Gas-phase Experiments Research Articles

Ionization of Anthracene Followed by Fusion in the Solid Phase under Intense Nonresonant Femtosecond Laser Fields

Fused anthracene cation and a series of anthracene-C n H m cluster-like products were observed on irradiatior with nonresonant femtosecond pulses at 1.4 μm. Ionization of polycrystalline anthracene revealed strikingly different features such as the absence of multiply charged molecular ions, the appearance of H + and C + a low intensity without the formation of C n H m fragment ions, C n H m -attached and detached molecular ions, and multimer cation formation, compared with the gas-phase experiments. The fusion reactions of anthraceno and C n H m fragments indicated the presence of novel reactions among high-density cations in the crystal produced by femtosecond laser irradiation.

Cyclododecasulfur as a Ligand: From Gas‐Phase Experiments to the Crystal Structures of [Cu(S12)(S8)]+ and [Cu(S12)(CH2Cl2)]+

Sag einfach S: Während massenspektrometrische Experimente für das Vorliegen von CuI-Komplexen mit Cycloschwefelliganden in der Gasphase sprachen, wurden solche Komplexes bisher in kondensierter Phase nicht nachgewiesen. Mithilfe von Quellen für „nacktes“ CuI mit schwach koordinierenden Anionen gelang nun die Synthese der ersten Cyclododecaschwefelkomplexe eines Metalls.

Cyclododecasulfur as a Ligand: From Gas‐Phase Experiments to the Crystal Structures of [Cu(S12)(S8)]+ and [Cu(S12)(CH2Cl2)]+

Cyclododecasulfur as a Ligand: From Gas‐Phase Experiments to the Crystal Structures of [Cu(S₁₂)(S₈)]⁺ and [Cu(S₁₂)(CH₂Cl₂)]⁺

Multiple Anharmonic Vibrational Probes of Sugar Structure and Dynamics

Quantum mechanical computations and molecular dynamics simulations are carried out for the simplest sugar glycolaldehyde to gain insight into the underlying force fields that determine the vibrational spectroscopic parameters relevant to structure and dynamics. The harmonic and anharmonic vibrational frequencies of the 3N--6 modes and their diagonal and off-diagonal anharmonicities are evaluated using the hybrid B3LYP functional in comparison with other high-level theories. Very good performance of B3LYP/6-31+G** is found in predicting the anharmonic frequencies by statistical analysis and by comparison to gas-phase experiments. Full cubic and semidiagonal quartic anharmonic force constants, the origin of the anharmonicities, the isotope dependence of the anharmonicities, and the polarizable continuum solvent effect on the anharmonicities are examined, in particular, for the CO, C-H, and O-H stretching modes. Site-dependent dynamical interactions between glycolaldehyde and water molecules in the hydration shells are examined by molecular dynamics simulations employing a set of molecular mechanical force fields developed on the basis of quantum mechanical computations. The statistical distributions and correlations of the fundamental transition frequencies and transition dipoles are obtained through instantaneous normal-mode analysis. The simultaneous assessment of multiple parameters of multiple vibrational probes shall prove useful in understanding the characteristics of sugar structure and dynamics expressed in two-dimensional infrared correlation spectra.

Electron attachment and electron ionization of acetic acid clusters embedded in helium nanodroplets

The effect of incident electrons on acetic acid clusters is explored for the first time. The acetic acid clusters are formed inside liquid helium nanodroplets and both cationic and anionic products ejected into the gas phase are detected by mass spectrometry. The cation chemistry (induced by electron ionization at 100 eV) is dominated by production of protonated acetic acid (Ac) clusters, Ac(n)H(+), although some fragmentation is also observed. In the case of anion production (at 2.8 eV electron energy) there is a clear distinction between the monomer and the clusters. For the monomer the dominant product is the dehydrogenated species, [Ac-H](-), whereas for the clusters both the parent anion, Ac(n)(-), and the dehydrogenated species, [Ac(n)-H](-), have similar abundances. A particularly intriguing contrast between the monomer and cluster anions is that helium atoms are seen attached to the latter whereas no evidence of helium atom attachment is found for the monomer. This surprising observation is attributed to the formation of acyclic (head-to-tail) acetic acid clusters in helium nanodroplets, which have more favourable electronic properties for binding helium atoms. The acyclic clusters represent a local minimum on the potential energy surface and in the case of the dimer this is distinct from the cyclic isomer (the global minimum) identified in gas phase experiments.

Photodissociation pathways of gas-phase photoactive yellow protein chromophores

The absorption dynamics of two model chromophores of the photoactive yellow protein were studied in gas-phase experiments. Using different time-resolving techniques with an overall sensitivity ranging from seconds down to a few nanoseconds, complex dynamics were revealed for the p -coumaric acid anion, involving both fragmentation and electron detachment as possible photoresponse channels. For the trans-thiophenyl-p-coumarate model, despite its more complex molecular structure, simpler decay dynamics showing only fragmentation were observed.

Photon-matter interaction at short wavelengths and ultra-high intensity – Gas-phase experiments at FLASH

In different focused beams at the new soft X-ray Free-electron LASer in Hamburg FLASH, ion time-of-flight spectroscopy on gas targets was performed. Irradiation levels above 1013 up to 1016 W cm-2 were achieved in the vacuum and extreme ultra-violet. The first group of experiments was performed on nitrogen molecules and neon and helium atoms at wavelengths around 30 nm, i.e. at photon energies around 40 eV. Absolute cross sections for one- and two-photon ionization could be derived. The second group of experiments was performed on different rare gases at the wavelength of 13.3 nm, i.e. at the photon energy of 93 eV. As an example, the generation of Xe21+ was observed which requires a total energy of at least 5 keV absorbed per atom, starting from neutral Xe, within the FLASH pulse duration of about 10 fs. Here, the situation might be beyond the multiphoton scheme and perturbation theory.

Diffraction in low-energy electron scattering from DNA: Bridging gas-phase and solid-state theory

Using high-quality gas phase electron scattering calculations and multiple scattering theory, we attempt to gain insights on the radiation damage to DNA induced by secondary low-energy electrons in the condensed phase, and to bridge the existing gap with the gas phase theory and experiments. The origin of different resonant features (arising from single molecules or diffraction) is discussed and the calculations are compared to existing experiments in thin films.

Intramolecular vibrational frequencies of water clusters (H2O)n (n=2–5): Anharmonic analyses using potential functions based on the scaled hypersphere search method

Potential energy functions (PEFs) in the form of the sixth-order polynomial concerned with intramolecular motions of (H(2)O)(n) (n=2-5) were constructed by the scaled hypersphere search (SHS) based polynomial fitting (SHS-PF) method, in which the numbers of ab initio calculations can be considerably reduced via the selection of sampling directions by the SHS method. Concerning fundamental frequencies of the dimer cluster, the root mean square error from gas-phase experiments was only 10 cm(-1). For the trimer, tetramer, and pentamer, computed anharmonic frequencies reproduced available gas-phase experimental frequencies of HOH bending, donor-OH stretching, and free-OH stretching excellently with errors of approximately 10-20cm(-1) on average. It follows that the SHS-PF method is promising for highly accurate anharmonic vibrational analyses of intramolecular modes of H-bond clusters.

Investigation of Monovalent and Bivalent Enantioselective Molecular Recognition by Electrospray Ionization-Mass Spectrometry and Tandem Mass Spectrometry

In this work is described the investigation of bivalent versus monovalent enantioselective molecular recognition in the context of enantioselective separations. Electrospray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) are used for evaluating enantioselective systems through the measurement of (1) relative solution-phase binding constants via titration and (2) relative gas-phase binding via collision threshold dissociation. In HPLC, a cinchonane-type chiral stationary phase (CSP) based on tert.-butylcarbamoylquinine provides vastly increased retention and enantioselectivity for separation of bivalent versus monovalent alkoxy-benzoyl-N-blocked leucine enantiomers. The bivalent enantiomers are able to span and simultaneously interact with multiple interaction sites on the CSP surface, leading to enhanced separation. ESI-MS titration measurements also show an increased avidity for binding between bivalent selector and bivalent selectand, compared with the monovalent system. However, enhanced enantioselectivities measured in HPLC for the bivalent system cannot be reproduced by MS due to inherent mechanistic differences. Assumed discrepancies in relative response factors also give rise to systematic errors which are discussed. The results of MS/MS gas-phase experiments show that enantioselectivity is essentially lost in the absence of solvation, but that dissociation thresholds can provide a measure of relative dissociation energy in the bivalent interaction system compared to the monovalent counterpart. Such measurements may prove useful and efficient in better understanding multivalent interactions, in line with current theoretical considerations of effective concentrations and ion trap effects. This is the first application of mass spectrometric methods for assessing increased avidity of binding in multivalent enantioselective molecular recognition.

From isolated molecules through clusters and condensates to the building blocks of life

Electron attachment processes in isolated molecules in the gas phase, in clusters and in molecules condensed on surfaces are reviewed, and a special section is dedicated to electron attachment to biologically relevant molecules. For isolated molecules in the gas phase the emphasis is on the comparison between direct dissociation from strongly repulsive σ* states and the more indirect predissociation processes associated with the initial occupation of a π* MO. These processes are discussed with regard to the temperature dependence of the dissociative electron attachment processes and with regard to its high bond selectivity. Specific examples of high bond and site selectivity at particular electron incident energies are discussed in terms of thermo-chemically controlled and state controlled selectivity of the dissociative electron attachment process. Different processes that are specific for electron attachment to molecules in clusters or condensed on surfaces are discussed for prototypical cases. These include suppression and enhancement of DEA channels, negative ion formation through “electron/exciton-complexes”, scavenging processes, and low energy electron induced synthesis in thin molecular films and in clusters. Finally we discuss gas phase electron attachment to the DNA and RNA building blocks, i.e., the nucleobases, the sugar unit and the phosphate group. The relevance of these gas phase experiments to actual DNA damage such as single strand breaks is discussed and first gas phase electron attachment experiments on larger DNA units are discussed.

Gas-Phase Synthesis and Reactivity of a Gold Carbene Complex

Electrospray ionization tandem mass spectrometry of a phosphonium ylid complex of gold produces an ion whose mass and gas-phase chemical reactivity indicate that it is a gold benzylidene complex. The complex, with a supporting NHC ligand, corresponds to a type of reactive intermediates which have been presumed to act in gold-catalyzed cyclopropanation reactions, but which have not been observed to date in solution or gas-phase experiments. A threshold CID experiment also yields thermochemical information on the formation of the gold carbene from the ylid complex precursor.

Polymerization in the Gas Phase, in Clusters, and on Nanoparticle Surfaces

Gas phase and cluster experiments provide unique opportunities to quantitatively study the effects of initiators, solvents, chain transfer agents, and inhibitors on the mechanisms of polymerization. Furthermore, a number of important phenomena, unique structures, and novel properties may exist during gas-phase and cluster polymerization. In this regime, the structure of the growing polymer may change dramatically and the rate coefficient may vary significantly upon the addition of a single molecule of the monomer. These changes would be reflected in the properties of the oligomers deposited from the gas phase. At low pressures, cationic and radical cationic polymerizations may proceed in the gas phase through elimination reactions. In the same systems at high pressure, however, the ionic intermediates may be stabilized, and addition without elimination may occur. In isolated van der Waals clusters of monomer molecules, sequential polymerization with several condensation steps can occur on a time scale of a few microseconds following the ionization of the gas-phase cluster. The cluster reactions, which bridge gas-phase and condensed-phase chemistry, allow examination of the effects of controlled states of aggregation. This Account describes several examples of gas-phase and cluster polymerization studies where the most significant results can be summarized as follows: (1) The carbocation polymerization of isobutene shows slower rates with increasing polymerization steps resulting from entropy barriers, which could explain the need for low temperatures for the efficient propagation of high molecular weight polymers. (2) Radical cation polymerization of propene can be initiated by partial charge transfer from an ionized aromatic molecule such as benzene coupled with covalent condensation of the associated propene molecules. This novel mechanism leads exclusively to the formation of propene oligomer ions and avoids other competitive products. (3) Structural information on the oligomers formed by gas-phase polymerization can be obtained using the mass-selected ion mobility technique where the measured collision cross-sections of the selected oligomer ions and collision-induced dissociation can provide fairly accurate structural identifications. The identification of the structures of the dimers and trimers formed in the gas-phase thermal polymerization of styrene confirms that the polymerization proceeds according to the Mayo mechanism. Similarly, the ion mobility technique has been utilized to confirm the formation of benzene cations by intracluster polymerization following the ionization of acetylene clusters. Finally, it has been shown that polymerization of styrene vapor on the surface of activated nanoparticles can lead to the incorporation of a variety of metal and metal oxide nanoparticles within polystyrene films. The ability to probe the reactivity and structure of the small growing oligomers in the gas phase can provide fundamental insight into mechanisms of polymerization that are difficult to obtain from condensed-phase studies. These experiments are also important for understanding the growth mechanisms of complex organics in flames, combustion processes, interstellar clouds, and solar nebula where gas-phase reactions, cluster polymerization, and surface catalysis on dust nanoparticles represent the major synthetic pathways. This research can lead to the discovery of novel initiation mechanisms and reaction pathways with applications in the synthesis of oligomers and nanocomposites with unique and improved properties.

Explosive Desorption and Fragmentation of Molecular Ion from Solid Fullerene by Intense Nonresonant Femtosecond Laser Pulses

Desorption of C 60 (+) and its dimer cation was investigated on irradiation with nonresonant femtosecond laser pulses at 1.4 mum. Ionization of solid C 60 revealed strikingly different features, such as the absence of multiply charged molecular ions, the emission of C (+) at low laser intensity, C 2 attachments, delayed ionization, and dimer cation formation, as compared with the gas phase experiments. The large kinetic energy distribution of ions found in this study indicated that the desorption process was mainly driven by an electrostatic mechanism rather than by thermal, photochemical, or volume expansion mechanisms. Singly charged C 60 emission by a Coulomb explosion due to the high density of C 60 (+) is suggested.

Photoemission and coincidence studies on gas-phase molecules

This article describes Young’s double-slit experiment using high-energy core-level photoemission from N2 molecules and experimental identification of interatomic Coulombic decay in Ar2 dimers after Auger decay using k-resolved electron–ion–ion coincidence spectroscopy, aiming to illustrate the leading edge of gas-phase experiments using synchrotron radiation.

Bimetallic Fe−Ni Cluster Alloys: Stability of Core(Fe)−Shell(Ni) Arrays and Their Role Played in the Structure and Magnetic Behavior

We present density functional theory as well as tight-binding calculations to analyze the stability and magnetic properties of small fcc and polyicosahedral Fe−Ni cluster alloys having different compositions and chemical orderings. In agreement with the gas-phase experiments of Parks et al. (Chem. Phys. 2000, 262, 151), we obtain in all our considered alloys that iron atoms prefer to accumulate in the core region of the structures leading to the formation of highly stable core(Fe)−shell(Ni) arrays. We found that the low energy atomic configurations of our fcc Fe−Ni clusters strongly depend on the chemical order within the structures. Interestingly, fcc particles with Fe-rich core regions are characterized by unusual and considerably expanded Fe−Fe bond lengths (∼2.7 Å), a result that induces a highly nonuniform relaxation profile in which both sizable expansions and contractions of the interatomic distance between neighboring atoms can be obtained. In contrast, disordered alloys and core(Ni)−shell(Fe) arrays are characterized by Fe−Ni bond lengths of ∼2.5 Å, being of the order of the arithmetic weighted average of the Fe−Fe and Ni−Ni pairs. A common feature of our results is that all our considered Fe−Ni clusters exhibit high-spin ground states (larger than the corresponding bulk alloy values for the same composition); however, we have found that iron clustering within the particles, the presence of chemisorbed hydrogens on the surface, as well as the existence of antiferromagnetic order in the iron-rich regions could lead to a sizable quenching of the average magnetization in the nanoalloys, which is in good agreement with the experimental data of Li et al. (J. Magn. Magn. Mater. 1997, 170, 339). Finally, we show that the value of the orbital-to-spin ratio in both fcc and polyicosahedral Fe−Ni clusters is very sensitive to the internal location of the Fe impurities, a result that suggests that X-ray magnetic circular dichroism experiments can be very useful to reveal precise features of the chemical order in magnetic cluster alloys.

Solvation of Copper(II) Sulfate in Binary Water/N,N-Dimethylformamide Mixtures: From the Solution to the Gas Phase

The solvation of copper(II) sulfate in binary mixtures of water and N,N-dimethylformamide (DMF) is studied by a combined approach using electrochemical studies in solution and a mass spectrometric assay of the solvated ions formed from these solutions upon electrospray ionization (ESI). In the condensed phase, the limiting transference numbers (t(+/-)(o)) and the apparent ion association constants (K(A)'s) of CuSO(4) have been determined in water/DMF solutions at 20 degrees C. The t(+)(o) values decrease with increasing DMF content, demonstrating a gradual solvation of Cu(2+) by DMF molecules. The association constants indicate that aggregation becomes more pronounced as the DMF content increases. In order to achieve complementary insight, the intrinsic interactions among the ions and solvent molecules are investigated in gas-phase experiments of the CuSO(4)/water/DMF system using ESI mass spectrometry. Under the conditions used, the dications [Cu(DMF)(n)](2+) (n = 3-6), [Cu(2)(DMF)(n)SO(4)](2+) (n = 2-7), and [Cu(3)(DMF)(n)(SO(4))(2)](2+) (n = 2-7), and the monocations [Cu(OH)(DMF)(n)](+), [Cu(DMF)(n)(HSO(4))](+) (both, n = 1-3), and [Cu(DMF)(n)](+) (n = 1, 2), are formed as the leading copper-containing cations. Likewise, polynuclear copper clusters observed in the anion ESI spectra support partial aggregation occurring in solution. The gas-phase studies clearly support the conclusions that (i) DMF is a highly preferred ligand for CuII in comparison to water and that (ii) DMF supports ion association for which the mass spectrometric data suggest the formation of polynuclear copper clusters.

Nature and magnitude of aromatic stacking of nucleic acid bases

This review summarises recent advances in quantum chemical calculations of base-stacking forces in nucleic acids. We explain in detail the very complex relationship between the gas-phase base-stacking energies, as revealed by quantum chemical (QM) calculations, and the highly variable roles of these interactions in nucleic acids. This issue is rarely discussed in quantum chemical and physical chemistry literature. We further extensively discuss methods that are available for base-stacking studies, complexity of comparison of stacking calculations with gas phase experiments, balance of forces in stacked complexes of nucleic acid bases, and the relation between QM and force field descriptions. We also review all recent calculations on base-stacking systems, including details analysis of the B-DNA stacking. Specific attention is paid to the highest accuracy QM calculations, to the decomposition of the interactions, and development of dispersion-balanced DFT methods. Future prospects of computational studies of base stacking are discussed.

Highly selective CO methanation catalysts for the purification of hydrogen-rich gas mixtures

High-throughput techniques based on emissivity-corrected infrared thermography (ecIRT) were used in the discovery and optimization of new catalysts for the purification of hydrogen-rich gas reformates by means of CO methanation. The application of an appropriate sequence of test gases in the high-throughput experiment (HTE) enabled us to screen for activity as well as selectivity. From literature and prescreening of about 2000 samples of highly diverse mixed oxides, we selected suitable elements for more detailed studies in the first generation. Among these 1000 samples, various doped Ni oxides proved most promising and were selected as leads for the subsequent evolutionary optimization based on selection and variation. Rapid optimization was achieved during three catalyst generations. The improvement in the catalysts in terms of CO activity and selectivity was confirmed at various stages during the developing process by conventional gas-phase experiments. Compared with a typical industrial methanation catalyst (Ru/TiO2), the Re2Y9Ni89Ox and Ti8Hf9Ni83Ox investigated here exhibit unique catalytic performances with respect to activity and selectivity, reducing or eliminating the problems of hydrogen loss by CO2 methanation. Strong evidence for good stability of the best catalysts was obtained by long-term HT screening for about 100 h. Characterization of selected materials is provided.

High-Valent Technetium Fluorides. Does TcF7 Exist?

The structures and stabilities of technetium fluorides in high oxidation states have been studied quantum-chemically at density functional theory (B3LYP) and coupled-cluster (CCSD(T)) levels. The calculations indicate clearly that technetium heptafluoride, TcF7, has a good chance of existence and preparation, thus providing the first heptafluoride in the 4d series. The [TcF6]+ cation, a potential precursor, is also computed to be thermochemically stable against gas-phase elimination reactions. The problem with such highly fluorinated complexes appears thus to be mainly in difficult synthetic access under typical condensed-phase conditions. Matrix-isolation techniques or gas-phase experiments appear to be better suited as starting points.