bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized Einstein relations for rotational and translational diffusion of molecules including spin
Authors: Hwang Lian-Pin , Freed Jack H.
Published: February 2003

In this work the validity of generalized Einstein relations, D (ω) = kT/Iβ (ω), where D (ω) and β (ω) are frequency-dependent diffusion and damping coefficients, is examined from a general point of view. It is shown that generalized Smoluchowski (S) equations for both translational and rotational diffusion involving the D (ω), which are defined in terms of simple correlation functions of the fluctuating forces and torques, follow from the appropriate generalized Fokker–Planck (FP) expressions when the translational and angular momenta are taken as rapidly relaxing and a coarse graining in time is introduced. It is found useful in this context to distinguish between those forces and torques fluctuating at rates faster than the diffusive-type motions of the B particle versus those fluctuating more slowly. The appropriate FP equation for rotational diffusion is also derived. Generalized FP and S equations are also derived for the semiclassical case where the molecule(s) examined contains spin degrees of freedom. A proper application of the correspondence principle leads to certain terms referred to as ’’spin-force’’ or ’’spin-torque’’ terms which have the property of tending to restore the spins to their thermal equilibrium value, an important feature usually lacking in semiclassical treatments. Some simple examples of the generalized S equations are given in terms of memory function approximations of the random force and torque correlation functions. The resulting expressions are seen to bear a close formal similarity to typical expressions developed for simple jump models in either orientational or translational space. It is suggested that recent frequency-dependent experiments, including some previously interpreted in terms of simple jump models, may be amenable to analysis in terms of generalized Einstein and S equations.
Temperature relaxation in a binary gas. I. Steady state solution
Authors: Fitzpatrick J. M. , Shizgal B.
Published: February 2003

A uniform mixture of two gases, initially with each component at different temperatures, is studied with regard to the equilibration of the temperatures. The extent of the perturbation of the velocity distribution functions of both components during the relaxation to equilibrium is determined. In this calculation it is assumed that the collision rate between unlike particles is much slower than either of the two collision rates between like particles and that a steady state is established quickly before substantial equilibration occurs. With this assumption the Boltzmann equations are linearized and uncoupled, the correction to the equilibrium rate of temperature relaxation is calculated, and the departure from equilibrium is determined. With the hard-sphere interaction for all binary elastic collisions, the magnitude of the deviations from equilibrium is determined as a function of the masses, concentrations, and initial temperatures of the two components.
Temperature, concentration, and molecular weight dependence of the twist elastic constant of cholesteric poly-γ-benzyl-L-glutamate
Authors: DuPré Donald B. , Duke Russell W.
Published: February 2003

Aspects of the behavior of the twist elastic modulus K22 of a lyotropic liquid crystal formed by the α-helical polypeptide poly-γ-benzyl-L-glutamate of three molecular weights (MW= 300 000, 310 000, and 550 000) in three solvents (dioxane, dichloromethane, and chloroform) were investigated as a function of concentration and temperature. We find that K22 in this system may be made to vary over a wide range by adjustment of solvent and polymer molecular weight. Only a slight concentration dependence of K22 throughout the liquid crystal range was, however, noted. The variations can be at least partially understood in terms of changing macromolecular interactions and recent statistical hard-rod theoretical treatments of liquid crystal elasticity. The functional form for the temperature dependence of the order parameter was also determined for the system through the clearing point.
The high pressure P
V
T properties of deuterium oxide
Authors: Fine Rana A. , Millero Frank J.
Published: February 2003

The high pressure isothermal compressibilities of deuterium oxide from 5 to 100 °C and 0 to 1000 bars applied or gauge pressure were determined from sound speed data. These compressibilities were used to derive an equation of state of the form V0P/(V0 − VP) = B + A1P + A2P2, where V0 and VP are the specific volumes at an applied pressure of zero and P; and B, A1, and A2 are polynomial functions of temperature. The compressibilities derived from this equation of state are consistent with those derived from the sound speed data to ±0.016×10−6 bar−1 over the entire pressure and temperature range (this is equivalent to ∼0.2 m sec−1 in sound speed). The 1 atm sound-derived compressibilities agree on the average to ±0.06×10−6 bar−1 with the direct measurements of Millero and Lepple. The P–V–T data from the sound-derived equation are compared with the high pressure work of Bridgman, Kesselman, Juza et al., and Emmet and Millero. Good agreement (average deviation of ±28×10−6 cm3 g−1) was found with the recent specific volume measurements of Emmet and Millero. The P–V–T properties of D2O are compared to pure water. D2O and H2O are shown to follow similar trends. Contrary to previous reports, the D2O/H2O ratios of the specific volumes and specific heats are shown to be functions of both temperature and pressure.
Temperature relaxation in a binary gas. II. Time dependent solution
Authors: Shizgal B. , Fitzpatrick J. M.
Published: February 2003

The time dependent Boltzmann equations describing the temperature relaxation in a hard-sphere gas are solved and the explicit time dependence of the distribution functions is determined. The time evolution of the perturbations of the distribution functions is studied in detail with a moment method. The range of validity of the earlier results obtained with a steady state assumption is determined. Provided that the intial temperature ratio is neither too large nor too small (102–10−2), it is found that the steady state assumption is valid only in the extreme disparate mass limit, i.e., m1/m2 of the order of 10−3–10−5. Qualitatively it appears that the ratio of the self-relaxation times to the temperature equilibration time must be of the order of 10−3–10−4 or smaller for a steady state to occur. Since the temperature relaxation rate is slow for this range of mass and initial temperatures, there is an extremely small perturbation of the distribution functions from the Maxwellian form. Perturbations of the distribution function which result in departures of a few percent from the equilibrium estimates of the temperature ratio or the temperature difference are found to occur for only the fastest relaxation rates. Estimates of these small departures from equilibrium based on the steady state assumption are very much in error.
Excitons and polaritons in polymers
Authors: Philpott Michael R.
Published: February 2003

The coupling of the lowest excited electronic states of a long chain of molecules to the radiation field gives rise to collective excitations with some of the characteristics of polaritons in three dimensional solids. In the model considered, the polariton spectrum has two branches. The states of the first branch have lifetimes up to λ/a times shorter than the transition in the isolated molecule and are therefore superradiant. Resonances in the scattering amplitudes of incident plane wave monochromatic fields locate the energies of the first branch. The other branch is subradiant, i.e., it does not radiate at all, and has a linear dispersion relation over the greater part of the interval 0<ω≲ω1, where ω1 is the exciton transition frequency, and corresponds to a photon localized about the axis of the chain. The problem of detecting polaritons in long molecular chains is briefly discussed and the theory of polaritons in polymers on a transparent substrate is outlined.
Lifetime studies of Na(4 2P) produced by photodissociation and quenched by iodine
Authors: Bersohn R. , Horwitz H.
Published: February 2003

Sodium atoms were generated in the 4 2P state by vacuum ultraviolet nanosecond flash photolysis of sodium iodide vapor. The fluorescence lifetime of the sodium atom was measured as a function of iodine pressure. The quenching cross section so derived is 175±22 Å2 which can be compared with 210 Å2 for the 3 2P state. This decline in the cross section becomes more dramatic in the case of K(5 2P) as shown by Earl and Herm. The basic reason is that these second excited states have such high energy that they lie above the ionic surface and the usual form of level crossing is impossible. To further clarify the problem it is essential to have more information on the final ionic products.
Padé approximant for the internal energy of a system of charged particles
Authors: Stell G. , Wu K. C.
Published: February 2003

A Padé approximant is used to extrapolate to high charge strength the series expansion in charge strength of the internal and free energies of a system of charged spheres. The numerical results for the internal-energy approximant are in excellent agreement with the Monte Carlo results of Larsen, especially at higher densities. The study indicates that the Padé approach is a promising method of treating the thermodynamics of more realistic fused-salt models.
Abstraction versus exchange in the reaction of D with HBr
Authors: Su H. Y. , White John M.
Published: February 2003

The photolysis of HBr–D2–Br2 mixtures at 185 nm has been investigated and the results have been analyzed to provide limits for hot deuterium atoms on the ratio of abstraction (D*+HBr →k8 DH+Br to exchange (D*+HBr →k9 DBr+H). For the hot deuterium atoms employed in these experiments, the ratio k8/k9 lies between 0.7 and 1.5. The results of quasiclassical trajectory calculations are shown to be in fairly good agreement with this experimental result. Comparison is made with a recent 300 °K thermal value of 3.2. Using molecular beam data for the exchange reaction cross section (1–10 Å2) we estimate the abstraction reaction cross section is of the same order of magnitude and lies in the range 0.2–15 Å2.
Studies of polyelectrolyte solutions containing mixtures of mono- and divalent counterions. III. Volume change on dilution
Authors: Škerjanc J. , Dolar D.
Published: February 2003

The volume change on dilution of aqueous solutions containing mixtures of polystyrenesulfonic acid and its magnesium salt have been determined at 25 °C by dilatometry for various equivalent fractions of the acid. Experimental values have been compared with those predicted by the cell model with cylindrical symmetry. A reasonable agreement between theory and experiment has been found.
Quantum mechanical studies of collision induced dissociation: Discretization theory
Authors: Wolken George
Published: February 2003

The collinear collision of an atom with a diatomic molecule A +  (B,C) is formulated quantum mechanically using the close coupling approach. Neglecting rearrangement collisions but including electronically adiabatic inelastic and dissociative processes, it is shown that the resulting continuously infinite set of coupled differential equations can be reformulated as a discrete set of coupled differential equations. The technique is analogous to a procedure developed to treat gas–solid energy transfer. However, the boundary conditions involving three free particles in the final state has no analog in the gas–solid formulation of the problem. These are formulated also in terms of the discretized equations, and various simplifications and symmetries of the equations are discussed.
Depolarized Rayleigh scattering and orientational relaxation of molecules in solution. IV. Mixtures of hexafluorobenzene with benzene and with mesitylene
Authors: Bauer D. R. , Brauman J. I. , Pecora R.
Published: February 2003

Measurements of the orientational relaxation times of the components of mixtures of hexafluorobenzene with benzene and mesitylene have been made by depolarized Rayleigh scattering. In both mixtures, there were strong ’’static’’ and ’’dynamic’’ cross correlation terms which affected the individual reorientation times. There was no significant contribution to the Rayleigh spectra from a long-lived complex in these mixtures.
Carbon-13 relaxation in multispin systems of the type AXn
Authors: Werbelow Lawrence G. , Grant David M.
Published: February 2003

The equations of time evolution which describe various aspects of longitudinal relaxation in methylene and methyl groups are derived on the basis of the Redfield–Bloch density operator theory of relaxation. It is assumed that the relaxation is dominated by intramolecular dipole–dipole interactions. The (cross-) correlation between the various internuclear vectors is explicitly taken into account; it is not assumed that the various pairwise interactions are additive. Although such correlation effects have long been dismissed, the present calculations demonstrate quite vividly such effects may be much more influential than is commonly thought, especially in the relaxation of carbon. Multiplet relaxation in the four-spin 13CH3 system is discussed in some detail with numerical calculations included to provide some compelling arguments against continued complacency regarding the need to consider multispin correlations. More general applications of the present theory await many inviting experimental studies.
Excited electronic states of HNC, hydrogen isocyanide
Authors: Schwenzer Gretchen M. , Schaefer Henry F. , Bender Charles F.
Published: February 2003

Walsh’s rules suggest that the excited states of HNC should have bond angles similar to the analogous states of HCN. To test this hypothesis, ab initio calculations have been carried out and compared to earlier calculations on HCN. The most surprising result is the prediction that several of the excited electronic states of HNC lie below the corresponding states of HCN. Also unanticipated are the unusually long CN bond lengths found for several of the lower excited states of HNC. The excited singlet states have bond angles qualitatively similar, although somewhat smaller, than those of HCN. For the triplet states, Walsh’s concept appears less successful, primarily owing to strong mixing between several different electronic configurations. Mulliken populations are used in the discussion of these results.
Resonance Raman spectra of Cu-1:3:5:7-tetramethyl porphin and Cu-1:2:3:4:5:6:7:8-octamethyl porphin
Authors: Sunder S. , Mendelsohn R. , Bernstein H. J.
Published: February 2003

Resonance Raman spectra of Cu-1:3:5:7-tetramethyl porphin and Cu-1:2:3:4:5:6:7:8-octamethyl porphin have been obtained both in dilute CS2 solution and as solids dispersed in KBr matrices using the Ar+ and tunable Rhodamine 6G dye lasers. The solution data for Cu-octamethyl porphin show that its rigorous molecular symmetry in solution is D4h, while the spectral data for Cu-tetramethyl porphin are consistent with C4h molecular symmetry. Assignments for some of the stronger bands are discussed by comparing the spectra of these two molecules with other planar metalloporphins and porphyrins.
Quality of Gaussian basis sets: Direct optimization of orbital exponents by the method of conjugate gradients
Authors: Kari Roy E. , Mezey Paul G. , Csizmadia Imre G.
Published: February 2003

Expressions are given for calculating the energy gradient vector in the exponent space of Gaussian basis sets and a technique to optimize orbital exponents using the method of conjugate gradients is described. The method is tested on the (9s5p) Gaussian basis space and optimum exponents are determined for the carbon atom. The analysis of the results shows that the calculated one-electron properties converge more slowly to their optimum values than the total energy converges to its optimum value. In addition, basis sets approximating the optimum total energy very well can still be markedly improved for the prediction of one-electron properties. For smaller basis sets, this improvement does not warrant the necessary expense.
On the anisotropy of the H2–H potential energy surface
Authors: Norbeck J. M. , Certain P. R. , Tang K. T.
Published: February 2003

There have been numerous attempts to calculate the potential energy of the H–H2 system. In this communication, we discuss previous estimates of the anisotropy of the surface, and present new calculated results for the leading anisotropic dependence.
Photodetachment of electrons from large molecular systems: Benzyl anion. An upper limit to the electron affinity of C6H5CH2
Authors: Richardson Jeffery H. , Stephenson L. M. , Brauman John I.
Published: February 2003

The relative cross section for the gas phase photodetachment of electrons has been determined for the benzyl anion in the wavelength region 870–1330 nm (1.43–0.93 eV). An ion cyclotron resonance spectrometer was used to generate, trap, and detect the negative ions and a 1000 W xenon arc lamp with a grating monochromator was employed as the light source. An upper limit to the electron affinity of the benzyl radical was determined to be 0.88±0.06 eV (20.4±1.5 kcal/mole).
Method for direct determination of localized orbitals
Authors: Levy Mel , Nee Tsun-Shi , Parr Robert G.
Published: February 2003

The orthogonalized Hartree method is put forward as a method for directly determining localized orbitals in atoms and molecules. To obtain the orbitals in this method one minimizes the energy functional for a Hartree product of orthonormal orbitals. If desired, the exchange energy for these orbitals is then subtracted. It is shown rigorously that this method gives orbitals more exchange-localized than can be obtained from the Hartree–Fock method, at a small cost in total energy. Specifically, if K represents the sum of the interorbital exchanges and E the total energy (including exchange), then 0 ? KorthHartree ? KHartree–Fock and 0 ? EorthHartree − EHartree–Fock ? KHartree–Fock − KorthHartree. The Edmiston–Ruedenberg method of localization minimizes KHartree–Fock, and so the orthogonalized Hartree method yields a K even smaller than the Edmiston–Ruedenberg K. From numerical evidence which is presented, previous work of others, and additional arguments, it is reasoned that the orthogonalized Hartree method, without exchange energy included, may provide good predictions of conformational energy changes in molecules—it is the orthogonality of the orbitals, not the exchange energy itself, which is more important for the prediction of energy changes. The near classical structure of the equations of the orthogonalized Hartree method is discussed.
Intermolecular potentials and isotope effects for molecular hydrogen–inert gas complexes
Authors: Kreek Holger , Le Roy Robert J.
Published: February 2003

An exact power series expansion is derived which interrelates the coefficients of the Legendre expansions for an atom–diatomic molecule potential function expanded about the centers of mass of two different isotopes of the diatomic. Its application to the anisotropic potential recently obtained for H2– and D2–Ar yields a potential for HD–Ar, which is then used to predict the transition frequencies in the infrared absorption spectrum of the latter. The relatively large discrepancies between these predictions and the recent measurements of McKellar [J. Chem. Phys. 61, 4636 (1974)] are attributed to the unusual sensitivity of such predictions to the shape of the isotropic potential in the region between its minimum at Re(0) and its zero energy turning point, σ(0).
Scalar equations for transport coefficients of a dilute gas in an applied field
Authors: Hunter L. W.
Published: February 2003

The linearized Waldmann–Snider equation leads to well-known tensor equations which determine the transport coefficients. In this paper, the equations for the shear viscosity and thermal conductivity in an applied magnetic field are discussed. Exact simplifications are derived, leading to an equivalent set of simpler equations which are scalar with respect to the symmetry group, C∞v, of the gas. These equations make it convenient to use larger basis sets when solving for the transport coefficients. Approximate solutions having the qualitative form discussed by Coope and Snider may be readily developed.
Vibrational energy transfer between DF(v=2) and CO2
Authors: Hinchen J. J. , Hobbs R. H.
Published: February 2003

Rates have been determined for the transfer of vibrational energy between DF(v=2), DF(v=1), and CO2 using a laser-induced fluorescence technique. DF, in DF–CO2 gas mixtures, was excited to both the v=1 and v=2 vibrational levels by absorption of radiation from a pulsed DF laser operating simultaneously on v (1–0) and v (2–1) transitions. By monitoring fluorescence intensity at 1.7, 3.5, and 4.4 μ, the temporal behavior of DF(v=2), DF(v=1), and CO2 (001) populations was followed. A computer program was used to solve the differential rate equations and a ’’best fit’’ with experimental data determined the rate parameters. Rate constants obtained in this manner from the fluorescence data are k=5.1×105 sec−1⋅torr−1 for the DF(v=2) →CO2 transfer and k=1.5×104 sec−1⋅torr−1 for CO2 (001) deactivation by DF.
The density expansion of the viscosity coefficient
Authors: Hanley H. J. M. , Haynes W. M.
Published: February 2003

Experimental viscosity data for gaseous argon, oxygen, and fluorine were analyzed as a function of density and temperature. Necessary conditions were applied to investigate if a given isotherm was consistent statistically with a given density function. It was found that the low temperature isotherms were consistent with the theoretically suggested density function, which involves terms logarithmic in the density. At higher temperatures, however, these logarithmic terms did not appear to be statistically significant.
Raman intensities of lattice modes and the oriented gas model
Authors: Burgos E. , Bonadeo H. , D’Allesio E.
Published: February 2003

The applicability of the oriented gas model to the calculation of Raman intensities of molecular crystals is investigated. The basic implications of the model and its consequences are discussed. The polarized spectra of only 3 out of 11 crystals considered are compatible with the model. For these, quantitative calculations based on calculated eigenvectors give reasonable agreement with experiment, while the others are in clear disagreement with available data. It is shown that assignments of unpolarized spectra based on the model’s predictions may easily be erroneous. Some of the parameters affecting the calculation are analyzed, confirming the conclusion that the oriented gas model is not a reliable tool for assignment of Raman lattice bands of molecular crystals.
Evidence for the dynamic disordering of the NH4 dipoles in the paraelectric phase of (NH4)2SO4 by proton spin thermometry
Authors: Knispel R. R. , Petch H. E. , Pintar M. M.
Published: February 2003

The proton Zeeman (T1) and dipolar (T1D) relaxation rates were measured in (NH4)2SO4 from 344 to 137 K. Below the phase transition (223 K) from 220 to 190 K, the ratio T1/T1D is about 2 and above Tc, from 224 to 280 K, T1/T1D is about 3. From an analysis of the rf field (H1) dependence of T1ρ at 236 and 210 K, this difference in T1/T1D is attributed to a contribution from cross relaxation to 14N which is comparable to the proton–proton relaxation in the paraelectric phase and negligible in the ferroelectric phase. The cross relaxation appears to be based on the dynamic disorder in the paraelectric phase. Above 300 K, the onset of ammonium diffusion, with activation energy 18±2 kcal/mole, is evident in the temperature dependence of T1D.
Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ-irradiated 2-methyltetrahydrofuran glass
Authors: Kevan Larry , Bowman Michael K. , Narayana P. A. , Boeckman Robert K. , Yudanov V. F. , Tsvetkov Yu. D.
Published: February 2003

Three specifically deuterated 2-methyltetrahydrofurans (MTHF) have been synthesized to delineate the proton (deuteron) arrangement and hence the molecular arrangement around a trapped excess electron in γ-irradiated glassy MTHF at 77 K. The electron–deuteron anisotropic hyperfine interaction was detected by its modulation of the two-pulse electron spin echo envelope. Comparison of experimental and simulated modulation patterns shows that the excess electron is located about 3.7 Å below the center of the plane of the ring carbons in MTHF on the side opposite the methyl group and that three or four MTHF molecules constitute the first solvation shell. A second moment analysis of the EPR linewidths of the excess electron in the variously deuterated MTHF’s independently supports the same structural conclusions. The shortest electron–proton distance is then ∼3.1 Å, which compares moderately well with previous, less accurate ENDOR results. This structure seems somewhat more expanded than that indicated by theoretical calculations based on the semicontinuum model, but does not seem incompatible with that theoretical model. A small positive isotropic coupling to the ring protons (∼1 MHz) is also indicated by the spin echo results.
Raman studies of rotational and vibrational relaxation in liquid fluoroform
Authors: DeZwaan Jack , Hess Doren W. , Johnson Charles S.
Published: February 2003

The rotational correlation functions and correlation times have been determined for liquid fluoroform from 135 to 296 °K by using depolarized Raman light scattering. Both the ν1(A1) hydrogen stretching mode and the ν3(A1) deformation mode were analyzed and found to give comparable results for the reorientational motion. The temperature dependence of the reorientational correlation times obtained can be described by an activation energy of 0.97 kcal/mole, and a comparison of these times with previously reported values of the deuterium spin–lattice relaxation times in CDF3 establishes a deuterium quadrupole coupling constant of ± (159±5) kHz. The isotropic linewidths of the ν1(A1) mode, which were found to be unusually large and completely insensitive to temperature, are reported and compared with the isotropic linewidths of the same vibrational mode in CDF3.
Quasiclassical trajectory studies using 3D spline interpolation of a
b i
n
i
t
i
o surfaces
Authors: Sathyamurthy N. , Raff L. M.
Published: February 2003

The accuracy of spline fitting procedures has been investigated for Morse potentials, Lennard-Jones potentials, a collinear D–Cl–H potential surface, and for three dimensional DIM (He–H+2) and a valence-bond D–Cl–H surface. The adequacy of 3D spline surfaces in quasiclassical trajectory studies has been examined for both the He–H+2 and D+HCl systems. It is found that while one dimensional spline fits are very accurate, this accuracy decreases substantially for a two-dimensional spline fit. There is an additional, but smaller, decrease in accuracy for three-dimensional splines. The surface gradients are found to be less accurately fitted than the corresponding function values. In general, (15×15×15) cubic spline fits are found to lack the necessary accuracy to produce a point-by-point match of a quasiclassical trajectory to that obtained on the original analytic surface. However, total reaction cross sections, energy partitioning distributions, and spatial scattering distributions computed on spline surfaces are found to be in good accord with those obtained from the full analytic surface. This suggests that in spite of their limited accuracy, spline fits may provide a very useful tool for interpolation of ab initio surfaces. The computer time requirements for use of 3D cubic splines in quasiclassical trajectory calculations are found to be within range of present computational facilities.
Spectroscopic studies on naphthalene in the vapor phase. II. Fluorescence quantum yields from single vibronic levels in the first excited singlet state—behavior of higher excited singlet states
Authors: Stockburger M. , Gattermann H. , Klusmann W.
Published: February 2003

Fluorescence quantum yields for a variety of single vibronic levels in the first excited singlet state of naphthalene-h8 and -d8 were measured under collision-free conditions. The interpretation of the data is given in terms of the present theory of intersystem crossing. The experimental results suggest that in cases of accidental degeneracy between vibronic levels in S1 and higher triplet states T2, T3 a resonance crossing mechanism is dominant. Fluorescence as well as intersystem crossing quantum yields were also measured on irradiation into higher singlet states. The results indicate that at higher photon energy a new decay channel opens up which is not intersystem crossing.
Experimental investigation of small He clusters
Authors: van Deursen A. P. J. , Reuss J.
Published: February 2003

The question of the existence of a bound state for He2 has been investigated by observing ionized Hen+ clusters produced in a low temperature molecular beam. The results agree with the prediction that there is no bound state for He2. (AIP)
Electronic structure of TTF–TCNQ complex
Authors: Cavallone Franco , Clementi Enrico
Published: February 2003

The electronic structure of the ground state of the TTF and TCNQ molecules have been computed in the Hartree–Fock approximation. The orbital energies and the total energies for the separated molecules are compared with those computed for the TTF–TCNQ complex at nine different geometrical conformations. The electronic charge transfer in the complex, the ionization potentials as well as some tentative conclusions on the electronic charge transfer mechanism in the (TTF–TCNQ)n complex are discussed.
Vibrational relaxation of NO in collisions with atomic oxygen and chlorine
Authors: Glänzer K. , Troe J.
Published: February 2003

The vibrational relaxation of NO(X 2Π) in the v=0, 1, and 2 states in collisions with Ar, NO, NO2, and electronic ground state O and Cl atoms has been studied in shoch waves. O and Cl atoms were produced in the thermal dissociation of NO2 or N2O at t?2700 K, and of ClNO at T?1700 K, respectively. BOth atoms were found to be very efficient catalysts for vibrational energy transfer to NO. The relaxation rates observed suggest a mechanism involving bound (NO2) * and (CINO) * collision complexes.
Collision free lifetimes of excited NO2 under very high resolution
Authors: Paech F. , Schmiedl R. , Demtröder W.
Published: February 2003

The output beam from a single mode argon laser, tunable around λ=4880 and 5145 Å, is crossed perpendicularly with a well collimated NO2 beam. The lifetimes of selectively populated hyperfine structure levels in electronically excited states have been measured under collision free in electronically excited states have been measured under collision free conditions at NO2 pressures below 10−4 torr, using pulse modulation of the laser beam and single photon counting techniques. The experimental results prove that several directly excited states have short lifetimes (1–3 μsec) which are determined mainly by fast internal conversions to neighboring excited states, but only partly by fluorescence to the ground state. There are at least two such vibronic states, which are populated by nonradiative transitions from the primarily excited state and which show fluorescence decay with lifetimes around 30 and 100 μsec. Comparison with other measurements indicates that the primarily excited state has 2B1 character; the fluorescence, however, is mainly emitted from 2B2 states.
Simulation of polymer chains on a tetrahedral lattice
Authors: Wall Frederick T. , Mandel Frederic , White R. Allen
Published: February 2003

A thorough analysis of partially restricted random walks on a tetrahedral lattice has been carried out to demonstrate the validity of Wall’s recent theory of polymer chain configurations. Tetrahedral lattice walks of order six (those for which ring closures corresponding to polygons of six of fewer vertices are forbidden) were studied both analytically and numerically. A set of forty-one different five-step configurations suffices for the numerical analysis; the total numbers of the configurations, together with the corresponding first and second moments, have all been determined with great accuracy. With increasing contour chain lengths, the dimensional distributions for all configurations become Gaussian and their mean square end-to-end separations, 〈r2〉n, become linear in the number of steps, n. As predicted by theory, the derivative of 〈r2〉n with respect to n becomes independent of the kind of configuration as n→∞. All parameters have been determined precisely and the results are interpreted in the light of their asymptotic trends.
Elastic scattering of 13 to 100 eV electrons from N2
Authors: Finn T. G. , Doering J. P.
Published: February 2003

The cross section for elastic scattering of electrons from N2 has been measured between 13 and 100 eV. The relative differential cross sections for elastic scattering and for excitation of the C 3Πu v′=0 and 1 bands were measured over the angular range from 20° to 90° in the 13–35 eV region and from 5° to 90° in the 40–100 eV region. The integrated cross sections were placed on an absolute basis by normalizing the C 3Πu v′=0 excitation cross section to the known emission cross section for the Second Positive system of nitrogen (C 3Πu→B 3Πg) and by calibrating the electron spectrometer with respect to other electron scattering results. The total elastic scattering cross section varies from 12.3×10−16 cm2 (±50%) at 13 eV to 3.9×10−16 cm2 (±50%) at 100 eV.
Electron–electron double resonance study of coherent and random rotational motion of methyl groups
Authors: Mottley C. , Kispert L. D. , Clough S.
Published: February 2003

Electron–electron double resonance (ELDOR) spectra are reported for the free radical CH3C̊ (COOH)2 in x-irradiated crystals of methylmalonic acid over the temperature range 10–50 K. At low temperatures tunneling sidebands are observed corresponding to simultaneous flips of the electron spin and changes in the tunneling rotation state of the methyl group. The previous assignment of weak satellite lines of the electron spin resonance (ESR) spectrum to these transitions is unequivocally confirmed, and sidebands obscured in the ESR spectrum are clearly observed in the ELDOR spectrum with the expected frequencies and intensities. The temperature dependence of the frequencies confirms the previous observation of the temperature dependence of the frequency of tunneling rotation of the methyl group. A sequence of changes with rising temperature in the relative amplitudes of lines in the ELDOR spectra is accounted for in a qualitative way as a consequence of the changing rates of several cross relaxation processes, all of which arise as a consequence of the modulation of the electron–proton hyperfine interaction by thermally activated rotation of the methyl group about its symmetry axis.
X-ray diffraction study of CaBr2 aqueous solutions
Authors: Licheri G. , Piccaluga G. , Pinna G.
Published: February 2003

CaBr2 aqueous solutions were investigated by x-ray diffraction in order to study the structure of hydrated ions. The experimental data were analyzed using a model describing coordinations around the ions only in terms of nearest-neighbor interactions. For the Br− ion, the octahedral coordination, already obtained in LiBr solutions, is confirmed. As regards the coordination of Ca++ ion, the experimental data can be interpreted both with six and eight coordination numbers.
Vibrational potentials and structures in molecular and solid carbon, silicon, germanium, and tin
Authors: Anderson Alfred B.
Published: February 2003

Using a molecular orbital method presented recently [J. Chem. Phys. 62, 1187 (1975)], a study is made of equilibrium distances, force constants, and binding energies for several states of diatomic Group IV A molecules. Close agreement exists with experiment for low-lying states of C2. The agreement between calculated and experimental properties for the 3πu and 3πg states of Si2 is ambiguous. The ground state of Ge2 is here predicted to be 3πu with Re=2.29 Å, ke=2.65 mdyn/Å, le=16 mdyn/Å, and De=57 kcal (65 experimentally). For Sn2 the 3πu and 3Σg− states are close in binding energy (48.6 and 48.7 kcal vs 45.2 from experiment). The former has Re=2.65 Å, ke=2.25 mdyn/Å, le=15 mdyn/Å2, and the latter has Re=2.77 Å, ke=1.44 mdyn/Å, and le=8 mdyn/Å2. A quantitative study of five atom tetrahedral clusters of these atoms leads to accurate determinations of bulk lattice parameters and stretching force constants. The origin of forces important to binding and interaction force constants is discussed. Structures and binding energies for three, four, and five atom clusters of tin are calculated. The binding energies follow experimental trends. The introduction of d orbitals to the basis set significantly improves binding energies for the larger clusters, but has little effect on calculated structures.
Electrochemical potentials for charged metal spheres in the Thomas–Fermi–Dirac jellium model
Authors: Goodisman Jerry
Published: February 2003

By using the Thomas–Fermi–Dirac approximation for the electronic charge density and the jellium approximation for the nuclei it is possible to compute the electronic properties of charged metal spheres of varying net electric charges. Even for small spheres (radius=5a0) it is found that the separation of the electrochemical potential of the electrons into a ’’chemical’’ part, which is independent of electrical potential, and an ’’electrical’’ part, given by the electron charge multiplied by the potential at the center of the sphere, is quite valid. Calculations of electron density and electrostatic potential as a function of distance from the center of the sphere are given for various net charges.
Spin lattice relaxation in an AM{Xn} system: Application to 13C relaxation in enriched molecules
Authors: London Robert E. , Matwiyoff Nicholas A. , Mueller Delbert D.
Published: February 2003

In 13C-enriched molecules, 13C–13C dipolar interactions can contribute significantly to the relaxation of 13C nuclei which are not directly bonded to protons. Comparison of the relaxation behavior of the 13C–13C multiplet lines with that of the singlet for which there are no 13C–13C dipolar interactions allows a quantitative analysis of this interaction. However, since 13C–13C dipolar interactions are necessarily weak, substantial contributions to the relaxation of nonprotonated carbon atoms are made by protons attached to adjacent atoms. The general features of the spin lattice relaxation behavior of an AM{X} system in which {X} represents the decoupled protons, A, the nonprotonated carbon, and M, the adjacent protonated carbon, have been derived. In general, the A relaxation will be nonexponential and dependent on the initial state of the Mz magnetization. A particularly useful application of these measurements on 13C-enriched systems is found in their sensitivity to internal and/or anisotropic motions. Such rotations affect the 13C–13C and 13C−1H dipolar interactions differently, depending on the particular geometry involved and the axis of rotation. These techniques have been applied to the observed relaxation behavior of 6-phosphogluconate enriched to the 79% level. In this case the contributions to the intramolecular dipolar relaxation of the carboxyl carbon consist primarily of the 13C–13C interaction with its adjacent carbon, the 13C−1H interaction with the proton on the adjacent carbon, and a significant contribution from the proton attached to the third carbon in the chain. For this case, it is found that the carboxyl relaxation will be, to a very good approximation, exponential and independent of the initial value of Mz. Furthermore, a significantly improved fit of the data results if anisotropic rotation effects are considered.
Electron spin resonance measurements of the spin Hamiltonian zero-field splitting parameter D as a function of temperature for trigonally distorted Cr3+⋅6H2O magnetic complexes in families of hydrated crystals. I
Authors: Manoogian A. , Leclerc A.
Published: February 2003

The spin Hamiltonian zero-field splitting parameter D is measured as a function of temperature for trigonally distorted Cr3+⋅6H2O magnetic complexes in families of hydrated crystals. The purpose of the study is to classify the isomorphs of the crystals via their magnetic behavior so that use can be made of similarities and differences between the members when analyzing data obtained from electron–nuclear double resonance (ENDOR) measurements. The D vs T curves are found to form well ordered patterns within a family of isomorphs, and the patterns are significantly different for each type of crystal. When the straight line part of the curves between 195 and 297 °K are extrapolated to high temperature, the existence of common crossing points is found. The orderly nature of the patterns is expected to provide an additional aid in describing the physical behavior of the crystals. The crystals studied are alums, guanidinium aluminum sulfate hexahydrate (GAlSH) and its isomorphs, and aluminum chloride hexahydrate (AlCl3⋅6H20). The Cr3+ ion is doped as a small impurity in most of the crystals but some concentrated crystals are also studied. Electron spin resonance measurements are made at X-band microwave frequencies (∼9.4 GHz) at the four temperatures 4.2, 77, 195, and 297 °K whenever possible.
Interpretation of 53Cr3+ electron–nuclear double resonance data obtained in hydrated crystals in terms of vibrational and static crystal distortion contributions to the spin Hamiltonian parameters. II
Authors: Leclerc A. , Manoogian A.
Published: February 2003

Electron nuclear double resonance (ENDOR) measurements of 53Cr3+ are extended to more isomorphs of guanidinium aluminum sulfate hexahydrate (GAlSH) and the alums. This is done because a satisfactory analysis of the varied ENDOR data obtained in the crystals can be obtained only by studying families of crystal isomorphs. An important relationship obtained in the combined double resonance studies at 4.2 °K is a plot of the quadrupole hyperfine parameter Q′ vs the zero field splitting parameter D. It is found that the Q′ points fall on a series of straight horizontal lines, indicating that Q′ is independent of D. The simple theory shows that if the trigonal distortion in the Cr3+⋅6H20 magnetic complexes is due to static effects alone then a direct proportionality between Q′ and D is expected. It is possible to give a satisfactory explanation to the experimental Q′ vs D diagram if the vibrational effect of the crystals is assumed to play an essential role. An analysis of the vibrational and static crystal distortion contributions to Q′ and D can be done in a consistent manner if the D vs T curves obtained in the crystals are also considered. Consequently both the Q′ vs D and D vs T plots can be explained separately, and a description of the physical behavior of the crystals made.
Vibronic spectra of the ozonide ion in the matrix-isolated M+O3− species
Authors: Andrews Lester
Published: February 2003

Matrix reactions of alkali metal atoms and ozone molecules at high dilution in inert gases produced a clear yellow film during condensation at 10–22 K on a sapphire plate which yielded vibronic spectra of the M+O3− species. Best resolution of up to 14 vibronic bands spaced 800–900 cm−1 apart was obtained for the lithium and sodium compounds. These bands formed a strong progression in ν1′ and the first six bands exhibited the presence of a weaker progression in ν2′. The band origin was located at 17 730 cm−1 for Li+16O3− and Li+18O3− in solid krypton. Harmonic and anharmonic vibrational constants were determined graphically for the excited electronic state.
Electron transport in mixtures of liquid methane and ethane
Authors: Bakale George , Tauchert Wolfgang , Schmidt Werner F.
Published: February 2003

The mobility of excess electrons in liquid mixtures of methane and ethane was measured over the entire range of composition at T=111 °K. Field dependent mobilities were observed at higher field strengths. The data are discussed on the basis of the quasifree and localized electron models.
Large isotope effects in photodissociation of polyatomic molecules
Authors: Band Yehuda B. , Freed Karl F.
Published: February 2003

The example of the photodissociation of HCN is used to demonstrate the existence of huge isotope effects in general classes of the photodissociation of polyatomic molecules. The isotopic ratio of photodissociation rates is of the form exp[(μ11/2−μ21/2) K], where μ1 and μ2 are the reduced masses of the atomic fragments (different isotopes), and where K is dependent on molecule and fragment normal modes as well as upon the interfragment repulsive potential. Conditions for the maximization of K are discussed.
Dynamic effects of phase transition of K3Fe(CN)6 on EPR Of Cr3+
Authors: Owens F. J.
Published: February 2003

A marked broadening of the resonances and a coalescing of the fine structure splitting of the EPR of the 3d3 state of Cr3+ is observed in the temperature range where previous data indicate the possibility of a phase change. The effects are attributed to motional reorientaion of the [Cr(CN)6]−3 complexes.
Effect of reagent electronic excitation on the chemical reaction Br(2P1/2,3/2)+HI
Authors: Bergmann K. , Leone S. R. , Moore C. Bradley
Published: February 2003

Br(2P3/2)+HI are found to react at room temperature to give HBr+I(2P3/2) with a rate constant of (1.0±0.3) ×10−11 cm3 molecule−1⋅sec−1. For Br+HI, electronic excitation of Br inhibits the reaction to HBr+I. Collisions with HI remove excited Br(2P1/2) with a rate of about 0.25×10−11 cm3 molecule−1⋅sec−1. This rate includes electronic quenching and electronic-to-vibration energy transfer as well as reaction. From correlation arguments the reaction of excited Br(2P1/2) with HI is expected to be much slower than that of ground state. The observed rate is comparable to that for quenching plus electronic-to-vibration transfer for Br(3P1/2) by HCl and HBr.
Nuclear magnetic relaxation of 31P in phosphorous halides
Authors: Leipert Thomas K. , Freeman Walter J. , Noggle Joseph H.
Published: February 2003

31P NMR spin–lattice relaxation time measurements in the laboratory frame, T1, and in the rotating coordinate frame, T1ρ, are reported for an equimolar mixture of PBr3 and PCl3. Relaxation times were measured as a function of temperature. T1ρ was measured also as a function of offset and lock-field strength. The lock-field dependent T1ρ technique is shown to be inadequate for exact determinations of scalar spin–spin coupling constants. We report for all four molecular species PBr3, PBr2Cl, PBrCl2, and PCl3 the following quantities: activation energies and frequency factors for chemical exchange and molecular reorientation and spin–lattice relaxation times for the bromines and chlorines. These quantities are obtained from the temperature dependent T1ρ measurements, which are shown to be suited for the study of chemical exchange processes when the exchange rates are in the order of 103–105 s−1. Using known values for quadrupolar coupling constants, we derive the temperature dependence of the reorientation correlation time. The known temperature dependence of the correlation time is in turn used for a complete analysis of the temperature dependent T1 measurements. All possible relaxation mechanisms contributing to T1 are identified and experimental values for the spin–rotation interaction constant and the chemical shift anisotropy are determined. The accuracy of the various parameters varies from a few percent to mere estimates.