Appearance Potential Measurements Research Articles

Positive cooperativity in binding: Spectroscopic characterization and dissociation energy of the anisole…(CH4)2 complex

We have previously reported on the spectroscopy and binding energy of the anisole…methane complex, which exhibits a dual mode of binding that involves both CH/O and CH/π interactions. In this work, we seek to examine cooperativity in binding through a study of the isolated anisole-(methane)2 complex, using a combination of experiments that include mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI) augmented with a complementary theoretical characterization. Using 2CAP and VMI, we derive the dissociation energies of the complex in ground (S0), excited (S1), and cation radical (D0) states. The experimental values from the two methods are in excellent agreement and are compared with selected theoretical values calculated using DFT and ab initio methods. The data show that the dissociation energy increases by some 10 % for the second methane relative to the first, with this trend being consistent across all three electronic states, indicating a cooperative binding effect where the initial solvation turns on binding of a second methane onto the opposite face. This lies in contrast to the aniline-(methane)2 complex, where recent studies have shown a negativity cooperativity, and these trends are examined.

Probing cooperativity in C-H⋯N and C-H⋯π interactions: Dissociation energies of aniline⋯(CH4)n (n = 1, 2) van der Waals complexes from resonant ionization and velocity mapped ion imaging measurements.

Recent studies of the weakly bound anisole⋯CH4 complex found a dual mode of binding, featuring both C/H⋯π and C/H⋯O noncovalent interactions. In this work, we examine the dissociation energies of related aniline⋯(CH4)n (n = 1, 2) van der Waals clusters, where both C/H⋯π and C/H⋯N interactions are possible. Using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity-mapped ion imaging (VMI), we derive the dissociation energies of both complexes in the ground (S0), excited (S1), and cation radical (D0) states. As the amide group is non-planar in the ground state, the optimized ground state geometry of the aniline⋯CH4 1:1 complex shows two isomers, each with the methane positioned above the aniline ring. The observed redshift of the electronic origin from the aniline monomer is consistent with TDDFT calculations for the more stable isomer, where the methane sits on the same face as the amino hydrogens. The dissociation energies of the 1:1 complex, obtained from 2CAP measurements, are in good agreement with the calculated theoretical values from selected density functional theory methods. VMI data for the 1:1 complex gave a binding energy value overestimated by ∼179 cm-1 when compared to the 2CAP results, indicating that dissociative ionization selectively populates an excited vibrational level of the aniline cation radical. Given that the electron donating ability of aromatic substituents trends as -NH2 > -OCH3 > -CH3, it is noteworthy that the strength of methane binding also trends in this order, as found by experiment (dissociation energies in kJ/mol: 6.6 > 5.8 > 4.5) and predicted by theory (PBE0-D3/def2-QZVPPD, in kJ/mol: 6.9 > 6.0 > 5.0). For the 1:2 complex of aniline and methane, calculations predict that the more stable conformer is the one where the two methane molecules lie on opposite faces of the ring, consistent with the observed redshift of the electronic origin. Unlike the anisole-methane 1:2 complex, which shows an enhanced dissociation energy for the loss of one methane in comparison with the 1:1 complex, here, we find that the energy required to remove one methane from the ground state aniline-methane 1:2 complex is smaller than that of the 1:1 complex, consistent with theoretical expectations.

C-H/π and C-H-O Interactions in Concert: A Study of the Anisole-Methane Complex using Resonant Ionization and Velocity Mapped Ion Imaging.

Noncovalent forces such as hydrogen bonding, halogen bonding, π-π stacking, and C-H/π and C-H/O interactions hold the key to such chemical processes as protein folding, molecular self-assembly, and drug-substrate interactions. Invaluable insight into the nature and strength of these forces continues to come from the study of isolated molecular clusters. In this work, we report on a study of the isolated anisole-methane complex, where both C-H/π and C-H/O interactions are possible, using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI). Using 2CAP and VMI, we derive the binding energies of the complex in ground, excited, and cation radical states. The experimental values from the two methods are in excellent agreement, and they are compared with selected theoretical values calculated using density functional theory and ab initio methods. The optimized ground-state cluster geometry, which is consistent with the experimental observations, shows methane sitting above the ring, interacting with anisole via both C-H/π and C-H/O interactions, and this dual mode of interaction is reflected in a larger ground-state binding energy as compared with the prototypical benzene-methane system.

- stacking vs. C-H/ interaction: Excimer formation and charge resonance stabilization in van der Waals clusters of 9,9'-dimethylfluorene.

Studies of exciton and hole stabilization in multichromophoric systems underpin our understanding of electron transfer and transport in materials and biomolecules. The simplest model systems are dimeric, and recently we compared the gas-phase spectroscopy and dynamics of van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9'-dimethylfluorene (F1) to assess how sterically controlled facial encumbrance modulates the dynamics of excimer formation and charge resonance stabilization (CRS). Dimers of fluorene and MF show only excimer emission upon electronic excitation, and significant CRS as evidenced in a reduced ionization potential for the dimer relative the monomer. By contrast, the dimer of F1 shows no excimeric emission, rather structured emission from the locally excited state of a tilted (non π-stacked) dimer, evidencing the importance of C-H/π interactions and increased steric constraints that restrict a cofacial approach. In this work, we report our full results on van der Waals clusters of F1, using a combination of theory and experiments that include laser-induced fluorescence, mass-selected two-color resonant two-photon ionization spectroscopy, and two-color appearance potential measurements. We use the latter to derive the binding energies of the F1 dimer in ground, excited, and cation radical states. Our results are compared with van der Waals and covalently linked clusters of fluorene to assess both the relative strength of π-stacking and C-H/π interactions in polyaromatic assemblies and the role of π-stacking in excimer formation and CRS.

Strength of π-Stacking, from Neutral to Cation: Precision Measurement of Binding Energies in an Isolated π-Stacked Dimer.

π-Stacking interactions are ubiquitious across chemistry and biochemistry, impacting areas from organic materials and photovoltaics to biochemistry and DNA. However, experimental data is lacking regarding the strength of π-stacking forces-an issue not settled even for the simplest model system, the isolated benzene dimer. Here, we use two-color appearance potential measurements to determine the binding energies of the isolated, π-stacked dimer of fluorene (C13H10) in ground, excited, and ionic states. Our measurements provide the first precise values for π-stacking interaction energies in these states, which are key benchmarks for theory. Indeed, theoretical predictions using ab initio and carefully benchmarked DFT methods are in excellent agreement with experiment.

Formation and Stability of Gaseous Ternary Oxides of Group 14–16 Elements and Related Oxides of Group 15 Elements: Mass Spectrometric and Quantum Chemical Study

AbstractThe formation of four hitherto unknown lead tellurium oxides – PbTeO3, PbTe2O5, Pb2TeO4 and Pb2Te2O6 – was observed in the gas phase by means of a mass‐spectrometric Knudsen‐cell method. Other ternary oxides of group 14–16 elements were not observed in the gas phase. The enthalpies of formation for these oxides were obtained experimentally by means of a mass‐spectrometric Knudsen‐cell method and confirmed theoretically by using quantum chemical (QC) calculations. Appearance potential measurements were conducted for PbTeO3 and Pb2TeO4. The structures of the ternary oxides that represent the minima on the potential‐energy surfaces were found by using QC calculations. The behaviour of the related compounds PbTeO3, X2O3, Pb2Te2O6 and X4O6 in the gas phase has been discussed (for which X = P, As, Sb, Bi).

Plasma polymerization chemistry of unsaturated hydrocarbons: neutral species identification by mass spectrometry

Radio frequency discharges ignited in low-pressure and pure hydrocarbon gases were investigated by mass spectrometry. The plasma process was applied to four unsaturated monomers: styrene C8H8, benzene C6H6, ethylene C2H4 and acetylene C2H2. The remote mass spectrometer location restricted species identification to neutral closed-shell molecules in their respective plasmas. Among the peaks in the mass spectra, those directly due to neutrals produced in the plasma were determined following a successful two-step methodology. Firstly, the use of low electron impact energy limited the fragmentation and strongly simplified the cracking patterns. Secondly, attribution of peaks directly due to neutrals was confirmed or ruled out by systematically measuring their appearance potential. In the case of styrene, not less than 48 new molecules were detected.The discussion of the observed stable by-products in each discharge suggested several radicals responsible for their production. Comparing the set of species among the four plasmas showed that the repeated addition of intermediates with one or two carbon atoms and with low H content dominated the chemistry. Under our conditions of intermediate to high W/FM (power over mass flow ratio), the gas-phase plasma polymerization then preferentially occurred through significant fragmentation and recombination. Finally, the measured appearance potentials during plasma provided estimation for the threshold ionization energy of several highly unsaturated hydrocarbons, useful for modeling.

Ultrafast ionization and fragmentation of molecular silane

The ionization and fragmentation of molecular silane is examined here with laser intensities ranging between $7\ifmmode\times\else\texttimes\fi{}{10}^{12}$ and $1\ifmmode\times\else\texttimes\fi{}{10}^{15}$ W/cm${}^{2}$ at $624$ nm. The ionization potential of silane determined using both multiphoton ionization (MPI) and tunneling ionization (TI) models agrees with the vertical ionization potential of the molecule. In addition, the application of the tunneling ionization model is extended here to the fragments of silane to determine their appearance potentials. MPI values for SiH${}_{3}{}^{+}$, SiH${}_{2}{}^{+}$, SiH${}^{+}$, Si${}^{+}$, as well as ${\mathrm{H}}_{2}{}^{+}$ and ${\mathrm{H}}^{+}$ are consistent with vertical potentials, whereas the TI measurements are found to be in accord with adiabatic potentials. The tunneling appearance potentials observed for the fragments ${\mathrm{H}}_{2}{}^{+}$ and ${\mathrm{H}}^{+}$ are lower than reported for other techniques. In fact, the appearance potential measurements for these species resulting from silane are lower than their ionization potentials. The fragmentation rate of silane is determined to be nearly $20$ times larger than the ionization rate. The main precursor for producing amorphous silicon ($a$-Si:H) thin films, SiH${}_{3}{}^{+}$ is the dominant fragmentation product making up roughly a third of the total ion yield, a substantial increase from other techniques.

Classical ultraviolet photoelectron spectroscopy of polymers

Although X-ray photoelectron spectroscopy of polymers was well established by Clark and coworkers in the 1970s, ultraviolet photoelectron spectroscopy of polymer films, was developed later. Previous to the 1970s, the first attempts to use ultraviolet light on polymer films took the form of appearance potential (valence band edge) measurements. Only some years later could the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. The development of what might be termed “classical ultraviolet photoelectron spectroscopy” of polymer films may be loosely based upon a variety of issues, including adapting thin polymer film technology to ultra high vacuum studies, the widespread use of helium resonance lamps for studies of solid surfaces, the combined advent of practical and sufficient theoretical–computational methods. The advent of, and the use of, easily available synchrotron radiation for multi-photon spectroscopies, nominally in the area of the near UV, is not included in the term “classical”. At the same time, electrically conducting polymers were discovered, leading to applications of the corresponding semiconducting polymers, which added technologically driven emphasis to this development of ultraviolet photoelectron spectroscopy for polymer materials. This paper traces a limited number of highlights in the evolution of ultraviolet photoelectron spectroscopy of polymers, from the 1970s through to 2008. Also, since this issue is dedicated to Prof. Kazuhiko Seki, who has been a friend and competitor for over two decades, the author relies on some of Prof. Seki's earlier research, unpublished, on who-did-what-first. Prof. Seki's own contributions to the field, however, are discussed in other articles in this issue.

Dissociative photoionization of ClN 3 using high-resolution synchrotron radiation: The N–Cl bond energy in ClN 3

High-resolution synchrotron-radiation-based photoionization mass spectrometry was applied to study the dissociative photoionization of ClN 3 under collision-free molecular beam conditions at ionization energies between 10 and 17 eV. No parent ion (ClN 3 +) could be detected under our experimental conditions. This suggests that the ground and excited states of ClN 3 + are weakly bound or repulsive, a conclusion supported by electronic structure calculations also reported here. We recorded photoionization yield spectra at m/ z = 49, 42, 35 and 14 from which we extracted the appearance potentials for NCl +, N 3 +, Cl +, and N +. The appearance potential of NCl + (10.17 ± 0.02 eV) observed here is close to the previously reported ionization potential of ClN 3 obtained form photoelectron spectroscopy. Using the theoretically calculated binding energy of ClN 3 + (0.2 eV), we derive an estimate of the adiabatic ionization potential of ClN 3 = 9.97 ± 0.02 eV. The measured appearance potentials for N 3 +, Cl +, and N + provide three independent determinations of the Cl–N bond energy in ClN 3, which agree within their respective error limits. The observations of this work are consistent with a new value of the N–Cl bond energy in ClN 3, D 0(Cl–N 3) = 1.86 ± 0.05 eV, 0.3 eV lower than previously reported values, which are however experimentally derived upper limits. The bond energy reported here is consistent with high level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit, which yield a value: D 0(Cl–N 3) = 1.87 eV.

A history of mass spectrometry in Australia

An interest in mass spectrometry in Australia can be traced back to the 1920s with an early correspondence with Francis Aston who first visited these shores a decade earlier. The region has a rich tradition in both the development of the field and its application, from early measurements of ionization and appearance potentials by Jim Morrison at the Council for Scientific and Industrial Research (CSIR) around 1950 to the design and construction of instrumentation including the first use of a triple quadrupole mass spectrometer for tandem mass spectrometry, the first suite of programs to simulate ion optics (SIMION), the development of early TOF/TOF instruments and orthogonal acceleration and the local design and construction of several generations of a sensitive high-resolution ion microprobe (SHRIMP) instrument. Mass spectrometry has been exploited in the study and characterization of the constituents of this nation's unique flora and fauna from Australian apples, honey, tea plant and eucalyptus oil, snake, spider, fish and frog venoms, coal, oil, sediments and shale, environmental studies of groundwater to geochronological dating of limestone and granite, other terrestrial and meteoritic rocks and coral from the Great Barrier Reef. Peter Jeffery's establishment of geochronological dating techniques in Western Australia in the early 1950s led to the establishment of geochronology research both at the Australian National University and at what is now the Curtin Institute of Technology in the 1960s. This article traces the history of mass spectrometry in its many guises and applications in the island continent of Australia. An article such as this can never be complete. It instead focuses on contributions of scientists who played a major role in the early establishment of mass spectrometry in Australia. In general, those who are presently active in the field, and whose histories are incomplete, have been mentioned at best only briefly despite their important contributions to the field.

Ionization energy measurements and electronic spectra for ThO

The ionization energy (IE) for ThO has been determined using photoionization efficiency and mass-analyzed threshold ionization measurements. An IE of 6.6038(12) eV was obtained, which was appreciably higher than the result from previous appearance potential measurements [6.1(1) eV]. The revised IE is 0.3 eV greater than that of atomic Th, indicating that neutral ThO is more tightly bound than ThO(+). The one-color two-photon resonant ionization spectrum of ThO was examined in the range of 315-370 nm. Rotationally resolved bands were recorded for three new electronic states (designated as E('),F('), and G(')). In addition, transitions to the A(')(v=1,2,3) levels and the N(v=2) level were observed for the first time. Ligand field theory predictions [L. A. Kaledin, J. E. McCord, and M. C. Heaven, J. Mol. Spectrosc. 164, 27 (1994)] were used to propose configurational assignments for 20 electronically excited states.

The electron-impact dissociative ionization ofCCl2F2

An investigation of the formation channels and properties of ionfragments following electron-impact dissociative ionization of theCCl2F2 molecule using electron kinetic energies in the 0–100 eV range isreported. Measurements of ion appearance potentials (APs) and nascenttranslational energy distributions were made on a supersonic expansion ofCCl2F2 in a time-of-flight mass spectrometer. A discussion of the correlation betweenthe channel APs, the precursor bond characters as calculated from thepopulation analysis, and the low-resolution photoelectron spectrum of theCCl2F2 molecule is presented.

Heat of formation of the CF 2++ dication: a theoretical estimate

Relative energies and ionization potentials of neutral and ionic fragments of CF 4 were calculated using the coupled cluster single double (triple)/correlation consistent triple zeta basis set ab initio method. The individual values were compared with available experimental and theoretical data to assess the accuracy of this approach for these particular systems. In addition, reaction enthalpies for possible fragmentation reactions were calculated. Maximum errors found were in the order of 5 kcal/mol (0.2 eV). The estimation of the heat of formation of the CF 2 ++ dication, derived from the calculations based on four different reactions, is 687 ± 5 kcal/mol (29.8 ± 0.2 eV). This value is considerably smaller than the one which follows from the appearance potential measurements (732 ± 7 kcal/mol).

Electron impact study of carbon disulphide micro-clusters

The production of carbon disulphide micro-clusters (CS 2) n , n⩽11, by adiabatic gas expansion of mixture of CS 2 vapour and Ar is studied by using an electron impact ion source and a double focusing sector field mass spectrometer. The appearance potentials for clusters n⩽9 decreased as n increased but was not a simple function of 1/ n. The appearance pressures of observed clusters showed a slowly increasing dependence on cluster size. Results of measurements of appearance potentials and appearance pressures of selected clusters are described for a range of operating conditions.

New measurements of appearance potentials and optical absorption of Na2F and Na3O

Oxides and fluorides of sodium clusters were produced by reacting a beam of metal clusters with N2O or SF6 in the region between nozzle and skimmer. The threshold for single photon ionization was measured by using a pulsed, frequency doubled visible dye laser or a UV dye laser and detecting ions with a quadrupole mass spectrometer. Resonant two-photon ionization was employed to record optical spectra. Comparisons with previous measurements and theoretical predictions are discussed.

In Vitro Stability of Poly(D,L-lactide) and Poly(D,L-lactide)/Poloxamer Nanoparticles in Gastrointestinal Fluids

Poty(D,L-lactide) (PLA) nanoparticles of various surface and bulk properties were prepared by a nanoprecipitation procedure and evaluated for their physical and chemical in vitro stability in simulated gastrointestinal fluids of 37°C. The influence of polymer characteristics and poloxamer 188 (POL 188) adsorption was studied. Physical stability was followed by visual appearance, particle size, and zeta potential measurements. Molecular weight changes were analyzed by gel permeation chromatography (GPC). Due to a sharp decrease in their negative zeta potential, poloxamer-free nanoparticles flocculated in simulated gastric fluid, irrespective of the polymer properties. Their physical stability in protein-free intestinal fluids increased with an increase in carboxy end group concentration of the PLA and thus, with an increase in their negative zetapotential. Protein effects at pH 7.5 were rather complex indicating a stabilizing effect of negatively charged proteins and a destabilizing effect of positively charged proteins. Poloxamer 188 adsorption sterically stabilized the nanoparticles against flocculation in gastric fluid, irrespective of the PLA characteristics. Physical stability of the PLA/POL 188 nanoparticles in intestinal fluids was affected by the PLA characteristics. Poloxamer 188 increased the physical stability of nanoparticles composed of hydrophobic PLA, irrespective of the proteins present. A gradual particle size increase could, however, be observed for PLA/POL nanoparticles composed of PLA with a high content of carboxy end groups, especially in combination with positively charged proteins. This effect is most likely due to a decrease in PLA/POL interactions resulting from the ionization of the carboxy end groups located on the nanoparticle surface and leading to conformational changes and/or a distinct desorption of POL 188. The chemical stability of PLA and PLA/POL nanoparticles depended on the glass transition temperature (TgH) of the hydrated polymer matrix. Enzymatic effects could not be detected. Nanoparticles with TgH > 37°C were chemically stable in both gastric and intestinal fluids at 37°C over a time period of more than 48 hr.

Cluster Beam Chemistry: Hydration of Nucleic Acid Bases; Ionization Potentials of Hydrated Adenine and Thymine

Gas phase hydration of adenine, cytosine, thymine, and uracil in supersonic molecular beams is examined by mass spectra and ionization potential measurements. In the beam source, designed for substances of low volatility that readily decompose on heating, a carrier gas (moist argon or pure water vapor) flows through a porous filter packed with the solid sample before exiting via a supersonic nozzle. With the electron impact energy below the ionization potential of water, progressions of cluster ions of the form Bm+(H2O)n with m = 1−3 and n as high as 15 are obtained. Cytosine, even with modest heating, loses two hydrogens and also undergoes tautomeric conversion to uracil. From appearance potential measurements and calibrations, the ionization potentials for B = adenine or thymine clusters (with m = 1, n = 0−3; m = 1 or 2, n = 0−1 or 2) were determined; relative to the neutral base monomer, hydration by three water molecules decreases the ionization potential by about 8%, or 0.7 eV.

A mass spectrometric study of the thermal decomposition of small quantities of AgNO 3, CsNO 3 and Cs 2CO 3

The thermal decomposition of minute quantities of AgNO 3, CsNO 3 and Cs 2CO 3 on platinum and on glass-like carbon was studied by use of a mass spectrometer. We observed that AgNO 3 decomposed at 700 K and Ag evaporated at T > 1100 K. In contrast, CsNO 3 evaporated mainly as molecules. Cs was found to be the main species in the gas flow from a Cs 2CO 3 filled Knudsen cell lined with glass-like carbon. The appearance potentials of the molecular species were determined as 10.5 eV for Cs 2 +, 7.0 eV for Cs 2O +, and 5.6 eV for Cs 2CO 3 +. Platinum catalyzes the decomposition of minute Cs 2CO 3 quantities to provide mainly cesium. A method for the fast and automatic measurement of appearance potentials was developed.

The Mechanism of the Plasma Induced Deposition of a-Ge and μc-Ge from Germane: The limits and Possible Alternatives

AbstractTime resolved mass spectrometry and the measurement of appearance potentials of various GeH -cationic fragments combined with selfconsistent theoretical modelling have been used to study the mechanism of plasma induced deposition of germanium. It is shown, that the high rate deposition occurs via germylene, GeH as the direct precursor. Digermane May play some role only at low deposition rates. Although we could reproduce the best values of theμτ product reported so far, no further improvement could be achieved.