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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•-. 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 ν3 IR frequencies indicate the following donor ordering: O2-[best donor] ≫ O•-> OH-> NO3-.

Herein, we explore the generation and characterization of the radical cations of glycylphenylalanylglycine, or [GFG]•+, formed via dissociative electron-transfer reaction from the tripeptide to copper(II) within a ternary complex. A comprehensive investigation employing isotopic labeling, infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations elucidated the details and energetics in formation of the peptide radical cations as well as their dissociation products. Unlike conventional aromatic-containing peptide radical cations that primarily form canonical π-radicals, our findings reveal that 75% of the population of the experimentally produced [GFG]•+ precursors are [GFα•G]+, where the radical resides on the middle α-carbon of the phenylalanyl residue. This unexpected isomeric ion has an enthalpy of 6.8 kcal/mol above the global minimum, which has an N-terminal captodative structure, [Gα•FG]+, comprising 25% of the population. The [b₂-H]•+ product ions are also present in a ratio of 75/25 from [GFα•G]+/ [Gα•FG]+, the results of which are obtained from matches between the IRMPD action spectrum and predicted IR absorption spectra of the [b₂-H]•+ candidate structures, as well as from IRMPD isomer population analyses. [ABSTRACT FROM AUTHOR]

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.

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.

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 and 18O 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.

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-.

Understanding of the nature and extent of chemical bonding in uranyl coordination complexes is crucial for the design of new ligands for nuclear waste separation, uranium extraction from seawater, and other applications. We report here the synthesis, infrared spectroscopic characterization, and quantum chemical studies of a molecular uranyl-di-15-crown-5 complex. The structure and bonding of this unique complex featuring a distinctive 6-fold coplanar coordination staggered sandwich structure and an unusual non-perpendicular orientation of the uranyl moiety are evaluated using density functional theory and chemical bonding analyses. The results provide fundamental understanding of the coordination interaction of uranyl with oxygen-donor ligands.

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-1 and a transition state barrier of 45 kJ mol-1 for 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.

We report mid-infrared spectra and potential energy surfaces of fouranionic, 2′-deoxynucleotide-5′-monophosphates (dNMPs) and the ionic DNA pairs[dGMP-dCMP−H]1−, [dAMP-dTMP−H]1− with a total charge of the complex equal to−1. We recorded IR action spectra by resonant IR multiple-photon dissociation (IRMPD)using the FELIX free electron laser. The potential energy surface study employed an onthe-fly molecular dynamics quenching method (MD/Q), using a semiempirical AM1method, followed by an optimization of the most stable structures using density functionaltheory. By employing infrared multiple-photon dissociation (IRMPD) spectroscopy incombination with high-level computational methods, we aim at a better understanding ofthe hydrogen bonding competition between the phosphate moieties and the nucleobases.We find that, unlike in multimer double stranded DNA structures, the hydrogen bonds inthese isolated nucleotide pairs are predominantly formed between the phosphate groups.This intermolecular interaction appears to exceed the stabilization energy resulting frombase pairing and directs the overall cluster structure and alignment.

Proton and hydrogen-bonded networks sustain a broad range of structural and charge transfer processes in supramolecular materials. The modelling of proton dynamics is however challenging and demands insights from prototypical benchmark systems. The intramolecular H-bonding networks induced by either protonation or deprotonation of 3-hydroxyglutaric acid provide intriguing case studies of correlated proton dynamics. The vibrational signatures associated with the fluxional proton bonding and its coupling with the hydroxyglutaric backbone are investigated here with infrared action ion spectroscopy experiments and Born–Oppenheimer molecular dynamics (BOMD) computations. Despite the formally similar symmetry of protonated and deprotonated hydroxyglutaric acid, the relative proton affinities of the oxygen centers of the carboxylic and carboxylate groups with respect to that of the central hydroxyl group lead to distinct proton dynamics. In the protonated acid, a tautomeric arrangement of the type HOCO·[HOH]+·OCOH is preferred with the proton binding tighter to the central hydroxyl moiety and the electronic density being shared between the two nearly symmetric H-bonds with the carboxylic end groups. In the deprotonated acid, the asymmetric [OCO]−·HO·HOCO configuration is more stable, with a stronger H-bonding on the bare carboxylate end. Both systems display active backbone dynamics and concerted Grothuss-like proton motions, leading to diffuse band structures in their vibrational spectra. These features are accurately reproduced by the BOMD computations. [ABSTRACT FROM AUTHOR]

Intramolecular radical and proton migration in the gas phase are important processes driving the dissociation reactions underlying common tandem mass spectrometry processes, such as collision-induced and electron-induced dissociation reactions (CID and ExD). Mechanistic insights in these processes requires experiments that probe the molecular structures of ions along the reaction pathways, usually combined with quantum-chemical calculations. The combination of ion trap mass spectrometry with IR laser spectroscopy, pioneered among others by Dieter Gerlich, provides a particularly effective method to explore details of the ion structures. In this work, we employ infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations to probe the reactant and product ion structures of a homolytic bond cleavage reaction. First, we employ IRMPD spectroscopy to establish that protonation of the 4-bromoaniline precursor occurs on the amine moiety and then that C-Br homolytic cleavage produces the π-radical cation of aniline. Transition-state calculations are performed to compare the various pathways that connect reactant and product ions, including both proton and radical transfer mechanisms. [ABSTRACT FROM AUTHOR]