bluuebunny/crossref_metadata_2025 huggingface dataset:
Clustering of water on hydrated protons in a supersonic free jet expansion
Authors: Searcy J. Q. , Fenn J. B.
Published: February 2004

This paper presents results concerning size distribution and growth in the gas phase of ions in the series H+(H2O)n. The measurements span a range of cluster sizes up to n =28, a domain not previously investigated quantitatively. The molecular beam sampling technique developed for this study promises to be a powerful tool in the study of ion nucleation phenomenon. Noteworthy observations include the unusual stability of the cluster with n =21 and the absence of trapped molecules of carrier gas within a cluster.
Theory of vapor phase nucleation in binary mixtures of water and sulfuric acid
Authors: Shugard W. J. , Heist R. H. , Reiss H.
Published: February 2004

An expression for the nucleation rate of a binary vapor mixture of H2O and H2SO4 at 25°C is derived which explicitly considers the contribution of H2SO4 hydrates to the droplet growth process. The continuum approximation used in earlier binary rate treatments has been abandoned in favor of treating the droplet composition in a discrete fashion. Rates of nucleation are calculated for a H2O–H2SO4 vapor mixture at 50%, 200%, and 300% relative humidities for various H2SO4 vapor activities. A finite nucleation rate is predicted with a relative humidity of 50% and a monomer H2SO4 vapor activity of 10−3. This translates to a vapor pressure of monomer H2SO4 of 3.6×10−7 mm Hg or 0.5 ppb of pure H2SO4 at 1 atm. At higher relative humidities (i.e., 300%), a finite nucleation rate is predicted for monomer H2SO4 concentrations of ∼0.04 ppt. This represents a vapor pressure of 3.2×10−11 mm Hg for monomer H2SO4.
Absorption, transmission, and reflection spectra of mono- and bimolecular layers
Authors: Philpott Michael R.
Published: February 2004

A microscopic theory of the optical properties of mono- and bimolecular layers is described. Intermolecular interactions are assumed to be of the point dipole type. It is shown how the refractive index of a mono- or bilayer may be defined and connections to the refractive index of a three dimensional crystal are established. At frequencies close to an absorption band, strong exciton-photon coupling is shown to result in excited states with very short radiative lifetimes.
ENDOR studies of the superhyperfine couplings of hydrogen-bonded protons. III. Protonated radical anions in irradiated L-valine hydrochloride monohydrate
Authors: Muto Hachizo , Nunome Keichi , Iwasaki Machio
Published: February 2004

The so-called carboxyl radical anion trapped in irradiated single crystals of L-valine hydrochloride monohydrate has been studied by ENDOR. The spectra exhibited two OHβ proton couplings, indicating that the anion is protonated to form a radical of the type R–Ċ(OH)2. The coupling tensors and their orientations clearly indicate that the amino proton in the neighboring NH3 group is transferred to the anion through the intermolecular hydrogen bond. Combining the results with those previously obtained for L-alanine and succinic acid, the variation of the dipolar tensor elements with the isotropic values has been examined for the OHβ proton couplings in this type of radical. It is then shown that the anisotropic coupling is approximately independent of the dihedral angle of the O–Hβ bond. The dipolar tensors were also calculated as a function of the dihedral angle, and the results are in good agreement with the observations.
Group theoretical approach to the configuration interaction and perturbation theory calculations for atomic and molecular systems
Authors: Paldus Josef
Published: February 2004

A formalism for an efficient generation of spin-symmetry adapted configuration interaction (CI) matrices of the N-electron atomic or molecular systems, described by nonrelativistic spin-independent Hamiltonians, is presented. The Gelfand and Tsetlin canonical basis for the finite dimensional irreducible representations of the unitary groups is used as an N-electron CI basis. A simplified Gelfand-type pattern pertaining to the N-electron problem is introduced, which considerably simplifies the canonical basis generation and, more importantly, the calculation of representation matrices of the (infinitesimal) generators of the pertinent unitary group in this basis. The calculation of the CI matrices for the above mentioned systems is then straightforward, since any particle number conserving operator may be written as a sum of n-degree forms in the unitary group generators. The computation of CI matrices for various Hamiltonians as well as the problems of the space-symmetry adaptation of the Gelfand-Tsetlin basis and of limited CI calculations are briefly discussed.
Moments and distribution functions for polymer chains of finite length. I. Theory
Authors: Flory P. J. , Yoon D. Y.
Published: February 2004

The persistence vector a ≡ 〈r〉, representing the configurational average of the end-to-end vector r for a chain molecule, and 〈g〉, the configurational average of the vector g extending from the beginning of the chain to its center of mass, are discussed. These and other vectors and related tensors are expressed in a reference frame defined by the first two bonds of the chain. Algebraic relations between the various vectors and tensors characterizing the spatial distribution of the chain are derived, and matrix generators for their evaluation are presented. The spatial distribution of the remote end of the chain in the reference frame embedded in its first two bonds is discussed in terms of the vector ρ = r − a representing the displacement from the mean position of the terminus of r. The density distribution function Wa(ρ) is developed in terms of the moments of vector ρ with the aid of three-dimensional tensor polynomials which are analogs of the Hermite polynomials.
Molecular beam scattering experiments on the reaction K + RbCl: Absolute angular distributions and integral cross sections
Authors: Aniansson G. , Creaser R. P. , Held W. D. , Holmlid L. , Toennies J. P.
Published: February 2004

The angular distributions of the reactive and nonreactive scattering for the system K + RbCl have been restudied in a molecular beam experiment. By absolutely measuring all the primary data, it was possible to obtain the absolute cross section for complex formation (σc = 124±21 Å2, ETrans = 0.18 eV) and the probabilities for reactive (Γr = 0.34±0.05; σr = 42±9 Å2) and nonreactive decay (Γn = 0.66±0.05; σn = 82±18 Å2) of the complex. The branching ratio Γ≡Γr/Γn = 0.52±0.11 agrees with previous experimental work and disagrees with theory. An error was found in the analysis of the earlier experiments. Both the formation of the complex and the energy disposal are now shown to be consistant with the ``loose'' complex model.
Intermolecular charge transfer in molecular crystals
Authors: Middlemiss K. M. , Santry D. P.
Published: February 2004

Charge transfer between sets of crystallographically inequivalent molecules within molecular crystals is discussed in the light of SCF perturbation treatment of intermolecular forces. It is shown that intermolecular charge transfer is a second and higher order effect. The theory is illustrated by calculations for the molecular charge distributions for ice II and various distorted formamide structures.
The effect of electric charge on the diffusion of macromolecules
Authors: Doherty P. , Benedek G. B.
Published: February 2004

We have determined the time for the relaxation of concentration fluctuations in solutions of bovine serum albumin as a function of the protein charge and the ionic strength of the solution. The relaxation rates for fluctuations whose wavelengths were of the order of the wavelength of light were determined from the temporal correlations in the intensity of the light scattered from the fluctuations. The experimental results are compared with the theory of M. J. Stephen. While the theory provides a qualitative understanding of the results, the restrictions inherent in the Debye-Hückel approximation prevent a quantitative agreement between the theory and the experimental data.
Quantum mechanical streamlines. I. Square potential barrier
Authors: Hirschfelder Joseph O. , Christoph Albert C. , Palke William E.
Published: February 2004

Exact numerical calculations are made for the scattering of quantum mechanical particles from a square two-dimensional potential barrier. This treatment is an exact analog of both frustrated total reflection of perpendicularly polarized light and the longitudinal Goos-Hänchen shift. Quantum mechanical streamlines (which are analogous to either classical mechanical trajectories or optical rays) are plotted. These streamlines are smooth, continuous, and have continuous first derivatives even through the classically forbidden region. The streamlines form quantized vortices surrounding each of the nodal points (which result from interference between the incident and reflected waves). Similar vortices occur in reactive collisions of H + H2 (McCullough and Wyatt; Kuppermann, Adams, and Truhlar) and undoubtedly play an important role in molecular collision dynamics. The theory for these vortices is given in a companion paper. A comparison is given between our numerical calculations and the stationary phase approximation (SPA). Although our incident wave packet has a half-width of more than one de Broglie wavelength in contrast to the SPA which replaces the diffuse beams by rays with delta function profiles, the agreement was surprisingly good for both the Goos-Hänchen shifts and for the reflection coefficients. However, we found that the Goos-Hänchen shift for the transmitted beam is significantly smaller than for the reflected beam, although in the stationary phase approximation these two shifts are equal. Furthermore, we found that the scattering from the potential barrier has very little effect on the shape of the wave packets. The power series expansion of the incident Debye-Picht wave packet ψI has an extremely small radius of convergence, whereas the power series for ψI*ψI has a radius of convergence of more than two de Broglie wavelengths. The imaginary velocity viis introduced into the Madelung-Landau-London hydrodynamical formulation of quantum mechanics. The corresponding imaginary streamlines will be considered in a forthcoming paper. The time-independent Schrödinger equation for real wavefunctions is reduced to solving the nonlinear first order partial differential equation: ℏ▿·vi = 2(E − V) + vi2. Here, vi is irrotational. This equation may lead to interesting new methods of solving the Schrödinger equation. It does lead to a generalization of the Prager-Hirschfelder perturbation scheme which invokes an electrostatic analogy. The philosophical implications of the hydrodynamical formulation of quantum mechanics are discussed. Penetration of a one-dimensional square potential barrier is used to demonstrate exactly how tunneling occurs by particles ``riding over the barrier'' à la Bohm. Cases are cited where quantum and classical mechanical motions are identical.
Ham effect in the 2T2u charge-transfer excited state of octahedral IrCl62−
Authors: Yeakel W. C. , Schatz P. N.
Published: February 2004

The Eg″ (2T2g)→ Eu″(2T2u)+Uu′(2T2u) allowed ligand-to-metal charge-transfer bands in octahedral ral IrCl62−(Cs2ZrCl6:Ir4+) which correspond to the excitation t2u(π)→t2g(d) show a highly resolved complex vibronic pattern over the region 22 900 – 24 500 cm−1 at liquid helium temperature. In this paper, it is shown that this spectrum can be interpreted in detail on the basis of a strong Jahn-Teller effect in the t2g molecular mode. The treatment is carried out in a basis spanning all vibronic functions through the first 11 quanta of the undisplaced t2g harmonic oscillator functions. All matrix elements involving both the first order Jahn-Teller Hamiltonian and spin-orbit coupling are included in a complete diagonalization. A large Ham effect quenches the spin-orbit coupling so that the no-phonon lines of the two spin-orbit components (Eu″ and U′u) are separated by ∼5–6 cm−1 rather than the expected value of ∼(3/4)ζCl≈440 cm−1. The calculated results explain semiquantitatively the over-all band spectrum in both absorption and MCD as well as the behavior of the no-phonon lines as a function of magnetic field (up to ∼70 kG) and temperature. There is additional low energy vibrational structure, undoubtedly due to lattice vibrations, the most important of which is probably Jahn-Teller active. The eg molecular vibration appears weakly, if at all. An alternative treatment assuming a strong Jahn-Teller effect in the eg mode does not satisfactorily account for the observed spectrum. Arguments are suggested to account for t2g rather than eg Jahn-Teller activity.
Infrared laser enhanced reactions: Spectral distribution of the NO2 chemiluminescence produced in the reaction of vibrationally excited O3 with NO
Authors: Braun Walter , Kurylo Michael J. , Kaldor Andrew , Wayne Richard P.
Published: February 2004

Vibrationally excited ozone, produced by CO2 laser radiation, was found to react significantly faster with NO than does thermal O3. The emission spectrum of the laser enhanced chemiluminescence from this reaction was measured from 520 to 810 nm. The lowest lying 12B2 state was identified as the primary source of NO*2 emission in the NO+O3 reaction. One quantum of vibrational excitation in the reactant O3 was found to introduce one quantum of vibrational energy in the product NO2 (12B2). The rate enhancement of the reaction channel producing NO2(12B2) as a result of vibrational excitation of O3 was 5.6±1.0. Thus, only about 50% of the available vibrational energy is used to enhance this reaction.
Molecular constants of some bent symmetrical XY2 molecules
Authors: Thirugnanasambandam P. , Mohan S.
Published: February 2004

On the basis of a revised set of symmetry coordinates, all the force constants (General Quadratic Valence Force Field) of 16 bent symmetrical XY2 type of molecules have been evaluated afresh by the Wilson's F-G matrix method using the Raman and infrared spectral frequencies. Certain simplifying considerations involving molecular dynamics have been made use of in solving the secular equation. The mean amplitudes have also been calculated at 298.16° K by Cyvin's method using the present values of the force constants.
Infrared and Raman spectra of polyethylene oxide in the low frequency region
Authors: Rabolt John F. , Johnson Kenneth W. , Zitter Robert N.
Published: February 2004

The far infrared and Raman spectra of polyethylene oxide (PEO) between 5 and 150 cm−1 has been measured for three crystalline samples with different degrees of polymerization. Lines are observed at 37, 52, 81, and 107 cm−1 in the infrared and at 37, 56, and 78 cm−1 in the Raman spectrum. Theoretical calculations show that the 81 and 107 cm−1 lines are assignable to the torsional motion of the PEO helical chain about the C–O bond. The absorptions at 37 and 52 cm−1 have not been previously reported or predicted by any normal mode calculations of the isolated 7/2 helix although they seem to arise from intramolecular rather than lattice vibrations.
Calculated photoionization cross sections and relative experimental photoionization intensities for a selection of small molecules
Authors: Rabalais J. Wayne , Debies Thomas P. , Berkosky Jeffrey L. , Huang Jan-Tsyu J. , Ellison Frank O.
Published: February 2004

The general equations for calculating photoionization cross sections of polyatomic molecules in the plane-wave and orthogonalized plane-wave approximations have been programmed for the electronic computer. Applications are described for the molecules H2, CH4, N2, CO, H2O, H2S, and H2CCH2 for incident photon energies from threshold to 1500 eV. Relative experimental photoionization band intensities for the above molecules are measured using NeI, HeI, and HeII resonance radiation. A simplified analysis of the intensities derived from electrostatic deflection analyzers is presented. The 2P3/2:2P1/2 intensity ratios of the rare gases are determined with the three modes of excitation mentioned above. An analysis of the resolution capabilities of an electrostatic deflection spectrometer as a function of electron kinetic energy is presented. Relative experimental photoionization band intensities for the above molecules obtained by ionization with the three uv sources and MgKa and A1Ka x-ray sources are compared with computed differential cross sections. Variations in computed cross sections as a function of the kinetic energy of the photoelectrons are discussed and possible interpretations are proposed.
Calculated angular distributions of photoelectrons using the orthogonalized plane-wave approximation
Authors: Rabalais J. Wayne , Debies Thomas P. , Berkosky Jeffrey L. , Huang Jan-Tsyu J. , Ellison Frank O.
Published: February 2004

The general equations for calculating angular distributions of photoelectrons within the orthogonalized plane-wave approximation have been programmed for the electronic computer. Applications are described for the molecules CH4, H2O, and N2 with incident photon energies from threshold to 1254 eV. Computed values of the asymmetry parameter β are compared to experimentally determined β values using HeI excitation. Large variations in β as a function of photon energy are obtained; possible interpretations are proposed.
Dipolar hard spheres: A Monte Carlo study
Authors: Patey G. N. , Valleau J. P.
Published: February 2004

The paper reports Monte Carlo computations for fluids of hard spheres with embedded point dipoles, at two densities and for a range of dipole moments. The results are compared with the predictions of the mean spherical and Onsager models and those of thermodynamic perturbation theory; the last appears to be most successful.
Matrix isolation infrared spectrum and vibrational potential function of sulfur monochloride
Authors: Noble Paul N. , Mei Lee
Published: February 2004

Infrared spectra of sulfur monochloride in various solid matrices were taken at 12°K. The sharpness of the band seen at 550 cm−1 compelled its assignment as the S–S stretching mode. Enrichment of samples with 37Cl led to the identification of the chlorine isotopic splittings on the two S–Cl stretching modes observed near 450 cm−1. A normal coordinate analysis was performed; on the basis of the best frequency fit, the band observed at 457.5 cm−1 was assigned to the symmetric S–Cl stretching mode and that at 449.0 cm−1 to the asymmetric mode. The force field derived from the normal coordinate analysis shows a weaker S–Cl bond than in SCl2, but essentially the same S–S bond strength as H2S2.
Excess energy dependence of radiationless transitions in naphthalene vapor: Competition between internal conversion and intersystem crossing
Authors: Fischer S. F. , Stanford A. L. , Lim E. C.
Published: February 2004

Experimental and theoretical evidence for effective internal conversion to the ground state from highly vibrationally excited states is presented for naphthalene-h8 and -d8. The evidence is based upon both the variation of the deuteron effect and strong exponential increase of the nonradiative decay rate as observed in the excess energy region between 10 000 and 18 000 cm−1. The calculations are only consistent with the experiments if it is assumed that the excess energy does not redistribute completely according to the statistical weighting among the normal modes but rather stays in the class of strongly accepting modes.
Decomposition of K and T matrices for inelastic scattering using variational principles
Authors: Conn Robert , Rabitz Herschel
Published: February 2004

A family of related approximate equations for K and T matrices applicable to inelastic scattering are derived from a variational principle using operator decompositions and projection operators. These approximations utilize the computational simplicity of first and second Born integrals but have the character of a resummed series such as to extend the usefulness of perturbation theory to stronger interactions and to provide information on multiquanta transitions. The approximations can be viewed as a reorganization of perturbation theory information to yield high-order nonperturbative results. The size of the matrices that enter the decomposition formulation is controlled by the states included in the projection operator. Coupling between these states is included to all orders and is not restricted to transitions between states coupled directly by the potential. The remaining degrees of freedom are treated in a renormalized Born approximation. The formulations can be developed for K or T, either fully off-shell, half off-shell, of fully on-shell, and the final approximations are rational in the potential strength. The approximations to the K matrix are Hermitian so that the resulting S matrix is unitary.
Fluorescence lifetimes of individual vibronic levels of partially deuterated benzenes: A further test of the theory of radiationless processes
Authors: Guttman Charles , Rice Stuart A.
Published: February 2004

This paper reports experimental studies of the nonradiative lifetimes of individual vibronic levels of several partially deuterated benzenes. The different deuterated species transform according to different symmetries, and therefore the number of totally symmetric modes differs from molecule to molecule. The observed rates of nonradiative decay fall into a regular pattern as a function of increasing deuteration. This observation, together with theoretical arguments, suggests that the accepting modes are, to a good approximation, local modes. Comparison of the predicted relative nonradiative rates with those observed is generally good, but not as accurate as for C6H6 and C6D6. The likely source of the discrepancy is discussed.
Nonradiative processes in p -C6H4F2 and m -C6H4F2
Authors: Guttman Charles , Rice Stuart A.
Published: February 2004

The nonradiative lifetimes and quantum yields of individual vibronic levels of m - and p -C6H4F2 have been experimentally determined. The data collected for the difluorobenzenes show great differences between the nonradiative decay processes in the para and meta compounds and similarity between these processes in meta -difluorobenzene and monofluorobenzene. It is suggested that the differences cited are a consequence of the different molecular symmetries and, as a result, the different possible accepting modes and the manner in which the excess energy distributes itself in the triplet manifold.
Relaxation of HF(v = 1) by various polyatomic molecules
Authors: Bott J. F. , Cohen N.
Published: February 2004

The vibrational relaxation of HF(v = 1) by various polyatomic molecules has been measured at 295°K by laser-induced fluorescence. The collision partners include N2O, SO2, H2S, COF2, BF3, NF3, CBrF3, CF4, SiF4, SO2F2, C2F6, C4F8, PF5, ClF, DBr, and SF6.
Microwave spectrum, structure, and dipole moment of 2,3,4,5-tetracarbahexaborane(6)
Authors: Pasinski John P. , Beaudet Robert A.
Published: February 2004

The structure of 2,3,4,5-tetracarbahexaborane (C4B2H6) has been determined from the microwave spectra of 10 isotopic species. The pentagonal pyramidal geometry of the molecule has been confirmed. The four carbon atoms lie in the pentagonal base; the short C–C bond lengths (1.43 Å) suggest pi-bonding character. A molecular dipole moment of 2.26 ± 0.06 D was determined from the observed Stark effect.
Electron-molecule scattering and molecular photoionization using the multiple-scattering method
Authors: Dill Dan , Dehmer J. L.
Published: February 2004

We adapt the multiple-scattering method to treat unbound electronic states of molecules in the independent electron approximation. An inhomogeneous linear system is derived whose solution yields the K matrix for the electron-molecule interaction. Using the K matrix, we derive continuum electronic wavefunctions by imposing boundary conditions corresponding to electron-molecule scattering and molecular photoionization, i.e., the wavefunctions satisfy the so-called outgoing-wave and incoming-wave normalization, respectively. These wavefunctions are then used to obtain expressions for elastic scattering and photoionization differential cross sections.
Kinetics of the reaction between oxygen atoms and carbon disulfide
Authors: Slagle Irene R. , Gilbert James R. , Gutman David
Published: February 2004

The open channels of the room temperature 0+CS2 reaction have been identified by studying this reaction in high-intensity undiscriminated crossed molecular beams using photoionization mass spectrometry to detect the products. Three open channels have been identified: O + CS2 → CS + SO (1); O + CS2 → OCS + S (2); and O + CS2 → CO + S2 (3). The overall rate constant (k) of the O + CS2 reaction as well as the branching ratio (k2/k) for Channel (2) were measured at 302°K using a fast-flow reactor. They are k=4.0×10−12 cm3 p−1 sec−1, and k2/k=0.093. Speculation on the branching ratios for Channels (1) and (3) are offered based on the results of the crossed beam experiments. The possible roles of OCS production by Channel (2) and CO production by Channel (3) in the performance of the O+CS2 chemical laser are mentioned.
Self-consistent time-dependent Hartree-Fock scheme
Authors: Jo/rgensen Poul , Oddershede Jens , Ratner Mark
Published: February 2004

We have derived a self-consistent time-dependent Hartree-Fock scheme based on a Green's function approach. The contour integral is performed analytically, which makes the computational effort per iteration comparable with a normal time-dependent Hartree-Fock calculation. The excitation energies are found as poles for the polarization propagator in the energy representation. The corresponding residues determine the transition moments. Comparison is made with other similar random-phase approximations. We have applied our scheme to the π-electron systems of trans -butadiene, pyridine, and benzene. The triplet spectra of those molecules especially are considerably improved over normal time-dependent Hartree-Fock results.
Angular correlations in linear copolymers from the compositional dependence of their dipole moments. II. Markovian typal sequences
Authors: Work Richard N.
Published: February 2004

An earlier treatment of the problem of extracting configurational information from experimental determinations of the compositional dependence of the dipole moments of copolymers containing random sequences is extended to include the case wherein typal correlations arise from pair dependent probabilities for the occurrence of the structural units in the copolymer. If the typal correlations are sufficiently strong, it is possible to evaluate certain, meaningful, linear combinations of terms containing the average cosines of the angles between the dipole moments of nearby pairs of structural units, as well as the angular correlation length M, measured by the number of structural units beyond which the average cosines are negligible. Conditions are given in order to be able to separate and evaluate the average cosines belonging to individual pairs of structural units.
Flash photolysis study of the gas phase recombination of hydroxyl radicals
Authors: Trainor Daniel W. , von Rosenberg C. W.
Published: February 2004

Rate constants are reported for the reactions OH+OH → H2O+O (where k1 is the rate constant) and OH+OH+N2 → H2O2+N2 (where k5 is the rate constant) measured in a room temperature flash photolysis experiment. Hydroxyl radicals were produced by the photodecomposition of water vapor, and time resolved absorption spectroscopy was utilized to determine the absolute OH concentration. Specific rate constants obtained are k1=2.1±0.2×10−12 cm3/sec and k5=2.5±0.3×10−31 cm6/sec.
Response of a quadrupolar spin system in zero applied field to a long, off-resonant radio-frequency pulse
Authors: Pratt J. C. , Raghunathan P. , McDowell C. A.
Published: February 2004

The response of a system of nuclear spins I=3/2 experiencing electric quadrupole couplings in zero applied field to a long, off-resonant radio-frequency pulse is investigated using simple time-dependent perturbation theory. It is found that the height of the transient following such a pulse varies with the distance from exact resonance, Δω, in the same manner as previously found for the case of a purely magnetically coupled spin system in high field, in which the rf pulse is of sufficient duration to allow the magnetization to become aligned along the effective field when viewed from the rotating frame. We find, in addition, that if a short resonant ``sampling'' pulse is applied after the transient from the long pulse has died down, the maximum height of the transient following this second pulse varies with Δω in the same manner as the response of the Zeeman system to this pulse sequence. Furthermore, with this two-pulse sequence, an echo-type transient is predicted to appear at a time 2τ, where τ is the time at which the sampling pulse is applied. We assume that the whole sequence is over in a time very much less than that characteristic of spin-lattice relaxation processes. The theory, which is extended to cover the general case of a nonaxially symmetric electric field gradient, is verified semiquantitatively by experiment.
Carbon-13 nuclear magnetic resonance in solid ammonium tartrate
Authors: Pines A. , Chang J. J. , Griffin R. G.
Published: February 2004

Chemical shielding tensors have been determined for 13C in single crystals of ammonium D-tartrate using proton-enhanced NMR. The unit cell contains two molecules, and in some orientations eight lines are resolved from the eight magnetically inequivalent carbon nuclei. Details of the experimental approach, equipment, and analysis are provided, and results are presented for three determinations on two single crystals. These indicate a precision of ±2 ppm in the determination of the principal values and ±3° in the orientation of the principal axes for the carboxyl groups, with lower precision for the hydroxyl groups. The orientations of the principal axes of the tensors are related to the crystallographic (a, b, c*) axes by employing orientation data from x-ray precession work in this study. The most plausible assignment of the tensors to particular 13C nuclei in the unit cell indicates that the heaviest shielding for 13C in the carboxyl groups occurs when H0 is perpendicular to the carboxyl plane and the lightest shielding when H0 is in the carboxyl plane bisecting the C–O bonds. For the hydroxyl 13C, the heaviest shielding occurs with H0 roughly along the C–O bond. The average principal shielding values are, for the carboxyl 13C, σ11=−110.5, σ22=−61.7, σ33=21.6; and for the hydroxyl 13C, σ11=41.9, σ22=48.0, σ33=74.0 in parts per million (±2) relative to an external reference of liquid benzene. The effects of hydrogen bonding on the tensors are discussed briefly.
Role of ions in vapor phase nucleation
Authors: O'Brien E. F. , Robinson G. W.
Published: February 2004

When the interaction between an impurity and a molecule of a nucleating substance is much stronger than the interaction between two molecules of that substance, the effects on nucleation are substantial. An example is an ionic impurity. In this paper the sources of these effects are quantitatively identified at the molecular level for a microcrystal model analogous to the one employed by McGinty for homomolecular nucleating clusters. The dominance of the potential energy contribution to the various thermodynamic functions for small clusters allows direct estimates of the effect of the impurity on these functions. If the functions are known for homomolecular clusters, detailed calculations need not be performed to obtain good estimates of the thermodynamic functions for impurity-containing (heteromolecular) clusters. When the potential energy contribution per molecule is very large, a deep minimum in the free energy of formation vs cluster-size function can occur at the size where the first nearest neighbor shell becomes filled. This leads to the formation of quasistable aggregates of small size which participate in the steady state nucleation process. Under certain thermodynamic conditions where this minimum is broad, a metastable aerosol is formed; under still other conditions where this free energy minimum is very shallow or nonexistent, nucleation is spontaneous, the rate depending only upon the collision frequency. The results presented here should not only lead to better intuition about the role played by ions or other strongly interacting impurities in upper atmosphere phenomena and in smog formation, but should also be useful in the understanding of heterogeneous nucleation in liquid and solid phases or at active sites on surfaces.
ENDOR studies of the superhyperfine couplings of hydrogen bonded protons. II. Protonated radical anion in irradiated succinic acid single crystals
Authors: Muto Hachizo , Nunome Keichi , Iwasaki Machio
Published: February 2004

The carboxyl radical anion trapped at 77°K in an irradiated single crystal of succinic acid has been studied by ENDOR in order to determine if the anion is protonated. The ENDOR spectra showed that the anion exhibited hyperfine couplings to the two OH protons. The principal values of the two coupling tensors are nearly the same and the principal directions are accounted for by the structure of R–Ċ(OH)2, that is, the protonated form of the anion. The location of the extra proton suggests that the proton is transferred from the neighboring molecule across the hydrogen bridge.
Theory of T1 relaxation measurements in pure nuclear quadrupole resonance for spins / = 1
Authors: Vega Shimon
Published: February 2004

A quantitative analysis of the effect of spin lattice relaxation rates on pure quadrupole resonance pulse experiments for spins with I = 1 is given. A model is presented in which the three dimensional relaxation problem is reduced to a two dimensional one. In this model any possible population distribution of the energy levels corresponds to a point in a plane, the ``ν plane.'' The response of the system to spin-lattice relaxation effects and to rf pulses are described as translations of points in this plane. The experimental signal intensities are directly related to the coordinates of those points. The following cases are treated quantitatively: (i) CW saturation, (ii) CW saturation followed by a 90° pulse, (iii) two-pulse sequences, and (iv) continuous steady state pulse sequences. For each case explicit equations for the signal intensity as function of the experimental parameters are given. The analysis is made for both single crystals and powders.
Electron excitation of the (0,0) second positive band of nitrogen from threshold to 1000 eV
Authors: Imami Mahmood , Borst Walter L.
Published: February 2004

The excitation of the 2P G (0,0) band of nitrogen (λ3371) by electron impact was studied from threshold (11 eV) to 1000 eV. In view of the serious discrepancies in the existing cross section measurements for this band at higher energies (> 30 eV), the effects due to high pressures and secondary electrons were carefully studied. Secondary electron effects were monitored by scanning the luminosity profiles in the collision chamber with a moveable optical detector assembly. The signal count rates were also monitored as a function of pressure. As a result of these checks, the present measurements were carried out at pressures in the 10−5 torr range. In order to increase the signal count rates at these pressures and high electron energies, the collision chamber was given the shape of an ellipsoid with highly polished interior surfaces. The electron gun was of the electrostatically focussed type. The signal count rates were compared with those from the 1N G (0,0) band at λ3914, for which the cross section is well established. As a result, it was found that the cross section for the 2P G (0,0) band decreases rapidly with increasing energy, but not rapidly enough to yield the expected E−3 dependence. The cross section has a value of 1.3×10−19 cm2 at 100 eV, which is about an order of magnitude smaller than most existing measurements.
Piezoelectric nonlinear optic CuGaSe2 and CdGeAs2: Crystal structure, chalcopyrite microhardness, and sublattice distortion
Authors: Abrahams S. C. , Bernstein J. L.
Published: February 2004

CuGaSe2 and CdGeAs2 crystallize in the chalcopyrite-type structure. The lattice constants of tetragonal CuGaSe2 are a = 5.5963 ± 0.0001 and c = 11.0036 ± 0.0002 Å at 298 °K, those of CdGeAs2 are a = 5.9432 ± 0.0001 and c = 11.2163 ± 0.0003 Å at 298°K. The space group is I4̄2d with four formulas in a unit cell. 1747 reflections were measured with PEXRAD for CuGaSe2 and 1892 for CdGeAs2 using MoKα x radiation. Least squares refinement of the structural parameters using 149 symmetry-independent CuGaSe2 and 300 CdGeAs2 structure factors led to final agreement factors of 0.035 and 0.038, respectively. CuGaSe2 is slightly Cu deficient: the value of the x coordinate in the final model is 0.2431 ± 0.0002, with considerable vibrational anisotropy. CdGeAs2 is stoichiometric: the final model has x = 0.2785 ± 0.0002, also with significant vibrational anisotropy, but less than that in CuGaSe2. The two compounds confirm the pattern of sublattice distortion in AIBIIIC2VI and AIIBIVC2V chalcopyrites previously found. The closest Cu–Se distance is 2.385 Å, Ga–Se is 2.435 Å, Cd–As is 2.629 Å and Ge–As is 2.430 Å, all ± 0.001 Å. The effective Debye characteristic temperatures are simply related to the reported microhardnesses of the chalcopyrites.
Reaction rate constants in steady-state hollow cathode discharges: N2 + H2O reactions
Authors: Howorka F. , Lindinger W. , Varney Robert N.
Published: February 2004

In a cylindrical hollow cathode discharge in N2 with traces of H2O, there is a distribution of ion types from the edge of the negative glow to the axis of the cylinder. This distribution was explored by using a small, radially movable probe with an entrance aperture to a mass spectrometer in the probe. Ions could then be extracted from the negative glow plasma from any desired radial location in the cylinder. The results show N2+ and N+ at the edge of the glow giving way to N2H+, H2O+, and H3O+ toward the axis. Clearly, ion-molecule reactions are proceeding in the field-free negative glow region. Quantitative rate constant measurements were obtained for formation and destruction of N2H+, H2+, N+, and the formation of H2O+ and H3O+. The measurements on N2+ and N+ formation by electron collisions give values for the density of high energy electrons in the negative glow. Adequate agreement is found with values obtained by other workers using other methods, where overlap occurs. The present new and different method thus lends support to older findings as well as providing new information.
Monte Carlo simulation of physical clusters of water molecules
Authors: Abraham Farid F.
Published: February 2004

The energy, specific heat, and radial density distribution of physical clusters containing 8, 27, and 64 water molecules have been calculated for 263.2 and 298.0°K using the Monte Carlo method. A pairwise water-water potential is used which has been constructed from Hartree-Fock calculations and corrected for correlation energy effects. For the larger clusters, a radial dipole ordering of the water molecules in the core of the clusters is found, the effect being more pronounced at the lower temperature. The results of these numerical experiments are compared with results obtained for bulk water and for clusters of 8 water molecules at zero temperature using the same water-water potential.
Ultrasonic study of critical mixing of hexane and β, β′-dichloroethyl ether
Authors: Bains Elizabeth M. , Breazeale M. A.
Published: February 2004

The velocity and attenuation of ultrasonic waves in the critical temperature region of a binary mixture of hexane and β, β′-dichloroethyl ether have been measured at 1, 3, 5, 7, and 9 MHz. By assuming that the attenuation per wavelength is made up of two terms, αλ =Af−1/4+Bf, we obtain good agreement with the Fixman theory for both αλ and α/f2. The magnitude of B, however, is only partially accounted for by our measured values of shear viscosity.
Calculated self- and foreign-gas-broadened linewidths for CH3D
Authors: Tejwani G. D. T. , Fox K.
Published: February 2004

Self-, N2−, O2−, and H2− broadened linewidths for CH3D have been calculated using the Anderson-Tsao-Curnutte theory. Interactions due to the dipole and octopole moments of CH3D were included. Linewidths have been computed for a wide range of angular momentum quantum numbers J and K, and temperature dependence has been obtained for 100 ≤ T ≤ 300°K.
Quantum ergodicity and vibrational relaxation in isolated molecules. II. λ-independent effects and relaxation to the asymptotic limit
Authors: Nordholm Sture , Rice Stuart A.
Published: February 2004

In this second article on quantum ergodic properties of simple anharmonic molecular models, we present further numerical calculations for the Barbanis and Henon-Heiles models. For the cases studied herein, the density of energy eigenstates is larger by about a factor of five than that used earlier. We show that the character of the low lying states of the model Hamiltonians is dominated by λ-independent effects which can be obtained by second order degenerate perturbation theory. Nondegenerate modifications of the above systems are again found to be highly nonergodic. We also discuss estimates of the time scale upon which the dynamics takes place. Two simple estimates are obtained on the basis of the ergodic properties and found to be reasonably accurate for the Barbanis system.
Study of the structure of molecular complexes. VIII. Small clusters of water molecules surrounding Li+, Na+, K+, F−, and Cl− ions
Authors: Kistenmacher H. , Popkie H. , Clementi E.
Published: February 2004

The two-body Hartree-Fock potential for the ion-water interaction and the two-body Hartree-Fock potential for the water-water interaction have been used in the pairwise additivity approximation to study the Li+(H2O)n, the Na+(H2O)n, the K+(H2O)n, the F−(H2O)n, and the Cl−(H2O)n complexes, where n =2,3,4, ⋯ ,10. The complex configurations have been constrained to have either symmetrical geometries around the central ion or to be free to assume the lowest energy configuration. For n smaller than 5 (depending on the specific ion in consideration), the symmetrical configuration is the lowest energy configuration. For higher values of n, some of the water molecules tend to form a second shell of solvated water around the ion. The configurational optimalization was carried out only at T =0°K; but for a small cluster containing only four molecules of water, calculations have been performed at T =298°K. From the study at 298°K we have computed the correlation functions gI–O, gI–H, gO–O, gO–H, and gH–H (where the subscript I is a shorthand notation for ``ion''). Correlation functions are reported for the cluster F−(H2O)n at T =298°K with n =27. By comparing the results obtained at T =0°K and with n =10, with those obtained at T =298°K and n =4 and finally with the results obtained at T =298°K and n =27, we feel confident that the conclusions (given below) will remain valid for n >27 and at t ≠0°K. The coordination numbers for the ion-water clusters are computed (approximatively) to be about 4 for Li+, between 5 and 6 for Na+, between 5 and 7 for K+, between 4 and 6 for F, and between 6 and 7 for Cl−. In the first solvation layer, the average ion oxygen distances are 1.9–2.0 Å for Li+, 2.3–2.4 Å for Na+, 2.8–2.9 Å for K+, 2.7–2.8 Å for F−, and 3.4–3.5 Å for Cl−. The validity of the pairwise additivity approximation has been tested for the small clusters ion (H2O)n with n =2,3 (and 4 and 5 for Li+ and F−).
An x-ray, spectroscopic, and magnetic study of the structure of nickel squarate dihydrate, NiC4O4 · 2H2O
Authors: Habenschuss Michael , Gerstein B. C.
Published: February 2004

The structure of nickel squarate dihydrate, NiC4O4 · 2H2O, has been investigated by initial susceptibility measurements, infrared spectroscopy, and single crystal x-ray diffraction techniques. The results indicate that the structure consists of Ni ions situated at the centers of the edges of a cube, with squarate ions in the cube faces. Each nickel is roughly octahedrally coordinated by four squarate oxygens, and by two water oxygens. The lattice constants were found to be a = b = c =8.0685A, with α = β = γ = 90° within experimental accuracy. The structure was found to be noncubic. A spin-orbit coupling parameter of −280 (30) cm−1, which is 86% of the free ion value, is obtained from the magnetic and spectroscopic results. The structure of this compound, which is isostructural with the Mn(II), Fe(II), and CO(II) squarates, is different than that proposed by two other sets of workers. All crystals investigated were found to be macroscopically twinned, and a reason for such twinning is proposed.
Macroscopic viscosity and the dielectric relaxation of aliphatic chain compounds
Authors: Crossley John
Published: February 2004

Dielectric constants and losses have been measured at up to eight frequencies between 1 and 35 GHz for six 1-bromoalkanes in three nonpolar solvents of different viscosity (η). The data have been analyzed for mean relaxation times (τ) which are compared with those previously obtained for liquid 1-alkenes, n-alkanones in several solvents, liquid 1-bromoalkanes and 1-bromoalkanes in cyclohexane solution. Relaxation times for 1-alkenes calculated from the viscosity dependence of the relaxation times for corresponding 1-bromoalkanes compare quite well with the experimental values. It would appear that the mechanisms of dipole reorientation for 1-bromoalkanes and 1-alkenes are similar. Plots of log τ against log η for several n-alkanones and 1-bromoalkanes are linear and the slopes for all the systems are similar.
On the reduction of many-body dielectric theories to the Onsager equation
Authors: Ramshaw John D.
Published: February 2004

An approximate theory for the dielectric constant ε of a dense polar fluid was derived by Ramshaw, Schaefer, Waugh, and Deutch (RSWD). In the present article, the RSWD theory is generalized and made rigorous by another method of derivation. The result is a rigorous expression for ε which differs from the RSWD expression by the presence of a fluctuation term. Both the rigorous expression and the RSWD expression are then specialized to the Onsager model. It is found that the rigorous expression for ε reduces to the Onsager equation, but that the RSWD expression does not because the fluctuation term is nonzero (and nonnegligible) for the Onsager model. The well-known discrepancy between the Onsager equation and the theory of Harris and Alder is found to have the same origin.
Analysis of rotational barriers in molecules on the basis of exchange perturbation theory
Authors: Lombardi E. , Tarantini G. , Pirola L. , Jansen L. , Ritter R.
Published: February 2004

Exchange perturbation theory of the Rayleigh-Schrödinger type in first and second orders is applied to a calculation of barriers of internal rotation about single bonds in molecules. The barrier is assumed to arise as a result of exchange interactions between nonbonded atoms; the electrons of these atoms are replaced by one effective electron per atom, with a Gaussian charge distribution. Spins of effective electrons on different atoms are taken to be parallel. The method is applied to a large variety of molecules; calculated results for the barriers are in substantial agreement with experiment. A decomposition of the interaction energy for different conformers and for the barrier itself is given in terms of two- and many-atom exchange interactions.