A study of the properties of hydrogen films on tungsten by the method of contact potentials

1937 ◽  
Vol 33 (3) ◽  
pp. 394-402 ◽  
Author(s):  
R. C. L. Bosworth
Keyword(s):  
Dipole Moment ◽  
Hydrogen Atom ◽  
Gas Phase ◽  
Atomic Hydrogen ◽  
The Other ◽  
Condensation Coefficient ◽  
Effective Dipole Moment ◽  
Tungsten Atom ◽  
Contact Potential ◽  
Hydrogen Molecules

In a study of the properties of hydrogen on tungsten by the method of contact potentials the following points have been established:(1) The contact potential of a 92% covered surface of hydrogen on tungsten against bare tungsten is 1·04 V., and the Richardson constants for such a surface are, approximately,A = 30, b = 5·60 V.(2) A film of deuterium is 20 m V. positive relative to a similar hydrogen film.(3) Over the range of temperatures and pressures used by Bryce in a study of the production of atomic hydrogen the films are nearly saturated, so that the production of atomic hydrogen is primarily due to a molecule striking a bare tungsten atom, one atom being adsorbed and the other going into the gas phase.(4)The condensation coefficient for hydrogen molecules on cold tungsten is 0·01.(5) The effective dipole moment of each hydrogen atom on the surface is −0·42 Debye unit and is independent of the fraction of the surface covered.

The mechanism of the production of atomic hydrogen by hot tungsten

1936 ◽  
Vol 32 (4) ◽  
pp. 653-656 ◽  
Author(s):  
J. K. Roberts ◽  
G. Bryce
Keyword(s):  
Temperature Variation ◽  
Atomic Hydrogen ◽  
Hydrogen Molecule ◽  
Hydrogen Pressure ◽  
The Other ◽  
Square Root ◽  
Important Process ◽  
Experimental Conditions ◽  
Tungsten Atom ◽  
Tungsten Surface

The fact that the rate of production of atomic hydrogen at a tungsten surface at a given temperature is proportional to the square root of the hydrogen pressure means either that the important process is the evaporation of atoms from an adsorbed film which over the whole range of experimental conditions is sparsely occupied, or that the production of atoms is in the main due to a process in which a hydrogen molecule strikes a bare tungsten atom in the surface, one atom being adsorbed and the other evaporating and the surface being almost completely covered over the whole range of experimental conditions. Either process leads to a temperature variation in the rate of atom production in agreement with experiment. A definite decision between the two processes cannot yet be made.

Activity Comparison of MCM-41 and V-MCM-4 Catalysts for Ethanol Selective Oxidation and DRIFTS Analysis

2009 ◽  
Vol 7 (1) ◽  
Author(s):  
Yesim Gucbilmez ◽  
Timur Dogu ◽  
Suna Balci
Keyword(s):  
Hydrogen Atom ◽  
Gas Phase ◽  
Selective Oxidation ◽  
The Other ◽  
Lattice Oxygen ◽  
Molar Ratio ◽  
Surface Species ◽  
Other Hand ◽  
Activity Comparison ◽  
Mcm 41

The activity of a V-MCM-41 catalyst with a V/Si molar ratio of 0.04 was compared with the activity of a pure MCM-41 catalyst and some DRIFTS studies were performed in order to understand the mechanism of selective oxidation of ethanol. V-MCM-41 was found to be much more active than pure MCM-41, favouring the formation of high amounts of ethylene over 300°C. MCM-41, on the other hand, was found to favour the formation of acetaldehyde over ethylene at all studied temperatures. It was shown by reaction findings that over V-MCM-41, ethylene was produced mainly in the presence of gas phase oxygen whereas acetaldehyde was produced using the lattice oxygen even in the absence of gas phase oxygen. DRIFTS studies indicated the formation of acetaldehyde molecules from acetate-like surface species which were formed by the removal of a hydrogen atom from the ?-carbon of chemisorbed ethoxy species by the help of the lattice oxygen. On the other hand, removal of a hydrogen atom from the ?-carbon of the ethoxy species, in the presence of gas phase oxygen, resulted in the formation of vinyl-like surface species which then yielded ethylene molecules.

Article

1998 ◽  
Vol 76 (2) ◽  
pp. 153-162
Author(s):  
P Kurunczi ◽  
K Becker ◽  
K Martus
Keyword(s):  
Hydrogen Atom ◽  
Organic Compounds ◽  
Cross Sections ◽  
Impact Energy ◽  
Atomic Hydrogen ◽  
Vacuum Ultraviolet ◽  
The Other ◽  
Dissociative Excitation ◽  
Balmer Series ◽  
Break Up

The vacuum ultraviolet (VUV) emissions of the hydrogen Lyman series following the dissociative excitation of the Si-organic compounds tetraethoxysilane (TEOS), tetramethylsilane (TMS), and hexamethyldisiloxane (HMDSO) by controlled electron impact were analyzed. Absolute photoemission cross sections from threshold to 200 eV and appearance potentials were determined for the Lyman-α and Lyman-β emissions for each of the three targets. The measured apparent emission cross sections at 100 eV impact energy for the Lyman-α lines are 6.0 x 10-19 cm2 for TMS, 6.5 x 10-19 cm2 for HMDSO and 1.1 x 10-18 cm2 for TEOS. The apparent Lyman-α cross sections for all three targets contained significant cascade contributions from the H Balmer series. Detailed studies of the near threshold regions indicated that several break-up mechanisms of the parent molecules contribute to the emissions. The lowest onsets for HMDSO, and TEOS were attributed to the removal of a single excited atomic hydrogen atom in the n=2 state. Additional onsets at higher energies could not be uniquely correlated in all cases with a particular break-up channel and (or) cascading, since the other fragments in these processes were not identified. The Lyman-β apparent emission cross sections were also measured and we found values of 1.3 x 10-19 cm2 for TMS, 1.6 x 10-19 cm2 for HMDSO, and 2.0 x 10-19 cm2 for TEOS at an impact energy of 100 eV. PACS Nos. 52.20Fs and 34.80Gs

scholarly journals The reaction of atomic hydrogen with hydrazine

1940 ◽  
Vol 175 (961) ◽  
pp. 164-186 ◽  
Keyword(s):  
Hydrogen Atom ◽  
Gas Phase ◽  
Atomic Hydrogen ◽  
Ammonia Molecule ◽  
Para Hydrogen ◽  
Hydrogen Atoms ◽  
Stationary Concentration ◽  
Essential Step ◽  
Photochemical Decomposition ◽  
The Subject

The reason for studying the reaction of hydrogen atoms with hydrazine is that a controversy has arisen in attempting to elucidate the mechanism of the photochemical decomposition of ammonia. It has been generally agreed that the ammonia molecule is decomposed to a hydrogen atom and an amine radical when it absorbs light around 2000° A. Presuming that the atomic hydrogen combines on the walls or in the gas phase it is possible to calculate what its stationary concentration ought to be under any given set of conditions. If, however, the stationary concentration is actually measured by using para-hydrogen as a detector, as was done by Farkas and Harteck (1934), it is found that the measured value is lower than the value calculated from the above assumptions. A number of suggestions, discussed in detail in the following paper, were made to explain this discrepancy, and among the most reasonable was that of Mund and van Tiggelen (1937) who suggested that the hydrazine known to be formed in the system removed such atoms more rapidly than would occur in the ordinary course of events. The result of their suggestion was the invention of elaborate schemes to explain the mechanism of the ammonia photolysis. As a further essential step in the ammonia problem it therefore seemed necessary to measure the efficiency of the reaction between hydrogen atoms and hydrazine. At the same time further information was also desirable about the photochemistry of hydrazine itself. This paper will therefore be concerned with this aspect of the subject. The results will then be discussed in the following paper together with a number of new experiments on ammonia in order that the mechanism of the ammonia reaction may be more fully established.

scholarly journals Structural and vibrational studies of equilenin, equilin and estrone steroids

2019 ◽  
Vol 9 (6) ◽  
pp. 4502-4516
Keyword(s):  
Dipole Moment ◽  
Gas Phase ◽  
Molecular Structures ◽  
The Other ◽  
Dihedral Angles ◽  
Vibration Modes ◽  
Frontier Orbital ◽  
Harmonic Force ◽  
Vibrational Studies ◽  
First Time

Three species associated with estrogens have been studied in this work, equilenin, equilin and estrone. Their molecular structures have been theoretically studied in gas phase with the hybrid B3LYP/6-31G* method. NBO, AIM and frontier orbital calculations were computed for the three species at the same level of theory. Higher dipole moment and volume were observed for estrone while equilin presents higher volume than equilenin but lower dipole moment value. Probably, the unsaturated C=C in the B ring of equilin could explain those differences. The differences observed in the properties could be clearly explained by differences in the dihedral angles. The analyses of MK and Mulliken charges evidence the higher variations on the C atoms common to the B, C and D rings in the three species. The mapped MEP surfaces show that both A and B rings of equilenin are different from the other ones because they have aromatic naphthalene core, as was evidenced experimentally. The NBO studies support the higher stability of equilin, in relation to equilenin and estrone while the AIM analyses reveal the higher stability for estrone. The gap values suggest that equilenin is the most reactive species due to its higher global electrophilicity value, in agreement with the higher stability observed for this species while the higher global nucleophilicty values are observed for equilin and estrone. Here, the harmonic force fields, scaled force constants and the complete assignments of 108, 114 and 120 vibration modes for equilenin, equilin and estrone, respectively are reported for first time.

scholarly journals Investigating the behaviors of corticosterone hormone in different solvents by using DFT calculations and experimental data

2019 ◽  
Vol 10 (1) ◽  
pp. 4876-4892
Keyword(s):  
Aqueous Solution ◽  
Dipole Moment ◽  
Dft Calculations ◽  
Gas Phase ◽  
Solvation Energy ◽  
The Other ◽  
Side Chain ◽  
Three Solutions ◽  
Vibrational Assignments ◽  
Methanol Solutions

In this work, structural, electronic, topological and vibrational properties of corticosterone hormone have been investigated in aqueous, ethanol and methanol solutions by using DFT calculations and experimental available infrared, attenuated total reflectance (ATR), Raman and Ultraviolet spectra. The properties predicted in the different solvents at the B3LYP/6-31G* level of theory were compared with those obtained in gas phase and, with others reported for steroids species at the same level of theory. The universal solvation model has evidenced higher solvation energy for corticosterone in aqueous solution and a higher value in methanol, as compared with the corresponding values to equilenin, equilin and estrone steroids in the same medium. Higher Mulliken charges on O atoms of C=O group of side chain are observed in the three solvents than the corresponding to C=O group of ring A while the MK charges on O atoms of OH group of ring C present higher values than the corresponding to O atoms of OH group of side chain. The natural bond orbital (NBO) studies have revealed a low stability of corticosterone in aqueous solution, as compared with the values in ethanol and methanol solutions, in total agreement with the higher solvation energy and dipole moment in this medium. On the other hand, the atoms in molecules (AIM) analyses support the lower stabilities of corticosterone in the three solutions because only five H bonds interactions different from of gas phase where six interactions are observed. The gap values suggests that corticosterone is most reactive in aqueous solution than the other solutions, as supported by the low stability and higher solvation energy and dipole moment values in this medium. This study shows clearly that the steroid species most reactive, equilenin and corticosterone, are characterized by a high global electrophilicity index value and low nucleophilicity index. Reasonable correlations in the predicted IR, Raman and UV spectra were observed, as compared with the corresponding experimental ones. Additionally, the complete vibrational assignments of all 159-vibration modes of corticosterone together with the harmonic force fields and force constants in the different media are for the first time presented.

A polycentric growth model of gallium arsenide epitaxial layers from the gas phase with simultaneous diffusion, adsorption and chemical reaction

1988 ◽  
Vol 53 (12) ◽  
pp. 2995-3013
Author(s):  
Emerich Erdös ◽  
Jindřich Leitner ◽  
Petr Voňka ◽  
Josef Stejskal ◽  
Přemysl Klíma
Keyword(s):  
Growth Rate ◽  
Gallium Arsenide ◽  
Epitaxial Growth ◽  
Gas Phase ◽  
Surface Reaction ◽  
Quantitative Description ◽  
The Other ◽  
Rate Equations ◽  
Impact Interaction ◽  
Partial Pressures

For a quantitative description of the epitaxial growth rate of gallium arsenide, two models are proposed including two rate controlling steps, namely the diffusion of components in the gas phase and the surface reaction. In the models considered, the surface reaction involves a reaction triple - or quadruple centre. In both models three mechanisms are considered which differ one from the other by different adsorption - and impact interaction of reacting particles. In every of the six cases, the pertinent rate equations were derived, and the models have been confronted with the experimentally found dependences of the growth rate on partial pressures of components in the feed. The results are discussed with regard to the plausibility of individual mechanisms and of both models, and also with respect to their applicability and the direction of further investigations.

2-Deoxyribose Radicals in the Gas Phase and Aqueous Solution. Transient Intermediates of Hydrogen Atom Abstraction from 2-Deoxyribofuranose

2005 ◽  
Vol 70 (11) ◽  
pp. 1769-1786 ◽  
Author(s):  
Luc A. Vannier ◽  
Chunxiang Yao ◽  
František Tureček
Keyword(s):  
Aqueous Solution ◽  
Hydrogen Atom ◽  
Gas Phase ◽  
Computational Study ◽  
Hydrogen Atom Abstraction ◽  
Oh Radical ◽  
Free Energies ◽  
Intramolecular Hydrogen ◽  
Solvation Free Energies ◽  
Transient Intermediates

A computational study at correlated levels of theory is reported to address the structures and energetics of transient radicals produced by hydrogen atom abstraction from C-1, C-2, C-3, C-4, C-5, O-1, O-3, and O-5 positions in 2-deoxyribofuranose in the gas phase and in aqueous solution. In general, the carbon-centered radicals are found to be thermodynamically and kinetically more stable than the oxygen-centered ones. The most stable gas-phase radical, 2-deoxyribofuranos-5-yl (5), is produced by H-atom abstraction from C-5 and stabilized by an intramolecular hydrogen bond between the O-5 hydroxy group and O-1. The order of radical stabilities is altered in aqueous solution due to different solvation free energies. These prefer conformers that lack intramolecular hydrogen bonds and expose O-H bonds to the solvent. Carbon-centered deoxyribose radicals can undergo competitive dissociations by loss of H atoms, OH radical, or by ring cleavages that all require threshold dissociation or transition state energies >100 kJ mol-1. This points to largely non-specific dissociations of 2-deoxyribose radicals when produced by exothermic hydrogen atom abstraction from the saccharide molecule. Oxygen-centered 2-deoxyribose radicals show only marginal thermodynamic and kinetic stability and are expected to readily fragment upon formation.

scholarly journals Preparation and the Microwave and Infrared Spectra of Vinyl Ketene

1979 ◽  
Vol 34 (11) ◽  
pp. 1269-1274 ◽  
Author(s):  
Erik Bjarnov
Keyword(s):  
Dipole Moment ◽  
Gas Phase ◽  
Infrared Spectra ◽  
Room Temperature ◽  
Normal Modes ◽  
Spectroscopic Constants ◽  
Electrical Dipole ◽  
Electrical Dipole Moment ◽  
Vacuum Pyrolysis ◽  
Modes Of Vibration

Vinyl ketene (1,3-butadiene-1-one) has been synthesized by vacuum pyrolysis of 3-butenoic 2-butenoic anhydride. The microwave and infrared spectra of vinyl ketene in the gas phase at room temperature have been studied. The trans-rotamer has been identified, and the spectroscopic constants were found to be Ã= 39571(48) MHz, B̃ = 2392.9252(28) MHz, C̃ = 2256.0089(28) MHz, ⊿j = 0.414(31) kHz, and ⊿JK = - 34.694(92) kHz. The electrical dipole moment was found to be 0.987(23) D with μa = 0.865(14) D and μb = 0.475(41) D. A tentative assignment has been made for 17 of the 21 normal modes of vibration

Methylene. III. Gas phase reactions with 1-chloropropane

1969 ◽  
Vol 312 (1509) ◽  
pp. 141-161 ◽  
Keyword(s):  
Gas Phase ◽  
Rate Constants ◽  
Hydrogen Abstraction ◽  
The Other ◽  
Gas Phase Reactions ◽  
Other Hand ◽  
Chlorine Substituent ◽  
High Degree ◽  
Singlet Methylene

The work described in this and the following paper is a continuation of that in parts I and II, devoted to elucidation of the mechanism of the reactions of methylene with chloroalkanes, with particular reference to the reactivities of singlet and triplet methylene in abstraction and insertion processes. The products of the reaction between methylene, prepared by the photolysis of ketene, and 1-chloropropane have been identified and estimated and their dependence on reactant pressures, photolysing wavelength and presence of foreign gases (oxygen and carbon mon­oxide) has been investigated. Both insertion and abstraction mechanisms contribute significantly to the over-all reaction, insertion being relatively much more important than with chloroethane. This type of process appears to be confined to singlet methylene. If, as seems likely, there is no insertion into C—Cl bonds under our conditions (see part IV), insertion into C2—H and C3—H bonds occurs in statistical ratio, approximately. On the other hand, the chlorine substituent reduces the probability of insertion into C—H bonds in its vicinity. As in the chloroethane system, both species of methylene show a high degree of selectivity in their abstraction reactions. We find that k S Cl / k S H >7.7, k T Cl / k T H < 0.14, where the k ’s are rate constants for abstraction, and the super- and subscripts indicate the species of methylene and the type of atom abstracted, respectively. Triplet methylene is discriminating in hydrogen abstraction from 1-C 3 H 7 Cl, the overall rates for atoms attached to C1, C2, C3 being in the ratios 2.63:1:0.

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