bluuebunny/crossref_metadata_2025 huggingface dataset:
Melting in two dimensions: Determination of phase transition boundaries
Authors: van Swol Frank , Woodcock Leslie V. , Neil Cape J.
Published: February 2003

Molecular dynamics computations are reported for systems of soft repulsive discs of the inverse twelfth-power potential and for Lennard-Jones atoms in two dimensions. In the case of soft discs the melting transition is obtained by assuming a first-order transition and subsequent application of Ross’s melting rule. The assumption is vindicated by additional computations for the 2-D L-J model in which the melting and freezing parameters are determined by direct MD computation along the isotherm T=0.8 ε/k and the isochore ρr20=1.0079. The results reaffirm the presence of first-order phase boundaries and refute the postulated existence of a ’’hexatic mesophase’’ bounded by second-order transitions. These two-dimensional models are seen to have much smaller discontinuities in density and entropy than their three-dimensional counterparts and to exhibit more pronounced premelting increases in the heat capacity. This is interpreted to be a consequence of the proximity of the mechanical instability points to the first-order thermodynamic melting transition for these two-dimensional systems. Previously reported lambdalike behavior of the heat capacity through the melting transition of the 2-D one-component plasma is consistent with this interpretation for an irreversible isochoric traversal of a two-phase region and hence suggests an ordinary, albeit weaker, first-order melting transition in this system also.
Electrical transport in doped polyacetylene
Authors: Park Y.-W. , Heeger A. J. , Druy M. A. , MacDiarmid A. G.
Published: February 2003

The results of an experimental study of electrical conductivity and thermopower in doped polyacetylene are reported. Included are measurements on both as-grown and partially oriented films doped with iodine and AsF5; [CH(AsF5)y]xand [CH(I3)y]x, where y covers the full doping range. The data indicate three important concentration regimes; the dilute limit (y<0.001), the transitional region (0.001<y<0.01), and the metallic state (y≳0.01). In the dilute limit, the transport is via carrier hopping; the mobility is small (∼5×10−5 cm2/V sec) and activated (ΔΕ=0.3 eV). The semiconductor to metal transition is evident in the data and leads to a qualitative change in temperature dependence of the conductivity and to finite zero temperature values above yc?0.01–0.02. The transport mobility increases by five to six orders of magnitude on going through the transitional region. In the metallic state, the high mobility (∼60 cm2/V sec, assuming unit charge transfer) provides evidence of the validity of a band theory approach with delocalized states in this disordered polymer. The transport in the metallic state is described as metallic strands separated by thin potential barriers. The main effect of orientation appears to be to alter the barriers. In particular, use of oriented cis-(CH)x starting material leads to significant improvement in conductivity due to smaller barrier widths and lower barrier heights. Analysis of the temperature dependence of the conductivity within this model leads to an estimate of the intrinsic conductivity in heavily doped metallic [CH(AsF5)y]x, σ?4×104Ω−1 cm−1 at room temperature.
Conformational energies and random-coil configuration of poly(3,3-dimethyl oxetane) (PDO)
Authors: Saiz E. , Riande E. , Guzmán J. , de Abajo J.
Published: February 2003

A sample of poly(3,3-dimethyl oxetane) was synthesized by cationic polymerization. A fraction having Mn=8500 and melting point of 45 °C was isolated and purified; its dipole moment, measured in benzene solutions at several temperatures in the interval 20–60 °C, yielded values of <μ2≳/nm2=0.249–0.297 and a temperature coefficient d(ln<μ2≳)/dT=4.4×10−3 K−1. Semiempirical potentials were used to compute conformational energies as function of rotations over the backbone bonds; the results of these calculations indicate that skeletal C–O bonds are always placed in trans conformation, whereas gauche states over C–C bonds have an energy of about 0.4 kcal mol−1 lower than their trans conformation, (Eσ?-0.4); a second order interaction energy Eω?1.3 kcal mol−1 arises in conformations g±g∓ over two consecutive C–C skeletal bonds because of the interactions between two consecutive oxygen atoms. Agreement between theoretical (calculated with the RIS theory) and experimental results of dipole moment and its temperature coefficient is achieved using Eω=1.3 and Eσ=−0.5 kcal mol−1.
Temperature dependence of the fluorescence decay of acridine measured with picosecond techniques
Authors: Shapiro S. L. , Winn K. R.
Published: February 2003

The fluorescence lifetime of acridine is measured as a function of temperature in three solvents. Whether the solvent is protic or aprotic, the same activation energy is extrapolated for energy dissipation. These results are inconsistent with a model used to describe electronic relaxation in acridine that depends solely on the energy gap between electronic states.
Some characteristics of the intravalence triplet–triplet electronic transition in HCN
Authors: Laidig William D. , Schaefer Henry F.
Published: February 2003

The precise theoretical predictions of the geometrical structures and relative energies of the two lowest triplet states of HCN are reported. (AIP)
On the nature of the sublimed phase of PCl5
Authors: Ripmeester J. A. , Wong P. T. T.
Published: February 2003

The sublimed ionic phase of Pcl5 was studied using 31P NMR spectroscopy, Raman spectroscopy and x-ray powder diffraction techniques. (AIP)
Mobilities and longitudinal diffusion coefficients for F− ions in Kr and Xe
Authors: Thackston M. G. , Eisele F. L. , Pope W. M. , Ellis H. W. , McDaniel E. W.
Published: February 2003

The ion mobility of F− ions in Kr and Xe was investigated under the influence of a uniform constant electric field. The ionic diffusion coefficient of F− ions has also been measured. (AIP)
Photodissociation of methylnitrite in the vacuum ultraviolet. I. Identification and quantum yields of electronically excited NO products
Authors: Lahmani F. , Lardeux C. , Lavollée M. , Solgadi D.
Published: February 2003

The fluorescence of the photofragments resulting from vacuum ultraviolet photodissociation of methyl nitrite (CH3ONO) in the gas phase has been studied using synchrotron radiation from Orsay Electron Storage Ring (ACO) as the source of excitation. In the spectral region between 1100 and 1600 Å where CH3ONO shows a diffuse absorption spectrum, the formation of NO in A 2Σ+ (v′=0,1,2), C 2Π and D 2Σ+ has been identified by time and energy-resolved spectra. The quantum yields of the photodissociation channels leading to the NO (A,C,D) states have been determined by comparison of the emission intensity with that of pure NO directly excited in A 2Σ+, C 2Π, v′=0, D 2Σ+, v′=0: φ (CH3ONO→NO A)=0.18±0.03 at λexc=1440 Å; φ (CH3ONO→NO C, v′=0)=0.07±0.02 at λexc=1380 Å; and φ (CH3ONO→NO D, v′=0)=0.05±0.02 at λexc=1200 Å.
Distribution of internal energy in the products of the reactions of 100 eV He+ with Xe
Authors: Jones E. Grant , Tiernan Thomas O. , Fee Darrell C. , Hughes B. Mason
Published: February 2003

The luminescence spectrum in the range from 50 to 890 nm which results from collision of a 100 eV beam of ground state He+ ions with Xe has been investigated. All observed emissions can be classified as Xe II transitions. Corrections of the spectra for cascading (amounting to about 30% of the total emission cross section) have been made in order to derive cross sections for direct production of Xe+* in the bimolecular collision. Comparisons of the cross section for radiative state production with the total cross section for charge transfer indicate that excited Xe+* states are formed with high efficiency. The internal energy distribution of Xe+* products maximizes at an energy corresponding to the conversion of <1 eV of translational energy into internal energy. For the near-resonant charge transfer reactions studied, a propensity to conserve the projection along the internuclear axis of the total electronic angular momentum is observed. The significance of the distribution of Xe+* states and the effects of kinetic energy are discussed.
Control of the motion of neutral molecules by a beam of electrons
Authors: Greene E. F. , Holber W. M. , Lambropoulos H. D.
Published: February 2003

Experiments show that a beam of electrons tends to trap or guide neutral molecules so their motion is preferentially parallel or antiparallel to that of the electrons. In one apparatus the beam of electrons is coaxial with a molecular beam. When the latter is KC1, the presence of the electrons increases the flux of molecules to the surface ionization detector by a factor of up to 103 over that obtained in the absence of the electrons. In another apparatus a beam of electrons in one chamber can direct molecules of the residual gas through a slit into another chamber where the detector is a mass spectrometer. The trapping effect is most marked when the number of molecules is small, as it is for weak molecular beams (fluxes less than 107 molecule cm−2s−1 for thermal beams) or when the residual gas is at a low pressure (<10−4Pa). A model based on the cumulative effect of many successive collisions of electrons with a molecule in the field of the beam of electrons provides a semi-quantitative interpretation of the measurements.
Direct observation of O(3P
J) in the photolysis of O3 at 248 nm
Authors: Amimoto S. T. , Force A. P. , Wiesenfeld J. R. , Young R. H.
Published: February 2003

The photodissociation of O3 at 248 nm has been studied following photolysis with a KrF excimer laser. The results of the current experiments, which were carried out in absorption using improved high-speed detection electronics, are in good agreement with previous direct observations of significant O(3PJ) photofragment yield following photolysis at 266 nm. A quantitative estimate of the quantum yield for O(1D2) production at 248 nm is 0.85±0.02. The implications of this observation for earlier studies of O(1D2) reactivity are discussed.
Application of semiempirical atom–atom potentials to crystals of acetylene
Authors: Filippini Giuseppe , Gramaccioli Carlo Maria , Simonetta Massimo
Published: February 2003

Semiempirical atom–atom potentials of the ’6-exp’ and ’Lennard-Jones’ type have been applied to crystals of acetylene, in order to account for various experimental data, such as sublimation energy, unit cell parameters, lattice vibration frequencies, etc. Most potentials which are generally satisfactory for hydrocarbons, do not account for the lattice vibration frequencies in acetylene: for the cubic phase, a good fit to several experimental data can be obtained using appropriate functions, provided that the sublimation energy is substantially lowered with respect to the experimental value. However, no potentials of this kind, at least in the very extensive range so far tried, can account for the properties and the very existence of the orthorhombic phase, as it is described in the literature. Before pointing to an inadequacy of the present form of these potentials, there is anyway the need of remeasuring the sublimation energy of acetylene, as well as collecting and refining single crystal data also for the orthorhombic phase.
The heat capacity and thermodynamic properties of potassium perrhenate and ammonium perrhenate from 8 to 304 K
Authors: Weir R. D. , Staveley L. A. K.
Published: February 2003

The heat capacities of KReO4 and NH4ReO4 from 8 to 304 K have been measured. The data show no obvious order–disorder phase transition but the curve for NH4ReO4 has a broad bump extending from about 100 to 250 K with a maximum around 200 K. This bump is absent from KReO4. For each salt, however, there is a tiny reproducible peak between 265–275 K, which is the result of impurity, probably water. An attempt has been made to calculate the normal curves of heat capacity against temperature for each salt, making use of spectroscopic information available for these crystals about the librational frequencies. For KReO4, the maximum difference between the experimental and calculated Cp values below 265 K is less than 0.8%. This excellent agreement is taken as evidence of an absence of any anomaly below 265 K. The case of NH4ReO4 is very different. Because the librational frequency ν6 of the NH4+ disappears from the spectrum at T≳180 K, the difference between the experimental and calculated Cp values was expressed as ΔCp=Clib NH4++Cextra, where the cextra refers to any other unaccounted contribution. When plotted against temperature, ΔCp rises steeply from 100 K to a sharp peak at 190 K (ΔCp=45 JK−1 mol−1) and then drops to 3 R (24.9 JK−1 mol−1) as 260 K is reached. Various models of restricted rotation of the ammonium ion are tested including that of a tetrahedron in a tetrahedral environment. All models tested so far fail to produce the peak in ΔCp. Our data are consistent with recent x-ray measurements suggesting that the hydrogen bonding present at 135 K disappears at room temperature and with NMR data showing a large amount of rotational motion of the NH4+ ion about 100 K.
A study of the 3p Rydberg transitions of the methylbenzenes by multiphoton ionization spectroscopy
Authors: Uneberg Gene O. , Campo Pamela A. , Johnson Philip
Published: February 2003

Two of the 3p Rydberg transitions of the methylbenzenes are seen in their multiphoton ionization spectra. These are assigned to states which include the 3pz orbital, having symmetries 1A1 and 1B2 in toluene and 1B1u and 1B2u in p-xylene. The origins of these states appear at 52839 and 54734 cm−1 in toluene. The 3s Rydberg transitions do not appear strongly in these spectra, indicating that the 3pz states have a longer lifetime with respect to radiationless transitions or dissociation.
Analysis of the spin–Hamiltonian parameters for Mn2+ in orthorhombic and axial symmetry sites of the alkali chlorides. Binding energies of the Mn2+ cation vacancy complexes
Authors: Oseguera V. U. , Murrieta S. H. , Medrano P. C. , Rubio O. J. , Ruíz-Mejía C.
Published: February 2003

The superposition model is used to analyze for the first time the second-order spin–Hamiltonian parameters b02 and b22 for the Mn2+ ion in orthorhombic C2v symmetry sites of the alkali chlorides LiCl, NaCl, KCl, and RbCl. Values for the intrinsic parameter ?2 and for the power-law exponent t2 are reported for the divalent manganese ion surrounded by chlorine ligands. In order to test the applicability of the superposition model a calculation was made of the quadrupole b02 term describing the 6S5/2 ground state splitting of the Mn2+ ion in axial symmetry sites of the alkali chlorides, using the intrinsic parameters ?2 and the power-law exponent t2 previously obtained from the analysis of the spin–Hamiltonian parameters corresponding to the orthorhombic symmetry site. In order to carry out the analysis mentioned above, a theoretical calculation was made of the lattice distortion around the impurity ion caused by its substitution for the host alkali ion in all the alkali halides. Values for the binding energy of the ground and first excited states of the impurity Mn2+ -cation vacancy pair are also reported.
Vibrationally inelastic integral cross sections for the scattering of He from H2. II. Transitions to the ν=2 level
Authors: Lin C. S.
Published: February 2003

Integral cross sections for the scattering of He from H2 on the Gordon–Secrest potential are reported. Cross sections for excitations to the second excited vibrational level ν=2 were calculated at 11 energies in the range of 2.5–3.0 hωe. Both types of transitions (0, j) →(2,j′) and (1,j)→(2,j′) are shown to have the characteristic structures previously reported for (0,j)→(1,j′). Its detail appears to depend on both Δν and the final vibrational quantum number. Analyses of the cross sections σ2j′←νj and σ1j′←0j where ν?1;j,j′? 4 bring out several interesting observations in the dynamics of collisional excitation.
Chemical dynamics of the reactions of O(1D2) with saturated hydrocarbons
Authors: Luntz A. C.
Published: February 2003

Nascent OH internal state distributions produced in the reactions of O(1D2) with saturated hydrocarbons have been measured by combining laser photolysis initiation of the reactions with laser induced fluorescence detection of the OH product. The OH rotational distributions are bimodal. One component corresponds to a broad distribution of high rotational states characterized by a linear surprisal. The other corresponds to population of only the lowest few OH rotational states. These are interpreted as due to insertion and abstraction, respectively. The insertion component dominates for small hydrocarbons (CH4, C2H6), while the abstraction component is the dominant mechanism for production of OH from larger hydrocarbons [C3H8, C(CH3)4]. The large vibrational and rotational surprisals for the insertion component imply that OH is produced by a prompt, non-RRKM decay of the alcohol collision complex. For the smaller hydrocarbons, insertion preferentially populates the lower Λ-doublet component of OH. Since the rotational state distribution for the abstraction component is quite similar to that observed in the reactions of O(3P) with saturated hydrocarbons, it is suggested that abstraction results from the 1A′ to 3A″ surface crossing in the entrance channel of the insertion path.
Multiphoton absorption of HF laser photons by molecules containing a hydroxyl group
Authors: McAlpine Robert D. , Evans D. K. , McClusky F. K.
Published: February 2003

Multiphoton absorption (MPA) of HF laser radiation has been studied, as a function of pressure (15 Pa to 1.3 kPa) and fluence (2 mJ/cm2 to 75 J/cm2) for the series: water, methanol, methan-d3-ol, ethanol, and 2,2,2-trifluoroethanol. As the group attached to the –OH is made more complex, the quasicontinuum occurs after fewer excitation steps, and under ’’collisionless’’ conditions, the same degree of multiphoton excitation is found to require a lower fluence. For water, at pressures between 73 Pa and 1.3 kPa, the cross sections are considerably lower than those for the other molecules, and MPA requires fluences in excess of ∼75 J/cm2. The remaining molecules divide into two groups, the ’’small’’ molecules (CH3OH and CD3OH) and the ’’large’’ molecules (C2H5OH and CF3CH2OH). For the small molecules at low pressures, the cross sections decrease with increasing fluence, an effect which is thought to be due to anharmonic bottlenecking. As pressure increases, the fluence dependence of the cross sections disappear. For the large molecules, anharmonic bottlenecking appears to be reduced, due to the greater density of states, and cross sections increase with increasing fluence according to the empirical form: σ(E, P)=K′Eb′Pa (where P is pressure, E is fluence and b′, a, and K′ are constants). The facility of HF laser-induced collisionless multiphoton dissociation of the –OH containing molecules is discussed in light of these results.
Collisional narrowing by polyatomic buffer gases in an optically pumped CH3F laser
Authors: Lawandy Nabil M. , Koepf Gerhard A.
Published: February 2003

The gain linewidth of an optically pumped CH3F molecular laser is observed to narrow with the addition of various polyatomic buffer gases. This is interpreted as collisional (Dicke) narrowing. The measurement is the first observation of collisional narrowing by polyatomic buffer gases. It is also the first observation of the effect in a laser oscillator. The effect was observed using a heterodyne mixing technique at the laser emission frequency of 604 GHz. Collision cross sections for SF6–CH3F and CS2–CH3F are obtained.
Angular momentum coupling schemes in the quantum mechanical treatment of P-state atom collisions
Authors: Aquilanti Vincenzo , Grossi Gaia
Published: February 2003

For the multichannel Schrödinger equations which arise in the quantum mechanical close coupling treatment of atomic collisions involving fine structure effects, alternative representations are developed by angular momentum algebra. The various representations are closely related to Hund’s coupling schemes for rotating diatomic molecules. Matrix elements for the electrostatic interaction and for the orthogonal transformations which connect the various representations, are given explicitly for the case when only one atom has internal angular momenta and follows LS coupling. The limit of large angular momenta, of interest under semiclassical conditions, is also considered. Some examples of applications to P atom collisions are discussed.
Resonant four wave mixing in molecular crystals
Authors: Hochstrasser R. M. , Meredith G. R. , Trommsdorff H. P.
Published: February 2003

Experimental studies are presented of the Raman and two-photon resonant effects in the third order susceptibility for benzene, naphthalene, and biphenyl crystals at 1.6 °K. The experiments consist of measurements of the polarized coherent light dispersion at ω3=2ω1−ω2 when the crystals are irradiated with two tunable lasers at ω1 and ω2. The frequencies ω1 and ω2 are chosen such that ω1−ω2 and 2ω1 match vibrational and electronic resonances, respectively, of the materials. The four wave mixing results obtained under definite polarization conditions are used in association with Raman scattering cross sections to find values for the nonresonant background third-order susceptibilities of the crystals and the two-photon absorption coefficients of various vibronic transitions. In addition the large dynamic range of these experiments has allowed us to obtain homogeneous (Lorenzian) damping parameters (Γ) for a number of vibrational levels of the electronic ground and excited states. In terms of the trace (αt2) and the anisotropy (βt) the following results were obtained: Benzene 154501, αt2 =8.6×10−51 (cm6 mol−1), βt2?0, Γ=0.7 cm−1; naphthalene 154201, αt2=2.8×10−50, βt2 =4.1×10−50, Γ=5.5 cm−1; biphenyl B3g←Ag, 0–0, βt2=3.6×10−49, Γ=0.9 cm−1. These calibration points can be utilized to obtain the absolute strengths of each of the many two-photon vibronic transitions observed previously in the two-photon fluorescence of these crystals. The damping parameters yield vibrational relaxation times in the range 0.4 ps for naphthalene 1542, to longer than 12.5 ps for the naphthalene ground state mode at 1383 cm−1. It is proposed that 1542 relaxes by fission into an electronic and vibrational exciton.
Infrared spectrum of formaldehyde in solid nitrogen. I. Monomer absorption
Authors: Nelander Bengt
Published: February 2003

Infrared spectra of matrices containing H2CO, HDCO, or D2CO have been studied in the 5–25 K interval. The results give evidence for the existence of a temperature dependent equilibrium between two trapping sites and for the existence of a C2-axial hindered rotation of formaldehyde.
Fermi resonances in solid N-methyl-acetamide
Authors: Dellepiane G. , Abbate S. , Bosi P. , Zerbi G.
Published: February 2003

The analysis of the low temperature vibrational spectrum of N-methyl acetamide recorded in our laboratory, has shown new features in the amide I–amide II and in the N–H stretching regions. In this paper an interpretation of the experimental data in terms of Fermi resonances is reported. For this purpose a theoretical treatment has been worked out and formulas are presented which allow the calculation of both intensities and frequencies in terms of anharmonic coupling constants. A satisfactory agreement between calculated and experimental data has been found.
Infrared spectra of hydrogen isotopes in potassium tantalate
Authors: Engstrom Herbert , Bates J. B. , Boatner L. A.
Published: February 2003

The spectroscopic constants of hydroxyl ions formed from hydrogen, deuterium, and tritium in a potassium tantalate (KTaO3) host have been determined. These isotopes were found to diffuse readily either into or out of KTaO3 single crystals. At T=77 K the infrared absorption band due to the OH−, OD−, or OT− stretching vibration is extremely narrow (i.e., ∼1.0, 0.8, and 0.7 cm−1, respectively). These results indicate that the interaction between the hydroxyl ions and the surrounding lattice is very weak. The optical properties of hydroxyl ions and the characteristic features of hydrogen diffusion in KTaO3 are compared with those determined previously for aluminum oxide (Al2O3) and rutile (TiO2).
Specific heats of the pyrochlore compounds Y2Mo2O7 and Y2Ru2O7
Authors: Blacklock K. , White H. W. , Gürmen E.
Published: February 2003

The specific heats of the pyrochlore compounds Y2Mo2O7 were measured below 20 K. The Y2Ru2O7 data were found to be well described by the expression C=γT+αT3 below 7 K with a γ value of 2.92±0.1 mJ/mole K2 and an α value of 0.236±0.015 mJ/mole K4. A correct description for the Y2Mo2O7 data was not obvious since there was, in addition to γT+αT3 contributions, a Schottky-like term with a maximum near 12 K plus a smaller contribution near 3.5 K of unknown origin. The γ and α values of 1.33±1 mJ/mole K2 and 0.973±0.06 mJ/mole K4, respectively, should be used with caution. Neutron diffraction data were taken at 300, 77, 13, and 4.2 K on Y2Mo2O7. The results indicated that no transition to an antiferromagnetically ordered state occurred down to 4.2 K.
Some remarks on the cluster expansion of macromolecules
Authors: Bruns Wolfgang
Published: February 2003

The perturbation expansion α2=1+C1z−C2z2+C3z3−+⋅⋅⋅ has originally been derived in the limit of a very large number of bonds (N→∞). Problems are discussed which arise when this formula is used for finite N. For this case it is shown that the coefficients of the expansion are no longer constant but are influenced by the kind of potential.
Rotational dynamics of rigid, symmetric top macromolecules. Application to circular cylinders
Authors: Tirado María M. , de la Torre José García
Published: February 2003

The formalism for the calculation of translational friction coefficients of symmetric top macromolecules presented in a previous paper [M. M. Tirado and J. García de la Torre, J. Chem. Phys. 71, 2585 (1979)] is here extended to the evaluation of rotational friction and diffusion coefficients. We show how the introduction of symmetry considerations leads to a great reduction of the computational requirements needed to solve the hydrodynamic interaction equations. We also obtain the translation–rotation coupling tensor from which the center of hydrodynamic stress can be obtained. For a rigid ring we have derived analytical solutions that reduce to those obtained by other authors when the unmodified interaction tensor is used. The general formalism is finally applied to the calculation of the rotational diffusion coefficients of circular cylinders modeled as stacks of rings and extrapolated to zero bead size. The resulting values are critically compared with those from earlier studies.
A flow tube investigation of the temperature dependence of the chemi-ionization reaction between antimony pentafluoride and benzyl chloride
Authors: Auerbach A.
Published: February 2003

The temperature dependence of the reaction between antimony penfluoride and benzyl chloride has been investigated using flow tube methods. Under the assumption that a single species of (SbF5)n is involved in the reaction, an activation energy of 22±2 kcal mol−1 is obtained. The bimolecular rate constant is estimated to be 10−16 cd molecule−1 sec−1 at 500 °K, and the reaction section 10−5 Å2. (AIP)
Molecular reorientation in liquid CF4
Authors: Garg S. K. , Ripmeester J. A. , Davidson D. W.
Published: February 2003

New values of the spin–rotation interaction constants for CF4 are used to obtain improved values of angular mometnum correlation times from 19F NMR spin–lattice relaxation times in liquid and gaseous CF4. Within limitations mainly imposed by uncertainties in the orientational correlation times derived from Raman line shape analysis, the J-diffusion limit of McClung’s extended diffusion model for spherical top molecules is applicable over a density range which extends from the liquid at the melting point through the critical point to the dilute gas.
Comments on Ohwada’s pairwise interaction model for polyatomic force fields
Authors: Anderson Alfred B. , Ohwada Ken
Published: February 2003

It is pointed out that the molecular electronic charge definition as a sum of rigid atoms and a flexible additional part by Anderson and Parr is neither an assumption nor an approximation as stated by Ohwada. Exact molecular energy formulas follow from this definition, and Ohwada’s parameterized pariwise force field attempts to fit both the pairwise energy component due to atom superposition and the nonpairwise part due to electron delocalization in polyatomic molecules.
Heat capacities of H2O+H2O2, and H2O+N2H4, binary solutions: Isolation of a singular component for C
p of supercooled water
Authors: Oguni M. , Angell C. A.
Published: February 2003

In an effort to examine further the suggestion that anomalies in the properties of supercooled water reflect the approach to a thermodynamic singularity in the vicinity of −45 °C, an attempt has been made to separate the total heat capacity of water into ’’anomalous’’ and ’’normal’’ components. The basis for such a separation used here is extrapolation to zero solute content of Cp of binary aqueous solutions from which the low temperature anomalies have disappeared. We have studied two closely related systems based on the second components H2O2 and N2H4. The low temperature molar Cp isotherms are very unusual. The extrapolations to zero solute of Cp isotherms in the two systems yield similar but not identical ’’normal’’ components for pure water. The corresponding alternative sets of ’’anomalous’’ Cp components vs temperature data, when best-fitted to the singular point equation Cp(anom)=A (T/Ts−1)−γ, yield within one degree, the same Ts value, 226 K, and exponents either side of the classical value of unity. When the value γ=1.0 is assigned to the anomalous components of the dilute aqueous H2O2 solutions, the best fit Ts values accurately mirror the behavior of the solution homogeneous nucleation temperatures.
Valence bond diatomics-in-molecules (DIM) treatment of collinear interactions of Group IIa and IIb metal atoms with hydrogen halides: Application to the CaHCl system
Authors: Isaacson Alan D. , Muckerman James T.
Published: February 2003

A recently developed formulation of the semiempirical valence bond diatomics-in-molecules (DIM) method is applied to the study of collinear MHX systems, where M is a Group IIa or IIb metal atom and X is a halogen atom. In particular, ground and excited state potential energy surfaces for reactions involving the three collinear arrangements of Ca, H, and Cl are considered. Explicit construction of the DIM Hamiltonian matrix is discussed. The resulting ground state surface for the collinear reaction Ca+ClH→CaCl+H, which exhibits a barrier of 34 kcal/mole at a rather stretched nuclear geometry, is shown to be in qualitative agreement with an ab initio surface for the analogous collinear Be+FH system. In addition, a description of the ’’harpooning’’ mechanism in the collinear reactions CaH+Cl→Ca+HCl and Cl+CaH→ClCa+H arises naturally from the calculations. Reaction pathways and reactant-to-product correlation diagrams are also discussed. A simple estimation of the sensitivity of the most prominent features of the calculated potential energy surfaces to the input diatomic fragment data demonstrates that these features cannot be attributed to errors made in those data.
Localized atomic orbitals for atoms in molecules. II. Diatomic molecules
Authors: Aufderheide Keith H. , Chung-Phillips Alice
Published: February 2003

Using a previously described method, localized atomic orbitals (LAOs) for atoms in molecules are found for the atoms Li, B, C, N, O, and F in the diatomic molecules LiH, Li2, LiF, BH, B2, BF, C2, CO, NH, N2, and F2. In all instances LAOs partition into sets of core, lone pair, and bonding orbitals which, except in C2, are arranged according to the hybridization schemes suggested by the localized molecular orbital (LMO) analyses of Edmiston and Ruedenberg. C2 is of special importance as the LMO result for this molecule is unorthodox and difficult to interpret whereas the LAO description is more lucid and enlightening. Generally, core LAOs are doubly occupied and lone pair LAOs are either unoccupied (Li) or doubly occupied. Bonding is usually described principally as the interaction of bonding LAOs on the adjacent, bonded atoms. Exceptions to these generalities are found for C2 (not a Lewis compound) and NH (considered in an excited state). The composition of core LAOs for atoms in molecules is found to be a function primarily of the atom itself and is rather insensitive to chemical environment of the atom. On the other hand, composition of the valence LAOs is found to vary in a meaningful manner reflecting changes in both the number and orbital arrangement of valence electrons.
Potential energy and dipole moment surfaces for simultaneous torsion and pyramidalization of ethylene in its lowest-lying singlet excited states: A CI study of the sudden polarization effect
Authors: Buenker Robert J. , Bonačić-Koutecký Vlasta , Pogliani Lionello
Published: February 2003

A b initio CI calculations have been carried out in a double-zeta AO basis for the simultaneous torsion and pyramidalization energy and dipole moment surfaces of the two lowest (V and Z) singlet excited states of ethylene. In nonpyramidalized geometries these two electronic states are found to undergo a potential crossing for a twisting angle of ϑ=82 °, with the Z(1A1) species being the more stable for the perpendicular D2d conformation. The dipole moments of these states are found to increase quite rapidly with pyramidalization for the entire range of twisting angle from ϑ=75 ° to ϑ=90 °, but this effect is found to reach a definite maximum in the neighborhood of the 82 ° crossing region for the unpyramidalized species and it is argued that these two phenomena are in fact closely related to one another. The CI results are found to be strongly dependent on the choice of one-electron basis in the significant portion of key structural regions and it is concluded that the use of natural orbitals optimized for one of the two nearly degenerate singlet states leads to excessively ionic charge distributions. Finally, the minimum-to-minimum energy difference (Te) between the ground and lowest singlet excited state is calculated to be 5.83 eV, suggesting a corresponding T0 value for this transition of 5.6–5.7 eV which is in very good agreement with McDiarmid’s recent experimental determination of this quantity.
Refined a
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o calculation of the potential energy surface of the He–H2 interaction with special emphasis to the region of the van der Waals minimum
Authors: Meyer Wilfried , Hariharan Puthugraman C. , Kutzelnigg Werner
Published: February 2003

A highly accurate ab initio surface for the interaction potential of the system HeH2 is computed. The method applied is essentially of CI type, but different calculations with different basis sets are performed for (1) the SCF and intrasystem correlations and (2) the intersystem correlation. The former is corrected for basis superposition errors (counterpoise method) and the latter is corrected by inclusion (or simulation) of triply substituted configuration. For either calculation basis saturation tests are performed, a criterion for the interbasis being that it accounts correctly for the van der Waals constants C6,C8 and C10 and their anisotropies on three successive levels of sophistication. The calculations cover the range from R=1.5a0 (0.8 Å) to R=∞ for the He–H2 distance, and r=0.9 a0 to 2.0 a0 for the H–H distance and the orientation angles 0 °, 45 °, 90 °.
The effect of a pair of vacancy defects on the free energy of harmonic crystals: Cell cluster method
Authors: Garza Cesar M. , Huckaby Dale A.
Published: February 2003

The change in the high-temperature Helmholtz free energy of harmonic crystals due to the interaction of a pair of vacancy defects was calculated by the cell cluster method. Third order results were obtained for the face-centered cubic lattice, and fourth order results were obtained for the two-dimensional triangular lattice.
The even–odd effect in liquid crystals: A collective or intrinsic molecular property
Authors: Lalanne J. R. , Lemaire B. , Rouch J. , Vaucamps C. , Proutiere A.
Published: February 2003

We have made the first observation that nematogens and smectogens exhibit physical properties characterized by even–odd alternation with the number n of carbon atoms in their alkyl tails, even when molecules are isolated. For the cyano-biphenyl series, when n?5, the alkyl tail is found to be all in the trans configuration. When n≳5, gauche isomers appear and an important even–odd effect is experimentally evidenced and discussed.
Nuclear quadruple parameters of vanadium in VB
Authors: Creel R. B. , von Meerwall E.
Published: February 2003

Using computer simulations of the quadruple-perturbed NMR powder pattern of 51VB, we confirm that the spectrum represents νQ≃125 kHz and η=0.95±0.03 rather than νQ≃250 kHz and η≃0, and we obtain refined estimates of the magnetic shift parameters.
Differential scattering of spin±1/2 polarized neutrons by chiral systems. III. Coupled oscillator model
Authors: Cox J. Neal , Richardson Frederick S.
Published: February 2003

The general theory of polarized neutron scattering by nonmagnetic chiral systems (see Ref. 1) is applied to a specific model system comprised of vibrating nuclei. The model scattering system is made up of two ’’spinless’’ nuclei constrained by external (crystal or molecular) force fields to vibrate as linear (harmonic) oscillators along mutually orthogonal, laboratory-fixed directions. Interactions between the two vibrating nuclei are introduced so that the dynamics of the two linear oscillators are coupled. This coupled oscillator system is constructed to reflect the chirality of the local force fields in the scattering medium. The vibrational states of the chiral coupled oscillator system lack parity, and it is shown that left-handed and right-handed polarized neutrons are scattered differentially by vibrational excitations (and de-excitations) within the system. Calculations are performed to estimate the magnitude of this differential polarized neutron scattering, and the detectability of this phenomenon is assessed.
Evaluation of thermal boundary layer interaction in shock tube sampling for kinetic studies
Authors: Voldner Eva C. , Trass Olev
Published: February 2003

In shock tube studies of reaction kinetics the gas to be analyzed is often leaked through a pin hole at the tube end wall into a mass spectrometer. Such samples may be influenced by the growing thermal boundary layer (Tbl) at the cold end wall. In order to assess the effect of this cooling on kinetic measurements, the temperature history of the gas sample reaching the analyzer must be known. This paper allows, based on independent orifice flow and Tbl solutions and their limited interaction, an evaluation of the extent of gas cooling. It shows that the simple flow hemisphere–Tbl overlay greatly overestimates cooling, that Tbl growth is retarded in the vicinity of the orifice, and that center-core sampling with a skimmer is strongly favored. It provides the kineticist with the basic tools required to assess these effects and gives some illustrative kinetic examples.
A stochastic-collision complex model theory of chemical reactions
Authors: Billing Gert D. , Eu Byung C. , Garisto-Zaritsky Nava , Nyeland Carl
Published: February 2003

In this paper a theory of chemical reactions is developed which combines stochastic theory and the Franck–Condon transition model. The theory assumes that collision complexes are formed and broken up in a manner similar to the Franck–Condon transition and the collision complexes evolve in time according to the Fokker–Planck equation derived previously for inelastic scattering. The model can be shown to include the RRKM theory as a special case. An illustrative calculation is performed with the example of the H+Cl2 reaction. The result is in qualitative agreement with the result by a numerical solution method.
Reaction of methyl radicals with O(3P), O2 and NO
Authors: Washida N.
Published: February 2003

The reactions of methyl radicals with atomic and molecular oxygen and nitric oxide were studied using a photoionization mass spectrometer. The methyl radicals were generated by the reaction of oxygen atoms with ethylene in a fast-flow reactor. In the measurement of the rate constant for the reaction of methyl radicals with oxygen atoms, the approach of the methyl radicals to their steady state concentration was measured for initial concentrations of ethylene of 0.1–1 mTorr. With less than 0.2 mTorr of ethylene, a rate constant of (1.38±0.46)×10−10 cm3 molecule−1 sec−1 was obtained. The rate constant for the reaction of methyl radicals with molecular oxygen was measured by two methods; Stern–Volmer plots and the steady-state approach method. Both results showed the existence of intercepts when the measured effective rate was extrapolated to zero pressure. The observed ratio of rates of the CH3+O2 to the CH3+O reaction was in agreement with the previous measurement by Washida and Bayes. The reaction of methyl radicals with nitric oxide showed no evidence of a two body process. The three body rate constant of this reaction was (2.4±0.8)×10−30 cm6 molecule−2 sec−1 with M=He.
A pulse radiolysis study of electron tunneling in an 8 M NaOH glass between 4 and 100 K
Authors: van Leeuwen Johan W. , Heijman Maritza G. J. , Nauta Henk , Casteleijn Gerard
Published: February 2003

Pulse radiolysis experiments have been performed in an 8 M NaOH glass between 4 and 100 K and in the time range 10−6 to 10 s. The spur radius was estimated from the decay of the trapped electrons on the red side of the absorption maximum. The spur radius was about 4 nm at 80 K and increased to about 6 nm at 10 K. We studied the decay of trapped electrons in the presence of the following electron scavengers: CrO2−4, Fe(CN)3−6, and NO−2. No temperature dependence was found between 10 and 100 K. To explain the results distance-dependent and/or time-dependent Franck–Condon factors have been introduced. We show that in the time range studied it is not possible to distinguish between direct tunneling and trap-to-trap tunneling. Dry electron scavenging efficiencies and encounter pair formation are expressed in S37. For CrO2−4, Fe(CN)3−6, and NO−2 we found that S37 was 45, 130, and 300 M, respectively.
Measurements of T2 in excited vibrational states of HCN
Authors: Charròn Melissa , Anderson Thomas G. , Steinfeld J. I.
Published: February 2003

Polarization relaxation times (T2) have been measured for the J=6, 9, and 10 l-doublet transitions in the (0110) vibrational level of HCN by means of microwave transient emission. Also, T2 values for relaxation induced by He, H2, and CH3F are obtained for the J=6 transition. The dephasing cross sections for foreign gas collisions are consistent with the force laws operative in each case. The HCN–H2 result provides a comparison with the recently measured cross section of the isoelectronic molecular ion system, HCO+–H2, and confirms the prediction of Green that rotational relaxation rates are only moderately faster for molecular ions.
Absolute measurements of diffusion coefficients by pulsed nuclear magnetic resonance
Authors: Hrovat Mirko I. , Wade Charles G.
Published: February 2003

NMR pulsed magnetic field gradient (?) methods to measure the self-diffusion coefficient rely upon an accurate measurement of ?. A technique has been developed which uses a deliberate mismatch between gradient pulses to obtain ? from the spin echo. This procedure provides an absolute measurement of self-diffusion coefficients in pulsed gradient experiments without the need for a reference sample.
Recoupling the angular momentum decoupling approximations
Authors: McLenithan Kelly , Secrest Don
Published: February 2003

A matrix Green operator is derived which significantly improves the coupled states approximation to scattering problems.(AIP)
Low temperature gas phase and matrix studies of the C6H3F3+ and C6H3Cl3+ radical cations
Authors: Bondybey V. E. , Sears T. J. , English J. H. , Miller Terry A.
Published: February 2003

Laser induced fluorescence excitation spectra of the radical cations C6H3F3+ and C6H3Cl3+ have been observed in the gas phase, but cooled to near liquid N2 temperature. This technique leads to greatly improved S/N and resolution compared to ambient temperature spectra. We have also observed electronic absorption spectra of these ions in a solid Ne matrix at ∼5°K. This spectrum, as well as the resolved emission spectrum, can be used to obtain accurate transition probabilities for various bands. The information obtained from both the low temperature gas phase and matrix experiments plays a critical role in determining the effects of Jahn–Teller distortion in the ground states of these ions.
Determination of S–O bond order in sulfur dioxide and dimethyl sulfite using a low-energy particle-accelerator technique
Authors: Powers D. , Olson H. G.
Published: February 2003

Energy loss from a beam of He+ ions due to inelastic collisions with vaporous sulfur dioxide and dimethyl sulfite has been measured. Application of the Bragg rule to these results suggests minimal d-orbital involvement in the S–O bonds for these compounds.
The effects of basis set quality and configuration mixing in a
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o calculations of the ionization potentials of the nitrogen molecule
Authors: Ermler W. C. , McLean A. D.
Published: February 2003

Systematic configuration interaction calculations of the adiabatic ionization potentials of N2(1Σg+) to the X 2Σg+, A 2Πu and B 2Σu+ states of N2+ give best results of 15.62, 16.41, and 18.93 eV compared with observed values of 15.576, 16.694, and 18.746 eV. The computed values are near the complete basis limit for a wave function containing single and double excitations from the Hartree–Fock configuration with the Langhoff–Davidson correction for unlinked cluster quadruple excitations. The critical dependence of ionization potentials on basis is demonstrated. Vertical ionization potentials at near the N2 ground state equilibrium internuclear separation are compared for a variety of basis sets using different computational methods. In particular, an important comparison is made with the EOM calculation of Herman, Yeager, Freed, and McKoy using a ’’double-zeta+polarization’’ basis identical to one used in this study. We conclude that agreement to within ∼0.2 eV can be expected for ionization potentials between good state of the art EOM and CI calculations. Potential curves computed from our best wave functions are tabulated since they should stand as reference data for some time; spectroscopic constants obtained from them are presented.
An a
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o study of the bonding in diatomic nickel
Authors: Noell J. Oakey , Newton Marshall D. , Hay P. Jeffrey , Martin Richard L. , Bobrowicz Frank W.
Published: February 2003

Hartree–Fock, GVB, and configuration interaction calculations were performed for diatomic nickel using an ab initio effective core potential. A basis set specifically optimized for the 3D state of atomic nickel is found to be far superior to the more common basis obtained from the 3F atomic state. Correlation effects are found to be significant in determining the bond energy. In particular, the two electrons of the s–s bond must be appropriately correlated. In addition, correlation effects which one would interpret as being principally intra-atomic in character are found to have a marked effect on the molecular properties. The theoretically predicted bond dissociation energy (De) of 43.4 kcal/mol is significantly lower than the experimental estimate of 55±5 kcal/mol. However, molecular partition functions calculated using the present results indicate that the experimental value should be revised downward to a value of ∼46±5 kcal/mol, in good agreement with our calculations. An interatomic distance of 4.27 bohr is computed and compared with experimental estimates. Spectroscopic parameters for dipole-allowed transitions from the ground state were determined from SCF and GVB calculations and discussed in relation to the experimentally observed visible and ultraviolet spectra attributed to Ni2.