bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation of Isotope Effects with Molecular Forces. II. Triatomic Molecules
Authors: Bigeleisen Jacob , Ishida Takanobu , Spindel William
Published: February 2004

The theory of the relationship between the reduced partition function ratio of isotopic molecules and molecular force constants in triatomic molecules is developed through the application of finite orthogonal polynomials. Detailed calculations are presented for 13C and 18O substitution in CO2 at 500 and 1000°K, and 2H and 18O substitution in H2O at 1000 and 2000°K. The quantum correction of order j is jth order in diagonal force constant elements, but essentially linear in off-diagonal force constant elements. Stretch-bend interaction force constants are typically of increasing importance to the higher quantum corrections. In some cases, such as 18O substitution in water, the stretch-bend interaction constant is an order of magnitude more important than the stretching force constant to all quantum corrections other than the simple first quantum correction. The quantum corrections are analyzed into contributions corresponding to the statistical mechanics of a single oscillating particle and coupling terms with other atoms in the molecule. The first quantum correction is always a purely statistical mechanical term. For D substitution in water, the higher-order quantum corrections are principally statistical mechanical in origin, whereas, for 18O substitution in water, these terms are predominantly coupling terms.
Cellular Method and the Calculation of Molecular Wavefunctions. I. A Generalized Method of Images
Authors: Fricker H. S. , Anderson P. W.
Published: February 2004

It is argued that the potential approximation and local physical viewpoint of the Cellular method, which is virtually obsolete in band theory, are well suited to the very different problem of calculating molecular orbitals. But the mathematics of the method, which are suspect even in metals, fail completely for covalent bonds. After discussing and illustrating these formal problems, we propose a technique for generating a highly overcomplete basis set in each cell from a Green's function for that cell, by placing source distributions on the neighboring atoms. The experience of the classical ``Method of Images'' suggests that a few point sources will be quite adequate to generate the wavefunction, so that the method is well suited to actual calculations. This is demonstrated in the succeeding paper where the technique is applied to the alkali molecules.
ESR and Optical Spectroscopy of the AlO Molecule at 4°K; Observation of an Al Complex and Its Interaction with Krypton
Authors: Knight L. B. , Weltner W.
Published: February 2004

The AlO molecule in its 2Σ ground state has been trapped in neon, argon, and krypton matrices at 4°K, and its ESR and optical spectra (infrared, visible, and uv) measured. In neon its magnetic parameters are g‖ = 2.0015(3), g ⊥ = 2.0004(3), A‖( 27Al) = 872(1) MHz, and A ⊥ ( 27Al) = 713(1) MHz. Δg⊥ is in accord with the magnitude and sign of the spin-doubling constant, γ0, determined in the gas phase. An infrared band at 924 cm−1 in neon (917 cm−1 in argon) is assigned to AlO: the large shift from the gas value of 965 cm−1 indicates a highly ionic molecule. Besides the strong B 2Σ←X 2Σ absorption bands near 4500 Å, a progression of weak bands have been observed between 5500 and 11 000 cm−1 and assigned to the A 2πi←X 2Σ transition of AlO. ESR spectra of a weakly bonded Al–X molecule (where X may be Al2O) are observed in concentrated matrices; its magnetic properties are found to be very dependent upon the matrix gas used. In krypton matrices, 83Kr hfs is also observed but disappears (reversibly) above about 35°K, suggesting the formation of a metastable Kr–AlX complex.
Radiative and Collisional Transitions between Coupled Vibrational Modes of CO2
Authors: Seeber Knut N.
Published: February 2004

Each of the two energy levels of CO2 at 1388 and 1285 cm−1 is an equal mixture of doubly excited bending mode and singly excited symmetric stretch mode. The collision numbers of these levels for CO 2–CO 2 collisions are calculated by using the mixed wavefunctions in conjunction with the SSH theory that has been expanded to include two-quantum transitions. This relaxation is shown to take place via various near resonant V-V processes at transition rates of the order of 10 4 sec −1 · torr−1. In particular, direct de-excitation of the lower laser level via singly excited bending mode is shown to be important. An explanation is given for transitions that involve a change of the vibrational angular momentum l, by Δ l = ± 2. Three independent experiments have measured rates in reasonable agreement with those calculated here Quite striking is the substantial disagreement between these three experiments on the one hand and the published interpretation of the Rhodes—Kelly—Javan (RKJ) experiment on the other. To resolve this discrepancy, the RKJ experiment is analyzed in detail, and a new interpretation is given that explains one of the observed transient effects as caused by stimulated two-photon processes and the other by rotational relaxation of the upper laser level.
Raman Study of Hydrogen Bonding and Long-Wavelength Lattice Modes in an L-Alanine Single Crystal
Authors: Wang C. H. , Storms R. D.
Published: February 2004

This paper presents a detailed polarization study of the Raman scattering spectrum from the lattice phonons of a single L-alanine crystal, with and without the presence of an external de electric field from 120 to 350°K. The scattering experiments were made with the incident Ar ion laser beam propagating along the 〈100〉, 〈010〉, and 〈001〉 directions of the crystal. The Raman spectra of four different scattering configurations associated with each crystal orientation were measured. The experimental results have shown the presence of a large orientational anisotropy as well as polarization anomalies in the lattice Raman spectrum of the L-alanine single crystal. By means of several different experiments (light transmission measurement, different excitation laser frequencies, temperature variations of the crystal, and de electric field study), several existing possibilities that might be associated with the polarization anomalies have been ruled out. To account for the observed effect, a picture of dynamic disorder due to the proton motion that exists within the hydrogen bond has been proposed. In addition to the spectral features associated with the lattice modes, the spectral bandwidth studies of the – NH 3 + libration and the –N–H stretching also give a result consistent with the proposed picture. Due to the nature of charge fluctuations present in a hydrogen bond, it is suggested that this phenomena should be present in all hydrogen-bonded molecular systems.
Arrival Time Distributions in High-Pressure Mass Spectrometry. II. Reactions of (H, D)3+ in Hydrogen-Deuterium Mixtures
Authors: Chang C. , Sroka G. J. , Meisels G. G.
Published: February 2004

Arrival time distributions of H3+ ions in hydrogen, D3+ ions in deuterium, and of these ions and their isotopically mixed analogs in equimolar mixtures of hydrogen and deuterium have been measured using a pulsed high-pressure ion source and individual ion flight time measurements. The ratio of the reduced mobilities of the hydronium ions in the pure gases was found to be 1.41, in good agreement with previous measurements and with the theoretical expectation of 2. Drift velocities are linear functions of E/P but do not go through the origin because mean residence times do not accurately correspond to the reciprocal drift velocities of the ion packets. In the mixtures, however, reduced mobilities of all isotopically substituted ions are identical even though a difference by as much as 13% is expected, approximately 3 times the standard deviation. This is interpreted in terms of a rapid H+ or D+ transfer, and a lower limit of 0.9× 10−10 cc sec−1 is established for the rate constant of the hydron transfer reactions.
Cross Sections for the Alkali-Metal-Halogen Molecule Reactions: Na, K, Rb, and Cs with I2
Authors: Edelstein S. A. , Davidovits P.
Published: February 2004

The reaction cross sections of alkali-metal atoms with I2 molecules were obtained by direct measurements of alkali-atom decay rates. The alkali atoms were produced in the presence of a known amount of I2 molecules, by photodissociating the alkali iodide salt with a short pulse of uv light. As the alkali atoms reacted with the I2 molecules their decay was monitored by observing the transmission of alkali atom resonance light through the vapor. The cross sections were computed from the decay rates. They are: Na: 97 Å2; K: 127 Å2; Rb: 167 Å2; Cs: 195 Å2. These results are in good agreement with the cross sections computed from a modified orbiting theory.
Low-Field Hyperfine Spectrum of CH4
Authors: Yi Pon-Nyong , Ozier Irving , Ramsey Norman F.
Published: February 2004

The hyperfine spectrum of CH4 has been observed in the low field limit by the molecular-beam magnetic-resonance method. Ten partially resolved lines were observed in the frequency region below 100 kHz. The lines associated with the nuclear quintet were found to be equally spaced with frequencies which depend only on the average spin-rotation constant ca. The lines associated with the nuclear triplet were found to form a complex spectrum with frequencies which depend not only on ca, but also on the anisotropy in the spin rotation matrix cd and on the tensor spin—spin coupling constant d. In the analysis of the triplet spectrum, the energies of the tensor interactions have been calculated in detail by applying the recent group theoretical treatment of Yi, Ozier, and Anderson. It was determined that ca = + (10.4 ± 0.1) kHz; cd = + (18.5 ± 0.5) kHz and d = + (20.9 ± 0.3) kHz. For these values, the calculated and experimental frequencies agree to within a rms deviation of 350 Hz.
Vibrational Relaxation in CO+He at Low Temperatures
Authors: Shin Hyung Kyu
Published: February 2004

Vibrational relaxation in CO+He in the temperature range from 100 to 300°K has been investigated by use of the WKB treatment of vibrational energy transfer with the inclusion of the effect of potential barrier penetration. The thermal average transition probability has been integrated numerically to obtain vibrational relaxation times. The calculated relaxation times agree satisfactorily with experiment over the temperature range considered.
Equation of State of Liquid CCl4 to 16 kbar—a Comparison of Shock and Static Experiments
Authors: Lysne P. C.
Published: February 2004

Shock compression experiments were used to obtain an enthalpy–pressure–volume equation of state of the Rice and Walsh form for liquid CCl4 in the pressure region below 16 kbar. Existing zero pressure data were used to introduce a temperature dependence into the specific heat at constant pressure and into the partial derivative of volume with respect to temperature at constant pressure. Sonic velocities and Hugoniot temperatures calculated from this equation of state agree well with data obtained by other investigators. Carbon tetrachloride apparently exists in the liquid state for the 1–20 μsec duration of shock experiments even though the equilibrium liquid–solid phase line has been crossed.
Study of Different Approximation in the Calculation of g Tensors: H2+
Authors: de Montgolfier Ph. , Harriman John E.
Published: February 2004

The theory of the g tensor in one-electron systems is briefly reviewed and calculations are done in several ways for the H2+ molecular ion in order to test approximations which must be made for larger systems. Approximate ground state wavefunctions are determined variationally using a power series in elliptic coordinates and as a linear combination of atomic orbitals. The first-order wavefunction with respect to the orbit-field perturbation is calculated variationally and by a Rayleigh–Schrödinger expansion and the second-order g tensor determined. Excited state functions and energies are approximated from the higher roots of the secular equations involved. The results of the various approximate calculations are compared and discussed. It is found that the LCAO method is rather poor and that two center integrals cannot be neglected. It also appears that a single spin–orbit parameter cannot be used for different orbitals on the same atom. The best result, obtained from the variationally determined first-order function, has gxx differing from the free electron value by −0.225 × 10−5.
Electric Field Gradients at 57Fe in ZnFe2O4 and CdFe2O4
Authors: Evans B. J. , Hafner S. S. , Weber H. P.
Published: February 2004

The nuclear quadrupole coupling constants and isomer shifts of 57Fe in the spinels ZnFe2O4 and CdFe2O4 were measured using the Mössbauer effect. The signs of the quadrupole coupling constants were determined from spectra which were taken in an applied magnetic field. The sign is negative in both spinels. The isomer shifts and Fe–O distances indicate that Fe3+ in ZnFe2O4 is somewhat more covalently bonded than in CdFe2O4. The external field gradients at the Fe3+ positions can be interpreted in terms of the ionic point-multipole model modified by some charge transfer between oxygen and the cations. The point charge contribution to the field gradient is positive in case of ZnFe2O4 and nearly zero in case of CdFe2O4; the predominant contribution is due to the electric dipole moments of the oxygen ions and is negative. The dipole polarizability of the oxygen ion which gave the best fit is αD = 0.8 Å3. The effect of charge transfer on the ionic field gradient is small.
Studies of the 3455- Å Triplet State of s-Triazine
Authors: Hochstrasser Robin M. , Zewail Ahmed H.
Published: February 2004

We present a confirmation of the assignment of the 28 935-cm−1 triplet state, A13″, of s-triazine. The high-field Zeeman effect is used to make this assignment which is consistent with the fact the transition is polarized perpendicular to the molecular plane. The optical spectrum at low magnetic fields yields the zero-field splitting D = −0.058 ± 0.024 cm−1. The sign and magnitude of D is consistent with an nπ* state in which one-center spin—spin interaction is a significant feature of the zero-field splitting.
Structure and Dipole Moment of the First Electronically Excited State of Pyridine N-Oxide
Authors: Hochstrasser Robin M. , Wiersma Douwe A.
Published: February 2004

A study is presented of the lowest-energy singlet—singlet transition of pyridine N-oxide in mixed crystals with p-dichlorobenzene at 4.2°K. Polarization and Stark effect measurements on the electronic orgin of the transition (at 29 599.6 cm−1) show that the transition is π*← π, 1B2← 1A1. The spectrum of d5-pyridine N-oxide is blue shifted by 123 cm−1 and its absorption and emission spectra are quite similar to those of h5. The vibrational analysis of the absorption and fluorescence spectrum, and the Franck—Condon patterns, suggest that the equilibrium geometry is little affected by electronic excitation. Stark effect measurements on the electronic origin band yield a value of Δ μ=0.71 ± 0.03 D (assuming E ≡ 0.60 Eeff) for the change in dipole moment on electronic excitation; where E and Eeff are the applied and effective Stark electric fields corresponding to an external field directed down the a′ crystallographic axis of the p-dichlorobenzene host crystal. This small change is much less than predicted by π-electron theory.
Random Lattice Calculations on Frenkel Excitons in Disordered Molecular Crystals—1B2u Naphthalene
Authors: Hong Hwei-Kwan , Kopelman Raoul
Published: February 2004

Using the recently acquired exciton dispersion relations for crystalline naphthalene, we have calculated the density-of-states functions for heavily doped isotopic binary mixed crystals of naphthalenes with arbitrary compositions and various energy separations (trap depths). This constitutes the first attempt to extend the negative factor counting (NFC) method, developed originally for lattice phonons, to a real physical system of three-dimensional molecular excitons. In most calculations, a total of 1280 molecules were included. The exciton interactions, which included both the translationally equivalent and the interchange equivalent ones, involved all 16 neighbors. Calculations based on the coherent potential approximation (CPA) were also performed for comparison. It was concluded that these two sets of calculations compared very well except in the split-band limit and at low concentrations. Under these conditions the cluster or conglomerate states become important and the computer-simulated density-of-states functions revealed some fine structure, which was completely indiscernible in the density-of-states function based on CPA. This fine structure is experimentally significant. The relationship between the Green's function method and the moment trace method was investigated in the light of these new results. Particularly, some of the lower moments were calculated for the density-of-states functions and compared with those calculated from the exact expressions in our previous paper. It was shown numerically that the CPA results indeed agree with the exact moments up to the seventh order.
ESR of Free Radicals in Ferroelectric and Antiferroelectric Arsenate Single Crystals
Authors: Gaillard J. , Constantinescu O. , Lamotte B.
Published: February 2004

The electron spin resonance of gamma irradiated single crystals of KH2AsO4, KD2AsO4, NH4H2AsO4, and ND4D2AsO4 has been investigated. New paramagnetic centers present in several varieties and which exhibit a strong coupling with one 75As atom have been described. At room temperature, their g tensor is nearly isotropic and their hyperfine couplings are axially symmetric. The annealing of these centers is examined and their behavior upon cooling down below Tc is described. All these centers which seem to belong to the same family have been identified as AsO32− species. Their different varieties must be ascribed to different environments coming either from various proton environments or from several associations with varied impurities.
Classical Study of Rotational Excitation of a Rigid Rotor: Li+ + H2
Authors: La Budde Robert A. , Bernstein Richard B.
Published: February 2004

Classical calculations are presented for a model problem simulating the rotational excitation of H2 (and D2) in collision with Li+, based upon the ab initio potential surface of Lester. The hydrogen molecule was taken to be a rigid rotor, even though this assumption becomes progressively worse as the collision energy is increased and the vibrational excitation threshold is crossed. However, the results should serve as a set of reference calculations on a well-defined system, suitable for comparison with semiclassical and quantal treatments. The influence of initial rotational state, molecular mass, and collision energy upon the angular distribution of the average rotational energy transfer and total inelasticity is evaluated and discussed. Most of the trends are due to simple dynamical effects and they also occur in computations using a rigid ellipsoid model for the diatomic molecule.
Spin-Free Computation of Matrix Elements. II. Simplifications Due to Invariance Groups
Authors: Junker B. R. , Klein D. J.
Published: February 2004

The spin-free equivalents of the antisymmetrized Löwdin projector, the valence-bond structure projector, and a number of other symmetry projectors are shown to be invariant to within a phase under the operation of elements of certain subgroups of the spin-free symmetric group, SN, associated with a spin-free N-electron system. These so-called invariance groups are used to demonstrate equalities among the Pauling numbers thereby reducing the number of Pauling numbers which must be computed. In addition the subgroup of SN whose elements leave a basis ket invariant to within a phase factor are described and used to eliminate redundancy of molecular integrals in the expression for expectation values. By the use of invariance groups of both the projectors and kets, matrix elements over spin-free operators are simplified by eliminating redundancy with respect to both the Pauling numbers and the molecular integrals.
Theory of Separability of Many-Electron Systems
Authors: Huzinaga S. , Cantu A. A.
Published: February 2004

Atomic and molecular systems are often intuitively separated into almost independent subsystems as, for example, the core and valence parts of an atom. Consequently, if this separation provides a good approximation, one can obtain the states of the system from the states of the subsystems which best represent the entire system. In the light of the work of McWeeny, in which one assumes strong orthogonality among subsystem wavefunctions, we determine an effective Hamiltonian for a given subsystem which should properly describe the states of that subsystem. Previous work is shown to have dealt with an improper effective Hamiltonian.
Nuclear Magnetic Resonance of the Aquated Proton. II. Chloroauric Acid Tetrahydrate. Phase Transitions and Molecular Motion
Authors: O'Reilly Donald E. , Peterson E. M. , Scheie C. E. , Williams Jack M.
Published: February 2004

Proton and deuterium nuclear magnetic resonance and chlorine-35 nuclear quadrupole resonance of chloroauric acid tetrahydrate, H5O2+AuCl4−· 2H2O has been studied from 180 to 300°K. Phase transitions are observed at 290°K (Solid I↔ Solid II) and at 218°K (Solid II↔ Solid III) in ordinary chloroauric acid tetrahydrate. In the deuterated compound the lower-phase transition shifts upward by 33° to 252°K; an upper-phase transition was not detected below 300°K. The chlorine-35 NQR consists of two lines (1:1 intensity ratio) above the Solid II—III phase transition and three lines (1:2:1 intensity ratio) below this transition. The proton spin—lattice relaxation time T1 of H5O2+AuCl4−· 2H2O exhibits a minimum at 273°K which is assigned to the interbond jump motion of the bridging proton in the H5O2+ ion. The activation energy for this process is 5.7 kcal mole−1 in Solid II and 11.7 kcal mole−1 in Solid III. Rotating frame relaxation times have minima at 200 and 233°K which are assigned to the interbond jump process and a proton exchange process, respectively. The proton exchange process involves rotation of the hydronium ion about the threefold axis and translational diffusion of water molecules between sites which differ in water molecule orientation by 90°. The deuteron resonance of a single crystal and polycrystalline D5O2+AuCl4−· 2D2O is interpreted in terms of the deuterium exchange process. This process has an activation energy of 10.4 kcal mole−1 in Solid II.
Heteropolar One-Electron Bond
Authors: Feinberg M. J. , Ruedenberg Klaus
Published: February 2004

Factors which are characteristic for covalent binding between different atoms are analyzed through an investigation of the one-electron heteronuclear bond. The examination of these model systems shows that the origin of binding can be understood by a partitioning of the binding energy in terms of five contributions with well defined conceptual meanings: Charge transfer from one atom to another, quasiclassical Coulombic interactions between the atoms, interference interactions between the atoms, atomic orbital contraction, and equalization of atomic orbital sizes. A stable bond is found to exist only if the nuclear charges differ by less than about 20%; a metastable bond exists if the difference in the nuclear charges lies approximately between 20%-40%; a purely repulsive binding energy curve results if the nuclear charges differ by more than about 40%. A variational analysis shows the reason for this behavior to be the high energetic cost of electronic charge transfer required to establish electron sharing. It is also confirmed that the interference energy is adversely affected by bond polarity. Finally, it is observed that, in these heteronuclear bonds, there exists a pronounced and explainable tendency for the atomic orbitals of the two parent atoms to become more similar in size upon molecule formation, while both contract. The basic origin of covalent binding is found to have the same explanation as in the homonuclear case.
Normal Coordinates: General Theory, Redundant Coordinates, and General Analysis Using Electronic Computers
Authors: Gwinn William D.
Published: February 2004

A system of doing normal coordinate analysis is described which is better suited for use with electronic computers than are current methods. Mass-weighted Cartesian coordinates are used, and rotation and translation are not separated until the matrix diagonalization procedure. The V matrix is generated numerically in a simple and accurate manner. Only one transformation is involved. The transformation which diagonalizes V is simply the transformation from Cartesian to normal coordinates. The problem of redundant coordinates does not exist in this proposed method, but the advantage of using linear potential constants in strained and constrained systems is retained.
Polarized Vibrational Spectra of Single Crystal Sodium Tetrafluoroborate
Authors: Bates John B.
Published: February 2004

The polarized Raman spectra of an oriented single crystal of NaBF4 were measured at 298 and 77°K in the region above 50 cm−1. The polarized infrared transmission and reflection spectra of single crystal NaBF4 were also measured at room temperature in the region above 300 cm−1. From the polarized vibrational spectra, the intramolecular modes were assigned, and estimates were made of the magnitude of the static field effects in crystalline NaBF4.
Perturbation Theory of Product Hamiltonians through Fourth Order
Authors: Rothman Laurence S. , Clough Shepard A.
Published: February 2004

Expressions are derived through fourth order in a V representation for the eigenvalues and off-diagonal elements of Hamiltonians expressible as sums of products of operators from two Hilbert spaces (V and R), H=0HV+0HR+ ∑ p(HVHR)p. The zeroth order Hamiltonian is assumed separable, and the eigenvalue differences in the V space are assumed to be an order of magnitude larger than the eigenvalue differences in the R space. The method involves successive contact transformations chosen to yield results in terms of matrix elements in the V space and operators in the R space. The technique allows for the exclusion of interactions between resonant states in the V space for subsequent numerical diagonalization.
Semiclassical Transition Probabilities for Rotationally Inelastic Scattering
Authors: Cross R. J.
Published: February 2004

Degeneracy averaged partial cross sections for rotationally inelastic scattering are calculated using semiclassical methods. Assuming a classical linear trajectory simple expressions are derived for the sudden limit, the classical limit, and first-order limit. A single uniform approximation is obtained which converges to all three limits. The partial inelastic cross section Qj′←j is given by DΣC2(jλj′;000) Aλ(d), where D and d are reduced parameters, C is the Clebsch–Gordan coefficient, and Aλ is a set of integrals depending on the potential form; the Aλ's are calculated for the ion–dipole and anisotropic van der Waals potentials and in the sudden limit for the dipole–dipole potential.
Vibrational Distribution Functions in Anharmonic Oscillators
Authors: Caledonia G. E. , Center R. E.
Published: February 2004

A model has been developed to calculate the steady-state vibrational distribution of an anharmonic oscillator under conditions of thermal nonequilibrium. The model includes vibration–vibration and vibration–translation collisional processes as well as radiative decay. Numerical calculations are shown to be in reasonably good agreement with experimental data obtained in electrically excited N2–CO mixing systems. It is shown that radiative decay may be the dominant loss mechanism in low-pressure N2–CO experiments.
Theoretical Electronic Transition Probabilities in Diatomic Molecules II. 13-Electron Sequence
Authors: Popkie H. E. , Henneker W. H.
Published: February 2004

Hartree–Fock (HF) electronic dipole-momentum and dipole-length transition moments, M(R) and M̄(R), respectively, are presented as a function of the internuclear distance R for the N2+(B 2Σu+ − X 2Σg+) first negative and (A 2Πu − X 2Σg>+) Meinel systems. They are compared with “experimental” M(R) and M̄(R) curves obtained by using relative band transition probability and upper-state lifetime measurements recently reported in the literature. Vibrational averages of both the theoretical and “experimental” transition moments are computed and used to calculate some relative band oscillator strengths fυ′υ″ / f00. The HF f00 value of 0.1370 for the first negative system differs from experiment by a factor of 5.6. For the Meinel system the HF f00 value of 1.065 × 10−2 is too large by a factor of 3.3 or 7.3 depending on whether the lifetime data of O'Neil and Davidson or that of Hollstein et al. are used to determine the experimental value. The HF transition-moment calculations at a single value of R are also reported for the (B 2Σu+ − X 2Σg+) and (A 2Πu − X 2Σg+) systems of C2− and for the (2Πi − X 2Σ+) and (2Πr − X 2Σ+) systems of CN, CO+, BO, BF+, and BeF. Electronic absorption oscillator strengths are presented and compared with experiment where possible.
New Model Potential for Pseudopotential Calculations
Authors: Simons Gary
Published: February 2004

The use of model potentials in pseudopotential calculations is discussed, and a set of desirable criteria for model potentials is suggested. The Hellmann potential and the Abarenkov and Heine potential are examined, and it is shown that both potentials are useful, but neither fully satisfies the suggested desiderata. A new potential of the form, V(r) = − Z / r + ΣlBlPl / r2, where Pl is the projection operator over the subspace of spherical harmonics of a given l, is proposed. A physical interpretation of the potential in terms of a “Pauli force” and a polarized core is given. Ionization energies for excited S, P, D, and F states for one-valence-electron atoms are calculated. The new potential is also applied to two-valence-electron atoms, and their calculated valence-state energies are shown to be in good agreement with experimental values.
Raman Spectral Studies of the Effects of Solutes and Pressure on Water Structure
Authors: Walrafen G. E.
Published: February 2004

Raman spectra excited by 4800-Å argon-ion-laser radiation and recorded photoelectrically have been obtained from a series of ternary aqueous solutions by back scattering and by 90° scattering using Cary and Jarrell–Ash double monochromators, respectively. The OD stretching region of HDO (and D2O), 2300–2800 cm−1, and the OH stretching region of H2O plus HDO, ∼ 2800–3800 cm−1, were examined for the following solutes: (1) NaPF6, (2) NaSbF6, (3) NaClO4, (4) NaBF4, (5) NaReO4, (6) NaMnO4 (infrared only), (7) NaIO4, (8) NaNO3, (9) NaClO3, (10) NaSCN, (11) KI, (12) CsF, (13) Na2SO4, (14) (C4H9)4NCl, and (15) Xe. (See Addendum for other solutes.) In addition, Raman spectra were obtained from HDO in H2O at pressures from 1 to 3790 bar at 25°C, and from a 5.0M solution of NaClO4 in HDO and H2O at pressures of 690 and 3450 bar at 25°C. Several pronounced intensity maxima were observed in the OD and OH stretching regions for salts of extremely strong acids, (1)–(4), and some qualitative correlations with the strengths of the corresponding acids were found for (1)–(10). Solutes (1)–(11) were also found to act as structure breakers in water, but (12) was found to enhance structure, and some hydrogen bonding was found for (13). Further, no significant spectral effects were noted for (15) when the Xe concentration of the molten clathrate hydrate was 0.8M at 70°C and 1720 bar. For (14), however, an intense low-frequency OD stretching component was observed near 2483 cm−1. The (C4H9)4N+ cation is the first found to produce pronounced effects in the OD and OH stretching regions, and the 2483 cm−1 OD stretching component appears to arise from a marked structure-making effect. The effect of pressure on the OD stretching contour of HDO in H2O was observed to be small. Only a slight decrease in the frequency of the nonhydrogen-bonded component centered near 2660 cm−1 was uncovered to pressures of 3790 bar, and no significant pressure effect was observed for the NaClO4 solution to 3450 bar. The high-pressure spectra indicate that pressure is not very effective in breaking the hydrogen bonds in water, and the filling of holes as the result of a redistribution of angles and distances in the hydrogen-bonded network is suggested as a more important effect.
Electrohydrodynamic and Dielectric Instabilities of Cholesteric Liquid Crystals
Authors: Helfrich W.
Published: February 2004

A model is proposed for electrohydrodynamic alignment effects in cholesteric liquid crystals. We derive and discuss a formula for the threshold voltage of such alignment, starting from the planar texture between two conducting plates. An improved formula for the threshold of the dielectric deformation of this orientation pattern is also given.
Dilute Solution Theory of Polymer Crystal Growth: A Kinetic Theory of Chain Folding
Authors: Sanchez Isaac C. , DiMarzio Edmund A.
Published: February 2004

A kinetic theory of polymer crystallization from dilute solution is formulated for linear chain molecules of finite molecular weight (monodisperse). Two models of crystal growth are considered; both are essentially “regular” chain folding type models. Formulas for the crystal growth rates are derived as a function of the fundamental rate constants associated with the various states of molecular crystallization. These rate constants are evaluated as a function of polymer concentration, molecular weight, crystallization temperature, and crystal thickness. Consideration of finite molecular weight molecules requires an understanding of how these molecules are incorporated into the crystal and what happens to chain ends. Attention is focused on these problems and a description of how “cilia” are formed in polymer crystals is given. A remarkable aspect of cilia formation is that the uncrystallized portion of a chain molecule which dangles in the solution can participate in nucleating a new growth strip (fold plane) on the crystal face—a kind of “self-nucleating” mechanism.
Electron Distributions for the Helium Atom–Hydrogen Molecule System
Authors: Gordon Michael D. , Secrest Don , Llaguno Claro
Published: February 2004

A study is made of the electron distribution generated by self-consistent field wavefunctions for the hydrogen molecule–helium atom system. The electron density is studied by means of electron density contour plots and plots of the difference between the electron density for interacting hydrogen molecule and helium atom systems and the noninteracting hydrogen molecule and helium atom. A population analysis technique is applied to these wavefunctions. Though the population analysis is a rather unrealistic method of distributing the electrons among and between the nuclei, it appears to give a qualitatively correct description of the electron distribution. The wavefunction studied gives a potential surface on which the H2 molecule tends to contract when the He atom approaches. This is true even when He approaches the H2 at an angle of 90° to the internuclear axis. The population analysis reflects this effect by showing an increase in the H2 bond population as He approaches.
Electron Diffraction Investigation of Dimethyl Diselenide
Authors: D'Antonio P. , George C. , Lowrey A. H. , Karle J.
Published: February 2004

The molecular structure of dimethyl diselenide has been determined by an electron diffraction investigation of the vapor. A computerized background correction routine, satisfying the positivity and area criteria, was employed to reduce the data and obtain a final molecular intensity curve. The values for a set of nine distances and amplitudes were obtained from a least-squares fit to the final molecular intensity curve. The bonded distances are rg(C–H) = 1.131 ± 0.008 Å, rg(C–Se) = 1.954 ± 0.005 Å, and rg(Se–Se) = 2.326 ± 0.004 Å. The corresponding amplitudes are l(C–H) = 0.079 ± 0.007 Å, l(C–Se) = 0.054 ± 0.003 Å, and l(Se–Se) = 0.056 ± 0.002 Å. The ∠CSeSe = 98.9 ± 0.02° and the ∠HCSe = 108.4 ± 0.8°. The errors are estimated to be at the 99% confidence level. The methyl groups are unsymmetrically placed with respect to the CSeSe planes. They are rotated about the C–Se bonds such that one of the HCSe planes in each CH3Se moiety makes an angle of 36.1 ± 6° with the respective CSeSe plane. Dimethyl diselenide assumes a stable skew conformation with a dihedral angle of 87.5 ± 4°. The molecule contains a twofold axis of symmetry passing midway between the two selenium atoms in the plane bisecting the dihedral angle.
Photodissociation of O3 in the Hartley Band. Reactions of O(1D) and O2(1Σg+) with O3 and O2
Authors: Gilpin R. , Schiff H. I. , Welge K. H.
Published: February 2004

The ultraviolet photolysis of O3 in the presence and in the absence of O2 has been investigated in the wavelength region ∼ 2375–2625 Å using flash photolysis and kinetic emission spectroscopy with a time resolution high enough to allow unambiguous identification of photodissociation products. The production of O(1D) was established by observing the forbidden D1→P3 emission at 6300 Å. The decay of the emission provided a value of the rate constant for the O(1D)–O3 reaction of (2.5 ± 1) × 10−10 cm3 molecule−1·sec−1 at 25°C. Analysis of the O(1D) decay showed that the rate of chain reactions in which O(1D) could be reproduced must be smaller than the O(1D)–O3 quenching rate by more than an order of magnitude. No emission was observed from O2(1Σg+) when O3 alone was photolyzed, indicating that the primary yield for O2(1Σg+) production is smaller than 1/20 of the O(1D) production. O(1D) was very efficiently quenched when N2 and O2 were added. O2(1Σg+) was observed by the Σg+1→Σg−3 emission when O3–O2 mixtures were photolyzed. The emission disappeared when N2 was added. The O2(1Σg+) production and decay rates were measured at various O3 and O2 concentrations. The results indicated that O2(1Σg+) was formed by energy transfer from O(1D) to ground state O2. The decay of the Σg+1 emission provided a value of the rate constant for the O2(1Σg+)–O3 quenching of (2.5 ± 0.5) × 10−11 cm3 molecule−1·sec−1. The relative quenching rate of O(1D) by O3 and O2 was about 5, yieding an O(1D)–O2 rate constant of 5 × 10−11 cm3 molecule−1·sec−1 with an uncertainty of a factor of 2. The dependence of the Σg+1 emission on the O2 pressure showed that the primary quantum yield of O2(1Σg+) is at least 20 times smaller than that of O(1D), in agreement with the previous result.
Hei and Heii Photoelectron Spectra and the Electronic Structures of XeF2, XeF4, and XeF6
Authors: Brundle C. R. , Jones G. R. , Basch H.
Published: February 2004

The high-resolution photoelectron spectra of XeF2, XeF4, and XeF6 have been obtained essentially free from any impurity features, and the ionization potentials up to 28 eV have been measured. The results are used, in conjunction with recent ab initio calculations, to assign the molecular orbital electronic structure of these molecules. Agreement between experiment and calculation is fairly good.
ESR Studies of an Intrinsic Trapped-Electron Center in X-Irradiated Alkali Borate Glasses
Authors: Griscom D. L.
Published: February 2004

Electron spin resonance studies have been carried out at X-band frequencies on a large number of glasses spanning the lithium, sodium, and potassium borate glass systems, following x irradiation at cryogenic temperatures (T ∼ 25 or ∼ 77°K). A new, intrinsic paramagnetic defect center characterized by a sizeable hyperfine interaction with boron [A (11B) ∼ 100 G] was observed, in addition to the more familiar boron–oxygen hole centers. The new center, which is readily destroyed by visible light or thermal annealing above ∼ 100°K, is shown to be of the trapped-electron type. By means of computer simulations of the spectra, a reasonably complete set of spin–Hamiltonian parameters is derived in spite of the obscuring effects of a substantial distribution in hyperfine coupling constants. Possible models for the new defect are considered and its potential usefulness as a “probe” of the structure of alkali borate glasses is discussed.
Thermodynamic Properties of the Fluid and Solid Phases for Inverse Power Potentials
Authors: Hoover William G. , Gray Steven G. , Johnson Keith W.
Published: February 2004

The two computer methods of Monte Carlo and lattice dynamics are used to determine fluid and face-centered-cubic solid thermodynamic properties for classical particles interacting with pairwise-additive inverse 4th, 6th, and 9th power potentials. These results, together with those already on hand for 12th power and hard-sphere potentials, provide a complete, and remarkably simple, description of the dependence of the pure-phase thermodynamics and the melting transition on the “softness” of the pair potential.
NMR Proton Second Moments in KH2PO4-type Crystals
Authors: Adriaenssens Guy J. , Bjorkstam John L.
Published: February 2004

Calculated second moments of the NMR proton lines for different cases of dynamic proton modes and static ordering in KH2PO4-type crystals show differences that are too small to be experimentally observable. It is shown, however, that a previously proposed disordered rotation of the H2PO4 group cannot account for the proton linewidth narrowing at the high-temperature (180°C) transition in KH2PO4. An incidental result of this investigation is the 14N nuclear quadrupole coupling constant in NH4H2AsO4, of 30.7 kHz.
Paramagnetic Resonance of Cu(II) Complexes in Deuterated Amino Acid Crystals: Dimethylalanine, α-Glycine, and l-Alanine
Authors: Fujimoto M. , Janecka J.
Published: February 2004

The ligand nitrogen hyperfine structure in paramagnetic resonance spectra of Cu(II) ions in crystals of three deuterated amino acids has been resolved. In dimethylalanine and α-glycine crystals, the copper complex structure is symmetrical, and the two nitrogen nuclei interact equally with the central ion. In contrast, the structure in l-alanine was found to be deformed, with only one amino group participating in the ligand hyperfine interaction. The g tensor and hyperfine coupling tensors for 63Cu, 65Cu, and ligand 14N nuclei were determined for the Cu(II) complexes in these materials.
Theoretical Model for the Differential Quenching Rates of CO Fluorescence by Ortho- and Parahydrogen
Authors: Sharma R. D. , Kern C. W.
Published: February 2004

The experimental measurements of Millikan and co-workers, which show that para-H2 quenches vibrationally excited CO faster than normal-H2 (14para, 34ortho) at temperatures below 500°K, are analyzed in the Born–Bethe approximation. Beginning with the observation that the rotational transition j = 2 → j = 6 in H2 has almost the same energy as the vibrational transition υ = 1 → υ = 0 in CO and invoking the assumption that long-range forces are solely responsible for the difference in quenching rates between the two spin species of H2, we find that the temperature dependence of the measured rate differences can be reproduced almost quantitatively by truncating the Born–Bethe series at second order and by using an interaction potential derived from a multipole expansion of nonoverlapping charge distributions. The implications of this result, and the assumptions on which it is based, are discussed in considerable detail.
Dynamics of Protons in Hydrogen-Bonded Systems: Propynoic and Acrylic Acid Dimers
Authors: Bournay J. , Maréchal Y.
Published: February 2004

This paper is part of a study of the origin of the abnormal infrared spectra of the stretching vibrations of protons in H-bonded species. An experimental ir study of the 2000–3000 cm−1 region of propynoic and acrylic acid dimers and of their deuterated analogs is presented. These ir spectra have been recorded with high resolution and intensity in order to be compared with a theoretical model which predicts positions and intensities of the lines. This model is briefly recalled, and the fit between theory and experiment is discussed. It is suggested that the theoretical model considered here, although in its first stage of development, represents the basic features of the mechanism of oscillation of protons in weak and moderate H bonds.
Molecular Orbital Studies of Hydrogen Bonds. III. C=O···H–O Hydrogen Bond in H2CO···H2O and H2CO···2H2O
Authors: Morokuma Keiji
Published: February 2004

Ab initio LCAO–MO–SCF calculation for H2CO···H2O is carried out with a minimal Slater basis set. The most stable conformation has an O···H distance of 1.89 Å with <C=O···H=− 64° and a stabilization energy of 3.5 kcal/mole, about a half of that for H2O···H2O. Nonlinear and π hydrogen bonds, H2CO···2H2O and the O···H–C hydrogen bond in H2O···HCHO, are also studied. An energy decomposition scheme is proposed and applied to H2CO···H2O and H2O···H2O. In the latter the electrostatic energy 8.0 kcal/mole, the exchange repulsion − 9.9 kcal/mole, the polarization and dispersion energy 0.3 kcal/mole, and the delocalization energy 8.2 kcal/mole are in good agreement with Coulson's estimates.
Depolarized Light Scattering from a Helix–Coil System
Authors: Marshall Alan G. , Pecora R.
Published: February 2004

The spectral distribution of the depolarized component of light scattered from a dilute solution of a macromolecule which exists in dynamic equilibrium between helix and coil forms is computed. The helix is treated as a cylindrically symmetric, optically anisotropic rigid rod, and the coil as a freely jointed chain of cylindrically symmetric, optically anisotropic segments. The duration of the helix–coil transition (as opposed to the “lifetime” for the process) is taken as short compared to characteristic times for molecular translation and rotation. It is assumed that a given segment on undergoing a transition loses all “memory” of its initial orientation—this assumption leads to a spectrum which consists of two Lorentzian lines whose widths are determined by the translational and rotational diffusion constants and by the lifetimes toward interconversion for the helix and coil forms. Under certain conditions, the low-angle scattering spectrum is a single Lorentzian whose width is primarily determined by the lifetime for disappearance of the helix form.
Theory of Atom–Diatom Collisions. IV. On Integral Equation Formalisms for Resonance Level Widths and Positions
Authors: Sams W. Neal , Kouri Donald J.
Published: February 2004

Integral-equation approaches to the level width and position of compound-state resonances are examined. A new description of the resonance state in terms of the eigenvectors and eigenvalues of the Hermitian operator QHQ + QHP[P / (E − PHP)] PHQ is explored in some detail. It is shown that one may utilize the formalism of Sams and Kouri to avoid expansions of the nonlocal effective potential in terms of approximate compound-state eigenfunctions. The procedure developed thus avoids difficulties such as neglect of the continuum of the closed channel projected Hamiltonian (or effective Hamiltonian) and should permit the evaluation of accurate level widths and positions by a single solution of a set of close-coupled inhomogeneous integral equations. These coupled equations are similar in nature to the original coupled open-and closed-channel scattering equations except that the inhomogeneity in the new equations is exactly that appropriate for describing the bound complex rather than an unbound scattering state. Finally, the integral equation technique may be used to study any of the various alternative descriptions of the compound state resonance.