bluuebunny/crossref_metadata_2025 huggingface dataset:
Validity of energy gap representations of rotationally inelastic cross sections between polar molecules
Authors: Alexander Millard H. , Jendrek Eugene F. , Dagdigian Paul J.
Published: February 2003

In a collaborative investigation of rotational energy transfer between polar molecules, we have reported experimental state-resolved cross sections for LiH( j=1)–HCl, DCl, HCN collisions [J. Chem. Phys. 71, 1670 (1979); 72, 6513 (1980)] and fully quantum cross sections for HF–HF collisions [J. Chem. Phys. 66, 1334 (1977) and to be published]. In the present article we apply both the exponential gap (EG) and power law (PL) parameterizations to the results of our experimental and theoretical study. Neither of these parameterizations is able to fit with high accuracy the HF–HF cross sections. Furthermore, this comparison reveals an inaccuracy in the statistical ’’prior’’ distribution which cannot be corrected by any multiplicative factor dependent on the energy gap. The fits were not greatly improved by the use of an alternative tensorial prior, suggested by group theoretic rather than statistical considerations. For the LiH scattering systems, the experimental cross sections can be fit extremely well by a power law dependence on the energy gap of the resolved LiH molecule. Unfortunately, it is improbable that either the PL or the EG fits to these partially averaged cross sections could be used to predict with any degree of accuracy the more fundamental fully resolved cross sections, as revealed by comparison with recently computed adiabatically corrected sudden cross sections [J. Chem. Phys. 71, 1683 (1979)].
Virial theorem for inelastic molecular collisions. Atom–rigid rotor scattering
Authors: Pack Russell T. , Hirschfelder Joseph O.
Published: February 2003

From the hypervirial theorem for a nonchemically reactive but otherwise arbitrary collision between two molecules, the virial theorem for the scattering of an atom by a nonvibrating (rigid rotor) diatomic molecule is derived and shown to be simply related to F. T. Smith’s collision lifetime matrix. It is found that there are two contributions to the collision lifetime: (1) The usual contribution due to the intermolecular potential and its derivative, and (2) a contribution due to the change in the internal energy of the molecule. The conditions under which the virial theorem is satisfied by the infinite order sudden (IOS), distorted wave (DW), and first Born (FB) approximations are also given.
The cyanamide–isocyanamide rearrangement
Authors: Vincent Mark A. , Dykstra Clifford E.
Published: February 2003

A b initio self-consistent field (SCF) and correlated SCEP calculations have been used to study the relative energies and structures of cyanamide, isocyanamide, and the transition state for the rearrangement reaction. Isocyanamide is found to be 53 kcal less stable than cyanamide, larger than the typical isocyanide–cyanide energy difference. Correlation effects account for 11 kcal of this energy difference. Isocyanamide is found to be pyramidal with an inversion barrier of 6 kcal, larger than for cyanamide. The activation barrier for rearrangement of isocyanamide into cyanamide is about 46 kcal, which tends to support the prospects for the existence of isolated isocyanamide. Equilibrium rotational constants of isocyanamide and cyanamide for the optimized structures are presented.
Rayleigh scattering from a methane–ethane mixture
Authors: Ackerson B. J. , Hanley H. J. M.
Published: February 2003

The Rayleigh linewidth of a methane (71.07%)–ethane mixture was measured by dynamic light scattering near the plait point. Results are reported for the concentration diffusivity and (preliminary only) for the thermal diffusivity. It was found that a large background contribution is present in the concentration diffusivity, in agreement with theoretical predictions. The thermal diffusivity is compared to a one-fluid model introduced previously. A critical temperature of 236.36±0.03 K and a critical pressure of 6.66±0.02 MPa was determined for the mixture.
Ro-vibronic species, overall allowed species, and nuclear spin statistical weights for symmetric top molecules belonging to the D
n
d and D
n
h (n≤6) point-groups
Authors: Weber Alfons
Published: February 2003

Nuclear spin statistical weight factors have been calculated for the rotational energy levels of symmetric top molecules belonging to the Dnd (n=2,3,4,5,6) and Dnh (n=3,4,5,6) point-groups. The ro-vibronic species for the levels have been calculated on the basis of Hougen’s classification scheme, using the point-group of the molecule. The Longuet–Higgins permutation–inversion-group was used to determine the overall allowed species permitted by the Pauli exclusion principle and the ramifications of inversion symmetry. The ro-vibronic species and the statistical weights are given in a set of easy to use tables. They are presented explicitly for all possible vibrational states of a totally symmetric electronic state. The weights are given in terms of the coefficients mα which occur in the expression ΓS=ΣmαΓαG for the reduction of the nuclear spin representation ΓS with respect to the irreducible representations ΓαG of the point-group G. General formulas for the characters of the nuclear spin representation are derived. These permit the numerical evaluation of the coefficients mα for a specific molecule by means of the standard group theoretical reduction formula. The results are general and are applicable to any molecule belonging to one of the symmetric top point-groups Dnd and Dnh with n?6, having an arbitrary number of atoms of the same or different kind, and for arbitrary values of the nuclear spins. The tables for the ro-vibronic species and for the statistical weights are arranged in such a manner that account can be taken of the splittings of level degeneracies due to rotation–vibration interaction. Extensions of the tables to include cases of nontotally symmetric electronic states are described. A few examples are worked out for molecules of increasing complexity and belonging to different point-groups to illustrate the ease of using the tables.
Dissociation energies of GdO, HoO, ErO, TmO, and LuO; correlation of results for the lanthanide monoxide series
Authors: Murad Edmond , Hildenbrand D. L.
Published: February 2003

High temperature gaseous reaction equilibria involving GdO, HoO, ErO, TmO, LuO, and certain reference oxides have been studied by mass spectrometry, using a molecular effusion beam source. From the reaction thermochemistry, the dissociation energies, D00, were derived as 169.5±3 kcal/mol (GdO), 144.1±3 kcal/mol (HoO), 143.9±3 kcal/mol (ErO), 121.8±3 kcal/mol (TmO), and 159.4±2 kcal/mol (LuO). Some of these values differ substantially from previous determinations. However, the new results together with several recent determinations and re-evaluations yield a consistent set of results for the entire series from LaO to LuO. Trends in D00 values across the series can be correlated remarkably well with changes in the 4fn6s2→4fn−15d6s2 electronic promotion energies in the gaseous metal atoms. Various aspects of the bonding are discussed.
Solitons, defect diffusion, and dielectric relaxation of polymers
Authors: Skinner James L. , Wolynes Peter G.
Published: February 2003

We propose that the sine-Gordon model provides a reasonable Hamiltonian for a single polymer chain in crystalline polyethylene. We show that the soliton solutions of this model are very similar to the twist defects proposed, on the basis of detailed conformational energy studies, by Mansfield and Boyd as a mechanism for the α-relaxation process in crystalline polyethylene. Allowing for the possibility that the soliton solutions describe Brownian motion, we make the connection to the theory of dielectric relaxation, showing that the macroscopic decay function is given approximately by the hopping correlation function previously derived by us. More generally, we argue that this decay function arises from the one-dimensional Brownian motion of unspecified defects. When the phenomenological friction constant is very large, we find agreement between our result and the theory of defect diffusion. We then briefly discuss dielectric relaxation in amorphous polymers, noting that our decay function is similar in several respects to the successful empirical decay function proposed by Williams and Watts.
Statistical mechanics of helical wormlike chains. XII. Multivariate distribution functions
Authors: Shimada Jiro , Yamakawa Hiromi
Published: February 2003

The multivariate distribution function of several vector distances each between two contour points for the helical wormlike chain is studied. General expressions for its moments are derived by a generalization of the operational method developed previously. The distribution function itself is constructed by the weighting function method on the basis of the first several moments to yield the well known Wang–Uhlenbeck theorem in the coil limit.
Theory of interionic forces in alkali halide molecules
Authors: Shanker Jai , Agrawal H. B. , Agrawal G. G.
Published: February 2003

A modified treatment of the interionic forces for alkali halide molecules is presented within the framework of Rittner’s model. The electronic polarizabilities of ions in the molecular state are calculated using an energy level analysis. The free ion polarizabilities used in Rittner’s model are replaced by the molecular state polarizabilities. The contributions arising from the van der Waals interactions and the mutual polarization of ions are recalculated. Values of the dipole moment, the binding energy, and the rotational and vibrational constants are estimated for twenty alkali halides using two alternative potential forms for the repulsion energy. The results are discussed in the light of experimental data.
Mobilities and longitudinal diffusion coefficients for K+ ions in nitrogen and argon
Authors: Takebe M. , Satoh Y. , Iinuma K. , Seto K.
Published: February 2003

We have constructed a drift tube with a movable ion source and measured the mobilities and longitudinal diffusion coefficients for K+ ions at 303 °K in N2 and at 305 °K in Ar in the pressure range 0.3–5.0 Torr, over the E/N range 4–346 Td in N2 and 3–320 Td in Ar. The zero-field reduced mobilities for K+ ions in N2 and Ar were determined to be 2.50±0.03 and 2.63±0.03 cm2/V sec, respectively. Both values are in excellent agreement with the values reported by Elford and Milloy. When our data are compared with the values obtained by Thomson et al. in N2 and the values obtained by James et al. in Ar over the entire E/N range, we find that the mean deviations are about 1.7%, independent of gas species and E/N. Our zero-field reduced mobilities are about 1.2% lower in both cases than the values compiled by Ellis et al. It is concluded that the discrepancy is due to a systematic error and is not caused by clustering reactions. The mean values of NDL over the E/N range 4–7 Td in N2 and 5–10 Td in Ar were found to be 1.96×1018 and 2.09×1018 cm−1 sec−1, respectively. Both values are about 7% higher than the values calculated from our mobility data by the generalized Einstein relation and from the same parameters reported by Pai et al.
The Jahn–Teller distortion in the cation radical of allene as evidenced by ESR
Authors: Takemura Yoshiko , Shida Tadamasa
Published: February 2003

It is found that the cation-radical of allene is distorted with a show angle of 30−40° in accord with Haselbach’s prediction.(AIP)
Trimer exciplex emission in benzopyrene
Authors: Abu-Zeid M. E. , Lopez J. R.
Published: February 2003

The authors study the emission spectra taken for 3,4,9,10-dibenzopyrene in benzene, when the main pulsed N2 laser beam (3371 Å) was used as a source of excitation.(AIP)
Experimental evidence for the universality of the pseudospinodal
Authors: Osman J. , Sorensen C. M.
Published: February 2003

Available experimental data 1–5 is used to test the existence of a universal pseudospinodal that describes all thermodynamic and transport properties of all liquid-gas and binary fluids systems with one curve.(AIP)
Cation quadrupole couplings in alkali metal nitrates: Rubidium, cesium, and potassium
Authors: Segel S. L.
Published: February 2003

Phase IV RbNO3 gives rise to three distinct 87Rb nuclear quadrupole couplings at 20°C. This observation supports the proposed crystal structure, trigonal, with space group P31. For phase II CsNO3, isomorphous with the rubidium salt, only an upper limit to the 133Cs nuclear quadrupole coupling can be assigned. Phase II KNO3 has the CaCO3 structure, orthorhombic, and only a lower limit to the 39K nuclear quadrupole coupling can be assigned.
Cross-correlation trajectory study of vibrational relaxation of HF (v=1–7) by HF (v=0)
Authors: Coltrin M. E. , Marcus R. A.
Published: February 2003

Results are presented for a three-dimensional quasiclassical trajectory study of the vibrational deactivation of vibrationally excited HF (v=1–7) by ground vibrational HF. A cross-correlation method of analysis is used to calculate probabilities and rate constants for V–V and V–RT transitions using trajectory results. Comparisons are made of calculated total deactivation rate constants (V–V plus V–RT) with experimental values. The V–RT dominates the relaxation for higher v states, and increases particularly rapidly with increasing v. Comparisons are made with recent classical-path calculations for this system, and in the use of Morse versus equivalent harmonic oscillator potentials.
Excitations and magnetic ground state in Co(pyr)2Cl2
Authors: Swüste C. H. W. , Phaff A. C. , de Jonge W. J. M.
Published: February 2003

The magnetic ground state and field induced magnetic arrays are derived by the method of Kudō and Katsura. From the critical fields a set of exchange parameters can be deduced. Spin cluster resonances are detected in the various phases as well as in the paramagnetic phase at T≳TN, where evidence for the existence of local two-dimensional arrays can be found.
Raman intensities of stretch oriented polyethylene and perdeuteropolyethylene. II. Electro-optical parameters for solid N-hydrocarbons
Authors: Abbate S. , Gussoni M. , Zerbi G.
Published: February 2003

In this paper use is made of the experimental Raman intensities of stretch-oriented polyethylene and perdeuteropolyethylene for the calculation of Raman electro-optical parameters (eop). The experimental data are reported in Paper I immediately before this paper. ∂αuv/∂Qi are given as linear combinations of symmetrized and internal Raman eop’s. From the experiments one set of eop’s has been derived and compared with other sets obtained from the Raman spectra of chemically similar molecules. The problem of the transferability of eop’s is discussed.
Simultaneous narrow-line Raman spectra of ground and excited electronic states
Authors: Decola P. L. , Andrews J. R. , Hochstrasser R. M. , Trommsdorff H. P.
Published: February 2003

The absorption and fluorescence spectra of pentacene in benzoic acid is studied at 1.6°K.(AIP)
Diffusional correlations in polymer dynamics
Authors: Pear M. R. , Northrup S. H. , McCammon J. A.
Published: February 2003

The authors present a simple theory which shows how viscous forces influence the character of conformational transitions in a polymer. (AIP)
Dynamic aspects of order–disorder phenomena
Authors: Lakkad S. C.
Published: February 2003

A molecule is considered to be in a ’’mean-field’’ behaving as an oscillator or a hindered rotator depending on its energy. The nematic-isotropic transitions which the molecule is undergoing are considered. (AIP)
Rotationally inelastic molecule–surface scattering in the sudden approximation
Authors: Gerber R. B. , Yinnon A. T. , Shimoni Y. , Kouri D. J.
Published: February 2003

Rotational and reorientational transitions in molecular collisions with solid surfaces are investigated by a model based on a sudden approximation with respect to both the rotational and the diffraction states that play a role in the scattering. The approximation developed leads to computationally simple expressions and provides detailed insight into the physical properties of the processes involved. A detailed quantitative study is made of the rotational state distribution produced by the collision, the variation of rotational excitation probabilities with the scattering angle, and related questions. A number of factorizations, sum-rule, and scaling properties are predicted for ‖ Sjmj,00;j′m′j′,mn  ‖2, the transition probability between the initial (jmj) and the final (j′m′j′) rotational states for scattering into the (mn) diffraction channel. The strongest sum rules and scaling laws are obtained using additional approximations beyond the sudden decoupling. Among the latter results: (1) The j,j′ dependence of ‖ Sj0,00;j’m’0,mn ‖2 is determined entirely by the difference variable Δj=j′−j. (2) The diffractive intensity distribution summed over all final rotational states is the same as that obtained for a mass-equivalent atom (with an interaction that is the orientation-averaged molecule–surface potential). (3) The rotational state distribution, summed over all diffraction states, equals that calculated from a corresponding flat surface. (4) All rotational transition probabilities for the (m,n) diffraction spot can be obtained from the diffraction–rotational transition probabilities in the (m,0) and (n,0) diffraction spots. The above and other properties are tested numerically in the framework of the full sudden approximation for a model of H2/LiF(001) in the energy range 0.5–0.9 eV. They are found to hold to excellent accuracy. Systematics of the results with regard to variation of the surface corrugation parameter are noted.
Theoretical analysis of the predissociation of the A 2Σ+ state of OH
Authors: Sink Michael L. , Bandrauk André D. , Lefebvre Roland
Published: February 2003

Multichannel scattering theory is used to calculate predissociation linewidths and energy shifts for the upper vibrational levels of the A 2Σ+ state of OH which is predissociated by three continua. Single channel numerical and semiclassical results are compared to results obtained from the full multichannel scattering matrix. The isolated resonance approximation for predissociated levels was thus carefully checked and was found to be valid for all levels. Results are presented for the predissociation probabilities of F1 and F2 components of the rovibronic levels of the A 2Σ+ state for v=0–2.
Laser photochemistry and hole-burning of chlorin in crystalline n-alkanes at low temperatures
Authors: Völker S. , Macfarlane R. M.
Published: February 2003

Photochemical hole-burning has been used to study selective and reversible phototautomerism in chlorin (dihydroporphin) incorporated in matrices of n-hexane, n-octane, and n-decane at 4.2 K. The energy difference between the S1↔S0 transitions of the two forms, distinguished by the position of the two inner protons, is ∼1500 cm−1; the stable tautomer absorbing near 6350 Å. The back reaction from the less stable form to the stable one is 103 times faster than the forward reaction, and under wide band excitation in the Soret band a dynamic equilibrium exists between the two forms. Very narrow holes (∼40 MHz) can be burnt in the inhomogeneous lines of both tautomers. The width of these holes is consistent with a limit imposed by the fluorescence lifetime of 8 ns.
A
b i
n
i
t
i
o projected-unrestricted Hartree–Fock calculation of some potential energy curves for carbonyl fluoride
Authors: Brewer Dana A. , Schug John C. , Phillips Donald H.
Published: February 2003

Some potential energy curves for CF2O were calculated using projected-unrestricted Hartree–Fock (PUHF) theory. The calculations employed a contracted [4s3p] Gaussian-type atomic orbital basis set. Bound states were found for the ? 1A1 and 1,3A2 states while the 1,3B1 and 1,3B2 states were repulsive in the valence representation. The merits of the PUHF treatment for excited states is discussed. The results are discussed in terms of available experimental information and previous calculations with particular emphasis on the question of the photolysis channels open in the solar spectral region.
The cyclic isomer of CO2
Authors: Feller David , Katriel Jacob , Davidson Ernest R.
Published: February 2003

The existence of cyclic isomers of O3 and NO2 has motivated a search for a cyclic isomer of CO2 on the energy surface corresponding to the electronic ground state. MCSCF and CI results were obtained using both 3-21G and DZP basis sets. A cyclic carbene isomer with RCO=2.46 bohr and &OCO=79° is predicted.
A
b i
n
i
t
i
o calculation of transition state normal mode properties and rate constants for the H(T)+CH4(CD4) abstraction and exchange reactions
Authors: Schatz George C. , Walch Stephen P. , Wagner Albert F.
Published: February 2003

We present ab initio (GVB–POL–CI) calculations for enough of the region about the abstraction and exchange saddle points for H(T)+CH4(CD4) to perform a full normal mode analysis of the transition states. The resulting normal mode frequencies are compared to four other published surfaces: an ab initio UHF–SCF calculation by Carsky and Zahradnik, a semiempirical surface by Raff, and two semiempirical surfaces by Kurylo, Hollinden, and Timmons. Significant quantitative and qualitative differences exist between the POL–CI results and those of the other surfaces. Transition state theory rate constants and vibrationally adiabatic reaction threshold energies were computed for all surfaces and compared to available experimental values. For abstraction, the POL–CI rates are in good agreement with experimental rates and in better agreement than are the rates of any of the other surfaces. For exchange, uncertainties in the experimental values and in the importance of vibrationally nonadiabatic effects cloud the comparison of theory to experiment. Tentative conclusions are that the POL–CI barrier is too low by several kcal. Unless vibrationaly nonadiabatic effects are severe, the POL–CI surface is still in better agreement with experiment than are the other surfaces. The rates for a simple 3-atom transition state theory model (where CH3 is treated as an atom) are compared to the rates for the full 6-atom model. The kinetic energy coupling of reaction coordinate modes to methyl group modes is identified as being of primary importance in determining the accuracy of the 3-atom model for this system. Substantial coupling in abstraction, but not exchange, causes the model to fail for abstraction but succeed for exchange.
On the computation of electronic correlation energies within the local approach
Authors: Stollhoff Gernot , Fulde Peter
Published: February 2003

A recently developed local approach to the computation of electron-correlation energies is extended and investigated in great detail. For that purpose it is applied to the Ne atom and the CH4 molecule for which correlation-energy calculations are available. It is shown that the local approach exhausts up to a few percent the correlation energy which is obtainable within a given basis set. A detailed comparison with conventional CI methods reveals the advantages of the present approach. Only a fraction of the two-electron matrix elements Vijkl has to be calculated, and the number of required configurations is drastically reduced. Furthermore, CH4 is used as an example to demonstrate that the correlation-energy calculations for large systems can be broken up into ones for small systems.
The surface tension of water: A Monte Carlo calculation using an umbrella sampling algorithm
Authors: Lee C. Y. , Scott H. L.
Published: February 2003

In this paper we describe a Monte Carlo calculation of the surface tension of water at 25 °C. The procedure used involves computing the free energy required to create a surface in a system of 256 water molecules with periodic boundary conditions imposed. In order to optimize computing time we have used a modified version of an umbrella sampling algorithm suggested by Torrie and Valleau. The excess surface free energy is computed in a series of steps, the most important of which consist of incremental calculations in which the interface is gradually created. Our final result is γ=97±6 dynes/cm, larger than the experimental value of 72 dyn/cm. Reasons for the rather large difference are discussed. Our result can be used as a basis for comparison of the surface tension of water/air interfaces with the surface tension of other aqueous interfaces calculated using the same technique.
Enhanced Raman spectroscopy of CO adsorbed on vapor-deposited silver
Authors: DiLella D. P. , Gohin A. , Lipson R. H. , McBreen P. , Moskovits M.
Published: February 2003

Raman spectra of CO condensed on vapor-deposited Ag at 11 K show two signals in the CO stretching region, one originating from a monolayer next to the metal, the other from the bulk CO film above it. In addition, a low-frequency band associated with the former was detected, as were overtones of both the low- and high-frequency bands of the surface CO. Raman scattering by the former was found to be some 106 times more intense than by the latter on a per molecule basis. The state of polarization of the tow signals was investigated as a function of angle of incidence, as was the excitation profile of the signal from the CO next to the metal, using the bulk CO signal as a reference. The behavior of the latter could only be understood when one took into account coherence effects arising from the superposition of rays directly incident on or scattered by a molecule with those reflected from the metal before impinging on, or after scattering from, the molecule. These effects could give rise to apparent polarizing or depolarizing effects and warrant attention in matrix isolation Raman spectroscopy as well as in surface Raman spectroscopy. Taking all observations into account, the enhanced Raman is suggested to be a form of resonance Raman by CO-covered, metal surface irregularities which act as ’’adsorbed,’’ giant cluster compounds, the metal core of which is large enough to sustain conduction electrons, thereby allowing the conduction electron resonance to become an electronic state of the pseudoadsorbed cluster. It is this electronic state which is in resonance with the incident illumination. The relationship between this electronic resonance and surface plasma resonance in an isolated metal sphere is pointed out. The expected enhancement of molecules adsorbed on several metals other than silver is estimated in the context of the model proposed. Group IB metals are shown to be premier among the enhancers, although Cu and Au should be most efficatious in the red.
Impulsive stochastic models of molecular relaxation and isomerization reactions
Authors: Berne B. J. , Skinner James L. , Wolynes Peter G.
Published: February 2003

We discuss several aspects of the problem of modeling the effect of collisions on isomerization reactions. We show how to generate stochastic dynamics for systems which suffer more than one type of collision, and derive the appropriate master equation for the phase space distribution function. We discuss a variety of different collision models and sketch how one might deduce the form and parameters of the model from experiment and molecular dynamics. Finally, we examine a one-dimensional model of an isomerization reaction in the presence of two types of collisions, finding that the reaction rate varies nonlinearly with the various collision rates.
Photodissociation of triatomic molecules: Application of the R-matrix propagation methods to the calculation of bound-free Franck–Condon factors
Authors: Kulander K. C. , Light J. C.
Published: February 2003

We present a method for calculating Franck–Condon factors for the photodissociation of triatomic molecules. We use the R-matrix propagation techniques developed by Light and Walker for reactive scattering processes. The method is used to study the collinear dissociation of a model CO2 system. Franck–Condon overlaps are calculated for three different initial states and probabilities for populating particular vibrational levels of the CO fragments are obtained. We present results for a range of photon energies. We find that the partial cross sections exhibit a series of resonant or Fano line shapes. The structure in the cross sections depends sensitively on the initial vibrational state, but the peak values are not significantly effected.
Rotational effects in the vibrational predissociation of X⋅⋅⋅H2 van der Waals molecules
Authors: Beswick J. A. , Requena A.
Published: February 2003

In this paper an approximate, analytic, three-dimensional quantum mechanical treatment of vibrational predissociation of triatomic van der Waals molecules X⋅⋅⋅BC, where X is a rare-gas atom and BC a normal diatomic molecule, is presented. The approximations used are valid for BC molecules with large rotational constants and weak-coupling complexes. Application is made to the X⋅⋅⋅H2 (X≡Ar, Kr, Xe) molecules for which ’’three-dimensional’’ potential energy surfaces exist.
Classical mechanics of a collinear exchange reaction: A direct evaluation of the reaction probability and product distribution
Authors: Pollak Eli , Child Mark S.
Published: February 2003

Instead of finding regions of reactivity in the asymptotic reactants (products) phase space, involving a two dimensional search, one may directly evaluate the boundary of reactivity bands. Here we provide a practical method, for the regime in which transition state theory is not exact, for directly evaluating such boundaries. The method is iterative, convergent and at each iteration step provides improved upper and lower bounds to the reaction probability. A numerical application to the hydrogen exchange reaction giving product distributions and reaction probabilities over a wide range of energies is provided. We find that the existence of two bounded trajectories that are not periodic is crucial to understanding the dynamics of the system.
NH3+ ĊH2 radical in α-glycine single crystal. Effect of the trapping matrix on the structure
Authors: Bonazzola L. , Iacona C. , Michaut J. P. , Roncin J.
Published: February 2003

The radical NH3+ ĊH2 is generated by a γ irradiation followed by a photolysis at 2600 Å of α-glycine single crystals at low temperature. From its study by EPR, this radical is shown to be nonplanar at the carbon site in this matrix.
Variationally determined multipole Cauchy moments
Authors: Bartolotti Libero J.
Published: February 2003

The hydrodynamic variational method for obtaining dynamic multipole polarizabilities is extended to determine multipole Cauchy moments. The hydrodynamic functional is replaced by an equivalent series of functionals, obtained by expanding the first-order functions in power series of the applied frequency. As an application of the method, multipole Cauchy moments for the ground states of atomic hydrogen and helium are determined by solving variationally the set of functionals. The resulting moments are then used to construct Padé approximants, from which long-range dispersion coefficients are determined.
Stochastic molecular dynamics study of t
r
a
n
s–g
a
u
c
h
e isomerization processes in simple chain molecules
Authors: Montgomery John A. , Holmgren Stephen L. , Chandler David
Published: February 2003

T r a n s–gauche isomerization rates for n-butane, n-pentane, and n-decane are determined from a stochastic molecular dynamics calculation of the reactive fluxes. Analysis of the deviations of these rates from the transition state theory predictions shows that the transmission coefficients differ markedly from unity for all physically realizable conditions. The dependence of the transmission coefficient on the collision frequency or friction constant parameterizing the stochastic element is discussed. A comparison between the stochastic dynamics and a true Newtonian molecular dynamics simulation of n-butane in a simple liquid solvent indicates that the reactive flux correlation function is significantly different than would be predicted by the high friction noninertial theory first discussed long ago by Kramers. For longer chain molecules, the isomerization rates depend upon which particular torsional angle is considered, and upon the configurational state of the neighboring angles. This dependence is analyzed. Finally, calculations are presented which address certain questions concerning the use of holonomic constraints in the dynamics of alkane chains.
Spin doubling in CaH
Authors: Cooper David L. , Richards W. Graham
Published: February 2003

The computation of Λ-doublet splittings in diatomic 2Π electronic states has become almost routine with accuracies of 1% obtainable for light hydrides. The spin−doubling which occurs in 2Σ states has received less attention: the only example seriously studied is HF+. Here this work is extended to CaH. Apart from the effects introduced by incorporating a third-row atom, CaH is of intrinsic value as its spectrum is of importance in resolving the puzzle of the apparent depletion of Ca in the interstellar medium relative to its abundance in stars.
On the structure and stability of bipyramidal B2O3
Authors: Snyder Lawrence C. , Wasserman Zelda
Published: February 2003

The structure of bipyramidal B2O3, which has often been postualted to be a major component of B2O3 vapor and liquid, has been computed using a 4–31 Gaussian basis set and the computer program Gaussian-70. Bipyramidal B2O3, of assumed D3h symmetry, is computed to have an equilibrium B–O bond length of 1.480 Å and an angle between boron bonds to oxygen of 62°. Based on computed total energies for boric acid, water and bipyramidal B2O3, the heat of formation of bipyramidal B2O3 in the gas phase from crystalline B2O3 is estimated to be +342.9 kcal/mole, in contrast to the observed heat of vaporization to gaseous B2O3 of +102.5 kcal/mole at 25 °C. We conclude that bipyramidal B2O3 is a very minor component of the B2O3 gas and liquid phases. (AIP)
Theoretical potential curves for excited states of ArH and the rate of collisional quenching of metastable Ar by H
Authors: Vance R. L. , Gallup G. A.
Published: February 2003

Energy curves for the ground state and the first seven excited states of ArH have been calculated using the multiconfiguration valence bond (MCVB) method. Important features of the excited curves include two distinct avoided crossings between the lowest Ar*H and the highest ArH* potential energy curves. Using these curves we have made a theoretical analysis of quenching of metastable Ar by collision with H at room temperature. Application of the Massey criterion indicates that the separation of the potential curves in the region of the crossing and the acceleration produced by the fall of the reactant channel potential curve from its asymptotic level combine to produce a relatively high probability for curve switching. A more quantitative estimate of the curve switching probability is given with the Landau–Zener formula and leads to a theoretical value of the quenching rate approximately ten times the experimental. In light of the many approximations involved this qualitative agreement is satisfactory and provides a rationale to explain the anomalously high rate constant for the quenching reaction. Structural features of the interacting potential curves are discussed in terms of the diabatic states involved.
Long range vibrational energy transfer to dielectric surfaces
Authors: Brus L. E.
Published: February 2003

Molecular vibrational energy transfer to the lattice vibrations of dielectric solids is investigated as a function of molecule to surface distance D. In the short range D−3 region, the rate shows an intense, narrow resonance at the surface vibrational polariton frequency. There is no resonance structure at the transverse optical frequency, where bulk absorption is maximized. The rates can be orders of magnitude faster than observed on reflective metallic surfaces in the infrared. The rates show a long range component extending beyond D? 1 μm which can be interpreted as transfer to lifetime broadened nonresonant vibrational polaritons. Experimental implications are considered in the cases of (a) vibrational deactivation in a gas–solid collision, and (b) chemical reactions near dielectric substrates or particles.
The HF energy distribution in the reaction of F atoms with vinyl radicals
Authors: Sloan J. J. , Watson D. G.
Published: February 2003

The reaction of F atoms with vinyl radicals has been measured by arrested relaxation infrared chemiluminescence techniques. The vibrational distribution in the HF product of the abstraction reaction is reported.
NMR nutation spectroscopy: A method for determining molecular geometry in amorphous solids
Authors: Yannoni C. S. , Kendrick R. D.
Published: February 2003

A NMR technique for direct determination of internuclear distances in amorphous solids is described and illustrated with a measurement of the carbon–carbon bond length in acetic acid at −190 °C. The result, 1.488 °A, is sufficiently close to the x-ray value of 1.478 °A to confirm the effectiveness of the overall approach. The pulse sequence used, and the ensuing spectral analysis, ensure a very direct measurement of internuclear distances in amorphour solids.
A Gaussian basis for Rydberg orbitals
Authors: Jungen M.
Published: February 2003

A formula is derived which allows the construction of Gaussian basis sets for the representation of atomic and molecular Rydberg orbitals. As an illustration calculations for the s, p, d, and f Rydberg series of the Li atom are presented up to n=8.
Comment on the Truhlar–Horowitz functional representation of the H+H2 potential surface
Authors: Johnson B. R.
Published: February 2003

The Truhlar-Horowitz H3 potentials subroutine is commented upon. It is contended that the TH functional representation has discontinuous derivatives for certain isosceles triangle configurations. (AIP).
Dissipative structures and broken symmetry
Authors: Walgraef D. , Dewel G. , Borckmans P.
Published: February 2003

The claim that nonlinear chemical instabilities leading to ’’dissipative structures’’ far from thermodynamic equilibrium are analogous to equilibrium phase transitions is further discussed.
On the theory of homogeneous and inhomogeneous line broadening. An exactly solvable model
Authors: Knapp E. W. , Fischer S. F.
Published: February 2003

The simultaneous influence of homogeneous and inhomogeneous line-broadening mechanisms are investigated for a reference system embedded in a medium. An analytically solvable model is constructed which includes the interaction of the reference mode with N strongly coupled environmental states (causing an inhomogeneity) and weakly coupled bath states. The modulation of the reference mode is caused by transitions between the strongly coupled perturber states. These may be switched on with a different rate than they are switched off. Consequently, effects of energy of activation and the change in entropy for environmental transitions on the line shape can be analyzed. The spectral function as well as the relaxation function are discussed as a function of the relevant parameters such as the number of perturbers, the lifetime T1, the temperature, and transition rates.
Elastic scattering and rotational excitation of a polyatomic molecule by electron impact: Acetylene
Authors: Thirumalai Devarajan , Onda Kunizo , Truhlar Donald G.
Published: February 2003

We have calculated differential, integral, momentum transfer, and partial cross sections for elastic scattering and rotational excitation of C2H2 by 10 eV electrons. The effective potential includes static, exchange, and polarization interactions calculated by the INDOX/1s method and the semiclassical exchange approximation with adiabatic polarization at large electron–molecule distances. The scattering is treated by well converged rotational close coupling using the centrifugal dominant scheme to select the channels included and including up to 32 coupled channels for a given total angular momentum. The calculated integral cross sections for pure elastic scattering and rotational excitation are 54.5 and 41.4a02 , respectively. These are much larger than the values (34.4 and 18.6a02) previously [K. Onda and D. G. Truhlar, J. Chem. Phys. 71, 5107 (1979)] calculated for the isoelectronic molecule N2 at this energy. This illustrates how the greater spatial extent of C2H2 greatly increases the cross sections for pure elastic and rotationally inelastic scattering.
A comparison between the LCAO–Xα and Hartree–Fock wave functions for momentum space properties of ammonia
Authors: Kaijser Per , Sabin John R.
Published: February 2003

Electronic wave functions have been determined with the LCAO–Xα and Hartree–Fock method for the same basis set (62 Gaussian type orbitals) and compared for momentum space properties for ammonia. The general qualitative features of the momentum density are equally described by the two methods, but in qualitative details and quantitative measures the methods are incomparable.
Theoretical studies of hydrogen bonding in liquid water and dilute aqueous solutions
Authors: Mezei Mihaly , Beveridge David L.
Published: February 2003

Monte Carlo computer simulations of liquid water and dilute aqueous solutions are analyzed in terms of the nature and extent of intermolecular hydrogen bonding. A geometric definition of the hydrogen bond is used. Calculations on liquid water at 25 °C, 37 °C, and 50 °C, were carried out based on the quantum mechanical MCY potential of Matsuoka, Clementi, and Yoshimine and at 10 °C based on the empirical ST2 potential. The effect of a dissolved solute on aqueous hydrogen bonding was studied for dilute aqueous solutions of Li+, Na+, K+, F−, Cl−, and CH4. The nature of the hydrogen bonding was characterized with quasicomponent distribution functions defined as a function of the intermolecular coordinates relevant to hydrogen bonding. The extent of the hydrogen bonding is described using a network analysis approach developed by Geiger, Stillinger, and Rahman. The results on the quasicomponent distribution functions show that the average hydrogen bond angle deviates with 10 °–25 ° from a linear form, quite independently of the potential function. The decrease in icelike character in liquid water as the temperature is increased is quantified. The structuration effects in solvent water for dilute aqueous solution of ionic and hydrophobic solutes are computed, and interpreted in terms of the Frank and Wen A, B and C regions of solvent. There is increased structure in the A region of both the ionic and apolar solutes, electrostrictive for the former and clathratelike for the latter. The B region is destructured in ionic solutions and shows increased icelike structural character in the solution of an apolar solute. The network analysis showed the existence of large space-filling hydrogen bonded networks. The occurrence of monomers was found to be negligibly small. These findings are in quantitative agreement with the analysis of molecular dynamics results by Geiger et al., based on an energetic hydrogen bond definition. Furthermore, the parameters of the distributions of the hydrogen bonded networks were found to be remarkably invariant to small change in the temperature, introduction of a solute and the change of the potential function. The average cluster size and related parameters are quite sensitive to the number of molecules considered. Overall, the analysis supports the validity of viewing liquid water in terms of a Pople continuum and Sceats–Rice random network model of water molecules interacting mainly via bent hydrogen bonds.
Thermochemistry of the gaseous fluorides of samarium, europium, and thulium
Authors: Kleinschmidt P. D. , Lau K. H. , Hildenbrand D. L.
Published: February 2003

The gaseous mono-, di-, and trifluorides of the lanthanide metals samarium, europium, and thulium were characterized thermochemically from high temperature equilibrium studies carried out by mass spectrometry. Reaction enthalpies and entropies were derived using second-law analysis throughout, and the results were used to evaluate the enthalpies of formation and bond dissociation energies (BDE) of the gaseous fluorides, and to obtain approximate values for the electronic entropies of the MF and MF2 species. The dissociation energies of the monofluorides D°0(SmF)=134 kcal/mole, D°0(EuF)=129 kcal/mole, and D°0(TmF)=121 kcal/mole, all ±2 kcal/mole, are in good agreement with values predicted by the Rittner electrostatic model, whereas values in the polyatomic fluorides show considerable variation and do not seem to follow any clear trends. Although the BDE values in some instances differ from previous estimates, their sums yield trifluoride heats of atomization that are in close accord with values derived from the vaporization thermodynamics of the solid trifluorides.
A new assignment of the ESR spectra obtained from a single crystal of dimethylnitramine photolyzed at low temperature
Authors: Michaut J. P. , Roncin J.
Published: February 2003

A new assigment is given for the radical generated by UV irradiation in a single crystal of dimethylnitramine. The structure of that radical is discussed on the basis of INDO calculations.
X-ray diffraction study of molten zinc chloride at 323 °C
Authors: Triolo R. , Narten A. H.
Published: February 2003

The x-ray diffraction pattern of liquid ZnCl2 has been measured. The distances and coordination numbers derived from the data indicate that the Cl− ions are closed packed, and that the Zn2+ ions are located in the tetrahedral holes typical of such a structure.
The G schemes of approximations to the thermal diffusion factors of Lorentzian and non-Lorentzian mixtures
Authors: Schwebel O. , Gouesbet G. , Le Toulouzan J. N. , Darrigo R.
Published: February 2003

In the Chapman–Enskog mathematical theory of nonuniform gases, the thermal diffusion factors are found from the solution of an infinite set of linear equations. Two methods of obtaining successive approximations to the solution of these equations have been developed, one by Chapman and Cowling, and the other by Kihara. The Kihara method is somewhat difficult to exactly interpret since it is too much based on a peculiar model, the so-called Maxwellian one. On the other hand, it is well known that the Chapman–Cowling series of approximation exhibits a rather poor convergence. So, a new scheme of approximation has been recently introduced in order to interpret plasma measurements previously reported. This scheme (the so-called G scheme) has been first developed in an almost-Lorentzian mixture, a situation too particular to make it really interesting unless it could be generalized and used for non-Lorentzian mixtures. This paper is concerned with such a generalization. The two first approximations to the thermal diffusion factors in an He/K gaseous mixture in the Chapman–Cowling and in the G schemes are computed, and results are discussed. The advantages of the G schemes are pointed out.
Polarization effects on rotational line strengths in collision-free polyatomic fluorescence spectra
Authors: Loge G. W. , Parmenter C. S.
Published: February 2003

Methods are given to calculate rotational line strengths in collision-free polyatomic electronic fluorescence spectra after single rotational levels are pumped with polarized (laser) excitation. Such fluorescence retains substantial polarization so that attempts to calculate line strengths with standard Hönl–London formulas or related procedures give significant errors. Illustrations with glyoxal S1→S0 fluorescence show good agreement between observed and calculated line strengths when polarization is accounted for. Specific formulas are given for the separate polarization components when partial depolarization of such fluorescence also occurs on account of external magnetic fields. Comparisons of field-free calculations with experimental data show that cross sections for collisional fluorescence depolarization (ΔM changes) in S1–S0 glyoxal collisions do not exceed those known for ΔJ, ΔK changes (about five times gas kinetic).