bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational relaxation of NO (v=1) by NO, N2, CO, HCl, CO2, and N2O from 300 to 600 K
Authors: Doyennette L. , Margottin-Maclou M.
Published: July 2002

The vibrational relaxation of NO, laser-excited to v=1, in collisions with NO or with other molecules, M=CO, N2, HCl, CO2, and N2O, has been studied by the laser-induced fluorescence technique, in the temperature range from 300 to 600 K. For the NO–M systems, the relaxation occurs through competitive far-from-resonance V–V transfer and V–T,R deexcitation processes. Measurements of the rate constants governing the temporal decay of the fluorescence intensity emitted by the laser-excited NO molecules were performed as a function of the molar fraction of NO. From these measurements, and those previously obtained on the same NO–M systems, but with the M molecules excited by laser, separate values, even if crude in some cases are deduced for the V–V transfer and V–T,R deexcitation rate constants. Except for NO–N2 and NO–CO, the efficiency of the V–T,R deexcitation of a molecule by collision on another one is shown to be as important as, or even greater than that of the V–V transfer from this molecule to the other one. For both NO–N2O and NO–CO2 systems, the experimental values of the V–V transfer rate constants are compared to values calculated on the basis of long-range multipolar interactions.
Highly excited vibrational levels of ‘‘floppy’’ triatomic molecules: A discrete variable representation—Distributed Gaussian basis approach
Authors: Bac̆ić Z. , Light J. C.
Published: July 2002

A novel, efficient, and accurate quantum method for the calculation of highly excited vibrational levels of triatomic molecules is presented. The method is particularly well suited for applications to ‘‘floppy’’ molecules, having large amplitude motion, on potential surfaces which may have more than one local minimum. The discrete variable representation (DVR) for the angular, bend coordinate is combined with the distributed (real) Gaussian basis (DGB) for the expansion of other, radial coordinates. The DGB is tailored to the potential, covering only those regions where V(r)<EMAX. The DVR permits a contraction of the primitive Gaussian basis to a small eigenfunction basis at each of the discretized values of the angular coordinate. It is shown for the floppy two-mode LiCN/LiNC system (fixed CN distance) that N lowest vibrational levels (N=131) can be converged to within 1 cm−1 (the lowest 117 to 0.1 cm−1) using only 3N basis functions. This appears to reduce the computational effort by a factor of 10–40 over standard methods. Moreover, only a very low order Gauss–Hermite quadrature, 3–5 points, is needed to evaluate each potential matrix element.
Dipoles induced by van der Waals interactions during collisions of atoms with heteroatoms or with centrosymmetric linear molecules
Authors: Bohr J. E. , Hunt K. L. C.
Published: July 2002

Using a reaction field model, we have derived symmetry-adapted expressions for the van der Waals dipoles of atom–heteroatom and atom-D∞h molecule pairs, with nonoverlapping charge distributions.The leading van der Waals contributions vary as R−7 in the intermolecular separation R and depend upon products of the polarizability ααβ(iω) of one molecule with a dipole–quadrupole hyperpolarizability Bαβ,γδ (0,iω) of the other, integrated over imaginary frequencies. We have developed new approximations for these integrals in terms of the static polarizabilities ααβ, the hyperpolarizabilities Bαβ,γδ, and the van der Waals energy coefficients C6 and C8 (both isotropic and anisotropic components for atom–molecule pairs). The approximations agree well with accurate perturbation results for two model systems. Applied to He⋅⋅⋅H2, our approximations give the first direct results for the leading van der Waals contributions to the dipole. In two symmetry components of the He⋅⋅⋅H2 dipole at long range, van der Waals effects are larger than induction effects; both should be included in fitting collision-induced roto-translational spectra.
Electronic transition moments for the A 2Σ+→X 2Π system of HCl+
Authors: Rodríguez A. , Campos J.
Published: July 2002

The emission spectra of 31 bands of HCl+(A 2Σ+→X 2Π) have been obtained using hollow cathode discharges. Relative intensities were determined by comparison with computer generated spectra. Treatment of the data within the r-centroid (r̄v′v″) approximation shows that the electronic transition moment decreases strongly for large internuclear distances (r centroid). The transition moment can be fitted by the expression R2e =0.543r̄ 3.29v′v″ ×exp(−2.69r̄ 2v′v″) in atomic units for 1.1<r̄v′v″ <1.8 Å.
Comment on constrained molecular dynamics of macromolecules
Authors: Toxvaerd So/ren
Published: July 2002

Molecular dynamics calculations for a free decane with and without bond and bond angle constraints show that the effect of freezing the bond angles is bigger—or of the same order of magnitude as the solvent effect from other decane molecules in a corresponding liquid decane. Especially the dynamical behavior, given by the trans–gauche transition rate is changed by the bond angle constraints, whereas only very small differences are obtained by freezing the bond lengths. The statistical means are first obtained after averaging over time intervals of the order nanoseconds.
The infrared spectrum of XeH+
Authors: Rogers S. A. , Brazier C. R. , Bernath P. F.
Published: July 2002

The Fourier transform emission spectrum of XeH+ was observed in the infrared region of the spectrum. The 1–0 and 2–1 vibration–rotation bands for 132XeH+, 131XeH+, and 129XeH+ were recorded from a nickel hollow cathode discharge in xenon and hydrogen. Molecular constants, including Re=1.602 813(6) Å, Be=6.560 686(50), αe=0.186 739(14), ωe=2269.9674(11), and ωexe=41.328 30(34) cm−1 for 132XeH+, were extracted from the line positions. Our work represents the first high-resolution detection of XeH+.
A
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t
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o calculations of correlation effects in t
r
a
n
s-polyacetylene
Authors: Liegener C.-M.
Published: July 2002

The fundamental band gap and the exciton binding energies of t-PA have been evaluated with the aid of ab initio Hartree–Fock plus Mo/ller–Plesset perturbation theoretical calculations. The irreducible self-energy part has been expanded up to third order and the irreducible vertex part has been expanded up to second order. The results support one of several controversial assignments of the ∼2 eV peak in the experimental absorption spectrum of trans-polyacetylene, namely that to interband transitions.
A thermodynamic analysis of solvation
Authors: Yu Hsiang-Ai , Karplus Martin
Published: July 2002

The free energy, energy, and entropy of solvation, relative to the pure liquid, are analyzed. By a coupling parameter integration it is shown that only averages over the solute–solvent interaction energy contribute to the free energy and that the solvent–solvent interaction term, which contributes the so-called cavity (solvent reorganization) term to the energy, is cancelled exactly by a corresponding term in the entropy. These terms exist even in the infinite dilution limit since they arise from the derivative of the free energy with respect to the solute density. Following the approach of Garisto et al. [J. Chem. Phys. 79, 6294 (1983)], the site–site Ornstein–Zernike integral equations and HNC closures are used to determine the derivatives of the distribution functions with respect to the density. This makes it possible to calculate the energetic and entropic contributions to the solvation free energy in the infinite dilution limit. The method is applied to pure solvent and to infinitely dilute aqueous solutions of cations, anions and neutral Lennard-Jones particles. The results are in agreement with numerical calculations of the thermodynamic quantities by use of finite difference values for the temperature derivatives. A simple empirical relation for the charge dependence of the solvation free energy is observed; it is shown for the case of an ion in a dipolar solvent, as typified by aqueous electrolyte solutions, that the free energy of solvation varies quadratically with the charge and is very nearly equal to one-half the solute–solvent portion of the solvation energy. Some discussion of the relation of the present results to entropy–enthalpy compensation and to computer simulations is given.
Application of projection techniques to the electron correlation problem
Authors: Becker Klaus W. , Fulde Peter
Published: July 2002

By applying projection techniques a unified description of weakly and strongly correlated electronic systems can be given. For that purpose the theory is formulated in Liouville instead of Hilbert space. By introducing an appropriate metric in terms of cumulants it is ensured that all energies are size consistent. An energy expression is derived which is a generalization of Goldstone’s linked cluster theorem for arbitrary Hamiltonians. It is suitable for applying the projection method of Zwanzig and Mori. The equations for the correlation energy within the Local Ansatz and within coupled electron pair approximation CEPA-2 follow straightforwardly from the present theory.
Electric double layer forces in the presence of polyelectrolytes
Authors: Åkesson Torbjörn , Woodward Clifford , Jönsson Bo
Published: July 2002

An electric double layer is studied by means of Monte Carlo simulations and mean-field theory. The counterions of the uniformly charged surfaces are modeled as flexible polyelectrolytes. For this particular model system it turns out that the traditional double layer repulsion becomes attractive for a wide range of systems. The main reason for this attraction is an entropically driven bridging mechanism, and its magnitude is significant compared to ordinary double layer or van der Waals forces. The polyelectrolyte Poisson–Boltzmann theory developed here behaves in a qualitatively correct manner, also predicting an attractive interaction extending over several nanometers. These results may have some relevance to technical and biological systems, where sometimes puzzling force behavior is seen in the presence of polyelectrolytes.
On the electronic structure of the NH radical. The fine structure splitting of the X 3Σ− state and the spin-forbidden (b 1Σ+, a 1Δ)→X 3Σ−, and the spin-allowed A 3Π→X 3Σ− and c 1Π→(b 1Σ+, a 1Δ), radiative transitions
Authors: Yarkony David R.
Published: July 2002

In this work the fine structure splitting of the X 3Σ− state of NH together with the spin-forbidden dipole-allowed radiative transitions

(b 1Σ+,a 1Δ)→X 3Σ− in that system are considered. In addition the spin-allowed A 3Π→X 3Σ− and c 1Π→(b 1Σ+,a 1Δ) transitions which provide valuable optical probes of the NH radical are studied. Symbolic matrix methods permit the use of large configuration state function (CSF) spaces (170–280×103 CSFs) in characterizing these effects. The fine structure splitting and spin-forbidden decay are described within the context of the Breit–Pauli approximation. In the determination of the fine structure splitting both ĤSO, the full microscopic spin–orbit and spin-other-orbit operator and ĤSS, the dipolar spin–spin operator, are considered through second order in pertubation theory. The

completely ab initio determination of λ0≡[E(X 3∑−1, v=0)−E(X 3∑−0+ , v=0)]/2, presented here gives λ0=0.903 (0.9198) cm−1 in good accord with the experimental value given parenthetically. The predicted radiative lifetimes for the v=n level of the A 3Π state τn (A 3Π)(corresponding to A 3Π, v=n→X 3Σ−) are τ0(A 3Π)=392[418±8, 453±10]ns and τ1(A 3Π)=438[420±35, 488±10] ns in good agreement with the experimental values given parenthetically. The predicted radiative rate for the v=0 level of the c 1Π state is somewhat slower than the total decay rate measured experimentally suggesting predissociation of even the lowest rotational levels. The radiative lifetime for the v=0 level of the a 1Δ state, τ0(a 1Δ) corresponding to the spin-forbidden dipole-allowed transition a 1Δ2→X 3∑−1 was found to be τ0(a 1Δ)=2.18(>1.9)s which compares favorably with the lower bound determined from matrix isolation experiments given parenthetically.

For the b 1Σ+→X 3Σ−(0,0) transition the ratio of the parallel to the perpendicular transition moment was found to be μ∥/μ⊥=−0.30 (−0.35±0.05) which again compares favorably with the experimental result given parenthetically. This result is qualitatively different from that in the isovalent systems NF, NCl, and NBr for which −μ∥‖μ⊥ >1. A partial explanation for this result in terms of molecular dipole moments is offered. The predicted radiative lifetime for the b 1Σ+, v=0→X 3Σ− transition τ0(b1 Σ+)=100 ms, which is in excellent agreement with the independent theoretical determination τ0(b 1Σ+)=97 ms of Marian and Klotz, is significantly longer than the most recent gas phase measurement τ0(b 1Σ+)=53(+17−13)ms.
Interface response function for a model of sodium: A molecular dynamics study
Authors: Tymczak C. J. , Ray John R.
Published: July 2002

Molecular dynamics studies of the liquid epitaxial crystallization and melting of a model of sodium have been carried out to determine the (001) velocity vs interface temperature and to investigate the microscopic growth and melting mechanisms. The growth involves the correlated growth of at least seven planes. We discuss two independent ways of determining the velocity of growth and melting from the simulation data. Details of how the interface temperature is determined and the variation of the system energy with time in both crystallization and melting are included. The maximum growth velocity is 150 m/s and occurs at approximately one-half the melting temperature. The system continues to crystallize to the lowest temperature investigated (81 K) and the growth is not diffusion limited. If conventional transition state theory is used to describe the simulation data then there is a slope discontinuity at the melting temperature with the melting velocities being much higher than predicted. The average structure factors of planes buried in the crystal vs temperature show similar discontinuities near the melting point and suggests that the higher melting velocities are associated with anharmonic softening.
Diffusion in supercooled liquids via normal mode analysis
Authors: Madan B. , Keyes T. , Seeley G.
Published: July 2002

A theory is developed for diffusion in supercooled liquids. The viewpoint taken is that the dynamics of system, represented by a point in configuration space, consists of long sojourns in the vicinity of the local minima of the total potential, interrupted by relatively infrequent barrier crossings to adjacent minima. An equation due to Zwanzig, plus a new relation for the hopping rate among the local minima, are combined in a theory for the self-diffusion constant D. All the quantities entering the two equations are shown to be derivable from the configuration-averaged densities of vibrational states, 〈ρ(ω)〉, introduced by us previously; the contribution of unstable modes plays a crucial role. Solution of the coupled equations reveals the existence of a critical temperature below which D vanishes. The theory is in excellent agreement with molecular dynamics values for D, over a wide temperature range in which D displays activated behavior, in Lennard-Jones argon at a reduced density of unity.
Experimental evidence of vibrational mode selectivity in the indirect predissociation of N2O+ A 2Σ+. Energy distribution of the diatomic fragment and comparison with a model prediction
Authors: Richard-Viard M. , Atabek O. , Dutuit O. , Guyon P. M.
Published: July 2002

The predissociation of the A 2Σ+ state of N2O+ has been reinvestigated with a threshold photoelectron–photoion coincidence experiment using synchrotron radiation as the excitation source and a newly designed spectrometer. The NO+ fragment vibrational distribution resulting from the predissociation of (1,0,0), (0,0,1), (2,0,0), (1,0,1), and (3,0,0) levels varies with vibrational excitation of the parent ion and in all cases is inverted and narrow. Both NO+(X 1Σ+)+N(4S0) and NO+(X 1Σ+)+N(2D0) limits are observed. Predissociation of the A state to the 2 4Σ− dissociative continuum is shown to proceed via an indirect mechanism involving the 1 4Π bound state. Close coupled quantum calculations were carried out with a simplified model for indirect predissociation in which the bending motion is neglected. They show the occurrence of vibrational population inversions in the fragment distributions, as observed experimentally.
Computer simulation of fluids interacting with fluctuating walls
Authors: Lupkowski Mark , van Swol Frank
Published: July 2002

We present a new method for performing molecular simulations of systems with planar interfaces in the presence of fluctuating walls. The technique is essentially the realization of a frictionless piston exerting an external potential on a fluid within a container. The simulation cell is constructed by placing walls at each of the two ends and allowing periodic boundary conditions in the other two directions. Instead of holding the walls rigid, as in a conventional NVT simulation, an external field is imposed on either one or both walls which are free to move. In this paper we focus on a dynamic implementation of the method. The simplest case is a constant external force which is equivalent to imposing a constant normal pressure. We test the method through comparisons with conventional NVT molecular dynamics simulations. The results of these calculations indicate that the new method is both practical and reliable.
Measurements of electric dipole moments on NO2
Authors: Heitz S. , Lampka R. , Weidauer D. , Hese A.
Published: July 2002

Electric dipole moments of the A 2B2 and X 2A1 states of NO2 were measured by high resolution laser spectroscopy in a supersonic molecular beam under a static electric field ranging up to 300 kV/cm. The measurements were performed on the qR(0)00 transition of band 99 at 593 nm. The results for the excited states (101) are μb(J′ = 1/2)=0.278(6)D, μb(J′ = 3/2)=0.254(6)D and for the ground state (000) is μb=0.289(6)D. A perturbation of the Stark pattern by an unknown background state could be localized at field strengths around 210 kV/cm.
Liquid crystalline properties of solutions of persistent polymer chains
Authors: DuPré Donald B. , Yang Shi-jun
Published: July 2002

A synthesis of theories for the orientational entropy and excluded volume interaction for persistent chain polymers at intermediate degrees of chain flexibility, when extended to liquidlike densities, is shown to satisfactorily account for experimental data on the volume fractions of polymer at coexistence of isotropic and anisotropic phases, the concentration dependence of the order parameter within the lyotropic liquid crystal and the osmotic pressure equation of state for several extended chain polymers. Results of this continuum model are also compared with the virial approximation and treatments of the macromolecule as a rigid rod.
A rate process with an entropy barrier
Authors: Zhou Huan-Xiang , Zwanzig Robert
Published: July 2002

This paper presents a simple dynamical model of a rate process in which the rate appears to be controlled by an entropy barrier, rather than an energy barrier. The model consists of independent particles moving in a two-dimensional region bounded by four reflecting disks. The particles collide elastically with the walls. A bottleneck separates the region into reactants and products. The extent of the reaction is followed by using computer simulations to get the time dependence of the number correlation function of reactants. The particle dynamics are either frictionless (inertial), moderately frictional (Langevin dynamics), or strongly frictional (Brownian dynamics). For small bottlenecks, the number correlation function generally decays in time as a single exponential. The transition rate in the frictionless limit is predicted correctly by microcanonical transition state theory. As the strength of the friction is increased, the rate changes to the diffusive limit without the usual Kramers turnover.
Dynamic solvent effects on electron transfer rates in the inverted regime: Ultrafast studies on the betaines
Authors: Åkesson Eva , Walker Gilbert C. , Barbara Paul F.
Published: July 2002

Transient pump–probe measurements have been made on betaine-30, the solvent polarity probe. Betaine-30 is composed of organic donor and acceptor groups, D+A−⇄kethνDA (excited state). The transient data has been analyzed to measure ket, the electron transfer (et) rate constant, in polar solvents for this inverted regime et. The observed rate constant ket,obs in various polar solvents is as much as a factor of 108 larger than is predicted by the standard, classical, implementation of the theory of Sumi and Marcus, using Kjaer and Ulstrup’s estimates for necessary parameters. In contrast, a quantum treatment of the vibrational modes by Jortner and Bixon, which includes the dynamic solvent effect, predicts that the rate should be approximately ket,JB≂1/〈τs〉. This is close to the experimental results (≂1012 s−1) in typical polar aprotic solvents. However, in highly viscous, slowly relaxing solvents, ket,JB≪ket,obs. This is taken as evidence that the efficient coupling modes for the et in these environments is not the ‘‘conventional solvent coordinate.’’ It is shown that a hybrid of the Jortner/Bixon and Sumi/Marcus theories can physically account for the results. The kinetic behavior of betaine-30 in slowly relaxing solvents is shown to be in the ‘‘diffusionless limit’’ of the hybrid, generalized Sumi–Marcus/Jortner–Bixon picture.
Modified Ar core a
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n
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o relativistic effective potentials for transition metals Sc through Cu
Authors: Fernandez Pacios L. , Botella Olcina V.
Published: July 2002

A b initio relativistic effective potentials (REP) for first row transition metals show large errors in excitation energies between s2dn, s1dn+1, and dn+2 states if only 4s and 3d electrons are included in the valence space (Ar core). These errors are diminished when the potentials are generated including also 3s and 3p electrons in the valence space (Ne core), but thus one of the main advantages of the REP procedure is lost. The origin of such errors in Ar core potentials is first analyzed in this work and a modification to their construction is proposed which, while retaining the Ar core choice, clearly improves the general performance of the potentials.
The Coulombic phase transition: Density-scaling Monte Carlo
Authors: Valleau John P.
Published: July 2002

The predicted low-temperature, low-density phase separation of Coulombic systems has been sought by applying the recently developed Monte Carlo technique of ‘‘density scaling’’ to a fluid of charged spheres. The existence of the transition is confirmed. As well as locating it, the method allows estimates of the thermodynamics of the transition.
Dynamics and spatial correlations of tetrapentylammonium ions in acetonitrile
Authors: Kunz W. , Turq P. , Bellissent-Funel M.-C. , Calmettes P.
Published: July 2002

Quasielastic neutron scattering (QENS) spectra were measured for a 0.43 M solution of n-tetrapentylammonium bromide in deuterated acetonitrile at 25 °C, 5 °C, and −15 °C. Values of the translational diffusion coefficient of the cations were inferred from these data using a simple model of translation and rotation. These values are significantly higher than the ones obtained by nuclear magnetic resonance (NMR) spin–echo measurements. The difference can be explained by the different time scales covered by QENS and NMR. QENS shows essentially the contribution of a second order electrophoretic effect to the diffusion coefficient whereas NMR encompasses both electrophoretic and relaxation effects. Consequently, the combination of both techniques allows the two effects to be separated. The relaxation contribution to the diffusion coefficient was calculated by brownian dynamics simulation and compared to the experimental results. The solvent-averaged ion pair potentials used for this computation were simultaneously adjusted to the thermodynamic and to the small-angle neutron scattering data by means of hypernetted chain (HNC) calculations.
Brownian dynamics simulation of electrooptical transients for solutions of rigid macromolecules
Authors: Antosiewicz Jan , Grycuk Tomasz , Porschke Dietmar
Published: July 2002

The overall rotational diffusion of rigid macromolecules in solution under rectangular electric field pulses is simulated by Brownian dynamics. We describe computer programs for the simulation of electrooptical transients without restrictions on molecular parameters or electric field strengths. The programs are used first for the calculation of electrooptical transients of molecules with cylindrical symmetry with induced or permanent dipole moments. The simulated data are consistent with analytical results, valid, e.g., for the limit of zero field strength, but have been extended to ranges, where analytical results are not available. Among the two time constants required for fitting of rise curves for permanent dipoles, the smaller one proves to be almost independent of the electric field strength E, whereas the larger one decreases strongly with increasing E; at high E values the two time constants are very close to each other. By comparison of simulated and experimental transients it is possible to analyze hidden contributions, e.g., of an induced dipole moment in the presence of a dominant permanent moment. The simulations are extended to the case of a molecule without symmetry, tRNA, which is used to characterize the hydrodynamic coupling of translational and rotational motion. We show that in this case the influence of hydrodynamic coupling on the dipole moment, the limiting reduced dichroism and the risetime constants derived from electrooptical experiments is very small (≤10%).
A general multireference configuration interaction gradient program
Authors: Shepard R. , Lischka H. , Szalay P. G. , Kovar T. , Ernzerhof M.
Published: July 2002

An efficient and general method for the computation of analytic energy gradients and energy response properties for general MRCI (multireference configuration interaction) and ACPF (averaged coupled pair functional) wave functions is presented. This methodology includes a general approach, based on successive orbital transformations, for the inclusion of the effects of various orbital resolution (canonicalization) constraints. Initial implementation in the columbus Program System demonstrates, particularly for large-scale multireference wave functions, that the additional computational effort required for the energy gradient is a small fraction of that required for the energy. For polyatomic molecules, the computational resources required for the energy gradient do not depend explicitly on the number of constituent atoms. This combination of features represents a major step forward in the computation and characterization of molecular potential energy surfaces.
Water structure in concentrated lithium chloride solutions
Authors: Tromp R. H. , Neilson G. W. , Soper A. K.
Published: July 2002

The radial pair distribution functions gHH(r) and gOH(r) (to a good approximation) of 1 and 10 m solutions of lithium chloride in water have been obtained from neutron diffraction. It turns out that the intermolecular water structure in a solution of 10 m is affected considerably by the presence of ions—the number of hydrogen bonds is about 70% lower than in pure water. The intermolecular water structure in 1 m lithium chloride as well as the intramolecular water structure in both 1 and 10 m lithium chloride is not distinguishable from that of pure water in any measurable extent.
Signs of the quadrupole coupling in 7LiNO3 from nuclear magnetic resonance at low temperature
Authors: Kuhns Philip L. , Waugh J. S.
Published: July 2002

The sign of the nuclear quadrupole coupling in a solid can be determined from the nuclear magnetic resonance (NMR) spectrum if the temperature is sufficiently low. This fact is illustrated by a measurement of e2qQ=+926 kHz for 7Li in LiNO3.
On the ab initio determination of higher-order force constants at nonstationary reference geometries
Authors: Allen Wesley D. , Császár Attila G.
Published: July 2002

Several complementary analyses have been performed in an investigation of the use of reference geometric structures which are not stationary at a given level of theory in the prediction of improved equilibrium anharmonic molecular force fields. Diatomic paradigms for the procedure were established by constructing empirical potential energy functions for the nitrogen and fluorine molecules which not only reproduce the available Rydberg–Klein–Rees data but also provide reliable derivatives through fourth order for ranges of 0.4 Å or greater around the equilibrium bond distance. For comparison, analogous curves were determined at the double-ζ plus polarization (DZP) restricted Hartree–Fock (RHF) level of theory, and the quartic force fields for N2 and F2 were also obtained at the experimental re structures using a (8s5p3d2f1g) basis set and the coupled-cluster singles and doubles method augmented by a perturbative contribution from connected triple excitations [CCSD(T)].

The results substantiate the ability of RHF theory to predict correlation-quality, higher-order force constants if an accurate reference geometry from experiment or a higher level of theory is employed. The theoretical foundations of this technique as applied to polyatomic molecular systems have been systematically explored. Mechanisms were analyzed which address the nonzero force dilemma by using various choices of internal coordinates to shift the equilibrium point of theoretical potential energy surfaces. Examples are presented in which the variations in predicted spectroscopic constants arising from different shift coordinate sets are non-negligible. A Cartesian projection scheme for higher-order force fields was developed and implemented to avert internal-coordinate dependences; formulas for higher-order projection matrices and higher-order derivatives of the external variables of a molecular system were concurrently derived. A formalism for the transformation of force fields between internal and Cartesian representations was also constructed which is applicable to arbitrary order. In addition to N2 and F2, case studies were performed on the F2O and N2O molecules, for which electron correlation effects are of unusual importance. Quartic force fields are reported for F2O and N2O at the DZP and TZ(2d1f) CCSD(T) levels of theory, respectively, which provide the best data sets currently available and facilitate the assessment of experimental force constants. The CCSD(T) results are reproduced remarkably well by RHF predictions at the experimental equilibrium structures of these molecules but not at the corresponding RHF optimum geometries. Finally, practical recommendations are made for predictions of higher-order force constants at nonstationary points.
Molecular dynamics study of the conformational properties of cyclohexadecane
Authors: Zhang Renshi , Mattice Wayne L.
Published: July 2002

Molecular dynamics has been used for the first time for the study of the conformational properties of cyclohexadecane, c-C16H32. By analyzing a long molecular dynamics trajectory (14.5 ns) at 450 K, equilibrium statistics such as the relative populations of different isomeric conformers and the probability ratios, p(gt)/p(tt), p(gg)/p(tt), and p(gg)/p(gtg), of different conformational segments, have been studied. The dynamic properties including the transition modes of gauche migration and gauche-pair creation, which have been reported before in n-alkanes, and the auto- and cross-correlations of the bond dihedral angles, have also been obtained. It was possible to make direct comparisons on some of the statistics with theory and experiment. Most of the results extracted from the molecular dynamics trajectory lie in between previously reported experimental and theoretical values. Many previously predicted conformers have been confirmed by our simulations. The results of the population probability of the most populated conformer seems to suggest that an earlier discrepancy between the theoretical works and an experimental work originates from insufficient samplings in earlier theoretical works, rather than from their inaccurate force field.
High resolution photoelectron spectroscopy and femtosecond intramolecular dynamics of H2CO+ and D2CO+
Authors: Niu Baohua , Shirley David A. , Bai Ying
Published: July 2002

High resolution helium Iα (584 Å) photoelectron spectra of H2CO and D2CO are reported. The present study reveals much new vibrational structure detail in the ionic first excited state of formaldehyde. Weak excitations of the ν3 (in H2CO) and ν1 (in D2CO) modes along with the strong excitations of the ν2 mode in the ionic first excited states are fully resolved for the first time. The weak excitations of the ν4 out-of-plane bending mode in the ionic ground and first excited states of formaldehyde cations indicate that they may have nonplanar equilibrium geometries. Strong isotope effects on vibronic (vibrational) couplings are observed in the cation first and second excited states. Vibrational autocorrelation functions are calculated from the high-resolution photoelectron spectra. The correlation functions calculated for the first electronic excited states show rather slow decay rate on the femtosecond time scale. The ultrafast decay of the formaldehyde cations implied by the correlation functions calculated for the third electronic excited states suggest that dissociation and intramolecular dynamic processes are the main decay pathways.
Zero kinetic energy photoelectron spectra of jet-cooled aniline
Authors: Song Xinbei , Yang Mo , Davidson Ernest R. , Reilly James P.
Published: July 2002

Zero kinetic energy electron spectra of aniline are recorded and a number of ion vibrational frequencies accurately measured. The adiabatic ionization potential of aniline is found to be 62 271±2 cm−1. Ab initio calculations provide insights into the bonding changes that take place in aniline upon ionization. Reasonable agreement between calculated and measured frequencies in the ground states of aniline neutral and ion is obtained.
High resolution infrared spectroscopy of pyrazine and naphthalene in a molecular beam
Authors: Hewett Kevin B. , Shen Meihua , Brummel Christopher L. , Philips Laura A.
Published: July 2002

The high resolution infrared spectrum of pyrazine and naphthalene were measured in a molecular beam in the vicinity of the C–H stretching transition. The rotational structure in the spectrum of pyrazine from 3065–3073 cm−1 reveals that the C–H stretch is coupled to one other vibrational mode in the molecule. The mode coupling is manifested in the spectrum as two overlapping vibrational bands. Each of these two bands are well modeled by an asymmetric top/rigid rotor Hamiltonian. The lack of any angular momentum dependence on the coupling indicates that the vibrations are coupled by an anharmonic mechanism. The magnitude of the coupling matrix element was determined to be 0.36 cm−1. The rotational structure in the spectrum of naphthalene from 3063–3067 cm−1 reveals that except for several local perturbations, the spectrum is well modeled by an asymmetric top/rigid rotor Hamiltonian. The local perturbations include transitions that are split into doublets as well as transitions that have been shifted from their expected positions. The magnitude of the average coupling matrix element for the doublets was determined to be 0.0016 cm−1. A comparison between the vibrational mode coupling in pyrazine and naphthalene indicates that mode coupling does not correlate with the density of states in the two molecules.
Structure, vibrational frequencies, and thermodynamic properties of hydrogen peroxide dimers: An ab initio molecular orbital study
Authors: Mó O. , Yáñez M. , Rozas I. , Elguero J.
Published: July 2002

High levels of ab initio molecular orbital theory were used to study the structures, binding energies, vibrational frequencies, and equilibrium constants of hydrogen peroxide dimers. The geometries of the different initial structures considered were optimized at the HF/6–311++G(2d,2p) level of theory. Five different stationary points have been characterized at this level, but only two of them were minima. The geometries of these two minima were refined at the MP2/6–311+G(d,p) level. Their vibrational frequencies, calculated at the same level of theory, show a sizeable redshift of the stretching vibrations of the proton donors. The global minimum corresponds to a six-membered ring of Ci symmetry, while the second minimum is a five-membered ring, which lies about 1.1 kcal mol−1 above the global one. The formation of the latter implies a considerable enhancement of the dipole moment. The binding energies of these two species were obtained at the QCISD(T)/6–311+G(2d,p) level using the MP2 optimized geometries. The equilibrium dimerization constants for hydrogen peroxide are considerably smaller than those for water, due to significant entropic effects. A topological analysis of the electronic charge densities of the dimers shows that both cyclic minima present weaker hydrogen bonds than noncyclic dimers.
Size-extensivity corrections in configuration interaction methods
Authors: Duch Wl/odzisl/aw , Diercksen Geerd H. F.
Published: July 2002

Limited configuration interaction methods suffer from size-extensivity errors. The origin and behavior of these errors is discussed and new versions of single and multireference corrections are presented. Accuracy of the new and various other size-extensivity corrections used in the literature is discussed and compared in a series of model calculations and calculations on small molecules. None of the commonly used multireference corrections restores the size extensivity of multireference configuration interaction calculations. Our correction behaves correctly for the special case of a reference state composed from all singly and doubly excited configurations. Formulas for size extensivity corrections in the variational-perturbation method are given and discussed.
Integrating the Car–Parrinello equations. I. Basic integration techniques
Authors: Tuckerman Mark E. , Parrinello Michele
Published: July 2002

In this paper and in a companion paper [M. E. Tuckerman and M. Parrinello, J. Chem. Phys. 101, 1316 (1994)] the problem of integrating the equations of motion in Car–Parrinello simulations is addressed. In this paper, new techniques for treating the constraint problem based on the velocity Verlet integrator and the Gaussian dynamics are presented. Questions of adiabaticity and temperature control are discussed, and it is shown how to combine the new techniques with the recently developed Nosé–Hoover chain thermostat method. All new techniques are described using the formalism of operator factorizations applied to the classical Liouville propagator. In the companion paper, the formalism and application of multiple time scale methodology in Car–Parrinello simulations are discussed.
Relativistic and correlation effects in CuH, AgH, and AuH: Comparison of various relativistic methods
Authors: Collins Charlene L. , Dyall Kenneth G. , Schaefer Henry F.
Published: July 2002

The effects of relativity on the bond lengths, dissociation energies, and harmonic vibrational frequencies of the 1Σ+ electronic ground states of the group IB hydrides CuH, AgH, and AuH have been evaluated with a variety of ab initio methods. These properties were investigated with moderately-sized basis sets at the self-consistent field Hartree–Fock (SCF-HF) level and with second-order Mo/ller–Plesset (MP2) perturbation theory for electron correlation. Comparisons were made between all-electron results using the nonrelativistic Hamiltonian, perturbation theory (PT-MVD) at first-order with only the one-electron nonfine-structure terms of the Breit–Pauli Hamiltonian, the spin-free Douglas–Kroll (DK) transformed Dirac Hamiltonian and the untransformed Dirac Hamiltonian, and results using two sets of relativistic effective core potentials (RECPs). The expected trends of bond length decrease, dissociation energy increase, and harmonic vibrational frequency increase with both relativity and correlation are found. Both sets of RECPs are shown to give good results, if accompanied by a reasonable basis set. The DK method is demonstrated to be an inexpensive, reliable approximation to the DHF method.
Intensities of forbidden pure torsional bands in S1–S spectra of toluenes
Authors: Walker Robert A. , Richard Erik , Lu Kueih-Tzu , Sibert Edwin L. , Weisshaar James C.
Published: July 2002

In toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck–Condon forbidden, pure torsional transitions m21, m30, and m41 invariably appear in S1–S0 absorption spectra. The intensities are typically 1%–5% of the allowed bands, m00 and m11. Determination of the preferred conformer in S1 as staggered or eclipsed relies on proper assignment of the upper state of the m30 band as 3a″1 or 3a″2 under the molecular symmetry group G12. In addition, inferences of the preferred conformer in S0 and D0 (ground state cation) from band intensities of fluorescence and threshold photoionization spectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both –CH3 and –CD3 rotors, we present experimental relative intensities for the three forbidden S1–S0 absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S1–S0 electric dipole transition moment predicts relative forbidden band intensities in quantitative agreement with experiment. This fixes the assignment of the upper state of the key m30 transition as 3a″1, establishes the preferred S1 conformer as staggered for all six molecules, and places earlier inferences of the preferred conformers in S0 and D0 on solid ground.
Dressed ion theory for bulk symmetric electrolytes in the restricted primitive model
Authors: Ennis Jonathan , Kjellander Roland , Mitchell D. John
Published: July 2002

The correlation functions of bulk symmetric electrolytes in the restricted primitive model are analyzed using the formalism of the dressed ion theory of Kjellander and Mitchell [Chem. Phys. Lett. 200, 76 (1992), J. Chem. Phys. 101, 603 (1994)]. An important result of this analysis is that the exact theory for the pair correlation functions can be cast in a form that is virtually identical to that of the Debye–Hückel theory provided one uses the renormalized charges of the ions—‘‘dressed ions’’—instead of bare ion charges. In the current work the (state dependent) charges of these dressed ions are investigated using the HNC approximation. The asymptotic decay of the pair correlation functions are analyzed in terms of the effective point charges of the ions and the effective permittivity of the electrolyte solution, concepts which are given rigorous and physically transparent definitions in the dressed ion theory. Several transitions between regimes with different qualitative behavior of the pair correlation functions are demonstrated, in addition to the well-known transition between monotonic and oscillatory damped decay. By means of an analysis of the singularities of the correlation functions in complex Fourier space, dressed ion theory provides a general method to determine these transitions from numerical pair correlation data and in this paper results are given for symmetric electrolytes with monovalent and divalent ions.
Microrheological modeling of weakly aggregated dispersions
Authors: Potanin A. A. , De Rooij R. , Van den Ende D. , Mellema J.
Published: July 2002

A microrheological model of aggregating dispersions is proposed in which the shear stress is estimated as the sum of hydrodynamic and structural parts. The former is attributed to the hydrodynamic cores of fractal aggregates, which behave as a suspension of impermeable spheres. The latter accounts for the forces transmitted by chains of particles linking neighboring aggregates into a transient network. To calculate the structural part the concept of fractal aggregation is incorporated into a transient network theory, to account for the creation and breakup of chains of colloidal particles connecting the aggregates. Rigid and soft chains are distinguished. The former have multiply connected backbones which deform as contorted elastic rods, while the latter have at least one soft junction and deform without elastic resistance until fully loaded. The contribution of the soft chains to the stress tensor is neglected. The calculations treat two different mechanisms for the evolution of rigid chains: a purely mechanical one, which corresponds to a shear-controlled structure built up in flow, and a thermal mechanism, which pertains to a quasiequilibrium structure undisturbed by shear. We calculate steady-shear viscosities in the former case and viscoelastic functions in the latter. The model can be fitted satisfactorily to the experimental results for a well-characterized polystyrene latex dispersion with physically acceptable parameters.
Rate coefficients for state-to-state rovibronic relaxation in collisions between NO(X 2Π, ν=2, Ω, J) and NO, He, and Ar at 295, 200, and 80 K
Authors: Islam Meezanul , Smith Ian W. M. , Wiebrecht Jörg W.
Published: July 2002

The state-to-state rates of collisional energy transfer within and between the rotational level manifolds associated with the Ω=1/2 and Ω=3/2 spin–orbit states of NO(X 2Π, ν=2) have been measured using an infrared–ultraviolet double resonance (IRUVDR) technique. NO molecules were initially prepared in a specific rovibronic level, for example, ν=2, Ω=1/2, J=6.5, by tuning the output from an optical parametric oscillator (OPO) to a suitable line in the (2,0) overtone band. Laser-induced fluorescence (LIF) spectra of the A 2Σ+–X 2Π (2,2) band were then recorded at delay times corresponding to a small fraction of the average time between collisions in the gas sample. From such spectra, the relative concentrations of molecules in levels populated by single collisions from the initially prepared state could be estimated, as could the values of the rate coefficients for the state-to-state processes of collisional energy transfer. Measurements have been made with NO, He, and Ar as the collision partner, and at three temperatures: 295, 200, and 80 K. For all collision partners, the state-to-state rate coefficients decrease with increasing ΔJ (i.e., change in the rotational quantum number and rotational angular momentum) and increasing ΔErot (i.e., change in the rotational energy). In NO–NO collisions, there is little propensity for retention of the spin–orbit state of the excited molecule. On the other hand, with He or Ar as the collision partner, transfers within the same spin–orbit state are quite strongly preferred. For transfers between spin–orbit states induced by all collision partners, a propensity to retain the same rotational state was observed, despite the large change in internal energy due to the spin–orbit splitting of 121 cm−1. The results are compared with previous experimental data on rotational energy transfer, for both NO and other molecules, and with the results of theoretical studies. Our results are also discussed in the light of the continuing debate about whether retention of angular momentum or of internal energy is the dominant influence in determining the rates of state-to-state rotational energy transfer.
Nature of optical transitions in conjugated oligomers. II. Theoretical characterization of neutral and doped oligothiophenes
Authors: Cornil J. , Beljonne D. , Brédas J. L.
Published: July 2002

The optical transitions in α-conjugated oligothiophenes are investigated on the basis of a theoretical approach, including correlation effects; the authors consider both the neutral and doped states. The results are able to provide a coherent interpretation of the sometimes contradictory experimental data. For the neutral oligomers, the authors report the evolution of the lowest energy, dipole-allowed triplet–triplet transition with increasing chain length.