bluuebunny/crossref_metadata_2025 huggingface dataset:
Mass spectrometric investigation of the dissociation energy of the molecule AlAu and estimated bond energies of some diatomic intermetallic compounds with gold
Authors: Gingerich K. A. , Blue G. D.
Published: February 2004

High-temperature Knudsen-cell mass spectrometry has been used to measure the gaseous equilibria over an aluminum-gold or aluminum-silver-gold alloy in a tantalum cell. The enthalpies for the reactions (1) AlAu(g) = Al(g) + Au(g), and (2) AlAu(g) + Au(g) = Au2(g) + Al(g) were measured. The following dissociation energy D°0, heat of sublimation Δ H°s,298, and heat of formation Δ H°f,298 were derived from these reaction enthalpies for gaseous AlAu: 77.0 ± 1.5, 98.4 ± 2.5, and 88.9 ± 2.0 kcal mole−1, respectively. The dissociation energy of AlAu(g) has been interpreted in terms of the Pauling model of a polar bond. This model has been used to calculate dissociation energies of selected intermetallic compounds between transition metals, Group III metals, and Group IV metals with gold, which are compared with available literature data.
Optical potentials in time-dependent quantum theory
Authors: White R. A. , Altenberger-Siczek A. , Light J. C.
Published: February 2004

The equations governing the exact quantum mechanical time evolution of a projected portion of the wave function are derived in terms of an optical potential. Although these equations are non-Markoffian, they are solved numerically for two types of approximate optical potential for the model problem of the linearly forced harmonic oscillator. It is shown that the perturbation optical potential is superior to normal second order perturbation theory and may be useful in semiclassical scattering problems, etc.
Low temperature optical spectra of Zn porphin in triphenylene
Authors: Kim Boris F. , Bohandy Joseph , Jen C. K.
Published: February 2004

The low temperature optical absorption spectra of single crystals of triphenylene doped with Zn porphin have been observed in detail. The spectra consisted of sharp lines, many of which were polarized. This is thought to be the first direct observation of polarized sharp line spectra of a porphyrin. Also observed were a number of weak anomalous absorption lines near the 0–0 transition, but at longer wavelengths. Two different site models were proposed to account for the polarization properties of the spectra. The implication of these results in the interpretation of porphyrin spectra are discussed.
Modulation of the dipolar echo maximum due to the chemical shift interaction in solids
Authors: Wang C. H.
Published: February 2004

This paper discusses the symmetry requirement for the one-cycle propagator associated with the design of the pulse cycle for resolving chemical shift in solids. The calculation of free induction decay and the dipolar echo modulation connected with the symmetrized WAHUHA cycle for a two spin system were carried out explicitly. The calculation is not available in the literature and should serve as a guide to interpreting the multiple pulse experiment in solids for like spins, especially when the two coupled nuclear spins considered are well separated from the others due to large interatomic distances.
Vibrational relaxation of hydrogen and deuterium fluorides
Authors: Hinchen J. J.
Published: February 2004

The vibrational relaxation of HF and DF has been studied using the laser fluorescence technique at 295°K. Rates were determined for self-relaxation of HF and DF and also for cross relaxation through transfer of vibrational energy between these molecules. The results are compared with relaxation of the isotopic combinations in HCl/DCl and HBr/DBr, and discussed with reference to the Moore and the Shin theories. Results are also given for the rates of vibrational energy transfer between DF–D2 and HF–H2 and for the relaxation of DF by Ar, He, H2, and N2 and for HF by Ar and He.
Oscillations, multiple steady states, and instabilities in illuminated systems
Authors: Nitzan Abraham , Ross John
Published: February 2004

The absorption of light by some but not all species of a chemical reaction, followed by a radiationless transition and ultimate conversion of light into heat on a time scale short compared to the chemical reaction time scale, is shown to give rise to the possibilities of multiple steady states, damped oscillations in state variables, hysteresis, and instabilities. All these phenomena are predicted to occur even for the simplest reaction A⇄ B, where only A absorbs light, and where the rate equation, with temperature dependent rate coefficients, is coupled nonlinearly to the equation for the rate of change of temperature. The theory is developed for both stationary and transient experiments. For the cyclic reaction mechanism A⇄ B⇄ C⇄ A, where again only A absorbs light, damped oscillations occur under isothermal conditions; the illumination, as described, effectively breaks microscopic reversibility. Both the kinetic and the thermodynamic analysis show the essential role of light in effectively breaking microscopic reversibility analogous to the net flux of reactants and of products across the boundary of an open system. In nonequilibrium relaxation experiments performed on illuminated systems with damped oscillations, both a frequency and a decay rate may be measured. The application of periodic perturbations leads to resonance effects.
Temperature dependence of the vibrational energy distributions in the reactions F + H2 and F + D2
Authors: Coombe Robert D. , Pimentel George C.
Published: February 2004

Using chemical laser methods, the behavior of the product vibrational distributions in the reactions F+H2→ HF+H and F+D2→ DF+D has been studied as a function of reactant temperature. Both the equal-gain and zero-gain techniques were employed. In the F + H2 system, k3/k2 and k2/k1 increase as temperature decreases, k2/k1 showing a much stronger dependence than k3/k2. Both the k3/k2 and k2/k1 temperature dependences fit an Arrhenius-type form, k3/k2= 0.39 exp (+117/R T) and k2/k1= 2.14 exp (+254/R T). An opposite behavior is observed for the F + D2 system: k3/k2 rises with increasing temperature. These results provide the first quantitative evidence that reaction product vibrational distributions are temperature dependent and they demonstrate the applicability of chemical lasers to such measurements.
Cross correlation effects in the nuclear spin-lattice relaxation of polycrystalline o-, m-, and p -dimethoxybenzene
Authors: Cutnell J. D. , Verduin Lans
Published: February 2004

Nuclear spin-lattice relaxation has been studied via pulsed NMR techniques in polycrystalline samples of o-, m-, and p-dimethoxybenzene. Nonexponential relaxation has been observed in each compound at temperatures such that the spin-spin relaxation time was constant at 12μsec and spin diffusion was rapid enough to maintain a single spin temperature. For each compound the nonexponentiality gradually decreased with decreasing temperature until a single spin-lattice relaxation time characterized the relaxation at the lowest temperatures studied, consistent with the Hilt-Hubbard theory for effects of cross correlations [Phys. Rev. 134, A392 (1964)]. Application of the theory yields correlation times and activation energies for hindered rotation of methyl protons. Values of 3.6 ± 0.1 and 1.9 ± 0.2 kcal/mole were determined for the activation energy for the para and meta isomer, respectively.
Conjecture on electron mobility in liquid hydrocarbons
Authors: Kestner Neil R. , Jortner Joshua
Published: February 2004

We propose that the mobility of electrons in liquid hydrocarbons can be explained by a physical model which implies that the medium is microscopically inhomogeneous with regard to electron transport. Rotational fluctuations result in high and low mobility regions. The effective medium theory is utilized to correlate all the available mobility data with Vo the energy of the quasifree electron in each liquid.
Isotope and mass weighting effects in collinear triatomic reactive collisions
Authors: Miller C. G.
Published: February 2004

A one-dimensional quantum mechanical reaction coordinate method has been developed by Rankin and Light [J. Chem. Phys. 51, 1701 (1969)] and Miller and Light [J. Chem. Phys. 54, 1635, 1643 (1971)] to study vibrational excitation of product molecules in reactive triatomic collinear collisions. In this paper the method is extended to study the isotopic reactions H+Cl2→HCl+Cl and D+Cl2→DCl+Cl. The observed results showed there is no change in the mean fraction of available energy entering product vibration. This is in agreement with previous experimental and classical trajectory study findings. The method is further used to study a reaction with a much different mass combination Cl+HI→HCl+I. We expect this reaction, from classical trajectory studies, to exhibit primarily ``mixed'' energy release. Also from classical studies, the different mass combinations for the reaction H+Cl2→HCl+Cl and the reaction Cl+HI→HCl+I are expected to lead to significantly different reaction dynamics. The one-dimensional quantum mechanical results are consistent with the classical results.
Proton exchange and molecular orientation of water in hydrated collagen fibers. An NMR study of H2O and D2O
Authors: Migchelsen C. , Berendsen H. J. C.
Published: February 2004

An experimental study of proton and deuteron magnetic resonance of the hydration of collagen as a function of water content, temperature, and the addition of salts has been made. From the temperature dependency of linewidths, correlation times for molecular rotation of water molecules and proton exchange times have been determined. For a water content of 45 g H2O per 100 g collagen the rotational correlation time is 3× 10−8sec at 25 °C, with an activation energy of 4.8 kcal/mole, and the proton exchange time is 1.3 × 10−4sec with an activation energy of 10 kcal/mole. Ammonium ions increase the proton exchange rate. The proton exchange most probably occurs between proton donating and proton accepting groups on the macromolecule, intermediated by hydrogen-bonded water molecules. It is shown that the exchange theory of Gutowsky, McCall, and Slichter is applicable to dipole coupling in anisotropic systems. From an analysis of the anisotropy of H2O and D2O rotation in terms of Saupe's parameters, is is concluded that a model with two specific water binding sites is consistent with experimental results.
External fields in the kinetic theory of gases
Authors: Schruben Dale L. , Condiff Duane W.
Published: February 2004

A new scaling procedure is introduced for the solution of kinetic equations containing strong field terms. The result is a rigorous Chapman and Enskog solution and anisotropic transport coefficients for a class of problems hitherto unsolved by CE techniques. The consequences are developed in detail for several relevant examples.
Investigation of the thermomagnetic torque in N2 between 75 and 300°K
Authors: Burgmans A. L. J. , Adair T. W.
Published: February 2004

The thermomagnetic torque in rarefied gases is caused by a bulk effect and contributions due to the boundary layer. The purpose of this paper is to determine the relative importance of these two groups of contributions. Therefore, data on the thermomagnetic torque has been extrapolated to the near hydrodynamic region in order to compare with the bulk contribution calculated from the second order Chapman-Enskog approximation. For N2 between 75 and 300°K the bulk effect is shown to be always the most important mechanism. The contributions from the boundary layer, however, become relatively more important for lower temperatures.
Photochemistry of fluoro(trifluoromethyl)benzenes. I. Photophysical processes of 1-fluoro-2-, 1-fluoro-3-, and 1-fluoro-4-(trifluoromethyl)benzenes in the gas phase
Authors: Al-ani Khalid E.
Published: February 2004

The photochemistry of the fluoro(trifluoromethyl)benzenes has been studied in the gas phase at wavelengths 248, 253.7, and 265 nm. Increase in the exciting wavelength caused an increase in fluorescence yields. At 265 nm the 1,4 isomer showed a yield of 0.23 compared to 0.35 and 0.32 for the 1,3 and 1,2 isomers, respectively. The mean radiative lifetime of the 1,4 isomer calculated from integrated absorption coefficients was found to be shorter than the mean radiative lifetimes for the 1,2 and 1,3 isomers. The sensitized phosphorescence of biacetyl and the Cundall methods were used to determine triplet-state yields. For the 1,2 isomer the sensitized phosphorescence of biacetyl yields were found to be 0.41, 0.33, and 0.2 at 265, 253.7, and 248 nm, respectively, compared to 0.71, 0.56, and 0.41. for the 1,4 isomer and 0.67, 0.42, and 0.33 for the 1,3 isomer. Agreement was satisfactory between the two methods.
Photochemistry of fluoro(trifluoromethyl)benzenes. II. Fluorescence spectra and quenching of singlet state emission in 1-fluoro-2-, 1-fluoro-3-, and 1-fluoro-4-(trifluoromethyl)benzenes in the gas phase
Authors: Al-Ani Khalid E.
Published: February 2004

The fluorescence spectra of 1-fluoro-2-(trifluoromethyl)benzene, 1-fluoro-3-(trifluoromethyl)benzene, and 1-fluoro-4-(trifluoromethyl)benzene were studied at various exciting wavelengths and pressures. At 248 nm the fluorescence spectra have so much structure that the appearances are essentially those of continua. At 266 nm, addition of 200 torr of cis -2-butene or increase of aromatic pressure causes the bands around 270–275 nm to disappear. Addition of biacetyl or 1,3-butadiene quenched the emission from the first excited state at 248, 253.7, and 265 nm. Butane and cis -2-butene enhanced the fluorescent emission at 248 nm, while cis -2-butene showed a small quenching of the fluorescent emission at 265 nm.
Variational calculation of continuum corrections to overlap
Authors: Weinhold F.
Published: February 2004

A method is derived for determining the accuracy of an approximate wavefunction by variationally calculating its overlap with the (unknown) true wavefunction. The variational flexibility serves to generalize a previous result due to Wang, and corresponds formally to the inclusion of continuum states in Weinberger-like modifications of the ordinary Eckart criterion for overlap. We use this procedure to calculate improved overlaps for two helium ground-state wavefunctions: (i) a simple screened-hydrogenic approximation, and (ii) a nearly exact 135-term Hylleraas-type function, in each case significantly surpassing the Eckart and Weinberger limits. A novel feature of these calculations is the evaluation of integrals over H3, the cube of the Hamiltonian operator.
ESR of spin 5/2 systems with axial symmetry and moderately large zero-field splittings. Application of line-shape calculations to the interpretation of randomly oriented microcrystallite spectra
Authors: Sweeney W. V. , Coucouvanis Dimitri , Coffman R. E.
Published: February 2004

Magnetic resonance line positions and intensities are calculated for the simplified spin-Hamiltonian H=D[Sz2−S(S+1)/3]+g μBH·Swith S=5/2. The resonance line positions and intensities are found by direct diagonalization of the matrix of ℋ and are plotted versus the axial angle θ and the magnetic field H. The region 1/2≲|D|/h  ν ≲ 2 is explored, where v=the microwave frequency, and line shapes integrated over all orientations are calculated using a Gaussian line shape with isotropic linewidth. The results are compared with the experimental EPR spectra of two crystallographically isomorphic high-spin trisdithio-oxalato-Fe(III) salts, one having |D|=0.26 GHz (treated by perturbation theory, including quartic spin operators), and the other |D|=6.8 GHz. Single crystal measurements were made for the former case, but only powder measurements of the latter system were possible. Calculation of the powder line shape for the |D|=6.8 GHz spectrum for both X-band and Ka-band EPR spectra gives good agreement with experiment and explains the increased multiplicity of lines in these and similar experimental spectra as due to strong mixing of all S = 5/2 basis states under the influence of the DSz2 operator. This gives rise to an unexpectedly large number of anomolous transitions, for which only a line shape properly averaged by integration over the unit sphere can give a reasonably accurate simulation of the experimental spectrum. The results also show that several previously published tables for the line positions of Fe(III) salts for the general axial or orthorhombic case must be used with care when interpreting powder or glass spectra, and spin-Hamiltonian parameter values inferred from them ought to be confirmed by some independent method. The results of this study are important for the interpretation of the EPR of biological and metal-ligand compounds which are difficult or impossible to obtain in crystalline form.
Intervalence transfer in biferricenium and biferricenylenium cations
Authors: Morrison William H. , Hendrickson David N.
Published: February 2004

Iron-57 Mössbauer spectra were obtained (300 and 4.2°K) for the singly oxidized ``mixed-valence'' molecules, biferricenium triiodide and biferricenylenium triiodide. The first compound showed two quadrupole-split doublets (Δ Eq≃ 2.0 and 0.4 mm/sec) while the last system only exhibited one quadrupole-split doublet (ΔEq = 1.7 mm/sec). Intervalence-transfer electronic absorption was detected for the biferricenium system at 5100 cm−1 and for the biferricenylenium system at 6790 cm−1. A discussion of the thermal rate of intervalence transfer in these systems is given.
Catalytic decomposition of ammonia on tungsten (100), (110), and (111) crystal faces
Authors: McAllister Jerome , Hansen Robert S.
Published: February 2004

The rates of catalytic decomposition of ammonia on (100), (110), and (111) single crystal faces of tungsten were measured over the temperature range 800–970 °K for ammonia pressures ranging from (0.5–100)× 10−3torr and for nitrogen and hydrogen pressures varying from (0–50) × 10−3torr. In all cases the rate of decomposition was of the form, rate=A+BPNH3(2/3), and was independent of nitrogen and hydrogen partial pressures. The constants A and B varied substantially with crystal face; the values of B for the (111), (100), and (110) faces were in approximate ratio 8.4:1.55:1 and these substantially established the decomposition rate for PNH3>5 × 10−3torr. Rate forms for NH3 and ND3 were compared on the (111) face at 860 °K, with results ANH3=AND3,BNH3≈ 1.47 BND3. The observed rate form is derived from a model involving nearly complete surface coverage by the species WN and small surface coverages by species W2N, W2N3H2, and WNH. The A term in the rate law is generated by the reaction, 2WN→W2N+(1/2)N2. This process was advocated as rate limiting in ammonia decomposition by Matsushita and Hansen and their rate law for this process obtained on polycrystalline tungsten furnishes an estimate of the A parameter in the same order of magnitude as that observed. The BPNH32/3 term results from a steady state balance of W2N3H2 and WNH decomposition reactions. Peng and Dawson have advocated decomposition of W2N3H as rate limiting in the decomposition of ammonia, and have also suggested WN dissociation may be rate limiting at low pressures, W2N3H dissociation at higher pressures. The present model therefore resembles that of Peng and Dawson in concept but not in detail.
Lower bound to the long-range interaction energy of two identical rare gas atoms in the restricted Hartree-Fock approximation
Authors: Lesk Arthur M.
Published: February 2004

The localized orbital description for a homonuclear diatomic molecule leads to a lower bound for the interaction energy of two identical rare gas atoms, computed in the restricted Hartree-Fock (RHF) approximation. At all sufficiently large internuclear separations, the lower bound is positive, implying that the interaction energy must approach zero from above. Therefore the RHF approximation cannot predict any asymptotic attractive interaction for systems consisting of two like rare gas atoms.
Planewise summation of point multipole-multipole interactions
Authors: Philpott Michael R. , Mahan Gerald D.
Published: February 2004

The interaction of a plane of static multipoles with another located outside the plane is shown to decrease exponentially with perpendicular separation. The retarded interaction is resolved into a short range static interaction and a long range oscillating interaction due to the radiation field of the plane.
Symmetry and structure of XeO2F2 by neutron diffraction
Authors: Peterson S. W. , Willett R. D. , Huston J. L.
Published: February 2004

The molecular symmetry and crystal structure of XeO2F2 have been determined from single crystal neutron diffraction data. The structure is orthorhombic (Cmcm) with a =6.443(3), b =6.288(3), c =8.312(3) Å, Z =4, and Dc =4.10 g cm−3. The neutron Patterson established the structure model which refined to wR (F)=0.069. The structure consists of molecular XeO2F2 units of C2ν symmetry bond together by weak Xe–O bridges. The configuration of XeO2F2 is that of a trigonal bipyramid with axial fluorine and equatorial oxygen atoms and one vacant equatorial position. the Xe–O and Xe–F distances are 1.714 and 1.899 Å and the O–Xe–O, O–Xe–F, and F–Xe–F angles are 105.7°, 91.6°, and 174.7°, respectively.
Low energy study of symmetric and asymmetric charge-transfer reactions
Authors: Smith David L. , Futrell Jean H.
Published: February 2004

A modified Varian ion cyclotron resonance (ICR) mass spectrometer was used to measure absolute rate constants for charge transfer from Kr+ to Kr and CO as well as from CO+ to Kr and CO. The technique of double resonance permitted measurement of the rate constants over the translational energy range from 0.1 to 10 eV. Ion motion in an ICR mass spectrometer was analyzed to give an average ion velocity which was used to convert the reaction rate constants to cross sections. These results are compared to the theoretical results of Rapp and Francis, with Langevin and with existing experimental results. One of the most striking results is that at a very low energy the cross sections for the crossed or asymmetric reactions are substantially greater than the cross sections for the symmetric charge-transfer reactions.
Extended diatomics in molecules calculations
Authors: Steiner E. , Certain P. R. , Kuntz P. J.
Published: February 2004

The diatomics in molecules method of estimating polyatomic energy surfaces is extended to include valence bond configuration interaction. The results are applied to H3, using a basis of covalent and ionic functions. The method is found to be stable to the addition of the ionic functions, but it is found to be necessary to assume that the overlap between orbitals centered on different atoms vanishes.
Nuclear magnetic resonance absorption and relaxation study of tert-butylamine and tert-butylamine clathrate deuterate
Authors: Allen P. S. , Khanzada A. W. K. , McDowell C. A.
Published: February 2004

Proton magnetic resonance absortion (at 16 MHZ) and spin-lattice relaxation time (at 26.46 MHz) measurements have been carried out for pure tertiary-butylamine, and tertiary-butylamine clathrate deuterate from 77°K to the melting points of these compunds. The absorption line measurements on t-butylamine show that the second moment changes from values expected for a rigid lattice to a plateau value consistent with the three methyl groups and the t-butyl group rotating at temperatures near 150°K. In the case of the deuterate, t-butylamine exhibited much more freedom of motion as judged from the second moment measurements. The motional behavior ranges from methyl and t-butyl group rotation to isotropic rotation of the whole molecules. The proton spin-lattice relaxation measurements on t-butylamine exhibited a broad minimum of 16 msec, which is thought to be mainly due to methyl group reorientation. An activation energy of <(3.2± 0.1) kcal/mole was calculated from T1 measurements at various temperatures and this is associated with the barrier hindering methyl group reorientation. The clathrate deuterate of this amine showed a very broad minimum of (32± 1) msec which is thought to arise from three types of motion; namely, methyl and t-butyl group rotation as well as isotropic rotation of the whole molecule. The limiting activation energies in the deuterate of (1.73± 0.03) kcal mole−1 and (2.5± 0.02) kcal mole−1 and therefore expected to be determined by methyl and t-butyl rotation and by t-butyl and isotropic rotation, respectively.
Electron paramagnetic resonance of Fe3+ in diopside and spodumene
Authors: Gaite J. M. , Michoulier J.
Published: February 2004

A study of the EPR spectrum of Fe3+ in diopside CaMg(SiO3)2 and spodumene LiAl(SiO3)2 has been made. It permits the calculation of all the constants of the spin Hamiltonian. We show that the pseudosymmetry axes of the fourth order tensor of the spin Hamiltonian are the same as the axes of the immediate environment of the Fe3+. From the fourth order constants of the spin Hamiltonian it is possible to obtain qualitative information about the Fe3+ environment. In both crystals, the Fe3+ is in the M 1 site. We show that the M 1 site is smaller and more distorted in spodumene than in diopside, and that in spodumene there is large tetragonal distortion along the M 1–01′ direction. It is seen that the information we obtain agrees very well with the crystallographic results.
Polarized electronic infrared spectra of CoCl2 single crystals
Authors: Hsu Edward C. , Stout J. W.
Published: February 2004

Polarized infrared absorption spectra of single crystal CoCl2 from 8 to 300 °K with light propagated parallel and perpendicular to the trigonal axis are reported. At 8°K spectra are presented in magnetic fields from 3 to 43 kG. Three polarized lines arising from magnetic dipole transitions are identified as transitions to electronic levels within the lower 4T1g manifold of a Co2+ ion. A line at 956 cm−1 (3 kG) and 960 cm−1 (43 kG) of oscillator strength 1.65× 10−8, and a second line at 977 cm−1 (3 kG) and 980 cm−1 (43 kG) of oscillator strength 2.5× 10−8, are transitions to components of a Kramers doublet. Their separation of 21 cm−1 measures the splitting of an excited electronic level of a Co2+ ion in the exchange field of ground state neighbors. A third line at 1007 cm−1 (3 kG) and 1010 cm−1 (43 kG) is the transition to the next higher Kramers doublet whose exchange splitting is not resolved. Calculations of positions, polarizations, and oscillator strengths of magnetic dipole transitions are in accord with the observations. A broad band is observed at 8 °K between 1100 and 1500 cm−1. A structured absorption between 300 and 600 cm−1 is reported in the unpolarized axial spectrum.
Distinguishable electron method for electronic structure calculations. VI. Application to frequency-dependent (dynamic) properties
Authors: Venanzi Thomas J. , Kirtman Bernard
Published: February 2004

The distinguishable electron method is extended to frequency-dependent (dynamic) properties. As in the static case, we derive accessible correlation expressions for the dynamic property which involve only uncorrelated wavefunctions that can, often, be obtained analytically. As an example, the dynamic polarizability of He and Li+ is calculated with somewhat better results than the coupled Hartree-Fock method and substantially greater ease.
ESR spectra and structures of NaSO2 and NaO2
Authors: Adrian Frank J. , Cochran Edward L. , Bowers Vernon A.
Published: February 2004

We have observed the electron spin resonance (ESR) spectra of NaSO2 and NaO2, trapped in argon at 4 °K, following their production by codeposition of a beam of sodium atoms with SO2 and O2, respectively, in a large excess of argon. Each spectrum is a triplet of quartets, the triplet structure arising from an orthorhombic g tensor (g), and the quartet structure resulting from the Na hyperfine (hfs) interaction. For NaSO2, the components of g are g1=2.0024, g2=2.0060, and g3=2.0101 where axis 1 is perpendicular to the SO2 plane and axis 2 is the SO2 symmetry axis. In this axis system, the Na hfs tensor of NaSO2 is A11=−1.49, A22=−1.22, A33=−1.85, A12=0.56, and A13=A23=0.00 Oe. For NaO2, g1=2.0029, g2=2.0063, g3=2.1112, and A1=−3.5, A2=−1.8, and A3=−3.4 Oe, where axis 1 is perpendicular to the NaO2 plane and axis 3 is the O–O bond. The magnitudes of the Na hfs splittings showed that each molecule is a bound ion pair. Comparison of the observed anisotropies in the Na hfs splittings with calculated anisotropic hfs constants led to a determination of the signs of the hfs constants and gave the following structural information. NaSO2 is nonplanar with the Na+ ion located 4.3 a0 from each of the two O atoms and 4.7 a0 from the S atom. NaO2 is a π radical, and the Na+ ion is located 4.0 a0 from each of the two O atoms.
Determination of proton affinity from the kinetics of proton transfer reactions. III. The measurement of the equilibrium constant at various temperatures
Authors: Hemsworth R. S. , Rundle H. W. , Bohme D. K. , Schiff H. I. , Dunkin D. B. , Fehsenfeld F. C.
Published: February 2004

The flowing afterglow measurements of the equilibrium constants for selected proton transfer reactions have been extended to temperatures other than room temperature in order to completely specify the thermodynamics of such reactions. The van't Hoff analysis of the data for the reaction CO2H++CH4⇄ CH5++CO2 results in a value for the standard enthalpy change, ΔH° (196 to 553°K), of −0.064±0.004 eV and for the standard entropy change, ΔS° (196 to 553°K), of +1.4±0.6 e.u. Studies of the reaction N2OH++CO⇄ COH++N2O led to a value of −0.151±0.009 eV for ΔH° (277 to 505°K) and a value of −1.8±1.1 e.u. for ΔS° (277 to 505°K). ΔH° is a direct measure of the difference between the proton affinities of CO2 and CH4 in the former case and of N2O and CO in the latter. The measured values for ΔS° yield differences in the standard entropies of the protonated species: S̄°298(CH5+)−S̄°298(CO2H+)= −5.2± 0.6 e.u and S̄°298(COH+)−S̄°298(N2OH+)= −7.1± 1.1 e.u. Reference to calculated values for the standard entropy of CH5+ and COH+ yields values for the standard entropy of CO2H+ and N2OH+ and some indication of the structure of these species.
Microwave spectrum, barrier to internal rotation, structure, and dipole moment of methane selenol
Authors: Thomas Clarence H.
Published: February 2004

The microwave spectra of the ground state and excited torsional states of several isotopic species of methane selenol have been observed. The barrier hindering internal torsion has been determined to be 1001±50 cal/mole. The structural parameters are C–Se= 1.959 Å, Se–H= 1.473 Å, C–H= 1.088 Å, ∠C–Se–H= 95.45°, ∠H–C–H= 110.00°, with the symmetry axis of the methyl group tilted 1.5° away from the C–Se internuclear line. The dipole moment components are μA=1.253 D, μB=0.314 D, μtotal=1.291 D. The implications of this barrier height determination for theories on the orgin of the barrier are discussed.
Chemical equilibrium of NH2−+H2⇄ H−+NH3 and the determination of D° (NH2–H)
Authors: Bohme D. K. , Hemsworth R. S. , Rundle H. W.
Published: February 2004

The proton transfer reaction NH2−+H2⇄ H−+NH3 has been investigated in a flowing afterglow system independently in both directions at 297°K. The rate constants were found to be kforward= (2.3± 0.5)× 10−11 cm3 molecule−1· sec−1 and kreverse= (9.2± 1.8)× 10−13 cm3 molecule−1· sec−1. The ratio of rate constants, kf/kr= 26−5+6 was found to be in agreement with the equilibrium constant, K= 27± 9, determined from equilibrium concentrations. The value for K corresponds to a standard free energy change, Δ G°297, of− 1.9± 0.2 kcal mole−1. A calculated value for the standard entropy change, ΔS°298, of −4.3± 0.5 cal deg−1· mole−1 leads to a value for the standard enthalpy change, Δ H°298, of−3.2± 0.3 kcal mole−1. Adoption of a recent photodetachment value for the electron affinity of NH2 leads to D°0(NH2–H)= 106.0± 1.1 kcal mole−1, D°298(NH2–H)= 107.4± 1.1 kcal mole−1, and Δ H°f,298(NH2)= 44.3± 1.1 kcal mole−1.
Kinetics of the vibrational relaxation of the HF–N2 system
Authors: Sentman L. H. , Solomon W. C.
Published: February 2004

Based on the suggestion that V-V transfers may be important in the vibrational relaxation of the HF–N2 system (Ref. 3), the experimental data for this system (Ref. 1) have been re-examined using the STK (Ref. 4) equations. A nonlinear regression analysis has resulted in the determination of τN2–HFν and τN2–HF. Sentman's theory agrees very well with the V-V relaxation time, but the V–R–T rate is an order of magnitude faster than the theoretical prediction. These results suggest that HF* relaxes via a V-V transfer to N2 with a subsequent V–R–T relaxation of N2*.
Can two-dimensional mesophases solidify?
Authors: Yorke Ellen D. , De Rocco Andrew G.
Published: February 2004

We investigate whether the orientational contribution to the free energy of a two-dimensional system of hard rectangular rods will assist in the formation of a solid phase from an anisotropic fluid phase. At a density of 77% of close-packed (where independent evidence exists for a hard-square fluid-solid transition) the orientational pressure and free energy are a small fraction of the corresponding values for the solid phase and furthermore the slope of the orientational pressure versus density acts against the formation of a solid phase. We conclude that a hard-square fluid-solid transition does not automatically imply a corresponding transition for hard rectangles.
Kinetics of small Ising systems. II. General formulation
Authors: Paul Edward
Published: February 2004

The time dependent behavior of small Ising systems in various configurations was followed by computer solutions of the kinetic equations. The inclusion of internal interactions leads to complex departures from internal equilibrium which depend upon the initial and final conditions as well as the interaction strength. The use of periodic boundary conditions on a series of small square lattice patches indicates that increased kinetic complexity obtains in larger systems.
Phase-space theory of electron detachment in slow atomic collisions
Authors: Shui Ven H. , Keck James C.
Published: February 2004

A classical phase-space theory for the excitation of electrons in slow atomic collisions is presented. An equation which governs the energy transfer between the electron and the heavy particles is derived for slow collisions (adiabatic approximation). This equation is analogous to the first law of thermodynamics for an open system. The theory contains only one adjustable parameter and gives excellent correlation for existing experimental measurements of detachment rate coefficients of F− + (Ar, N2, CO) and Br− + Ar. Possible modifications to the present theory are summarized. It is anticipated that the theory should be applicable to other electronic rate processes such as ionization and charge transfer.
EPR studies of radiation damage in metal acetates: The hydrated lead (II) salt
Authors: Roberts H. C. , Eachus R. S.
Published: February 2004

An EPR study of x-irradiated lead acetate trihydrate at 77 °K has strengthened the identification of two recently reported species, Pb1+ and PbH2+. The first of these results from radiation-induced electron trapping by coordinated Pb2+ ions, and the second center consists of a hydrogen atom that is weakly associated with a plumbous ion in the crystal lattice. The radical PbCH32+ was also observed. A detailed study of the Pb1+ center reveals that the diagonal components of the 207Pb hyperfine coupling tensor are very large and anisotropic, and that the characteristic g tensor is almost isotropic. These data are interpreted in terms of a bonding scheme that locates the unpaired electron in a hybridized molecular orbital that possesses 6s, 6p, and possibly some 5d, character. A comparison of our data for Pb1+ with published data for Pb3+ reveals striking differences.
Molecular orbital investigation of chemisorption. I. Hydrogen on tungsten (100) surface
Authors: Anders Leon W. , Hansen Robert S. , Bartell L. S.
Published: February 2004

The relative bonding energies of hydrogen chemisorbed at three symmetric sites on a W(100) surface were obtained by means of the extended Hückel molecular orbital theory (EHMO). The preferred site for hydrogen chemisorption was found to be the single coordination number (1 CN) site or the site above a surface tungsten atom. The W(100) surface was represented by finite arrays of tungsten atoms which were shown to be adequate for obtaining semiquantitative results. The basis set for the calculations contained the valence orbitals of tungsten and, initially, the 5p orbitals which were nonbonding but provided the necessary repulsion at small internuclear separation. The repulsive energy provided by these orbitals was replaced by an analytical exponential repulsive energy term. This allowed the 5p orbitals to be omitted from the basis set to simplify computation. Functionally, the energy change for the reaction Wn + H → Wn H was calculated for various assumed configurations of the Wn H ``molecule.'' The bonding between tungsten atoms was found to be changed as a result of Wn H formation, and the change varied with hydrogen position. Energy barriers to surface diffusion were also calculated and found to agree reasonably with experimental values.
Theoretical study of H2+ spectroscopic properties. II. The 2p πu electronic state
Authors: Beckel Charles L. , Shafi Mohammad , Peek James M.
Published: February 2004

Vibration-rotation eigenenergies are computed for the (2pπu) 2Πu state of H2+. Spectroscopic constants Yij are found from Dunham power series expansions of the vibrational potential and also from the eigenenergies. The Yij are much smaller in magnitude than H2+ ground state values because of the 2p πu broadened potential and larger equilibrium separation re. The eigenenergies are used to determine Bν, Dν, and Hν for all bound vibrational states, i.e., for 0 ≤ ν ≤ 11. Both the ΔG and Bν curves have negative curvature at all ν. The Dν and Hν curves are similar in shape to the respective curves for the H2+ ground state. The Dν's decrease slowly at low ν but increase rapidly near dissociation. The Hν curve has a small negative initial slope; the Hν's decrease very rapidly, becoming negative, at high ν. Origin wavelengths are listed for a few H2+ 2p πu−1sσg bands predicted to be intense on the basis of the classical Franck-Condon principle; these lie in the Lyman α region.
Theoretical study of H2+ spectroscopic properties. III. The 3d σg electronic state
Authors: Shafi Mohammad , Beckel Charles L.
Published: February 2004

Numerical integration of the Schrödinger equation is used to determine vibration-rotation eigenvalues for the (3d σg)2Σg+ state of H2+ up to ν = 58. Dunham power series expansions are used to determine the equilibrium separation re, potential coefficients an, and spectroscopic constants Yij. The eigenvalues are used to determine ΔG, Bν Dν, and Hν. The ΔG and Bν curves have positive curvatures at low and high ν and negative curvatures at intermediate ν; points of inflection for both curves are at very nearly the same values of ν. The Dν curve is qualitatively similar to those for 1s σg and 2p πu up to ν = 25; above ν = 30 the 3d σg Dν's reach a maximum and then apparently tend to zero as ν → ∞. As ν approaches 25, Hν values appear to be heading negative; with further increase in ν the Hν's rise sharply and then appear to go to 0 as ν→∞.
Accurate dissociation energies from rotational predissociation and long-range forces: B 1Π LiH
Authors: Way Kermit R. , Stwalley William C.
Published: February 2004

A new approach for determination of accurate dissociation energies from experimental predissociation data and the calculated long-range potential is described. In this case long-range predissociation in the rotational manifold of only a single vibrational level is required to yield a precise dissociation limit. The case of B 1Π LiH is considered in detail and a dissociation limit of 34 492.5 ± 0.5 cm−1 is obtained, 2.5 cm−1 lower and 4 times more precise than the best previous result. Also presented is a reasonably good representation of the potential energy curve for the B 1Π LiH molecule, constructed by an isotopically combined Rydberg-Klein-Rees method based on the concept of mass-reduced quantum numbers: η ≡ [ν+12]/μ and ξ≡J(J + 1)/μ.
Depolarized Rayleigh scattering and orientational relaxation of molecules in solution. III Carboxylic acids
Authors: Alms G. R. , Bauer D. R. , Brauman J. I. , Pecora R.
Published: February 2004

Depolarized Rayleigh linewidths have been measured for several carboxylic acids dissolved in carbon tetrachloride and water. The correlation times derived from the spectra are determined mainly by over-all molecular reorientation and are relatively insensitive to molecular internal rotations and changes in chemical state. The dependence of the reorientational correlation times on intermolecular interactions such as hydrogen bonding and chemical effects such as dimerization is studied. It has also been found that the polarized and depolarized Rayleigh spectra are insensitive to the rate of the reaction CF3COOH⇄CF3COO− + H+ in aqueous solutions of CF3COOH.
Model for calculating spin-orbit interactions with applications to photoelectron spectroscopy
Authors: Berkosky Jeffrey L. , Ellison Frank O. , Lee Ting Ho , Rabalais J. Wayne
Published: February 2004

A semiempirical model has been developed for estimating spin-orbit interactions in molecular ion states derived by ionization of closed-shell neutral molecule states. A perturbation approach is used with an effective spin-orbit interaction operator Hso and approximate Mulliken-Wolfsberg-Helmholz molecular orbital (MO) wavefunctions. Effective spin-orbit parameters ζv were obtained by interpolation with respect to atomic charges derived from the MO calculations. The He I photoelectron spectra of the PX3(X = Cl, Br, I) and PYX3(X = Cl, Br; Y = O, S) series have been measured. Splittings observed in some of the ionization bands of this series have been successfully interpretated as spin-orbit splittings using the newly developed model.
Three-state computer calculation of chemi-ionization in oxygen-alkali collisions
Authors: van den Bos J.
Published: February 2004

A model of three coupled states, based on the (pseudo) crossing of molecular potential curves, has been applied to the process of chemi-ionization (ion-pair formation) for oxygen atoms colliding with Na, K, Rb, or Cs. The semiclassical impact parameter method has been employed in the calculation of total and differential cross sections for oxygen kinetic energies of from 300 eV to 10 keV. It is shown that rotational coupling of the magnetic substates is of considerable importance.
Classical phase space description of rotationally inelastic scattering
Authors: Pierre Anthony G. St.
Published: February 2004

This article is concerned with establishing a general formalism for rotationally inelastic scattering. The method for achieving this is within the scope of the Prigogine-Résibois formalism, as recently applied by Miles and Dahler, and Eu. The starting point is the Liouville equation of motion for the two-body system comprised of a linear rotor and a structureless particle. Many parallelisms with quantum scattering theory encountered by these authors, such as evolution operators for handling the time development of the system, the Lippmann-Schwinger equation for governing the steady-state distribution, and the optical theorem continue to appear also in this investigation. In addition, there arise here for consideration some new features, such as polarizations, and correlations in the orientation and angular momentum, which have their origins in the internal degrees of freedom of the rotor. Finally, a perturbation type of solution for the distribution function is set up, and expressions for transition probabilities and cross sections are obtained.
Rotationally inelastic molecular scattering. Computational tests of some simple solutions of the strong coupling problem
Authors: Tsien Thomas P. , Parker Gregory A. , Pack Russell T
Published: February 2004

Partial cross sections (opacity functions) for rotational transitions in atom-diatom collisions are computed in the infinite-order sudden (IOS) approximation and compared with accurate close-coupling (CC) calculations. Agreement is good in the dominant coupling (small total angular momentum J) region. Simple methods for calculating integral inelastic cross sections are discussed, and it is found that accurate cross sections can often be computed very simply, even when large numbers of channels are coupled together, by using IOS or first-order sudden (FOS) approximations for small J and CC or exponential Born (EBDW) methods for large J.
14N Nuclear quadrupole resonance in tetrasulfur tetranitride
Authors: Benlian David , Gourdji Michel , Guibé Lucien
Published: February 2004

The 14N nuclear pure quadrupole resonance has been observed in solid S4N4. The spectrum consists of 12 lines assigned to the resonance of nitrogen atoms located at four inequivalent crystallographic sites. The measured quadrupole coupling constants are discussed in terms of the electronic populations of the valence orbitals on the nitrogen atoms. As compared, on a simple sp2 hybridization model, to standard pyridinic compounds, π type orbitals appear to be much more populated in S4N4. It is also shown that the temperature dependence of the ν+ line frequencies is caused by both lattice and intramolecular motions; the deduced frequencies are consistent with the results of ir and Raman spectra.
Phase transition associated with a soft mode of molecular libration in crystal
Authors: Chihara Hideaki , Nakamura Nobuo , Tachiki Masashi
Published: February 2004

A simple theory is presented to account for phase transitions of molecular crystals that accompany a small entropy change and occur over a wide temperature range; for example, P2Cl10 undergoes a transition that extends from 75 to 110 °K with the entropy change of 1.67 J °K−1 · mol−1. The theory assumes that molecules gradually change their orientation with temperature at the expense of an increase in the potential energy, which is counterbalanced by an increase in entropy resulting from a change in frequency of librational modes coupled anharmonically to the uniform tilt of the molecules. The theory predicts first or second order transitions of small entropy change depending on the shape of the potential energy curve.
Heat capacity of solid tetrachloro-p -benzoquinone (chloranil) between 11 and 300 °K. Phase transition at 92 °K
Authors: Chihara Hideaki , Masukane Kazuyuki
Published: February 2004

Molar heat capacity of solid chloranil was measured between 11 and 300 °K with particular attention to the phase transition at 92 °K. The heat and entropy of the transition were 38 J mol−1 and 0.41 J °K−1 · mol−1. The transition is of a lambda shape starting at 70 °K and ends at 100 °K. The anomalous heat capacity curve was explained by the soft librational mode theory. There is another large transition at about 570 °K. The shape and magnitude of the transitions are very sensitive to the long-range regularity of the structure and depend on the method of preparation.