combination of IRMPD spectroscopy and computational methods. For (9eG:1mC)H+, the dominant structure is a Hoogsteen type complex with the proton covalently bound to N3 of 1mC despite this being the third best protonation site of the two bases; based on proton affinities N7 of 9eG should be protonated. However, this structural oddity can be explained considering both the number of hydrogen bonds that can be formed when N3 of 1mC is protonated as well as the strong ion-induced dipole interaction that exists between an N3 protonated 1mC and 9eG due to the higher polarizability of 9eG. The anomalous dissociation of (9eG:1mC)H+, forming much more (1mC)H+ than would be predicted based on the computed thermochemistry, can be explained as being due to the structural oddity of the protonation site and that the barrier to proton transfer from N3 of 1mC to N7 of 9eG grows dramatically as the base pair begins to dissociate. For the (9eG:1mC)M+; M = Li+, Na+, K+, Rb+, Cs+ complexes, single unique structures could not be assigned. However, the experimental spectra were consistent with the computed spectra. For (9eG:1mC)Li+, the lowest energy structure is one in which Li+ is bound to O6 of 9eG and both O2 and N3 of 1mC; there is also an interbase hydrogen bond from the amine of 1mC to N7 of 9eG. For Na+, K+, and Rb+, similar binding of the metal cation to 1mC is calculated but, unlike Li+, the lowest energy structure is one in which the metal cation is bound to N7 of 9eG; there is also an interbase hydrogen bond between the amine of 1mC and the carbonyl of 9eG. The lowest energy structure for the Cs complex is the Watson-Crick type base pairing with Cs+ binding only to 9eG through O6 and N7 and with three hydrogen bonds between 9eG and 1mC. It also interesting to note that the Watson-Crick base pairing structure gets lower in Gibbs energy relative to the lowest energy complexes as the metal gets larger. This indicates that the smaller, more densely charged cations have a greater propensity to interfere with Watson-Crick base pairing than do the larger, less densely charged metal cations.

Metallated gas-phase structures consisting of a deprotonated and an intact histidine (His) ligand, yielding M(His-H)(His)+, where M = Zn and Cd, were examined with infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). In parallel, quantum chemical calculations identified several low-energy isomers for each complex. Experimental action spectra were compared to linear spectra calculated at the B3LYP level of theory using the 6-311+G(d,p) and def2-TZVP basis sets for the zinc and cadmium complexes, respectively. For both Zn and Cd species, the definitive assignment is complicated by conflicting relative energetics, which were calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels. Spectral comparison for both species indicates that the dominant conformation, [Nα, Nπ, CO-][CO2-](NπH+), has the deprotonated His chelating the metal at the amine nitrogen, π nitrogen of the imidazole ring, and the deprotonated carbonyl oxygen and that the intact His ligand adopts a salt-bridge bidentate binding motif, coordinating the metal with both carboxylate oxygens. There is also evidence for a conformation where the deprotonated His coordination is maintained, but the intact His ligand adopts a more canonical structure, coordinating with the metal atom at the amine nitrogen and π nitrogen, [Nα, Nπ, CO-][Nα, Nπ]gtgg. For both metallated species, B3LYP, B3P86, and B3LYP-GD3BJ levels of theory appear to describe the relative stability of the dominant zwitterionic species more accurately than the MP2(full) level.

The macrocycle valinomycin displays an outstanding ability in cation binding and carriage across hydrophobic environments (e.g., cell membranes) and constitutes a central landmark for the design of novel ionophores for the regulation of biochemical processes. Most previous investigations have focused on the capture of metal cations (primarily K+). Here, we address the versatility of valinomycin in the encapsulation of molecular ions of small and moderate size, with NH4+ and H4PO4+ as case studies. A combination of infrared action vibrational spectroscopy and quantum chemical computations of molecular structure and dynamics is employed with the two-fold aim of assessing the dominant H-bonding coordination networks in the complexes and of characterizing the positional and rotational freedom of the guest cations inside the cavity of the macrocycle. Valinomycin binds NH4+ with only moderate distortion of the C3 configuration adopted in the complexes with the metal cations. The ammonium cation occupies the center of the cavity and displays two low-energy coordination arrangements that are dynamically connected through a facile rotation of the cation. The inclusion of the bulkier phosphoric acid cation demands significant stretching of the valinomycin backbone. Interestingly, the H4PO4+ cation achieves ample positional and rotational mobility inside valinomycin. The valinomycin backbone is capable of adopting barrel-like configurations when the cation occupies a region close to the center of the cavity, and funnel-like configurations when it diffuses to positions close to the exit face. This can accommodate the cation in varying coordination arrangements, characterized by different H-bonding between the four POH arms and the ester carbonyl groups of the macrocycle.

In a previous study [Int. J. Mass Spectrom. 2010; 297: 67–75], the asymmetric O=U=O stretch (ν3) was measured for anionic uranyl complexes with composition [UO2(X)3]-, X = Cl-, Br- and I-. Within this group of complexes, the ν3 frequency red-shifts following the trend I > Br > Cl, suggesting concomitant weakening of the U=O bonds. However, a value for [UO2(F)3]- was not measured, which prevented a comprehensive comparison of measured ν3 positions to computed frequencies from density functional theory (DFT) calculations. Because the shift in ν3 is predicted to be most dramatic when X = F, we revisited these species using infrared multiple-photon photodissociation spectroscopy. As in our earlier study, a modest red-shift to the ν3 vibration of ∼ 6 cm-1 was observed for X = I-, Br-, and Cl-, and the position of the frequency follows the trend I- > Br- > Cl-. The value measured for [UO2(F)3]- is ∼43 cm-1 lower than the one measured for [UO2(Cl)3]-. Overall, the trend with respect to ν3 position is reproduced well by computed frequencies from DFT.

We report the fingerprint IR spectra of mass-isolated gaseous coordination complexes of 2,2′-bipyridine (bpy) and 1,4,8,11-tetra-azacyclotetradecane (cyclam) with a copper ion in its I and II oxidation states. Experiments are carried out in a quadrupole ion trap (QIT) mass spectrometer coupled to the FELIX infrared free-electron laser. Dications are prepared using electrospray ionization (ESI), while monocations are generated by charge reduction of the dication using electron transfer-reduction (ETR) in the QIT. Interestingly, [Cu(bpy)2]+ can also be generated directly using ESI, so that its geometries as produced from ETR and ESI can be compared. The effects of charge reduction on the IR spectra are investigated by comparing the experimental spectra with the IR spectra modeled by density functional theory. Reduction of Cu(II) to the closed-shell Cu(I) ion retains the square-planar geometry of the Cu–cyclam complex. In contrast, for the bis–bpy complex with Cu, charge reduction induces a conversion from a near-square-planar to a tetrahedral geometry. The geometry of [Cu(bpy)2]+ is identical to that of the complex generated directly from ESI as a native structure, which indicates that the ETR product ion thermalizes. For [Cu(cyclam)]+, however, the square-planar geometry of the 2+ complex is retained upon charge reduction, although a (distorted) tetrahedral geometry was predicted to be lower in energy. These differences are attributed to different barriers to rearrangement.

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, AnVO2(C2O4)−, where An = Pa or U, are essentially actinyl ions, AnVO2+, coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, AnVO(OH)2(C2O4)−. The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas PaVI is inaccessible, UVI is very stable. The UVO2(C2O4)− complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O2 to yield UO4– and two CO2 molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, UVIO22+, coordinated in the equatorial plane by two equivalent O atoms each having formal charges of −1.5 and U–O bond orders intermediate between single and double. The unreactive nature of PaVO2(C2O4)− toward O2 is a manifestation of the resistance toward oxidation of PaV, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO4–, reacts with water to yield UO5H2–. Infrared spectra obtained for UO5H2– confirm the computed lowest-energy structure, UO3(OH)2–.

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.

The observed variety of metal-ion complexation sites offered by peptides reflects a basic tension between charge solvation of the ion by Lewis-basic chelating groups versus amide nitrogen deprotonation and formation of metal–nitrogen bonds. Gas-phase models of metal-ion coordination can illuminate the factors governing this choice in condensed-phase proteins and enzymes. Here, structures of gas-phase complexes of Ni(II) and Cu(II) with tri- and tetra-peptide ligands are mapped out using a combination of Infrared Multiple Photon Dissociation (IRMPD) spectroscopy and density functional theory (DFT) computations. The two binding modes give distinctive IRMPD signatures, particularly in the diagnostic region 1500–1550 cm−1. Previous observations have suggested that Ni(II) complexes preferentially show the iminol rearrangement pattern (Im) giving low-spin square-planar geometries with metal-ion bonds to deprotonated amide nitrogens. In contrast, alkaline earth metal ion complexes prefer amide carbonyl oxygens chelating the metal ion with pyramidal geometry (charge-solvation, CS). Surprisingly, it is shown here that the Gly4 complexes are CS bound, in contrast with the expectation of Im binding. It is suggested that CS binding is actually a normal Ni(II) and Cu(II) binding mode to simple peptides lacking participating side chains. Three factors are suggested to influence the choice between CS and Im binding patterns: (1) presence of an accessible side-chain Lewis-basic proton interaction site (FGGF, FGG and HAA complexes); (2) short chain length of the peptide leading to a shortage of accessible carbonyl oxygen sites for CS binding, (AAA, FGG and HAA complexes); (3) outright deprotonation of the ligand giving net negatively charged Im[Ni2+(Gly4–3H+)]− and Im[Ni2+(Ala3–3H+)]− complexes, which have a triply-deprotonated ligand. IRMPD spectra of [Cu2+Gly4]2+ and [Cu2+(Gly4–3H+)]− complexes suggest that their structures are similar to their Ni2+ analogs.

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.

The gas-phase InfraRed Multiple Photon Dissociation (IRMPD) spectra in the fingerprint region of the anion (m/z 163) and mono-deprotonated homodimer of 9-methyl-guanine (2, m/z 329) and its d5 analogue (2-d5) are reported here. The anionic dimer is reverse Watson-Crick paired with the hydrogen bonding pattern ADD/DAA (where A stands for acceptor and D stands for donor), in which the site of negative charge is unambiguously found to be N1. The match between experimental vibrational spectra and DFT-computed spectra is good with the exception of the region between 2000 and 2900 cm-1 that contains symmetrical and antisymmetrical N-H stretches of the adjacent hydrogen-bond donor-donor pair.

Infrared ion spectroscopy (IRIS) requires reproducibility and standard protocols in order to be fully appreciated as an analytical tool. An important parameter in IRIS measurements is the infrared laser power (energy per pulse), which is often difficult to control precisely on a day-to-day basis. We have developed an automatic variable attenuator to keep the laser power at a well-defined value, independent of IR frequency and time, while maintaining full overlap of the laser beam with the ion cloud in our quadrupole ion trap mass spectrometer. The device has been used to record IR spectra of the metabolite prostaglandin D2 at different laser pulse energies, allowing us to assess the effects on the IRMPD spectra of excessive ion depletion, a threshold pulse energy below which dissociation is not observed, and unobserved fragments due to electron detachment or fragmentation to below the low mass cut-off of the trap.

Gas-phase infrared multiple-photon dissociation (IRMPD) spectra are recorded for the protonated dye molecules indigo and isoindigo by using a quadrupole ion trap (QIT) mass spectrometer coupled to the free electron laser for infrared experiments (FELIX). From their fingerprint IR spectra (600—1800 cm–1) and comparison with quantum-chemical calculations at the density functional level of theory (B3LYP/6-31++G(d,p)), we derive their structures. We focus particularly on the question of whether trans-to-cis isomerization occurs upon protonation and transfer to the gas phase. The trans-configuration is energetically favored in the neutral forms of the dyes in solution and in the gas phase. Instead, the cis-isomer is lower in energy for the protonated forms of both species, but indigo is also notorious for not undergoing double-bond trans-to-cis isomerization, in contrast to many other conjugated systems. The IR spectra suggest that protoisomerization from trans to cis indeed occurs for both dyes. To estimate the extent of isomerization, on-resonance kinetics are measured on diagnostic and common vibrational frequencies to determine the ratio of cis-to-trans isomers. We find ratios of 65–70% cis and 30–35% trans for indigo versus 75–80% cis and 20–25% trans for isoindigo. Transition-state calculations for the isomerization reactions have been carried out, which indeed suggest a lower barrier for protonated isoindigo, qualitatively explaining the more efficient isomerization.

A charge-tagged phenyl pyruvic acid derivative was investigated by tandem-MS, infrared (IR) ion spectroscopy and theory. The tailor-made precursor ions efficiently lose CO2 in collision induced dissociation (CID) experiments, offering access to study the secondary decay reactions of the product ions. IR ion spectroscopy provides evidence for the formation of an enol acid precursor ion structure in the gas phase and indicates the presence of enol products formed after CO2 loss. Extensive DFT computations however, suggest intermediate generation of hydroxycarbene products, which in turn rearrange in a secondary process to the enol ions detected by IR ion spectroscopy. Quantum mechanical tunneling of the hydroxycarbene can be excluded since no evidence for aldehyde product ion formation could be found. This finding is in contrast to the behavior of methylhydroxycarbene, which cleanly penetrates the energy barrier to form exclusively acetaldehyde at cryogenic temperatures in an argon matrix via quantum mechanical hydrogen tunneling. The results presented here are attributed to the highly excited energy levels of the product ions formed by CID in combination with different barrier heights of the competing reaction channels, which allow exclusive access over one energy barrier leading to the formation of the enol tautomer ions observed.

Chelation complexes of the histidine-containing tripeptides HisAlaAla, AlaHisAla, and AlaAlaHis with Ni(II) and Cu(II) having a -1 net charge are characterized in the gas phase by infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory calculations. We address the question of whether the gas-phase complexes carry over characteristics from the corresponding condensed-phase species. We focus particularly on three aspects of their structure: (i) square-planar chelation by the deprotonated amide nitrogens around the metal ion (low-spin for the Ni case), (ii) metal-ion coordination of the imidazole side chain nitrogen, and (iii) the exceptional preference for metal-ion chelation by peptides with His in the third position from the N-terminus, as in the amino terminal Cu and Ni (ATCUN) motif. We find that square-planar binding around the metal ion, involving bonds to both deprotonated backbone nitrogens, one of the carboxylate oxygens and the N-terminal nitrogen, is the dominant binding motif for all three isomers. In contrast to the condensed-phase behavior, the dominant mode of binding for all three isomers does not involve the imidazole side chain, which is instead placed outside the coordination zone. Only for the AlaAlaHis isomer, the imidazole-bound structure is also detected as a minority population, as identified from a distinctive short-wavelength IR absorption. The observation that this conformation exists only for AlaAlaHis correlates with condensed-phase behavior at neutral-to-basic pH, in the sense that the isomer with His in the third position is exceptionally disposed to metal ion chelation by four nitrogen atoms (4N) when compared with the other isomers. These results also emphasize the divergence between the conformational stabilities in the gas phase and in solution or crystalline environments: in the gas phase, direct metal binding of the imidazole is overall less favorable than the alternative of a remote imidazole that can act as an intramolecular H-bond donor enhancing the gas-phase stability.

In uranyl coordination complexes, UO2(L)n2+, uranium in the formally dipositive [O=U=O]2+ moiety is coordinated by n neutral organic electron donor ligands, L. The extent of ligand electron donation, which results in partial reduction of uranyl and weakening of the U=O bonds, is revealed by the magnitude of the red-shift of the uranyl asymmetric stretch frequency, ν3. This phenomenon appears in gas-phase complexes in which uranyl is coordinated by electron donor ligands: the ν3 red-shift increases as the number of ligands and their proton affinity (PA) increases. Because PA is a measure of the enthalpy change associated with a proton-ligand interaction, which is much stronger and of a different nature than metal ion-ligand bonding, it is not necessarily expected that ligand PAs should reliably predict uranyl-ligand bonding and the resulting ν3 red-shift. Here, ν3 was measured for uranyl coordinated by ligands with a relatively broad range of PAs, revealing a surprisingly good correlation between PA and ν3 frequency From computed ν3 frequencies for bare UO2 cations and neutrals, it is inferred that the effective charge of uranyl in UO2(L)n2+ complexes can be reduced to near zero upon ligation by sufficiently strong charge-donor ligands. The basis for the correlation between ν3 and ligand PAs, as well as limitations and deviations from it, are considered. It is demonstrated that the correlation evidently extends to a ligand that exhibits polydentate metal ion coordination.

Collision-induced dissociation of isotopically labelled protonated pentaglycine produced two abundant [b5]+ ions, the products of the loss of water from the first and second amide groups, labelled [b5]+I and [b5]+II. IRMPD spectroscopy and DFT calculations show that these two [b5]+ ions feature N1-protonated 3,5-dihydro-4H-imidazol-4-one structures. 15N-Labelling established that some interconversion occurs between these two ions but dissociations are preferred. For both ions, DFT calculations show that the barrier to interconversion is slightly higher than those to dissociation. Dehydration of protonated hexaglycine produced three imidazolone ions. Ions [b6]+I and [b6]+II exhibit analogous CID spectra to those from [b5]+I and [b5]+II; however, the spectrum of the [b6]+III ion was dramatically different, showing losses predominantly of a further water molecule or cleavage of the second amide bond to give the glycyloxazolone (a deprotonated [b2]+ ion, labelled GlyGlyox (114 Da)) from the N-terminus. Protonated polyglycines [Glyn + H]+, where n = 7-9, all readily lose at least one water molecule. The corresponding [bn]+ ions lose either a further water molecule, an oxazolone from the N-terminus or a truncated peptide from the C-terminus. The number of amino acid residues in the latter two eliminated neutral molecules provides insight into the location of the imidazolone in the peptide chain and which oxygen was lost in the initial dehydration reaction. From this analysis, it appears that water loss from the longer protonated polyglycines is predominantly from the central residues.

Infrared multiple photon dissociation (IRMPD) experiments and electronic structure computations have been used to explore the structures and energetics of binding in gas phase sodium cyclopentadienyl complexes of the general form NanCpn–1+. Computational work for the analogous anionic NanCpn+1– complexes reveals nearly identical energetics for the loss of neutral NaCp units from both cationic and anionic species leading to the conclusion that the binding in the gas phase species is largely electrostatic, arising primarily from ion–ion and ion–dipole interactions. This is supported by an examination of the molecular orbitals of these species, which show no orbital overlap between the Na and Cp moieties. Observation of peaks in the IRMPD spectra, which do not correspond to any of the computed linear absorption frequencies, strongly suggest the contribution of overtone and combination bands.

Proton transfer in the conjugate acid cation of a symmetric diether, 1,5-dimethoxy-3,3-dimethylpentane (1), is explored both theoretically and experimentally via IRMPD spectroscopy. The optimized geometry of the cation has one oxygen pyramidalized (sum of the three angles Σ = 345°), while the other oxygen has a virtually planar geometry (if one includes the strongly hydrogen bonded O⋯H, the three bond angles sum to Σ′ = 357°). Assuming that this geometry is not preserved upon intramolecular proton transfer from one oxygen to the other, two alternatives remain: either the two oxygens trade geometries (Σ and Σ′ change places via a transition state that has a mirror plane of symmetry) or else both oxygens become pyramidalized. Because the asymmetric stretch couples with other vibrational modes of the cation, special attention is paid to the classical turning points at the zero point level corresponding to the lowest frequency vibration that shows a deuterium isotope effect, which occurs experimentally around 895 cm−1. This stretching vibration has a lower frequency than the bands assigned to O⋯H O bends, and it shifts to still lower frequency in the deuteronated ion. Because a normal modes calculation matches this vibration, we conclude that the potential energy surface matches a single-well and does not correspond to a double-well minimum.

Gas-phase ion chemistry methods that capture and characterize the degree of activation of small molecules in the active sites of homogeneous catalysts form a powerful new tool to unravel how ligand environments affect reactivity. A key roadblock in this development, however, is the ability to generate the fragile metal oxidation states that are essential for catalytic activity. Here we demonstrate the preparation of the key Ni(I) center in the widely used cyclam scaffold using ion–ion recombination as a gas-phase alternative to electrochemical reduction. The singly charged Ni+(cyclam) coordination complex is generated by electron transfer from fluoranthene and azobenzene anions to doubly charged Ni2+(cyclam), using the electron-transfer dissociation protocol in a commercial quadrupole ion trap instrument and in a custom-built octopole RF ion trap. The successful preparation of the Ni+(cyclam) cation is verified through analysis of its vibrational spectrum obtained using the infrared free electron laser FELIX.

© 2017 American Chemical Society. Recent efforts to activate the strong uranium-oxygen bonds in the dioxo uranyl cation have been limited to single oxo-group activation through either uranyl reduction and functionalization in solution, or by collision induced dissociation (CID) in the gas-phase, using mass spectrometry (MS). Here, we report and investigate the surprising double activation of uranyl by an organic ligand, 3,4,3-LI(CAM), leading to the formation of a formal U6+chelate in the gas-phase. The cleavage of both uranyl oxo bonds was experimentally evidenced by CID, using deuterium and18O isotopic substitutions, and by infrared multiple photon dissociation (IRMPD) spectroscopy. Density functional theory (DFT) computations predict that the overall reaction requires only 132 kJ/mol, with the first oxygen activation entailing about 107 kJ/mol. Combined with analysis of similar, but unreactive ligands, these results shed light on the chelation-driven mechanism of uranyl oxo bond cleavage, demonstrating its dependence on the presence of ligand hydroxyl protons available for direct interactions with the uranyl oxygens.

© 2016 American Chemical Society. Gas-phase organoactinyl complexes possessing discrete An-C bonds (An = U, Np, Pu) were synthesized in a quadrupole ion trap by endothermic decarboxylation of [AnO2(O2C-R)3]-anion complexes in which a formally AnO22+actinyl core is coordinated by three carboxylate ligands, with R = CH3(methyl), CH3CC (1-propynyl), C6H5(phenyl), C6F5(pentafluorophenyl). Decarboxylation and competing ligand loss were studied computationally by density functional theory complementing experiment. Although decarboxylation was computed to be the energetically most favorable process in all cases, reduction from An(VI) to An(V) via neutral ligand loss was often prevalent, particularly for An = Np, Pu, presumably resulting from barriers associated with decarboxylation. Comparative hydrolysis rates of the An-C bonds were experimentally determined, and the chemical properties of these bonds were analyzed by the quantum theory of atoms in molecules. The measured hydrolysis rates differed by up to 3 orders of magnitude: the fastest was for [(CH3CC)UO2(O2C-CCCH3)2]-and the slowest for [(C6F5)PuO2(O2C-C6F5)2]-. There is a general correlation between hydrolysis exothermicity and hydrolysis rate. Prototypical hydrolysis reaction pathways computed for R = CH3(An = U, Np) reveal a mechanism in which an outer-sphere water becomes inner-sphere concomitant with transfer of an H atom to yield an OH ligand and CH4, with a net energy release of 170 kJ mol-1and a transition state barrier of 45 kJ mol-1for An = U. Infrared multiphoton dissociation spectra of selected complexes were acquired to confirm the predicted structures by agreement between the computed and observed vibrational frequencies. The experiment and theory results provide an evaluation of the comparative propensities for formation of the organoactinyls as a function of actinide and carboxylate and an assessment of the nature and stability toward hydrolysis of the primarily ionic An-C bonds.

The vibration–rotation dynamics of acetylene are computed from a spectroscopic Hamiltonian with 468 parameters fit to 19,582 vibration–rotation transitions up to 13,000 cm−1 of vibrational energy. In this energy range, both the bending and the CH stretching vibrations can reach large amplitudes, but the maximum energy remains below the threshold for isomerization to vinylidene. In contrast to the behavior at energies below 5000 cm−1 [Mol. Phys. 108, 1115 (2010)], excitation of single bright states leads, in almost all cases, to computed intramolecular vibrational redistribution (IVR) that is irreversible on the timescales investigated. Hierarchies of IVR processes on timescales ranging from 20 fs to 20 ps result when different bright states are excited. Different parts of the vibrational quantum number space are explored as a result of the four different classes of coupling terms: vibrational l-type resonance, anharmonic resonances, the rotational l-type resonance, and Coriolis couplings. The initial IVR ...