Vibrational properties of anhydrous and partially hydrated uranyl fluoride.

Uranyl fluoride (UO2F2) is a hygroscopic powder with two main structural phases: an anhydrous crystal and a partially hydrated crystal of the same R3¯m symmetry. The formally closed-shell electron structure of anhydrous UO2F2 is amenable to density functional theory calculations. We use density functional perturbation theory (DFPT) to calculate the vibrational frequencies of the anhydrous crystal structure and employ complementary inelastic neutron scattering and temperature-dependent Raman scattering to validate those frequencies. As a model closed-shell actinide, we investigated the effect of LDA, GGA, and non-local vdW functionals as well as the spherically averaged Hubbard +U correction on vibrational frequencies, electronic structure, and geometry of anhydrous UO2F2. A particular choice of Ueff=5.5 eV yields the correct U-Oyl bond distance and vibrational frequencies for the characteristic Eg and A1g modes that are within the resolution of experiment. Inelastic neutron scattering and Raman scattering suggest a degree of water coupling to the lattice vibrations in the more experimentally accessible partially hydrated UO2F2 system, with the symmetric stretching vibration shifted approximately 47 cm-1 lower in energy compared to the anhydrous structure. Evidence of water interaction with the uranyl ion is present from a two-peak decomposition of the uranyl stretching vibration in the Raman spectra and anion-hydrogen stretching vibrations in the inelastic neutron scattering spectra. A first-order dehydration phase transition temperature is definitively identified to be 125 °C using temperature-dependent Raman scattering.

Infrared multiple photon dissociation action spectroscopy of deprotonated DNA mononucleotides: gas-phase conformations and energetics.

The gas phase structures of the deprotonated 2'-deoxymononucleotides including 2'-deoxyadenosine-5'-monophosphate (dA5'p), 2'-deoxycytidine-5'-monophosphate (dC5'p), 2'-deoxyguanosine-5'-monophosphate (dG5'p), and thymidine-5'-monophosphate (T5'p) are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical electronic structure calculations. The measured IRMPD action spectra of all four deprotonated DNA mononucleotides exhibit unique spectral features in the region extending from ~600 to 1800 cm(-1) such that they can be readily differentiated from one another. The measured IRMPD action spectra are compared to the linear IR spectra calculated at the B3LYP/6-311+G(d,p) level of theory to determine the conformations of these species accessed in the experiments. On the basis of these comparisons and the computed energetic information, the most stable conformations of the deprotonated forms of dA5'p, dC5'p, and T5'p are conformers where the ribose moiety adopts a C3' endo conformation and the nucleobase is in an anti conformation. By contrast, the most stable conformations of the deprotonated form of dG5'p are conformers where the ribose adapts a C3' endo conformation and the nucleobase is in a syn conformation. In addition to the ground-state conformers, several stable low-energy excited conformers that differ slightly in the orientation of the phosphate ester moiety were also accessed in the experiments.

Infrared multiple photon dissociation spectroscopy of cationized asparagine: effects of metal cation size on gas-phase conformation.

Gas-phase structures of cationized asparagine (Asn) including complexes with Li(+), Na(+), K(+), Rb(+), Cs(+), and Ba(2+), as well as protonated Asn, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser. Experimental spectra for the alkali metal cation complexes exhibit systematic trends, whereas spectra for Ba(2+)(Asn) and H(+)(Asn) are more distinct. To identify the structures formed experimentally, measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level with several effective core potentials and basis sets evaluated for the heavy metal systems. The dominant conformation ascertained for complexes with the smaller metal cations, Li(+)(Asn) and Na(+)(Asn), is a charge-solvated, tridentate [N,CO,CO] structure that binds the metal cation with the amine group of the amino acid backbone and to the carbonyl oxygen atoms of the backbone and amino acid side chain. For the larger alkali metal cation complexes, K(+)(Asn), Rb(+)(Asn), and Cs(+)(Asn), an additional charge-solvated, tridentate [COOH,CO] structure that binds the metal cation with the two oxygen atoms of the backbone carboxylic acid group and the carbonyl oxygen atom of the Asn side chain may also be present. The Ba(2+)(Asn) spectrum is characteristic of a single charge-solvated [N,CO,CO] conformation, in contrast to Gly, Trp, Arg, Gln, Pro, Ser, Val, and Glu, which all take on a zwitterionic structure when complexed to Ba(2+). In no case do the cationized Asn complexes show definitive evidence of forming a zwitterionic structure in the complexes studied here. For H(+)(Asn), a mixture of two [N,CO] structures, which differ only in the orientation the side chain and are calculated to be nearly identical in energy, explains the experimental spectrum well.

Temperature dependence of reactions involving electron transfer in K(np)/C2Cl4 collisions.

Electron transfer in K(np)-C(2)Cl(4) collisions, which leads to formation of both Cl(-) and C(2)Cl(4)(-) anions, is investigated as a function of target temperature over the range of 300-650 K. Measurements at high n (n approximately 30) show that the likelihood of Cl(-) production increases rapidly with temperature indicating the presence of a dissociation barrier. The data yield an activation energy of approximately 0.1 eV. A broad distribution of product C(2)Cl(4)(-) lifetimes is observed that extends from microseconds to milliseconds, this distribution moving toward shorter lifetimes as the target temperature is increased. The measured lifetimes are consistent with the predictions of quasiequilibrium theory. Studies at low n (n approximately 14) show a substantial fraction of the product K(+)-Cl(-) and K(+)-C(2)Cl(4)(-) ion pairs is electrostatically bound leading to creation of heavy-Rydberg ion-pair states. Variations in target temperature lead to changes in kinetic energy of relative motion of the reactants that can result in marked changes in the fraction of ion pairs that is bound, especially at low Rydberg atom velocities. In the case of bound K(+)-C(2)Cl(4)(-) ion pairs a few percent subsequently dissociate by the conversion of internal energy in the anion into translational energy of the ion pair. Analysis of the data points to a mean energy conversion of approximately 60-90 meV, much less than the available excess energy of reaction, approximately 0.7 eV.

Impact of Sodium Cationization on Gas-Phase Conformations of DNA and RNA Cytidine Mononucleotides.

Gas-phase conformations of the sodium-cationized forms of the 2'-deoxycytidine and cytidine mononucleotides, [pdCyd+Na]+ and [pCyd+Na]+, are examined by infrared multiple photon dissociation action spectroscopy. Complimentary electronic structure calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) level of theory provide candidate conformations and their respective predicted IR spectra for comparison across the IR fingerprint and hydrogen-stretching regions. Comparisons of the predicted IR spectra and the measured infrared multiple photon dissociation action spectra provide insight into the impact of sodium cationization on intrinsic mononucleotide structure. Further, comparison of present results with those reported for the sodium-cationized cytidine nucleoside analogues elucidates the impact of the phosphate moiety on gas-phase structure. Across the neutral, protonated, and sodium-cationized cytidine mononucleotides, a preference for stabilization of the phosphate moiety and nucleobase orientation is observed, although the details of this stabilization differ with the state of cationization. Several low-energy conformations of [pdCyd+Na]+ and [pCyd+Na]+ involving several different orientations of the phosphate moiety and sugar puckering modes are observed experimentally.

Infrared multiphoton dissociation spectroscopy of cationized serine: effects of alkali-metal cation size on gas-phase conformation.

The gas-phase structures of alkali-metal cation complexes of serine (Ser) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. Spectra of Li+(Ser) and Na+(Ser) are similar and relatively simple, whereas Cs+(Ser) includes distinctive new IR bands, and K+(Ser) and Rb+(Ser) exhibit intermediate behavior. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. On the basis of these experiments and calculations, the only conformations accessed for the complexes to the smaller alkali-metal cations, Li+ and Na+, are charge-solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain, M1[N,CO,OH]. For the cesiated complex, a band corresponding to a zwitterionic structure, ZW[CO2-], is clearly visible. K+(Ser) and Rb+(Ser) exhibit evidence of the charge-solvated analogue of the zwitterions, M3[COOH], in which the metal cation binds to the carboxylic acid group. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally.

Infrared multiphoton dissociation spectroscopy of cationized threonine: effects of alkali-metal cation size on gas-phase conformation.

The gas-phase structures of alkali-metal cation complexes of threonine (Thr) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser in conjunction with quantum chemical calculations. Spectra of Li+(Thr) and Na+(Thr) are similar and relatively simple, whereas K+(Thr), Rb+(Thr), and Cs+(Thr) include distinctive new IR bands. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. For the smaller metal cations, the spectra match those predicted for charge-solvated structures in which the ligand exhibits tridentate coordination, M1[N,CO,OH], binding to the amide and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain. K+(Thr), Rb+(Thr), and Cs+(Thr) exhibit evidence of the charge-solvated complex, M3[COOH], in which the metal cation binds to the carboxylic acid group. Evidence for a small population of the zwitterionic analogue of this structure, ZW[CO2-], is also present, particularly for the Cs+ complex. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. The present results are similar to those obtained previously for the analogous M+(Ser) complexes, although there are subtle distinctions that are discussed.

Negative ions of nitroethane and its clusters.

Valence and dipole-bound negative ions of the nitroethane (NE) molecule and its clusters are studied using photoelectron spectroscopy (PES), Rydberg electron transfer (RET) techniques, and ab initio methods. Valence adiabatic electron affinities (EA(a)s) of NE, C(2)H(5)NO(2), and its clusters, (C(2)H(5)NO(2))(n), n=2-5, are estimated using vibrationally unresolved PES to be 0.3+/-0.2 eV (n=1), 0.9+/-0.2 eV (n=2), 1.5+/-0.2 eV (n=3), 1.9+/-0.2 eV (n=4), and 2.1+/-0.2 eV (n=5). These energies were then used to determine stepwise anion-neutral solvation energies and compared with previous literature values. Vertical detachment energies for (C(2)H(5)NO(2))(n)(-) were also measured to be 0.92+/-0.10 eV (n=1), 1.63+/-0.10 eV (n=2), 2.04+/-0.10 eV (n=3), and 2.3+/-0.1 eV (n=4). RET experiments show that Rydberg electrons can be attached to NE both as dipole-bound and valence bound anion states. The results are similar to those found for nitromethane (NM), where it was argued that the diffuse dipole state act as a "doorway state" to the more tightly bound valence anion. Using previous models for relating the maximum in the RET dependence of the Rydberg effective principle number n(max)(*), the dipole-bound electron affinity is predicted to be approximately 25 meV. However, a close examination of the RET cross section data for NE and a re-examination of such data for NM finds a much broader dependence on n(*) than is seen for RET in conventional dipole bound states and, more importantly, a pronounced [l] dependence is found in n(max)(*) (n(max)(*) increases with [l]). Ab initio calculations agree well with the experimental results apart from the vertical electron affinity value associated with the dipole bound state which is predicted to be 8 meV. Moreover, the calculations help to visualize the dramatic difference in the distributions of the excess electron for dipole-bound and valence states, and suggest that NE clusters form only anions where the excess electron localizes on a single monomer.

Influence of Transition Metal Cationization versus Sodium Cationization and Protonation on the Gas-Phase Tautomeric Conformations and Stability of Uracil: Application to [Ura+Cu]+ and [Ura+Ag].

The gas-phase conformations of transition metal cation-uracil complexes, [Ura+Cu]+ and [Ura+Ag]+, were examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical calculations. IRMPD action spectra were measured over the IR fingerprint and hydrogen-stretching regions. Structures and linear IR spectra of the stable tautomeric conformations of these complexes were initially determined at the B3LYP/6-31G(d) level. The four most stable structures computed were also examined at the B3LYP/def2-TZVPPD level to improve the accuracy of the predicted IR spectra. Two very favorable modes of binding are found for [Ura+Cu]+ and [Ura+Ag]+ that involve O2N3 bidentate binding to the 2-keto-4-hydroxy minor tautomer and O4 monodentate binding to the canonical 2,4-diketo tautomer of Ura. Comparisons between the measured IRMPD and calculated IR spectra enable elucidation of the conformers present in the experiments. These comparisons indicate that both favorable binding modes are represented in the experimental tautomeric conformations of [Ura+Cu]+ and [Ura+Ag]+. B3LYP suggests that Cu+ exhibits a slight preference for O4 binding, whereas Ag+ exhibits a slight preference for O2N3 binding. In contrast, MP2 suggests that both Cu+ and Ag+ exhibit a more significant preference for O2N3 binding. The relative band intensities suggest that O4 binding conformers comprise a larger portion of the population for [Ura+Ag]+ than [Ura+Cu]+. The dissociation behavior and relative stabilities of the [Ura+M]+ complexes, M+ = Cu+, Ag+, H+, and Na+) are examined via energy-resolved collision-induced dissociation experiments. The IRMPD spectra, dissociation behaviors, and binding preferences of Cu+ and Ag+ are compared with previous and present results for those of H+ and Na+. Graphical Abstract ᅟ.

Temperature dependence of negative ion lifetimes.

The autodetachment lifetimes of SF6-* and C6F6-* ions formed by charge transfer in K(np)/SF6, C6F6 collisions are measured as a function of target temperature over the range of approximately 300-600 K with the aid of time-of-flight techniques and a Penning ion trap. At room temperature only formation of long-lived SF6 -* ions with lifetimes tau >or similar to 1 ms is seen. As the temperature is increased the lifetime of these long-lived ions is reduced, some having lifetimes as short as approximately 0.4 ms. The appearance of a short-lived, tau