bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical determination of the nuclear spin–spin coupling tensors in NH−2, NH+4, BH−4 molecular ions
Authors: Lazzeretti P. , Rossi E. , Taddei F. , Zanasi R.
Published: February 2003

Coupled Hartree–Fock perturbation theory has been employed to evaluate the nuclear spin–spin coupling constants between geminal hydrogen and directly bonded X, H atoms. Full tensor matrices have been calculated ab initio, adopting SCF wave functions of near Hartree–Fock quality, determining the average trace and the anisotropy. The results are discussed and compared with those reported for similar systems and with corresponding experimental results.
Instability of a coupled rotor-phonon system
Authors: Hüller Alfred , Raich John C.
Published: February 2003

The dynamic suceptibility of a free rotor is calculated in the high temperature limit. The result is used to study a system of rotors coupled to a phonon mode of frequency ω(q) with a coupling constant g(q). It is shown that the coupled system has an instability at a temperature Tc = g2(q)/2ω2(q). The calculated spectra exhibit either soft phonon behavior or a sharp central component, depending on the ratio of the phonon energy to the average, rotational energy and the strength of the coupling.
On the structure of methyl alcohol at room temperature
Authors: Magini M. , Paschina G. , Piccaluga G.
Published: February 2003

Methyl alcohol has been examined at room temperature by means of x-ray diffraction technique. The radial distribution curve straightforwardly shows O ⋅⋅⋅ O interactions at about 2.8 Å, which indicate that extensive hydrogen bonding is occurring in liquid methanol. The other prominent peaks of the radial curve fulfill the zig–zag chain model found in solid methanol. A direct analysis of the structure function leads to the conclusion that the average number of nearest oxygen neighbors per oxygen atom lies between 1.40 and 1.55. On the contrary, no unique conclusion, as far as the shape of the chain is concerned, can be obtained through the x-ray scattering data.
Photoionization mass spectrometry of NH2OH: Heats of formation of HNO+ and NOH+
Authors: Kutina R. E. , Goodman G. L. , Berkowitz J.
Published: February 2003

Energy-dependent mass spectra are presented for the most prominent ions produced upon photoionization of hydroxylamine. Two distinct thresholds are oberved for mass 31, which we attribute to onsets for formation of HNO+ and NOH+. The corresponding heats of formation are ΔH°f,0 (HNO+) = 256.8±1.4 kcal/mol and ΔH°f,0 (NOH+) = 274.8±0.7 kcal/mol. The thresholds for mass 32, which we tentatively ascribe to H2NO+, corresponds to 224.6±0.2 kcal/mol for its heat of formation. Each of these entities can therefore be produced in an exothermic reaction of H+3 with NO, and represent mechanisms for depleting NO in the interstellar clouds. Experimental breakdown diagrams are generated and compared with predictions of QET. It is found necessary to invoke a three-body decomposition to explain some observations at higher energy. A theory is presented to accommodate this process within the framework of QET.
An experimental procedure for ESR studies of rare gas matrix isolated molecular cation radicals: 12CO+, 13CO+, 14NH3+, and 15NH3+
Authors: Knight Lon B. , Steadman J.
Published: February 2003

A generally applicable technique is described that enables the ESR study of molecular cation radicals isolated in neon matrices at 4 K. Preliminary evidence indicating the possible use of the method for the study of ion-neutral reactions is briefly discussed. The ’’gaslike’’ nature of the trapped matrix ions is demonstrated by a comparison with gas phase results for 13CO+, one of the few radical cations studied by high resolution microwave spectroscopy. Application of the method to the following radical cations is presented in this report: 12CO+, 13CO+, 14NH3+, and 15NH3+. The ions are produced during neon matrix deposition by photoionization with an open tube neon discharge source (16.8 eV).
NMR hole-burning: A study of slow molecular rotations in glassy glycerol
Authors: Kuhns Philip L. , Conradi Mark S.
Published: February 2003

Chemical shift anisotropy produces inhomogeneous broadening of NMR lines in glassy and polycrystalline solids. Holes can be burned into such lines, tagging molecules which are at certain orientations. Subsequent molecular reorientations result in a spectral diffusion which is not related to spin-spin interactions. By measuring the broadening and recovery of the hole as a function of time, detailed knowledge of the reorientations is obtained (e.g., mean jump rate and angular jump size). The reorientation rate in supercooled glycerol was followed with this technique from ∼101 to 10−2 s−1. This rate agrees well with higher frequency dielectric results and the location of the glass transition temperature. It was determined that the mean angular jump size is greater than 45°. The hole recovery curves were not exponential, but fit the Williams–Watts function exp−(t/τ)β with β = 0.5. The spin-spin contribution to spectral diffusion is very slow, making this technique widely applicable for the study of slow motions, particularly in organic solids and polymers. The hole-burning experiment is compared with related NMR techniques.
Two-photon ionization processes of PO in a C2H2/air flame
Authors: Smyth Kermit C. , Mallard W. Gary
Published: February 2003

Laser induced two-photon ionization processes are observed for PO in a premixed C2H2/air flame in the 302–334 nm wavelength region. The ionization signal obtained as a function of energy shows distinct band heads, which correspond to one-photon absorption from the ground state to the resonant intermediate B 2Σ+ state. A second photon of the same energy then either (1) excites PO to high-lying electronic states from which collisional ionization occurs or (2) directly photoionizes the PO molecule, depending upon the total excitation energy available. Several new bands are assigned on the basis of calculated spectra. The intensities of the observed ion signals vary in an unexpected fashion for transitions which reach the same vibrational level of the resonant intermediate state. Further analysis indicates that absorption of the second photon makes a significant contribution to the overall intensity of some of the observed bandheads. This result is attributed to the presence of short-lived autoionizing states which lie close to the ionization limit, despite some uncertainties encountered in calculating Franck–Condon factors for absorption of the first photon. It is estimated that PO can be easily detected at a mole fraction of 1×10−8 with a laser pulse energy of 100 μJ.
The structure and hyperfine constants of HF–Cl2
Authors: Baiocchi F. A. , Dixon T. A. , Klemperer W.
Published: February 2003

Molecular beam electric resonance studies on two isotopes of HF–Cl2 have enabled the following spectroscopic constants to be determined:               These constants are best interpreted in terms of an equilibrium structure for HF–Cl2 in which the three heavy atoms form a linear array with an F–Cl bond length of 2.96 Å. The proton is off-axis with an H–F–Cl angle of 125°. The chlorine quadrupole coupling constants show a charge distortion of the chlorine submolecule. Precise interpretation of eQq is hindered by lack of a value for the coupling constant of free 35Cl2.
On the role of certain rotational invariants in crystal-field theory
Authors: Leavitt Richard P.
Published: February 2003

We examine a perturbation scheme in which the effect of the crystal-field Hamiltonian in mixing free-ion states is represented in second order by an effective operator acting only on the free-ion state under consideration. The shifts in the centers of gravity of the free-ion levels caused by J mixing is derived in terms of three parameters s2, s4, s6, which are quadratic rotational invariants. The second moment of the Stark levels corresponding to a particular free-ion state is described by these same parameters in the absence of J mixing. Using Ho3+ as an example, we show how these parameters can be extracted from experimental data in a simple manner and how they can be used to correct the data for J-mixing effects.
Dynamic corrections for neutron scattering of water in the free molecule approximation
Authors: Blum L. , Rovere M. , Narten A. H.
Published: February 2003

We develop a quantum mechanical theory for the inelasticity or Placzek correction for an asymmetric top rigid rotator, and give a formula to compute this correction for neutron scattering. We have, however, used a sphericalized model which satisfies Mecke’s sum rule to compute the correction for H2O and D2O. Since this rigid rotor model fails to satisfy the exact first sum rule, we introduce a form that does satisfy this condition. Good agreement with the large momentum exchange (k) behavior in the case of D2O is obtained. However, for the H2O this is not so and adjustable parameters have to be introduced to obtain the correction asymptotic behavior.
Neutron diffraction study of light and heavy water mixtures at 25 °C
Authors: Thiessen W. E. , Narten A. H.
Published: February 2003

Neutron diffraction data of very high statistical accuracy for four mixtures of light and heavy water containing 0.01%, 35.79%, 67.89%, and 99.75% deuterium are presented. The dynamic corrections described in a preceding article yielded a self-consistent static coherent structure function for heavy water. For the other mixtures an empirical correction method was used. The four total structure functions were analyzed to yield the three partial structure functions aOO(k), aOH(k), and aHH(k). The function aOO(k) is not significantly different from previously obtained x-ray diffraction results. The partial structure functions for hydrogen interactions will provide a sensitive test for proposed models of liquid water.
19F dynamic nuclear polarization studies on the system CF2Cl–CFCl2/tritertiary butyl phenoxyl radical: Evidence for indirect chlorine participation in enhancing fluorine scalar rates
Authors: Chandrakumar N. , Narasimhan P. T.
Published: February 2003

Dynamic nuclear polarization experiments were performed on the system CF2Cl−CFCl2/ trilertiary Bulyll Phenoxy radicals. The enhancement scalar compling is explained by correlations with molecule electronic structure. (AIP)
Temporary negative ions in the chloromethanes CHCl2F and CCl2F2: Characterization of the σ* orbitals
Authors: Burrow P. D. , Modelli A. , Chiu N. S. , Jordan K. D.
Published: February 2003

The electron transmission spectra of chlorometers CHCl2F and CCl2F2 are presented. The electron affinities are evaluted compared with thoses calculated using self-consistent field methods. (AIP)
Third and fourth virial coefficients of hard hyperspheres of arbitrary dimensionality
Authors: Joslin C. G.
Published: February 2003

Results for the third and fourth virial coefficients of hard hyperspheres of arbitrary dimensionality obtained by Luban and Baram are derived much more simply. We give a closed-form expression for B4(⧠) in 2N dimensions (N an integer).
Photodissociative channels at 1216 Å for H2O, NH3, and CH4
Authors: Slanger Tom G. , Black Graham
Published: February 2003

H(2S) and O(1D, 3P) yields have been measured from photolysis of H2O, NH3, and CH4 at 1216 Å. From H2O, it is confirmed that the H2+O(1D) channel occurs with a yield of 0.1, but in addition there is a yield of 0.12 for the channel giving 2H(2S)+O(3P). The remaining process is H+OH production. For NH3, the NH+2H(2S) channel accounts for at least 90% of the dissociation. For CH4, combination of our data with earlier work on H2 and CH production at 1236 Å leads to the conclusion that the two major dissociative channels generate 1CH2+H2 and CH2+2H(2S), with comparable probabilities. CH and CH3 production is at most minor. The yields at 1216 Å for the three-fragment processes O+2H, NH+2H, and CH2+2H, exhibit a dependence on the available excess energy.
Electronic structure of linear polyacenes in the SCF–RPA method
Authors: Baldo Marcello , Grassi Antonio , Pucci Renato , Tomasello Pasquale
Published: February 2003

The excited states of the π-electron system of linear polyacenes are studied with the self-consistent field–random phase approximation (SCF–RPA) scheme. The semiempirical Pariser–Parr–Pople (PPP) model is used. The parametrization is derived by fitting the benzene and it is held fixed throughout the polyacene series. Substantial improvement with respect to Tamm–Dancoff results [J. Chem. Phys. 24, 250 (1956)] for the singlet oscillator strengths is obtained. It is pointed out that the ratio between the experimental oscillator strength of the strong 1B3u transition and the total f-sum rule strength shows a drastic drop for naphthacene and pentacene. It is argued that this behavior is associated with a deep modification of the π-electron correlation structure of these two molecules with respect to the smaller ones. The calculated energies are compared with experimental and Tamm–Dancoff values. The assignments of Meyer et al. [J. Chem. Phys. 56, 801 (1972)] for the triplet excited states of naphthalene and anthracene are confirmed. Theoretical implications of the results are discussed and possible developments and improvement of the theory are indicated.
Anisotropy of the nuclear spin–spin coupling tensor in water, ammonia, and methane molecules
Authors: Lazzeretti Paolo , Zanasi Riccardo
Published: February 2003

The anisotropy of the nuclear spin–spin coupling tensor relative to H–H and X–H (X = O, N, C) interactions is investigated through extended ab initio calculations on the molecules H2O, NH3, and CH4. Spin–orbit, spin–dipolar, and Fermi contact-spin–dipolar cross terms are accounted for in the coupled Hartree–Fock perturbative scheme. The major role played by the Fermi contact-spin–dipolar cross term is ascertained. A large discrepancy emerges between theoretical and experimental anisotropy for the CH coupling, which evidences the need for further careful analyses of the measured data.
Calculation of polarization potentials and three-body induction effects from perturbation theory
Authors: Haymet A. D. J. , Oxtoby David W.
Published: February 2003

The polarization potential method, well known in the theory of electron–atom scattering, is used to study the interactions in a polar fluid. Calculations are presented for the interaction between a fluoride ion and a proton in liquid HF, using an approximate perturbation theory which involves the average energy denominator approximation. This approximation is tested by comparison with the results of a novel exact perturbation theory approach using ab initio finite field ionic wave functions, and found to be reasonably accurate. The calculations show an ’’induction energy enhancement’’ at intermediate distances which is not present in the previously proposed empirical polarization potentials of Stillinger and David. An ’’ionic plus polarization’’ model of the chemical bond in HF is discussed. The perturbation method is extended to calculate three-body induction energies. The predicted equilibrium structure of the HFH+ ion agrees poorly with full quantum mechanical calculations, identifying a severe limitation of those polarization methods which omit covalent bonding effects. The strengths and weaknesses of the polarization potential method for polar liquids are discussed.
Mobilities of ground-state and metastable O+, O2+, O2+, and O22+ ions in helium and neon
Authors: Johnsen Rainer , Biondi Manfred A. , Hayashi Makoto
Published: February 2003

The ionic mobilities of O+, O2+, O2+, and O2+2 in helium and neon have been measured using a selected-ion drift apparatus (SIDA). It is found that the mobilities of both O+ and O+2 ions in the metastable states (2D or 4Πu) are measurably smaller than those of the same ions in their ground states. The detection and identification of metastable ions was carried out by using known, state-selective ion–molecule reactions. A similar mobility differentiation of ground-state and metastable ions was not observed for the O2+ and O2+2 ions.
A pulse radiolysis study of electron transfer in ethylene glycol–water glasses between 10 and 160 K
Authors: van Leeuwen Johan W. , Hollestelle Harm H.
Published: February 2003

Pulse radiolysis experiments have been performed in ethylene glycol–water glasses between 10 and 160 K and in the time range 10−5–10 s. In these glasses the decay kinetics of the trapped electrons is nearly independent of the temperature for the scavengers CrO42−, Fe(CN)63−, NO2−, and cytochrome c. However, in the presence of H+ the decay of the trapped electrons is strongly temperature dependent. This is ascribed to the coupling of the electron transfer to low frequency medium modes (h/ω<100 cm−1), because the proton has no intramolecular modes of vibration.
Diffusion of ion implanted aluminum in silicon carbide
Authors: Tajima Yo , Kijima Kazunori , Kingery W. D.
Published: February 2003

Diffusion of aluminum in silicon carbide was studied by Al implantation into single crystal SiC and subsequent profile analyses by secondary ion mass spectrometry (SIMS). The bulk diffusion coefficient of Al at temperatures between 1350 and 1800 °C was determined to be D(cm2/s) = 1.3×10−8 exp (−231 kJ/mol/RT). The results were characterized by a low activation energy and a low pre-exponential constant compared with previously reported results. Dislocation enhanced diffusion was suggested from the appearance of the tails observed in the annealed concentration profiles.
The second order concentration corrections to the mutual diffusion coefficient of Brownian macroparticles
Authors: Phillies George D. J.
Published: February 2003

From the initial time dependence of the dynamic structure factor S(k,t), we obtain a general form, valid through second order in the concentration, for the mutual diffusion coefficient Dm. All effects of direct and hydrodynamic interactions (other than dynamic friction) are taken into account, including the three-point ’’0seen’’ hydrodynamic tensor Tijl, whose analytic form is obtained here for the first time. The concept of the reference frame correction is re-examined. The usual factor (1−φ) for transition from the fundamental hydrodynamic frame to the volume-fixed frame is argued to be a low-concentration approximation. The general form for Dm is evaluated for a model system of hard spheres. With the use of the auxiliary assumption (relaxed herein) that all hydrodynamic interaction tensors satisfy ∇ ⋅ t = 0, use of our new general method [J. Stat. Phys. 28, 673 (1982)] for reducing N-particle cluster integrals to (N−1)-dimensional integrals shows Dm = D0 (1−0.875φ−19.53φ2).
Photochemical holes: A spectral probe of the amorphous state in the optical domain
Authors: Friedrich J. , Wolfrum H. , Haarer D.
Published: February 2003

The optical line shape features of dye molecules are used for probing some low temperature properties of amorphous solids. The experimental technique is photochemical hole burning (PHB); the dye molecule is 1,4-dihydroxyanthraquinone. The results obtained for ethanol glasses and boric acid glasses are as follows: The optical linewidth obeys a T2 law. It has two contributions: one reversible contribution due to fast dephasing processes, and one irreversible contribution due to ground state redistribution processes. The latter contribution will be interpreted in terms of the TLS model (two-level system) which has been commonly used to describe the amorphous state. One main conclusion is that the energy asymmetry distribution of the TLS systems is rather narrow (several cm−1). A second conclusion follows from the absence of a sizeable isotope effect upon deuteration of the samples. We must assume that proton tunneling is not the main mechanism involved in the TLS dynamics.
Dynamic behavior of silica dispersions studied near the optical matching point
Authors: Kops-Werkhoven M. M. , Fijnaut H. M.
Published: February 2003

Photon correlation spectroscopy experiments on silica particles dispersed in cyclohexane over a large concentration range and near the optical matching point have been explained by the occurrence of two different relaxation modes: one mode due to the collective diffusion of the particles and the other mode due to the self-diffusion. This is in agreement with the theoretical considerations of Weissman and Pusey. In the diluted region, information has been obtained about the behavior of the self-diffusion coefficient at zero wave vector (K) as a function of the particle volume fraction (φ). We found that the following relation exists: Ds(K = 0) = D0(1 + k′sφ), where D0 is the diffusion coefficient at infinite dilution and the constant ks′ has a value of −2.7±0.5. The concentration dependent behavior of the collective and self-diffusion coefficient has further been compared with the results of sedimentation and viscosity measurements. At high particle concentrations we observed that the relation between collective diffusion and sedimentation by means of the generalized Einstein relation has been confirmed qualitatively. The self-diffusion coefficient appeared to be inversely proportional to the viscosity of the solution. The sedimentation experiments have been found to be in agreement with the theories of Reed and Anderson over the whole concentration range, while at high particle volume fractions the theory of Lekkerkerker seemed to reasonably fit. Finally, we observed at φ≳0.40 that numerically, the values of the collective and self-friction coefficient are nearly the same.
Defect structure of NH4+ β-alumina studied by NMR
Authors: Arribart H. , Carlos H. , Sapoval B.
Published: February 2003

The behavior of proton NMR absorption spectra in nonstoichiometric NH+4 β-alumina single crystal has been studied between 90 and 230 K, below the onset of translational motion narrowing. Below 140 K the signal is the superposition of a broad line due to motionless NH+4 ions in mid-oxygen sites and of a narrow line due to rapidly rotating ions in Beevers–Ross sites. Above 160 K all the ions rotate and both lines merge into a single line. A detailed quantitative analysis of the second moments of these lines enables us to show that the defect related to the nonstoichiometry is made of two pairs of NH+4 ions in displaced mid-oxygen sites. Perfect agreement between theory and experiment is found if one takes into account small displacements of the Beevers–Ross ions from their high symmetry sites, the size and the shape of the NH+4 ions, and the specific orientations of the displaced Beevers–Ross and mid-oxygen ions. This study shows in particular that the population of anti-Beevers–Ross sites is vanishingly small at the temperatures where the translational motion is slow, showing that there is a close link between ionic conductivity and anti-Beevers–Ross occupancy.
An NMR study of the solid phase of trifluoromethane
Authors: Watton A. , Pratt J. C. , Reynhardt E. C. , Petch H. E.
Published: February 2003

Nuclear magnetic absorption line shape and spin-lattice relaxation (T1, T1ρ) measurements have been made on the 1H and 19F nuclear species in solid CHF3 over the temperature range 4.2 to 120 K. The dominant motional mechanism has been found to be a threefold reorientation of the molecules with activation energy 4.3±0.2 kcal/mol and pre-exponential Arrhenius factor in the range 4.9×10−17–8.9×10−17 s. An additional low temperature mechanism has been associated with transitions between the librational states of the molecules. The threefold reorientation is discussed in terms of a possible crystal structure, and is also related to the nonexponential relaxation observed in the 1H T1 data.
The excitation of O(1S) in the reaction between N2(A 3Σ+u) and O(3P)
Authors: Piper Lawrence G.
Published: February 2003

The rate constant for the excitation of O(1S) in the reaction between N2(A,v′ = 0) and O(3P) has been measured in a discharge-flow reactor at room temperature to be (2.1±0.4)×10−11 cm3 molecule−1 s−1. Combining this measurement with the previously determined rate constant for total quenching of N2(A) by O shows that the fraction of all quenching events which lead to O(1S) excitation is 0.75±0.13. These results are at variance with rate constants estimated from some auroral models and indicate that some revision of auroral models is in order.
Test of the linear sum rule for vibrational energy transfer by trajectory calculations
Authors: Truhlar Donald G. , Blais Normand C.
Published: February 2003

Vibrational relaxation rate constants for H2 in a bath of Ar are computed for v = 0, 4, 6, 10, and 13 and a rotational-translational temperature of 4500 K. The relaxation rate increases by a factor of 5.5 as v is raised from 0 to 10, then decreases by 33% as v is raised to 13. The v dependence of the vibrational relaxation violates a linear sum rule that is required to hold for an information theoretic synthesis of state-specific rate constants from bulk relaxation data.
Exponential depression as a test of estimated decay parameters
Authors: Isenberg Irvin , Small Enoch W.
Published: February 2003

A new test for judging the goodness of estimated decay parameters is presented. The test is based on the fact that a convolution is invariant under exponential depression. In the absence of significant error the estimated parameters will then remain constant as the degree of depression is varied over a finite range. In the presence of error, the parameters will vary. Up to now, no test has existed to see if moment index displacement corrects errors to a satisfactory extent in any given analysis. It has always been necessary to have some a priori knowledge of the type of error that limited the analysis. The test presented here removes that requirement. In addition, it is shown that the test performs better than a visual inspection of residual and autocorrelation plots in judging analyses when decays are closely spaced, even in the absence of nonrandom errors. The test is useful in accepting or rejecting analyses, with or without automatic error correction, in helping to discriminate between different models of sample decay, and in tuning pulse fluorometers for optimal performance. The test is, in principle, independent of the method of moments; it may be used with any method which needs only a small amount of computer time, and which is a statistically resistant procedure.
The lambda invariance test: A characterization of exponential decays
Authors: Lee John W.
Published: February 2003

The lambda invariance test is a requirement that estimated decay parameters to be locally flat as a function of the degree of exponential depression. This paper presents a converse theorem: Suppose one calculates decay parameters as if they represented a multiexponential decay, but without any a priori knowledge that the decay is, in fact, multiexponential. Then, if the parameters are locally flat, the decay is, in fact, multiexponential. It is shown that global flatness is not required. Only local flatness is needed.
Two-particle vibrational excitations in molecular crystals
Authors: Klafter Joseph , Jortner Joshua
Published: February 2003

In this paper we utilize a two-particle band model to describe cooperative vibrational excitations and vibrational overtones in molecular crystals of diatomic molecules. The cooperative vibrational excitation involves a two-vibron continuum, while the overtone corresponds to a two-vibron bound state. The problem of two-particle vibrational excitations was shown to be equivalent to the perturbation of a band by a local potential, whose strength is determined by the intramolecular anharmonicity. Model calculations were performed for a three-dimensional, semicircular density of states, resulting in transparent expressions for the energetics and intensities, which were utilized subsequently to provide a self-consistent analysis of the available experimental results. In all these cases, the local perturbation is sufficiently strong to ensure the appearance of a bound overtone state outside the two-particle band. Finally, we consider the effects of structural disorder on two-vibron states, which provide a diagnostic tool for the identification of two-particle bound states and of two-particle continua.
Interchain vibrational coupling in phase II (hexagonal) n-alkanes
Authors: Casal Hector L. , Mantsch Henry H. , Cameron David G. , Snyder Robert G.
Published: February 2003

The intense methylene scissoring and rocking bands near 1460 and 720 cm−1 in the infrared spectra of the odd n-alkanes C17–C27 show doubling in phase II, the so-called hexagonal phase. This doubling, which is shown to be due to interchain vibrational coupling, is highly dependent on temperature and chain length. A possible explanation for this dependency is offered in terms of a combination of static and dynamic factors associated with more or less constrained rotational/twisting motions of the chains about their long axes.
Classical chaos and quantum simplicity: Highly excited vibrational states of HCN
Authors: Lehmann Kevin K. , Scherer George J. , Klemperer William
Published: February 2003

Direct overtone spectra of H 12C 14N, H 13C 14N, and H 12C 15N have been measured between 15 000 and 18 500 cm−1 with a precision of 0.001 cm−1. These were obtained using intracavity photoacoustic spectroscopy, with a fully automated laser system. The spectra are unperturbed. The transition energies and rotational constants are in good agreement with predictions of first order anharmonic constants. Classical trajectories for HCN have been computed on the best experimentally parameterized potential, and found to be stochastic 12 990 cm−1 above the ground state. Quantal density of states were computed for HCN and show that if extensive vibrational coupling occurs, the observed states would be highly perturbed. The simplicity of the observed states is shown to be expected given a Franck–Condon type limitation on significantly perturbing states. The results show the inapplicability of classical dynamics for predicting the dynamics of molecular vibrations.
Piecewise analytical integration of chemical rate equations. I. The algorithm
Authors: Bienstock Sergio , Gordon Roy G.
Published: February 2003

A piecewise analytical method for the integration of chemical rate equations is introduced and developed. The method solves the usual set of coupled chemical rate equations generated by first- and second-order kinetic laws, including quadratic recombination terms. The recombination terms are evaluated exactly, while the coupling coefficients between different chemical species are approximated, to any predetermined accuracy, by either piecewise constant, linear, or quadratic functions of time. Piecewise analytical solutions are found in terms of known and readily computable functions (either error functions or quantum mechanical harmonic oscillator functions). From these solutions, analytical time averages are derived, and then used interatively to refine the accuracy of the solutions by a simple direct predictor-corrector scheme. The method provides numerically stable solutions to any (preset) desired accuracy, even for large sets of coupled kinetic equations.
Raman spectra and phase transitions in O2PF6
Authors: Griffiths J. E. , Sunder W. A.
Published: February 2003

Evidence of the existences of two stable phases of solid O2PF6 with a phase transition at −67 °C was found by a temperature-dependent Raman study (0–2000 cm−1) of the polycrystalline material. The dynamics of the O+2 cation were examined in terms of the relaxation times associated with the O+2 stretching modes in the phases. The O+2 ion in phase I (T≳−67 °C) appears to enjoy greater rotational freedom than in phase II. No evidence of an anticipated third phase was found down to −138 °C.
Monte Carlo studies of water monolayer clusters on substrates: Hexagonal AgI
Authors: Ward Richard C. , Holdman Jon M. , Hale Barbara N.
Published: February 2003

A Monte Carlo procedure is used to study the stability and structure of small water clusters adsorbed on model basal and prism faces of hexagonal AgI. Effective pair potentials for the H2O–AgI interaction [B. N. Hale and J. Kiefer, J. Chem. Phys. 73, 923 (1980)] and the revised central force potentials for the H2O–H2O interaction [F. H. Stillinger and A. Rahman, J. Chem. Phys. 68, 666 (1978)] are used to determine average internal cluster binding energy and average cluster–substrate binding energy for water clusters containing 6, 24, and 44 water molecules at temperatures 240, 265, and 298 K. The results (using an effective substrate point charge of 0.4e) give between 5 and 6 kcal/mol per molecule for each of the above average binding energies. The iodine exposed basal AgI face organizes the H2O into five- and six-membered rings centered around the exposed I atoms. The prism face appears less effective in promoting ring structure and gives, in general, lower average internal cluster binding energies. The basal AgI face positions the water molecules (in H2O–substrate high binding sites) about 2.8 A apart while the water–water interactions orient the H2O dipole moments into configurations favorable for hydrogen bonding. The resulting six-membered ring structure is similar to that in ice Ih (crinkled) basal face layers, and appears to be qualitatively the same for all the temperatures studied.
Conformational state relaxation in polymers: Time-correlation functions
Authors: Hall Carol K. , Helfand Eugene
Published: February 2003

A study is made of relaxation processes in polymer molecules which proceed via conformational transitions of the chain backbone from one rotational isomeric state to another. The conformational time-correlation functions are determined for several models of the process. Each model allows for the occurrence of both independent and cooperative conformational transitions. The resulting correlation functions contain a modified Bessel function which is associated with the diffusional nature of the process. This functional form has recently proven useful in fitting time-correlation functions determined in polymer simulations, which indicates that it will be of value in fitting data obtained in relaxation experiments on polymers.
Temperature and thickness dependence of the 0–15 eV electron transmission spectra of rare gas films
Authors: Sanche L. , Perluzzo G. , Bader G. , Caron L. G.
Published: February 2003

The electron transmission spectra for Kr, Ar, and Xe films deposited on a polycrystalline platinum substrate are reported. (AIP)
Dynamics of ion–molecule recombination. II. An alkali ion and a water molecule
Authors: Swamy Kandadai N. , Hase William L.
Published: February 2003

The Monte Carlo classical trajectory method is used to study collisions of alkali ions I+ with H2O and D2O. Formation of the vibrationally excited intermediates I+ (H2O) and I+ (D2O) requires energy transfer from relative translation to the water molecule’s vibrational and rotational degrees of freedom. This energy transfer probability is inefficient and decreases exponentially with increase in relative translational energy nearly independent of the initial impact parameter. The energy transfer probability is strongly dependent upon the alkali ion, but only has small variations between H2O and D2O. The inefficient energy transfer manifests itself kinetically in recrossing of the dividing surface separating the hydrated ion and the two reactants. As a result, the bimolecular rate constant for hydrated ion formation is smaller than that predicted by variational transition state theory. Several different statistical and collision theories are compared, and it is found that an activated complex theory expression for the bimolecular rate constant can be derived within the framework of collision theory and the average molecular interaction (AMI).
Optimization of vibrational coordinates, with an application to the water molecule
Authors: Thompson Todd C. , Truhlar Donald G.
Published: February 2003

A new method is introduced for calculations of vibrational structure of molecules. The method consists of self-consistent field (SCF) calculations in which not only the modals, but also the coordinates, are optimized. The coordinates are assumed to be orthogonal linear combinations of normal coordinates. The SCF method with optimized coordinates is illustrated by an application to the symmetric stretch–bend interaction in the water molecule. The rms error of the SCF energy levels with optimized coordinates is only 42% of the rms error of the normal-coordinate SCF energies.
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o MO studies of electronic states and Mössbauer spectra of high-, intermediate-, and low-spin Fe(II)-porphyrin complexes
Authors: Obara Shigeru , Kashiwagi Hiroshi
Published: February 2003

A b initio LCAO SCF MO calculations were carried out on Fe-porphine, Fe-porphine-pyridine, and Fe-porphine-pyridine-carbonmonoxide which are typical intermediate-, high-, and low-spin complexes, respectively. Mössbauer spectra of ferrous porphyrins were analyzed theoretically by the evaluation of isomer shift (IS), quadrupole splitting (QS), sign and direction of the principal component of electric field gradient tensor, and asymmetry parameter for the complexes. The calculated values have a good agreement with the experimental ones for the low- and high-spin complexes. The ground state of Fe-porphine, which remains undetermined so far, is assigned as the 3Eg state which is described by mixing of the two configurations, (dxy)2 (dxz,dyz)3 (dz2)1 and (dxy)1 (dxz,dyz)3 (dz2)2. A new relation between Mössbauer spectra and electronic structure is found. The field gradient along the perpendicular axis to the porphyrin plane is determined by two factors, the d-electron configuration and a constant term depending on axial ligands.
Coadsorption studies of CO and H2 on ZnO
Authors: Griffin G. L. , Yates J. T.
Published: February 2003

We have studied the adsorption of pure CO and CO:H2 mixtures on powdered ZnO using the combined techniques of transmission infrared spectroscopy and temperature programmed desorption (TPD). When CO is adsorbed alone, the vibrational frequency ωCO decreases from 2192 to 2178 cm−1 with increasing CO coverage, and a repulsive CO–CO interaction is observed in the TPD spectrum. When CO is adsorbed on an H2-covered surface, ωZnH decreases from 1709 to 1653 cm−1, ωOH increases from 3490 to 3523 cm−1, and the zero-coverage limit of ωCO increases from 2191 to 2196 cm−1. There is also an increase in the CO adsorption energy due to an attractive CO–H2 interaction. Analysis of TPD spectra for CO yields an expression for the CO adsorption energy as a function of CO and H2 coverage: ΔHCOads (kcal/mol) = 12.2−0.16 nCO+0.08 nH2, where nCO and nH2 are the coverages in μmol/gm. We attribute the coverage dependence of the CO adsorption energy, as well as the observed IR frequency shifts, to both electrostatic and chemical interactions between adsorbates. The former arise from dipole interactions between neighboring species, while the latter are due to ’’through-substrate’’ inductive effects associated with the electron donating/withdrawing properties of each adsorbate.
Laser photocatalytic isomerization and hydrogenation of olefins in the gas phase
Authors: Whetten Robert L. , Fu Ke-Jian , Grant Edward R.
Published: February 2003

We report the first observation of homogeneous organometallic photocatalysis in the gas phase. Gaseous mixtures of iron pentacarbonly with 1-pentene and hydrogen for hydrogenation (with accompanying isomerization) are irradiated with the pulsed output of a N2 (337 nm) or XeF (351 nm) excimer laser. We observe quantum yields, calculated as number of conversions per photon absorbed, as high as 400 for isomerization and 18 for hydrogenation. Kinetic and spectroscopic evidence suggest that the photocatalytic mechanisms operating in the gas phase closely parallel those of the corresponding, better characterized liquid phase systems. (AIP)
The lowest a 3Σ+u state of Cu2
Authors: Bondybey V. E.
Published: February 2003

The lowest a 3Σ+u state of Cu2 in solid neon is populated following laser excitation of the A 1Π or B 1Σ+u states by the predissociation-cage recombination sequence and observed and characterized by means of the a 3Σ+u→ X 1Σ+g phosphorescence.
Molecular beam-laser induced fluorescence experiments on hydrogen abstraction from amines by O(3P)
Authors: Kleinermanns K. , Luntz A. C.
Published: February 2003

Using crossed molecular beam-laser induced fluorescence experiments, we have probed nascent internal state distribution and dynamic energy thresholds of OH formed in the reactions of O(3P) with amines. The results imply that the mechanism for OH formation is direct abstraction from a C–H bond. This is contrary to an earlier suggestion that OH is formed as a result of an addition mechanism.
Participation of two-dimensional hindered internal rotations in activated complexes
Authors: Pacey Philip D.
Published: February 2003

It is postulated that the bending motions in an activated complex, of the form AB2, may be treated as a two-dimensional internal rotation hindered by a sinusoidal potential function. The Shrödinger equation for these degrees of freedom takes the form of the oblate spheroidal equation. For various values of the barrier to internal rotation, this equation has been solved to find the lowest 231 energy levels. A series expansion has been found for the energies of the bound states. The contributions of these degrees of freedom to the heat capacity, the enthalpy function, and the free energy function have been calculated. Approximations to the latter quantities are also deduced and are shown to be valid in certain temperature ranges. This type of motion has been incorporated into activated complex theory. Replacement of the usual harmonic bending potential by a sinusoidal one has the following effects: (i) the concept of reaction path degeneracy is replaced by nondegenerate states of opposite symmetry, (ii) the zero point energy of the complex is decreased, (iii) at low temperatures, partition functions, activation energies, and Arrhenius plot curvature increase more rapidly with increasing temperature, (iv) at high temperatures, partition functions and activation energies increase less rapidly and curvature declines with increasing temperature. At high temperatures, the expression for the rate constant has the same form as the expression from simple collision theory. Expressions for the collision theory steric factor and activation energy are deduced. As an example, calculations are performed for the reaction of D with H2.
Angular aspects of electron correlation and the Coulomb hole
Authors: Boyd Russell J. , Yee Main C.
Published: February 2003

The simple physical picture of the Coulomb hole due to Coulson and Neilson has been extended to reveal the shape of the Coulomb hole in the ground state of helium. The approach is based on a pair distribution function h(r12,r1,α) which depends on the position of a reference electron and the angle between the nucleus, the reference electron, and a second electron. Following a discussion of the properties of this new function, the angular features of the Coulomb hole are studied in terms of the difference between the values of the pair distribution function obtained from good approximations to the exact and to the Hartree–Fock wave functions. The Coulomb hole is essentially spherically symmetric when the reference electron is close to the nucleus. As the reference electron is removed to larger radial distances, substantial polarization appears. The polarization is greater away from the nucleus than toward the nucleus.
Classical dynamics of energy transfer between bonds in A
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Authors: Sibert Edwin L. , Reinhardt William P. , Hynes James T.
Published: February 2003

A discussion of energy transfer betwen bonds is presented for ABA triatomics for which a local mode picture affords a good zeroth-order description. A model of H2O is used to illustrate the discussion. It is shown that the energy transfer between the bonds can be readily understood both qualitatively and quantitatively in terms of the dynamics of a twofold hindered rotor. The transformation to this representation leads to a simple picture of the transition from ’’local mode’’ to ’’normal mode’’ behavior and to simple analytic expressions for the energy transfer rates. These predicted rates are found to be in excellent agreement with trajectory calculations. Isotope effects are discussed for the H2O–D2O pair.
Complete active space (CAS) SCF and externally contracted multireference CI studies of atomic and molecular properties. Static dipole polarizabilities of F, F−, and Ne
Authors: Nelin Connie , Roos Björn O. , Sadlej Andrzej J. , Siegbahn Per E. M.
Published: February 2003

Accurate calculations of the dipole polarizability of F−, Ne, and F are performed by using the complete active space (CAS) SCF and externally contracted multireference configuration interaction (MC-CCI) methods within the finite-field perturbation scheme. The CAS SCF method gives the following polarizability results: 14.4, 2.63, and 3.68 a.u. for F−, Ne, and F(2P), respectively. The corresponding results obtained from the MC-CCI calculations based on the CAS SCF eigenvectors are: 15.1, 2.64, and 3.52 a.u. The anisotropy of the dipole polarizability calculated for the fluorine atom equals 0.38 a.u. according to the CAS SCF method and 0.55 a.u. according to the MC-CCI scheme.
The stability of RbH2O and RbHF: A Hartree–Fock study
Authors: Broughton J. Q. , Bagus P. S.
Published: February 2003

The stability of RbHF and RbH2O complexes has been investigated by performing Hartree–Fock calculations on Rb ⋅ HF, H ⋅ RbF, and Rb ⋅ H2O species in various orientations. Heats of interaction of 2–5 kcal/mol are typically obtained. In each system, the free radical prefers to approach the dipolar molecule at the negative end. This preference cannot be explained by a simple point-dipole induced-dipole model; the finite size of the dipole is important and produces the directional sensitivity. These results indicated that such species should be stable in no-collision environments; such as the ultrahigh vacuum conditions in which RbH2O has been observed.
An efficient algorithm for calculating a
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o energy gradients using s, p Cartesian Gaussians
Authors: Schlegel H. Bernhard
Published: February 2003

A method is presented for the calculation of analytical first derivatives of the two electron integrals over s- and p-type Cartesian Gaussian basis functions. Formulas are developed for derivatives with respect to the positions of the nuclei and basis functions (for use in geometry optimization) and with respect to the exponents of the primitive Gaussians (for use in basis set optimization). Full use is made of the s = p constraint on the Gaussian exponents. Contributions from an entire shell block are computed together and added to the total energy derivative directly, avoiding the computation and storage of the individual integral derivatives. This algorithm is currently being used in the ab initio molecular orbital program gaussian 80.
Density-functional theory for the solid alkali cyanides
Authors: LeSar R. , Gordon R. G.
Published: February 2003

Structural properties of the solid alkali cyanides were studied with a recently reported ab initio theory based on the Gordon–Kim electron gas model. The low temperature crystal structures of LiCN, NaCN, KCN, RbCN, and CsCN were examined. The average error in lattice parameters is 2.6% and in lattice dissociation energy 1.7%. For the NaCN and KCN systems, the energy differences between the ferroelectric and antiferroelectric structures were examined and the antiferroelectric structure was found to be more stable at zero temperature by about 0.5–0.8 kcal/mol. The high pressure phases of KCN were studied and the phase diagram partially mapped out at 0 K. The crystal structure of RbCN at low temperatures was studied, and the orthorhombic Pmmn structure found to be more stable than the monoclinic Aa structure. The heat of formation of RbCN was calculated at 0 K to be about 31.4±7 kcal/mol. The cyanide electron density was found to shift from the C atom towards the N atom in all crystals studied, when the free space CN− ion is incorporated into a crystal.