Difference In Ionization Potential Research Articles

Multicomponent Oxides in Selective Oxidation of Alkanes Theoretical Acidity versus Selectivity

Semi-empirical relationships between the ‘optical basicity’ Λ (so-called after Duffy) of solid oxides and the ‘thermodynamic’ selectivity in mild and total oxidation of hydrocarbons have recently been set up. They can be used to determine the optimum acidity of a solid catalyst or to account for its observed selectivity in a given reaction. The oxidic MxOz catalysts were ranked by means of the electron-donor power of oxygen which is represented by the optical basicity Λ. The difference of ionization potential I of molecules when the reactant becomes the product, which represents the variation of electron-donor power during the reaction, was used to rank reactions. Plotting ΔI against Λ for each ‘reaction/selective catalyst’ couple results in straight lines, the equation of which depends on the chemical nature of reactant (alkane and alkyl-aromatics, alkenes and aromatics, alcohols) and on the deepness of oxidation (ammoxidation, mild oxidation, total oxidation). The correlations are used to discuss the behaviour of V- and Mo-based, simple and multicomponent oxide catalysts, in the mild oxidation of C2 and C3 hydrocarbons.

Trap-controlled hole transport in small molecule organic semiconductors

The influence of trap concentration on hole transport is investigated by an optical time-of-flight method for the amorphous small molecule organic semiconductor N,N′-bis(1-naphtyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamin (α-NPD) doped with neutral hole traps by codeposition of 4,4′,4″-tris-[N-(1-naphtyl)-N-(phenylamino)]-triphenylamine (1-NaphDATA). α-NPD doped with 120ppm 1-NaphDATA exhibits nondispersive hole transport like undoped α-NPD, but trap-controlled with reduced mobility. The trap depth derived from the mobility decrease coincides with the ionization potential difference of α-NPD and 1-NaphDATA. The transition to dispersive transport for increasing trap concentration to 1160ppm is explained by an energetic relaxation of optically generated charge carriers within a density of states broadened by traps.

Positive hole transfer between organic molecules of different classes in freon matrices

The processes of positive charge (hole) transfer between organic molecules of different classes occurring upon irradiation of their frozen freonic solutions at 77 K were studied by ESR spectroscopy. The efficiency of positive hole transfer was characterized by the “redistribution coefficient”, which was determined as ratio of concentrations of the radical cations of different compounds at equal concentrations of the parent neutral molecules. A very effective positive hole transfer to toluene was shown to occur for alkane-toluene pairs (K > 10). Meanwhile, much lower K values (∼ 2 to 3) were observed for alkane-alkene pairs, in spite of a rather high difference in the ionization potentials (ca. 1 eV). This effect was explained by the conformation dispersion and distribution of environment configurations of ionized molecules for the alkane-alkene pairs. An effective positive hole transfer (K ∼ 6) was observed for the pair dimethyl ether-acetone, where the conformation dispersion was absent, although the difference in ionization potentials for this pair was only 0.3 eV.

Computational study on the donor-adducts of phosphinidene complexes

The bonding of adducts between electron donors (D), such as NH 3 and PH 3, with phosphinidenes RP: (R = H, CH 3, NH 2, PH 2, PF 2) was examined using density functional theory calculations. The structures were optimized at the B3LYP/6-311++G ∗∗ level of theory. The main interest of this work is to find the appropriate donor that can stabilize the phosphinidene. Accordingly, the magnitude of bond strength correlates well with the difference in ionization potential ( ΔIP) of the orbitals involved in the D−P interaction.

Direct Laser Ablation and Ionization of Solids for Chemical Analysis by Mass Spectrometry

A laser ablation/ionization mass spectrometer system is described for the direct chemical analysis of solids. An Nd:YAG laser is used for ablation and ionization of the sample in a quadrupole ion trap operated in an ion-storage (IS) mode that is coupled with a reflectron time-of-flight mass spectrometer (TOF-MS). Single pulse experiments have demonstrated simultaneous detection of up to 14 elements present in glasses in the ppm range. However, detection of the components has produced non-stoichiometric results due to difference in ionization potentials and fractionation effects. Time-of-flight secondary ionization mass spectrometry (TOF-SIMS) was used to spatially map elemental species on the surface and provide further evidence of fractionation effects. Resolution (m/Δm) of 1500 and detection limits of approximately 10 pg have been achieved with a single laser pulse. The system configuration and related operating principles for accurately measuring low concentrations of isotopes are described.

Effect of molecular properties on the performance of polymer light-emitting diodes

The performance of a single layer polymer light-emitting diode depends on several interdependent factors, although recombination between electrons and holes within the polymer layer is believed to play an important role. Our aim is to carry out computer experiments in which bipolar charge carriers are injected in polymer networks made of poly(p-phenylene vinylene) chains randomly oriented. In these simulations, we follow the charge evolution in time from some initial state to the steady state. The intra-molecular properties of the polymer molecules obtained from self-consistent quantum molecular dynamics calculations are used in the mesoscopic model. The purpose of the present work is to clarify the effects of intra-molecular charge mobility and energy disorder on recombination efficiency. In particular, we find that charge mobility along the polymer chains has a serious influence on recombination within the polymer layer. Our results also show that energy disorder due to differences in ionization potential and electron affinity of neighbouring molecules affects mainly recombinations that occur near the electrodes at polymer chains parallel to them.

Structural Symmetry Study of Some Protonated Dimer Systems

AbstractThe protonated water dimer has a symmetric structure 2 in contrast to the asymmetric structure of the protonated ammonia dimer 1. To make a general understanding of factors affecting structural symmetry in these protonated dimer systems, we carried out a systematic theoretical study using the B3LYP/6‐31+G** method on several D‐A‐D systems with D as the electron‐pair donors NH3, H2O, and CO and A as the electron‐pair acceptors H+ and Li+. We found the magnitude of the first bonding energy for the monomer (BE(1)) is decisive for the symmetry of the D‐A‐D system. A larger BE(1) leads to a smaller BE(2), the bonding energy of D‐A with the second donor. This gives rise to a stronger tendency towards asymmetry. The magnitude of BE(1) correlates well with the difference in ionization potential (ΔIP) of the orbitals involved in the D‐A interaction. The donor‐donor distance (RD‐D) also plays an important role for the asymmetry, since it is also capable of modifying the ratio of BE(2)/BE(1). The parameter χ, correlated to system asymmetry, defined as RD‐D/RD‐A is a useful parameter insensitive to the type of the system studied, with RD‐A referring to the bond distance in the monomer.

CHARGE CARRIER MOBILITY IN VACUUM-SUBLIMED DYE FILMS FOR LIGHT-EMITTING DIODES STUDIED BY THE TIME-OF-FLIGHT TECHNIQUE

The Charge carrier mobility has been examined for a representative organic hole transport material TPD and a host material CBP, doped with a fluorescent dye, rubrene and Ir(ppy)3, by time-of-flight (TOF) technique. Decreasing of hole mobilities of the dye-doped films can be explained by a carrier-trapping model and the ionization potential difference of TPD and CBP. We successfully measured an electron mobility of the rubrene-doped TPD films, and Ir(ppy)3-doped CBP films, while the mobility could not measured non-doped TPD and CBP films. The EL process of the dye-doped systems is discussed in terms of the electron transfer properties of emission layer.

Application of atmospheric pressure ionization mass spectrometry to cover gas analysis in fast reactors

This paper proposes to apply atmospheric pressure ionization mass spectrometry to on-line real-time monitoring gas analysis in fast reactors. The experimental results have shown that the quantitative analysis of the low ppt level can be achieved for all isotopes of krypton and xenon contained in argon except for the species, 78Kr, 80Kr, 124Xe and 126Xe that suffer interference by cluster ions. The excellent sensitivity is attributed to an ion concentration effect in an atmospheric pressure ionization process driven by the difference in ionization potential between argon and krypton or xenon. The detection limits (3 σ) are estimated to be 20 ppt for 84Kr and 2.3 ppt for 132Xe in the present condition.

Structural symmetry study of some bis‐adduct of an acceptor with two donors

AbstractThe protonated water dimer has a symmetric structure 1 in contrast to the asymmetric structure of protonated ammonia dimer 2. To make a general understanding of structural symmetry difference for these D–A–D systems, we carried out a systematic theoretical study with the B3LYP/6‐31+G** method. We considered the electron donors (D) of the lone pair on either side of BF, CO, and N2 molecules, as well as the electron acceptors (A) like Li+, CH3+, SiH3+, and PH2+ beside the proton. We found the magnitude of the first bonding energy between D and A [BE(1)] is decisive for the symmetry of the D–A–D system. A larger BE(1) leads to a smaller BE(2), the bonding energy of D–A with the second donor. This gives rise to a stronger tendency toward asymmetry. The magnitude of BE(1) correlates well with the difference in ionization potential (ΔIP) of the orbitals involved in the D–A interaction. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

Hole Traps in DNA

Sequences of guanines, GG and GGG, are known to be readily oxidized, forming radical cations, i.e., hole traps, on DNA. The trapping probability of GG is less than that of GGG. Lewis et al. (J. Am. Chem. Soc. 2000, 122, 12037) have used measurements on synthetic hairpins to determine the free energy liberated when a hole goes from the radical cation G(+) to GG or to GGG. They find these free energies to be of the order of thermal energy at room temperature, in contradiction to the expectation by many of much greater trap depths. We have calculated the wave function of a hole on G, on GG, and on GGG surrounded by adenines, as in the Lewis et al. experiments, using a simple tight-binding model. We find that to account for the shallow traps found by them it is necessary that the difference in ionization potentials of contiguous guanine and adenine be smaller by about 0.2 eV than the 0.4 eV found for isolated bases. Using this value and taking into account polaron formation, we find the wave functions of holes trapped on G, GG, or GGG to extend over approximately 6 sites (bases) and with energy level differences in good agreement with the values found by Lewis et al.

Mechanism of Charge Migration through DNA: Molecular Wire Behavior, Single-Step Tunneling or Hopping?

In this work the mechanism of migration of positive charges through donor−DNA−acceptor systems is studied using a quantum mechanical model based on the tight-binding approximation. For DNA bridges containing only adenine-thymine (AT) base pairs the difference in ionization potential between the donor moiety and the AT base pairs (i.e., the injection barrier) is shown to determine the mechanism by which the charge migrates from the donor to the acceptor. For an injection barrier of 0.55 eV, corresponding to a guanine radical cation as the hole-donor, a β-value of 0.85 A-1 is found. This agrees reasonably with the value of β = 0.7 A-1 deduced from experimental studies on these sequences. For this injection barrier (0.55 eV) the charge density on the AT bridge was found to be very small, which is characteristic for charge transfer by single-step tunneling. For lower injection barriers the charge density on the AT bridge becomes substantial and the charge moves through the bridge according to a bandlike mecha...

Effect of extrinsic traps on thermally stimulated luminescence in molecularly doped polymers

The low-temperature thermally stimulated luminescence (TSL) technique has been applied to study the influence of shallow and deep traps on localized state distribution in charge transport polymer systems. Effect of shallow traps is studied in molecularly doped polymer (MDP), as tri-p-tolyamine (TTA)-doped polystyrene in which a small concentration of di-p-anisyl-p-tolylamine (DAT), acting as a weak hole trap, was added (Et=0.15 eV). It was found that deep trap is created when DAT is doped into poly(N-epoxypropylcarbazole) (PEPC) (Et=0.5 eV) due to the difference in ionization potential of these materials. The results are described in terms of the Gaussian disorder model, which provides reasonable understanding of all trends observed in the TSL. The effect of extrinsic shallow trapping on TSL properties can be reasonably interpreted in terms of the effective energetic disorder and the TSL results are in good agreement with those obtained earlier by charge transport studies. The effect of percolative motion of charge carriers within manifold of trapping states on TSL properties of deep trap-containing polymers was found for the first time.

Ab Initio Quantum Mechanical Study of Metal Substitution in Analogues of Rubredoxin: Implications for Redox Potential Control

Substitution of different metals into the redox sites of metalloproteins is a means of studying the structure of the native protein and of varying the redox properties of the protein. The implicit assumption is often made that metal substitution changes only intrinsic properties of the redox site such as the ionization potential without altering the surrounding protein or solvent. However, if this is not true, structural studies of metal-substituted proteins will not reflect the native protein and the differences in redox potential upon metal substitution will not be simply the differences in ionization potential of the redox sites because of perturbations in the extrinsic electric field. Here, we present an ab initio unrestricted Hartree−Fock quantum mechanical study of metal substitution in the [M(SCH3)4)]2-/1- analogue, where M = Fe, Co, Ni, and Zn, of the protein rubredoxin. Variations in several physical properties were determined and compared to experimental data. Upon metal substitution, only minor variations in geometry, atomic spin, and atom-centered partial charges of the redox site are observed. However, significant variation is found in the energies of reduction, on the order of 100−1000 mV. This indicates that when such substitutions are made into an Fe−S metalloprotein, little change will occur in the interactions between the metal site and the surrounding protein and thus the surrounding protein structure and the resultant electric field will not change. Thus, the structure is relevant to the native protein and the redox properties are mainly determined by the variations in the intrinsic ionization potential of the metal site and not the extrinsic field of the surrounding protein and solvent.

On the heats of formation of trifluoromethyl radical CF3 and its cation CF3+

The CF+ and CF3+ fragment ion yield curves from C2F4 have been remeasured by photoionization mass spectrometry. Fits with appropriate model curves yield the appearance potentials AP0(CF3+C2F4)=13.721±0.005 eV and AP0(CF+/C2F4)=13.777±0.005 eV and an accurate difference in ionization potentials, IP(CF)−IP(CF3)=0.055±0.003 eV. With the existing photoelectron value IP(CF)=9.11±0.01 eV, this produces IP(CF3)=9.055±0.011 eV. The CF3+ fragments from CF3Cl, CF3Br, and CF3I have also been remeasured, and their ion yield curves fitted with model functions. The experimentally derived AP0(CF3+/CF3Cl)<12.867±0.008 eV has been found to be only an upper limit. The Analogous CF3+CF3+ fragment yield curves from CF3Br and CF3I produce AP0(CF3+/CF3Br)=12.095±0.005 eV and AP0(CF3+/CF3I)=11.384±0.005 eV, leading to D0(CF3−Br)=70.1±0.3 kcal/mol (70.8±0.3 kcal/mol at 298 K) and D0(CF3−I)=53.7±0.3 kcal/mol (54.3±0.3 kcal/mol at 298 K). Based on tabulated values for ΔHf∘(CF3Br) and ΔHf∘(CF3I), which appear to be inconsistent by ∼1 kcal/mol, a compromise value of ΔH298 f∘(CF3)=−111.4±0.9 kcal/mol (−110.7±0.9 kcal/mol at 0 K) is selected, resulting in ΔH298f∘(CF3+)=97.4±0.9 kcal/mol (98.1±0.9 kcal/mol at 0 K). Additionally, IP(CF4)≡AP0(CF3+/CF4)=14.67±0.04 eV can be inferred. From data on C2F4, ΔH298f∘(CF)=62.5±1.1 kcal/mol (61.7±1.1 kcal/mol at 0 K) can be deduced. Many earlier literature values for appearance potentials of CF3+ from CF3X, leading to very low ΔHf∘(CF3+) and/or IP(CF3) values, are demonstrated to be in error.

The Third Ionization Potentials of Lanthanides and Major Orientations of 4f Orbital Clouds.

Considerations are given to the third ionization potentials of lanthanides with general regularities and certain irregularities. Main attention is paid to those irregularities, which are found to be explicable in terms of stabilization energy due to steric symmetry of configuration of 4f electrons. It is reasoned that stabilization energy for Ho is about twice that for Nd, because Nd is sterically symmetrical with respect to the unpaired f electron system only, while Ho is sterically symmetrical concerning both the unpaired and paired systems of f electrons. The observation about the ionization potential in question can be regarded to be consistent with this inference. Small differences in ionization potential between Nd and Pm and between Ho and Er are also reasonable. Further, the fact that the difference between Ho and Er is smaller than that between Nd and Pm is in line with the assumed configurations of 4f electrons. Briefly, a kind of deceleration of general tendency of increase of ionization potential takes place at Nd-Pm for light lanthanides and at Ho-Er for heavy lanthanides, owing to the stabilization energy ascribable to the three-dimensional symmetry of major directions of f electron clouds.

Entrainaient and demixing in subsonic argon/helium thermal plasma jets

The velocity, temperature, entrained air fraction, and Ar/He concentration profiles were measured in a subsonic thermal plasma jet using an enthalpy probe and mass spectrometer. Through interaction with the surrounding atmosphere, air is quickly entrained into the jet, resulting in rapidly decreasing velocities and temperatures. Due to the difference in ionization potential, a significant diffusive separation or demixing of Ar and He is also observed in the large temperature gradients present. Near the exit of the torch, in the jet center, the relative He concentration is enhanced by approximately 50% over that of the premixed feed gases. Demixing occurs primarily in the discharge region and torch nozzle. As jet mixing progresses in the downstream direction, the Ar to He ratio approaches the initial input ratio.

Hole drift mobility in organopolysilane doped with trap-forming organic compounds

The carrier transport in poly(phenylmethylsilane) films doped with trap-forming organic compounds with different ionization potentials has been investigated. The carrier transport in the doped systems is discussed as trapping events in the transport-active host polymer. Zero-field activation energy E0 corresponds well to the ionization potential difference between the additives and host polymer. The field and temperature dependencies of the mobilities have been analyzed in the framework of Gill’s [W. D. Gill, J. Appl. Phys. 43, 5033 (1972)] expression. Its characteristic parameter T0, at which the field dependence disappears, has been found to depend on trap depth and concentration, and an explicit expression interrelating them has been experimentally derived.

Reactions of Ar+, Ne+, and He+ with SiF4 from thermal energy to 50 eV c.m.

Guided ion-beam techniques are used to measure the cross sections for reaction of SiF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Charge transfer followed by loss of F atoms are the sole processes observed. All SiF+x (x=0–4) products are observed, except for SiF+4 from reaction with Ne+ and He+, and Si+ from reaction with Ar+. At high energies, the dominant products are SiF+3 in the Ar system, and SiF+ in both the Ne and He systems. There is some evidence in the Ne system for an excited state of SiF+3 at 5.7 eV. In the Ar+ and Ne+ reactions, the observed energetics are consistent with literature thermochemistry, but with He+, reaction barriers are observed. A value of ΔH0f,298 (SiF+3)=−30.1±0.9 kcal/mol is derived, which is in agreement with previous values but is much more precise. The observed product distributions and energetics are explained by consideration of the potential energy surfaces and the difference in ionization potentials of the rare gases. Finally, the relationships of these reactions to plasma deposition and etching are discussed.

Free hole transfer and capture in non-polar hydrocarbons studied by the OD ESR method

OD ESR spectra taken in very dilute solutions of 2,5-diphenyloxazol (PPO) in p-xylene, benzene, n-pentadecane under ionizing irradiation show a narrow line from solvent holes at the background of the line from PPO radical ions. The parameters for spectral collapse have been used to estimate the charge-transfer rate constants for moving holes in p-xylene and n-pentadecane. The hole capture efficiency has been shown to depend strongly on the ionization potential difference between the solvent and the hole acceptor.