bluuebunny/crossref_metadata_2025 huggingface dataset:
Matrix reactions of Na, K, Rb, and Cs atoms with N2O: Infrared spectra and geometries of K2O, Rb2O, and Cs2O
Authors: Spiker Robert C. , Andrews Lester
Published: February 2004

Reactions of the heavy alkali metal atoms with N2O have been studied using the matrix reaction technique. No sodium oxides were produced; however, the metals K, Rb, and Cs yielded products with infrared absorptions appropriate for assignment to ν3 of the M–O–M high-temperature molecules. The identification of these molecules is based on oxygen isotopic shifts and on experiments codepositing two different alkali metals. Apex angle calculations indicate that K2O and Rb2O have valence angles in the 160°–180° range and that the Cs2O angle is approximately 130°–140°. This trend is rationalized on the basis of increasing metal-metal bonding with the more polarizable alkali cation. The KO and CsO fundamentals were observed at 384 and 314 cm−1 in solid nitrogen.
The simple exponential distribution for initial electron-positive ion separation distances as seen in the γ radiolysis of alkanes
Authors: Abell G. C. , Funabashi K.
Published: February 2004

A numerical solution of the Smoluchowski equation for diffusion, reaction, and neutralization of ion pairs in a dielectric medium in the presence of a scavenger has been applied for analysis of recent electron scavenging experiments in alkanes. It is shown that these experiments can be quantitatively understood in terms of a simple exponential distribution of initial electron-positive ion separation distances. The credibility of this distribution is considerably enhanced when it is employed, in conjunction with Onsager's solution for diffusion of an ion pair in the presence of an external field, to calculate ion yields as a function of field strength. The agreement with experiment is once again excellent. The attenuation parameter which characterizes our initial exponential distribution is related to the liquid structure factor and gas phase molecular scattering cross sections through the multiple scattering theory of Lax. The wave nature of electron motion is explicitly considered in a discussion of the physical meaning of ionization in liquids.
On the analysis of glory scattering data for the extraction of information on the interatomic potential well
Authors: Bernstein R. B. , LaBudde R. A.
Published: February 2004

With the availability of a large (and ever increasing) body of experimental data on the velocity dependence of the absolute total cross section Q(v) for atom-atom collisions and improved observations of well-resolved glory undulations Δ Q(v), it is desirable to reconsider the problem of ``inversion'' of such data to yield information on the interatomic potential V(r). In the absence of differential cross section measurements there is a practical limit to the amount of information on V(r) available from the total cross sections, even when observations of good accuracy, e.g., ±0.5% in Q(v), are in hand over an extended energy range. It is important to develop optimal means of data processing for the most efficient utilization of glory extrema velocities and amplitudes. Several commonly used procedures for data inversion are no longer adequate to deal with the high quality experimental results now becoming available. The present paper describes new and improved methods for data utilization which take full advantage of the accuracy of the glory scattering data.
Deactivation of N2 (A3Σu+, v=0–7) by ground state nitrogen, ethane, and ethylene measured by kinetic absorption spectroscopy
Authors: Dreyer J. W. , Perner D.
Published: February 2004

Following the excitation of nitrogen and nitrogen/additive mixtures by single electron pulses the metastable nitrogen A3Σu+ is detected in its vibrational levels v=0–7 by absorption spectroscopy. The additives are ethylene, ethane, and NO. In pure nitrogen the prevailing process is a vibration-vibration (V-V) energy transfer process of the N2 A, v≥2 with Δv=2 to groundstate nitrogen. The rate constants increase with higher quantum numbers i.e., improving energy resonance up to a limiting collisional efficiency for this process of 10−2. The kinetic behavior of the vibrational states v=5, 6, 7 indicates strongly a collision induced intersystem crossing with the N2 B3Πg state. The rate constants for quenching of N2 A by ethane exhibit a strong increase with rising vibrational quantum number. Ethylene is generally more reactive and shows no strong dependence on the vibrational level. This is interpreted as a fast quenching of the electronic state by ethylene. While this latter process is relatively slow with ethane there exists a fast relaxation of the higher vibrational states by V-V energy transfer with Δv=1. The deactivation of N2 A, v=0, 1 by NO is investigated because of the importance of this process as indicator for N2 A. The rate constants for the two levels are found to be (2.8± 0.5)× 10−11 and (6.6± 1.2)× 10−11cm3/sec, respectively.
Delayed fluorescence of organic mixed crystals: Temperature independent triplet-triplet annihilation in biphenyl host
Authors: Misra T. N.
Published: February 2004

The investigation on the delayed fluorescence of phenanthrene guest and of naphthalene guest in biphenyl host mixed crystals has been extended to liquid helium temperature. In the very low-temperature range, the intensity as well as the decay and the buildup characteristics of delayed luminescence are temperature independent. It has generally been observed that even at this very low temperature, phosphorescence buildup is faster than its decay whereas delayed fluorescence buildup is slower than the decay.

It has been concluded that, although at higher temperature guest triplet exciton migration is an activated process and occurs through the host triplet exciton band, at low temperature, direct exchange interaction is probably the main mechanism of triplet energy transfer in biphenyl host.
Thermal diffusion factor for the H3e−H4e system in the quantum region
Authors: Taylor W. L.
Published: February 2004

The composition dependence of the thermal diffusion factor for H3e−H4e mixtures was measured at nine temperatures ranging from 5.5 to 240°K in a seven-tube trennschaukel. A pronounced dependence upon the helium-3 content was observed which increased with decreasing temperature and which was also linear in 1/αT, in accord with theory. The experimental data were compared with theoretical thermal diffusion factors calculated quantum mechanically using several different helium potentials. Both our experimental and theoretical results indicate a rather weak temperature dependence with αT rising slowly with decreasing temperature in the temperature region investigated. This is in sharp constrast to the results of van der Valk, who observed a rapid rise from 40°K down to 14°K. On the basis of the ``sensitive'' regions of the transport properties to the intermolecular potential, as reported by Hanley and Klein, we prefer either Beck's or the Morse-VDD potentials on the basis of the data above 50°K. However, we were unable to discriminate among potentials from the 5.5°K experiments since the ``insensitive'' region extended to a much lower temperature than anticipated.
Shock tube study of DF vibrational relaxation
Authors: Bott J. F. , Cohen N.
Published: February 2004

The vibrational relaxation of DF behind incident shock waves in the temperature range 800–4000° K has been studied by monitoring the 3.5-μm infrared emission. Relaxation times reduced to standard pressure were obtained for mixtures containing 1%–10% DF in Ar and for mixtures containing N2, H2, and F atoms. These data were used for calculation of the relaxation times of DF due to these various gases. The relaxation data can be summarized by P τDF–DF=1.4× 10−3exp (63.7/T1/3)  μsec·atm (above 2000°K), P τDF–Ar=7.1× 10−3exp (128.6/T1/3) μsec·atm, P τDF–H2=1.9× 10−2exp (35/T1/3) μsec·atm. The values of P τDF–N2 fell between those for Ar and D2. The deactivation rate of DF by F was measured to be 1.5× 1013cc/mole·sec. The measurements were compared to the predictions of several theories of vibration-to-translation and vibration-to-rotation energy transfer.
Temperature dependence of the 31P excitation transfer cross section of helium
Authors: Hunter William W. , Leinhardt T. E.
Published: February 2004

Inelastic collisions between helium atoms in 31P and 11S states can result in radiationless transfers of excitation. Measurements were made to determine the cross section of the excitation transfer and its variations with the relative velocity of interacting atoms. The collision process was investigated in low density (1015–1018 atoms/cm3) helium gas at four different temperatures between 296°K and 22°K. A 30 KeV electron beam provided primary excitation of the 31P state. The cross sections were found to be 1.6(± 0.3)× 10−15cm2 at 296°K, 1.3(± 0.8)× 10−15cm2 at 214°K, 1.1(± 0.2)× 10−15cm2 at 82°K, and 1.7(± 0.3)× 10−15cm2 at 22°K. A comparison of the experimental results with the predictions of the Stueckelberg theory for imperfect resonance excitation transfer was made. Results indicate possible dipole-quadrupole and quadrupole-quadrupole interactions.
Calculation of crystal vibrations of benzene
Authors: Taddei G. , Bonadeo H. , Marzocchi M. P. , Califano S.
Published: February 2004

The complete vibrational spectra of crystalline C6H6 and C6D6 have been calculated using semiempirical atom-atom potential functions. The calculation uses a free molecule normal coordinate basis including Eckart mass-weighted coordinates. The following factors affecting the calculated freuqencies have been investigated in some detail: (i) intermode mixing among internal vibrations and between internal and external modes, (ii) inclusion of the first derivative of the atom-atom potential with respect to interatomic distances, (iii) number of atom-atom interactions, (iv) fulfillment of equilibrium conditions, and temperature effects. The Jacobian of the crystal frequencies, equilibrium conditions, and heat of sublimation with respect to the individual atom-atom parameters have been evaluated and a refinement of the potential parameters is presented. The agreement between the calculated frequencies and available experimental data is satisfactory specially for lattice frequencies. Transition dipole-dipole calculations lead to poor agreement with observed frequencies thus showing the model to be inappropriate for crystalline benzene.
Calculation of dispersion relations and frequency distribution of crystalline benzene
Authors: Bonadeo H. , Taddei G.
Published: February 2004

The dispersion relations for crystalline benzene at 138°K have been calculated using semiempirical atom-atom potential functions, for many relevant directions of the k vector in the Brillouin zone. The importance of mixing between lattice and low-lying internal vibrations is investigated. The semiempirical atom-atom potential functions have been employed for calculating the density of state according to the interpolation method of Gilat and Raubenheimer [Phys. Rev. 144, 390 (1966)].
Fluorescence of saturated hydrocarbons. III. Effect of molecular structure
Authors: Rothman William , Hirayama Fumio , Lipsky Sanford
Published: February 2004

Fluorescence spectra and fluorescence quantum yields have been determined for a wide variety of alkanes, cycloalkanes, and polycycloalkanes excited at 147 and 165 nm. Correlations between emission characteristics and molecular structure are noted and discussed.
Raman scattering study of the disorder-order phase transition in NH4Cl. II
Authors: Wang C. H. , Wright R. B.
Published: February 2004

A complete Raman scattering study of an NH4Cl single crystal at both sides of Tλ has been carried out and new results are reported. The study includes measurements and interpretations of the Raman spectral bandwidths, intensities and frequencies of the lattice and internal modes as a function of temperature. By the careful examination of the polarization properties of various scattering spectral components, it is suggested that well-defined Raman selection rules are operative in the orientationally disordered crystal.
Use of translational energy measurements in the evaluation of the energetics for dissociative attachment processes
Authors: Harland P. W. , Franklin J. L. , Carter D. E.
Published: February 2004

The translational energies of negative ions formed by dissociative resonance capture processes from CO, NO, CO2, and SO2 have been measured as functions of excess energy. The sums of the translational energies of O− and the corresponding neutral from CO and NO were found to be equal to the electron energy above onset over a range of about 1 eV. At higher energies, the translational energies dropped down from the expected values because of loss of the more energetic ions to the walls. With CO2 and SO2 the total translational energy was always E*/N, where E* is excess energy and N the number of vibrational modes, 3 in each case. The measurement of translational energy has also helped in interpreting the states involved in the various processes and in computing the ground state thermochemical properties of the decomposition products.
Pulse radiolysis study of argon-nitrogen mixtures. Measurement of the rate constant of metastable argon de-excitation by nitrogen
Authors: Calvé J. Le , Bourène M.
Published: February 2004

The postluminescence of N2(C 3Πu→ B 3Πg) induced by the transfer of energy to nitrogen by 3P2 metastable argon atoms obtained by the pulsed radiolysis of argon-nitrogen mixtures has been studied spectrophoto-electrically. The emission decay lifetime τ is controlled by the destruction of metastable argon atoms. The variation of 1/τ as a function of the argon or nitrogen pressure is used to determine the rate constants k1(Ar*+N2)=3± 0.3 × 10−11cm3sec−1 and k2(Ar*+2Ar)=6± 0.4 × 10−33cm6sec−1. The general reaction process is discussed. The results obtained seem to show that the metastables are formed mainly from the dissociative recombination of Ar2+ ions.
Dynamics of polymer molecules in dilute solutions
Authors: Bixon Mordechai
Published: February 2004

In this paper we apply the methods of linear response theory for analyzing the dynamics of flexible polymer molecules in a dilute solution. For simplicity, the analysis is limited to ring (closed chain) molecules. Our starting point is Kirkwood's generalized diffusion equation, from which we extract, with the aid of a proper projection operator, linear relaxation equations for the normal modes. These equations for the fluctuation dynamics of the molecule enable one to evaluate correlation function expressions for transport coefficients. The dynamical viscosity of a dilute polymer solution is evaluated according to this method. On the assumption of Gaussian statistics our results reduce to those derived from the Rouse-Zimm model.
On the electron impact spectra of CH4 and CF4
Authors: Harshbarger William R. , Lassettre Edwin N.
Published: February 2004

The electron impact spectra of CH4 and CF4 have been observed for electrons having incident energies of 400 and 500 eV and at scattering angles from 0° to 5°. The oscillator strength of the CH4 spectrum in the region from 8.55 to 10.95 eV was determined to be 0.28±0.04. For the CF4 spectrum, the oscillator strength was measured as 0.024±0.003 in the region from 11.95 to 13.03 eV and 0.36±0.03 in the region from 13.03 to 14.90 eV. The major features of the spectra of these two compounds are assigned as transitions from molecular orbitals of the ground state to Rydberg orbitals of the molecule. This assignment implies that the first allowed transition in the spectrum of CH4 is split due to the Jahn-Teller distortion of the excited state. The dependence of the differential cross section for electron scattering upon scattering angle for such Jahn-Teller transitions is analyzed and compared with the observed relative intensities in the spectra.
Effect of molecular reorientation in urea on the 14N PNQR linewidth and relaxation time
Authors: Zussman A.
Published: February 2004

The theory of the influence of molecular C=O reorientation in urea on the 14N PNQR linewidth and spin-lattice relaxation time is presented. Pulse techniques are used to measure the inverse linewidth, spin phase memory time and spin-lattice relaxation time of the two high frequency transitions at several temperatures. The relaxation time T1z of the low-lying transition is measured using double radiation methods. At room temperature the spin-lattice relaxation times are highly anisotropic, with T1z three orders of magnitude shorter than T1x and T1y. The experimental results are consistent with a molecular C=O reorientation. The activation energy for this motion is ΔE=11.7±2 kcal/mole.
Charge transfer exciton fission in anthracene crystals
Authors: Schwob H. P. , Williams D. F.
Published: February 2004

Magnetic field modulation of anthracene prompt electroluminescence has been observed for the temperature range 20–80°C. The modulation shows the same crystal orientation dependence as delayed fluorescence but is opposite in sign and smaller in magnitude. These effects are discussed in terms of charge transfer exciton fission.
Self-consistent-field wavefunctions for complex molecules. The approximation of partial retention of diatomic differential overlap
Authors: Halgren Thomas A. , Lipscomb William N.
Published: February 2004

A new approach, based on partial retention of diatomic differential overlap over an orthogonalized basis, is described for approximating LCAO SCF molecular orbital wavefunctions at the minimum basis set level for closed-shell molecules containing hydrogen and first-row atoms. The SCF equations are solved explicitly, retaining all one-electron integrals and approximating two-electron Coulomb integrals, hybrid integrals, and exchange integrals of the forms (iAjA | iAjA) and (iAjB | jAjB) for centers A and B. Single-center averaging processes otherwise required for rotational invariance are avoided by the use of local atomic-centered axes which are unique in anisotropic environments. The result is accuracy comparable to that of much more elaborate methods such as STO-3G, in computing times only moderately longer than for simpler methods based on neglect of differential overlap such as CNDO and INDO. Both unparameterized and parameterized methods are reported. Comparison of parameterized results with ab initio SCF results for a large number of molecules indicates that F-matrix elements are given to standard deviations of 0.007 a.u., density matrix elements to 0.007 electrons, orbital populations and atomic charges to 0.01 or 0.02 electrons, orbital energies to 0.01 a.u., and total energies to 0.03 a.u. Optimal geometries and computed force constants also agree well with reference SCF results. Computing times in seconds for the IBM 360/91 are 0.2+0.006N2+0.00015N3+0.0000012N4 for molecules with minimum basis sets of 2–64 atomic orbitals (N).
Gauge invariance, current conservation, and GIAO's
Authors: Epstein S. T.
Published: February 2004

The well-known relationship between gauge invariance and current conservation is exhibited within the usual quantum mechanical formalism. It is then shown that the use of Gauge Invariant Atomic Orbitals does not necessarily lead to the expected current conservation. The reason is found to lie in the constrained nature of the gauge invariance which is provided by the use of GIAO's. It is concluded that this invariance is, of itself, no argument in favor of their use.
Classical trajectory studies of long-lived collision complexes. I. Reaction of K atoms with NaCl molecules
Authors: Kwei G. H. , Boffardi B. P. , Sun S. F.
Published: February 2004

We have studied the reaction K+NaCl→KCl+Na using classical trajectory techniques on an analytic potential energy surface fit to the ground-state semiempirical pseudopotential surface of Roach and Child. A total of 8000 trajectories were calculated in order to study product angular distributions, energy partitioning and complex lifetimes as a function of collision energy. The ``snarled'' trajectories, the shapes of the center-of-mass angular distributions, and the disposal of energy all provide evidence that reaction proceeds via the formation and subsequent decomposition of long-lived complexes. The calculated laboratory angular distribution of KCl product and the magnitude of the reaction cross section are in reasonably good agreement with experimental results from molecular beam studies by Miller, Safron, and Herschbach. However, the predicted ratio of reactive-to-nonreactive complex decompositions is too large and, since the surface used has too weak a long-range K–Na attraction, these results support the earlier suggestion that many nonreactive decompositions result from linear complexes formed with the wrong orientation. Comparison with other trajectory calculations reemphasize the importance of the height and shape of the entrance and exit-channel barriers in determining both complex formation and reaction dynamics.
Vibrational intensities of F2CO, Cl2CO, and Br2CO
Authors: Bruns Roy E. , Nair R. Kesavan
Published: February 2004

The complete neglect of differential overlap (CNDO) approximate wavefunctions for Cl2CO have been applied to the calculation of the derivatives of the dipole moment with respect to the symmetry coordinates. Although it was not possible to choose unique sets of derivatives in the A1 and B1 species, the acceptable experimental derivatives are very close in magnitude permitting a comparison and interpretation of these derivatives with those found previously for F2CO. Differences in the derivatives of F2CO and Cl2CO are used to predict the preferred experimental derivatives for Br2CO. The effect of d orbitals in the calculation of the Cl2CO derivatives is examined. The experimental results for the out-of-plane CCl and CBr bends indicate that these derivatives are positive and opposite in sign to the in-plane derivatives, although the Cl2CO out-of-plane bending derivative is very close to zero.
Ultrasonic investigation of the nematic-isotropic phase transition in MBBA
Authors: Eden Don , Garland Carl W. , Williamson R. C.
Published: February 2004

An ultrasonic investigation of the relaxation processes associated with the nematic-isotropic phase transition has been carried out on the liquid crystal p-methoxybenzylidine-p-n-butylaniline (MBBA). Simultaneous measurements of the sound velocity and absorption in samples of MBBA without any preferred orientation have been made at eight frequencies from 0.3 to 23 MHz. The temperature range from 23 to 77°C was studied, with special attention given to the region near the transition at ∼44°C. Far from the transition the absorption and velocity dispersion can be well described by a single relaxation process for which the relaxation time and the relaxation strength both increase substantially as the transition is approached. In the immediate vicinity of the transition the acoustic properties are characteristic of a multiply relaxing fluid.
Ionic soft sphere parameters from Hartree-Fock charge densities
Authors: Carlson Edward H.
Published: February 2004

Soft sphere parameters (R, ρ) that describe the charge density ρc in the outer region of an ion through the expression ρc(r)=exp[−4.3−(r−R)/ρ] a.u. are obtained for 32 ions from charge densities given by good Hartree-Fock (HF) calculations. These are compared to those previously obtained for 178 closed shell and transition ions from Hartree-Fock Slater (HFS) charge densities. Systematic differences are noted, and it is recommended that the soft sphere parameters from HF calculations be used for the halide ions and noble gas atoms and those from HFS for the remaining ions.
Ab initio potential energy surface for linear H3
Authors: Liu B.
Published: February 2004

The configuration interaction (CI) method has been applied to determine an accurate potential energy surface for linear H3. The calculated surface is believed to lie no more than 0.8 kcal/mole and no less than 0.2 kcal/mole above the exact surface, a significant improvement over all previous calculations. The calculation yields a linear symmetric saddle point at an internuclear separation of 1.757 a.u. The saddle point energy is 9.8 kcal/mole above the calculated energy of an isolated hydrogen atom and a hydrogen molecule, and 10.28 kcal/mole above the exact energy of the isolated systems. An estimated lower bound for the barrier height is 9.5 kcal/mole. Comparison of the calculated H3 potential surface with the best semiempirical surface and the best previous ab initio surface shows good qualitative agreement. However, there are quantitative differences, whose effect can only be determined by accurate dynamical calculations. The method employed in the H3 calculation was designed to approach the complete CI result with a severely truncated CI expansion. It attempts to establish, computationally, convergence patterns that can be reliably extrapolated to the complete CI limit. Thus, the calculations reported here also serve the purpose of evaluating a computational method applicable to more complicated systems.
Photoionization of HOF with mass analysis
Authors: Berkowitz J. , Appelman E. H. , Chupka W. A.
Published: February 2004

The ionization yields of HOF+, O+, and OH+ have been studied as a function of wavelength in the photoionization of HOF. The first ionization potential of HOF is 12.71±0.01 eV. The threshold for O+ formation, 14.34 eV, leads to Δ Hf00 (HOF)=−22.8± 1 kcal/mole. The threshold for OH+ formation, 15.07 eV, leads to a value for Δ Hf0 (HOF) in good agreement with the above result if one of two discrepant values for Δ Hf0 (HO+) is chosen. This choice implies that the ionization potential for OH is 12.9o±0.05 eV. The proton affinity of OF is 5.82 eV. Pronounced structure in the ionization yield curves is interpreted as vibrational autoionization in the region near threshold, and as electronic autoionization at higher photon energies.
Theory of reactive scattering. IV. Simple model for endoergic and exoergic collinear reactive collisions
Authors: Evers N. S. , Kouri Donald J.
Published: February 2004

An extension of the simple collinear reactive scattering model recently studied by Baer and Kouri is proposed which allows for the investigation of many additional features of chemical interest. The model is used to describe the reaction, A+BC→ AB+C, where the mass of the central atom B is assumed infinite. The asymptotic limit of the potentials of the initial and final channels differ, allowing for the study of endoergic and exoergic effects on reaction probabilities. The equations for the coupled channel operators are employed to develop a set of algebraic equations involving quantities that correspond to the transmission and reflection coefficients. While the derivations and final expressions are somewhat more cumbersome than the results of Baer and Kouri for the simplest model, the extension does not destroy the analytic nature of the solution procedure. The solution of this model is especially interesting since the equations for analogous three-dimensional models can be reduced to essentially the equations derived here.
Relative importance of delocalized and pseudolocalized lattice vibrations in impurity vibron relaxation
Authors: Small Gerald J.
Published: February 2004

The novel dependence of the fluorescence spectrum of tetracene in p-terphenyl on the pseudolocalized vibrations produced by absorption is examined in detail. The results together with the fluorescence spectra obtained by selective excitation of the tetracene zero-phonon vibration bands are used to determine the importance of pseudolocalized vibrations in impurity vibron relaxation. Relative to the system delocalized phonons they are found to be unimportant. This result is interpreted in terms of the wave packet nature of pseudolocalized vibrations, electron-phonon coupling, and the system anharmonicity.
Electron-electron double resonance of irradiated dimethylmalonic acid, α-aminoisobutyric acid, and L-alanine single crystals: The role of methyl substituents
Authors: Kispert Lowell D. , Chang Kichoon , Bogan Carolyn M.
Published: February 2004

Electron-electron double resonance (ELDOR) spectra of the (CH3)2ĊCOOH radical in irradiated single crystals of dimethylmalonic acid and α-aminoisobutyric acid, the (CH3)2ĊCOOD radical in irradiated N,N-dideutero-α-aminoisobutyric acid single crystals, and the CH3ĊHCOOD radical in irradiated single crystals of deuterium substituted L-alanine (CH3CHND2COOD) were measured over a temperature range from 170 to 370°K. Two competing relaxation mechanisms, an electron-nuclear dipole relaxation (END) and a cross relaxation mechanism, resulted in an observed reduction in the allowed ESR lines above 283°K with ELDOR lines observed at aH,2aH, 3aH, and 4aH. Below 283°K an enhancement of the ESR line occurred at aH, a reduction at 2aH and negligible ELDOR intensity at higher multiples of aH. The qualitative aspects of the temperature dependence of the ELDOR spectrum of (CH3)2ĊCOOH can be explained in terms of the correlation times of the intramolecular dynamic processes. Approximate calculations based on these mechanisms give reasonable agreement with experiment.
Electronic states of 1,3,5-(sym)-triazine. I. Absorption spectra of pure and mixed crystals
Authors: Bernstein E. R. , Smalley R. E.
Published: February 2004

Spectra of pure and isotopic mixed crystals of sym-triazine are presented which indicate that the 3455 Å triplet state is E″3(nπ*). These same results confirm previous assignment of the state at 3330 Å as E″3(nπ*). Absorption spectra of sym-triazine in sym-trioxane chemical mixed single crystals are presented and further corroborate these conclusions. Out-of-plane (parallel) vibrations of sym-triazine are considerably affected by crystal fields in both crystal systems. It is demonstrated that assigned state symmetries are not inconsistent with previously obtained experimental results on these states. The nature and type of intermolecular interactions and exciton effects present in the triazine system are briefly discussed. A synthetic route is described which yields substantial quantities of high purity sym-triazine, an absolutely essential part of present and forthcoming work on this system.
Monte Carlo calculations of reaction rates and energy distributions among reaction products. III. H+F2→ HF+F and D+F2→ DF+F
Authors: Wilkins Roger L.
Published: February 2004

A three-dimensional classical trajectory calculation is made of the collision dynamics of the exothermic reactions H+F2(v, J)→ HF(v′, J′)+F and D+F2(v, J)→ DF(v′, J′)+F by means of a modified London-Eyring-Polanyi-Sato (LEPS) potential energy surface. Trajectory calculations are used to establish the anti-Morse parameters for a modified LEPS potential-energy surface which produce the best agreement with previous experimental measurements of reaction rates and energy distributions among reaction products. Very good agreement is obtained with the experimental overall rate constant, the experimental prediction that the maximum vibrational level population of HF (v′) is achieved in v′ = 5, and the mean fraction of available energy entering vibration of the newly formed HF bond. The maximum vibrational level population of DF (v′) is achieved in v′ = 8. The mean fractions (f̄v′=Ev′/Etotal and f̄R′=EJ′/Etotal) of the total available energy entering vibration plus rotation are (1) for H+F2, f̄v′+f̄R′=(0.54+0.02)=0.56. (2) for D+F2, f̄v′+f̄R′=(0.56+0.02)=0.58. The relative rate constants for both reactions have a slight temperature dependence. Rate constants are presented for reactions directly into specific vibrational states of HF and DF.
The molecular Zeeman effect of vinylene carbonate, maleic anhydride, acrolein and the benzene isomers, 3,4-dimethylenecyclobutene and fulvene
Authors: Benson R. C. , Flygare W. H.
Published: February 2004

The molecular Zeeman effect has been observed in vinylene carbonate, maleic anhydride, and acrolein yielding the molecular g values, magnetic susceptibility anisotropies, molecular quadrupole moments, and other magnetic parameters. The susceptibility results are compared with a number of other open-ring compounds and nonconjugated ring compounds in order to establish a set of group magnetic susceptibility anisotropies which can be added to give the molecular magnetic susceptibility anisotropy. The group values of the out-of-plane minus the average in-plane susceptibility are (in units of 10−6 erg/G2·mole): –––C–(sp3 carbon)=1.6 ± 0.8,=C(sp2 carbon) = −4.4 ± 0.4,––=O(carbonyl oxygen)=−6.5 ± 0.7, andO(ether oxygen)=2.0 ± 1.5.––The appropriate sum of these values is in good agreement with experiment on a large number of molecules. The rotational molecular Zeeman effect has also been observed in 3,4-dimethylenecyclobutene and fulvene. For 3,4-dimethylenecyclobutene, the g values are gaa=−0.0703 ± 0.0007, gbb=−0.0532 ± 0.0007, and gcc=0.0023 ± 0.0007, the magnetic susceptibility anisotropies (in units of 10−6 erg/G2 · mole) are 2χaa−χbb−χcc=22.1 ± 0.7 and 2χbb−χcc−χaa=21.2 ± 0.6, and the molecular quadrupole moments (in units of 10−26 esu cm2) are Qaa=4.0± 1.2, Qbb=4.0± 1.2, and Qcc=−8.0± 1.8 where the a and b axes are in the plane and the b axis is the dipole moment axis. For fulvene, the g values are gaa=−0.1059 ± 0.0014, gbb=−0.0482 ± 0.0007, and gcc=0.0219 ± 0.0007; the magnetic susceptibility anisotropies (in units of 10−6 erg/G2 · mole) are 2χaa−χbb−χcc=35.9 ± 0.7 and 2χbb−χaa−χcc=38.1 ± 1.1; and the molecular quadrupole moments (in units of 10−26 esu cm2) are Qaa=5.8 ± 1.4, Qbb=3.6 ± 1.6, and Qcc=−9.4 ± 2.1, where the a and b axes are in the plane and the a axis is the dipole axis. The Zeeman parameters in these two molecules are compared to benzene and other ring compounds by the use of the local group anisotropies determined from the open-ring and nonconjugated ring compounds as described above.
Calculations of Hartree-Fock polarizabilities for some simple atoms and molecules, and their practicality
Authors: Gutschick Vincent P. , McKoy Vincent
Published: February 2004

Hartree-Fock electric polarizabilities have been calculated for H2, He, Li, Be, LiH, and N2. Perturbation theory with all the coupling terms was employed variationally for the first five, using a variety of basis sets for each. Each basis for the perturbation calculations was composed of a zero-order set, plus a first-order set (appropriate to the direction of polarization, for the molecules). The two sets are disjoint to ensure identical zero-order functions for the two molecular polarizability components and, hence, reliable anisotropy values. Nonorthogonal theory as formulated by Das and Duff [Phys. Rev. 168, 43 (1968)], assuming exact zero-order orbitals, was used for LiH. For practical reasons, the nitrogen molecule was treated by the fully self-consistent approach which does not distinguish orders of perturbation. The results for all six species are in very good agreement with experiment, reflecting both a reliable choice of polarization functions and, more significantly, the basic accuracy of the Hartree-Fock method for the static charge distributions, both unperturbed and perturbed by an electric field.
ESR study of uv-irradiated single crystals of dimethylglyoxime: Pairwise formation of isolated dissimilar iminoxy radicals
Authors: Muto Hachizo , Iwasaki Machio
Published: February 2004

Radicals produced by uv light in single crystals of dimethylglyoxime have been studied by ESR, in order to compare with those produced by high energy radiations. It has been found that a pair of dissimilar isolated iminoxy radicals is formed with approximately the same relative yield. The nitrogen hyperfine coupling and g tensors for the two species are considerably different from each other, but the relative orientation of the magnetic tensors differs only by a few degrees. The cause of this difference is tentatively attributed to the mode of radical formation, that is, one is formed from the O–H bond scission through excitation and the other is formed from the abstraction reaction by the hydrogen atom. The results are discussed in relation to the triplet radical pairs and isolated iminoxy radicals produceed by high energy radiations.
Local orientational order in ice
Authors: Stillinger Frank H. , Cotter Martha A.
Published: February 2004

A cluster expansion approach for the Kirkwood orientational correlation factor gk in ice has been examined. The expansion parameter measures the extent to which neighboring molecules obey bonding requirements of the ``ice rules.'' For cubic ice, leading terms have been evaluated in the series for the separate contributions to gk from the first three shells of neighbors. These series tend to confirm the validity of a recent Monte Carlo estimate of gk, but they also suggest that orientational order in cubic and hexagonal ices has long range and is analogous to local order of critical point pair correlations in Ising models. As a result, the cluster expansion for gk itself likely converges very slowly, if at all.
Gross dynamics of a high pressure helium discharge
Authors: Peatman Wm. B. , Barach John P.
Published: February 2004

The spectroscopic and plasma characteristics of a brief (200 nsec) repetitive discharge in 50 torr of helium are approximately determined. Time resolved spectroscopic analysis of the divers atomic and molecular species is employed over the entire discharge period from 100 nsec prior to breakdown to 115 μsec afterwards. Discussion of the discharge is focused on three distinct periods: the arc period, comprising the first 200 nsec after initiation of the excitation pulse; the plasma or cool period, 200–1100 nsec; and the afterglow, 1100 nsec and beyond. Coupled differential equations representing the time dependent concentration of each species are solved approximately and are found to be in general agreement with the spectral data. The energy storage modes for the several periods are obtained. Particular comments on the 600 Å bands of the helium molecule are made.
Fluorescence quenching by the stable free radical di-t-butylnitroxide
Authors: Green James A. , Singer Lawrence A. , Parks Joel H.
Published: February 2004

Bimolecular rate constants (kQ) were measured for the fluorescence quenching of a series of aromatic hydrocarbons by di-t-butylnitroxide (DTBN). In methylcyclohexane, measured values of kQ were close to the diffusion limit with only a 6.5-fold variation through the hydrocarbon series. A study of the viscosity dependence of the quenching reaction, in the Smoluchowski manner, suggests an interaction distance of 4–6 Å. A comparison is made of the efficiency of the DTBN and O2 fluorescence quenching reactions. Possible mechanisms are discussed.
Variational formalism and intermolecular forces in OCS lattice dynamics
Authors: Aung Soe , Strauss Herbert L.
Published: February 2004

A variational formalism for lattice dynamics in molecular crystals (consisting of ionic or neutral molecular units) is developed. We take account of the fact that in the presence of large-amplitude motions, the librational and translational degrees of freedom in the crystal are in general not separable, even at the zone center in crystals with centrosymmetric sites. This formalism has been brought to practical realization and applied to the OCS crystal as a test case using an intermolecular potential function for OCS–OCS determined from gas-phase and crystal-phase experimental data. The determination of the potential function yields information on the intermolecular forces in the crystal, and a discussion of these forces is given. Theoretical results for the zero-point energy of the crystal and for the spectroscopic frequency of the ``librational'' transition at zero wave vector are compared with available experimental data. The comparison indicates the essential validity of the intermolecular potential function and the variational formalism employed. It is found that, as expected, a strong coupling exists between the librational and translational degrees of freedom in the crystal. The results obtained show that the methods used are powerful tools for circumventing the harmonic approximation in the lattice dynamics of molecular crystals.
Lattice sum transformations and potential function expansions in lattice dynamics
Authors: Aung Soe , Strauss Herbert L.
Published: February 2004

Mathematical techniques are presented for the treatment of both harmonic and anharmonic lattice dynamics in molecular crystals. We first give Ewald-type transformation formulas for the accurate and rapid evaluation of the slowly convergent lattice sums associated with long-range isotropic and anisotropic intermolecular force laws. The nature of these lattice sums is discussed in light of the properties of the transformed sums. We then give a systematic procedure for obtaining the Taylor series expansion of the intermolecular potential functions, in terms of the translational displacements of molecular centers of mass from their equilibrium positions. This expansion is valid for arbitrarily large orientational displacements. The need for such an expansion arises since the intermolecular potential functions are naturally expressed in molecular-pair coordinates but for many applications these potential functions must then be written in terms of lattice-fixed or space-fixed coordinates. Although the procedure involved is discussed from the viewpoint of lattice dynamics, further applications in the investigation of intermolecular dynamics of weakly bound molecular complexes and adsorption of molecules on solid surfaces are anticipated.
Theory of chemically induced dynamic electron polarization. I
Authors: Pedersen J. Boiden , Freed Jack H.
Published: February 2004

A general and detailed analysis is given of the phenomenon of chemically-induced dynamic electron polarization (CIDEP) by means of the stochastic-Liouville method in accordance with the earlier preliminary report. The finite-difference technique employed permits rapid and convergent solutions without requiring any untoward assumptions on the nature of the models. The dependence of the polarization on the exchange interaction J(r), the Larmor frequency differences between the interacting pair of radicals, diffusion rates, and rates of spin-selective chemical reactions are given in detail. It is shown that models in which J(r) is taken to decay exponentially with r, the radical-separation distance of the radical pair, yield results which are distinctly different from those for a contact exchange model, when J0 [the value of J(r) when r is at the distance of closest approach] is appreciable. The former, more realistic model yields substantial polarizations asymptotically independent of J0, but larger the slower the decrease of J(r) with r; the contact exchange model, however, rapidly goes to zero with increasing J0. These asymptotic values of polarization are predicted to be as high as 10–40 times the equilibrium polarizations (Peq) for sensible values of the relevant parameters, while for values of J0 yielding maximum polarizations (generated at the formative reaction), they can be greater than 100 Peq. These results are of the correct order for agreement with recent experiments. The polarizations have been related to the CIDEP intensities that one may observe for typical schemes of radical production, reaction, and relaxation in order to allow a comparison of the theoretical predictions with experiment.
Classical rotators on a linear lattice
Authors: Freasier B. C. , Runnels L. K.
Published: February 2004

Exact thermodynamic properties are computed for a model of rotating, coplanar molecules with centers defining a regular one-dimensional array. The interaction included is that between nearest neighbors, represented by a truncated expansion in Legendre functions. The problem reduces to an integral eigenvalue equation, and consequently the free energy must depend analytically on temperature.
Low frequency Raman spectra of ammonium halides in the disordered phases I and II
Authors: Couzi Michel , Sokoloff J. B. , Perry C. H.
Published: February 2004

The low frequency Raman spectrum of single crystals of NH4 Cl, NH4 Br, and NH4 I has been measured in disordered phases I and II. Particular attention has been paid to the polarization selection rules. The temperature dependence of the intensity of the disorder induced spectrum on entering ordered phase III or IV has also been measured. The data for disordered phase II in NH4 Cl were interpreted by performing a calculation of the Raman spectrum, based on the theory of Loveluck and Sokoloff of Raman scattering from phonons in a disordered system, using the lattice dynamical calculations performed by Teh. Excellent agreement is obtained with the major features of the observed polarization selection rules. The two polarization configurations are shown to pick out either NH4+ vibrations or Cl− vibrations alone. The implications for phase II in NH4Br are also discussed in the light of these calculations. A brief discussion of NH4I in phase I is also given.
Monte Carlo calculations of reaction rates and energy distribution among reaction products. II. H+HF(v) → H2 (v′) + F and H+HF(v) → HF(v′) + H
Authors: Wilkins Roger L.
Published: February 2004

Rate constants are calculated for the reactions of atomic hydrogen with vibrationally excited hydrogen fluoride by analyzing the results of classical trajectories on a semiempirical London-Eyring-Polanyi-Sato (LEPS) potential energy surface. Monte Carlo procedures are used to start each collision trajectory. Reaction rate constants are presented for direct reactions into specific vibrational states of the product H2 and HF molecules. By means of this calculation, it is predicted for the reactant HF molecule in the ν = 3 state that 11.3% of the mean fraction of available energy will become vibrational energy in H2, 1.2% will become rotational energy in H2, and 87.5% will become translational energy in the products. As the vibrational energy of the reactant HF molecule increases, the mean fraction of available energy which becomes vibrational energy of the product H2 molecule increases. For example, if the reactant molecule HF is in the v = 6 state, it is found that the mean fraction of available energy that will become vibrational energy in H2 is 35.1%, 8% will become rotational in H2, and 56.9% will become translational energy in the products. For the reactant HF molecule in the v = 3 state, 45% of the mean fraction of available energy will become vibrational energy in the product HF molecule, 0.4% will become rotational energy in HF, and 54.8% will become translational energy in the products. The de-excitation of HF by H atoms is due to vibration-translation energy transfer. Results of this trajectory calculation, indicate that (1) multiple quantum jumps are significant in the reactions of H atoms with vibrationally excited HF, and (2) chemical effects provide an important mechanism for the efficient relaxation of vibrationally excited HF by H atoms. Both theory and experiment indicate that the rate of deactivation of vibrationally excited HF by H atoms is very fast. Rate constants are provided for many reactions that have not been measured experimentally.