bluuebunny/crossref_metadata_2025 huggingface dataset:
New Measurements of the Raman and the Far-Infrared Spectrum of S2Br2
Authors: Bradley Eugene B. , Frenzel Charles A. , Mathur Maya S.
Published: February 2004

The Raman spectrum of S2Br2 has been excited with a 6-mW He–Ne laser operating at 6328 Å, and new depolarization ratios of the Raman bands have been obtained. The Raman band at 68 cm−1 corresponding to the torsional mode is found to be polarized in disagreement with an earlier Raman investigation; the torsional mode is far-infrared active at 70 cm−1. The Raman band at 534 cm−1 is found to be polarized and of type A in contrast to earlier work in which this band was found to be depolarized and of type B. The Raman band at 304 cm−1 is found to be depolarized and of type B whereas in an earlier investigation the band was reported as polarized and of type A. The new Raman and infrared data have been correlated with the right-angle configuration of C2 symmetry and new assignments have been made. Six coincident Raman- and infrared-active fundamentals are now known from this work and the work of others.
Distorted-Wave Theory for Collisions of an Atom and a Diatomic Molecule
Authors: Miller William H.
Published: February 2004

The general collision of an atom and a diatomic molecule (interacting through a single potential surface) is treated in the distorted-wave Born approximation (DWBA). The scattering amplitude in the helicity representation is given by Eqs. (9)–(11) and is seen to be an intuitive generalization of the usual partialwave expansion for potential scattering. This general result contains only three-dimensional integrals which must be evaluated, in contrast to the six-dimensional space of the system in its center of mass. Both distorted wavefunctions suggested by Karplus and Tang (free molecular functions and adiabatic molecular functions) are treated and lead to the same form for the scattering amplitude. Several semiclassical approximations are presented with regard to the sum over partial waves. These lead to physically interesting results, and a numerical example is presented to indicate how accurate they may be expected to be.
Electron Impact Spectra of Ethane and Other Saturated Hydrocarbons
Authors: Lassettre Edwin N. , Skerbele Ausma , Dillon Michael A.
Published: February 2004

Electron impact spectra for ethane are reported at accelerating voltages from 50 to 180 V and scattering angles to 9°. Spectra of methane at 0° and 6° and for propane and butane at 0° are reported at an accelerating voltage of 50. Spectra of perdeutero ethane are reported at 0° and accelerating voltages of 50 and 100. A normal coordinate analysis of an excited electronic state of ethane is carried out using the frequencies observed in C2H6 and C2D6. A model, suggested by semiempirical molecular orbital theory, leads to the conclusion that the C–C internuclear distance decreases on excitation. The model is tested by means of an application of the Franck–Condon principle based on the normal coordinate analysis and the model proves to be self consistent if (a) the C–C internuclear distance decreases to 1.39 Å and (b) the bond angle C–C–H is increased 10° above the tetrahedral. The significance of this result in the theory of chemical bonding is discussed.
Rotational Spectra of Substituted Diazirines. I. Dimethyldiazirine
Authors: Wollrab James E. , Scharpen LeRoy H. , Ames Donald P. , Merritt James A.
Published: February 2004

Ground-state rotational spectra of (CH3)2CN2 and (CD3)2CN2 have been assigned, and the derived structural parameters are: r(NN) = 1.235 ± 0.005 Å, r(CC) = 1.499 ± 0.01 Å, r(CN) = 1.490 ± 0.01 Å, ∢C–C–C = 119.7 ± 0.5°. The internal rotation splittings for (CH3)2CN2 yield a barrier to internal rotation of 1129 ± 20 cal mole−1 and an angle of 120.4 ± 0.4° between the top axes. For (CH3)2CN2 the quadrupole coupling constants (MHz) are χaa = −0.94 ± 0.8, χbb = 3.20 ± 0.4, and χcc = −2.27 ± 0.4, while for (CD3)2CN2 χaa = −1.21 ± 0.8, χbb = 3.44 ± 0.4, and χcc = −2.23 ± 0.4. Analysis of the Stark effect gave a dipole moment of 2.19 ± 0.07 D.
Helium–Hydrogen Liquid–Vapor Equilibrium to 100 atm
Authors: Sneed C. M. , Sonntag R. E. , Van Wylen G. J.
Published: February 2004

Liquid–vapor phase compositions are given for 11 isotherms of the system normal-hydrogen–helium over the range 15.5° to 29.8°K at pressures to 1500 lb/in.2 In this range, the mixture critical curve is located, the freezing points are shown, and the locus is determined for the barotropic phenomenon (gravitational phase inversion). Two isotherms are duplicated with the substitution of parahydrogen.
Effects of Molecular Reorientation in Solid Methane on the Quasielastic Scattering of Thermal Neutrons
Authors: Sköld Kurt
Published: February 2004

Recent neutron-scattering measurements on solid methane show that the elastic peak is broadened in the high-temperature phase (T > 20.4°K) and that the broadening depends on the momentum transfer in a complicated manner. In the present paper it is suggested that the broadening is due to frequent stepwise reorientation of the molecules. The widths predicted by this model are in qualitative agreement with the experimental results. The average time between successive rotational jumps is estimated to be about 10−12 sec at T = 22.1°K.
Fine Structure in Energy-Distribution-Difference Ionization-Efficiency Curves
Authors: Collins Jerome H. , Winters Robert E. , Engerholm Gail G.
Published: February 2004

Refinements in the experimental application of the energy-distribution-difference (EDD) method for resolving fine structure in ionization-efficiency curves are presented. These include data acquisition and computer-processing techniques by which first-differential EDD curves can be obtained with a good signal-to-noise ratio. Using these techniques, a study has been made of the fine structure observed in the first-differential EDD ionization-efficiency curves of C2H2+ and Xe+ near their thresholds. Distinct “steps” are resolved in the first-differential ionization-efficiency curve of C2H2+. The energy separations between these steps agree quite well with energy separations between vibrational levels of the C2H2+ ion determined by photoionization techniques. However, the relative cross sections for these processes determined by electron impact are drastically different from those reported for photoionization. Possible explanations for these differences are discussed. The electron-impact results for xenon indicate that a large portion of the cross section between the P3/22 and P1/22 states of Xe+ is due to autoionization from known Rydberg levels.
Theoretical Study of the Geometry and Spectrum of Nitrous Oxide
Authors: Peyerimhoff Sigrid D. , Buenker Robert J.
Published: February 2004

A series of ab initio SCF MO calculations is carried out for the nitrous oxide molecule as a function of internuclear angle in both NNO and NON arrangements. Investigation of the results of these calculations indicates that this molecule favors the nonsymmetric NNO arrangement mainly as a result of the net stabilization its occupied π MO's derive from having the more electronegative oxygen atom at the terminal position of the system. Generalization of this point leads to a qualitative explanation for the almost universal preference of triatomic molecules for equilibrium structures in which the most electropositive atom of the three lies at the middle position of the system. The calculations are also employed to discuss the applicability of general rules governing questions regarding the geometry of symmetric triatomic molecules to analogous problems concerning the nonsymmetric members of this family. Finally, a large-scale CI calculation for the linear equilibrium geometry of NNO is carried out in order to obtain a detailed list of assignments for the absorption lines in the electronic spectrum of this molecule.
Theory of Temperature-Dependent g-Tensor Splittings in p-Phenylenediamine–Chloranil
Authors: Soos Zoltán G.
Published: February 2004

A theory is developed to interpret the unusual temperature dependence of the g-tensor splittings—the electron paramagnetic resonance lines narrow slightly as they merge into a single line—observed in the paramagnetic organic crystal p-phenylenediamine–chloranil. Dipolar interactions are shown to delocalize the magnetic excitations over magnetically inequivalent chains of radicals. The g tensor for a delocalized excitation is calculated in terms of the g tensors of the radicals over which it is delocalized. The splittings decrease, and finally vanish, as the interchain dipolar field becomes sufficiently strong to delocalize the excitation entirely. The dipolar field strength depends on temperature through the strong magnetic dilution arising from large, antiferromagnetic, intrachain exchange. The theory, which accounts for the entire temperature range of the spectrum, is contrasted with the qualitatively different line merging predicted by random modulation among fixed frequencies (i.e., localized excitations), which necessarily leads to an initial line broadening greater than the line shift.
Zeeman Study of the 35Cl NQR Spectrum of Chlorobenzene
Authors:   Dinesh, Narasimhan P. T.
Published: February 2004

A Zeeman study of the nuclear quadrupole resonance spectra of 35Cl using frozen samples of chlorobenzene has been carried out at 77°K. The experimental lower limit obtained for the asymmetry parameter (η) at the chlorine site is 0.10 ± 0.01. A theoretical estimate of the η value using Bersohn's analysis yields a value of 0.092 in agreement with experiment.
Vibration–Rotation Interaction Effects in Calculated Franck–Condon Factors. I. The Ionization of H2 and D2
Authors: Villarejo Don
Published: February 2004

Computed Franck–Condon factors for the ionization of H2 and D2 are reported which include previously neglected vibration–rotation interaction effects. Eigenfunctions were obtained by direct solution of the Schrödinger equation in which the exact centrifugal potential is explicitly included. Only υ″ = 0 and small K″ states of the neutral molecule have been considered and attention is confined to those transitions for which |K″ − K| = 0, 2. The results show a clear dependence of the computed Franck–Condon factor on rotational quantum number. For K″ = K, this effect is small except for transitions to the uppermost vibrational states of the ion. For |K″ − K| = 2, the effect is substantial even for transitions to low-lying vibrational states of the ion. By using a sum rule, it is shown that the probability of dissociative ionization exhibits a similar dependence on rotational state. Comparison of the present results for the case K″ = K = 0 with previous computation shows that adiabatic corrections for nuclear motion, explicitly included in this work, have a wholly negligible influence on computed Franck–Condon factors.
Internal Rotation in Butane
Authors: Hoyland James R.
Published: February 2004

Quantum-mechanical calculations within the Hartree–Fock framework and using two Gaussian basis sets are carried out on several conformations of the butane molecule. The smaller basis set predicts a transgauche barrier of 3.536 kcal/mole, an energy difference of 0.822 kcal/mole between the gauche and trans forms, and a barrier of 6.821 kcal/mole for interconversion of the two gauche conformations. The values of these quantities as given by the larger basis set are 3.619, 0.761, and 6.834 kcal/mole. Taking the angle of rotation about the central bond to be 0 for the trans conformation, the gauche form is predicted to occur at 110.9° for the smaller and at 111.3° for the larger basis set. The barrier to internal rotation of a methyl group is predicted to be 2.92 kcal/mole and 2.94 kcal/mole for the smaller and larger basis sets, respectively.
Si–Te System: Partial Pressures of Te2 and SiTe and Thermodynamic Properties from Optical Density of the Vapor Phase
Authors: Brebrick R. F.
Published: February 2004

The optical density between 2400 and 7000 Å of the vapor in equilibrium with Si–Te samples containing between 10 and 100 at.% Te was measured for sample temperatures between 500° and 940°C. The partial pressures of Te2(g), pTe2, were obtained from the optical densities at 4357, 5000, and/or 5500 Å, where SiTe(g) does not absorb. A plot of pTe2 vs 103 / T shows the existence of a single compound, Si2Te3(c), whose homogeneity range falls within outer limits of 59.45 and 60.50 at.% Te and ends at a peritectic temperature of 892°C. X-ray powder diffraction patterns indicate the Si2Te3(c) phase is the same as that previously identified by some authors as SiTe2(c). The net SiTe(g) optical densities at 2708 and 2893 Å were determined and found to be in constant ratio to one another. For 0.4 Si(c) + 0.6 Te(1)→Si0.4Te0.6(c), the standard Gibbs free-energy change is ΔGf° = − 5.754 + 3.904(10−3)T kcal / g-atom between 604° and 892°C. For the reaction Si(g) + 12Te2(g) → SiTe(g), it is ΔGf° = − 74.86 + 15.77(10−3)T kcal / mole (700°–892°C).
Photionization of Ytterbium: 1350–2000 Å
Authors: Parr Albert C. , Elder Fred A.
Published: February 2004

Using a photoionization mass spectrometer, the photoionization efficiency of ytterbium has been measured from 1350–2000 Å. The spectrum is dominated by line structure which tentative analysis indicates is due to the following configurations: 4f145dnp, 4f146pnd, 4f136s2nd, and 4f135d6snd. The ionization potential derived for Yb I from Rydberg series is 6.25 ± 0.01 eV.
Binary Diffusion and Bulk Flow through a Potential-Energy Profile: A Kinetic Basis for the Thermodynamic Equations of Flow through Membranes
Authors: Manning Gerald S.
Published: February 2004

The flow of a two-component solution through a potential-energy profile is analyzed in detail. The thermodynamic flow equations of Kedem and Katchalsky are derived, along with the Onsager reciprocal relation—the equality of the osmotic-flow coefficient and the ultrafiltration coefficient. Expressions are obtained for the solute permeability ω and the Staverman reflection coefficient σ. It is demonstrated that contributions from equilibrium partitioning and “frictional” effects both arise from the characteristics of the energy profile. The role of the pressure gradient as the driving force for osmotic flow is stressed.
Molecular Beam Kinetics: Magnetic Deflection Analysis of Scattering of Alkali Atoms from Polyhalide Molecules
Authors: Gordon R. J. , Herm R. R. , Herschbach D. R.
Published: February 2004

An inhomogeneous magnetic field has been used to measure the ratio of reactively scattered alkali halide molecules (diamagnetic) to elastically scattered alkali atoms (paramagnetic) for several systems, including Cs + Br2, CH3I, CHCl3, CCl4, SiCl4, and SnCl4; K + CC14 and SiCl4; and Na + CH3I. For comparison, the scattering of the nonreactive systems Cs+cyclohexane and Na+propane was also studied. The results verify previous work which employed two surface ionization filaments to distinguish between reactive and elastic scattering. In particular, it is confirmed that the laboratory angular distribution of KCl from the K + CCl4 reaction is bimodal. To enhance the alkali beam intensity, a “Laval slit” was used and the oven operated at pressures well above the effusive flow region. This source produces marked deviations from the Maxwell–Boltzmann velocity distribution. It was found that the Stern–Gerlach deflection patterns and the variation of transmission with field strength for such beams can be accurately calculated by use of a suitable modification of the velocity distribution (empirically calibrated in separate velocity-analysis experiments).
Collision-Induced Dissociation on HD+ by Rare Gases
Authors: Rozett R. W. , Koski W. S.
Published: February 2004

Absolute cross sections and kinetic isotope ratios for the dissociation of MD+ into protons and deuterons when striking helium, neon, argon, krypton, and xenon are reported up to 50-eV incident kinetic energy. The calibration of the tandem mass spectrometer is described in detail. The spectator mechanism for ion–molecule reactions has been elaborated to account for the periodic maxima observed in the cross sections of these collision-induced reactions.
Direct Determination of the Parameters for Gd3+ in YVO4 and YPO4 from Variable-Frequency EPR
Authors: Urban Wolfgang
Published: February 2004

A new variable-frequency EPR method for determining crystal-field parameters directly from measurements at special magnetic fields is described. The method is applied to trivalent gadolinium in yttrium vanadate and yttrium phosphate. The parameters of the corresponding spin Hamiltonian are given.
Characterization of Ground-State Wavefunctions by Measured Electronic Properties. I. 119Sn Mössbauer Isomer Shifts
Authors: Unland Mark L. , Letcher John H.
Published: February 2004

A particular type of LCAO MO wavefunction is applied to the problem of correlating experimental 119Sn Mössbauer isomer shifts with calculated electron densities at the nucleus. The results are found to be sensitive to the choice of basis functions, molecular geometry, and method of setting basis orbital exponents. Because of this sensitivity, only a rough correlation of the isomer-shft data is obtained when conventional methods of setting orbital exponents are used. A set of wavefunction parameters which result in good correlation of the 119Sn data is found and, for each molecule, a quantitative measure of the sensitivity of the calculated electron densities to each parameter is obtained.
Conformational Analysis of Macromolecules. V. Helical Structures of Poly-L-aspartic Acid and Poly-L-glutamic Acid, and Related Compounds
Authors: Yan Johnson F. , Vanderkooi Garret , Scheraga Harold A.
Published: February 2004

Previous energy calculations for isolated (single-stranded) homopolymer polyamino acids, to find the most stable regular (helical) conformations, have been extended to poly-L-aspartic acid, poly-L-glutamic acid, and several related compounds. Since rotation about all single bonds (including the acid and ester groups of side chains) was taken into account, it was necessary to obtain expressions for the torsional potentials for these internal rotations. The computed helix sense was found to agree with the experimental one in all cases except one, using a single set of potential functions and parameters. The various contributions to the total energy are all important in their influence on the conformation. For most of the polymers studied, there are only four principal side-chain conformations of low energy, two for the right-handed and two for the left-handed helix sense. Essentially the same set of four conformations was found for poly-L-aspartic acid and all of its esters; a different set of four conformations was found for poly-L-glutamic acid and its esters. For any given polymer, the conformation of lowest energy (of the four general conformations) depended upon the details of the specific side chain involved. The side chain conformations can be classified as being either transverse, for which the side chains wrap tangentially about the backbone at right angles to the helix axis, or longitudinal, for which the side chains lie more nearly parallel to the helix axis. For most polymers, one transverse and one longitudinal conformation of low energy was found for each helix sense. For aspartate polymers with short side chains, the longitudinal conformations had lower energy than the transverse ones, but the converse was true for aspartate polymers with longer side chains (viz., the n-propyl and benzyl esters). This effect was primarily the result of competition between the torsional and nonbonded energies; the unfavorable torsional energy in the transverse conformation was compensated by a favorable nonbonded energy, provided that the side chain was long enough to lead to a large nonbonded side chain-to-backbone interaction. For the glutamate polymers, a longitudinal conformation (different from that of the aspartate polymers) was preferred in all cases for the right-handed helix sense, but a transverse one was of lower energy for the left-handed helix sense. Poly-l-phenylalanine was found to have essentially the same conformations as the previously reported conformations for poly-l-tyrosine; the χ1 and κ2 values for these conformations are close to those found for poly-l-aspartic acid. This conformational similarity can be accounted for in terms of the geometrical similarity between the side chains of phenylalanine, tyrosine, and aspartic acid.
Theoretical Study of the Low-Energy Photoionization of Large Molecules: Benzene
Authors: Johnson Philip M. , Rice Stuart A.
Published: February 2004

In this paper we use a linear combination of phase-shifted atomic continuum factions to represent the unbound orbitals of large molecules. These functions take into account the interference arising from phase coherence between atomic centers with finite internuclear separations, an important effect at low energies. The effect of vibrations and configuration interaction on oscillator strengths calculated from these wavefunctions is discussed. A significant rounding (in energy) of the ionization threshold is seen to be caused by vibrational effects, and it is suggested that configuration interaction in large molecules may result in radiationless decay without ionization even when the molecules are excited to a continuum state. The results of sample calculations pertaining to benzene are presented in the form of differential oscillator strength curves in which interference effects are either included or neglected. Existing experiments give some support to the results of these calculations, but there are important discrepancies between calculation and some of the observations.
ESR Spectrum of HO2 in Solutions of Hydrogen Peroxide in Water at 77°K
Authors: Wyard S. J. , Smith R. C. , Adrian Frank J.
Published: February 2004

The electron spin resonance (ESR) spectrum of the perhydroxyl radical (HO2) has been observed in a hydrogen peroxide–water matrix at 77°K. The radical was produced by either ultraviolet or ionizing radiation, and the initial poor resolution of the spectrum was greatly improved by careful annealing of glassy samples. Accurate measurements of g factors and hyperfine splittings were made at both 9 and 6 GHz. The following parameters, obtained by computer analysis, were found to give the best fit to the spectrum at both frequencies: g1 = 2.0353, g2 = 2.0086, and g3 = 2.0042; |A1| / h = 39.2, |A2| / h = 10.0, and |A3| / h = 43.9 MHz. Experiment and theory both show that axis 1 lies along the O–O bond. Theoretical considerations also suggest that axis 2 lies in the molecular plane, that axis 3 is perpendicular to the molecular plane, and that the values of the hyperfine splitting constants are negative. The ESR spectrum of DO2 was observed in a deuterated sample, and computer analysis gave parameters in reasonable agreement with those for HO2, allowing for the difference in nuclear magnetic moment. The parameters for HO2 in H2O2–H2O are similar to those for HO2 in argon, the small differences being attributed to matrix effects. However, the rotation of the radical about its O–O bond, which was observed for HO2 in argon, does not occur in the H2O2–H2O matrix. This is attributed to hydrogen bonding in the H2O2–H2O matrix.
Raman Spectra of Polycrystalline H2S and D2S
Authors: Miller R. E. , Leroi G. E.
Published: February 2004

The Raman spectra of H2S and D2S have been obtained in all three solid modifications, using both conventional and laser excitation. The spectra in the two higher-temperature phases are consistent with their known disordered crystal structures. Previously undetected vibrational structure also has been obtained for both the internal and lattice vibrations in the lowest-temperature phase (III); an assignment of the entire spectrum is proposed. Conclusions about the crystal structure of this phase, which has not been previously established, can be drawn from the combined results of x-ray, electron diffraction, infrared, far-infrared, and Raman investigations. It is shown that a C4h structure with eight molecules per unit cell on C1 sites fits all the data for Phase III of H2S and D2S.
Method for the Determination of Asymmetric Crystalline Electric-Field-Gradient Tensors with Application to CS2MnCl4·2H4O
Authors: Jonge W. J. M. De , Srivastava G. N. , Parker Paul M.
Published: February 2004

A procedure is described which allows convenient determination of electric-field-gradient tensors by nuclear magnetic-dipole–electric-quadrupole resonance in single crystals. The procedure is illustrated through application to the cesium quadrupole resonance in CS2MnCl4·2H2O.
Intrinsic Photoconduction in Pyrene Crystals
Authors: Chaiken Robert F. , Kearns David R.
Published: February 2004

The dc bulk photoconductivity of “pure” pyrene crystals and pyrene crystals doped with perylene has been investigated in the uv region (230–400 mμ). From a study of the spectral response of the hole and electron photocurrents and the effects of varying crystal temperature, the threshold for intrinsic photoconduction is located at about 4.1 eV (300 mμ). With incident photon energies less than 4.1 eV, the generation of holes and electrons is by an extrinsic, impurity-dependent process, which probably involves migration of monomeric singlet excitons to surface or near-surface impurity sites where ionization occurs.
Nuclear Relaxation in the Benzene–PMMA System
Authors: Anderson J. E. , Liu Kang-Jen
Published: February 2004

The nuclear relaxation of benzene protons in the benzene–polymethylmethacrylate (PMMA) system has been examined at concentrations up to 35 wt% PMMA. Systems prepared from isotactic and syndiotactic PMMA were studied. The polymer samples ranged in molecular weight between 4 × 104 and 2thinsp;× 106. The concentration dependence of the benzene spin–lattice relaxation time is independent of the mw and tacticity of PMMA and decreases with increasing polymer concentration. The benzene spin–lattice relaxation of samples containing 4 × 104 mw PMMA was resolved into dipolar contributions from: (1) intermolecular pairs of protons; (2) intermolecular pairs of benzene protons; (3) intermolecular benzene–polymer proton pairs. Selective deuterium substitution was used to separate these effects. T2, the benzene spin–spin relaxation time, showed sensitivity to the molecular weight and stereoregularity of the polymer. In contrast to the behavior found in nonpolymeric solutions, (T2 / T1) ratios much greater than unity were observed (greater than 40 in extreme cases). These results cannot be explained on the basis of solution viscosity. Rather, they reflect the microscopic inhomogeneity of concentrated polymer solutions, and the migration of solvent molecules between local environments of different polymer concentrations. A qualitative mathematical description of the migration process and its effects on T2 is presented.
Electron Paramagnetic Resonance of X-Ray-Irradiated Single Crystals of Potassium Cobalticyanide, K3Co(CN)6
Authors: Lin W. C. , McDowell C. A. , Ward D. J.
Published: February 2004

X-ray irradiation of single crystals of undoped diamagnetic potassium cobalticyanide K3Co(CN) has been shown to produce paramagnetic species. The electron paramagnetic spectra indicate that the eight-line hyperfine interactions observed (I = 72for59Co) collapse into one set when the magnetic field is parallel to any one of the three crystallographic axes labeled a, b, or c. The widths of the hyperfine lines vary and the linewidths increase as the values of the nuclear magnetic spin quantum number (MI) decreases. The experimental results have been interpreted in terms of an appropriate spin Hamiltonian to yield the principal values of the axially symmetric g and hyperfine interaction tensor A. The results obtained are g‖ = 2.010 ± 0.005, g⊥ = 2.170 ± 0.005; A‖ = 83.5 ± 2 G, and A⊥ = 26.9 ± 2 G. A theoretical estimate of the g values obtained by using the method described by Pryce leads to the conclusion that the paramagnetic center is most likely due to a species with d7 configuration formed by the odd electron going into a dz2 orbital followed by a weak tetragonal distortion to symmetry D4h of the ensuing structure as the result of the Jahn–Teller effect. An alternative possibility is that the original anion loses a CN group to form a paramagnetic species with symmetry C4v.
Dipole Shielding Factor in Hartree–Fock Theory
Authors: Chang Edward S.
Published: February 2004

In the coupled Hartree–Fock approximation, the dipole shielding factor is proved to be identically N / Z for any atomic S state. This suggests that variational-perturbation methods which yield poor values of the shielding factor may give unreliable Hartree–Fock values for other atomic properties such as the polarizability.
Environmental Effects on Phosphorescence. IV. Triplet Decay of Halonaphthalenes in Compressed PMMA
Authors: Baldwin B. A. , Offen H. W.
Published: February 2004

The effects of high pressure (0–32 kbar) on the phosphorescence lifetime τp of seven monohalogen-substituted naphthalenes, dispersed in polymethylmethacrylate (PMMA), have been measured at room temperature. It is observed that the pressure-induced shortening of the lifetimes becomes less significant in those halonaphthalenes which possess the heavier atoms, i.e., the greater the intramolecular heavy-atom effect the less influential are environmental perturbations. “Activation volumes,” ranging in magnitude between − 0.25 < ΔV‡ < 0.0 Å3/molecule, were computed from linear lnτp-vs-P plots. Qualitative arguments are given to support the observed magnitudes and trends in the pressure-induced volume effects on triplet decay.
Spin Hamiltonian of 51V2+ in MgO, as Derived from Electron Nuclear Double Resonance
Authors: de Wijn H. W. , Schrama A. H. M.
Published: February 2004

The electron nuclear double resonance spectrum of 51V2+ diluted in a single crystal of MgO has been measured at 4.2° and 77°K. The spin-Hamiltonian parameters have been determined, including an additional hyperfine contribution of the type S3I. Perturbation theory up to the fourth order has been used to calculate the latter term as a result of interactions of the ground orbital singlet with higher orbital levels.
Rate of Vaporization of Arsenic Single Crystals and the Vaporization Coefficient of Arsenic
Authors: Rosenblatt Gerd M. , Lee Pang-Kai
Published: February 2004

Rates of vaporization from (111) cleavage faces of arsenic single crystals have been measured over the temperature range 508°–580°K with a recording vacuum microbalance. Expressing the rates as Langmuir vapor pressures (PL), the results are R lnPL(atm) = −(43.99 ± 1.47) / T + (37.98 ± 2.72). At 550°K, PL = 6.60 × 10−10atm. Comparison with literature equilibrium data yields a vaporization coefficient, αυ = PL / PE = 4.6 × 10−5 at 550°K, which increases with temperature, αυ = exp (−10921 / RT). The results are consistent with a vaporization mechanism in which the slow step involves formation of As4 molecules and occurs prior to surface diffusion and desorption. Ancillary measurements on vaporization of wrapped arsenic crystals, powdered arsenic, and Knudsen-effusion experiments are presented. These data and previous vaporization studies on powdered arsenic are correlated using the concept of effective vaporizing areas. Some implications of the results for the equilibrium vapor pressure of arsenic, the interpretation of Knudsen-effusion experiments, and the absolute entropy of As4(g) are discussed.
P, v, T Equation of State for Polyethylene
Authors: Pastine D. John
Published: February 2004

A P, v, T equation of state is theoretically constructed for polyethylene of arbitrary crystallinity. This is done by constructing, separately, equations of state for purely crystalline and purely amorphous polyethylene and then combining the two according to the degree of crystallinity. Very little experimental data is used in constructing either the crystalline or amorphous equations of state. Comparisons of the predicted P, v, T relations with experiment are quite favorable.
Quantum-Mechanical Study of Some Three-Center Two-Electron Systems. II. A Natural-Orbital, Electron-Population, and Energy Analysis
Authors: Banyard K. E. , Tait A. D.
Published: February 2004

Configuration-interaction wavefunctions for a series of pseudomolecular ions of the form ZHZ+2Z−1, where Z = 1.0 (0.4) 2.2, were reformulated in terms of natural orbitals. Consequently, changes in the electron density could be investigated, as a function of Z and the bond angle ZĤZ, by means of the population analysis of Mulliken. Contour diagrams were also determined for the charge density evaluated in the plane of the molecule. The total energy for each system was analyzed in terms of the kinetic energy, nuclear attraction energy, and the electron and nuclear repulsion energies. The results are presented graphically as a function of the bond angle θ for each value of Z. For Z = 1.0, i.e., H3+, the occupation numbers for the natural orbitals revealed the united atom character of this system, and the contour diagrams indicated that, in the equilibrium configuration, the “bonds” were directed from each nucleus towards the centroid of the positive charges. As θ was increased beyond 60°, it was found that the “central” proton within H3+ began to dominate the system. For Z = 2.2, the ions approach most closely to their separated systems of minimum energy as θ tends to 180°. The contour diagrams and the electron-population analysis also show that the ZHZ+2Z−1 ions change from a three-center to, basically, a two-center system and a strongly perturbing proton as the values for Z and θ increase and decrease, respectively. The θ dependence of the kinetic energy is perhaps the most interesting of the energy curves. By associating a decrease in kinetic energy with an increase in spatial freedom for the electrons, it was possible to understand the θ variations of all the energy components for each value of Z. The present examination has allowed us to observe, in detail, changes which occur in the electron density and energy components for a series of three-center two-electron systems when θ and Z are allowed to vary.
Refractive Index of Gaseous and Liquid Hydrogen
Authors: Diller Dwain E.
Published: February 2004

The refractive index of gaseous and liquid hydrogen has been measured by an interferometric method at temperatures between 15° and 298.15°K and at pressures up to 230 atm. The measurements have been analyzed in terms of the density and temperature dependence of the Lorentz–Lorenz function, L–L ≡ [(nλ2 − 1) / (nλ2 + 2)]ρ−1, where nλ is the refractive index at λ = 5462 Å and ρ is the fluid density in grams per cubic centimeter. The precision and reproducibility of L–L is better than 0.05% in most cases. L–L for gaseous parahydrogen first increases with increasing density to a maximum and then decreases to values below the low-density limit. L–L is also slightly temperature dependent; the low-density limit increases with increasing temperature; the maximum on the L–L isotherms decreases with increasing temperature. L–L for saturated liquid parahydrogen decreases with increasing density by about 0.1% at temperatures between 15° and 32°K. The difference in L–L for normal and parahydrogen is consistent with previous theoretical and experimental estimates of the molecular-polarizability difference.
Microwave Spectrum of cis-2-Butene
Authors: Sarachman T. N.
Published: February 2004

The microwave spectrum of the cis-2-butene molecule has been studied between 20 and 40 Gc/sec. Analysis of internal-rotation splittings yields an effective threefold barrier of 0.73 kcal/mole, or 256 cm−1. The principal moments of inertia of the normal isotopic species are 31.479, 98.370, and 123.659 amu·Å2. The electric dipole moment is 0.257 D and lies along the b axis of the molecule.
Some Aspects of Electron Correlation in Open-Shell States. 21P and 23P Helium
Authors: Yin Moon-Lung , Silverstone Harris J.
Published: February 2004

Accurate wavefunctions for the 23P and 21P states of helium have been analyzed into pair, polarization, and semi-internal correlation effects. The dynamical, transferable pair-correlation energies account for approximately 90% of the total correlation energies. The special open-shell effects, symmetry polarizations, and semi-internal correlation, account for half the remaining correlation energy. It is shown computationally that the polarization and semi-internal correlation energies can be computed separately from the pair energies to about 96% accuracy.
Melting of DNA
Authors: Lehman G. W. , McTague J. P.
Published: February 2004

An exact statistical-mechanical treatment is given for the nearest-neighbor coupled model of DNA, allowing for the difference in bonding stability of the two base pairs, adenine–thymine and gaunine–cytosine. Treatments are given for random as well as pair-correlated sequences, both for finite and infinite degrees of polymerization. In addition, sequence-ensemble average thermal properties are compared with the corresponding properties of single molecules. The breadth in temperature of the helix–coil transition is shown to be a marked function of base composition. It is also shown that nonrandomness in nearest-neighbor base sequence of the magnitude observed in typical bacterial and viral DNA's has little effect on the thermodynamic properties.
Phosphorescence and the True Lifetime of Triplet States in Fluid Solutions
Authors: Tsai S. Cheng , Robinson G. W.
Published: February 2004

Phosphorescence has been observed in a highly purified fluid solution of naphthalene in 3-methylpentane (3-MP). The phosphorescence lifetime of C10H8 in 3-MP at −45°C was found to be 0.49 ± 0.07 sec, while that of C10D8 under identical conditions is 0.64 ± 0.07 sec. At this temperature 3-MP has the same viscosity (0.65 CP) as that of benzene at room temperature. It is believed that even these long lifetimes are dominated by impurity-quenching mechanisms. Therefore it seems that the radiationless decay times of the lowest triplet states of simple aromatic hydrocarbons in liquid solutions are sensibly the same as those in the solid phase. A slight dependence of the phosphorescence lifetime on solvent viscosity was observed in the temperature region, −60° to −18°C. This has been attributed to the diffusion-controlled quenching of the tiplet state by a residual impurity, perhaps oxygen. Bimolecular depopulation of the triplet state was found to be of major importance over a large part of the triplet decay. The lifetime of triplet C10H8 at room temperature was also measured in highly purified benzene by means of both phosphorescence and triplet–triplet absorption. The lifetime was estimated to be at least 10 times shorter than that in 3-MP. This is believed to be due not only to residual impurities in the solvent but also to small amounts of impurities produced through unavoidable irradiation by the excitation source. In agreement with this idea, lifetime shortening caused by intense flashes of light is readily observed. This latter result suggests that experiments employing flash lamp techniques are not suitable for these kinds of studies.
hfs–Zero-Field Paramagnetic Resonance of ·CH Radicals
Authors: Birkle M. , Schoffa G.
Published: February 2004

An introduction to the method of hfs–zero-field paramagnetic resonance is given in this article and spectra of the radicals of barbituric acid, malonic acid, cyclohexane-1, 1-diacetic acid, fumaric acid, and α-alanine are shown. From these spectra the hfs-coupling-tensor values were determined.
Photoconduction of Pyranthrene in Benzene
Authors: Prock Alfred , Zahradnik Robert
Published: February 2004

Pyranthrene in benzene solution shows a large photoconduction in the visible region that extends far beyond its spectral absorption region. Present studies using both dc and chopped-light ac methods and cells of two kinds of geometry show that this is not a bulk phenomenon, but rather is a single-photon process involving interaction between the electode and an adsorbed pyranthrene layer. The charge carriers produced are faster than pyranthrene ions alone are expected to be in solution.
Statistical Theory of Dissociation and Recombination for Moderately Complex Molecules
Authors: Keck James , Kalelkar Ashok
Published: February 2004

The dissociation and recombination of moderately complex molecules in a background of inert atoms has been investigated using a master equation with transition probabilities and spontaneous decay rates obtained from the statistical theory of reaction rates. A relatively simple expression has been derived for the steady-state dissociation rate constant which is valid at all pressures for molecules containing three or more atoms. This expression has been used with reasonable success to correlate the available experimental data for 21 molecules having from three to six atoms.
Spectroscopic Evidence for Slow Vibrational and Electronic Relaxation in Solids. The Vegard–Kaplan and Second Positive Systems of N2 in Solid Rare Gases
Authors: Tinti D. S. , Robinson G. W.
Published: February 2004

The spectra emitted from N2-doped rare-gas solids at low temperatures, irradiated with x rays, have been investigated in detail. The molecular N2 emission seen in the spectral range 2300–5000 Å consists of the Vegard–Kaplan (VK) A 3Σu+ → X1 Σg+ and the second positive C 3Πu → B 3Πg band systems. The latter is observed only in solid Ne. Forbidden transitions of N atoms and at times of impurity O atoms are also seen. Both N2 band systems show emission from υ′ > 0, establishing a relatively slow vibrational relaxation rate in the A 3Σu+ and C 3Πu states in solid rare gases. For the VK system the relative intensity of emission from υ′ > 0 is strongly dependent on the rare-gas host and on the N2 concentration. No υ′ > 0 is seen for a Ne host, whereas in Ar, Kr, and Xe the relative intensity of υ′ > 0 increases with decreasing N2 concentrations, with a maximum of υ′ = 6 being found in these three hosts. Decay times for isolated VK bands were measured and appeared exponential within experimental error. From the measured lifetimes it is concluded that the vibrational relaxation time in the A 3Σu+ state of N2 in solid rare gases lies between 0.4 and 3.3 sec. The effect of temperature on the relaxation rate was found to be small over the range 1.7°–30°K. The measured lifetime of the A 3Σu+ state in solid Ne is 3.3 sec, which lies within the range of values reported for the purely radiative lifetime of gaseous N2. A close connection between the processes discussed in this paper and the detailed elementary reactions of radiation chemistry is pointed out.
Reactions of Nitrogen–Hydrogen Radicals. III. Formation and Disappearance of NH Radicals in the Photolysis of Ammonia
Authors: Mantei Kenneth A. , Bair Edward J.
Published: February 2004

The rates of formation and disappearance of the NH biradical in the ground and first-excited vibrational states were measured by kinetic spectroscopy following flash photolysis of mixtures of ammonia and inert gases. The delay in the appareance of NH following a 25-μsec photolysis flash shows that it is formed by secondary reactions of the primary photolysis products, either NH2 + NH2 or NH2 + H. A large proportion of the observed NH is initially formed in the υ″ = 1 vibrational state, which disappears an order of magnitude faster than NH(υ″ = 0). The disappearance of NH(υ″ = 0) is pressure dependent and in the falloff region at 1 torr, 300°K. The rate of disappearance is rapid and is consistent with NH + NH3 insertion. At the highpressure limit the rate constant for this reaction is k4 = 1.0 ± 0.3 × 1010liter/mole·sec. The reaction has a small negative energy of activation. The high rate of NH disappearance accounts for the small range of conditions under which it can be observed. Implications of the results to the mechanism of biradical insertion reactions are discussed.
Relaxation Spectra and Association Constants of 1:1 Charge-Transfer Complexes
Authors: Hayman H. J. G.
Published: February 2004

The possibility of determining the association constant of a 1:1 charge-transfer complex AB in solution from the concentration dependence of the relaxation time is discussed. If small quantities of termolecular complexes A2B and AB2 are also present there will be two additional relaxation times corresponding to the decomposition rate constants of these complexes. It is shown that if A and B are aromatic molecules whose two sides act completely independently so as to form an infinite series of sandwich complexes (model 3), then the usual relaxation techniques would detect only one relaxation time whose concentration dependence would be indistinguishable from that of a system containing only one complex, AB. If such a system were mistakenly assumed to contain only one complex AB, relaxation methods would lead to the correct value for the dissociation rate constant of AB but to an association rate constant equal to half the true value. The association constant found would thus be equal to half its true value, just as if it had been determined spectrophotometrically. The complete relaxation spectrum of model 3 and the corresponding normal reaction coordinates were calculated; the reciprocal relaxation times form three infinite series, the spacing in each series being twice the dissociation rate constant of AB. If the rate constants of reactions involving isolated molecules of A or B differ slightly from those of the corresponding reactions involving complexes of A and B, then perturbation theory shows that in principle three relaxation times, instead of one, should be observable; in practice, difficulties would arise since these relaxation times are all of similar magnitude.
Saturation Studies of N2− in Irradiated Sodium Azide
Authors: Brezina Gerald W. , Gelerinter Edward
Published: February 2004

Saturation studies of N2− in sodium azide (NaN3) were performed at temperatures of 4.2° and 2°K and at a frequency of 9 GHz. The product T1T2 was found to be approximately 10−11 sec2. The assumption that the width of the partially resolved hyperfine structure is equal to the width of the homogeneously broadened packet allowed estimations of T2 as 10−8 sec and T1 as 10−3 sec. This assumption, justified in the text, also allowed one to calculate an upper limit on the ratio (a) of the homogeneous to inhomogeneous linewidths of 0.07. The saturation data yielded values of 0.4–0.8 for this ratio. The spin-diffusion time, T21, was shown to be the order of 10−5 sec, so that spin diffusion was a dominant relaxation mechanism. This was used to explain apparent ambiguities in the values of a. The relaxation times were highly anisotropic when the dc magnetic field was nearly parallel to the crystalline c axis. Further experiments are needed before one can understand this variation.
Vibrational Assignment of Carbazole from Infrared, Raman, and Fluorescence Spectra
Authors: Bree A. , Zwarich R.
Published: February 2004

The infrared, Raman, and fluorescence spectra of carbazole are reported with as complete polarization information as possible. An experimental assignment of fundamentals is given and is reliable only for A1 species, being incomplete or uncertain for the other symmetry species. Some lattice modes of the carbazole crystal were found both in the infrared and Raman spectra, and a partial assignment made. A crude normal-coordinate calculation was made using a force field transferred from phenanthrene for the in-plane problem and from benzene and anthracene for the out-of-plane problem. The observed and calculated frequencies are compared, and good agreement is found for the A1 fundamentals.
Structural Studies of Polyesters. II. Far-Infrared Spectra of Aliphatic Polyesters: Comparison with α-Polyamides
Authors: Tadokoro Hiroyuki , Kobayashi Masamichi , Yoshidome Hideo , Tai Kazuo , Makino Daisuke
Published: February 2004

Polarized far-infrared spectra of aliphatic polyesters [–(CH2)z–CO–O–O]n with z = 1, 2, and 5 (z = 4, nonpolarized) and α-polyamide (nylon 6 and its N-deuteroderivative) have been measured in the region up to 80 cm−1. All these polymers have essentially planar zigzag conformations. The normal vibrations of these polymers were calculated for the far-infrared regions by using the Urey–Bradley force field with some modifications. For the series of polyesters, several far-infrared bands were found to appear systematically at nearly the same positions with similar intensities. The similarities between these series of bands of the polyesters and those of polyamides were interpreted from the point of view of the normal modes of vibrations.