bluuebunny/crossref_metadata_2025 huggingface dataset:
Self-consistent molecular orbital methods. XXIII. A polarization-type basis set for second-row elements
Authors: Francl Michelle M. , Pietro William J. , Hehre Warren J. , Binkley J. Stephen , Gordon Mark S. , DeFrees Douglas J. , Pople John A.
Published: February 2003

The 6-31G* and 6-31G** basis sets previously introduced for first-row atoms have been extended through the second-row of the periodic table. Equilibrium geometries for one-heavy-atom hydrides calculated for the two-basis sets and using Hartree–Fock wave functions are in good agreement both with each other and with the experimental data. HF/6-31G* structures, obtained for two-heavy-atom hydrides and for a variety of hypervalent second-row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first-row compounds, where limiting Hartree–Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF/6-31G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split-valance 3-21G basis set for both normal- and hypervalent compounds. Normal-mode vibrational frequencies derived from 6-31G* level calculations are consistently larger than the corresponding experimental values, typically by 10%–15%; they are of much more uniform quality than those obtained from the 3-21G basis set. Hydrogenation energies calculated for normal- and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences.
Hypervertex approximations for fluids. I. General formalism and the linear hypervertex approximation for symmetric fluids
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

A method for generating Dyson equations for two-particle fluid correlation functions is applied to the case of ionic and polar fluids to systematically yield a sequence of approximations that go beyond the lowest-order gamma-ordered approximation. The first two such approximations are given here. For the simplest class of such fluid models—those having hard-core potentials and a high degree of symmetry (e.g., the restricted primitive model, dipolar spheres)—the first of these approximations, the linear hypervertex approximation, is used to derive explicit expressions for thermodynamic quantities and the dielectric constant in terms of the hypervertex function, the hard-sphere reference correlation function, the perturbing pair potential, and the state of the fluid (e.g., the temperature and density of species).
Electron-spin relaxation and molecular dynamics in liquids. II. Density dependence
Authors: Zager Stephen A. , Freed Jack H.
Published: February 2003

A pressure-dependent ESR relaxation study of the probe PD-tempone dissolved in toluene-d8 is described. Extensive results on rotational relaxation, in which τR(T,P) is varied over more than two orders of magnitude, are presented. These results are found to be inconsistent with a simple Stokes–Einstein type η/T behavior modified with a nonzero intercept. However, the data were successfully fit by the empirical form τR= cηβ(ρ−p̄)/T, where β is the isothermal compressibility, c is a constant, and p̄ is an (empirical) reference density whose inverse can be thought of as an ’’expanded volume’’. Thus, ρ−? is a measure of the strength of the anisotropic intermolecular interactions acting on the probe, while β−1 may be thought of as a measure of the total intermolecular interactions. These results for a solute of molecular size somewhat greater than that of the solvent molecules exhibit some differences when compared to a previous NMR study on neat toluene-d8. The ε parameter introduced by Freed and co-workers to fit their nonsecular spectral densities is found to be independent of temperature and pressure, and it is pointed out that this could be consistent with an intermolecular fluctuating torque model.
A reference standard for iodine-127 Mössbauer spectroscopy
Authors: Batchelor Raymond J. , Birchall Thomas , Myers Ronald D.
Published: February 2003

We have examined the iodine-127 Mössbauer spectra of a number of potential single line absorbers with a view to establishing one of these as a reference material. It is concluded that KI is the most acceptable material for this purpose.
Three-iron clusters in iron–sulfur proteins: An EPR study of the exchange interactions
Authors: Gayda Jean-Pierre , Bertrand Patrick , Theodule François-Xavier , Moura José J. G.
Published: February 2003

A detailed discussion of the Mössbauer data on the three-iron clusters in the iron–sulfur proteins from Azotobacter vinelandii and Desulfovibrio gigas [T. A. Kent, B. H. Huynh, and E. Münck, Proc. Natl. Acad. Sci. U.S.A. 77, 6574 (1980).] has shown that the iron atoms were coupled by antiferromagnetic interactions. The three coupling constants should be equal within a factor 2. We present an EPR investigation of the magnetic properties of the cluster from the iron–sulfur protein from D. gigas (ferredoxin II) in the temperature range 5.5 to 50 K. The main characteristics of the EPR spectrum are the appearance of a new component above 8 K, the rapid decrease of the intensity, and the relaxation broadening which appears above 16 K. The temperature dependence of these properties has been systematically studied. The results are well interpreted with an Heisenberg Hamiltonian. We found that the exchange coupling constants are about −20 cm−1 and that their relative differences do not exceed 20%.
Electron attachment by chlorine donors in e-beam pumped laser gas mixtures
Authors: Kligler D. , Rozenberg Z. , Rokni M.
Published: February 2003

The electron attachment rate constants of HC1, CCl4, CH3Cl, CH2Cl2, NOCL, and C1F have been measured in electron-beam excited mixtures of the chlorine donor in a majority buffer gas. The measurements were made by monitoring the electron current decay after termination of the e-beam pulse, in conditions resembling an e-beam controlled laser discharge. The possible enhancement of attachment due to vibrational excitation of the chlorine donor under e-beam pumping was studied by performing measurements with e-beam currents of 15 and 500 mA/cm2. With the exception of methyl chloride, no enhancement was observed. HC1 attachment was also measured at 5 A/cm2 by a steady-state technique, and vibrational enhancement was still insignificant. The implications of these results for XeCl laser kinetics are discussed.
Atomic and ionic core binding energies of selected levels in the alkaline earths from x-ray photoelectron spectroscopy and Dirac–Fock calculations
Authors: Perera J. S. H. Q. , Frost D. C. , McDowell C. A. , Ewig C. S. , Key R. J. , Banna M. S.
Published: February 2003

Selected core levels in the alkaline-earth atoms have been measured by photoelectron spectroscopy using AlKα (1486.6 eV) radiation. The binding energies obtained [Ca2s (447.5 eV), Ca2p1/2 (359.6 eV), Ca2p3/2 (356.0 eV), Sr3d (142.6 eV), Ba3d3/2 (803.5 eV), Ba3d5/2 (788.1 eV), Ba4d3/2 (100.9 eV), and Ba4d5/2 (98.3 eV)] are compared with single-configuration Dirac–Fock ΔSCF calculations and it is found that the computed values are sometimes too high by more than 2 eV thus indicating more correlation in the hole state than in the ground state. In some cases the discrepancy was found to decrease in the doubly charged alkaline-earth cations, of which we report semiempirical binding energies, primarily as a result of the removal of the outer two electrons. Extra-atomic relaxation in the metal was estimated as the difference between the total energy of the atomic hole state and the hole state plus a screening electron added to the first unoccupied atomic level. Comparison is made with experiment and with other theoretical and semiempirical estimates of atom–metal shifts.
LMR spectra of DO2 tuned by the Coriolis-induced avoidance of Zeeman level crossings
Authors: Uehara Hiromichi
Published: February 2003

In an extension of the study of high-field LMR spectra [H. Uehara and K. Hakuta, Chem. Phys. Lett. 58, 287 (1978); J. Chem. Phys. 74, 969 (1981)], it is found that level mixing due to the Coriolis interaction also induces the avoidance of Zeeman level crossings and brings about unexpected high-field LMR spectral lines. The presence of Coriolis-induced avoidance between 55, F2 Zeeman levels of the (010) state and 54, F1 Zeeman levels of the (001) state of the DO2 radical has been confirmed and LMR spectral lines of the ν2 band of DO2 tuned by the Coriolis-induced avoidance have been identified by use of CO2 laser lines in the 9.8 μm region. The present detection of the Coriolis-induced Zeeman spectrum provides support for the analysis of the Coriolis interaction of DO2 by McKellar [A. R. W. McKellar, J. Chem. Phys. 71, 81 (1979)].
The effect of pressure and freezing on luminescence. I. 3-hydroxyflavone and indole
Authors: Salman O. A. , Drickamer H. G.
Published: February 2003

The effect of pressure and freezing on the radiative and nonradiative rates from the excited states of 3-hydroxyflavone and indole has been studied. A series of solvents which have a wide range of dielectric constants (1.96–38.3) and which freezes at 295 K in the available pressure range were used. The radiative rates (kr) correlate well with the low frequency dielectric constant of the media (ε). Clear evidence of this correlation is demonstrated at the phase transition where both kr and ε drop sharply in polar solvents while in nonpolar solvents kr and ε stay almost constant. The nonnradiative rates (knr) appear to depend on solute–solvent collisions. As the viscosity of the media increases, the number of these collisions decreases causing a drop in knr. A brief discussion on the tautomerization process of 3-hydroxyflavone is presented in the Appendix.
A study of polymer conformation in turbulent flow
Authors: Armstrong Rob , Jhon Myung S.
Published: February 2003

Turbulent drag reduction by the addition of a small amount of polymer is studied using the bead-string model for the polymer moleculses. (AIP)
General second-order MCSCF theory for large CI expansions
Authors: Lengsfield B. H.
Published: February 2003

Second-order MCSCF theory is presented in a fashion which is capable of treating large CI expansions. This formalism is then extended to encompass a state-average MCSCF procedure. Sample calculations on Mo2 and the ethyl radical, which involve 1698 and 2302 CSFs, respectively, are presented and a number of approximate schemes discussed.
Statistical mechanics of electrolytes and polyelectrolytes. II. Counterion condensation on a line charge
Authors: Ramanathan G. V. , Woodbury C. P.
Published: February 2003

Starting from the BBGKY hierarchy and using the method described in paper I, the statistical mechanics of a polyion in a dilute ionic solution is studied. It is shown that if the length of the polyion is much larger than the distance of closest approach, ’’counterion condensation’’ occurs. If the length of the polyion is comparable to the Debye length or larger, then the critical charge density is shown to be independent of the ionic strength. If the length is much smaller than the Debye length, the critical value is shown to depend logarithmically on the ionic strength and the length of the polyion. Expressions are derived for the critical charge density and the distribution of the counterions around the polyion.
Vibrational relaxation of hexafluoride compounds: MoF6, ReF6, SeF6, and WF6
Authors: Bass Henry E. , Jensen Verner , Ezell Jean
Published: February 2003

Ultrasonic attenuation has been measured in MoF6, ReF6, SeF6, and WF6 at 293 K. In all cases, a single relaxation process was observed and attributed to the total vibrational energy of the relaxing molecule. Isothermal relaxation times were found to be 2.8×10−8, 8.8×10−9, 2.9×10−7, and 2.3×10−8 s atm for MoF6, ReF6, SeF6, and WF6, respectively. Assuming series relaxation, those relaxation times can be converted to the number of collisions required for relaxation Zvib. The values of Zvib vary with the energy of the lowest vibrational mode as expected from the simple Lambert–Salter correlation.
Lattice vibrations of a monolayer of n-butane adsorbed on graphite
Authors: Kim Hyunyong , Danner Horace R.
Published: February 2003

The incoherent inelastic neutron spectrum of a monolayer of n-butane adsorbed on graphite has been analyzed. The system corresponds to a two-dimensional lattice with a unit cell having one n-butane molecule and four substrate atoms of infinite mass. The potential energy matrix for the vibration is set up in Cartesian coordinates with both intramolecular valence force fields and intermolecular pairwise potentials. The calculated frequency dispersions and hydrogen amplitudes are used to calculate the neutron scattering cross sections. They are summed for all normal modes and folded with both the instrumental resolution function and the incident beam profile to form a composite time-of-flight spectrum. The comparison of the calculated spectrum with observation provides a set of intermolecular force constants. The best agreement is obtained with the hydrogen–substrate and the hydrogen–hydrogen constants of 0.04 and 0.003 mdyn/Å, respectively.
Quantum dynamics of the Henon–Heiles system
Authors: Bixon Mordechai , Jortner Joshua
Published: February 2003

In this paper, we investigate the quantum dynamics of wave packets of bound states in the nonlinear Henon–Heiles (HH) system. The time evolution of a wave packet was separated into two processes involving (a) the motion of its first moments and (b) its spreading. On the time scale when wave packet spreading can be neglected, we were able to establish relations between wave packet quantum dynamics expressed in terms of the initial-state population probability P(t) and classical trajectory dynamics. We have demonstrated that the initial states, which are characterized by a periodic time evolution of P(t) over many periods, are related to the five elliptic periodic orbits in the HH system. The decay of P(t) over a few vibrational periods is related to the classical features of quasiperiodic trajectories and can be accounted for in terms of overlap reduction effects. When the center of the initial wave packet is moved further away from the classical periodic orbits, P(t) is characterized by a single initial peak followed by a ’’noisy’’ background. There is no qualitative distinction between the ’’short-time’’ behavior of a wave packet initiated in distant integrable regions and in classical stochastic regions. For long times, wave packet spreading effects set in. Quantum mechanical estimates for the spreading time τs, which depends on the initial conditions, were provided. For wave packets initiated on periodic trajectories, τs is very long, becoming shorter as one moves away from the fixed points. The quantum mechanical long time (t≫τs) behavior of P(t) exhibits small-amplitude noisy time dependence for all initial conditions. This behavior may be described as ’’quantum mechanical stochasticity’’ and is related to the universal feature of irregularity of the spectral resolution of all wave packets in a nonlinear system. From this point of view, all wave packets exhibit ’’quantum stochastic behavior,’’ but there are vast quantitative differences for different initial conditions that are manifested in terms of the values of τs and of the average noisy amplitude. It should be emphasized that this ’’quantum stochasticity’’ is not related to classical stochasticity phenomena.
Gamma-lithium iodate structure at 515 K and the α-LiIO3 to γ-LiIO3, γ-LiIO3 to β-LiIO3 phase transitions
Authors: Liminga R. , Svensson C. , Albertsson J. , Abrahams S. C.
Published: February 2003

The crystal structure of γ-LiIO3 at 515 K has been determined by profile refinement of x-ray diffraction powder data measured with a Hägg–Guinier focusing camera. The space group is Pna21, with lattice constants at 515 K of a = 9.422(2), b = 5.861(2), and c = 5.301(2) Å. A model was derived from the Patterson function and by similarities with the α-LiIO3 structure and refined by the method of least squares. For all 59 reflections resolved by the profile refinement method, RF = 0.109. The structural rearrangement at the α-LiIO3 to γ-LiIO3 phase transition involves ionic displacements of about 0.5 Å, with consequent rotations of the iodate ion by about 20° and elongation of the oxygen octahedron about the Li+ ion. The symmetry change from point group 6 to mm2 is accompanied by unit cell doubling and an increased separation between layers of iodate ions normal to [100]. A further unit cell doubling occurs at the γ-LiIO3 to β-LiIO3 phase transition, with reorientation of iodate ions to form inversion centers and tetrahedral interstices about the Li+ ions.
Analytic solutions of gelation theory for finite, closed systems
Authors: Donoghue Edward
Published: February 2003

The theory of gelation as formulated by Donoghue and Gibbs for polycondensing, f-functional units in a finite, closed system is solved analytically to obtain both exact formulas and the more useful asymptotic expansions of these. Both forms of solution are derived from exact contour integral representations that follow immediately from the basic generating function presented earlier. The two first-order saddle points of the integrands of these representations, at α and at 1/(f-1), coalesce at the critical extent of reaction αc to form a second-order, or ’’monkey’’ saddle. A known technique that is valid in such circumstances, as the more familiar method of steepest descents is not, is applied to derive asymptotic expansions that approximate the integrals uniformly well at all extents of reaction, rather than diverging at αc as the steepest descents expansions do. The uniform expansions are applied to finite systems to obtain both approximations to the mean polymer size distribution that accurately reproduce the bimodal distribution of coexisting sol and gel at extents of reaction above αc, rather than diverging at the molecular weight of the gel, and approximations to the moments of these distributions that are uniformly valid at all extents of reaction, rather than diverging at αc. The uniform expansions are applied to the infinite system in two limits. Taken in the limit as N→∞ at constant extent of reaction, they reproduce all the well-known results for the infinite system, and extend these somewhat. In this limit, the uniform expansions agree with the steepest descents expansions, which thus also produce the above results. However, because of the divergencies at αc, the common limiting expressions do not describe the gel. Taken in the N→∞ limit with α and (k-NWg) /N2/3 fixed, the formula for km̄k/N1/3, as given by the uniform expansions, is shown to give the correct analytic expression for the limiting ensemble average gel distribution. The gel fraction of a macroscopic system is shown from the new distribution to consist of a single molecule of limiting size NWg(α), the weight of gel as given by Stockmayer. The limiting weight average molecular weight of the system above αc, which is shown to be [NWg(α)]2, is thus due entirely to the one gel molecule.
The interpenetration of two chain polymers in a good solvent
Authors: Witten T. A. , Prentis J. J.
Published: February 2003

The interpenetration of two excluded-volume chain molecules of different size in dilute solution is studied via scaling and renormalization methods. The chains are found to interpenetrate much more strongly than smoothed-density models suggest, in accordance with recent work by Khokhlov. The pair correlation funtion g(r) goes to zero at the origin only as a weak power of r. This power is related to Des Cloizeaux’s exponents ϑi describing intrachain correlations. The power is also related to the scaling exponents of star polymers. The mutual excluded volume MSL of two chains with greatly different length is proportional to the volume of the smaller chain and to the mass of the larger. Thus MSL is much smaller than a smoothed density model would predict. We discuss which chain correlations give rise to this small MSL. The universal coefficient relating MSL to the radius of gyration of the smaller chain is strongly dependent on the dimension d of space, according to our second-order expansion in 4-d. The interpenetration behavior predicted here affects measurable thermodynamic, scattering, and physical–chemical properties of the solution.
Excited state geometry changes from preresonance Raman intensities: Isoprene and hexatriene
Authors: Myers Anne B. , Mathies Richard A. , Tannor David J. , Heller Eric J.
Published: February 2003

A method is presented for using a single preresonance Raman spectrum and an absorption spectrum to obtain changes in equilibrium geometry upon electronic excitation. The relative displacements along each of the vibrational normal coordinates are obtained from the Raman intensities, while the overall scaling of the displacements is determined by the absorption band shape. The absorption spectra, as well as Raman excitation profiles, are calculated using either a sum over vibronic states or a formally equivalent time-dependent method [S.-Y. Lee and E. J. Heller, J. Chem. Phys. 71, 4777 (1979)]. The time-dependent method is computationally much faster than the vibronic sum for large multidimensional systems. Our analysis, which assumes isolated molecules and separable, harmonic surfaces, yields a good fit to the vapor phase absorption spectrum of trans-hexatriene with a Lorentzian linewidth of 175 cm−1. However, the diffuse absorption spectrum of isoprene cannot be adequately reproduced using Lorentzian line shapes, even when all 33 normal modes are included. Finite temperature and excited state frequency changes are also found to have little effect on the calculated band shapes. These results suggest that inhomogeneous broadening may be a major factor, but calculations using Gaussian broadening fail to accurately reproduce the experimental spectrum.
Hydrogen bonding in liquid formamide. A low frequency Raman study
Authors: Faurskov Nielsen O. , Lund P.-A. , Praestgaard E.
Published: February 2003

Low frequency Raman spectra from 20 to 400 cm−1 of liquid HCONH2, HCOND2, and HCO 15NH2 were studied. Experimental data were directly transformed to the R(v̄) representation, previously described. Assignments of low frequency bands are discussed based on a comparison between frequencies for the isotopic species and measurements of depolarization ratios. Chain structures are the dominant species. An out-of-plane intermolecular mode involving atoms in the hydrogen bond was assigned. The frequency for this mode decreased with increasing temperature.
Multiple excitations and charge transfer in the ESCA N1s (NO2) spectrum of paranitroaniline. A theoretical and experimental study
Authors: Ågren Hans , Roos Björn O. , Bagus Paul S. , Gelius Ulrik , Malmquist Per-Åke , Svensson Svante , Maripuu Rein , Siegbahn Kai
Published: February 2003

The N1s (NO2) ESCA spectrum of paranitroaniline (PNA) is analyzed theoretically and experimentally. New monochromatized high-resolution spectra in the vapor (gas) and condensed phases are presented. In the condensed phase spectrum two distinct peaks are discerned, whereas the gas phase spectrum shows a drastically different feature in which the two peaks are reduced to an asymmetric band. The gas phase spectrum has been interpreted based on large-scale MCSCF wave functions calculated using a complete configuration expansion in the π valence orbital space. Emphasis is placed on the identification of features due to ’’shake’’ effects. Multiple excitations in the charge transfer excited states, as well as configuration near-degeneracy effects in the NO2 group, are found to be crucial for this identification. Our results are different from an earlier many-body, Green’s function, treatment of PNA. We find that the charge transfer shake states lie higher in energy (shakeup) and not lower (negative shakeup).
Radiationless transitions in electron donor–acceptor complexes: Position-dependent deuterium isotope effects on S1→S internal conversion of 1:1 and 2:1 complexes of methyl-substituted benzenes with tetracyanobenzene
Authors: Lim B. T. , Okajima S. , Chandra A. K. , Lim E. C.
Published: February 2003

The deuterium isotope effects on S1→S0 internal conversion of alkyl benzene/tetracyanobenzene EDA complex depend on the stoichiometric ratio of the donor (alkyl benzenes) to the acceptor (tetracyanobenzene) molecules. Thus, the nonradiative decay rate of the 2:1 complex is more greatly affected by ring deuteration than by methyl deuteration of the donor molecule, in sharp contrast to the very dominant methyl deuteration effect in the 1:1 complex. A theoretical consideration of the binding in the termolecular (2:1) complex suggests that the different isotope effect and the smaller S1→S0 internal conversion rate in the 2:1 complex arise primarily from the smaller cationlike character of the donor molecule in the excited singlet state of the termolecular complex.
Electron-spin relaxation and ordering in smectic and supercooled nematic liquid crystals
Authors: Meirovitch Eva , Igner Dan , Igner Eva , Moro Giorgio , Freed Jack H.
Published: February 2003

We report on careful line shape studies of slow motional and orientation dependent ESR spectra of a deuterated liquid-crystal-like spin probe dissolved in a benzilidene-derivative (40,6) and in cyanobiphenyl derivative (S2 and 5CB) liquid crystals. The simulation of the ESR spectra is based on the Lanczos algorithm recently applied by Moro and Freed in a general and efficient formulation of slow motional and ordering effects on ESR line shapes. With 40,6 which exhibits monolayer smectic phases, we find that the main change in the spin relaxation upon passing from the nematic to the smectic A phase consists of changes occuring in ordering attributable to packing forces on functional groups. Such ordering effects appear to be further enhanced in the SB phase with consequent alterations in dynamics. With S2, which exhibits an interpenetrating bilayer smectic A phase, we find unusual ESR spectra in that phase which may be simulated on the basis of a model of cooperative distortions static on the ESR time scale, and superimposed on individual molecular reorientation. This mode is interpreted as a collective chain distortion which affects the orientational distribution of the piperidine ring of the spin probe. A similar phenomenon is observed in the supercooled nematic phase of 5CB, which is aligned by an electric field, and evidence is also found that the reorientational dynamics of this ring are affected by interaction with local cooperative modes in the liquid crystal (i.e., a SRLS mechanism previously proposed by Freed and co-workers). Some microscopic characteristics of liquid crystals revealed by this and previous ESR spin probe studies are discussed.
Direct current-induced second harmonic generation by chiral molecules
Authors: Lam Y. T. , Thirunamachandran T.
Published: February 2003

A theory of second harmonic generation by optically active molecules in the presence of an external, static, electric field is presented. It is shown that the intensity of the second harmonic beam depends on the helicity of the incident radiation. Expressions for the intensity are derived using electric dipole, electric quadrupole, and magnetic dipole interactions, and the intensity dependence on helicity is given as a circular intensity differential ratio.
Surface Raman scattering from effervescent magnetic peroxyborates
Authors: Walrafen G. E. , Krishnan P. N. , Hokmabadi M. , Griscom D. L. , Munro R. G.
Published: February 2003

Surface Raman scattering using a spinning technique was investigated for solid NaBO3⋅4H2O and NaBO3⋅H2O, as well as for electron bombarded peroxyborates, for peroxyborates heated for various times and at temperatures for 110–180 °C, and for solid Na2O2 and BaO2. The Raman spectra indicate that the breakdown of peroxy groups is accompanied by the formation of trapped molecular O2. Quantitative Raman intensity data were also obtained as functions of heating time at 115 °C for the 1556 cm−1 line from O2 and for the 890 and 705 cm−1 lines whose intensities scale with the peroxy concentration. These intensity data were treated by logistics theory, and they were found to be consistent with a second-order autocatalyzed forward reaction dependent on the product of the peroxy and O2 concentrations, plus a first-order reverse reaction dependent only on the O2 concentration.
Slow chain reactions of Br2 and Cl2 with HI: Multiple state analysis and vibrational relaxation of HBr(v = 2) and HCl(v = 1–4)
Authors: Dolson David A. , Leone Stephen R.
Published: February 2003

Two chain reactions of the general form X+HI→k1HX(v⩽vmax) +I, I+X2→k2IX+X(X =Br, Cl) are studied by realtime detection of infrared chemiluminescence from the vibrationally excited HX(v) products. Both are characterized by k2≪k1. These reactions are initiated by pulsed UV laser photolysis of the diatomic halogens at 355 nm in a flow cell apparatus at 295 K. Observations are made on Δv = −1 fluorescence from individual vibrational levels, and the results are treated with a complete mathematical analysis for the production of multiple vibrational states and their subsequent individual decays. Effects of vibrational cascading on the measurements of k2 are discussed. The chain propagation rate coefficients determined by this technique for k1(X=Br, Cl) and k2(X=Br, Cl) are 8.9(±1.3)×10−12, 1.4(±0.3)×10−10, 3.4(±0.8)×10−13, and 8.5(±1.1)×10−17 cm3 molecule−1 s−1, respectively. The deactivation rate coefficients for HBr(v = 2) and HCl(v = 1,2,3,4) by HI are 1.7(±0.2)×10−12, 1.43 (±0.05)×10−13, 6.3(±0.5)×10−13, 7.0(±2.4)×10−13, and 3.2(±0.7) ×10−12 cm3 molecule−1 s−1, respectively. Vibrational deactivation rate cofficients for HCl(v = 1,2,3,4) by Cl2 are 5.2(±0.3)×10−15, 2.2(±0.3)×10−14, 4.3(±4.2)×10−14, and 2.8(±1.5)×10−13 cm3 molecule−1 s−1, respectively. The vibrational deactivation efficiencies of HCl(v=1−4) by HI and Cl2 scale approximately as vn, where n = 2.1(±0.2) and 2.8(±0.2), respectively.
Polymorphism of crystalline poly(hydroxymethyl) compounds. VI. The crystal structure of 2-hydroxymethyl-2-nitro-1,3-propanediol
Authors: Sake Gowda D. S. , Rudman Reuben
Published: February 2003

O2NC(CH2OH)3 is triclinic from room temperature to 347.4 K, where it forms an orientationally disordered fcc phase I. The room temperature form (phase II) has two molecules per unit cell (asymmetric unit) in space group P1, with a=6.203(2), b=9.823(3), c=6.095(2) Å, α=91.68(1), β=109.59(1), γ=89.74(1)°, Dx=1.440 g/cm3, and Dm=1.458 g/cm3. Although phase II is triclinic, it approximates a body-centered monoclinic cell, which is easily related to the high-temperature phase I fcc unit cell. In phase II, a total of 728 independent reflections were refined to a final agreement factor of 0.0304. The nitro groups on both crystallographically independent molecules are disordered and do not participate in the three-dimensional hydrogen-bonded network (which does involve all the hydroxymethyl groups).
Multiphoton ionization detection of NO scattered from solid surfaces
Authors: Hayden J. S. , Diebold G. J.
Published: February 2003

This paper describes experiments where resonantly enhanced multiphoton ionization was used to determine rotational state distributions of NO scattered from a copper surface in a high vacuum. A pulsed valve produced a high intensity beam of NO seeded in He with a well-defined translational energy and a rotational temperature of 7 K. With a 0.28 eV incident beam and a surface temperature of 77 K the scattered beam was found to have a rotational temperature of 230 K. When the surface temperature was raised to 300 K, the scattered beam temperature approximated that of the surface. For the surface at either temperature, significant population of the X 2Π3/2 state was found. The variation of the scattered beam temperature with surface temperature is in qualitative agreement with calculations based on the hard cube model.
The relationship between the modified Flory rubber elasticity theory and the Dossin–Graessley small strain model
Authors: Gottlieb Moshe
Published: February 2003

The formal relationship between the Dossin–Graessley small strain model and Flory’s revised rubber elasticity theory is established. A closed form equation is given for h the suppression of junction fluctuation parameter, as a function of Flory’s parameter κ and ζ.
Tritium isotope separation by multiphoton dissociation of CTF3 under CO2-laser pulses
Authors: Mévergnies M. Nève de , Verhoeven F. , del Marmol P. , Koch G.
Published: February 2003

Isotopic separation of tritium by CO2 laser induced multiphoton dissociation of tritiated fluoroform is reported. It is found that maximum dissociation occurs around 1060 cm−1, that is close to the center of P branch of ν2 (CTF3). (AIP)
An effect of potential shallowing and the determination of barriers to internal rotation (in electron diffraction)
Authors: Scharfenberg Peter
Published: February 2003

A relationship between the molecular potential ofinternal rotation and its substitute (a parabola), shallowed in the surrounding of the energy minimum, as determined from the fitting of electron diffraction data, is pointed out by the principle of maximum similarity. The numerical approach introduced allows to determine the barrier of internal rotation from the scaling factor of the parabola as well as from average torsional angles related to it. The scaling factor itself is shown to be a lower boundary of the rotational barrier. (AIP)
A second-order phase transition in Mn(CO)5Br
Authors: Adams David M. , Ekejiuba Isaiah O. C.
Published: February 2003

Raman spectra of Mn(CO)5Br are reported at hydrostatic pressures of up to 50 kbar. Existence of a phase change is shown by changes of slope in frequency/pressure plots. The phase transition is a second-order transition. (AIP)
Determination of Arrhenius parameters for unimolecular reactions of chloroalkanes by IR laser pyrolysis
Authors: Dai Hai-Lung , Specht Eliot , Berman Michael R. , Moore C. Bradley
Published: February 2003

A simple and reliable method is elaborated for accurate measurements of thermal rate constants of homogeneous gas phase unimolecular reactions. A pulse of CO2 laser radiation was used to multiphoton excite SiF4 sensitizer molecules and consequently produce temperatures in the range 1100–1400 K. Expansion of the heated gas column quenches pyrolysis reactions on a 10 μs time scale. There are no hot surfaces to induce chemistry. HCl elimination from C2H5Cl, Ea=57.4 kcal/mol and log A(s−1)=13.8, was used as an internal temperature standard. For the molecular elimination CCl3CH3 → HCl+CCl2CH2, Ea=49.5±1.3 kcal/mol and log A(s−1)=13.1±0.3, were determined. In these experiments the major decomposition products of CHCl2CH2Cl are HCl and cis- or trans-CHClCHCl with Ea=58.5±2, log A=14.1±0.4 and Ea=59.5±2, log A=13.9±0.4, respectively. HCl elimination to give CCl2CH2 and C–Cl bond breaking to CHClCH2Cl radical have higher activation energies. The method is generally useful for kinetics at high temperature.
Atom–diatom processes in helium: The multiple-scattering description of elastic and dissociative collisions
Authors: Haftel M. I. , Lim T. K.
Published: February 2003

Elastic and dissociative scattering of He on He2 are studied for an incident laboratory energy of 1 K. We apply Faddeev-AGS multiple-scattering theory in momentum space to the analysis. We show that this leads to a simple justification for the importance of the ‘‘complex-formation mechanism’’ in termolecular recombination, the time reverse of collision-induced dissociation. Our work, in which separable expansions of two phenomenological He–He potentials are used, predicts significant magnitude differences in the cross sections derived from the interactions and also verifies the validity of the ‘‘peaking approximation’’ for hyperthermal-energy elastic collisions.
Born–Oppenheimer energy surfaces of similar molecules: Interrelations between bond lengths, bond angles, and frequencies of normal vibrations in alkanes
Authors: Lifson Shneior , Stern Peter S.
Published: February 2003

CH bond lengths, HCH and HCC bond angles, and CH symmetric and asymmetric stretching frequencies in alkane molecules are placed into four groups according to their occurrence in CH4, –CH3, CH2, and –CH, and are seen to vary in a regular fashion. The physical rationale offered for these variations relates them to balanced interactions between adjacent orbitals of CH and CC bonds, which are assumed to be common to all energy surfaces of alkane molecules. The regular variations are quantitatively reproduced by a consistent force field of alkanes, which in place of the usual harmonic stretching potentials uses only two Morse potentials, one for the CH bond, common to all four groups, and one for the CC bond. The correlated variation in bond lengths and bond angles, due to orbital interactions, is represented mainly by stretch–bend, stretch–stretch, and bend–bend cross terms. The resulting stretching frequencies, being dependent upon the second derivative of the Morse function, decrease with increasing bond length. The new force field yields bond lengths, bond angles, and vibrational frequencies, and reproduces the observed trend in their variation, mostly to within experimental accuracy. Remaining deviations are attributed to vicinal and higher order nonbonded interactions. Methane is included as a member of the alkane family and the new force field accounts successfully for its vibrational frequencies.
Dipole moments, molecular ionization potentials, and asymptotic form of electron density in hydrogen halides and other heteronuclear diatomic molecules
Authors: March N. H. , Mucci J. F.
Published: February 2003

From available experimental data, it is demonstrated that the electric dipole moments of hydrogen halides and other heteronuclear diatomic molecules are strongly correlated with molecular ionization potentials. By means of the Euler equation of density functional theory, solved far from nuclei, it is shown that the asymptotic form of the electron density is determined by the spherically averaged density and by the ratio dipole moment to ionization potential.
The Z dependence of the total energy, energy components, orbital energy, and MO coefficients for mononuclear, isoelectronic species
Authors: Murdoch Joseph R. , Magnoli Douglas E.
Published: February 2003

Previous workers have shown that in the limit Z→∞, the total energies, Vne, T, and total orbital energies (E0) of a series of isoelectronic, mononuclear structures are quadratic functions of Z, while Vee is a linear function of Z. In the present paper, a new perturbational treatment is presented which is based on a zero-order solution corresponding to Z=Z0, where Z0 is arbitrary. For first-order corrections to Hartree–Fock wave functions, the same limiting behavior (E,Vne, T, E0≡quadratic functions of Z; Vee≡linear function of Z) is seen as for the high-Z limit (Z→∞). For higher-order corrections, a completely different pattern of behavior emerges. For the ten-electron series (Z=9,18), E and T remain highly quadratic with respect to Z (to within 7 kcal), but Vne, Vee, E 0 exhibit large oscillations (∼60 kcal) around a quadratic function. This behavior is analyzed by applying the virial and Hellmann–Feynman theorems to the total energy, the Hellmann–Feynman theorem to Hartree–Fock orbital energies, and by expressing perturbation expansions for Vne and E in terms of Cheybshev polynomials. The results provide a demonstration of the fact that the total energy and orbital energy may show a different pattern of behavior with respect to substituent effects. The present work has important implications for perturbational treatments, such as Walsh’s rules, frontier orbital theory and orbital symmetry correlations, which are based on the behavior of the orbital energy rather than the total energy.
The CNDO/S3 crystal orbital model: Definition and application to polyacetylene
Authors: Ford W. K. , Duke C. B. , Paton A.
Published: February 2003

The CNDO/S3 model, a spectroscopically parametrized complete neglect of differential overlap molecular orbital method, is extended to encompass the description of radical cation states in infinite periodic quasi-one-dimensional molecules. Detailed model calculations of the energy bands and density of valence electron states (DOVS) are given for macromolecules of trans polyacetylene. These calculated quantities are shown to correspond directly to predictions extrapolated from sequences of CNDO/S3 calculations for finite polyenes, and to describe accurately both results of ab initio calculations and measured valence electron photoemission spectra for polyacetylene. The consequences of introducing electron–electron interactions into a noninteracting electron model are studied systematically by examining in turn the Hückel, Hubbard, CNDO–SCF, and ab initio limits of the calculated energy bands and DOVS. Comparison of the results of these calculations with measured photoemission spectra for polyacetylene reveals that electron–electron interactions exert a minor effect on the interpretation of these spectra. Since the CNDO/S3 model has previously been shown to provide a quantitative description of the valence electron photoemission of (gas-phase) finite polyenes, the present analysis of macromolecules of trans polyacetylene provides an explicit link between the interpretation of these spectra for individual finite polyene molecules and the analysis of the corresponding spectra obtained from films of polyacetylene.
On the functional derivative of the kinetic energy density functional
Authors: Bartolotti L. J. , Acharya P. K.
Published: February 2003

The functional derivative of the kinetic energy Ts[ρ↑,ρ↓] of a noninteracting system of particles with density ρ=ρ↑+ρ↓ is evaluated in terms of the Kohn–Sham spin orbital densities ρiσ and Lagrange multipliers εiσ(σ=↑ or ↓). Of particular interest is the functional derivative of TΔ[ρ↑,ρ↓] =Ts[ρ↑,ρ↓]−Tw[ρ↑,ρ↓], where Tw[ρ↑,ρ↓] is the Weizsäcker kinetic energy functional: δTΔ[ρ↑,ρ↓]/δρσ =(1/8ρσ){𝒥Nσi=1[(∇ρiσ ⋅  ∇ρiσ)/ρiσ −(∇ρσ ⋅ ∇ρσ)/ρσ]} −(1/ρσ)(𝒥Nσi=1 εiσρiσ −μσρσ). This quantity is used to analyze the approximate kinetic energy density functional proposed by Acharya et al.
Polymorphism of crystalline poly(hydroxymethyl) compounds. IV. The structures of crystalline and plastic 1,1,1-tris(hydroxymethyl) propane
Authors: Sake Gowda D. S. , Federlein Nancy , Rudman Reuben
Published: February 2003

H5C2C (CH2OH)3 forms an orientationally disordered crystalline (ODIC) phase I as it is cooled through its melting point and remains in this phase until approximately 0 °C, whence it transforms to an ordered phase II. However, phase I does not form on heating phase II, and phase II forms spontaneously if phase I is left standing at room temperature. Thus, the ODIC phase is metastable. Phase II is monoclinic, P21/n, a=8.431(2), b=9.598(2), c=9.190(2) Å, β=98.96(1)°, Z=4, Dx 1.213 g/cm3, and Dm=1.175 g/cm3. A total of 1153 independent reflections were refined using least-squares analysis to a final agreement factor of 0.039. The molecular centers of the four molecules in the unit cell are located, with respect to each other, in the same positions as the four molecules in the fcc ODIC phase I (with a=9.210(6) Å and Z=4). Thus, the reversible transition between phases I and II occurs with minimum distortion of the lattice. In phase II, three hydroxymethyl groups form a tightly bonded three-dimensional network.
Polymorphism of crystalline poly(hydroxymethyl) compounds. V. X-ray diffraction investigation of crystalline and plastic 2-bromo-2-nitro-1,3-propanediol
Authors: Sake Gowda D. S. , Rudman Reuben
Published: February 2003

Between room temperature and its melting point (401 K), BrNO2C(CH2OH)2 exhibits one phase transition, at 367 K, to form an orientationally disordered crystalline (ODIC) phase I. The ordered form phase II is monoclinic Cc, a=8.020(3), b=9.659(2), c=8.965(2) Å, β=91.00(1)°, Z=4, Dx=1.913 g/cm3, and Dm=1.845 g/cm3. A total of 596 independent reflections were refined using least-squares techniques to a final agreement factor of 0.031. The structure can be described in terms of a three-dimensional hydrogen-bonded network of interconnected, parallel chains, with the molecular centers of the four molecules in the unit cell in an approximate tetrahedral arrangement reminiscent of a face-centered unit cell. Yet, the ODIC phase I is body-centered cubic [a=7.11(6) Å, Z=2]. The relationship between the fcc and bcc structures is reviewed in terms of the phase II–phase I transition.
Correlation between the observed infrared stretching frequency and the bond character of the Si–H bond
Authors: Sahu S. N. , Shi T. S. , Ge P. W. , Corbett J. W. , Hiraki A. , Imura T. , Tashiro M. , Singh Vijay A.
Published: February 2003

It is shown that the empirical correlation between the infrared (IR) stretching frequency of the Si–H bond in substituted silane molecules (SiH3R1, SiH2R1R2, and SiHR1R2R3 with Ri the substituted molecular fragment) can be understood on a fundamental basis as the dependence of frequency on the bond order (S component) of the bond.
A determination of hyperfine constants for the RCONHO radical in randomly oriented samples of x-irradiated hydroxamic acids
Authors: Shields Howard , de Lyon Terence , Chiu FonJen , Hamrick Phillip J.
Published: February 2003

RCONHO radicals are identified in x-irradiated acetohydroxamic acid, hydroxyurethane, and propiohydroxamic acid. The hydrogen and nitrogen hyperfine coupling constants for these radicals are evaluated from ESR spectra taken of polycrystalline samples. The characteristic spectrum of randomly oriented RCONHO radicals is described. Equations for the coupling constants in terms of the width of the spectrum are given.
Yields and mean free paths of photoelectrons from liquid hexamethyl phosphoric triamide
Authors: Birkhoff R. D. , Hubbell H. H. , Ashley J. C. , Painter L. R.
Published: February 2003

Monochromatic photons between 775 and 500 Å were incident almost normally (15°) on the surface of liquid hexamethyl phosphoric triamide (HMPT). Incident and reflected beam strengths were measured with a double ion chamber filled with argon and the current leaving the cup of liquid was determined. After subtracting a portion of the current due to Ar+ drift into the cup, we obtained the photocurrent and, hence, the absolute photoemission yield. The yield falls from 9% at 16 eV to 5% at 25 eV. Such a falloff with increasing photon energy is expected from theory. The values are about double those calculated from theory using a three-step model for the photoemission and using the Born approximation to calculate electron mean free paths. Conversely, if the experimental yields are used in the three-step model, the electron mean free paths calculated from the data range from about 935 Å at 16 eV to about 80 Å at 24 eV and are about two to three times the theoretically predicted values.
The relative Raman intensities of ν(M–L) A1g and E
g modes for some dML6 anions of O
h symmetry
Authors: Luknar-Gabor Noemi , Kettle Sidney F. A.
Published: February 2003

The observation that unusual relative ν(M–L) Raman intensities are associated with a t62g configuration has prompted calculations on d0 systems based on the hypothesis that charge transfer transitions to the t2g shell make a dominant contribution to the corresponding Raman intensities of these systems. A combination of Albrecht and Wolkenstein–Eliashevich models leads to the prediction of relative Raman intensities in good qualitative agreement with those observed experimentally.