bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular reorientation in p-fluoranil as studied by 19F NMR and potential profile calculations
Authors: Ripmeester J. A. , Boyd R. K.
Published: February 2003

The temperature dependence of the 19F NMR second moments and spin–lattice relaxation times studied between 200 and 440 K reveal the presence of large amplitude molecular reorientation in solid p-fluoranil. The lack of disorder in the crystal structure and the apparent motion invariance of the 19F chemical shielding tensor require the motion to be a 180° flip. The reasonableness of such a motional model was confirmed by potential profile calculations using a Lennard-Jones potential function with parameters adjusted to give approximate agreement with the experimentally derived value for the reorientational activation energy of 8.5±0.4 kcal/mole.
Softness expansion of gaseous transport properties. I. Dilute gases
Authors: Rainwater J. C.
Published: February 2003

Transport properties of dilute gases are studied both analytically and numerically for an inverse power interparticle potential. The mathematical dependence of transport properties on the softness parameter x=n−1 is emphasized, and nonanalytic dependences in x, of the form x2 lnx, are found. The derivation uses Gislason’s series expansion for the scattering angle, and confirms and extends a previous result of Baroody. Equilibrium properties are also shown to be nonanalytic in x, but there the nonanalytic contributions are normally insignificant. The results cast doubt on the utility of effective hard-sphere diameter methods for transport theory. In addition, new insight is gained into classical scattering by inverse power potentials.
Determination of rotational relaxation parameters for OCS in H2, CO2, and CH3F by linewidth measurements
Authors: Thrash P. M. , Schwendeman R. H.
Published: February 2003

Absorption of microwave radiation as a function of frequency has been measured under conditions of low and moderate incident power for dilute gaseous samples of OCS in H2, CO2, and CH3F. The line shape of the J=2←1 transition near 24 326 MHz was studied for each of the samples. From the low-power line shapes, values of the linewidth parameters Δν/p were obtained as follows: OCS/H2, 6.07±0.10 MHz/Torr; OSC/CO2, 5.26±0.10 MHz/Torr; and OCS/CH3F, 11.45±0.20 MHz/Torr. The power-broadened line shapes were analyzed in two ways. By fitting the lines to a superposition of power-broadened M components, values of (T1/T2)0=1.05, 1.03, and 0.97, all ±0.10, were obtained for OCS in H2, CO2, and CH3F, respectively. By fitting the line shapes to a single Lorentzian, corresponding values of qT1/T2=1.32, 1.30, and 1.23, all ±0.13, were obtained.
Ultrasonic measurement of the vibrational relaxation in liquid SO2
Authors: Fenner D. B. , Bowen D. E. , Eastman M. P.
Published: February 2003

Measurements of the absorption and velocity of ultrasound in liquid sulfur dioxide have been made over a wide range of temperatures, greatly extending the temperature range of the measurements of Bass and Lamb on the same system. The data are complete now from 203 to 323 K in the frequency range from 6 to 90 MHz. All the data have been fit satisfactorily to a single-relaxation description, and give a temperature-independent relaxation frequency of 22.5±1 MHz. The relaxation amplitude has a strong temperature dependence of the Arrhenius type, with an energy of activation of ∼3 kcal/mole, the temperature behavior of which can be satisfactorily calculated on the basis of reported vibrational frequencies of the SO2 molecule. The velocity of sound, as measured, is a linear function of the temperature. Measurements are also reported for dilute mixtures of cyclohexane in SO2 where no shift in the measured relaxation frequency is observed, contrary to the usual situation for vibrational relaxations.
HTFFR kinetics studies of the Al/SO2 reaction from 700 to 1600 K. Implications for D(Al–O)
Authors: Fontijn Arthur , Felder William
Published: February 2003

Measurements of the kinetics of the bimolecular Al/SO2 reaction over the temperature range 700 to 1600 K have yielded a k (T) = (1.6±0.8) ×10−10 exp[(=2000±300)/T]cm3 molecule−1  s−1. This k (T is significantly larger than that for the Al/CO2 reaction in the same temperature range. This temperature dependence indicates with 95% probability that the Al–O dissociation energy ?126 kcal mol−1, somewhat higher than most often thought. AlO laser fluorescence has been used to identify AlO+SO as a major, and probably the only, product channel. A k of 2×10−12 cm3 molecule−1 s−1 is measured for the AlO/SO2 reaction near 940 K.
Mechanism of thermal electron attachment in O2–C2H4, O2–CO2, and O2–neopentane mixtures
Authors: Kokaku Yu-ichi , Hatano Yoshihiko , Shimamori Hiroshi , Fessenden Richard W.
Published: February 2003

Thermal electron attachment of O2 in O2–C2H4, O2–CO2, and O2–neopentane mixtures has been investigated at room temperature, using a microwave conductivity technique combined with pulse radiolysis. The measurements have been extended to a higher pressure region (∼850 Torr) than previous observations in order to compare them with the results of an electron swarm method at very high pressures. From low pressure data, the values of (2.0±0.3) ×10−30, (3.2±0.3) ×10−30, and (7±1) ×10−30 cm6/molecule2 sec are determined for the overall three body attachment rate constants of O2 with the stabilizing partners C2H4, CO2, and neopentane, respectively. In each case, the effective rate constant continued to increase with increased density and exceeded those predicted by the Bloch–Bradbury mechanism by sizeable amounts. The excess attachment is suggested to involve pre-existing van der Waals complexes such as (O2⋅C2H4). Some quantitative conclusions which follow from this mechanism are given.
Studies of H+, H+2, and H+3 dissociative ionization fragments from methane, ethane, methanol, ethanol, and some deuterated methanols using electron-impact excitation and a time-of-flight method incorporating mass analysis
Authors: Burrows M. D. , Ryan S. R. , Lamb W. E. , McIntyre L. C.
Published: February 2003

Kinetic energy distributions and relative intensities of energetic (≳1eV) H+,H+2, and H+3 dissociative–ionization fragments from methane, ethane, ethanol, and methanol have been measured using pulsed electron-impact excitation and a time-of-fight method incorporating a new mass analysis technique. The various deuterated analogs of H+, H+2, and H+3 fragments were also observed from CH3OD, CD3OH, and CD3OD. Results for these deuterated fragments show that energetic H+2 and H+3 fragments originate primarily from the −CH3 group, rather than from H atom extraction from both the−CH3 and −OH groups. Comparisons among the kinetic energy distributions of these light fragments from CH3OH, CD3OD, and CD3OH reveal no major isotope effects for fragment energies exceeding 2 eV.
A treatment of open shells
Authors: Bouscasse L. , Jaffé H. H.
Published: February 2003

The minimization conditions for open shells in RHF theory are obtained without use of LaGrange multipliers. The formulation given for these conditions permits an easy way of defining coupling operations and is particularly adapted to a given problem.
The singlet and triplet state rotational potential surfaces for dihydroxycarbene
Authors: Feller David , Borden Weston Thatcher , Davidson Ernest R.
Published: February 2003

Potential surfaces have been computed for rotation of the OH groups in C(OH)2 for both the lowest singlet and triplet state. The minima on the singlet surface occur at planar geometries, which represent maxima on the triplet surface. This result is interpreted in terms of different modes of pi electron donation from oxygen being favored in the two states. Evidence is presented that shows pi electron donation is chiefly responsible for making dihydroxycarbene a ground state singlet.
On first-row diatomic molecules and local density models
Authors: Dunlap B. I. , Connolly J. W. D. , Sabin J. R.
Published: February 2003

The total Xα energy accurate to 0.3 eV is computed for H2, B2, C2, N2, O2, CO, and F2. Relative to experiment, the Xα model (α=0.7) is accurate to within ΔRe=0.1 bohr, ΔDe=2 eV, and Δωe=300 cm−1 for these molecules. Except for the lightest first-row diatomic molecules, the Xα and experimental dissociation energies are bracketed by those of the Hartree–Fock model (from below) and the Local Spin Density model (from above).
Flexible vs rigid constraints in statistical mechanics
Authors: Helfand Eugene
Published: February 2003

Rigid constraints fix the value of some degrees of freedom, such as bond lengths or angles, in the allowed motion of a system. ’’Flexible constraints’’ is a term which describes the limit as the amplitudes of vibration in these coordinates are made to approach zero by infinitely increasing appropriate force constants. The statistical properties of flexibly and rigidly constrained systems are, in general, different. How the difference arises is illustrated in terms of a simple model. A suggestion of Fixman on how to compensate for the difference in terms of an effective potential is rederived for the statics of the model. This paper examines dynamical problems more deeply. The quantum mechanical case is also discussed.
A comparison of the vibrational structures of ethanol, acetic acid, and acetaldehyde adsorbed on alumina
Authors: Evans H. E. , Weinberg W. H.
Published: February 2003

The adsorption on aluminum oxide of acetaldehyde (CH3CHO) and acetic acid (CH3COOH) has been studied by utilizing inelastic electron tunneling spectroscopy (IETS) to probe the vibrational structures of the surface species over the spectral range between 240 and 4000 cm−1. Results for temperatures between 295 and 475 K are compared with our previous IETS results for ethanol (CH3CH2OH) adsorption on alumina up to 575 K. Although all three hydrocarbons form acetate complexes with the surface under certain conditions, slight differences in bonding and structure are evident in the IET spectra. The formation of some bulk aluminum hydroxide is observed also in certain cases. Observed variations in surface concentrations, bulk hydration, and the nature of surface hydroxyl groups are related to previous experimental results. Mechanisms are substantiated for the initial adsorption and subsequent reaction of each hydrocarbon with the surface.
Vibrational population relaxation of compressed nH2 fluid in the 15–110 K range
Authors: Chateau M. , Delalande C. , Frey R. , Gale G. M. , Pradère F.
Published: February 2003

Experimental observations of the population relaxation time of the first excited vibrational level of nH2 have been made in the fluid state over a very wide range of temperature and pressure (15–110 K, 0–500 atm). These extensive measurements yield new insight into the fundamental parameters governing energy relaxation in simple fluids.
High pressure studies of poly (N-vinylcarbazole) excimer emission in polymer films
Authors: Chryssomallis George , Drickamer H. G.
Published: February 2003

The excimer fluorescence properties of poly (N-vinylcarbazole) (PVCA) dissolved in a variety of polymer films were investigated. The existence of two interchain excimers emitting from frozen preformed sites in the film was observed. Large hydrostatic pressures were used in order to change the local environment of the two sites. Changes with pressure of the ratio of respective intensities are interpreted according to a hopping singlet exciton diffusion model with the two sites acting as competing traps. There are two distinct mechanisms, one predominant at low and the other at high pressures. The assumption that a small rearrangement between the two excimers is possible at low pressures is valid since both glass transition temperatures and overall intensity changes support it. The change of volume in the transformation between the two excimers was calculated for the different films and is equal to or less than 3% of the monomer molar volume.
The generalized moment method for transport phenomena in dense fluids
Authors: Eu Byung Chan
Published: February 2003

Based on the kinetic equation reported previously, the generalized moment method is presented for dense fluids in this paper. The equations of change are presented for stress tensor and heat flux in nonuniform dense fluids and by solving them up to second order by a perturbation method, constitutive relations and various linear and nonlinear transport coefficients are obtained. Comparison of the transport coefficients with the counterparts in the generalized Chapman–Enskog (GCE) method shows that two results agree with each other if the first Chapman–Enskog approximants are used for the GCE results and some potential energy contributions are neglected in the molecular expressions for fluxes. Discussions are given regarding the constitutive relation for the Stokes fluids and the non-Newtonian behavior of real fluids.
Comment on the negative ion states of fluorobenzenes
Authors: Jordan K. D. , Burrow P. D.
Published: February 2003

It is contended that several assignments given by the model employed by Frazier et al1 should be reconsidered especially of the anion states aniline phenol and anisole. The symmetry assignments of the anion states of fluorobenzene and p difluorobenzend as given by this model are incorrect. (AIP)
Comment in reply to K. D. Jordan and P. D. Burrow’s ’’Comment on the negative ion states of fluorobenzenes’’
Authors: Christophorou L. G. , Schweinler H. C.
Published: February 2003

The comments of K.D. Jordan and P. D. Burrows on the symmetry assignments of anion states of fluorobenzene is termed inadequate. Another assignment is suggested (AIP)
Tensorial factorization and rotationally inelastic collisions
Authors: Alexander Millard H.
Published: February 2003

Since the transition- or T-operator is a scalar it can be expanded as a sum of products of operators which transform as spherical tensors. Consequently the T matrix for rotationally inelastic atom–molecule collisions can be factored into products of reduced matrix elements in the internal (rotational) and relative (orbital) degrees of freedom. This basic factorization, which is independent of specific dynamical approximations, leads to generalized cross section scaling relations. In the sudden limit these reduce to the expressions derived earlier by Goldflam, Kouri, and Green [J. Chem. Phys. 67, 5661 (1977)]. From previously computed T matrices one can extract partial opacities corresponding to the various tensor orders which contribute. This is done for the case of Ar–N2 collisions. The spherical tensor factorization is extended to the more complex case of collisions between two diatomic molecules. Recent energy-gap models are discussed in light of the tensorial analysis developed here.
Photoabsorption cross sections of CH3F, CHF3, CH3Cl, and CF2Cl2 from 175 to 760 Å
Authors: Wu C. Y. Robert , Lee L. C. , Judge D. L.
Published: February 2003

Using synchrotron radiation as a continuum background, the absorption cross sections of CH3F, CHF3, CH3Cl, and CF2Cl2 were measured in the 175–760 Å region using a double ionization chamber. Discrete structure was observed in CHF3 and CH3Cl and attributed to the Rydberg states converging to the known ion states. In CHF3, Rydberg states convergin to the (3e)−1 and (5a1)−1 ion states with averaged vibrational frequency of 910 cm−1 were observed in the 610–640 Å region. Another Rydberg state converging to the (4a1)−1 ion state with an averaged vibrational frequency of 1080 cm−1 was also observed in the 530–540 Å region. In CH3Cl, a Rydberg state converging to the (3a1)−1 ion state with a vibrational frequency of 2330 cm−1 was observed in the 580–620 Å region. No vibrational structure was observed in CH3F and CF2Cl2.
Precise determination of differential electron scattering cross sections. I. The apparatus and the N2 results
Authors: Fink M. , Moore P. G. , Gregory D.
Published: February 2003

Total (elastic plus inelastic) differential cross sections of 30, 40, and 50 keV electrons incident on N2 have been measured in the angular range of 9–200 mrad. The accuracy achieved was about 0.1%. The apparatus is discussed in extensive detail. The data are used to determine the molecular structure parameters and then matched to the independent atom model composed of Hartree–Fock atoms. In a second comparison, configuration interaction atoms are utilized. The difference curve of s4(Iexpt−Itheo) is constructed and integrated in order to determine the molecular binding energy. The value varies from 17 eV to 10 eV depending on the matching procedure chosen. Since the cross sections were measured at various incident electron energies, the validity of the first Born approximation could be investigated. It was found that the Born theory holds to within the uncertainty in the data. Thus, recent discrepancies found in Compton profile measurements must be attributed to a failure of the binary encounter theory.
Precise determination of differential electron scattering cross sections. II. CH4, CO2, CF4
Authors: Fink M. , Schmiedekamp C. W. , Gregory D.
Published: February 2003

Precise total (elastic plus inelastic) differential cross sections have been measured for CH4, CO2, and CF4 with 50.4 keV incident electrons. The accuracies are 0.1% in the intensities and ±2 arc sec in the scattering angles. The structure parameters have been evaluated. The bond distances agree well with previous results, although the mean amplitudes of vibration are in general larger than those calculated from harmonic force fields. The difference curves comparing the experimental cross sections to IAM (independent atom model) predictions are discussed utilizing CI atomic potentials in the IAM. The ’’binding’’ energies have been determined and are found to be consistently several electron volts above those evaluated via the heat of combustion.
Stability analysis of steady states in a catalytic pellet
Authors: Ibáñez J. L.
Published: February 2003

Topological degree and Liapunov potential techniques are used to study the stability of the steady states found in an isothermal n-order chemical reaction within a catalytic pellet.
Regularities in nuclear–nuclear potential energy of molecules at equilibrium
Authors: Mucci J. F. , March N. H.
Published: February 2003

Regularities in the nuclear–nuclear potential energy Vnn for molecules at equilibrium are exposed. The motivation for the study is afforded by relations between the various energy terms which follow from the simplest form of density functional theory. The first of these relates Vnn to the total electronic kinetic energy T and the electron–electron potential energy Vee by Vnn/T−Vee/T=−1/3. A further relation also follows between Vnn, T, and the electron–nuclear potential energy Ven, namely 2Vnn/T+Ven/T=−7/3, equivalent to a relation suggested previously by Politzer. Attention is then focused on a density functional treatment of tetrahedral and octahedral molecules, in which the model is adopted of smearing the outer nuclei uniformly over a sphere. While the model is too crude to be quantitively useful in calculating the observed properties of these classes of molecules, it is first shown that the above scaling relations are exactly obeyed. Secondly, Vnn is related to T in the model and thirdly Vnn is displayed as a function of the total number of electrons and the charge on the central atom, always at equilibrium. Motivated by the above model, we have studied empirical results for Vnn for tetrahedral and octahedral molecules, and examined T/N, where N is total number of electrons. Also Vnn vs N shows regularities as anticipated from the model. Finally, for light molecules with N?24 we have used selected self-consistent field calculations to (a) verify the scaling relations and (b) demonstrate the relation between Vnn and T/N, and Vnn and N, as for the tetrahedral and octahedral molecules.
Dielectric behavior of n (p-octyloxybenzylidene) p-toluidine (OBT) in nematic mesophase at radio frequencies
Authors: Gupta G. K. , Agarwal V. K. , Bahadur B.
Published: February 2003

Dielectric permittivity and loss have been measured in the frequency range 0.5–3.5 MHz in nematic and isotropic phases of OBT. Measurements in the nematic region with a magnetic field greater than saturation value acting parallel to the rf electric field exhibit a Debye type dispersion of ε∥ with relaxation frequency 5.3 MHz at 71° C and activation energy of 27.0 kcal/mole. The results can be explained on the basis of the molecular rotation about a transverse axis occurring under hindering nematic potential. Measurements also indicate that OBT behaves as a positive dielectric material at low frequencies and a negative dielectric material at higher frequencies.
Submillimeter wave absorption in a homologous series of liquid crystals
Authors: Venugopalan S. , Prasad S. N.
Published: February 2003

The far-infrared absorption spectra of the first seven homologs (C1–C7) of the p,p′-di-n-alkoxyazoxy-benzene series of liquid crystals have been studied in the range 30–200 cm−1. Comparison of the different spectra in the isotropic phase shows that the intensity of absorption is maximum in C1. In C2 and C3, the intensity as also the peak position of the central band decrease as compared to C1. A broad and shallow absorption profile without any central band is observed in C4–C7. The various factors that can possibly contribute to the far-infrared absorption in the fluid phases of these mesogens are considered. It is found that the Poley absorption may be the dominant factor in C4–C7, but it can not account for the distinct changes associated with the central band in C1–C3. The observed decrease in the intensity and the shift in the peak position of this band on going from C1 to C2 strongly suggest that this band originates from the torsion of the end alkoxy groups around the phenyl-oxygen bonds. This assignment is also consistent with (i) the virtual disappearance of the central band in C4–C7 (ii) the dichroic behavior of the band in C1 in the homogeneously aligned nematic phase and (iii) the considerably reduced intensity of absorption in the liquid phase of the related, but nonmesomorphic compound p,p′-azoxytoluene. We conclude that in the composite absorption profiles due to a superposition of the Poley and torsional bands, the latter contribution becomes increasingly important for the lower homologs of this series.
A partial vibrational energy transfer map for cyclopropane
Authors: Fujimoto Gordon T. , Weitz Eric
Published: February 2003

Laser induced infrared fluorescence studies have been performed on cyclopropane. Molecules were excited from the ground state to the ν10 level of cyclopropane using a Q-switched CO2 laser. Fluorescence was observed from the ν6, ν8, ν9, ν11, ν10+ν11, and ν5+ν10 levels of cyclopropane. When analyzed as single exponentials, the rise-time rates of fluorescence from the ν9 and ν11 levels are 312±30 and 302±32 msec−1  Torr−1 respectively. The effect of rare gas collisions on the fluorescene rise-time of the ν9 and ν11 levels was also measured. A model is proposed to account for equulibrium processes among the ν9, ν10, ν11 levels. Fluorescence measurements from the ν6, ν8, ν10+ν11, and the ν5+ν10 levels exhibit a multiple exponential decay. The implications of this with respect to the V–V equilibration of the vibrational manifold are discussed. SSH ’’breathing sphere’’ calculations were performed for V–T/R and various V–V processes.
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o calculation of the first order interaction energy in excited dimers. The H2O–H2O and H2O–Ne dimers
Authors: van Hemert Marc , van der Avoird Ad
Published: February 2003

In order to interpret the solvation blue shift in the 1A1→1B1 UV band in H2O (observed in liquid water, ice, and H2O embedded in rare gas matrices) we have made ab initio calculations of the first order electrostatic and exchange interaction energies in H2O–H2O and H2O*–Ne dimers, after extending the usual symmetry adapted perturbation expressions to excited state molecules.We have found this blue shift to be caused mainly by the enlarged exchange repulsion between the excited H2O molecule and its neighbors, originating from the extended (Rydberg) character of the excited 1B1 state. The orientational dependence of this exchange repulsion has been calculated and correlated with the spatial distribution of the Rydberg state. The transition-dipole resonance interaction was found to be of little importance. These results were confirmed by supermolecule MO calculations on the ground state and excited dimers which showed moreover, an enlarged polarization of the excited H2O ( compared with the ground state), as well as some other effects that may be artifacts from the supermolecule treatment of excited dimers.
Raman study of N-nitrodimethylamine single crystal. Evidence for a structural phase transition
Authors: Rey-Lafon Madeleine
Published: February 2003

Low frequency Raman spectra of oriented single crystals of N-nitrodimethylamine and its perdeuterated derivative have been recorded using polarized light. The observed frequencies have been assigned to the different symmetry species of the unit-cell factor group. The study of the temperature dependence of the spectra has shown the occurrence of a structural phase transition. This later involves a very weak entropy of transition, about 0.04 cal deg−1 mole−1, and is accompanied by a soft mode, which implies a displacive character. The soft mode has been observed in the Raman spectrum of the low temperature phase only; the analysis of the spectra corresponding to the different elements of the polarizability tensor suggests the existence of a superstructure in the low temperature phase.
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o model calculations for pyridine, pyrazine, and their protonated forms
Authors: Butkus Aldona M. , Fink William H.
Published: February 2003

Application of an ab initio calculation for construction of molecular wave functions from pattern fragments is reported for pyridine, pyridinium, pyrazine, and the mono and diprotonated forms of pyrazine. The technique involves the transfer of entire pattern fragment wave functions after their initial generation in a manner such that the local environment of the final molecule is present. Generalization of the Mulliken population analysis to include contributions from Kekulé structures is discussed. Populations are reported for the molecules treated and compared to published ab initio results.
Transformation characteristics and orientation relations among the phases of solid oxygen
Authors: DeFotis Gary C.
Published: February 2003

Transmission Laue photographs of single crystal gamma and preferentially oriented polycrystalline beta and alpha oxygen samples are described and analyzed. Parent beta and daughter alpha oxygen patterns are observed to be remarkably similar, but also exhibit characteristic differences. Spot positions are essentially unchanged from beta to alpha oxygen patterns; however, beta oxygen spots either transform into larger alpha oxygen spots or split into two or three alpha oxygen spots. The reverse is observed on transforming from alpha to beta oxygen. These observations are interpreted as a consequence of the threefold symmetry of beta oxygen, i.e., beta oxygen may be deformed in each of three ways to form alpha oxygen. Using the measured spot positions and relative intensities, including an estimation of spot wavelengths in favorable cases, an analysis of the patterns leads to the determination of the alpha oxygen orientations, and their relative amounts, present in three different preferentially oriented samples. The second component of the full orientation relation, (010)α parallel to one of (1̄2.0)β, (21̄.0)β or (1̄1̄.0)β, is determined, in addition to that found in earlier work, (001)α parallel to (00.1)β. A theory of the beta to alpha transformation which accounts for these and other features is proposed; it involves essentially a shear, with small dilatations, parallel to the (00.1)β layer and in one of the three directions [1̄2̄0]β, [210]β or [1̄10]β. The strain matrix for the transformation is derived. No unique orientation relationship was found in the gamma to beta transformation. In particular, a previously proposed mechanism requiring {111}γ to be parallel to (00.1)β was found not to hold for three samples examined.
Magnetic field effects on the A band in KCl:Tl+ crystals
Authors: Tsuboi Taiju
Published: February 2003

The observed MCD line shape of the A band in Kcl:Tlt is reported. The g−value and election−lattic coupling constant are also reported. (AIP)
An improved analysis of EPR g-values in low spin Fe(i
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i) heme complexes
Authors: Sato Mitsuo , Ohya Toshie
Published: February 2003

An improved method of g−value analysis is reported for low spin Fe III heme complexes. The importance of including of including t42e terms for evaluation of crystal field splittings is discussed. (AIP).
Extended x-ray absorption fine structure (EXAFS) of dispersed metal catalysts
Authors: Via G. H. , Sinfelt J. H. , Lytle F. W.
Published: February 2003

Extended x-ray absorption fine structure (EXAFS) studies were made in a series of highly dispersed metal catalysts to obtain structural information on the metal clusters present. Clusters of osmium, iridium, and platinum dispersed on silica or alumina were investigated. The metal clusters in the catalysts constituted 1.0 wt.% of the total mass. Chemisorption measurements indicated that the metal dispersions approached unity, where dispersion is defined as the ratio of surface metal atoms to total metal atoms in the clusters. When EXAFS data on the metal clusters are compared with data on the corresponding bulk metals, the lower average coordination number of the metal atoms in a cluster is evident. The decrease in coordination number is accompanied by an increase in the root mean square deviation of interatomic distance about the equilibrium value.
Charge distributions and chemical effects. XXI. A new energy formula for molecules
Authors: Fliszár Sándor
Published: February 2003

The energy of atomization of molecules in their hypothetical vibrationless state at K, ΔEa*, is conveniently partitioned into nonbonded and bonded contributions and the latter are further partitioned into individual bond terms εij, i.e., ΔEa* (bond) =Σεij. A straightforward application of Politzer’s formula for molecular energies yields εij=εij0+aijΔqi+aij Δqj, where the Δq’s are the increments in net atomic charges on atoms i and j relative to those of a reference ij bond whose εij value is ε0ij. The aij’s are given by aij= (∂Ei/∂Ni)0 −(3/7) Zj<rij−1≳, where Ei is the valence shell electronic energy of atom i, Ni its electron population, and Zj is the effective nuclear charge of the j atom bonded to i. The derivative is evaluated for Ni=N0i, i.e., the population on i in the reference bond, and 〈rij−1〉 is the average inverse distance between nucleus j and the Ni electrons of atom i. The calculated ΔE*a’s of saturated hydrocarbons agree with their experimental counterparts, within ±0.157 kcal/mol (average deviation).
Electronic structure of homoleptic transition metal hydrides: TiH4, VH4, CrH4, MnH4, FeH4, CoH4, and NiH4
Authors: Hood Diane M. , Pitzer Russell M. , Schaefer Henry F.
Published: February 2003

A b initio molecular electronic structure theory has been applied to the family of transition metal tetrahydrides TiH4 through NiH4. For the TiH4 molecule a wide range of contracted Gaussian basis sets has been tested at the self-consistent-field (SCF) level of theory. The largest basis, labeled M(14s 11p 6d/10s 8p 3d), H(5s 1p/3s 1p), was used for all members of the series and should yield wave functions approaching true Hartree-Fock quality. Predicted SCF dissociation energies (relative to M+4H) and M–H bond distances are TiH4 132 kcal, 1.70 Å; VH4 86 kcal, 1.64 Å; CrH4 65 kcal, 1.59 Å; MnH4 – 36 kcal, 1.58 Å; FeH4 0 kcal, 1.58 Å; CoH4 27 kcal, 1.61 Å; and NiH4 18 kcal, 1.75 Å. It should be noted immediately that each of these SCF dissociation energies will be increased by electron correlation effects by perhaps as much as 90 kcal. For all of these molecules except TiH4 excited states have also been studied. One of the most interesting trends seen for these excited states is the shortening of the M–H bond as electrons are transferred from the antibonding 4t2 orbital to the nonbonding 1e orbitals.
Photodissociation of vibrationally excited hydrogen bromide
Authors: Zittel P. F. , Little D. D.
Published: February 2003

A two-step laser photodissociation technique was used to measure the cross section for photodissociation of HBr(v=1) at 258.9 nm. The cross section was found to be (5.5±1.1) ×10−20 cm2/molecule and is 34±7 times larger than the cross section for photodissociation of HBr(v=0) at the same wavelength. A theoretical estimate of the ratio of cross sections, based on a ’’reflection approximation’’ treatment of the UV absorption spectrum of HBr, gave poor agreement with the experimental result. The limitations of simple approximations to the photodissociation process in the threshold region of a photodissociative transition are discussed.
Diffusion coefficients for Br2 in NO and Br in Ar at 298 °K
Authors: Zittel P. F. , Little D. D.
Published: February 2003

A two-step laser photodissociation technique was used to generate Br atoms in NO or Ar mixtures. Time resolved mass spectrometric detection of Br2, formed by fast homogeneous recombination in the NO mixtures, or by a heterogeneous wall reaction in the Ar mixtures, led to a determination of diffusion coefficients for Br2 in NO and for Br in Ar at 298 °K. The reported values are DBr2−NO = (0.11±0.01) cm2/sec and DBr–Ar= (0.21±0.04) cm2/sec at 1 atm. General solutions to the diffusion and heterogeneous reaction equations for the geometry of this experiment are presented.
Collision-induced transfer rates connecting fine-structure levels in S2(B 3Σ−u,v′=4)
Authors: Caughey Thomas A. , Crosley David R.
Published: February 2003

Rates for collision-induced transfer between a single rotational level (N′=40, J′=41) and other individual rotational levels within the υ′=4 vibrational level of B 3Σ−u diatomic sulfur are reported. The S2 was excited into the B state by the radiation from an atomic Zn lamp, and rotationally resolved fluorescence measurements were used to determine the rates for the collision partners He, Ar, and Xe.
Two-determinant numerical self-consistent atomic orbitals: Application to 11B
Authors: Spagnolo Frank A. , Burke Edward A.
Published: February 2003

Two determinants derived from the spin functions αβαβα and βαβαα are used to determine numerical atomic orbitals for the ground state of 11B. The total energy is calculated to be −24.5401 a.u. as compared to the experimental value of −24.66 a.u. and the lowest value obtained for a single determinant numerical orbital calculation is −24.5291 a.u. While this energy value does not compare favorably with the lowest value obtained using analytical orbitals (−24.6392 a.u.) it is argued that numerical orbitals have the added benefit of being readily reproducible and extendible to more complex systems. Furthermore, after correcting for omissions in the original Hamiltonian and spin representations, the radio of hyperfine coupling constants: a1/2/a3.2 is calculated to be 5.063 as compared to the experimental value of 4.993 and a value of 5.009 obtained in one of the more accurate analytic orbital calculations, indicating that the present results are reasonably well-suited for the detailed representation of atomic structure.
Electronic spectrum of the diacetylene radical cation in solid rare gases
Authors: Bondybey V. E. , English J. H.
Published: February 2003

The matrix isolation and laser induced fluorescence techniques are applied to the study of the electronic spectrum of C4H+2 and its deuterated varieties. The ion is produced in solid Ne in several distinct sites by VUV photoionization of the parent diacetylene. The major site shows a vibronically well resolved X2Πg→A2Πu fluorescence spectrum. Comparison of the fundamental frequencies with the gas phase values shows in most cases negligible perturbation by the solid medium (<0.05%). Study of the fluorescence excitation spectra provides new information about the upper state vibrational potential function.
Electron transfer bands of the divalent 3d transition series ions in LiCl
Authors: Simonetti John , McClure Donald S.
Published: February 2003

The electron transfer absorption bands of the divalent transition metal ions V, Cr, Mn, Fe, Co, Ni, and Cu in LiCl host crystals have been measured up to the absorption edge of LiCl at 1500 Å. The band assignments and average energies of electron transfer are given. The average energies can be fitted to empirical equations containing I2, the second ionization potential; for example, ?=C−I2, where C=22.4±0.5 eV. By comparing the values of C for several host chlorides, it is shown that C depends on the properties of the MCl6−4 cluster much more than on the host crystal. The relationship of the electron transfer energy to the photoionization energy and to the band levels of LiCl are discussed.
Relativistic effects in a
b i
n
i
t
i
o effective core potential studies of heavy metal compounds. Application to HgCl2, AuCl, and PtH
Authors: Basch Harold , Topiol Sid
Published: February 2003

A method is described for obtaining l-dependent relativistic effective core potentials (ECPs) from Dirac–Fock self-consistent field atomic wave functions. These potentials are designed for use in nonrelativistic (NR) valence electron Hartree–Fock calculations on atoms and molecules. The novel aspect of this approach involves the averaging of the separately generated nodeless pseudovalence atomic orbitals belonging to each of the spin–orbit j components for every l value greater than zero. For the mercury and gold atoms the ECPs obtained here are shown to be virtually identical in the valence region with the corresponding ECPs obtained by Hay et al. [J. Chem. Phys. 69, 984 (1978)] using different type relativistic (R) atomic wave functions. Ab initio valence electron calculations using relativistic potentials on Hg, Au, HgCl2, and AuCl show corresponding agreement with all electron calculations, experimental values, and previous relativistic core potential results. For the Pt atom relativistic state energy calculations give good agreement with experiment, whereas the NR approach gives grossly incorrect values for relative multiplet energies, including the wrong ground state. These errors are reflected in valence electron calculations on the PtH diatomic molecule where using the NR potential predicts incorrectly a 2Σ ground state. On the other hand, using the R potential the proper 2Δ ground state is obtained along with good agreement with experiment for the equilibrium bond distance and stretching frequency. Both the R and NR type wave functions in their respective ground states predict the major bonding orbital on the Pt atom to be the 5d.
New vibrational quantum number assignments for the UV absorption bands of ozone based on the isotope effect
Authors: Katayama D. H.
Published: February 2003

A 3m normal incidence vacuum spectrograph has been used to obtain the absorption spectra of 16O3 and 18O3 in the 3100–3600 Å region. The band head positions of the absorption bands in this region (the so called Huggins bands) for normal and isotopic ozone at room and dry ice temperatures have been measured and the isotope shifts determined. The observation of two previously unreported bands in the cooled ozone spectrum at wavelengths longer than the 3514 Å band for normal ozone, as well as the isotope shifts show that the previous assumption that the 3514 Å band is the (000) ′← (000) ″ transition is incorrect. Vibrational quantum number assignments are made based on the isotope shifts and the vibrational constants for the upper electronic state of the Huggins bands are determined for 16O3 and 18O3. The absorption bands in the region near the O2(a 1Δg)+O(1D) dissociation threshold appear to be perturbed since their energy spacings are irregular compared to those for the longer wavelength bands.
Franck–Condon factors for the B–X system of S2
Authors: Anderson William R. , Crosley David R. , Allen John E.
Published: February 2003

A frequency-doubled tunable dye laser has been used to selectively excite individual rotational levels within the vibrational levels υ′=2, and 6–9, of the B3Σ−u state of S2. Measurements of the intensity in emission of all the (υ,υ″) bands permit determination of the absolute Franck–Condon factors qv′v″. Combined with previous studies, this work completes the set of qv′v″ for bound–bound transitions within the B–X system. A simple pictorial representation of the Franck–Condon factors is presented, and conclusions may be drawn concerning overlap with the B-state wave functions above the predissociation limit.
An anomaly in the oxidation rate of magnetite at its Curie temperature
Authors: Gallagher P. K. , Gyorgy E. M. , Bair H. E.
Published: February 2003

The rate of oxidation of Fe3O4 to Fe2O3 was measured in oxygen over the range of 500–720°C by following isothermal weight changes. Rate constants were evaluated at 22–28% conversion using numerous kinetic rate laws. Arrhenius plots of the rate data exhibit a significant perturbation in the region of the Curie temperature, Tc. This perturbation is relatively independent of the particular mechanistic model or rate law used. There are no significant changes in the Arrhenius parameters (activation energy, 1.2 eV) well above and below Tc. The specific heat and thermal expansion of Fe3O4 were also measured in the vicinity of Tc. Implications of the irregularity in the rate are discussed.
Lattice dynamics in solid oxygen
Authors: Kobashi K. , Klein M. L. , Chandrasekharan V.
Published: February 2003

Lattice dynamical calculations for the bulk α, β, and γ phases of solid O2 and for the monolayer α and β phases have been made in the harmonic approximation. In the α and β phases, atom-atom 6−12 potentials are employed. In the γ phase, effective potentials are used between molecular centers and only the translational lattice vibrations are calculated. It is found that Laufer and Leroi’s potential parameters give two k=O frequencies at 42.7 and 43.6 cm−1 in the bulk α-O2, and at 40.7 cm−1 for the degenerate k=0 modes in the β phase. The observed Raman lines for α-O2 at 43 and 79 cm−1, which are both known to exhibit isotope shifts, are thus tentatively assigned to an accidentally degenerate line and a two-phonon band, respectively, In view of the possible contribution from anharmonic effects, the agreement of the calculation with experiment (48-51 cm−1) in β-O2 may be better than it seems. For the bulk γ-O2, a discrepancy is observed between the calculated elastic constants and those derived from Brillouin scattering experiments. This discrepancy may be due to the neglect of translation-rotation coupling. In the monolayer O2, Raman active modes at 28.3 and 40.6 cm−1 for the α phase, and 31.9 cm−1 for the β phase are predicted.
EPR and ENDOR of triplet state diphenyl-h10 in diphenyl-d10 single crystals. Structural implications
Authors: Hutchison Clyde A. , Kemple Marvin D.
Published: February 2003

The results of EPR and ENDOR spectroscopic measurements for the lowest energy triplet state of diphenyl-h10 molecules dilutely substituted for host molecules in diphenyl-d10 single crystals at 1.9 °K are reported. Our data indicate that the triplet state diphenyl-h10 molecules do not possess inversion symmetry but that any concomitant deviation from zero for the value of the angle between the normals to the mean planes of the two phenyl groups is necessarily less than 0.008π (1.5°). We present a discussion of this result in the light of recent results concerning the low temperature structure of the diphenyl crystal.
Short range caging effects for reactions in solution. II. Escape probability and time dependent reactivity
Authors: Northrup Scott H. , Hynes James T.
Published: February 2003

Short range caging effects arising from the potential of mean force and a spatial dependence of the relative diffusion coefficient are examined for three probes of solution reactions. The escape probability that potentially reactive molecules escape each other rather than react is found to be depressed at small initial separations and enhanced at large initial separations when compared to predictions of a continuum diffusion description. Two measures of time dependent reactivity, namely the ’’time dependent rate constant’’ and the probability that initially separated members of amolecular pair remain unreacted at time t, are found to exhibit nonnegligible short range effects which can be interpreted in terms of ’’caging.’’
Hyperpolarizabilities of substituted conjugated molecules. III. Study of a family of donor–acceptor disubstituted phenyl-polyenes
Authors: Zyss J.
Published: February 2003

In Parts I and II of this series, we have made use of a finite field perturbated INDO scheme to compute molecular hyperpolarizabilities, to discriminate through simple symmetry considerations between respective σ and π electronic contributions, and to test these ideas on smaller size benzene monosubstitutes. Following the same approach, we report here investigation of a family of disubstituted donor–acceptor benzenes of increasing molecular size [NH2–C6H4– (CH=CH)n−NO2, with n=0,1 and 2] and of great experimental interest in nonlinear optics (second harmonic generation). Supposedly responsible for the higher value of the second order hyperpolarizability (tensor βijk) of this type of molecules, the charge transfer process is explored here and its importance confirmed. Results are compared to experimental data with the help of a simple ’’two level’’ charge transfer model to account for frequency dispersion and found in satisfactory agreement with previously noted tendencies. Also confirmed here are the chain length influence on βijk and the prevailing part played by π electrons in second order nonlinear processes of conjugated molecules. This study is seen to bring a theoretical credit to the ever growing experimental interest in large charge transfer conjugated molecules in the field of nonlinear optics material research.
A simple procedure for the improvement of CI convergence in molecular systems
Authors: Cooper Ian L. , Pounder Christopher N. M.
Published: February 2003

A simple scheme for the construction of improved virtual orbitals for use in SCF+CI wave functions is described and applied to the HCN molecule. The resultant CI energy lies well below that obtained from the use of those natural orbitals constructed from the one-electron density arising from an identical CI calculation with the canonical SCF orbitals. The method relies, for its applicability, on the use of a CI wave function containing only a limited number of nondiagonal double excitations, and should therefore have considerable potential in the application of CI methods to larger molecules, where the incorportion of all double excitations into the wave function is unrealistic.
The intramolecular potential of water molecules engaged in hydrogen bonding from analysis of the overtone spectrum of ice I
Authors: Sceats Mark G. , Rice Stuart A.
Published: February 2003

An analysis of the fundamental and overtone spectra of ice Ih is presented. It is shown that the major features of the spectra of H2O, D2O, and H2O/D2O mixtures can be modeled by a theory which asserts that there is no change in the anharmonic diagonal stretching terms in the intramolecular potential upon hydrogen bonding. The changes in the anharmonic constant arise from the change in the OH stretching amplitude. Both the harmonic and anharmonic terms can be expressed in terms of a hydrogen bond parameter η which can be extracted from the observed OH stretching frequency alone. The influence of intermolecular coupling in dimers and pure crystals gives rise to new spectral features which can be assigned using the model.