Experimental Threshold Energies Research Articles

Hydration energies of calcium hydroxide cation, CaOH+(H2O)x (x = 1–6), studied using guided ion beam tandem mass spectrometry

Hydration energies of CaOH+(H2O)x, x = 1–6, were obtained using threshold collision-induced dissociation (TCID) with xenon (Xe) as conducted with a guided ion beam tandem mass spectrometer (GIBMS). The primary reaction pathway observed for all complexes is the loss of one water ligand, followed by the loss of additional water molecules at higher collision energies for x > 1. The kinetic-energy-dependent cross sections for dissociation of CaOH+(H2O)x complexes were modeled to obtain 0 K binding energies after accounting for lifetime effects, energy distributions, and pressure effects. Except for x = 5, experimental threshold energies measured through TCID agree well with theoretical hydration energies determined using B3LYP/6-311+G(d,p) structure geometries followed by single point energies calculated at B3LYP, B3P86, M06, and MP2(full) levels of theory with a 6-311+G(2d,2p) basis set. B3LYP-GD3BJ and ωB97XD calculations were also used, with the former yielding results having the best agreement with the threshold energies extracted from the analysis of the TCID cross sections.

Thermodynamics and Reaction Mechanisms for Decomposition of a Simple Protonated Tripeptide, H+GGA: From H+GGG to H+GAG to H+GGA.

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated glycylglycylalanine (H+GGA) with Xe using a guided ion beam tandem mass spectrometer. Kinetic energy dependent cross sections for nine ionic products were obtained and analyzed to provide 0 K barriers for the five primary products, [b2]+, [y1 + 2H]+, [b3]+, [y2 + 2H]+, and [a1]+; and four secondary products, [a2]+, [a3]+, high-energy [y1 + 2H]+, and CH3CHNH2+, after accounting for multiple ion-molecule collisions, the internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-GD3BJ/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the five primary and three secondary products (with the mechanism of the other secondary product previously established). Geometry optimizations and single point energy calculations of reactants, products, intermediates, and TSs were performed at several levels of theory. These theoretical energies are compared with experimental threshold energies and found to give reasonable agreement, with B3LYP-GD3BJ and M06-2X levels of theory performing slightly better than MP2 and better than B3LYP. The results obtained here are compared with previous results for decomposition of H+GGG and H+GAG to probe the effect of changing the amino acid sequence. Methylation in H+GGA has a significant effect on the competition between the primary sequence products, [b2]+ and [y1 + 2H]+, suppressing the [b2]+ cross section by raising its threshold energy, while enhancing that of [y1 + 2H]+ by lowering its threshold energy.

Cis-trans isomerization is not rate determining for b2 ion structures: A guided ion beam and computational study of the decomposition of H+(GlyProAla)

Decomposition of the protonated tripeptide, GlyProAla (H+GPA), through collision-induced dissociation with Xe in a guided ion beam tandem mass spectrometer (GIBMS) is examined. Cross sections for the formation of [b2]+, [y2 + 2H]+, [a1]+, [b3]+, [a3]+, [a2]+, and H+(1-pyrroline) fragment ions were collected as a function of kinetic energy. Analysis of these cross sections include consideration of the effects of multiple collisions, dissociation lifetimes, reactant internal and kinetic energy distributions, competition among channels, and evolution into sequential channels. Structures and molecular parameters of reactants, transition states, intermediates, and products were identified using the B3LYP/6-311+G(d,p) level of theory, with single-point energy calculations performed at the B3LYP, B3P86, and MP2(full) levels of theory using a 6-311+G(2d,2p) basis set. Good agreement between experimental threshold energies and those calculated for rate-limiting transition states allow key reaction pathways for the formation of each product to be identified. The MP2(full) level of theory is found to agree best with the experimental results. Notably, even though H+GPA starts with a cis-peptide bond, it forms an oxazolone as its [b2]+ fragment and the absence of a diketopiperazine [b2]+ fragment is verified by other key observations.

Thermodynamics and Reaction Mechanisms for Decomposition of a Simple Protonated Tripeptide, H+GAG: a Guided Ion Beam and Computational Study.

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated glycylalanylglycine (H+GAG) with Xe using a guided ion beam tandem mass spectrometer. Kinetic energy dependent cross sections for nine ionic products were observed and analyzed to provide 0K barriers for the six primary products: [b2]+, [y1 + 2H]+, [b3]+, CO loss, [y2 + 2H]+, and [a1]+; and three secondary products: [a2]+, [a3]+, and CH3CHNH2+, after accounting for multiple ion-molecule collisions, internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-GD3BJ/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the six primary and one secondary products (where the other two secondary products have mechanisms previously established). Geometry optimizations and single-point energy calculations were performed at several levels of theory. These theoretical energies are compared with experimental threshold energies and are found to give reasonably good agreement, with B3LYP-GD3BJ and M06-2X levels of theory performing better than other levels. The results obtained here are also compared with previous results for decomposition of H+GGG. The primary difference observed is a lowering of the threshold for the [b2]+ product ion and a concomitant suppression of the directly competing [y1 + 2H]+ product, the result of specific methylation of the [b2]+ product ion.

Structural, magnetic, vibrational and optical studies of structure transformed spinel Fe2+-Cr nano-ferrites by sintering process

This investigation presents the optical, structural, vibrational and magnetic studies of the cubic spinel Fe2+CrFeO4 nano-ferrites, which transformed to maghemite structure by annealed at 250 °C for 3 h and to hexagonal hematite at temperatures T ≥ 500 °C in air. These nanomaterials were synthesized by the co-precipitation route and sintered at different temperatures. A split of the lattice constant a to a and c occurred for T ≥ 500 °C due to the transformation to hexagonal hematite, where c/a ≈ 2.7. The density and trend of crystallite size (R) values showed increase against T, whereas the calculated and experimental specific surface area, threshold energy, Debye temperature and elastic wave velocities showed decrease. The trend of dislocation density, inter-chain distance, distortion parameter, saturation magnetization (Ms), squareness, remnant magnetization, magnetic moment and anisotropy constant was decreased with T. The main IR absorption bands; ν1 and ν2 of the nanomaterials were observed in the IR spectra where their positions proved dependence on T. The porosity (P), strain (ɛ), force constants, elastic and stiffness constants and coercivity revealed dependence on T. Variation of all deduced parameters with T revealed the structure transformation from cubic to hexagonal hematite. The optical band gab energy (Eg) showed increase as T increases. Eg, S and ɛ proved dependence on R, whereas Ms proved dependence on P. So, Dx, Ms and Hc proved effect on the elastic wave velocities.

Thermodynamics and Reaction Mechanisms of Decomposition of the Simplest Protonated Tripeptide, Triglycine: A Guided Ion Beam and Computational Study.

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated triglycine (H+GGG) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Kinetic energy-dependent cross-sections for 10 ionic products are observed and analyzed to provide 0 K barriers for six primary products: [b2]+, [y1 + 2H]+, [b3]+, CO loss, [y2 + 2H]+, and [a1]+; three secondary products: [a2]+, [a3]+, and [y2 + 2H - CO]+; and two tertiary products: high energy [y1 + 2H]+ and [a2 - CO]+ after accounting for multiple ion-molecule collisions, internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-D3/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the six primary and one secondary products. Geometry optimizations and single point energy calculations were performed at several levels of theory. These theoretical energies are compared with experimental energies and are found to give reasonably good agreement, in particular for the M06-2X level of theory. This good agreement between experiment and theory validates the reaction mechanisms explored computationally here and elsewhere and allows identification of the product structures formed at threshold energies. The present work presents the first measurement of absolute experimental threshold energies of important sequence ions and non-sequence ions: [y1 + 2H]+, [b3]+, CO loss, [a1]+, and [a3]+, and refines those for [b2]+ and [y2 + 2H]+ previously measured. Graphical Abstract ᅟ.

Threshold energies for single-carbon knockout from polycyclic aromatic hydrocarbons.

We have measured absolute cross sections for ultrafast (femtosecond) single-carbon knockout from polycyclic aromatic hydrocarbon (PAH) cations as functions of He–PAH center-of-mass collision energy in the 10–200 eV range. Classical molecular dynamics (MD) simulations cover this range and extend up to 105 eV. The shapes of the knockout cross sections are well-described by a simple analytical expression yielding experimental and MD threshold energies of EthExp = 32.5 ± 0.4 eV and EthMD = 41.0 ± 0.3 eV, respectively. These are the first measurements of knockout threshold energies for molecules isolated in vacuo. We further deduce semiempirical (SE) and MD displacement energies, i.e., the energy transfers to the PAH molecules at the threshold energies for knockout, of TdispSE = 23.3 ± 0.3 eV and TdispMD = 27.0 ± 0.3 eV. The semiempirical results compare favorably with measured displacement energies for graphene (Tdisp = 23.6 eV).

Reconstructing the WIMP velocity distribution from direct dark matter detection data with a nonnegligible threshold energy

In this paper, we investigate the modification of our expressions developed for the model-independent data analysis procedure of the reconstruction of the (time-averaged) one-dimensional velocity distribution of galactic weakly interacting massive particles (WIMPs) with a nonnegligible experimental threshold energy. Our numerical simulations show that, for a minimal reconstructable velocity of as high as [Formula: see text] km/s, our model-independent modification of the estimator for the normalization constant could provide precise reconstructed velocity distribution points to match the true WIMP velocity distribution with ≲ 10% bias.

Re-print of “Theoretical Investigation and Reinterpretation of the Decomposition of Lithiated Proline and N-Methyl Proline”

Lithium cation complexes of proline (Pro) and N-methyl proline (NMP) have been collisionally activated with xenon in a guided ion beam tandem mass spectrometer (GIBMS). In addition to the loss of the intact ligand, Pro and NMP, we observed two prominent fragmentation pathways involving the loss of (CO+LiOH) and (CO+H2O). Quantum chemical calculations at the B3LYP/6-311+G(d, p) level are used to explore the reaction mechanisms of these Li+(Pro) and Li+(NMP) fragmentations. Complete potential energy surfaces including all intermediates and transition states are elucidated for the two fragmentation processes in both systems. Theoretical molecular parameters for the rate-limiting transition states are then used to analyze the experimental data. The experimental threshold energies are compared with single point energies calculated at six different levels of theory. Reasonable agreement between experiment and some levels of theory indicate that loss of the intact amino acid competes with the loss of CO over a tight transition state in both Li+(Pro) and Li+(NMP) systems. Once CO is lost, efficient loss of H2O can occur at lower or comparable energy and is followed at somewhat higher energies by loss of LiOH, both proceeding by loose transition states. Overall, we find that MP2(full)/6-311+G(2d, 2p) gives the best agreement with the experimental threshold energies and the qualitative characteristics of the competing reactions. This study refines the bond energy of Li+ to Pro by considering competition with CO loss and lowers the value previously published by 24±12kJ/mol, with methylation of proline increasing the bond energy to Li+ by 9±15kJ/mol.

Theoretical investigation and reinterpretation of the decomposition of lithiated proline and N-methyl proline

Lithium cation complexes of proline (Pro) and N-methyl proline (NMP) have been collisionally activated with xenon in a guided ion beam tandem mass spectrometer (GIBMS). In addition to the loss of the intact ligand, Pro and NMP, we observed two prominent fragmentation pathways involving the loss of (CO+LiOH) and (CO+H2O). Quantum chemical calculations at the B3LYP/6-311+G(d, p) level are used to explore the reaction mechanisms of these Li+(Pro) and Li+(NMP) fragmentations. Complete potential energy surfaces including all intermediates and transition states are elucidated for the two fragmentation processes in both systems. Theoretical molecular parameters for the rate-limiting transition states are then used to analyze the experimental data. The experimental threshold energies are compared with single point energies calculated at six different levels of theory. Reasonable agreement between experiment and some levels of theory indicate that loss of the intact amino acid competes with the loss of CO over a tight transition state in both Li+(Pro) and Li+(NMP) systems. Once CO is lost, efficient loss of H2O can occur at lower or comparable energy and is followed at somewhat higher energies by loss of LiOH, both proceeding by loose transition states. Overall, we find that MP2(full)/6-311+G(2d, 2p) gives the best agreement with the experimental threshold energies and the qualitative characteristics of the competing reactions. This study refines the bond energy of Li+ to Pro by considering competition with CO loss and lowers the value previously published by 24±12kJ/mol, with methylation of proline increasing the bond energy to Li+ by 9±15kJ/mol.

α-Particle Clustering from Expanding Self-Conjugate Nuclei within the Hartree-Fock-Bogoliubov Approach

The nuclear equation of state (EOS) is explored with the constrained Hartree-Fock-Bogoliubov approach for self-conjugate nuclei. It is found that beyond a certain low, more or less universal density, those nuclei spontaneously cluster into A/4 α particles with A the nucleon number. The energy at the threshold density increases linearly with the number of α particles as does the experimental threshold energy. Taking off the spurious c.m. energy of each α particle almost gives agreement between theory and experiment. The implications of these results with respect to α clustering and the nuclear EOS at low density are discussed.

Electron ionization and dissociation of aliphatic amino acids

We present experimental and theoretical study of electron ionization and dissociative ionization to the gas phase amino acids valine, leucine, and isoleucine. A crossed electron/molecular beams technique equipped with quadrupole mass analyzer has been applied to measure mass spectra and ion efficiency curves for formation of particular ions. From experimental data the ionization energies of the molecules and the appearance energies of the fragment ions were determined. Ab initio calculations (Density Functional Theory and G3MP2 methods) were performed in order to calculate the fragmentation paths and interpret the experimental data. The experimental ionization energies of parent molecules [P](+) 8.91 ± 0.05, 8.85 ± 0.05, and 8.79 ± 0.05 eV and G3MP2 ionization energies (adiabatic) of 8.89, 8.88, and 8.81 eV were determined for valine, leucine, and isoleucine, respectively, as well as the experimental and theoretical threshold energies for dissociative ionization channels. The comparison of experimental data with calculations resulted in identification of the ions as well as the neutral fragments formed in the dissociative reactions. Around 15 mass/charge ratio fragments were identified from the mass spectra by comparison of experimental appearance energies with calculated reaction enthalpies for particular dissociative reactions.

Collision-induced dissociation of MO + and MO 2+ (M = Ta and W): Metal oxide and dioxide cation bond energies

The collision-induced dissociation (CID) of TaO +, WO +, TaO 2 +, and WO 2 + with Xe along with reactions of TaO + and WO + with O 2 are studied as a function of kinetic energy using guided ion beam tandem mass spectrometry in order to elucidate the thermochemistry of the MO 2 + species. The kinetic energy dependences for the CID reactions show endothermic behavior, whereas the MO ++O 2 → MO 2 + + O reactions proceed near the collision limit indicating exothermic processes. Analyses of the endothermic CID reaction cross sections yield 0 K threshold energies in eV of E 0(Ta +–O) = 7.01 ± 0.12, E 0(W +–O) = 6.72 ± 0.10, E 0(OTa +–O) = 6.08 ± 0.12, and E 0(OW +–O) = 5.49 ± 0.09. The nature of the bonding in MO + and MO 2 + is discussed and compared for Ta and W and analyzed using theoretical calculations at the B3LYP/HW+/6-311 + G(3df) level of theory. Bond energies for all MO + and MO 2 + species are calculated using geometries calculated at this level as well as BHLYP and CCSD(T) levels and the Stuttgart–Dresden (SDD) and the Def2TZVPP basis sets. Reasonable agreement between the theoretical bond energies and experimental CID threshold energies for TaO +, WO +, TaO 2 +, and WO 2 + is found. Potential energy surfaces for the reaction of the metal cations with O 2 are also calculated at the B3LYP level of theory and reveal additional information about the reaction mechanisms.

Influence of the Charge State on the Structures and Interactions of Vancomycin Antibiotics with Cell‐Wall Analogue Peptides: Experimental and Theoretical Studies

Charge matters! The charge state significantly influences the conformation and the binding energy between vancomycin antibiotic and bacterial cell-wall analogue peptides (see figure). Surface-induced dissociation (SID) studies provide a quantitative comparison between the stabilities of different charge states of the complex.In this study we examined the effect of the charge state on the energetics and dynamics of dissociation of the noncovalent complex between the vancomycin and the cell-wall peptide analogue N(alpha),N(epsilon)-diacetyl-L-Lys-D-Ala-D-Ala (V-Ac(2)LKdAdA). The binding energies between the vancomycin and the peptide were obtained from the RRKM (Rice, Ramsperger, Kassel, Marcus) modeling of the time- and energy-resolved surface-induced dissociation (SID) experiments. Our results demonstrate that the stability of the complex towards fragmentation increases in the order: doubly protonated<singly protonated<deprotonated. Dissociation of the singly protonated and singly deprotonated complex is characterized by very large entropy effects, which indicate a substantial increase in the conformational flexibility of the resulting products. The experimental threshold energies of (1.75+/-0.08) eV ((40.3+/-1.8) kcal mol(-1)) and (1.34+/-0.08) eV ((30.9+/-1.8) kcal mol(-1)) obtained for the deprotonated and singly protonated complexes, respectively, are in excellent agreement with the results of density functional theory calculations. The increased stability of the deprotonated complex observed experimentally is attributed to the presence of three charged sites in the deprotonated complex, as compared with only one charged site in the singly protonated complex. The low binding energy of (0.93+/-0.04) eV ((21.4+/-0.9) kcal mol(-1)) obtained for the doubly protonated complex suggests that this ion is destabilized by Coulomb repulsion between the singly protonated vancomycin and the singly protonated peptide comprising the complex.

Threshold collision-induced dissociation of diatomic molecules: A case study of the energetics and dynamics of O2− collisions with Ar and Xe

The energetics and dynamics of collision-induced dissociation of O2- with Ar and Xe targets are studied experimentally using guided ion-beam tandem mass spectrometry. The cross sections and the collision dynamics are modeled theoretically by classical trajectory calculations. Experimental apparent threshold energies are 2.1 and 1.1 eV in excess of the thermochemical O2- bond dissociation energy for argon and xenon, respectively. Classical trajectory calculations confirm the observed threshold behavior and the dependence of cross sections on the relative kinetic energy. Representative trajectories reveal that the bond dissociation takes place on a short time scale of about 50 fs in strong direct collisions. Collision-induced dissociation is found to be remarkably restricted to the perpendicular approach of ArXe to the molecular axis of O2-, while collinear collisions do not result in dissociation. The higher collisional energy-transfer efficiency of xenon compared with argon is attributed to both mass and polarizability effects.

Elastic scattering cross sections for 6Li and 7Li scattering by aluminum, silicon and titanium below 12 MeV at angles of 140 ∘ and 170 ∘

Elastic scattering cross sections for 6Li and 7Li scattering by natural aluminum, silicon and titanium have been measured in the energy range of 4–11 MeV. Scattering angles were 140 ∘ and 170 ∘ for 6Li and 140 ∘ for 7Li. The threshold energies above which Rutherford backscattering becomes invalid have been determined. Above this threshold the cross sections decrease rapidly, rendering backscattering analysis impractical. For 7Li scattering by Al and Si the observed threshold values were higher than for 6Li scattering. This behavior was reversed for scattering by Ti. The findings were systematic for both scattering angles. The present cross section and threshold data have been compared with the available literature data. The experimental threshold energies are compared with results obtained by analytical models based on nuclear potential perturbation calculations. For all targets studied, the experimental 6Li threshold energy values exceed the calculations. In the case of 7Li the comparison yields less systematic results.

Theoretical Studies of Structures and Ionization Threshold Energies of Water Cluster Complexes with a Group 1 Metal, M(H2O)n (M = Li and Na)

Water cluster complexes with a group 1 metal atom, M(H2O)n (M = Li and Na), for n = 3−6 were studied with ab initio MO methods. The singly occupied molecular orbitals (SOMOs) are classified into three types: surface, quasi-valence, and semi-internal. It is the isomers of structures with semiinternal SOMO that are responsible for the observed convergence of the ionization threshold energy. They are the ion-pair complexes, M+(H2O)m·(H2O)l· , and their vertical ionization energies (VIEs) are determined by the local structure of and the electrostatic potential from the cation M+(H2O)m. The model also explains why the experimental ionization threshold energy converges at n = 4.

Conformational Analysis of the Molecular Complexes of Sodium with Methanol and 1,2-Ethanediol: Photoionization and ab Initio Molecular Orbital Studies

The molecular complexes of the sodium atoms with methanol and 1,2-ethanediol were generated in a flow reactor and studied with photoionization mass spectrometry. The photoionization efficiency spectra of these complexes were measured and the ionization threshold energies were determined by the method of the Watanabe plots. The conformation-dependent bond dissociation energies and ionization energies of these complexes were also calculated by ab initio molecular orbital methods. The photoionization threshold energies of the sodium/methanol complex was determined to be 4.30 ( 0.02 eV, a value in good agreement with the ab initio prediction. Guided by the theoretical results, the experimental photoionization threshold energy of the cyclic conformers of the sodium/1,2-ethanediol complex was determined to be 3.95 ( 0.02 eV and that of the conformers with the concerted intramolecular H-bond was determined to be 4.32 ( 0.01 eV. The bond dissociation energy of the sodium/methanol complex was determined to be 0.28 eV. The interplay of the sodium complexation bonding and the intramolecular hydrogen bonding and its effects on both the bonding energies and the ionization energies of the conformers were discussed. The conformation-dependent physical properties of the corresponding cationic states of these complexes were also reported. Comparisons of the physical and chemical properties among the sodium/1,2-ethanediol, sodium/1,2-dimethoxyethane, and potassium/1,2-ethanediol complexes were emphasized in this study.

Elastic scattering cross sections of protons by copper, molybdenum, silver and tin near the Coulomb barrier

Cross sections for elastic scattering of protons by natural copper, molybdenum, silver and tin have been measured in the energy range of 2.3–6.5 MeV at typical Rutherford backscattering angles of 135° and 165°. The threshold energies have been determined above which the Rutherford cross sections become invalid. The threshold energies obtained are 3.5 and 3.4 MeV for copper, 5.1 and 5.0 MeV for molybdenum, 5.0 and 5.8 MeV for silver, and 6.1 and 6.2 MeV for tin at angles of 135° and 165°, respectively. Resonances or the rapid decrease of the cross sections above the thresholds render practical proton backscattering analysis impossible. A literature survey has been carried out to compile relevant energy threshold data for the present target mass and scattering angle regions. Considerable experimental and theoretical interest has recently been focused at the non-Rutherford cross sections. Comparing the experimental non-Rutherford threshold energies with theoretical calculations, we find the data clearly exceeding the predictions of simple classical formulations, while the optical model approach yields more realistic threshold energy values.

Ionization Energies of ClO and Cl2O2

The ionization energies of chlorine oxide (ClO) and its dimer (Cl2O2) have been measured using monochromatic synchrotron radiation in the 10−20 eV energy regime in combination with photoionization mass spectrometry. The threshold energy of ClO+ (m/z = 51) is found at 10.85 ± 0.05 eV, whereas the ionization threshold of Cl2O2+ (m/z = 102) occurs at 11.05 ± 0.05 eV. The experimental values are compared to results from ab initio calculations, where three stable isomers of Cl2O2 are considered: dichlorine peroxide (ClOOCl), chloryl chloride (ClClO2), and chlorine chlorite (ClOClO). The results indicate that the experimental threshold energy of Cl2O2+ is due to adiabatic ionization of ClOOCl. The heat of formation of ClOOCl+ is derived from the ionization threshold: ΔfH0298(ClOOCl+) = 1203 ± 12 kJ/mol. Evidence for the occurrence of less stable Cl2O2 isomers, such as ClClO2 and ClOClO, is obtained from appearance energy measurements of the OClO+ (m/z = 67) fragment cation. The shift in ionization energy of C...