Decrease In Ionization Potential Research Articles

Oxidation of methylsilenes with molecular oxygen. A matrix isolation study

The photochemical and thermal oxidation of 1-methylsilene (2-silapropene) (1a), 1,1-dimethylsilene (1b), and 1,1,2-trimethylsilene (1c) has been investigated in O 2 -doped argon matrices. All silenes 1 are easily photooxidized in matrices containing more than 1% O 2 , but trimethylsilene (1c) is the only silene that exhibits thermal reactivity toward oxygen at temperatures as low as 20-40 K. The photochemical reactivity increases from 1a to 1c with increasing number of methyl groups at the double bond and decreasing ionization potential. Key intermediates in both the photochemical and the thermal oxidation of 1 are siladioxetanes 9

VEH electronic band structure of poly(phenylsilane)

The electronic structure of all- trans syndiotactic and isotactic poly(phenylsilane) has been calculated using the valence effective Hamiltonian (VEH) method. The effects of attachment of the phenyl group on the electronic properties of polysilane are analysed in detail. The VEH results show a decrease of ionization potential and an increase of electron affinity which determine an important reduction of the bandgap. These features are correlated with σ−π and σ ∗−π ∗ interactions between the silicon backbone and the phenyl group.

Volume of activation for the radiationless deactivation of singlet oxygen in aromatic hydrocarbons

Phosphorescence lifetimes of singlet oxygen ( 1 O 2 ; 1 Δ g ) were measured in six aromatic hydrocarbons at pressures of up to 400 MPa. The lifetimes decreased significantly with increasing pressure in the solvents examined. The observed activation volumes, ΔV D * , for the quenching by solvent were in the range from -8.3 cm 3 /mol (benzene) to -20.7 cm 3 /mol (mesitylene), and they decreased with decreasing ionization potential of solvents

Charge-transfer structures of aromatic electron donor-acceptor complexes with titanium tetrachloride. Ground-state and excited-state spectroscopy for redox processes

Aromatic hydrocarbons spontaneously form a series of related electron donor-acceptor (EDA) complexes with titanium tetrachloride, both as a neat liquid and as a solute diluted in either cyclohexane or dichloromethane. The accompanying charge-transfer absorption bands (hv CT ) of the [ArH, TiCl 4 ] complexes follow the Mulliken correlation with the decreasing ionization potentials of various benzene, naphthalene, and anthracene donors (ArH)

A low-energy reactive-ion gun using an ECR ion source and acceleration-deceleration system

A low-energy reactive-ion gun has been developed as a sputtering gun for high-depth-resolution analysis. It consists of an electron cyclotron resonance ion source and an acceleration-deceleration system. The current density, the energy distribution, and the amount of impurity particles were measured to characterize the ion gun. The current density was estimated by measuring the ion current and the beam diameter for various ion beams. The maximum ion current density at 100 eV is 8.42 μA/cm2, and this means that it takes 19 s to sputter atoms on the monolayer of 1015 atoms/cm2 assuming that each ion sputters an atom. The energy width calculated from the full width at half maximum of the energy distribution was very narrow, 2 eV for the 15-eV beam. The neutralization efficiencies evaluated by measuring secondary-ion emission under bombardment by only neutral particles were 2.9, 0.5, and 0.33 for 1000-eV Ar+, O2+, and BF2+, respectively. This order corresponds to the decreasing ionization potentials of the ion species.

Matrix effects of easily ionized elements on the spatial distribution of electron number densities in an inductively coupled plasma using an optical fibre probe and a photodiode array spectrometer

The effects of alkali and alkaline earth metals on the spatial distribution of electron number densities (ne) in an inductively coupled plasma have been studied. An optical fibre probe was used to sample the spatial position of the plasma and the photodiode array spectrometer to detect the Hβ line. The resultant lateral profiles were subjected to an Abel inversion. An on-line intelligent background correction method was used to correct the background under the Hβ line, whose full width at half maximum was then calculated automatically. Graphs of three-dimensional spatial distributions of ne in the presence of K, Na and Ca are compared with that of water. At an observation height of 7.2 mm above the load coil, the ne increased with decreasing ionization potentials of the elements. At an observation height of 9.6 mm, no measurable increase of ne is observed except for K.

THE PROMOTER EFFECT OF ALKALI METAL TO SUPPORTED IRON CATALYST IN CARBON MONOXIDE HYDROGENATION, THE INFLUENCE OF ALKALI ON CARBON DEPOSITIONS

The promoter effect of alkali metals on CO hydrogenation was studied on a series of silica supported iron catalysts. Mossbauer spectra of the catalysts show that iron is well dispersed on the silica support. The decrease in chemisorption of carbon monoxide on addition of promoters reveals that alkali covers part of the iron metal surface. The production of olefins, carbon dioxide and long chain hydrocarbons increases with decreasing ionization potentials of the alkali metals added. Hydrogenative decarbidation of catalysts after CO/H2 reaction show that the hydrogenation of the carbon depositions on iron catalysts can be well explained by a series reaction model. The relative reactivity of carbon depositions on well dispersed iron catalysts are strongly affected under the influences of alkali. Addition of alkali metals to iron catalyst may depress the hydrogenation ability of the catalysts.

UV photoelectron and quantum mechanical characterization of DNA and RNA bases: valence electronic structures of adenine, 1,9-dimethyl-guanine, 1-methylcytosine, thymine and uracil

HeI UV photoelectron spectra of adenine, 1,9-dimethylguanine, 1-methylcytosine, thymine and uracil have been interpreted with the aid of results from ab initio SCF-MO calculations employing a double zeta 4–31G basis set, and HAM/3 semiempirical calculations. Ionization potentials from results of the SCF-MO calculations have been obtained through the application of Koopmans' theorem. For cytosine and guanine, problems associated with the occurrence of more than one tautomeric form in the gas phase have been circumvented by examining methyl substituted derivatives of these bases. For adenine, 1,9-dimethylguanine, thymine and uracil the current assignments, based on the results of the 4–31G SCF and HAM/3 calculations, are essentially the same as those provided in earlier studies. For 1-methylcytosine, the results of the current calculations and recent findings concerning the tautomerism of cytosine in nonpolar environments has led to new assignments. The vertical ionization potentials of the first (π 1), second (π 2), third (π 3) and fourth (π 4) highest occupied π orbitals in adenine are 8.48, ≈9.6, ≈10.5, and 12.10 eV, respectively. The ionization potentials of the first through third (n 1-n 3) lone-pair orbitals are ≈9.6, ≈10.5, and 11.39 eV. In 1,9-dimethylguanine, the π 1, π 4 and π 5 orbitals have ionization potentials of 8.09, ≈11.0, and 12.31 eV, while the π 2 and π 3 orbitals have ionization potentials in the range between 9.3 and 10.2 eV. Values for the ionization potentials of the n 1 and n 2 orbitals lie in the range between 9.3 and 10.2 eV, and the value for the n 3 orbital is ≈11.0 eV. In 1-methylcytosine, the ionization potentials of the π 1 through π 4 orbitals are 8.65, ≈9.7, 11.53, and 12.40 eV; the values for the n 1 and n 2 orbitals lie in the range 9.5 to 10.0 eV. For uracil, the ordering of the ionization potentials is π 1 (9.59 eV), n 1 (10.11 eV), π 2 (10.56 eV), n 2 (11.16 eV), and π 3 (12.63 eV). In thymine, the energetic ordering of orbitals is the same as in uracil and the corresponding ionization potentials have values of 9.18, 10.03, 10.39, 10.82 and 12.27 eV. An examination of DNA and RNA alkylation patterns occurring in reactions with dimethyl-sulfate, methylmethanesulfonate and N-methyl- N-nitrosourea indicates that the reactivity of nucleotide bases towards methylating reagents increases as the base ionization potentials decrease.

Mechanism of the oxidation of methylbenzenes by Pd(II)

1. The oxidation of methylbenzenes (toluene, p-xylene, mesitylene, durene) by palladium- (II) to diarylmethanes in CF3COOH proceeds according to a mechanism of one-electron transfer. In addition, there is an oxidation of arenes to diaryls according to the well-known two-electron mechanism, including electrophilic substitution in the ring of the arene. 2. The contribution of the one-electron mechanism increases with decreasing ionization potential of the arene and with increasing acidity of the medium. 3. Complexes of palladium(I) (probably dimers) are intermediates of the one-electron oxidation of methylbenzenes to diarylmethanes.

Single electron capture by Ar 3+ from rare-gas atoms

Translational energy spectra for single electron capture collisions of 9 keV Ar 3+ ions with He, Ne, Ar, Kr and Xe were measured, using a double-focusing mass spectrometer. The data show a clear trend that as target ionization potential decreases, the degree of excitation of the fast product ions increases. There is clear evidence of the presence of first and second metastable states in the primary ion beam. The cross-sections for single electron capture into particular states of Ar 2+, for the collision of Ar 3+ with He, are reported for the impact energy range 6–18 keV.

Equilibrium structures, proton affinities, and ionization potentials of the fluoroacetones

Theoretical calculations on the fluoroacetones and their conjugate acids exhibit an inverse dependence of the proton affinity on the number of fluorine substituents. The good correlation between increasing proton affinity and decreasing ionization potential is attributed to the observation that the ionization of a nonbonding electron is essentially localized at a single atom whieh is also the site of protonation. The best quantitative agreement between the theoretical and experimental proton affinities is obtained at the 6-31G* level, while the STO-3G basis set leads to a satisfactory account of the additivity of fluorine substituents. The calculations corroborate the experimental interpretation of the preferred conformations of p-fluoroacetone molecules in terms of the presence of an intramolecular hydrogen bond between the carbonyl proton and the F on the adjacent carbon. Only one type of equilibrium conformation is predicted for the fluorinated acetones, whereas two types of equilibrium conformations are predicted for the conjugate bases.

Rates of reaction of alkyl radicals with ozone

The rate constants for a series of alkyl radicals reacting with ozone have been measured at 298 K and 2 torr by using a photoionization mass spectrometer. The radical (R) + ozone rate constants (in units of 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/) are 2.53 +/- 0.54 for CH/sub 3/, 25.3 +/- 5.8 for C/sub 2/H/sub 5/, 24.4 +/- 5.9 for 1-C/sub 3/H/sub 7/, 46.5 +/- 10.6 for 2-C/sub 3/H/sub 7/, and 54.5 +/- 11.4 for t-C/sub 4/H/sub 9/. As was observed for the R + O/sub 2/ reactions the R + O/sub 3/ reactions show a trend of increasing rate constant k/sub 1/ with decreasing ionization potential (IP) of the radical. When log (k/sub 1/) vs. IP(R) - EA(O/sub 2/ or O/sub 3/) is plotted, where EA represents electron affinity, the two families of rate constants are brought into close proximity. In addition, rate constants for fluorescence quenching of substituted benzenes by O/sub 2/ are cited which extend the log (k) vs. IP-EA plot to lower ionization potential. Adiabatic channel model calculations have been done by using a long-range potential resulting from the perturbation between the R/sup +/ + O/sub 3//sup -/ and R + O/sub 3/more » surfaces. These calculations give good agreement with the experimental cross sections and they predict rate constants faster than those derived from the classical harpoon model at low IP-EA.« less

Supported alkali catalysts for steam gasification of carbonaceous residues from petroleum

Steam reforming of petroleum residues was investigated at 1173 K and atmospheric pressure in a fluidized-bed flow system using a series of alumina-supported catalysts of alkali metal compounds. The catalytic activity increased with decreasing ionization potential of the catalytic alkali metals: Cs >; K > Na > Li. Catalysis was elucidated in terms of an oxygen transfer mechanism proposed for the catalysis by alkali metal compounds in graphite-or char-steam reaction.

ChemInform Abstract: ARRHENIUS PARAMETERS FOR THE ALKOXY RADICAL DECOMPOSITION REACTIONS

Arrhenius parameters for the decomposition reactions of various alkoxy radicals are evaluated. The recommended values for tert-butoxy radical decomposition reaction are log A (s−1) = 14.1, Ea = 15.3 kcal/mol. The parameters give reasonably self-consistent results with related rate constants, product distributions of hydrocarbon oxidation reactions, and the values reported in different laboratories in different environments. A reworking of data by Batt et al. on RO + NO and RO + NO2 leads to a lowering of his A factors for these reactions by a factor of 5. It also leads to a similar lowering of the A factors for RO decompositions. The intrinsic activation energies E for the addition of alkyl radicals R to the carbonyl group are not only found to be a function of the exothermicity of reaction, but they are also dependent on the nucleophilicity of the alkyl radicals. For each radical R (Me, Et, i-Pr) the data can be fit to an Evans–Polanyi plot E = a + b (δH), where a uniform slope b = 0.58 can be found for all R. The intercepts a are then found to decrease nearly linearly with the decreasing ionization potential of R, namely, a = 2.1 [IP] – 6.2 (kcal/mol), with the ionization potential given in electron volts.

Arrhenius parameters for the alkoxy radical decomposition reactions

AbstractArrhenius parameters for the decomposition reactions of various alkoxy radicals are evaluated. The recommended values for tert‐butoxy radical decomposition reaction are log A (s−1) = 14.1, Ea = 15.3 kcal/mol. magnified image The parameters give reasonably self‐consistent results with related rate constants, product distributions of hydrocarbon oxidation reactions, and the values reported in different laboratories in different environments.A reworking of data by Batt et al. on RO + NO and RO + NO2 leads to a lowering of his A factors for these reactions by a factor of 5. It also leads to a similar lowering of the A factors for RO decompositions. The intrinsic activation energies E for the addition of alkyl radicals R to the carbonyl group are not only found to be a function of the exothermicity of reaction, but they are also dependent on the nucleophilicity of the alkyl radicals. For each radical R (Me, Et, i‐Pr) the data can be fit to an Evans–Polanyi plot E = a + b (δH), where a uniform slope b = 0.58 can be found for all R. The intercepts a are then found to decrease nearly linearly with the decreasing ionization potential of R, namely, a = 2.1 [IP] – 6.2 (kcal/mol), with the ionization potential given in electron volts.

Electron transfer energies and dipole moments of alkyl halides and amines from an electrostatic model

Energies of the electron transfer reaction CH3X+RX+→CH3X++RX and dipole moments of molecules RX, where R is an alkyl group and X = Cl, Br, I, or NH2, are calculated for an electrostatic model in which each atom is characterized by a fixed charge and polarizability located at the position of the nucleus. The calculation takes into account the mutual polarization of the atoms in the molecular field. Atom polarizabilities are obtained from previous applications of the atom–dipole interaction model to molecular polarizabilities. Atom charges are assumed to be nonzero only for X and Cα, and these are optimized to fit 13 experimental electron transfer energies from first ionization potentials and 17 gas-phase dipole moments for selected alkyl compounds. Energies of 19 transfer reactions and dipole moments of 29 molecules are calculated, showing satisfactory overall agreement with experiment. The results suggest that the electrostatic interaction of the fixed charges with the polarizable atoms is responsible for the trend toward decreasing ionization potential with increasing size of the R group.

ChemInform Abstract: A MOLECULAR ORBITAL STUDY OF SILYLATED CARBENES

The geometry and electronic properties of methylene, methylcarbene, carboxycarbene, phenylcarbene, trimethylsilylcarbene, silylcarbene, carboxysilylcarbene, and phenylsilylcarbene have been determined by MINDO/3 calculation. Introduction of a silyl group results in considerable decrease in ionization potential of carbenes. The electron affinity is increased to some extent by such substitution. The computation suggests a bent geometry for optimized triplet trimethylsilylcarbene structure, in conflict with a recent postulate that the ground-state molecule has an essentially linear structure.

A Molecular Orbital Study of Silylated Carbenes

Abstract The geometry and electronic properties of methylene, methylcarbene, carboxycarbene, phenylcarbene, trimethylsilylcarbene, silylcarbene, carboxysilylcarbene, and phenylsilylcarbene have been determined by MINDO/3 calculation. Introduction of a silyl group results in considerable decrease in ionization potential of carbenes. The electron affinity is increased to some extent by such substitution. The computation suggests a bent geometry for optimized triplet trimethylsilylcarbene structure, in conflict with a recent postulate that the ground-state molecule has an essentially linear structure.

Correlation between selectivity in photochemical cross-cycloaddition and ionization potential of the reacting olefins

The selectivity (the ratio of crossadducts to homodimers) in the photochemical [2 + 2] cross-cyclo-addition of a 1 : 1 mixture of several series of (E)-olefins (1) and methyl p-nitro-(E)-cinnamate (2) was found to increase regularly with decreasing ionization potential of (1).

Photoelectron spectra of substituted oxiranes and thiiranes. Substituent effects on ionization potentials involving σ orbitals

The state assignments corresponding to the second and third ionization potentials of oxirane have been the subject of some uncertainty due to the great sensitivity of Koopmans' theorem predictions to the type of calculation performed for this molecule. In this study, the assignments made by Basch et al. for oxirane are confirmed through measurement of the photoelectron spectra of methyl-, 1,1-dimethyl-, 1,2-dimethyl, ethyl-, vinyl-, phenyl-, chloromethyl-, and fluoromethyloxirane. The second and third ionizations of thiirane are confirmed to be of the opposite order from those in oxirane by correlations of these values with those reported here for methyl-, vinyl, and methoxymethylthiirane.In these compounds, mono-substitution by alkyl groups causes a decrease in ionization potential not obviously related in a simple way to the type of orbital involved. Thus, assignments cannot be made straightforwardly by observing ionization potential changes caused by hyper-conjugating or inductive substituents. Alkyl group substitution in both oxirane and thiirane lowers IP's in the same order: a2 > b2 > a1 > b1. The effects of alkyl and heteroalkyl groups on the four lowest IP's of oxirane are linearly related to the electronegativities of the groups, and to the influence of alkyl and heteroalkyl substituents on the π IP of ethylene.Comparisons of the IP's of ethylheteranes and dimethylheteranes are quite useful in determining the site of localization of orbital density in the various orbitals.Conjugating substituents, which for symmetry reasons, selectively interact with only a few orbitals make definite assignments possible. Comparisons of experimental ionization potential changes and those predicted by Koopmans' theorem using ab initio STO-3G calculations are in good agreement.