Difference In Ionization Energy Research Articles

Conformer-dependent vacuum ultraviolet photodynamics and chiral asymmetries in pure enantiomers of gas phase proline

Proline is a unique amino-acid, with a secondary amine fixed within a pyrrolidine ring providing specific structural properties to proline-rich biopolymers. Gas-phase proline possesses four main H-bond stabilized conformers differing by the ring puckering and carboxylic acid orientation. The latter defines two classes of conformation, whose large ionization energy difference allows a unique conformer-class tagging via electron spectroscopy. Photoelectron circular dichroism (PECD) is an intense chiroptical effect sensitive to molecular structures, hence theorized to be highly conformation-dependent. Here, we present experimental evidence of an intense and striking conformer-specific PECD, measured in the vacuum ultraviolet (VUV) photoionization of proline, as well as a conformer-dependent cation fragmentation behavior. This finding, combined with theoretical modeling, allows a refinement of the conformational landscape and energetic ordering, that proves inaccessible to current molecular electronic structure calculations. Additionally, astrochemical implications regarding a possible link of PECD to the origin of life’s homochirality are considered in terms of plausible temperature constraints.

Activity correlation among singlet-oxygen quenching, free-radical scavenging and excited-state proton-transfer in hydroxyflavones: Substituent and solvent effects

Substituent and solvent effects on activities of singlet-oxygen (1O2) quenching, free-radical scavenging and excited-state proton-transfer (ESPT) in hydroxyflavones were investigated by means of laser, stopped-flow and steady-state spectroscopies together with density functional calculations. Some correlations were found among the three reaction activities. Substitution of OH group at the 5-position in hydroxyflavones increases the ionization energy, hinders the electron transfer from the molecule to 1O2 and free-radicals, and causes deactivation of the 1O2 quenching and free-radical scavenging in ethanol, although the ESPT is very efficient. OH substitution at the 7-position has a negligible influence on the 1O2 quenching and free-radical scavenging activities, but induces solvent-assisted ESPT, which is deactivated by substitution of a catechol structure at B-ring. A synergetic effect between the electron transfer and ESPT-induced potential-surface distortion is likely present on the 1O2 quenching and free-radical scavenging activities. The 1O2 quenching activity of 3,3’,4’-trihydroxyflavone monotonously increases with increasing water concentration in the ethanol solution, whereas addition of a small amount of water respectively activates and deactivates the ESPT and free-radical scavenging, whose activity changes are saturated by addition of a large amount. The reason for these results can be explained in terms of the nodal-plane model, ionization energy difference and potential-curve distortion/displacement with regard to the ESPT and solvation.

Structural and electronic properties of Gen and EuGen-1 nanoclusters: A full potential DFT study

In this work, the stability, structure and electronic properties of the nanoclusters of germanium (Gen) and europium atom doped germanium clusters (EuGen-1) with n=2 to 12, 15 and 20 were investigated. First, the stability of nanoclusters such as Gen and EuGen-1 was addressed using FHI-aims as a software package based on the density functional theory. Then the lowest-energy structures were selected for calculating the first vertical ionization with the symmetry adapted cluster-configuration interaction General-R (SAC-CI-General-R) method. The results of this research show that there is a good agreement between calculation and experiment ionization potential for Gen nanoclusters. Generally, the analyses of binding energies show that increasing the size of nanoclusters leads to more stability for nanoclusters. The most stable nanoclusters for EuGen can be created with exchanging the Eu atom in the most stable Gen+1 nanoclusters, but there is an exception for n=11 case. Here, the second difference in energy (∆2E) and gap energy are computed for the stable nanoclusters. The results of ionization energy and second difference in energy confirm that Ge7 and Ge10 also EuGe8 and EuGe10 have the most stability.

What a little branching can do – Dissociative photoionization of two butanol isomers

The fragmentation processes of two internal energy-selected C4H10O+ isomers, 1-butanol and isobutanol cations, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The first dissociation channel leads to the formation of C4H8+ ions (m/z 56) by water loss in both isomers. Using statistical energy distribution and rate models including the isomerization of the parent ion, the 0 K appearance energies (E0) were determined to be 10.347 ± 0.015 eV and 10.57 ± 0.05 eV for 1-butanol and isobutanol, respectively. The second dissociation channel, the formation of CH3OH2+, quickly overtakes the water-loss channel in isobutanol with an E0 of 10.61 ± 0.02 eV. It appears only as a minor channel in 1-butanol with an E0 of 10.74 ± 0.09 eV. The methanol-loss channel, forming propylene ion, opens up at E0 = 10.94 ± 0.04 eV and 10.72 ± 0.02 eV in 1-butanol and isobutanol, respectively. The next two fragmentation pathways correspond to complementary pair formation of C3H7(+/o) and CH2OH(o/+). The former is assigned as the isopropyl (ion) in both butanol isomers. The channel corresponds to simple bond cleavage in isobutanol with the E0 difference corresponding to the ionization energy difference of the fragments at E0 = 10.97 ± 0.05 eV (C3H7+) and at E0 = 11.11 ± 0.20 eV (CH2OH+). However, there is an internal hydrogen shift necessary in 1-butanol and, therefore, the complementary ions appear at the same E0 = 11.10 ± 0.03 eV, which corresponds to a shared rate limiting transition state. The sequential dissociation product from m/z 56, C3H5+, appears above 11.6 eV as a minor channel in both isomers. Finally, we determined −279.1 ± 1.6 kJ mol−1 as the 298 K heat of formation of gaseous isobutanol. We also propose revised ionization energies for the two butanols isomers based on composite method calculations.

Study on Quantitative Analysis of Carbon and Nitrogen in Stoichiometric θ-Fe3C and γ′-Fe4N by Atom Probe Tomography

The quantitative analysis performance of carbon and nitrogen was investigated using stoichiometric θ-Fe3C (25 at% C) and γ'-Fe4N (~20 at% N) precipitates in pulsed voltage and pulsed laser atom probes. The dependencies of specimen temperature, pulse fraction, and laser pulse energy on the apparent concentrations of carbon and nitrogen were measured. Good coincidence with 25 at% carbon concentration in θ-Fe3C was obtained for the pulsed voltage atom probe by considering the mean number of carbon atoms per ion at 24 Da and the detection loss of iron, while better coincidence was obtained for the pulsed laser atom probe by considering only the mean number of carbon at 24 Da. On the other hand, a lack of nitrogen concentration in γ'-Fe4N was observed for the two atom probes. In particular, the pulsed laser atom probe showed a significant lack of nitrogen concentration. This implies that a large amount of 14N2+ was obscured by the main iron peak of 56Fe2+ at 28 Da in the mass-to-charge spectrum. Regarding preferential evaporation or retention, carbon in θ-Fe3C exhibited little of either, but nitrogen in γ'-Fe4N exhibited definite preferential retention. This result can be explained by the large difference in ionization energy between carbon and nitrogen.

Unveiling the Ionization Energy of the CN Radical.

The cyano radical is a ubiquitous molecule and was, for instance, one of the first species detected in astrophysical media such as comets or diffuse clouds. In photodissociation regions, the reaction rate of CN+ + CO → CN + CO+ is one of the critical parameters defining nitrile chemistry. The enthalpy of this charge transfer reaction is defined as the difference of ionization energies (EI) between CN and CO. Although EI(CO) is known accurately, the EI(CN) values are more dispersed and deduced indirectly from thermodynamic thresholds only, all above EI(CO), leading to the assumption that the reaction is fast even at low temperature. Using a combination of synchrotron radiation, electron/ion imaging coincidence techniques, and supporting ab initio calculations, we directly determine the first adiabatic ionization energy of CN at 13.956(7) eV, and we demonstrate that EI(CN) < EI(CO). The findings suggest a very slow reaction in the cold regions of interstellar media.

Pauli magnetic susceptibility of bilayer graphene and hexagonal boron-nitride

We study the contribution of s and p orbitals on the Pauli magnetic susceptibility (PMS) and density of state (DOS) of the following three structures (1) bilayer graphene (2) bilayer boron-nitride (BN) and (3) bilayer graphene-BN within a two-band tight-binding Harrison Hamiltonian and the Green's function technique. It is shown that in all three cases, the contribution of s and px or py orbitals have no states around the Fermi level, while for bilayer graphene and graphene-BN the total DOS and DOS of pz orbital appear to be a linear function around this level. We show explicitly that for bilayer BN the contribution of pz orbital does not have states around the Fermi level, because of ionization energy difference between the boron (B) and nitrogen (N) atoms. We find that the bandwidth of s, px or py is more extension than case of pz orbital as a result of the Van-Hove singularities in the DOS. This leads to consideration of the PMS in two, low and high temperature, regions.

Selective charge asymmetric distribution in heteronuclear diatomic molecules in strong laser fields

In this paper we study double-ionization-induced charge asymmetric dissociation (CAD) in heteronuclear diatomic molecules. In CO we find a selective charge distribution in two CAD channels, i.e., ${\mathrm{C}}^{2+}+\phantom{\rule{1.0pt}{0ex}}\mathrm{O}$ is abundantly produced but $\mathrm{C}+{\mathrm{O}}^{2+}$ is nearly nonexistent. This cannot be explained by the ionization energy difference between the two channels alone. Our study shows that the ${\mathrm{C}}^{2+}+\mathrm{O}$ channel is sequentially formed through an intermediate state ${\mathrm{C}}^{+}+\mathrm{O}$ and the selective charge distribution is the result of electron distribution in CO when exposed to intense laser fields.

Analysis of exchange energy at long range for heteronuclear diatomics

The present paper shows that the exchange energies of heteronuclear diatomics can be well fitted with simple analytical expressions. For heteronuclear diatomics with two atoms having large difference of ionization energies, the exchange energies can be closely fitted with − C R b e − 4R/(b + 2). In cases where the constituent atoms have similar ionization energies, − C R b e − 7R/(b + 1) fits the exchange energies very well. In addition, the parameter b, in general, can be well estimated with the atomic ionization energy.

The surface effect on the p-type conductivity of Li-doped ZnO film

By using the first-principles method, we study the relative stabilities and the thermal ionization energies of the doped Li (LiZn) in the different atomic layers for both the non-polar and polar surfaces. Our calculations indicate that the LiZn in the surface region is more stable than in the ZnO bulk, and the thermal ionization energy of the LiZn in the surface region is considerably bigger than in the ZnO bulk. So, the surface of ZnO film degrades the p-type conductivity of the Li-doped film significantly, which is important for the p-type doping in the low-dimensional ZnO system. Furthermore, we find that the observed difference in thermal ionization energy of LiZn between a surface and bulk actually stems from the different distributions of the electrostatic potentials between a surface and bulk.R66

Density Functional Theory Study of Conformation-Dependent Properties of Neutral and Radical Cationic l-Tyrosine and l-Tryptophan

Conformation-dependent properties of L-tyrosine and L-tryptophan in neutral and radical cations were studied by using the density functional theory (DFT) with a new density functional M05-2X. The results are compared with those obtained by using the conventional DFT (B3LYP). Results obtained by both types of DFT were in qualitative accord, including the existence of two conformational subgroups and their subgroup-dependent adiabatic ionization energy and hydrogen bonding. On the other hand, quantitative differences were found between the two DFT methods as well: the M05-2X method successfully reproduced experimental adiabatic ionization energy, whereas the B3LYP functional consistently yielded significantly lower values by 0.2-0.3 eV. More importantly, natural bond orbital (NBO) analysis for cationic conformers showed that all conformers of L-tyrosine and L-tryptophan undergo charge localization upon ionization regardless of the presence of intramolecular hydrogen bonding, unlike the case of L-phenylalanine that was treated earlier by other studies. Different degrees of charge localization among all three aromatic amino acids are explained by employing a simple model in which the aromatic amino acid is assumed to consist of two submoieties of distinct cationic core: the backbone and aromatic side chain. The difference in adiabatic ionization energy between these two submoieties is found to govern the degree of charge localization.

Ionization of decaborane with controlled hydrogen content by charge transfer from ambient gas

ABSTRACTWe present the ionization of decaborane (B10H14) and formation of hydrogen- and boron-contents-controlled B10-yHx+ through the charge transfer from ambient gas ion to decaborane molecules in an external quadrupole static attraction ion trap. The charge transfer energy is estimated from the experimentally observed products. PBE0/6-311+G(d)//B3LYP/6-31G(d) level of DFT calculations are conducted to investigate the mechanism of charge transfer from ambient gas ion. The calculation of the difference of ionization energies and mismatch of orbital energies between decaborane and ambient gas reveals the mechanism of ionization.

Investigation of adenine, uracil, and ribose phosphate thin films prepared by electrospray in vacuum deposition using photoemission spectroscopy

Previous results showed significant ionization energy differences between thin films of the four ribonucleic acid (RNA) polynucleotides. The experiments reported here aim at the investigation of the origin of these differences. Since the ribose phosphate backbone is common to all RNA nucleotides, the nucleobases are the most likely candidate defining the ionization energy of RNA. Consequently, experiments were performed to investigate the electronic structure and ionization energies of thin films of two nucleobases (adenine and uracil, representative for purines and pyrimidines), and ribose phosphate. These experiments were performed using x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) in conjunction with an electrospray based in vacuum multi-step deposition technique. The presented results clearly demonstrate a significant ionization energy difference between the two nucleobases, qualitatively matching the previous results on the homopolymers of adenosine and uridine (poly rA, poly rU). As expected, the ionization energy of the prepared ribose phosphate thin films was much larger than those of the nucleobases.

Internal Energy Deposition in Electron Capture Dissociation Measured Using Hydrated Divalent Metal Ions as Nanocalorimeters

Extensively hydrated divalent metal ions are used as nanocalorimeters to measure the internal energy deposition resulting from electron capture. For M(H2O)322+, M = Mg, Ca, Sr, and Ba, two dissociation pathways are observed: loss of a water molecule from the precursor (∼6%) owing to activation from blackbody photons or collisions with residual background gas, and loss of between 9 and 11 water molecules from the reduced precursor formed by electron capture. The binding energy of a water molecule to the reduced precursor ions is estimated to be approximately 10 kcal/mol. From the distribution of water molecules lost, corrected for residual activation, the average and maximum internal energy deposited into these ions is determined to be ∼4.4 and ∼4.8−5.2 eV, respectively. The average internal energy deposition does not depend significantly on metal ion size (∼0.1 eV difference for Mg to Ba) despite the large (5.0 eV) difference in second ionization energies for the isolated atoms. Similar results were obtained for Ca(H2O)n2+, n = 30 and 32, suggesting that neither the water binding energy nor the recombination energy changes significantly for clusters of this size. The recombination energy is roughly estimated from theory to be about 4.5 eV. These results show that these ions are not significantly activated by inelastic non-capture collisions with electrons and that the vast majority of the recombination energy resulting from electron capture is converted into internal energy of the reduced precursor ions, indicating that these ions dissociate statistically.

Mechanism of the Diphenyldisulfone-Catalyzed Isomerization of Alkenes. Origin of the Chemoselectivity: Experimental and Quantum Chemistry Studies

Polysulfone- and diphenyldisulfone-catalyzed alkene isomerizations are much faster for 2-alkyl-1-alkenes than for linear, terminal alkenes. The mechanism of these reactions has been investigated experimentally for the isomerization of methylidenecyclopentane into 1-methylcyclopentene, and theoretically [CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) calculations] for the reactions of propene and 2-methylpropene with a methanesulfonyl radical, MeSO2*. On heating, polysulfones and (PhSO2)2 equilibrate with sulfonyl radicals, RSO2*. The latter abstract allylic hydrogen atoms in one-step processes giving allylic radical/RSO2H pairs that recombine within the solvent cage producing the corresponding isomerized alkene and RSO2*. The sulfinic acid, RSO2H, can diffuse out from the solvent cage (H/D exchange with MeOD,D2O) and reduce an allyl radical. Calculations did not support other possible mechanisms such as hydrogen exchange between alkenes, electron transfer, or addition/elimination process. Kinetic deuterium isotopic effects measured for the (PhSO2)2-catalyzed isomerization of methylidenecyclopentane and deuterated analogues and calculated for the H abstraction from 2-methylpropene and deuterated analogues by CH3SO2* are consistent also with the one-step hydrogen transfer mechanism. The high chemoselectivity for this reaction is not governed by an exothermicity difference but by a difference in ionization energies of the alkenes. Calculations for CH3SO2* + propene and CH3SO2* + 2-methylpropene show a charge transfer of 0.34 and 0.38 electron, respectively, from the alkenes to the sulfonyl radical in the transition states of these hydrogen abstractions.

The current–voltage characteristics of heterostructures formed by MEH-PPV spin-coated on n-type GaAs and n-type porous GaAs

P–N heterostructures are formed by depositing the poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) MEH-PPV on n-type GaAs(100) substrate and on n-type porous GaAs. The elaborated heterostructures are studied by current–voltage measurements. Thermionic emission is used to model the heterostructures behaviors and to extract parameters as ideality factor and zero bias barrier high. Such model also appears to be useful as a new approach for calculating hole concentration in MEH-PPV deposited on n-type GaAs ( N a = 2.2 × 10 17 cm − 3 ). The zero bias barrier height in both heterostructures were found to be close to the ionization energy difference of isolated MEH-PPV and n-type GaAs and the ideality factor values are found to be high. Such high values are suggested to be due to the existence of high density of trap. This is preliminary evidenced by calculating the trap density using space charge limited conductivity (SCLC) characterized by an exponential distribution of trapping levels in both heterostructures.

Influence of ionization energy on charge-transfer ionization in matrix-assisted laser desorption/ionization mass spectrometry

In this study, non-polar matrices are used in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) to analyze selected non-polar analytes. Our hypothesis is that gas-phase charge-transfer reactions between matrix and analyte are responsible for the generation of analyte radical molecular ions. Following this hypothesis, the ionization energies of the matrices and analytes should have a direct influence on the production of radical molecular cations of the analytes. Several non-polar analytes, including ferrocene and ferrocene derivatives, trans-stilbene, triphenylphosphine, 2,2′-methylenebis(6-tert-butyl-4-methylphenol), biphenyl and 1,4-bis(methylthio)benzene were studied using positive-ion mode MALDI-TOFMS. The results of these studies demonstrate that formation of the radical molecular cation depends on the difference in ionization energies between the matrix and the analyte. The propensity for charge-transfer ionization, as opposed to proton-transfer ionization, for these analytes, was confirmed using atmospheric pressure chemical ionization mass spectrometry. Charge-transfer ionization using non-polar matrices in MALDI-MS is a suitable method for the characterization of a number of non-polar, thermally labile analytes.

Carrier scattering by neutral divalent impurities in semiconductors: Theory and experiment

We have developed a theoretical model describing carrier scattering by divalent impurities in semiconductors. The mobility predicted by the model based on the scattering of electrons by helium atoms shows excellent agreement with the low-temperature mobilities measured for three Ge samples doped with different double acceptors; Be, Zn, and Hg. We show that the scattering cross sections of these double acceptors are the same despite the large difference in ionization energies. This supports our assumption that the contribution of the central-cell potential to neutral impurity scattering is negligible.

Phospholes with Reduced Pyramidal Character from Steric Crowding. 2. Photoelectron Spectral Evidence for Some Electron Delocalization in 1-(2,4-Di-tert-butyl-6-methylphenyl)-3-methylphosphole.

Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra had shown no difference in ionization energies (IE) for simple phospholes and their tetrahydro derivatives (both around 8.0-8.45 eV). Calculations of the Koopmans IE at the Hartree-Fock 6-31G level for 1-methylphospholane showed that, as is known for nitrogen, planarization at phosphorus markedly reduced the ionization energy value (8.74 to 6.29 eV). A reduction in IE also occurred on planarizing 1-methylphosphole, but to a lesser extent, being offset by increased electron delocalization (8.93 to 7.16 eV). This suggests that experimental comparison of IE for the unsaturated and saturated systems could be used to detect the presence of electron delocalization in the former. The IE experimentally determined for the crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was 7.9 eV, the lowest ever recorded for a phosphole. The corresponding phospholane had IE 7.55 eV. The difference in the values is attributed to electron delocalization in the phosphole. Calculations performed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)phosphole showed that the P-substituent adopted an angle of 55.7 degrees (DFT/6-31G level; 57.6 degrees at the HF/6-31 level) with respect to the C(2)-P-C(5) plane (for P-phenyl, 67.1 degrees and 68.3 degrees, respectively).

Acidities, Proton Affinities, and Other Thermochemical Properties of Hypohalous Acids HOX (X = F−I): A High-Level Computational Study

The acidities, proton affinities, ionization energies, dissociation energies, and heats of formation of the hypohalous and hydrohalic acids have been calculated at the G2 level of theory. Where reliable experimental data are available, our results are generally in good agreement but in other cases our predictions serve to fill important gaps. The calculated gas-phase acidities of the hypohalous acids (1507.9 (HOF), 1490.0 (HOCl), 1490.6 (HOBr), and 1487.0 (HOI) kJ mol-1 at 298 K) agree well with available experimental data and are close to one another (lying within a range of 20.9 kJ mol-1), showing that the nature of the halogen has relatively little impact on their acidity. In contrast, the ΔHacid values for the hydrohalic acids HX increase by 232.9 kJ mol-1 in going from HF to HI. Hypohalous acids are more acidic than water. In addition, hypofluorous acid is a slightly stronger acid than HF. However, other hypohalous acids are weaker than the hydrohalic acids HX (X = Cl−I). The calculated proton affinities at oxygen (HOX → H2OX+: 565.9 (F), 641.9 (Cl), 678.0 (Br), and 724.7 (I) kJ mol-1 at 298 K) and at the halogen (HOX → HOXH+: 488.7 (F), 581.5 (Cl), 601.0 (Br), and 642.3 (I) kJ mol-1 at 298 K) are larger than PA(HX) values (484.0 (F), 561.5 (Cl), 584.8 (Br), and 626.0 (I) kJ mol-1 at 298 K) for all the halogens. The HOXH+ structures are higher in energy than the O-protonated forms, H2OX+. The ionization energy (IE) values for HOX decrease from HOF (12.71 eV) to HOI (9.89 eV) in a manner parallel to that found for the IE values for HX (X = F−I). The IE(HOX) values are all smaller than the corresponding IE(HX) values, but the IE difference decreases substantially in going from F to I. The G2 heats of formation for the hypohalous acids (−88.3 (HOF), −76.0 (HOCl), −58.3 (HOBr), and −48.9 (HOI) kJ mol-1 at 298 K) show good agreement with available experimental values.