bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of Pade approximants in the construction of diabatic potential energy curves for ionic molecules
Authors: Jordan K. D. , Kinsey J. L. , Silbey R.
Published: February 2004

For most ionic molecules, sufficient experimental information is not available for the construction of an RKR potential curve; thus, model potentials have been employed which use some of the experimental information in their construction (e.g., Born-Mayer and Rittner potentials). In this paper, we present a simple method for constructing model potential curves which allows one to use all the available experimental information and to easily incorporate new information as it becomes available. This method, employing a Padé approximant technique, is capable of predicting the value of higher Dunham coefficients to within experimental accuracy from a knowledge of the lower ones. Consequently, it also predicts some spectroscopic properties from a knowledge of other data to much better accuracy than the Rittner or Born-Mayer models.
ESR study of free radicals in irradiated single crystals of potassium hydroxylamine disulfonate
Authors: Holmberg Robert W. , Wilson B. J.
Published: February 2004

Two free radicals formed when single crystals of potassium hydroxylamine disulfonate are irradiated with gamma rays at 77°K were studied by ESR. The ESR spectra result from a predominant center (radical A) exhibiting large 14N and 1H hyperfine splittings and from lesser amounts of Fremy's radical [ȮN(SO3)22−]. The g and hyperfine tensors of these radicals have been evaluated and are discussed. Radical A, whose identity remains in doubt, may be regarded as the precursor to Fremy's radical; on warming its lines decay and those of Fremy's radical are formed. Lines from radical A decrease in strength and those of the hydroxyl radical (ȮH) grow in strength when irradiated crystals are photolyzed at 77°K. The reverse reaction occurs in the dark; the hydroxyl radical disappears and radical A is reformed.
Optimized cluster theory correction to the van der Waals model of mixtures
Authors: Sung Stephen H. , Chandler David , Alder Berni J.
Published: February 2004

The correction to the van der Waals mean field model for mixtures is shown to be significant compared to the excess thermodynamic properties of mixing hard spheres and square-well molecules under typical experimental conditions. Calculation of this correction to higher powers in the perturbation series of the attractive potential relative to the kinetic energy but in the mean field approximation, which leads to the one fluid model, results in an order of magnitude error. On the other hand, the optimized cluster theory, which reorders the perturbation series by graph theoretical techniques, provides quantitative predictions of the correction except near the critical line. The cluster theory is sufficiently accurate that it predicts heat capacities of mixtures larger than those of the pure components at the same volume and temperature as the mixture, in nearly quantitative agreement with the results obtained from molecular dynamics.
Random coupling model for molecular dissociation
Authors: Heller Eric J. , Rice Stuart A.
Published: February 2004

In this paper, we propose and solve sequential coupling models for molecular dissociation of the Rice-McLaughlin-Jortner (RMJ) type in which the usual assumption of constant coupling among the states is replaced by an assumption of random coupling. The counter-intuitive nonsequential branching behavior found previously for constant coupling is eliminated and we find completely sequential time dependence which obeys the phenomenological rate equations. We isolate the features of constant vs random coupling which give rise to the branching vs sequential behavior in terms of simple physical models and considerations of the coherence properties of the wavefunction. It is concluded that constant coupling is inappropriate for most molecules, and that the random coupling assumption has the effect of validating the use of a random phase approximation which in turn causes the molecule to decay as if each quasibound molecular level is coupled to its own continuum. Our conclusions do not change when we solve an extended model with many continua, with each molecular level coupled to each continuum.
Semiclassical theory of atom-solid-surface collisions: Application to He–LiF diffraction
Authors: Doll Jimmie D.
Published: February 2004

The semiclassical theory of gas-solid-surface collisions is applied to the diffractive He–LiF system. It is shown that uniform semiclassical results are in good agreement with high quality quantum-mechanical results. The present study suggests that classically forbidden contributions should not always be ignored when both classically allowed and forbidden contributions are present. The experimental results of O'Keefe et al. are examined.
Perturbation corrections to Koopmans' theorem. II. A study of basis set variation
Authors: Chong Delano P. , Herring F. Geoffrey , McWilliams Denis
Published: February 2004

Ordinary third-order Rayleigh Schrödinger perturbation theory is used to evaluate vertical ionization potentials of closed-shell molecules. The variation in the accuracy of the method as the size of the basis set increases is investigated for water. From this study, the smallest basis compatible with reasonable accuracy is selected as the best compromise between cost and accuracy. This basis is used to calculate the vertical ionization potentials of H2O, CH2, NH3, H2CO, F2O, and CO. The results are compared to those obtained from a double-zeta basis set.
NbO molecule: ESR and optical spectra in inert matrices at 4 °K; establishment of its ground electronic state as 4Σ
Authors: Brom J. M. , Durham C. H. , Weltner W.
Published: February 2004

NbO molecules, produced by the vaporization of a mixture of Nb metal and solid Nb2O5 or by passing O2 gas over hot Nb metal, were trapped in solid Ne or Ar at 4°K and investigated by ESR and optical spectroscopy. Analysis of the ESR spectrum established that the molecule has a 4Σ ground state (as derived from a σδ2 configuration) exhibiting large hyperfine (hf) splittings from interaction with the 93Nb (I=92) nucleus. Essentially all lines were accounted for as effectively ΔMI=0, ±1, and ±2 transitions (or as multiple trapping sites). Hyperfine and g tensor elements have the following values in a neon matrix: g∥=2.0023(10), g⊥=1.9577(10), |A∥(93Nb)|=1587(1) MHz, and |A⊥(93Nb)|=1647(1) MHz. The magnetic parameter b ≅ 0.19 cm−1 found in the gas phase optical spectrum is here determined to be b =0.165(1) cm−1. These magnetic parameters are in accord with the expected electronic properties of NbO (assuming the A values are positive) and are compared with those of VO. Both Nb16O and Nb18O have been investigated by optical spectroscopy, and the spectra correlate well with gas phase data. The 4Σ−←X 4Σ− blue system appears at 461.4 nm in a Ne matrix and contains several vibrational perturbations. At least five systems of bands appear between 530 and 740 nm. It is conjectured that the progression with (0,0) at 655.3 nm is a 4Σ←X 4Σ transition and that the remaining bands arise from a 4Π←X 4Σ system where the upper state is partially Hund's case (c) coupling. In the infrared the vibrational transition of NbO appears as a triplet at 965.5, 973.0, and 977.1 cm−1 in neon; a similar triplet was recently observed in argon by Green, Korfmacher, and Gruen.
Laser photoluminescence and photopredissociation of Rb2
Authors: Brom J. M. , Broida H. P.
Published: February 2004

Laser-photoluminescence studies of Rb2 indicate that the C 1Πu state of the molecule is strongly predissociated. Both fixed frequency ion lasers and a tunable cw dye laser have used been to excite the photoluminescence at pressures from 1 to 100 torr of metal vapor contained in a heat-pipe oven. The photoluminescence spectra show that the A Σu+1 and B 1Πu states also are populated following absorption by the C 1Πu state. Recombination collisions between Rb 52P, formed in the predissociation of Rb2, and Rb 52S are proposed to account for the A→X and B→X emission. A strongly pressure dependent quasicontinuum emission is observed in the 540–610 nm region. This emission is attributed to transfer from the C 1Πu state into an unidentified state followed by transitions to high-lying vibrational and continuum levels of the ground state.
Some mathematical properties of magnetic resonance line shapes
Authors: Alexander S. , Baram A. , Luz Z.
Published: February 2004

The present paper deals with some mathematical properties of Freed's method of calculating magnetic resonance line shapes in the slow tumbling region. It is first shown that this method is related to a moment expansion of the spectrum and a general expression for the spectral moments is given. A modification of Freed's approach of solving the spectrum of an axial Hamiltonian is then derived: Rather than truncating the infinite set of Freed's equation, we obtain an approximate solution of the discarded part (using the asymptotic values of the Clebsh-Gordan coefficients) which are substituted into the retained part of the set. This results in a significant improvement in the convergence of the spectrum and at the same time enables one to compute powder spectra (without motion). Finally the more complicated problem of a spectrum due to a nonaxial Hamiltonian is discussed. A method is derived to transform this ``two-dimensional'' problem into a one-dimensional one with lower order.
Molecular charge transfer. II. Experimental and theoretical investigation of the role of incident-ion vibrational states in O2+–O2 and NO+–NO collisions
Authors: Moran T. F. , Flannery M. R. , Cosby P. C.
Published: February 2004

Charge-transfer cross sections are measured for the O2+–O2 and NO+–NO systems and are compared with those calculated from a theoretical multistate impact parameter model for which the resulting coupled first-order differential equations are solved numerically. Convergence in the theoretical total cross sections is achieved by the systematic introduction of as many as 54 product states to the wavefunction expansion for the total system. At high kinetic energies (∼2 keV) the computed cross sections are controlled predominantly by the magnitude of the vibrational overlaps between the reactant and product states, and multiquantum transitions do occur with high efficiency. At lower energies, the energy defects and vibrational overlaps of the various product channels tend to control the reaction pathways. Total charge-transfer cross sections measured as a function of reactant ion vibrational and translational energy are found to be in general accord with the computed multistate cross sections. In the low velocity limit, an approximation which assumes that all product channels with small energy defect are degenerate with the incident channel provides a reasonable description of the charge-transfer process at ion kinetic energies less than 200 eV.
Energy partitioning by mass spectrometry: Chloroalkanes and chloroalkenes
Authors: Kim K. C. , Beynon J. H. , Cooks R. G.
Published: February 2004

HCl elimination from vibrationally exicted C2–C4 chloroalkane and chloroalkene ions has been studied by ion kinetic energy spectrometry. The kinetic energy release during unimolecular decomposition was measured from the width of the metastable peak and used, together with other data, to infer the reaction mechanism. An average kinetic energy release of 438–614 meV was associated with 1,2-elimination except in the chloroethane molecular ion. A much smaller kinetic energy release (∼ 30 meV) was associated with the 1,3-elimination mechanism. In a few cases, composite metastable peaks were observed due to HCl elimination via two competitive reaction channels. In one such instance, involving the ion C4H7Cl+·, several different methods of generating the ion were used. This had a considerable effect on the relative weights of the two pathways but only a small effect upon the kinetic energy releases. The thermochemistry of many of the 1,2-elimination reactions has been determined from ionization and appearance potential measurements on the chloroalkanes and chloroalkenes and their dehydrochlorination products. In combination with the ion kinetic energy measurements, this has yielded direct information on the energy partitioning pattern in these reactions. Data for the four-centered elimination reaction in a large number of chloroalkane molecular ions show that (i) a large fraction of the kinetic energy release originates from the reverse activation energy, (ii) approximately 25% of the reverse activation energy is funneled into the translational energy of separation of the products, and (iii) differences in the energy partitioning behavior caused by modification of the reactant are small provided that such modification does not alter the reaction mechanism. In studying the 1,2-elimination reaction RRKM calculations were performed in order to provide an estimate of the nonfixed energy of the activated complex and its contribution to the kinetic energy release. Complete thermochemical characterization of the 1,3-dehydrochlorination has not been possible. It is suggested, however, that the reverse activation energy is small and that a substantial fraction of the kinetic energy release arises from the nonfixed energy of the activated complex.
X-ray and neutron diffraction from diatomic liquids
Authors: Sandler S. I. , DasGupta A. , Steele W. A.
Published: February 2004

The analysis of x-ray and neutron diffraction data for studying the structure of diatomic liquids is critically examined. Using calculated pair distribution functions for a model diatomic fluid, ``experimental'' x-ray and neutron intensity scans are constructed. The importance of the orientational correlations to the scattering intensity, and its effect on the interpretation of the intensity, is considered in detail. A method of assessing the orientational correlation contribution to experimental diffraction data is suggested, and the circumstance for which this contribution may be large is identified. It is found that approximate Fourier inversion procedures, and simple models of the liquid state which neglect the coupling between radial and orientational correlations, cannot be used to extract accurate information about the center-of-mass radial distribution function from diffraction data. The limited utility of the atomic radial distribution function for polyatomic molecules, and a problem which arises in calculating this function from diffraction data, are both discussed. A method is suggested for unfolding the structure of a dense molecular fluid from x-ray and neutron diffraction data which is based upon a theory of molecular fluids proposed by Steele and Sandler [preceding paper]. A test of this method indicates that it yields reasonably accurate information about both the radial and orientational correlations in diatomic liquids.
OH chemiluminescent radiation from lean hydrogen-oxygen flames
Authors: Davis M. G. , McGregor W. K. , Mason A. A.
Published: February 2004

The OH radiation intensity in the 3064 Å band, the temperature, and the OH concentration were measured as a function of the height above the face of a premixed H2/O2 flat flame burner. The burner was operated fuel lean at subatmospheric ambient conditions. Examination of possible reaction mechanisms that could produce chemiluminescent OH radiation from these hydrogen-lean flames led to results consistent with those obtained in hydrogen-rich flames. The termolecular rate coefficient for the reaction H+OH+OH→H2O+OH* was found to be (2.3±0.9) × 10−32 cm6 molecules−2 · sec−1. The value represents an upper bound for the rate coefficients of the reactions which might contribute to the excitation of OH.
Raman spectra of solid benzene under high pressures
Authors: Ellenson William D. , Nicol Malcolm
Published: February 2004

Raman spectra of benzene I and benzene II have been obtained to pressures of 40 kbar at 77 and 295 K. Eight lattice mode frequencies were observed for benzene I, including one feature of the spectrum that has not been previously reported. At 45 kbar and 77 K, lattice mode frequencies of benzene I are about twice their atmospheric pressure values. The spectra of some internal modes as functions of pressure are reported. The lattice dynamics of the benzene crystals were studied by calculations using atom-atom potentials to predict the wavenumbers and normal coordinates of the lattice modes. A simple change-of-polarizability argument was used together with structural direction cosines and calculated normal coordinates to estimate the relative intensities expected for the Raman spectra. The results of the intensity and dynamical calculations were combined to make a complete assignment of the lattice mode Raman spectra of both phases of benzene.
Heat pulse modulation of molecular crystal phosphorescence
Authors: Hunter S. J. , Parker H. , Francis A. H.
Published: February 2004

The modulation of low temperature (2°K) molecular crystal phosphorescence by submicrosecond duration heat pulses is discussed in connection with the study of rates of triplet energy transfer and trap thermalization. A phenomenological discussion of the dynamics of heat pulse propagation in molecular solids is presented and estimates of the thermal relaxation time of the lattice are obtained. It is suggested that these times are sufficiently short to permit investigation of the dynamics of exciton energy transport in molecular crystals. The details of the technique are discussed and results from several experiments are presented. Additionally, heat pulse modulation of the electron paramagnetic spin distribution of radiative triplet states is discussed and some preliminary results presented.
Self-consistent calculation of the phonon-libron spectrum in α-N2
Authors: Raich J. C. , Gillis N. S. , Anderson A. B.
Published: February 2004

The temperature dependence of the librational and translational lattice modes in α nitrogen is calculated on the basis of two approximate treatments. The random phase approximation is shown to be unsatisfactory in predicting the temperature dependence of the librational frequencies. The self-consistent phonon approximation is found to be a definite improvement over the random phase approximation. The first model uses quadrupolar interactions alone, the second an atom-atom potential. Although the atom-atom potential is moderately successful in fitting observed properties, the evidence for the necessity of a more complete potential, containing quadrupolar as well as short range and long range anisotropic terms, is discussed.
Self-consistent phonon calculation of the β phase and the α-β transition in solid nitrogen
Authors: Raich J. C. , Gillis N. S. , Koehler T. R.
Published: February 2004

The self-consistent phonon approximation of anharmonic lattice dynamics is applied to solid β nitrogen. Completely disordered molecular orientations and a Lennard-Jones 6–12 intermolecular potential are assumed. The quantities calculated are the phonon spectrum and the thermal expansion as a function of temperature at zero pressure. The α-β fcc-hcp transition temperature is estimated on the basis of two models for the molecular ordering in the β phase. Although the computations are intended primarily to explore the application of the self-consistent phonon method to a treatment of structural phase transitions in simple molecular crystals, comparison is made with existing experimental data.
Studies in molecular dynamics. XIV. Mass and size dependence of the binary diffusion coefficient
Authors: Alder B. J. , Alley W. E. , Dymond J. H.
Published: February 2004

The velocity autocorrelation function and the binary diffusion coefficient of a single hard sphere test particle whose mass and size is less than that of the solvent are computed in the dense fluid phase. Large deviations from the Enskog theory of uncorrelated motion are found as the diffusion coefficient vanishes for the lighter and larger test particle at higher densities. Comparison with experimental values leads to the conclusion that the correlated motions observed in hard spheres correspond well to those observed in real systems.
K absorption structure of the nitrogen atom in the nitric oxide molecule
Authors: Morioka Y. , Nakamura M. , Ishiguro E. , Sasanuma M.
Published: February 2004

The absorption spectrum of the nitric oxide molecule in the 30 Å region has been observed by using a 2 m grazing incidence spectrograph and synchrotron radiation from a 1.3 BeV electron synchroton as a background source. The spectrum has discrete structure and is similar to the spectrum, near the K edge, of the nitrogen molecule. The absorption structure is analyzed by drawing an analogy between the electron configuration of the states formed by the excitations of a (σ1s)N electron in nitric oxide to the outer orbitals and the electron configuration of the states formed by the excitations of the outermost orbital electron of oxygen molecule. The K absorption edge of the nitrogen atom in nitric oxide is determined to be 409.9±0.1 eV.
Scattering of He and Ne atoms from a (001) surface of sodium fluoride
Authors: Wood B. , Mason B. F. , Williams Brian R.
Published: February 2004

The scattering of He and Ne atoms from a cleaved (001) surface of NaF has been studied at different surface temperatures and incident beam energies. The geometric peak shape expected for the beam detector arrangement was compared to the experimentally observed peaks under different conditions of surface temperature and incident energy. The (00) peak height was followed as a function of surface temperature for two values of the incident momentum, in order to extract some information about the role of the attractive part of the interaction potential during a collision. Finally, the peak heights of several diffraction peaks were determined as a function of the incident energy. Several minima were observed in the scattering pattern, which can be associated with bound state resonances and which have been the subject of considerable interest recently. In this case, several maxima were found which can also be associated with bound state resonances. The (00) peak, observed in the scattering of Ne atoms, also showed maxima and minima when the height was monitored, as a function of the incident energy.
Contact electrification between mercury and polyethylene: Effect of surface oxidation
Authors: Hays Dan A.
Published: February 2004

The contact electrification between mercury and polyethylene films has been measured under a variety of conditions. For freshly prepared films, the mercury contact charging of polyethylene is low, ≈1 nC/cm2. For films exposed to the laboratory air for a few hours, the contact charging increases to ≈20 nC/cm2. A brief exposure of the polyethylene to air near a corona, or ozone from a generator, also increases the contact charging. The infrared spectra of unsaturated polyethylene crystals display the formation of ozonides when the crystals are exposed to ozone or the laboratory air. Since there is a strong correlation between the appearance of ozonides and an increase in contact charging, we believe that a definite charge exchange site on polymers has been identified for the first time. To substantiate our observation, we find that ozonization of saturated polyethylene does not significantly increase the contact charge exchange. Analogous results have been obtained for lower molecular weight hydrocarbons. The experimental results are discussed in terms of a model involving polymer surface states.
Heat capacity, entropy, and enthalpy of 242PuF3 from 10 to 350 °K
Authors: Osborne Darrell W. , Flotow Howard E. , Fried Sherman M. , Malm John G.
Published: February 2004

Heat capacity measurements have been made on a 17.5 g sample of 242PuF3 from 10 to 350°K. The measurements from 20 to 350°K were made by the quasiadiabatic method with discontinuous electrical heating. Temperature drifts in the region 18–58°K with no electrical heating were compared with the measured heat capacities to evaluate the radioactive self-heating rate, (1.206±0.004)×10−4 W/g Pu. Heat capacities from 10 to 20°K were obtained from this radioactive self-heating rate and observations of the temperature drifts with no electrical heating. A transition was observed between 110 and 122°K, with an associated excess entropy of 0.777±0.027 J °K−1 · mole−1 and an excess enthalpy of 92.2±3.4 J mole−1. Magnetic susceptibility measurements showed that PuF3 undergoes anti-ferromagnetic ordering at 9.0±0.5°K. At 298.15°K the heat capacity Cp°, entropy S°, enthalpy H° - H0°, and Gibbs energy divided by temperature (G° - H0°)/T are, respectively, 92.64±0.28 J °K−1 · mole−1, 126.11±0.36 J °K−1 · mole−1, 18015±54 J mole−1, and −65.69±0.18 J °K−1 · mole−1. Evidence for radiation damage was observed after the compound was stored overnight in contact with a reservoir at 4.2°K, and the radiation damage annealed between 10 and 14°K.
Radical pair formation in n -decane and n -decane-d22 single crystals by γ irradiation. Evidence by electron spin resonance for pairwise trapping at six different distances
Authors: Gillbro T. , Lund A.
Published: February 2004

Radicals and radical pairs in γ-irradiated single crystals of n -decane, n -decane-d22, and n -octane-d22 have been studied by means of electron spin resonance. A simple device was constructed to permit orientation of single crystals in the magnetic field at subambient temperature. In n -decane two different radical species, CH3CH(CH2)7CH3, R(I), and −CH2CHCH2−, R(II), are produced. The hyperfine coupling tensor for the α proton of the R(I) radical has the principal elements Axx = 11.7 G, Ayy = 35.2 G, and Azz = 19.6 G. In n -decane-d22 only one species of radical, −CD2CDCD2−, RD(II), is formed. The radical pairs in n -decane and n -decane-d22 consist of two R(II) or RD(II) radicals. In n -decane-d22, six different radical pairs, differing in their fine-structure splitting, are observed, and of the total amount of radicals about 3% are paired at 77 K. Each radical pair is characterized by a certain temperature above which it is unstable. A short distance between the radicals constituting the pair implies a low degree of thermal stability. The radical pairs are believed to disappear by recombining to form dimers. The mechanism of radical pair formation is discussed.
Fluorescence and zero-field level crossing spectroscopy of the C 2Σ+ state of NS
Authors: Silvers Stuart J. , Chiu Chi-Lian
Published: February 2004

Fluorescence and zero-field level crossing (Hanle effect) spectra have been observed for the C2Σ+ state of NS. The NS radical is generated in a microwave discharge and flowed into a fluorescence cell. Optical excitation is by means of both an NS emission lamp and the Ba II 2304.235 Å atomic line. The latter coincides with absorption lines of the 0–0 band of the γ (C2Σ+−X2Π) system, exciting fluorescence from apparently several N′ levels. A Hanle effect with this fluorescence is analyzed by making assumptions about the contributions of different N′ levels and their g values. A radiative lifetime estimate of 6.5 nsec for C2Σ+(ν′ = 0) results.
Proton and carbon-13 hyperfine interaction in the electron spin resonance spectrum of krypton-isolated ethynyl radical at low temperatures
Authors: Jinguji M. , McDowell C. A. , Raghunathan P.
Published: February 2004

The electron spin resonance (ESR) spectrum of the ethynyl radical, ĊCH, isolated in a krypton matrix at 4.2°K, has been measured. The radical was produced by the photolysis of acetylene with radiation from microwave discharge in hydrogen gas. The observation of the hydrogen atom ESR spectrum suggests that the mechanism of the formation of ethynyl radical is the photolytic decomposition, C2H2+hv → C2H+H, and this has been confirmed by the photolysis of C2D2 and 13C2H2. Analysis of the ESR spectra leads to the following g and hyperfine interaction parameters, with the parallel direction being taken along the C–C bond: g∥=2.0026±0.00005, g⊥=2.0017±0.00005; A∥(H)=18.1±0.1 G, A⊥(H)=15.1±0.1 G; A∥(13Cα)=357.2±0.5 G, A⊥(13Cα)=317.3±0.5 G; A∥(13Cβ)=67.7±0.5 G, A⊥(13Cβ)=50.6±0.5 G; a(H)=16.1±0.1 G; a(13Cα)=331.2±0.5 G; and a(13Cβ)=55.9±0.5 G. The above hyperfine interaction data are compared with corresponding theoretical estimates presently available.
Study of nitrogen-14 pure nuclear quadrupole resonance in 1,3,4-oxadiazole
Authors: Guibé Lucien , Gourdji Michel , Péneau Alain , Nygaard Lise
Published: February 2004

The nitrogen-14 nuclear quadrupole resonance has been investigated in 1,3,4-oxadiazole. The observed quadrupole coupling constants are compared to the values from microwave spectroscopy (MW) and discussed in terms of valence orbital populations on the nitrogen atom using principal directions of the electric field gradient tensors as defined by the MW results. The temperature dependence of the resonance frequencies ν(T) show no phase transition between 4.2 and 246°K where the lines begin to disappear. Discussion of the ν(T) curves reveals the contribution of crystal thermal motions at about 85 cm−1 and of intramolecular motions at about 590 cm−1 which is compared with the value of 625 cm−1 obtained by ir spectroscopy. Deuteration of the sample brings no appreciable change of the resonance frequencies.
Ab initio evaluation of correlation contributions to the true π-electron Hamiltonian: Ethylene
Authors: Iwata Suehiro , Freed Karl F.
Published: February 2004

The first ab initio calculations of the correlation contributions to the matrix elements of the true effective π electron Hamiltonian are presented for ethylene. It is shown that there is an additional core ``correlation'' energy in the valence singlet excited (V) state that has no counterpart in the triplet (T) and ground (N) states. This correlation energy contributes to the reduction of the V–T separation and the contraction of the π*g orbital of the V state. Its neglect in ordinary Pariser-Parr-Pople theory, etc., results in a serious cancellation of errors in traditional semiempirical valence theories. When the correlation contributions are evaluated by second order perturbation theory, the final true parameters are much less dependent on the basis set (inclusion of diffuse orbitals) than are the original ``theoretical'' integrals.
Free volume and chain constraints in cis-1,4-polybutadiene as observed by high resolution NMR linewidth
Authors: Cohen-Addad J. P. , Faure J. P.
Published: February 2004

High resolution proton magnetic spectroscopy of cis-1,4-polybutadiene is used to identify properties observed by NMR on molten polymers. It is shown that temporary constraints exerted on chains and due to entanglements can be qualitatively observed from low speed sample rotation. Also, resonance lines of the two types of protons of the monomer unit are shown to be well shaped according to the same function A|1+[(ω-ωo)/Δ]3/2|−1; this line shape holds whatever the dilution and the temperature above the melting point of the solvent-polymer system. Interpretation of the concentration and temperature dependence of the linewidth is shown to be compatible with a free volume description. Several values of free volume parameters are given. It is concluded that NMR spectra are sensitive not only to the chemical structure of monomer units but also to segmental chain motions.
Photoelectron spectra and molecular properties. XXXVI. (H3 C)3 B, (H3 C)2 BF, (H3 C)2 BN(CH3)2, and (H3 C)2 CC (CH3)2: The use of ionization potentials in assigning uv spectra
Authors: Fuss W. , Bock H.
Published: February 2004

Helium(I) photoelectron spectra and far uv spectra of R3 B, R2BF, R2 BNR2, and R2 CCR2 (R=CH3) are assigned. Subtracting uv excitation energies from corresponding ground state ionization potentials lead to term values that compare favorably with π→π* and π→σ* states of the olefin. Some term values can be satisfactorily calculated using the Rydberg series formula.
Lennard-Jones interaction for hexagonal layered crystals
Authors: Green J. F. , Bolland T. K. , Bolland J. W.
Published: February 2004

A very wide range of extremely high precision single-plane hexagonal lattice sums were calculated by computer. These sums are necessary to the Lennard-Jones interaction potential theory for hexagonal layered crystals. As an application, interplanar spacing, c-axis lattice energy, c-axis elastic moduli, and their change with compressive strain were obtained for all known graphites having ordered stacking. Theoretical results consistently compared well with available experiment, except for cases involving c-axis shear or in-plane translation, where serious underestimation was the rule. Substantial errors in an earlier major calculation for ABA graphite, also using precise lattice sums, became apparent. While it was concluded here that the Lennard-Jones interaction potential is satisfactory otherwise, it fails completely to describe correctly changes in graphite related to atomic movements in the c plane. This limited success may, indeed, be merely incidental rather than fundamental.
High temperature negative ions. Electron impact study of tetratomic phosphorus vapor
Authors: Bennett S. L. , Margrave J. L. , Franklin J. L.
Published: February 2004

Appearance potentials and ion translational energies have been measured for the negative ions P, P2−, and P3− formed by dissociative resonance capture of P4. The following thermochemical values (all in kcal mole−1) have been obtained: Δ H °f,298 (P2−) = 37.2 ± 4.8; E.A. (P2) = 5.5 ± 5.3; and Δ H°f,298 (P3−) = 38.2 ± 4.5. The P− ion appears to be formed in its first excited state on low-energy electron impact of P4, and the derived heat of formation of P3, ΔH°f,298 (P3,g) =59.4±4.0, is proposed. The measurement of the increase in translational energy across resonance gave α (where α serves as the fraction of the oscillators in the transition complex which are effective for the distribution of the excess energy) = 0.43 for the three processes studied.
Vibrational relaxation of N2 and CO2(001) by alkali metal atoms
Authors: Benson R. C. , Benard D. J. , Walker R. E.
Published: February 2004

A new technique, based on fluid modulation in a low-pressure fast-flow apparatus, has been used to measure the vibration-to-translation relaxation rates of N2 and CO2(001) by alkali metal atoms. Basically, the technique is to fluid-modulate trace amounts of CO2 which are added to a fast-flowing alkali metal-active N2 mixture. The extremely fast resonant collisional exchange between N2(ν=1) and CO2(001) inverts the CO2(001) level, as in a fluid mixing laser, which is then probed with a CO2 laser. By measuring the optical gain with and without the alkali metal, the vibrational relaxation rates can be determined. This method enables gain changes as small as 0.1% to be measured. The effective rate, keff=kN2–M +0.914(XCO2/XN2) kCO2–M, has been determined for XCO2/XN2=0.071 and M = Li, Na, K, and Cs: Li, keff=1500±500 torr−1 · sec−1; Na, keff=1400±200; K, keff=4600±1000; Cs, keff=7900±2000. In the case of Na, XCO2 was also varied on order to determine the individual rates: kN2–Na=920±100 torr−1 · sec−1 and kCO2–Na=8100±2000 torr−1 · sec−1. These rates differ by orders of magnitude from theoretically predicted values.
EPR and ENDOR studies of hyperfine interactions in solutions of stable organic free radicals
Authors: Dalal N. S. , Kennedy D. E. , McDowell C. A.
Published: February 2004

ENDOR has been used to measure hyperfine couplings for all protons in solutions of the stable free radicals picryl-N-aminocarbazyl (PAC) and α, γ-bisdiphenylene-β-phenyl allyl (BDPA). The assignment of the measured couplings was aided through computer stimulation of the ENDOR spectra. The intensities of the ENDOR lines were not found to be proportional to the number of contributing protons, and the correction factor suggested by Allendoerfer and Maki was found to be necessary. The values of T2 thus obtained are ∼0.1 μsec and ∼0.05 μsec for PAC and BDPA in mineral oil at room temperature. A further check in the assignments was obtained through comparison of the experimentally observed high resolution EPR spectra with those stimulated on a computer using the proton couplings determined through ENDOR. This also yielded the nitrogen hyperfine couplings in PAC. These results complement our earlier assignment of hyperfine couplings in DPPH and confirm our earlier observations that large ENDOR enhancements can be obtained in nitrogen-centered, strongly exchanging radicals by using solvents with high viscosity and low dielectric loss and by optimizing radical concentration for large EPR signal but lower exchange rates. Well degassed mineral oil at about room temperature appears to be a good solvent. Temperature dependence of the ENDOR spectra shows dynamical changes in line splittings in DPPH and PAC. A model involving the presence of hindered rotations about the Nα–Nβ and the Nβ–Cpicryl bonds helps explain these observations. These results complement the available results on hindered rotations of picryl groups in the related nitroaromatic amines. The present work shows that for paramagnetic systems where NMR might not be successful, ENDOR could be a valuable technique for obtaining information on molecular dynamics. The ENDOR results on BDPA also resolve a controversy between earlier EPR and Knight shift studies.
Magnetic relaxation and quadrupole splitting for liquid crystalline solutions of poly-γ-benzyl-L-glutamate in CDCl3 and CD2Cl2
Authors: Fung B. M. , Martin Thomas H.
Published: February 2004

The spin-lattice relaxation time (T1) of protons and deuterons and the deuterium quadrupole splitting in liquid crystalline solutions of poly-γ-benzyl-L-glutamate (PBLG) in CDCl3 and CD2Cl2 are reported. The proton T1 of PBLG has a strong frequency dependence; as a function of temperature, it shows a minimum at high frequencies. The data cannot be conveniently handled by a rigid ellipsoid model, but calculations based upon a log-Gaussian distribution in the correlation time show reasonable agreement with experimental data. The deuterium quadrupole splittings of CDCl3 and CD2Cl2 are linear functions of the reciprocal temperature, with slopes independent of concentration. T1 for deuterium in the solvents does not show any anomaly in the concentration range in which the phase transition occurs.
Negative ion contamination of electron impact spectra
Authors: Lassettre Edwin N. , Huo Winifred M.
Published: February 2004

Since two particles with the same charges and the same kinetic energies are transmitted equally by electrostatic velocity analyzers regardless of mass, it follows that negative ions produced on electron impact with a gas can interfere with the electron impact spectrum of the gas. Possible contribution of negative ions to the electron impact spectrum of water is studied by theoretical means. It is concluded that H− ions can contribute peaks to the spectrum in the apparent energy loss range 4–6 eV if the spectrum is produced by electrons with kinetic energy in the range 5–7 eV. With incident kinetic energies of 15 eV or higher negative ion contamination is not important. In general, negative ion interference can be important, especially for hydrogen containing compounds, when (a) the kinetic energy of incidence is very low and (b) when the electron spectrometer employed contains only electrostatic lenses and analyzers. These considerations suggest that negative ion spectra might be studied as a means of detecting excited quantum states of radical fragments especially for hydrogen containing substances which give H− on collision with electrons.
Multiple collision effects in the vibrational relaxation of molecules in the liquid phase
Authors: Keizer Joel
Published: February 2004

Vibrational relaxation in fluids is examined using a simple stochastic model of a relaxing oscillator. As an example of multiple collision effects, the coupling between translational and relative motion which occurs for an asymmetric oscillator in a viscous fluid is investigated. A semiclassical calculation of the relaxation constant in this model suggests that for some molecules this effect may be important in the liquid phase.
Chemical relaxation in a radiation field
Authors: Kapral Raymond
Published: February 2004

The relaxation processes occurring in a chemically reacting gas irradiated by a monochromatic light source are investigated. For the case of a bimolecular isomerization, the decay of the progress variable of the reaction to the steady state produced by the illumination is studied. A macroscopic equation for this quantity is derived from the quantum mechanical Boltzmann equation for this system by projection operator methods. The derivation also provides microscopic expressions for the rate coefficients which depend on time (or frequency). The rate coefficients which characterize the one way fluxes in the steady state are compared with the observed rate coefficients. Calculations for specific models of the isomerization indicate that significant differences between these quantities occur at all values of the light intensity. In addition, the time evolution of the part of the rate coefficient which depends on the coupling between the chemical and internal relaxation processes is studied and compared with the decay of the progress variable.
Raman study of ferric nitrate crystalline hydrate and variably hydrated liquids
Authors: Sharma Shiv Kumar
Published: February 2004

A Raman investigation on crystalline ferric nitrate enneahydrate (FNEH) has shown that NO3− and Fe(H2O)63+ ions are present in two distinct environments in the monoclinic unit cell of the crystal. In FNEH melt, it has been found that NO3− and Fe3+ ions form a complex in which water is also coordinated to the Fe3+ ion. Raman evidence is presented which indicates that NO3− ion in the nitrate complex is coordinated to the Fe3+ ion in monodentate fashion, with an appreciable covalency in the Fe3+–ONO2− bond. The addition of water to FNEH melt causes dissociation of the nitrate complex. This complex almost ceases to exist when the water content of the melt is raised to 28 mole of water per mole of the salt. A weak polarized band at ∼407 cm−1 in the Fe(NO3)3 · xH2O system (x ≥ 28) has been attributed to the presence of the hydrolysis product of Fe3+ ions. The effect of dilution and of temperature on this band has also been investigated. It is suggested that the hydrolysis product of the salt exists in the form of a polymeric species.
Deuterium magnetic resonance study of a smectic liquid crystal
Authors: Luz Z. , Hewitt R. C. , Meiboom S.
Published: February 2004

The compound terephthal-bis(butylaniline) (TBBA) has been prepared with selective deuteration on the central benzene ring and the two methine positions. The deuterium magnetic resonance of this material was studied as a function of temperature and sample rotation in the nematic, smectic A, smectic C, and smectic B phases. It is concluded that in the nematic and smectic A phases the direction of molecular alignment (the ``director'') makes an angle of 8° with the para axis of the central benzene ring. There is also good evidence that the molecular conformation is not planar, i.e., the outer benzene rings are rotated out of the plane of the central ring. Results for the smectic C phase confirm a model in which the long axes of the molecules are free to realign while keeping a constant angle with the smectic planes; there is no indication that in this phase rotation of the molecules about their long axes is inhibited, as suggested by a recent theory. Observations on the smectic B phase suggest that its structure resembles the smectic C in some aspects, but results are less conclusive in this case.
Regularities in calculated dipole moment derivatives of first row diatomic hydrides
Authors: Bruns Roy E. , Person Willis B.
Published: February 2004

Calculated dipole moment derivatives from CNDO/2 wavefunctions for the first row diatomic hydride (XH) molecules are compared with the derivatives calculated by Cade and Huo from a near Hartree-Fock calculation. In making this comparison, a remarkably simple straight-line relationship is found to exist between these dipole moment derivatives and the atomic members (Zx) of the X atoms. In the CNDO framework, this linear relationship is due to a nearly linear relationship between q1 (the equilibrium charge on each atom in XH) and Zx. Finally, the results from the CNDO/2 and Hartree-Fock calculations of ∂p/∂R are compared with the values from INDO and CNDO/2D calculations.
Transferability of intrageminal correlation energies and occupation coefficients
Authors: Levy Mel
Published: February 2004

It is shown, within the separated pair approximation, that the occupation coefficient vector and the intrageminal correlation energy may both be obtained by diagonalizing a matrix whose diagonal elements are the natural orbital promotion energies and whose nondiagonal elements are the intrageminal exchange integrals over natural orbitals. In the examined minimal basis example, this matrix for the OH geminal in H2O turns out to be almost identical to the matrix for the OH geminal in H2O2. The corresponding relative OH correlation energies and the corresponding OH occupation coefficients consequently match closely in the two systems. Furthermore, it is demonstrated that these close matchings are possible because the difference in the nuclear attraction part of the promotion energy, obtained when H2O is compared to H2O2, is practically cancelled by the difference in the intergeminal repulsion part of the promotion energy. Different definitions for the intrageminal correlation energy are analyzed in terms of this cancellation.
Dispersion states and orientation of aromatic solutes in monocrystalline matrices of n -heptane
Authors: Lamotte M. , Joussot-Dubien J.
Published: February 2004

Solid solutions of coronene, perylene, and pyrene in monocrystalline n -heptane have been prepared by slow cooling. The solutes exhibit dichroic absorption, fluorescence, and phosphorescence spectra that make evident an orientation of aromatic molecules in the paraffinic crystal lattice. Molecular aromatic planes are found to be nearly perpendicular to the growth axis, while their largest axis is oriented in the direction of alkane chains. The sites in which the molecules are trapped appear to be similar to the ones obtained with low concentrated quickly frozen solutions. Although aggregates could be present, they are not observed.
One-dimensional Ising model: Kinetic studies
Authors: Lacombe R. H. , Simha Robert
Published: February 2004

We investigate methods to deal with the mathematical diffculties inherent in the general model. Two approaches are taken, namely closure and extrapolation from short to large chains. Exact solutions on short chains indicate that the closure approximation employed earlier for infinite chains is satisfactory under a variety of conditions and should provide a valuable computational technique. Moreover, analytic results are presented, which elucidate the reasons behind the success of the numerical approximations. At present it appears that the extrapolation calculations, employing cyclic chains, were not carried far enough to predict the infinite chain results obtained by the earlier approximate calculations. The rigorous short chain studies are of intrisic interest since they yield rarely available data. They also suggest and provide a check on certain special cases of the general kinetics. In particular, a model where only one nucleation event is allowed, very closely approximates the general results in highly cooperative cyclic chains, at least for small chain lenghts. A possible tractable generalization of the single nucleation model is suggested.
Nuclear spin resonance and relaxation of 14N in N, N1-dimethylpiperazine
Authors: Tzalmona A. , Kaplan A.
Published: February 2004

A pulsed free decay nuclear magnetic resonance system has been used to study the 14N nuclear quadrupole resonance in N, N1-dimethylpiperazine. The quadrupole frequencies and spin-lattice relaxation times are measured from 77°K up to the melting point (∼276°K). The relaxation times are determined by the rotation of the CH3 group (120–200°K) and the hindered rotation of the whole molecule (above 200°K). The spin relaxation of the liquid state is also reported.