bluuebunny/crossref_metadata_2025_split huggingface dataset:
Study of the Electronic Structure of Molecules. I. Molecular Wavefunctions and Their Analysis
Authors: Clementi Enrico 
Published: January 2005

In this paper we have summarized the complete multiconfiguration self-consistent-field theory (CMC SCF LCAO MO or CMC for short) as given from the analysis of Veillard and the author (published elsewhere).
In the analysis of the CMC theory it is shown that the CMC technique allows determination of the pair and pair—pair correlation energy and retains the physical simplicity of the Hartree—Fock functions. The CMC theory differs from usual many-body techniques in so far as it does not take the Hartree—Fock energy as zero-order energy. It has been shown that the CMC wavefunction can be cast in the form of a new type of ``single determinant'' (the ``CMC determinant'') with elements which are different from the standard elements of the Slater determinant. The existence of the CMC determinant provides the formal justification for the use of a single determinant in semiempirical work aiming at exact prediction.
The analysis of the CMC wavefunctions is then continued in terms of a population analysis which to a large extent follows Mulliken's work. Then the energy expression of the SCF MO and CMC techniques are analyzed in terms of bond-energy diagrams. These are special partitions of the total energy with immediate reference to structural chemistry. Finally, the bond-energy diagrams are briefly discussed in their relation to bond-energy transferability, vibrational analysis, and other chemical phenomena. This paper introduces both the notation and the terminology we shall use in describing all-electron computations for the NH3, HCl reaction surfaces, and for pyrrole, pyridine, pyrazine, fluorobenzene, benzene, and a few other molecules presented in the following papers of this series.
Temperature Dependence of Singlet—Triplet Intersystem Crossing in Anthracene Crystals
Authors: Adolph J. , Williams D. F. 
Published: January 2005

The formation of triplet excitons from singlet excitons via a radiationless transition (intersystem crossing) is studied in anthracene crystals. Using blue exciting light with 430 mμ&gt;λ&gt;350 mμ the temperature dependence of the intersystem crossing is found to follow an exponential law with an activation energy of 800±50 cm−1.
It is concluded that this transition starts from a vibrationally excited single state and involves the second triplet state. The total energy of this vibrational state is about 26 100 cm−1 in good agreement with that suggested by Kellogg, 26 050 cm−1, for an excited triplet level.
No emission from the triplet—triplet transition could be observed.
Vacuum-Ultraviolet Photolysis of Solid Propane at 20° and 77°K
Authors: Rebbert R. E. , Ausloos P. 
Published: January 2005

The photolysis of CD3CH2CD3, CH3CD2CH3, and C3H8—C3D8 mixtures has been investigated at 1470 Å (8.4 eV), 1236 Å (10 eV), and 1048–1067 Å (11.6–11.8 eV) at both 20° and 77°K. On the basis of the isotopic analysis of the products it is concluded that the following primary fragmentation processes occur: C3H8+hv→C3H7+H→C3H6+H2→C2H6+CH2→C2H5+CH3→C2H4+CH4.Hydrogen, methane, and ethane are mainly formed by elimination processes from a single carbon atom which results in the simultaneous formation of a carbene in each case. In this respect the primary processes occurring in the solid phase are the same as those reported to occur in the gas phase. The elucidation of the primary processes in the condensed phase is facilitated because of the inhibition of secondary decomposition of the internally excited fragments formed in Processes A to E. For instance, the ethylidene and propylidene rearrange quantitatively to ethylene and propylene, respectively.
The occurrence of geminate disproportionation and combination leads to difficulties in determining the relative quantum yields of the primary processes A and D. It could, however, be concluded on the basis of isotopic analyses that the C–H and C–C cleavage processes, A and D, increase in importance when the energy of the incident photon is increased.
Although propane ions, which have an ionization energy of 11.07 eV in the gas phase, must be produced at 1048–1067 Å, no concrete information could be obtained as to the fate of these ions in the solid phase. The data, however, show that decomposition of the superexcited molecules may be the major source of the hydrocarbon products formed at these wavelengths.
Vaporization of Dimolybdenum Carbide
Authors: Fries R. J. 
Published: January 2005

The vaporization behavior of Mo2C was investigated over the temperature range 2125° to 2550°K by Langmuir experiments utilizing an induction heated, ultrahigh vacuum thermobalance. A congruently vaporizing composition of MoC0.49 was found at temperatures from 2250° to 2300°C. The total evaporation rate increases and the heat of evaporation decreases as the carbon-to-metal ratio of the sample decreases from 0.49 to 0.44.
A mass-spectrometric examination of the evaporate from MoC0.49 did not reveal any MoxCy molecules. Therefore, evaporation by decomposition is assumed. If monatomic gaseous species and unit evaporation coefficients are also assumed, the pressures over MoC0.49 are logPMo(atm)=7.112±0.139−(3.420±0.033) (104/T),logPC(atm)=6.351±0.139−(3.420±0.033) (104/T).
The errors quoted are standard deviations. The heat of formation at 298°K of MoC0.49 is −4.9 kcal/mole from third-law calculations, in good agreement with the −5.5 kcal measured calorimetrically.
Theoretical Investigations of Translation—Rotation Energy Transfer: (H2, He) and (D2, He) Systems
Authors: Raff L. M. 
Published: January 2005

The classical analysis of translation—rotation energy transfer in (H2, He) and (D2, He) systems is reported. The accurate, a priori potential surface obtained by Krauss and Mies has been employed in a generalized, three-body Monte Carlo calculation to obtain, as a function of temperature, relaxation times and collision numbers for the 0→2 and the 2→0 rotational transitions in both H2 and D2. A negative dependence upon temperature is obtained in each case. The relaxation times for the 0→2 transition in H2 range from 7.725×10−8 sec at 100°K to 3.491×10−8 sec at 700°K. For the 2→0 deexcitation the corresponding values are 9.00×10−8 and 4.30×10−8 sec. When a van der Waals attraction term, whose magnitude is estimated from PVT data, is added to the Krauss—Mies surface, the values for the 0→2 transition decrease to 3.1×10−8 sec at 100°K and 2.0×10−8 sec at 700°K. Corresponding decreases occur for the 2→0 transition. Calculated relaxation times for D2 are 50%—100% smaller than those obtained for H2. These results are in satisfactory agreement with available experimental data and appear to be more accurate than previous theoretical results.
The energy-transfer mechanism has been investigated and found to be smooth and continuous with transfer occurring in about 10−13 sec. The most favorable approach angle for transfer is found to be about 60°.
Two other empirical potential surfaces have been employed to estimate the effect of potential well depth and the assumption of pairwise additivity of molecular potentials. It is found that as the well depth increases, the relaxation time decreases in a manner previously predicted by Parker. The assumption of pairwise additivity is found to yield results for the relaxation times which are low by about a factor of 2. The magnitude of this error, coupled with calculations employing off-center, pairwise potentials, indicates that the repulsive centers in H2 are located about 0.16 Å from the nuclei.
LEED Studies of the Adsorption Systems W(100) +N2 and W(100) +N2 +CO
Authors: Estrup P. J. , Anderson J. 
Published: January 2005

Low-energy electron-diffraction (LEED) techniques have been used to study the room-temperature interaction of nitrogen with a tungsten (100) surface. Adsorption to saturation results in the formation of a c(2×2)—N structure in which half of the surface sites are occupied, and coverage determinations show the adsorption to be atomic. The complete c(2×2) structure lowers the work function of the surface by 0.65 eV. Rapid desorption occurs at temperatures above ∼1200°K and gives only a single β peak.
With simultaneous adsorption of N2 and CO the adsorbed species interact to give a series of LEED patterns, none of which can be formed by the individual gases. CO adsorption on the c(2×2)—N structure causes the ½½ diffraction spots to disappear and raises the work function to +0.30 eV above that of the clean surface. Subsequent heating of the saturated surface produces a ¼-order LEED pattern. From these results it is concluded that the interaction of CO gives rise to a new binding state β* for nitrogen.
Theory of Collision-Broadened Ion Cyclotron Resonance Spectra
Authors: Beauchamp J. L. 
Published: January 2005

A formalism for calculating collision-broadened ion cyclotron resonance spectra is developed in detail. The Boltzmann transport equation describing the time variation of the arbitrary function of ion velocity X (v) is extended to include resonant charge transfer. Then with X (v) =v and X (v) =v2, equations describing, respectively, the velocity and energy of an average particle are developed. The solution of these equations is used to obtain the power absorption of the ions from the observing radio-frequency electric field.
The effects of collisions are treated in terms of specific ion—molecule interactions. The equations of motion for a mixture of ions, A and B, are coupled by the collision term which accounts for the charge-transfer process A++B⇄A+B+. As numerical examples the collision frequencies are evaluated for the rare gases and used to calculate absorption spectra for Ar+ in Ar and mixtures of He and Ne isotopes. The formalism permits the straightforward examination of the effects of electric-field strength, pressure, and temperature on the line shapes.
Optical Spectra of Orientationally Disordered Crystals. I. Theory for Translational Lattice Vibrations
Authors: Whalley E. , Bertie J. E. 
Published: January 2005

There is a class of crystals in which the molecules are arranged on or near regular positions in space, but are irregularly oriented. The irregular orientation of the molecules frequently perturbs only slightly the translational lattice vibrations of the crystal, that would be perfectly periodic if the irregular orientation were absent. For a first approximation, then, the crystal can be considered to have mechanically regular vibrations. In particular, any normal vibration can be described in terms of a wave vector k that is the wave vector of the dominant Fourier component of the vibration. The irregular orientation of the molecules will cause the dipole-moment change due to a displacement of a particular molecule to depend in magnitude and direction upon its own orientation and upon the orientation of its neighbors. The crystal is thus in a sense electrically irregular.
The theory of the optical spectra due to the translational lattice vibrations of these mechanically regular, electrically irregular crystals is discussed assuming that the translational and rotational vibrations are not coupled. All the vibrations are active in both infrared and Raman spectra, and to a first approximation, for a crystal with only one nearest-neighbor central interaction, the intensity of absorption or of Raman scattering by any particular normal vibration of the crystal that is active only because of the irregularity of the orientations (there may, of course, be vibrations that are active in the absence of electrical irregularity) is proportional to the square of its frequency. For more realistic models, the relationship is more complicated. The optical spectra of these crystals provides, therefore, information about the spectrum of lattice vibrations. In particular, features in the optical spectrum probably arise mainly from features in the vibrational spectrum, and it may be possible to identify corresponding features.
In addition, these crystals are intermediate between perfect crystals, which are mechanically and electrically regular, and vitreous solids, which are mechanically and electrically irregular. The study of their spectra should therefore provide an approach to understanding the spectra of vitreous phases.
Infrared Lattice Vibrations of KNiF3 from Reflection Data at 300° and 90°K
Authors: Balkanski M. , Moch P. , Teng M. K. 
Published: January 2005

Elementary group-theoretical considerations predict three infrared-active vibrational modes in KNiF3 which crystallizes with the cubic perovskite structure. Reflectivity measurements at 300° and 90°K, in the spectral range extending from 80 to 600 cm−1, allow the observation of the above three modes corresponding to the wavenumbers: 148, 244, 441 cm−1 for T=300°K and 144, 248, 452 cm−1 for T=90°K. In addition, we observe a side band near 500 cm−1 at 300°K and 520 cm−1 at 90°K.
Assuming that the dispersion can be interpreted in terms of independent damped oscillators, the reflection data have been analyzed using both the Kramers—Kronig relationships and the classical dispersion formula. The intensity of the extra band and some feature of the vibrational absorption spectrum in the visible range suggest that this extra band may arise from a one-phonon process.
The oscillator parameters associated with each observed mode and those deduced from the two methods of analysis are in good agreement with temperature-dependence considerations.
Relativistic Corrections to the Electron Density at the Nuclear Surface and to the Alkali Halide Overlap Integrals
Authors: Hafemeister D. W. 
Published: January 2005

The electron density at the nuclear surface and the alkali halide overlap integrals have been calculated with Hartree—Fock, Hartree—Fock—Slater, and the relativistic Dirac—Slater SCF wavefunctions. Both the full Slater exchange and ⅔ of it have been used.
The SCF relativistic enhancements of the electronic density at the nuclear surface are in good agreement with the enhancements predicted by Racah and by Rosenthal and Breit. The Slater exchange approximation slightly overestimates the density at small radii for valence electrons, but gives excellent results for core electrons.
The alkali halide overlap integrals are extremely dependent on the amount of Slater exchange used, thus implying that this approximation is less satisfactory for large radii. Our results indicate that the multiplicative factor modifying the Slater exchange should be between ⅔ and 1 for the calculation of overlap integrals. The overlap integrals for the heavier alkali halides are reduced by about 10% by relativistic effects.
Vibronic Coupling. V. Several Theorems Concerning the Eigenfunctions and Spectra of the Vibronic Oscillator
Authors: Fulton Robert L. 
Published: January 2005

Three theorems concerning the nature of vibronic coupling in a simple system are proven. These are that the eigenfunction of the lowest state of the negative system (a ``forbidden'' system in many dimers) is always of one sign, that the lowest eigenvalue of the negative system is lower than the lowest eigenvalue of the positive system (usually an allowed system in a dimer), and that the lowest eigenvalue of the negative system is lower than the lowest eigenvalue in the absence of coupling between the units comprising the dimer.
Some physical implications of these theorems concerning the radiative lifetimes of certain excited states are derived and discussed.
Origin of Rotational Barriers. I. Many-Electron Molecular Orbital Wavefunctions for Ethane, Methyl Alcohol, and Hydrogen Peroxide
Authors: Fink William H. , Allen Leland C. 
Published: January 2005

An investigation into the origin of rotation barriers has been carried out with the aid of LC(Hartree—Fock) AO MO SCF calculations on CH3CH3, CH3OH, and H2O2. The general shape and magnitudes of the barriers are reasonably well represented by the wavefunctions and one may conclude that the origin of rotational barriers is to be found within the framework of the Hartree—Fock approximation. It is also found that study of several molecules, particularly ones of low symmetry, is required for a detailed understanding of barriers.
The electronic energy has been decomposed into one- and two-electron parts. The relative phase and relative amplitude of these two components provide the best invariants available for analyzing the barrier mechanism. On the other hand the nuclear—nuclear repulsion is shown not to be an appropriate invariant. A physical picture of the barrier mechanism constructed from the electronic-energy decomposition depicts the barrier origin as a balance between the interactions of lone pairs, bonds, and the protons.
Reaction of O(1D) with Methane
Authors: DeMore W. B. , Raper O. F. 
Published: January 2005

The reaction of O(1D) with CH4 has been studied by photolysis (2537 Å) of O3–CH4 mixtures dissolved in liquid argon at 87°K. The quantum yields of O3 decomposition and the product yields based on O3 decomposed were measured. The products found were CH3OH, CH2O, and H2. Added O2 was found to lower the quantum yields of O3 decomposition and to increase the product yields. Experiments with added 18O2 were carried out to trace the origin of the products. At 6000 Å, which can only give O (3P), no photo-reaction could be detected.
The results indicate that the O(1D)—CH4 reaction has essentially zero activation energy and proceeds by three paths: O(1D)+CH4+M→CH3OH+M,O(1D)+CH4→CH2O+H2,O(1D)+CH4→CH3+OH.
The effect of added O2 is attributed to scavenging of methyl radicals which otherwise react with O3. The reaction of CH3 with O3 has zero Ea and occurs at a rate approximately equal to the reaction of CH3 with O2. The magnitude of the scavenging effect shows that at least two O3 molecules are decomposed by each CH3. By contrast, OH radicals do not react with O3 in these experiments, indicating a nonzero activation energy for the reaction OH+O3→HO2+O2.
Rings and Screws of Sulfur and Other Divalent Elements
Authors: Tuinstra F. 
Published: January 2005

A conformational study of all a priori possible molecular forms of divalent elements with restricted rotation has been carried out using the matrix formalism introduced by Eyring. The calculations are confined to those forms which have a single value of the bond distance, a single value of the bond angle β, and a single value of the dihedral angle γ.
The way in which the regular conformations are built up by repetition of a ``module'' (i.e., a definite sequence of right and left turns) may be of two types: (i) the helical conformations, for which a relation between the pitch and the order of the helix and the angles β and γ is derived, for any finite module, the special case of pitch zero leading to ``improper rings,'' where the values of β and γ form a discrete set; (ii) the proper rings, characterized by a certain kind of sequence of the matrices in the module, making the pitch zero, independently of the value of β (or γ). The closure of the ring requires a relation between β and γ, which may be written down for any finite module of this kind.
For every module the correspondence between the symmetry of the sequence of matrices and the symmetry of the molecule is given. The study is applied to sulfur especially.
Nonempirical Calculations of the Isotropic Hyperfine Splitting Constants of 13C, 14N, and 17O in Simple Radicals
Authors: Chung Alice L. H. 
Published: January 2005

The isotropic hyperfine splitting constants in ESR spectra for simple radicals 13C1H3, 14N1H2, and 17O1H have been calculated nonempirically by using a configuration-interaction method correct to the second order in the energy matrix elements. Included in the treatment are all the singly excited configurations plus certain types of doubly excited configurations. Each configuration is expressed in terms of the self-consistent-field molecular orbitals and virtual orbitals derived for the ground state. The molecular orbitals are expanded about a single center with an extended basis set of Slater-type orbitals.
The calculated hyperfine splitting constants for 13C, 14N, and 17O are found to be 2.7, 0.9, and −0.5 G, respectively, when all the virtual orbitals with symmetry species A1′ or A1 are used; the values are improved to 40.6, 19.0, and −35.4 G, respectively, when certain virtual orbitals with undesirable characteristics are excluded. Though not to be given serious consideration because of the nature of the wavefunctions employed, calculations for 1H are carried out and the values are found to range from −4 to −7 G.
Attention is called to the possible danger in using virtual orbitals indiscriminately for calculations on ground-state properties other than energy. Based on the experience gained from this work we suggest that only the orbitals that are better approximations to the excited-state molecular orbitals be used.
ESR and Optical Spectroscopy of ScO, YO, and LaO in Neon and Argon Matrices; Establishment of Their Ground Electronic States
Authors: Weltner W. , McLeod D. , Kasai P. H. 
Published: January 2005

ScO, YO, and LaO were vaporized from the solid oxides at temperatures near 2500°K and trapped in neon and argon matrices at 4° and 20°K. ESR spectra of these matrices show that the ground states of all of the molecules are 2Σ. The measured magnetic parameters for the three molecules are as follows: ScO, g=2.00±0.01, A=2.01±0.01 kMc/sec; YO, g=2.003±0.002, A=0.803±0.003 kMc/sec; LaO, g=2.01±0.01, A=3.89±0.01 kMc/sec. The A values then yield nuclear hyperfine-structure separations (Δν) of 0.27, 0.03, and 0.52 cm−1, respectively, and explain the large splittings observed in the optical spectra of ScO and LaO.
The optical spectra of the matrices exhibit the two transitions A2Π←X2Σ and B2Σ←X2Σ observed in the gas spectra of these molecules. The bands are exceptionally broad and shifted in the matrix relative to the gas, much more so than in the matrix spectra of other transition-metal diatomic oxides. Illumination of the matrices with tungsten light leads to a gradual disappearance of the absorption bands and loss of color. The ESR and infrared signals are similarly affected. An explanation of this bleaching phenomenon is offered which involves diffusion and reaction of excited M—O molecules in the matrix. New bands appear in the LaO—neon matrix during illumination and are tentatively attributed to LaO molecules in more ``gas-like'' sites.
Flow and Diffusion of Gases in Porous Media
Authors: Mason E. A. , Malinauskas A. P. , Evans R. B. 
Published: January 2005

A generalized treatment of gas transport in porous media is presented as developed on the basis of the ``dusty-gas'' model, a model in which a porous medium is described as consisting of uniformly distributed, giant molecules (dust) held stationary in space. The problem is broken down into a series of special cases which involve the various combinations of gradients in composition, pressure, and temperature.
Equations are given for the description of several well-known phenomena. These include isobaric, isothermal diffusion; diffusion under the influence of a pressure gradient; Poiseuille's flow equation, including the Knudsen minimum; the Kramers—Kistemaker effect; thermal transpiration; and the effect of pressure on the thermal-diffusion factor. The results are likewise applicable to capillaries by a suitable substitution for geometric parameters.
Nuclear Relaxation in CHFO
Authors: MacLean C. , Mackor E. L. , Hilbers C. W. 
Published: January 2005

The present paper deals with experiments on the nuclear relaxation of CHFO from −142° to −20°C. The relaxation is analyzed in terms of intra- and intermolecular interactions that induce the relaxation. The deuterated compound has likewise been investigated as the pure liquid and in dilute solution in hexadeuteroacetone.
Spin—rotation interaction is found to be the dominant relaxation mechanism of fluorine in the high-temperature region. At the lowest temperatures the relaxation is practically exclusively dipolar in origin; intramolecular dipolar interactions are dominant.
Quadrupolar relaxation of the deuterium nuclei manifests itself in the transverse relaxation rates of the fluorine spins.