bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Timing of the Relaxation from Selected Excited States; Beta-Naphthylamine
Authors: Schlag E. W. , Weyssenhoff H. v.
Published: February 2004

The fluorescence of β-naphthylamine vapor in the low-pressure region was timed directly using a high-speed laboratory light modulator. Emission was from defined, and variable quantum states initially pumped with high-intensity monochromatic light. The energy of irradiation was swept over a range of 10 000 cm−1, encompassing two excited electronic manifolds. The results then constitute a direct timed measurement of the relaxation from individual vibronic states as the energy of excitation is continuously increased. This is then a spectrum of vibronic lifetimes which is discussed in terms of various theories for electronic relaxation.
Defect Phosphorescence of Pyrazine Crystal
Authors: Azumi Tohru , Nakano Yasuko
Published: February 2004

The phosphorescence spectrum of crystalline pyrazine is observed in the temperature range of 1.6–15°K. The analysis of the fine structure, dependence on sample preparation, effect of zone refining, decay characteristics, and other data lead to the definite conclusion that the observed phosphorescence is coming from a number of crystalline defects. This is in contrast to the interpretation by Moomaw and El-Sayed who stated that the observed fine structures are due to the coupling with the ground-state phonons. A model for defect phosphorescence is set up. The observed temperature dependence as well as the decay behaviors are satisfactorily interpreted in terms of this model. Finally, an attempt is made to distinguish two types of crystal-line defects, the structural defects and impurity-induced resonance defects; the observations seem to be in favor of the latter.
General Model of Anharmonic Interaction Force Constants in Polyatomic Molecules
Authors: Machida Katsunosuke , Overend John
Published: February 2004

A model, previously postulated in a two-dimensional form and applied to the discussion of anharmonic stretch–stretch interaction force constants in triatomic molecules, has been extended to the polydimensional case. In this form it has been used to discuss the anharmonic interaction force constants coupling the stretching and bending coordinates. Numerical values of the model parameters have been evaluated for CO2 and HCN.
Charge Distribution and Nuclear Quadrupole Interactions in Ionic Crystals
Authors: Betsuyaku Hiroshi
Published: February 2004

A general expression for the electric field gradient (EFG) at the positive ion nucleus in an ionic crystal with polyatomic negative ions has been derived in the frameworks of the LCAO–MO approximation and the multipole expansion theorem of the electrostatic potential. The contribution to the field gradient in the present treatment is analyzed in terms of the multipole moments well defined in molecular orbital theory; the expression for the field gradient due to the atomic charges located on each atom of the polyatomic ion provides just what Bersohn derived on the assumption of the point-charge model, and the contribution due to higher multipole moments describes a deviation from it. The expression derived on the basis of this type of point-multipole model has been applied to the evaluation of the field gradient at the 23Na nucleus in a single crystal of sodium nitrite (NaNO2), and the results have been used to make clear the underlying assumptions and the limitations of the simple point-charge model. The conclusions derived from the present calculation are: (1) the field gradient in an ionic crystal depends too sensitively on the electron charge distribution of the polyatomic ions adjacent to the nucleus under study to be predicted by the point-multipole model, and so (2) the evaluation of the field gradient needs a careful treatment of the charge distribution of the closest ions including overlap and covalent bonding effects.
Direct Method of Calculating the Grüneisen Parameter γ Based on Shock-Wave Measurements of Metals
Authors: Huang Y. K.
Published: February 2004

On the basis of the well-established relationship U = A + Bu (U denoting shock velocity, u particle velocity, and A, B experimental constants), it is shown that the following explicit formula can be used for the evaluation of the Grüneisen parameter: γ = {U(2U − A)[A + (B − 1)U] − (13AU)(U − A) − BA3} / U(U − A)(U + A). This is a short cut of its kind. Although its resultant values of γ reflect some differences similar to those between the Slater and Dugdale–MacDonald schools, the present method of computation can lead to a satisfactory evaluation of the equation-of-state data. In addition, direct calculation of sound velocity behind the shock front turns out to exhibit an interconnection between static and dynamic compressibilities. Such a link should be of value for pursuit to gain basic understanding of high pressures.
Theory of the Vibrational Structure of Molecular Excitons. Soluble One-“Phonon” Models
Authors: Philpott Michael R.
Published: February 2004

Solutions of several models of vibronic coupling in molecular crystals are presented using the techniques of second quantization. In the first, the VEP model, vibronic exciton V transmutes into another vibronic exciton E by emitting a vibrational exciton P (the phonon). Stable V excitons are regarded as bound states of E and P. If Pauli commutators are replaced by Bose ones, the VEP Hamiltonian admits two constants of motion, and the Hilbert space breaks into sectors belonging to distinct eigenvalues of the constants. Weak and strong coupling correspond to stable and unstable V excitons, respectively. For the latter case, decay of V into E–P scattering states is briefly considered. Explicit formulas are given for the E–P scattering states for stable and unstable V. In the second model, interactions neglected in the first are crudely approximated by a separable pairing interaction. A new bound state of E and P results, illustrating the influence of the previously ignored terms in creating bound states. The third model is based on the Hamiltonian of Rashba in which electronic and vibrational excitation are treated separately. In this model, exciton and phonon numbers are constants and a solution exists if terms comparable to those neglected in the VEP model are ignored. When simplified the secular equation is found to have the same form as an earlier solution of the VEP model. Other avenues of investigation opened by the present work are discussed.
Mössbauer Studies of Bonding in Iron Porphyrin–Ligand Systems
Authors: Moss Thomas H. , Bearden Alan J. , Caughey Winslow S.
Published: February 2004

Mössbauer and electron paramagnetic resonance (EPR) spectra of several iron porphyrins have been obtained. The Mössbauer data for a series of high-spin pentacoordinated iron (III) compounds establish that both the electric field gradient and the zero-field splitting of the ionic Sz2 states follow the order fluoro<acetato<azido<chloro<bromo. The correlation of the two parameters can be qualitatively rationalized by considering the strong perturbing influence of the porphyrin ring on the relatively large high-spin ion. The implications of the data for recent detailed theoretical models of the iron porphyrins are discussed. In pyridine, chloroprotohemin displays a Mössbauer spectrum with unusual temperature dependence and quadrupole splitting like that observed for acid methemoglobin. This suggests that these features may arise because of stronger interactions between orbitals of the more nearly planar metal ion with the porphyrin ring in hexacoordinated iron porphyrins. The isomer shifts of the Fe (II) dipyridine porphyrins are nearly the same as those of the Fe (III) forms, indicating that very extensive electron delocalization from the iron takes place in the former case. The formal valence change does not, then, result in a large difference in charge localized on the iron.
Vibrational Energy Transfer at Gas–Solid Interfaces
Authors: Hunter T. F.
Published: February 2004

Linear semiclassical vibrational energy-transfer theory is applied to the interaction of vibrationally excited CO2, N2O and CH4 with solid NaCl and of N2 with glass. The model shows energy transfer to be slow during specular reflection but fast when the molecules are adsorbed and migrating over the surface. Values for the condensation coefficients are derived from the experimental energy-transfer probabilities, yielding 0.22, 0.37, and 0.88–1.0, for CO2, N2O, and CH4 on NaCl, respectively. The theory substantiates existing evidence that CH4 rotates freely about at least one axis on a NaCl surface, and suggests that this rotation is involved in the vibrational energy degradation. Vibrationally excited N2 on glass is deactivated mainly through interaction with surface OH groups.
Semiempirical, Pseudopotential Calculation of Alkali–Noble-Gas Interatomic Potentials
Authors: Baylis W. E.
Published: February 2004

A relatively simple semiempirical method is suggested for calculating interatomic potentials of ground and low-lying excited states of diatomic systems composed of an alkali atom and a ground-state noble-gas atom, and its applicability to other systems is discussed. Two types of interaction are included: an electrostatic interaction which yields the asymptotic R−6 behavior characteristic of van der Waals potentials, and a pseudointeraction which represents the effect of the Pauli exclusion principle on overlapping electron states. For the electrostatic interaction, the noble-gas atom is treated as a polarizable dipole with two parameters, namely a polarizability α and a “radius” r0. The value of r0 is chosen by adjusting the depth of the ground-state well to agree with recent scattering data. To determine the interatomic potentials, the total Hamiltonian including spin–orbit coupling is diagonalized in a limited basis set of atomic orbitals. Results are presented and compared with previous calculations and experiments.
Analysis of Electron Correlation in Two-Electron Systems. I. H−, He, and Li+
Authors: Banyard K. E. , Baker C. C.
Published: February 2004

Because they form the beginning of an isoelectronic series possessing electrons with antiparallel spins, H−, He, and Li+ lend themselves well to a study of electron correlation. The study is of three wavefunctions. Two introduce correlation: one by configuration interaction, the other by including Hylleraas-type correlation factors. For comparison, the third function is based on the Hartree–Fock approach. The correlation within the wavefunctions is demonstrated by presenting two-particle density difference maps, ΔD(r1; r2), relative to the uncorrelated approach, and graphs of the radial density D(r). Amongst the quantities calculated from the different treatments are 〈cosγ12〉, where γ12 is the angle subtended by the electrons at the nucleus, 〈r1·r2〉, the coherent x-ray scattering contribution f00(X), and 〈rn〉, where − 2 ≤ n ≤ 4. Results are compared throughout with those from a more accurate wavefunction. In addition, it was possible to study the effects of radial and angular correlation separately by producing the natural expansion for one of the correlated functions. The main conclusion is that as Z increases, angular correlation replaces radial correlation in being more important. Further, s-type correlation terms in the wavefunction cause the density to spread, whereas angular terms generally contract it. All such correlation effects tend to decrease with increasing Z.
Far-Infrared Spectra of Ring Compounds. IV. Spectra of Compounds with an Unsymmetrical Potential Function for Ring Inversion
Authors: Carreira L. A. , Lord R. C.
Published: February 2004

The infrared spectra of gaseous trimethylenimine, trimethylenimine-N-d, 2,5-dihydropyrrole, 2,5-dihydropyrrole-N-d, and cyclopentene oxide have been observed in the range 300–25 cm−1. The N–H bonds of trimethylenimine and 2,5-dihydropyrrole and the oxygen atom of cyclopentene oxide introduce asymmetry into the potential function for inversion of the ring in these molecules. In consequence the eigenvalues of the inversion vibrations are quite irregular but can be inferred from the spectra and fitted to a potential function of the form V(x) = ax4 − bx2 + cx3, where x is the ring-inversion coordinate. Interpretation of the far-infrared spectrum also enables assignment of the satellite fine structure of certain of the mid-infrared fundamentals as difference bands involving the inversion vibration.
Density Expansion of Frequency Dependent Transport Coefficients: Application to the Spin–Rotational Interaction and Other Relaxation Processes
Authors: Wang Chin Hsien , Oppenheim Irwin
Published: February 2004

The binary collision expansion technique is utilized to obtain low-density expressions for frequency dependent transport coefficients. In particular, the Laplace transform of a correlation function expressed in terms of the dynamical variable of the internal coordinates of polyatomic molecules interacting with angular dependent forces as well as with central forces is obtained. An application to the calculation of the spin–lattice relaxation time due to the spin–rotational interaction for diatomic molecules is treated in detail.
LCAO–MO–SCF Calculations Using Gaussian Basis Functions. III. Determination of Geometry by SCF Calculations, CF2
Authors: Sachs Lester M. , Geller Murray , Kaufman Joyce J.
Published: February 2004

Results are presented of an LCAO–MO–SCF investigation of the geometry of the CF2 molecule. It is shown that a well-chosen 582p Gaussian set is sufficient to determine reliably the geometry of triatomic molecules. A Walsh-type orbital diagram is given which shows a number of features which differ from the predictions of Walsh. The bonding in CF2 is discussed in terms of the Walsh diagram and the population analysis results of a calculation using a more extensive 985p basis set.
Systematic Trends in the Coupling Constants of Directly Bonded Nuclei
Authors: Jameson Cynthia J. , Gutowsky H. S.
Published: February 2004

The indirect coupling constant JXN has been observed for the magnetic nuclei in 50 different pairs of directly bonded X–N atoms. A synopsis is given of the reported values along with the corresponding reduced constant KXN = (2π / ℏγXγN)JXN which depends only on the molecular electronic structure. There are three nuclei, N=1H, 13C, and 19F for which KXN is now known for 15 or more different nuclei X, enough that trends are visible in the dependence of KXN upon the position of X in the periodic table. The sign of KXN(positive for H2) changes across the table somewhat between Groups V and VI, the sense of the change for N=19F being the reverse of that for N=1H and 13C. Furthermore, there is a marked increase in the magnitude of KXN with increasing nuclear charge of atom X in each Group, for negative as well as positive coupling constants. The significance of these observed trends is considered. The Ramsey theory for the electron coupling of the nuclear spins includes orbital, spin-dipolar, and contact contributions. For directly bonded atoms, the orbital contribution is zero unless there is multiple bonding, the tendency for which decreases with increasing Z in a given Group. The spin-dipolar contribution increases with increasing Z; however, it is positive, and the values calculated are an order of magnitude smaller than those found experimentally for KXN. A model is presented attributing the observed trends to the contact contribution, which depends upon the nature of the bonding orbitals employed by each atom in the bond. If both atoms employ ns orbitals in the bond, the direct contact interaction term, which is positive, dominates. The bonding of Group VII and, to a lesser degree, Group VI atoms employs primarily p orbitals. In such atoms the contact interaction is indirect, involving polarization of the core s electrons and a change in sign of the term. The contact term with inclusion of such core polarization effects provides a model consistent with the data available. The model is used to predict the signs, in some cases also the magnitudes, of several coupling constants not yet observed. For example, in NF3, OF2, and F2, we expect KNF to be negative, KFF to be positive, and KOF probably to be positive. A number of features are discussed including the coupling in highly ionic bonds such as the Rb, Cs, and H fluorides, and the relationship of the model to nuclear hyperfine interactions in atoms and ions with unpaired spins and in organic free radicals.
Application of the Refined Model of Nonhomogeneous Kinetics to the Reactions of Electrons during the γ Radiolysis of Ethanol and 2-Propanol. Energies and Entropies of Activation of the Reactions of Solvated Electrons with Alcohols and with Water
Authors: Jha K. N. , Freeman G. R.
Published: February 2004

The yields of nitrogen and hydrogen from the radiolysis of solutions of nitrous oxide in ethanol and in 2-propanol at temperatures from about − 100° to + 140° have been calculated by the refined model of the nonhomogeneous kinetics of ionic reactions in irradiated alcohols. The calculated yields agree satisfactorily with those measured experimentally. The value G(total ionization)= 4.3 was assumed for both liquids. In “absolutely pure” ethanol the values of G(H2) and G(esolv−)fi were estimated to be, respectively, 5.3 and 1.7 at − 112°, 5.3 and 1.5 at + 25°, 6.3 and 1.4 at 90°, and 6.9 and 1.2 at 145°. For the decomposition of the solvated electron esolv−→ROsolv− + H [Eq. (8)] in ethanol at 25°, k8 = 1 × 105sec−1 E8∼4.6 kcal/mole, and ΔS8†∼−21 cal/deg·mole. In “absolutely pure” 2-propanol the values of G(H2) and G(esolv−)fi were estimated to be, respectively, 4.1 and 1.2 at −85°, 4.9 and 1.3 at + 25°, and 5.4 and 1.0 at 140°. For the decomposition of the solvated electron in 2-propanol at 25°, k8 = 3 × 105sec−1, E8∼6.3 kcal/mole, and ΔS8†≈ − 14 cal/deg·mole. In each of the liquids methanol, ethanol, 1-propanol, 2-propanol, and water, Reaction (8) has an activation energy approximately equal to that of dielectric relaxation, which is also approximately equal to that of diffusion. It is concluded that Reaction (8) is approximately thermoneutral and that the enthalpy of solvation of RO− is more than 70 kcal/mole more negative (exothermic) than that of the electron in each of the five liquids. The activation energy E8 might be used either to rearrange the solvent molecules about the reaction site or for the migration of the electron to a suitably oriented site. The large negative entropy of activation is related to the specific structure required by the transition state to increase the solvation energy of the negative species by more than 70 kcal/mole.
Chemical-Shift Concertina
Authors: Ellett J. D. , Waugh J. S.
Published: February 2004

The phase-alternated experiment in liquids, a multiple-pulse NMR experiment capable of scaling chemical shifts, is examined theoretically and experimentally. The theory of the experiment is worked out using both the average-Hamiltonian and classical magnetic-dipole techniques, and the results from the two methods are compared. The effects of nonideal pulse cycles are discussed. Theoretical results are compared with experimental data, and a high-resolution NMR spectrum with scaled chemical shifts is presented.
Low-Energy Electron-Impact Study of the First, Second, and Third Triplet States of Benzene
Authors: Doering John P.
Published: February 2004

The six lowest excited states of benzene have been investigated in the gas-phase free molecule at low pressure by electron-impact spectroscopy. Incident electron energies of 13.6 and 20.0 eV and scattering angles from 9° to 80° were used. Three singlet–singlet transitions at 5.0, 6.2, and 6.9 eV were identified. These transitions agree with the results of optical absorption and higher-energy electron-impact experiments. In addition, three triplet states were observed at 3.9, 4.7, and 5.6 eV. The positions of the first two triplet states agreed with optical data on solid benzene and threshold electron-impact experiments. The third triplet state at 5.6 eV was assigned on the basis of the relative intensity of the transition at various scattering angles. The ratio of the intensity of this transition to the allowed singlet–singlet transition at 6.9 eV was in a constant proportion to the corresponding ratio for the first triplet state (3.9 eV) at all scattering angles. The spacings of the first and second and second and third triplet states in benzene were determined to be 0.80 and 0.85 eV. The difference in the spacing is not significant with respect to experimental error.
Electron Densities from Gas-Phase Electron Diffraction Intensities. I. Preliminary Considerations
Authors: Kohl D. A. , Bartell L. S.
Published: February 2004

The intensity of electrons and x rays scattered by a freely rotating molecule is determined, in the kinematic approximation, solely by the nuclear–nuclear, electron–nuclear, and electron–electron radial distribution functions of the molecule. Although these functions are one-dimensional, the latter two contain some information about the three-dimensional distribution of electrons in the molecule because the electrons are distributed relative to several nuclear reference positions and the spatial distribution of the nuclei is known. The purpose of this series of papers is to investigate the extent to which this information can be deciphered. Although published accounts have purported to show that the electron density ρ(r) can be determined uniquely from the scattered intensity, we demonstrate that, in fact, the transfomation is not unique. Nevertheless, if certain, not unreasonable, restrictions are imposed upon the form of ρ(r), it becomes possible to make fairly detailed inferences about the three-dimensional character of the density. We propose a procedure which, although not guaranteeing a unique transformation, provides a means for deriving chemically significant knowledge about the molecular electron density from experimental gas-phase intensities.
Electron Densities from Gas-Phase Electron Diffraction Intensities. II. Molecular Hartree–Fock Cross Sections
Authors: Kohl D. A. , Bartell L. S.
Published: February 2004

Differential cross sections for electron scattering based on molecular Hartree–Fock electron densities are compared with cross sections based on the independent-atom approximation for the molecules C2, N2, O2, F2, and CO. The results show that bonding effects on the electron density manifest themselves to the extent of several percent in the scattered intensity at small scattering angles. Furthermore, molecule-to-molecule variations in the shifts of electron density are clearly reflected in variations in the functional form of the scattered intensity. A comparison of the calculated intensities for N2 and O2 with preliminary experimental intensities suggests that electron scattering techniques now in development should be able to provide information about bonding and electron correlation effects competitive in accuracy with that of current quantum-mechanical calculations.
Crystal-Field Spectra of d3,7 Ions. V. Tetrahedral Co2+ in ZnAl2O4 Spinel
Authors: Ferguson J. , Wood D. L. , Van Uitert L. G.
Published: February 2004

Absorption and fluorescence spectra of Co2+ in the tetrahedral (Td) site of ZnAl2O4 spinel (gahnite) at low temperatures have been observed and analyzed. The resulting energy-level diagram includes four quartet states and nine doublets, and has been interpreted first in terms of a weak field formalism without spin–orbit coupling. In this analysis the “free-ion” levels for Dq = 0 were deduced and compared with those for a free ion outside the influence of the crystalline environment. The reduction in the electrostatic interaction parameters F2 and F4 in the crystal is due to the effect of covalency. A second approach has been used to analyze the fine structure of the energy levels in terms of a strong field spin–orbit formalism. The fine structure can be explained for all the bands on the basis of simple spin–orbit splitting, but the possibility of a Jahn–Teller distortion of the A41a state cannot be entirely eliminated.
Structure of the Alkali Hydroxides. V. The Infrared Spectra of Matrix-Isolated RbOH, RbOD, NaOH, and NaOD
Authors: Acquista N. , Abramowitz S.
Published: February 2004

The infrared spectra of matrix-isolated RbOH, RbOD, NaOH, and NaOD have been observed. Both the alkali-metal–oxygen stretch ν1 and the bending mode ν2 have been assigned for each species. The metal–oxygen stretching mode is found at 354.4, 345, 431, and 422 cm−1 for RbOH, RbOD, NaOH, and NaOD, respectively; the bending mode ν2 is observed at 309.0, 229, 337, and 250 cm−1, respectively. Combination of the results of this study with the microwave measurements for RbOH and RbOD indicates an essentially linear structure for these species. The isotope shift for ν2 observed in NaOH coupled with reasonable bond lengths indicates an equilibrium configuration of NaOH which probably does not deviate significantly from linearity. The assumption of a linear model with a harmonic bending potential yields force constants of 0.046 × 10−18 and 0.053 × 10−18 Nm (0.046 and 0.053 mdyn·Å) for RbOH and NaOH, respectively. These results are consistent with our previously reported results for CsOH.
Heat Capacity of Lutetium from 6 to 300°K
Authors: Gerstein B. C. , Taylor W. A. , Shickell W. D. , Spedding F. H.
Published: February 2004

The heat capacity of lutetium has been measured from 6 to 300°K. The results differ from a previous measurement in this temperature range by more than the accuracies of the measurements. The difference is discussed in terms of impurity effects. The thermodynamic functions are tabulated for the temperature range studied.
Vibronic Spin–Orbit Interaction in the Phosphorescence Spectrum of Phenazine
Authors: Pavlopoulos T. G.
Published: February 2004

The polarization of the phosphorescence spectrum (PS) of phenazine in 3-methylpentane and EPA as solvents was measured. If the strong 0, 0 band, which was negative polarized, is disregarded, the other bands are found about equally positive and negative polarized. This indicates the presence of a rather strong in-plane component of the emitting oscillator. The Raman and infrared spectra and a sharp PS of phenazine in n-heptane as a solvent were recorded. A vibrational analysis of these spectra was performed and the results compared with the polarization of the PS of phenazine in 3-methylpentane and EPA. All negative-polarized lines of the PS could be assigned to Raman frequencies and all positive-polarized lines to infrared modes. The presence of the positive-polarized lines (out-of-plane modes) of appreciable intensity indicated rather strong vibronic spin–orbit (SO) coupling (second-order perturbations) in phenazine. The possible coupling schemes could be: S(π,π*)↔ lim SOT(n,π*)or T(σ,π*)↔ lim OP vibrT(π,π*) or, S(π,π*)↔ lim OP vibrS(n,π*)or S(σ,π*)↔ lim SOT(π,π*). Both coupling schemes involve out-of-plane vibrations (OP vibr).
Structure of the Delta Phase in the Argon–Oxygen System
Authors: Barrett C. S. , Jordan Truman H. , Meyer L.
Published: February 2004

The crystal structure of the δ phase of the argon–oxygen system has been investigated using powder techniques. This phase, which exists in the composition range of 60%–90% O2, has a body-centered cubic unit cell with a0 = 13.23 Å. The a0 for the phase closely approximates 2a0 for γ-O2, and the diffraction patterns have marked similarities. The structure was found to be a superlattice or a close analog of the γ-O2 structure with definite segregation of the Ar and O2 within the unit cell. Although the exact nature of this segregation was not determined, the basic type is presented and three possible modifications of this type that meet specific requirements are discussed. The composition limits for the phase are understandable in terms of the occupancy of the sites in the proposed structure.
LCAO–MO–SCF Calculations Using Gaussian Basis Functions. IV. The Helium Adduct of Lithium Hydride, HeLiH
Authors: Kaufman Joyce J. , Sachs Lester M.
Published: February 2004

Population analyses of wavefunctions calculated for LiH indicated that the Li-atom site was severely electron deficient, thus favoring complexing to a species with an unshared electron pair, even on a rare-gas atom. Calculations were performed for linear HeLiH as a function of HeLi distance at several fixed LiH distances. The minimum EHF in each case was found at an HeLi distance of about 4.0 bohr, and in each case the system is bound relative to the separated products He+LiH. With the HeLi distance fixed at the equilibrium position 4.0 bohr, variation of the LiH distance gave an EHF minimum at 3.05 bohr, close to the equilibrium distance of 3.015 bohr in LiH itself. The Hartree–Fock binding energy of HeLiH relative to He+LiH was estimated to be 0.092 eV. A calculation of the zero-point vibration energy of the He–Li bond indicates that HeLiH is bound by 0.08 eV with respect to dissociation along the He–Li bond. The HeLiH system appears to be intermediate between a true compound and an extremely weak van der Waals complex. The higher rare-gas complexes of LiH are expected to be more stable.
Potential Flows of Dilute Polymer Solutions by Kramers' Method
Authors: Bird R. Byron , Johnson Millard W. , Curtiss Charles F.
Published: February 2004

Kramers' method for describing the hydrodynamic behavior of a dilute solution of polymer molecules has been extended to include the “bead–spring” models. The connection between previous results of Kramers (for the bead–rod model), and of Rouse and Zimm (for the bead–spring model) is established. Finally, an expression is obtained for the elongational viscosity; it is found that the elongational viscosity increases with elongational rate, as was found earlier by Lodge (for a network theory) and by Roscoe (for a theory of suspensions).
Microwave Studies of Collision-Induced Transitions between Rotational Levels. VI CH3OH and CH3NH2
Authors: Lees R. M. , Oka T.
Published: February 2004

The techniques of high-power microwave double and triple resonance have been applied to the study of collision-induced transitions between rotational levels of CH3OH and CH3NH2. Experiments have been carried out both for the pure gases and for mixtures with excesses of foreign gases, in particular He and H2. The results of the experiments and their quantitative analysis are summarized as follows: (i) Collision-induced transitions in pure CH3OH were observed between rotational levels connected by μa and μb dipole selection rules ΔJ = 0, ±1, ΔK = 0, ±1. No collision-induced transitions were observed in pure CH3NH2. (ii) In CH3OH–He, CH3OH–H2, and CH3NH2–He mixtures collision-induced transitions with ΔJ>1 have been observed strongly. This result was interpreted as showing that for collision-induced transitions in these mixtures the value of ΔJ is rather arbitrary while ΔK = 0 is preferred. (iii) Collision-induced transitions between different torsional levels A↔E, A+↔A−, and υ = 1↔0 were searched but never found. This result shows that these transitions are orders of magnitude smaller than transitions in a single torsional level not only in pure sample but also in mixtures. For several four-level systems in pure CH3OH, observed results are compared with values derived from resistance network analogs to the four-level systems and dipolar rate constants calculated using Anderson's theory.
Molecular Magnetic Susceptibility Tensor in Sulfur Dioxide
Authors: Bhattacharyya Pranab K. , Dailey Benjamin P.
Published: February 2004

The diagonal elements in the magnetic susceptibility tensor in the principal molecule-fixed inertial axis system in SO2 have been determined by observing the molecular rotational Zeeman effect with a high-resolution microwave spectrometer. The signs of gaa, gbb, gcc have been found to be negative from theroetical considerations. The principal values of the magnetic susceptibility tensor, paramagnetic susceptibility tensor, and diamagnetic susceptibility tensor are, respectively: χaa = − 14.4 ± 1.2, χbb = − 19.6 ± 0.5, χcc = − 20.6 ± 0.6; χaap = 38.9 ± 0.3, χbbp = 129.3 ± 1.4, χccp = 139.5 ± 1.2; χaad = − 53.3 ± 1.2, χbbd = − 148.9 ± 1.5, χccd = − 160.1 ± 1.3 (all in units of 10−6 erg/G2·mole). The ground-state averages of the sum of the squares of the electronic coordinates, and approximate values of the diagonal components of the quadrupole moment tensor have also been determined.
Critical Slowing-Down Process of Dielectric Relaxation in Antiferroelectric Cu(HCOO)2·4H2O
Authors: Makita Y. , Seo I.
Published: February 2004

The real and imaginary parts of the dielectric constants of antiferroelectric Cu(HCOO)2·4H2O were measured in its transition temperature region. The measurements were made by using a capacitance bridge for the frequency range between 3 × 102 and 3 × 106 Hz and an admittance meter at the microwave frequency of 1 × 109Hz. The experimental results obtained for εb′ and εb″ were analyzed both for the paraelectric and antiferroelectric phases on the basis of time-dependent statistics in a molecular-field approximation. It is shown that the dielectric relaxation observed in the paraelectric phase can be described to good approximation by monodispersive process, while in the antiferroelectric phase it can be described qualitatively by a monodispersive process. The increase of frequency results in the temperature shift of (εb″)max upwards in the paraelectric phase. This can well be accounted for as due to dominant contribution of a relaxation time τ0 to the observed relaxation time τ. The latter (τ) is characterized by critical slowing-down process near the transition temperature, while the former (τ0) does not relate to this process.
Orienting Polar Molecules in Molecular Beams. Symmetric Tops
Authors: Brooks Philip R. , Jones Eugene M. , Smith Ken
Published: February 2004

Symmetric-top molecules exist in various orientations in even very weak electric fields, in contrast to diatomic molecules which are oriented only in fields so large as to be impractical. For symmetric tops these orientations can be separated in an inhomogeneous electric field, and calculations are presented for the separating properties of a hexapole field. The transmission and the final distributions of velocity and orientations have been calculated for several different molecules over a range of conditions. Experimental transmission measurements were made and are in good agreement with calculations. By fitting experimental points to calculated transmission curves dipole moments in good agreement with literature values have been determined. The molecules separated by the hexapole field are experimentally shown to make adiabatic transitions into a homogeneous electric field, providing evidence that the molecules can be oriented in the laboratory reference frame.
Molecular g Values, Magnetic Susceptibilities, Molecular Quadrupole Moments, and Sign of the Electric Dipole Moment in Cyclopropene
Authors: Benson R. C. , Flygare W. H.
Published: February 2004

The high-field rotational Zeeman effect has been studied in cyclopropene and 1,2-dideuterocyclopropene. The molecular g values for cyclopropene are gaa = −0.0897 ± 0.0009, gbb = −0.14915 ± 0.00016, and gcc = 0.05363 ± 0.00077. For the deuterated species they are gaa = −0.0689 ± 0.0010, gbb = −0.13718 ± 0.00015, and gcc = 0.4538 ± 0.00018. The magnetic susceptibility anisotropies, in units of 10−6 erg/G2·mole, are 2χaa − χbb − χcc = 7.1 ± 0.6 and −χaa + 2χbb − χcc = 26.8 ± 0.4. Although only the relative signs of the g values are determined experimentally, the above signs are conclusively assigned on the basis of the molecular quadrupole moments. The a axis is perpendicular to the double bond, and the a and b axes are in the molecular plane. The molecular quadrupole moments in cyclopropene, in units of 10−26 esu·cm2, are Qaa = −0.4 ± 0.4, Qbb = 2.4 ± 0.3, and Qcc = −2.0 ± 0.6. Using the known molecular structure, the diagonal elements of the paramagnetic susceptibility tensor and the anisotropies in the second moment of the electronic charge distribution may be calculated, and they are χaap = 56.1 ± 0.3, χbbp = 80.7 ± 0.2, χccp = 93.4 ± 0.2, all in units of 10−6 erg/G2·mole; and 〈a2〉 − 〈b2〉 = 4.26 ± 0.2, 〈b2〉 − 〈c2〉 = 7.75 ± 0.2, and 〈c2〉 − 〈a2〉 = −12.01 ± 0.2, all in units of 10−16 cm2. Although the bulk susceptibility is unknown, a theoretical value of 〈c2〉 in conjunction with the measured second-moment anisotropies enables the diagonal elements of the diamagnetic and total magnetic susceptibility tensors to be predicted. These are χaad = −82.3 ± 0.3, χbbd = −100.4 ± 0.3, χccd = −133.3 ± 0.3, χaa = −26.2 ± 0.4, χbb = −19.7 ± 0.4, χcc = −39.9 ± 0.4, and χAv = −28.6 ± 0.5, all in units of 10−6 erg/G2 mole. The sign of the electric dipole moment is obtained from accurately measured g values in the two isotopic species, and gives a polarity of +⊲−.
One-Dimensional Model for Aqueous Solutions of Inert Gases
Authors: Lovett Ronald A. , Ben-Naim A.
Published: February 2004

A one-dimensional model for aqueous solutions of inert gases is presented for which the partition function may be calculated exactly. The water is treated as a mixture of monomeric water molecules and various icelike aggregates. It is shown that clustering is promoted by an inert gas only if the inert-gas molecules can occupy interstitial sites in the clusters. Numerical illustrations are given of the effect of an inert gas upon the cluster equilibrium and of the dependence of the enthalpy and entropy of solution upon the numerical parameters of the theory.
Optical Spectra of Orientationally Disordered Crystals. III. Infrared Spectra of the Sound Waves
Authors: Whalley E. , Labbé H. J.
Published: February 2004

The theory of the absorption of light by the translational vibrations of orientationally disordered crystals has been extended to low frequencies where the vibrations are short-wavelength sound waves whose density of states can be represented by the Debye model. Both the integrated intensity of a normal vibration and the density of states are proportional to the frequency squared, so the absorptivity is proportional to the fourth power of the frequency. The proportionality constant can be related to the difference between the limiting low- and high-frequency permittivities of the translation band. The absorptivity of ice has been measured below 45 cm−1 in order to test the theory. As predicted, it is approximately proportional to the fourth power of the wavenumber. At low frequencies the absorptivity becomes approximately proportional to the square of the wavenumber and is probably caused by two-phonon difference bands involving high-frequency acoustic phonons. The microwave absorptivity reported by others appears to be consistent, at least approximately, with this square line extrapolated, which suggests that it also is due mainly to two-phonon difference bands. The absorptivity is less than predicted due to two approximations in the theory, namely, the neglect of the strong orientational correlation in ice and the assumption that R defined by Eq. (20) is independent of the type of internal coordinate.
Two-Electron Homopolar Molecule: A Test for Spin-Density Waves and Charge-Density Waves
Authors: Falicov L. M. , Harris Robert A.
Published: February 2004

The eigenstates of the one-band Hamiltonian for a two-electron homopolar molecule can be exactly determined. The exact solution for the ground state and all its properties can thus be used as a standard to assess the accuracy and validity of several commonly used approximations: the normal molecular-orbital ground state, the Heitler–London states, the spin-density waves and the charge-density waves. The most general unrestricted self-consistent field approximations are studied in both the Hartree and Hartree–Fock approximations and are compared with one another as well as with the exact results. New and simple correlated states are also proposed and tested: a symmetrized spin-density wave and a symmetrized charge-density wave. These new states are easily obtained from the corresponding Hartree–Fock states, and whenever they exist, they are extremely good approximations to the exact ground state. General consequences and extensions to more complicated systems can be inferred from this study.
Reactions of H2O3 in the Pulse-Irradiated Fe(II)–O2 System
Authors: Sehested Knud , Bjergbakke Erling , Rasmussen Ole Lang , Fricke Hugo
Published: February 2004

G[ Fe (III)] is measured in pulse-irradiated O2-saturated solutions of 20 to 160 μMFe(II), at the pH's 0.46, 1.51, and 2.74 H2SO4 and HClO4 and with dose rates between 1 and 8 krad/1 μsec pulse. Based on homogeneous kinetics, the results are interpreted by a system of 18 reactions. The formation of the hydrogen sesquioxide H2O3 as an intermediate in the reaction OH + HO2→H2O3→H2O + O2 is confirmed. In the absence of Fe(II), G(H2O3) varies from 2.04 at pH 0.46 to 1.57 at pH 2.74. The rate constant k[H2O3+Fe(II)] = 6.0 × 1 4M−1·sec−1 at the pH's studied. The direct reaction, OH + HO2 = H2O + O2, does not take place. In H1SO4, at pH 0.46, 151, and 2.74 and in HClO4 at pH 0.46 and 1.51, the complexes of Fe(III) produced by the radiation reach equilibrium before they react with HO2. However, equilibrium is not reached in HClO4 at pH 2.74, where the effective rate constant k(HO2+Fe(III)] is 6.2 times its value when Fe(III) is in its equilibrium form. This ratio is constant over the dose rate range studied. With dose rates up to 64 krad/1.4 μsec pulse G[Fe(III)] and G(H2) were measured in the air saturated 1-mM FeSO4 dosimeter and G[Fe(III)] in the O2 saturated 10-mM FeSO4 dosimeter, each in 0.4MH2SO4. G[Fe(III)] decreases in both dosimeters less rapidly with increasing dose rate than was found by earlier workers. The principal reason for the decrease is the failure of O2 to scavenge the H atoms at high dose rates. Excellent agreement is obtained between experiment and theory except at high dose rates, where the calculated G[Fe(III)]'s are slightly too high in both dosimeters.
Ab Initio Studies of Orbital Hybridization in Polyatomic Molecules
Authors: Petke J. D. , Whitten J. L.
Published: February 2004

Single-determinant molecular orbital wavefunctions are constructed for a variety of polyatomic molecules using well-known techniques of hybridization and bond orbital formation. Such concepts generally are employed for qualitative purposes; however, in the present study the emphasis is on a quantitative comparison of energies and wavefunctions with results of ab initio SCF treatments. All electrons are included in the formulation, and all integrals over basis functions are evaluated accurately in order to permit rigorous use of the variational theorem. Minimization of the total energy of a molecule with respect to hybrid angles and bond polarity parameters is found to give energies and wavefunctions in rather good agreement with the minimal LC(Hartree–Fock) AO SCF limit for the systems investigated. Hybridization and bond polarity parameters are reported for a number of systems, and these results provide an assessment of the validity of certain widely used qualitative concepts. The applicability of the procedure in predicting useful molecular wavefunctions and SCF starting vectors is discussed.
NMR Spectra of Some CF3-Substituted Triphenylphosphines. Evidence for “Through-Space” P–F Spin–Spin Coupling
Authors: Miller Gerald R. , Yankowsky A. W. , Grim S. O.
Published: February 2004

In the ortho-substituted compounds (o-CF3Φ)3P, (o-CF3Φ)2PΦ, and (o-CF3Φ)PΦ2, JPF is 55.00 ± 0.17 Hz, 52.82 ± 0.12 Hz, and 53.41 ± 0.20 Hz, respectively. In contrast, we could not observe any phosphorus–fluorine coupling in the compounds (m-CF3Φ)3P and (p-CF3Φ)3P. These results, together with the very short phosphorus–fluorine distance in the ortho-substituted compounds, strongly suggest that the P–F coupling results from a “through-space” mechanism in the ortho-substituted compounds. This hypothesis is supported by the strong positive temperature dependence found for JPF, which is similar to that found for JFF in compounds in which the “through-space” mechanism is believed to make a major contribution to the fluorine–fluorine coupling constant.
Energies and Expectation Values for Be by the Transcorrelated Method
Authors: Handy N. C.
Published: February 2004

Further calculations are reported using the transcorrelated method for energies and wavefunctions recently proposed by Boys and the author. In this method, approximate solutions to the Schrödinger equation are found by obtaining approximate solutions to the transcorrelated wave equation (C−1HC − W)Φ = 0, where C = Πi>jf(rij, ri, rj) and Φ = A(φ12φ22···φN2). The advantage of working with wavefunctions which explicitly include electron correlation seems to outweigh the difficulties of working with a nonvariational procedure. Two arbitrary parameters, similar to the Slater orbital exponents, occur in the expansion functions used here for f(rij, ri, rj. For nearly all values of these parameters, a nine-term expansion for f(rij, ri, rj) gives at least 82% of the correlation energy, and a 19-term expansion gives 88%. The transcorrelated orbitals φ are within 6% of the self-consistent field orbitals and arguments are given which suggest that the true transcorrelated orbitals are much closer than this. It is also suggested that the method used here and in the previously reported LiH calculations is not the best, but that the variant used in the Ne calculations is preferable. In the second part, it is demonstrated how expectation values can be determined for transcorrelated wavefunctions without having to evaluate 3N-dimensional integrals, and without using cluster expansion-type formula and approximations for them. The method used the Hellmann–Feynman formula, and this is considered reasonable because of the great accuracy of the transcorrelated wavefunction. The general agreement between the expectation values reported here for Be by this method, and those obtained by configuration-interaction-type procedures, upholds this view. The transcorrelated method can therefore be used for the ab initio determination of highly accurate energies, wavefunctions, and expectation values.
Electric Field Effects on the Optical Rotatory Power of a Compensated Cholesteric Liquid Crystal
Authors: Baessler H. , Laronge Thomas M. , Labes M. M.
Published: February 2004

By studying the optical rotatory power (OR) of a compensated cholesteric helical structure, it is possible to observe both thermally induced helical inversion and electric field perturbations of the helix. For a cholesteryl chloride–cholesteryl myristate 1.75:1 mixture, the OR changes sign at T = T(nematic) = 42° increasing to infinity prior to the inversion. Upon application of a dc electric field parallel to the helix axis, the OR decreases continuously and reversibly by a factor of 4 in the field range 0.25Fu<F<Fu, where Fu denotes the critical field required to induce a nematic transition. This effect cannot be explained as a macroscopic reordering of the material, but is rather due to a decrease in pitch accompanying a conical helical perturbation. This perturbation arises from bending forces tending to align the dipole moments of cholesteryl chloride in the field direction. When a dc field is applied perpendicular to the helix axis, the pitch diverges logarithmically to infinity as F approaches the critical value for inducing a transition.
Energy Transfer in Tb3+-Activated Cerium(III) Compounds
Authors: Blasse G. , Bril A.
Published: February 2004

The luminescence of Tb3+-activated CeF3, CePO4, CeBO3, and CeAlO3 has been investigated. Energy transfer from the Ce3+ host lattice to the Tb3+ ion occurs by Ce3+–Ce3+ and Ce3+–Tb3+ transfer. From a study of the Ce3+ luminescence in the analogous La compounds it can be concluded that the probability of the former transfer is different for these host lattices. An analogy with Eu3+-activated host lattices is pointed out.
ESR Study of HĊO in Single Crystals of Formic Acid at 77°K
Authors: Holmberg Robert W.
Published: February 2004

Formyl radicals, HĊO, are formed when crystals of formic acid which have been gamma irradiated at 77°K are warmed to 195°K. An ESR study of these radicals in single crystals of formic acid has been made at 77°K. The g tensor (principal values 2.0037, 2.0023, 1.9948) and the hyperfine tensors for the proton (principal values 337, 346, 378 MHz) and for 13C (principal values 341, 438, 317 MHz) were determined. These tensors are discussed in relation to the structure of the formyl radical.