bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure of alpha-CF4 at low temperature
Authors: Pep̀e Gérard , Gay Jean-Marc
Published: July 2002

The structural data on the molecular packing of alpha-CF4 found in the literature are contradictory. The crystal static energy is calculated in order to determine the orientation of the CF4 molecule in the cell. The results agree with molecular dynamics calculations previously reported in the literature, whereas the interpretation of x-ray measurements must be somewhat changed. Moreover, the x-ray pattern calculated from the proposed packing is closer to the observed one.
Exact wave packet propagation using time-dependent basis sets
Authors: Kucar J. , Meyer H.-D.
Published: July 2002

The use of a time-dependent basis set for solving the time-dependent Schrödinger equation is proposed. The basis set is a generalization of the harmonic oscillator functions. The proposed method is similar to the approaches formulated by Lee and Heller and by Coalson and Karplus. However, in the present method the time dependence of the basis functions is determined differently. Numerical studies on the quartic oscillator and on the Morse potential are carried out to investigate the performance of the various methods.
Tetracritical and novel tricritical points in sulfur solutions: A Flory model for polymerization of rings and chains in a solvent
Authors: Corrales L. René , Wheeler John C.
Published: July 2002

A model for equilibrium polymerization of rings and chains in a solvent is solved in a Flory-like approximation. The presence of polymeric rings leads to interesting new kinds of phase equilibrium and higher order critical points. We find a higher order critical point analogous to a tetracritical point in a corresponding magnet, at which four phases come into simultaneous equilibrium, as well as novel tricritical points where three critical lines meet at a cusp. The resulting phase diagrams give improved agreement with those of sulfur solutions with cis-decalin and with ortho-xylene. The model has interesting consequences for the predicted behavior of magnets as well. It reduces to the earlier theories of Tobolsky and Eisenberg, Scott, Wheeler, and Pfeuty, and Petschek, Pfeuty, and Wheeler in various limits, but exhibits new behavior not seen in any of the previous theories.
Molecular dynamics for discontinuous potential. IV. Lennard-Jonesium
Authors: Chapela Gustavo A. , Scriven L. E. , Davis H. T.
Published: July 2002

Argon, represented as a Lennard-Jones fluid, is simulated using a discontinuous version of the potential. The effect of discretization is thoroughly investigated in terms of the number of discontinuities and the cutoff diameter. Radial distribution functions, pressure, energy, self-diffusion coefficients, and velocity autocorrelation functions are used in order to evaluate this effect. A reduction of five times on computing time can be achieved by varying the cutoff radius and the number of discontinuities without much departure from the physical representation of Lennard-Jonesium atoms. The results indicate that discontinuous molecular dynamics is a viable alternative method to Monte Carlo and continuous molecular dynamics in simulating the properties of simple fluids.
The singlet–triplet splittings in AsH+2, SbH+2, and BiH+2 and bond energies and ionization potentials of AsH2
Authors: Balasubramanian K.
Published: July 2002

The three low-lying electronic states (1A1, 3B1, 1B1) and their five spin–orbit states of AsH+2, SbH+2, and BiH+2 are investigated using complete active space MCSCF/second-order configuration interaction/relativistic CI schemes (CASSCF/SOCI/RCI). In addition the X 2B1 ground state and the excited 2A1 state of AsH2 and the X 3Σ− state of AsH are studied at the same levels of theory. The CASSCF/SOCI calculations yield De (HAs–H)=69.1 kcal/mol and De (AsH)=62.4 kcal/mol in excellent agreement with experimental values of D0(HAs–H)=66.5 kcal/mol and D0(As–H)=64.6 kcal/mol obtained by Berkowitz recently. The adiabatic CASSCF/SOCI ionization potential of the X 2B1 state of AsH2 to form the X 1A1 state of AsH+2 is 9.25 eV in comparison to an experimental value of 9.44 eV obtained by Berkowitz and Cho. The X 1A1–3B1 separations of AsH+2, SbH+2, and BiH+2 are calculated as 22, 31, and 35 kcal/mol, respectively. All the three ions were found to have bent equilibrium structures. The spin–orbit effects are found to be very significant for both BiH+2 and SbH+2, which changed the bond angle of 3B1(A1) to a considerable extent by contamination with 1A1. The relativistic density matrices and dipole moments are also obtained for all the species from the RCI wave functions.
Calculations of nonadditive effects by means of supermolecular Mo/ller–Plesset perturbation theory approach: Ar3 and Ar4
Authors: Chal/asiński G. , Szczȩśniak M. M. , Cybulski S. M.
Published: July 2002

Nonadditive, multibody effects arising in the supermolecular Mo/ller–Plesset perturbation theory (MPPT) (IMPPT) calculations are classified and interpreted in terms of the exchange, induction, deformation, and dispersion contributions, as defined by the perturbation theory of intermolecular forces. As an example the many-body effects in the equilateral Ar trimer and tetrahedral Ar tetramer, calculated through the MP4 level of theory with extended basis [7s4p2d], are reported and discussed. It is stressed that the ‘‘Heitler–London-exchange plus dispersion’’ model for nonadditive effects is too attractive mainly because of the neglect of the second-order exchange contribution.
Photodissociation of 2-bromoethanol and 2-chloroethanol at 193 nm
Authors: Hintsa Eric J. , Zhao Xinsheng , Lee Yuan T.
Published: July 2002

2-Bromoethanol and 2-chloroethanol were photodissociated in a molecular beam at 193 nm. Only one primary reaction channel was observed, elimination of the halogen atom, with an average translational energy release of 33 kcal/mol. In the case of 2-bromoethanol, some of the C2 H4 OH partner fragment survived and some underwent secondary dissociation to produce C2 H4 and OH. The surviving C2 H4 OH contained up to 43 kcal/mol of internal energy, far more than the expected C2 H4 -OH bond energy of ∼28 kcal/mol. The initial C–Br recoil occurs with a large exit impact parameter and leaves most of the internal energy in C2 H4 OH rotation, creating rotationally metastable fragments. The angular distributions of the secondary C2 H4 and OH products were strongly forward–backward peaked with respect to the primary (C2 H4 OH) velocity vector, consistent with the decay of a long-lived complex in which the total angular momentum is perpendicular to the velocity vector and mainly carried away as orbital angular momentum. This effect is analogous to that observed in the decay of similar long-lived complexes in crossed molecular beam experiments.
A thermochemical model for shock-induced reactions (heat detonations) in solids
Authors: Boslough Mark B.
Published: July 2002

Recent advances in studies of shock-induced chemistry in reactive solids have led to the recognition of a new class of energetic materials which are unique in their response to shock waves. Experimental work has shown that chemical energy can be released on a time scale shorter than shock-transit times in laboratory samples. However, for many compositions, the reaction products remain in the condensed state upon release from high pressure, and no sudden expansion takes place. Nevertheless, if such a reaction is sufficiently rapid, it can be modeled as a type of detonation, termed ‘‘heat detonation’’ in the present paper. It is shown that unlike an explosive detonation, an unsupported heat detonation will decay to zero unless certain conditions are met. An example of such a reaction is Fe2 O3 +2Al+shock→Al2 O3 +2Fe (the standard thermite reaction). A shock-wave equation of state is determined from a mixture theory for reacted and unreacted porous thermite. The calculated shock temperatures are compared to experimentally measured shock temperatures, demonstrating that a shock-induced reaction takes place. Interpretation of the measured temperature history in the context of the thermochemical model implies that the principal rate-controlling kinetic mechanism is dynamic mixing at the shock front. Despite the similarity in thermochemical modeling of heat detonations to explosive detonations, the two processes are qualitatively very different in reaction mechanism as well as in the form the energy takes upon release, with explosives producing mostly work and heat detonations producing mostly heat.
Analysis of nondynamical correlation in the metal–ligand bond. Pauli repulsion and orbital localization in MnO−4
Authors: Buijse Marten A. , Baerends Evert Jan
Published: July 2002

The nondynamical correlation error in first row transition metal complexes is studied through calculations on the permanganate ion. The source of the error is the well-known Hartree–Fock failure in the weak-interaction limit, which is shown to exist for both the metal–ligand and the ligand–ligand bonds: the metal–ligand and the ligand–ligand distances are large compared to the size of the metal 3d and ligand 2p atomic orbitals (AO’s). Pauli repulsion between ligand orbitals and 3s/3p orbitals prevent the metal–ligand and ligand–ligand distances to become small enough for efficient overlap and bonding. In multiply bonded systems the Hartree–Fock error does not show up in excessive electron repulsion, but leads to localization of the bonding orbitals (which sometimes requires symmetry breaking), resulting in a loss of covalent character. It is shown how, in the MnO−4 ion, the bonding electrons of E symmetry are localized on the oxygens while the T2 electrons are localized on the metal. The mechanism behind this (unphysical) localization is studied in detail, making use of a simple model system. The covalent character is reintroduced in configuration interaction or multiconfiguration self-consistent-field calculations: density is transferred from the ligand to the metal in the E bonds and vice versa in the T2 bonds. The total metal 3d occupation, however, remains unchanged. Several configuration selection schemes in the space of bonding, nonbonding, and antibonding orbitals are tested with the purpose to recover a large fraction of the nondynamical correlation error but still retain a manageable wave function. It is shown that the ‘‘nonbonding’’ O2p orbitals play an important role in the correlation process and cannot be excluded (kept closed) in a correlated calculation if quantatively correct results are required.
Back to the roots of ‘‘channel three’’: Rotationally resolved spectra of the 6113 band of C6H6
Authors: Riedle E. , Weber Th. , Schubert U. , Neusser H. J. , Schlag E. W.
Published: July 2002

Rotationally resolved fluorescence excitation and resonance enhanced multiphoton ionization (MPI) spectra of the 610130 one-photon band of benzene at the onset of ‘‘channel three’’ are reported. The fluorescence decay is monitored after rotationally selected excitation and a large variation of the nonradiative decay time (<1 to 6.8 ns) is found for the different rotational states at the vibrational excess energy of 3287 cm−1 in S1. The rotational structure of the fluorescence excitation spectrum and the MPI spectrum measured with delayed laser pulses differ considerably. All observed lines of the MPI spectrum were assigned and the rotational line structure can only be understood with a model which incorporates interference between different decay channels. Due to this interference, particular rotational states decay fairly slowly and give rise to lines in the spectrum while states with neighboring rotational quantum numbers decay rapidly and are therefore not found in the spectrum. The previously reported drastic increase of the electronic, nonradiative decay of benzene in this region of excess energy, which led to the postulation of ‘‘channel three,’’ cannot be confirmed. Instead, the optically excited rovibronic states are thought to be coupled to background states within S1 which are themselves broadened due to strong coupling to the highly excited S0 electronic state rather than due to an unknown (‘‘channel three’’) or isomerization process.
Reactions of N(4S) atoms with NO and H2
Authors: Koshi M. , Yoshimura M. , Fukuda K. , Matsui H. , Saito K. , Watanabe M. , Imamura A. , Chen Congxiang
Published: July 2002

Reactions of N(4S) atoms with NO and H2 have been investigated using direct detection of N atoms by the atomic resonance absorption technique in a shock tube apparatus, where N(4S) is generated by photodecomposition of NO by 193 nm laser radiation behind reflected shock waves. The rate constant of the reaction, N+NO→N2+O (1) has been determined using pseudo first-order kinetic analysis to be k1=(1.3±0.3)×1013 (cm3 mol−1 s−1) over 1600–2300 K temperature range, which agrees very well with the estimation by Baulch et al. [Evaluated Kinetic Data for High Temperature Reactions (Butterworths, London, 1973), Vol. 2]. No (or very small) activation energy of this process was confirmed. Also, the rate constant of the reaction, N+H2→NH+H (2) has been decided by adding H2 to NO–Ar mixtures; it is k2=(2.8±0.2)×1014 exp(−Ea/RT) (cm3 mol−1 s−1), where Ea =33±7 kcal/mol. A quantum mechanical calculation performed in order to determine the mechanism of this reaction suggests that the reaction N(4S)+H2→NH+H proceeds via a direct abstraction of H atom from H2, and it gives calculated activation energy which is in good agreement with the present experiment.
Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. III. SF6→F−+SF+5
Authors: Mitsuke Koichiro , Suzuki Shinzo , Imamura Takashi , Koyano Inosuke
Published: July 2002

Ion-pair formation from photoexcitation of SF6 has been studied by negative-ion mass spectrometry using synchrotron radiation in the 11.27–31.0 eV photon energy range. Negative ions F−, SF−6, and SF−5 have been observed. The appearance energy of the F− ion is about 1 eV higher than the thermochemical threshold for the formation of the pair of the ground state ions F−(1Sg) and SF+5(X̃1A1). The peak features observed in the F− efficiency curve are interpreted as resulting from transitions to neutral excited states with the 1T1u symmetry which effectively couple with ion-pair states through avoided potential surface crossings. The peaks assigned to diffuse Rydberg states are distinctively enhanced in the F− efficiency curve, probably because of large transition probabilities from the dissociative Rydberg states to the ion-pair states. In contrast, the excited states of valence type autoionize in a short period and have quite small branching to the ion-pair channel. Consequently, the corresponding peaks are markedly suppressed in the F− spectrum. Assignments of the peak features in the previous photoabsorption spectra are also performed by using the term values for related Rydberg and virtual valence orbitals. Other negative ions observed, SF−6 and SF−5, are produced by resonance capture of low energy electrons emitted by photoionization of the parent molecules, and are not of major concern of the present study.
The low-lying electronic states of protonated C2, CCH+
Authors: Koch Wolfram , Frenking Gernot
Published: July 2002

Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) calculations have been performed on the energetically lowest lying electronic states of protonated C2 (CCH+), using extended Gaussian basis sets of the general contraction type.The 3Π state has been unequivocally identified as the ground state of CCH+, followed by 3Σ−, which is 0.34 eV higher in energy. The lowest linear singlet state is 1Π. However, this state is not stable towards Renner–Teller distortion. A bent structure of 1A′ symmetry is 0.21 eV lower in energy than 1Π and represents the most stable singlet CCH+ cation, 0.91 eV higher than X 3Π. A 1Δ and two 1Σ+ states have also been studied. They are 1.13, 1.54, and 3.55 eV higher in energy than the 3Π ground state. The activation barriers of the degenerate hydrogen migration for ground state CCH+, which can occur on the 3A″ or the 3A′ surface, are computed to be 1.27 and 2.43 eV, respectively, with C2v transition structures. For the lowest singlet CCH+ cation, the 1A1 C2v structure is not a transition state, but represents a very shallow energy minimum, 0.26 eV higher than the 1A′ minimum. The saddle point has Cs geometry and lies 0.30 and 0.04 eV above the 1A′ and 1A1 minima. Our results are compared to earlier theoretical data, obtained at significantly lower levels of ab initio theory, and with the experimentally known translational energy spectrum of CCD+.
The energy separation between the classical and nonclassical isomers of protonated acetylene. An extensive study in one- and n-particle space
Authors: Lindh Roland , Rice Julia E. , Lee Timothy J.
Published: July 2002

The energy separation between the classical and nonclassical forms of protonated acetylene has been reinvestigated in light of the recent experimentally deduced lower bound to this value of 6.0 kcal/mol. The objective of the present study is to use state-of-the-art ab initio quantum mechanical methods to establish this energy difference to within chemical accuracy (i.e., about 1 kcal/mol). The one-particle basis sets include up to g-type functions and the electron correlation methods include single and double excitation coupled-cluster (CCSD), the CCSD(T) extension, multireference configuration interaction, and the averaged coupled-pair functional methods. A correction for zero-point vibrational energies has also been included, yielding a best estimate for the energy difference between the classical and nonclassical forms of 3.7±1.3 kcal/mol.
Spectroscopic properties of OCS and OCCl+ by Mo/ller–Plesset perturbation theory and configuration interaction
Authors: Peterson Kirk A. , Mayrhofer Rudolph C. , Woods R. Claude
Published: July 2002

A b initio calculations have been carried out on the stretching potential energy surfaces of OCS and the isoelectronic positive ion OCCl+. The methods used included fourth order Mo/ller–Plesset perturbation theory with single, double, and quadruple substitutions (MP4SDQ), singles and doubles configuration interaction (CI-SD), and the latter with the addition of a size-consistency correction [designated CI-SD(s)]. Identical calculations were also carried out on the diatomics CO, CS, and CCl+, and these results were used to correct the CI-SD(s) potential energy functions of OCS and OCCl+. Stretching vibrational band origins have been calculated variationally for both OCS and OCCl+, and those for OCS obtained from the corrected CI-SD(s) potential function are within 0.4% of experiment for transitions up to 5000 cm−1. The ground state rotational constant of OCCl+ is predicted to be 5955±15 MHz, while the band origins of the ν1 and ν3 stretching fundamentals are predicted to lie at 793(10) and 2282(20) cm−1, respectively. Electric dipole moments have been calculated from the CI-SD wave functions, and rotationless dipole moment matrix elements have been derived from the analytical dipole moment functions and the vibrational wave functions obtained from the variational calculations. The ν3 band of OCCl+ is predicted by CI-SD to be fairly strong with a band strength of 1490 cm−2 atm−1, while the ν1 band is weaker by more than 1 order of magnitude.
The dipole moment of water. II. Analysis of the vibrational dependence of the dipole moment in terms of a dipole moment function
Authors: Shostak Shelley L. , Muenter J. S.
Published: July 2002

The electric dipole moment measurements on H2O, presented in the previous paper, are analyzed here. First, centrifugal distortion effects were removed from the observed moments. The rotationless moments were then considered in terms of a dipole moment function for water. The moments were combined with first dipole derivatives to determine second derivatives of the moment with respect to normal coordinates. This procedure indicated inconsistencies between existing first derivatives and the permanent moments, which were attributed to approximations made when the first derivatives were obtained from the infrared intensity data. A combined analysis of the original transition moment data and the permanent moments yielded consistent first and second derivatives, but with less accuracy than the input data. The overall results include: the first accurate determination of the equilibrium value, and vibrational energy dependence, of the dipole moment; new second derivative values; and an increased awareness of the need for more complete analysis of the vibrational properties of water.
Dynamic viscoelasticity of dilute polyelectrolyte solutions
Authors: Hodgson Donald F. , Amis Eric J.
Published: July 2002

Dynamic storage and loss moduli of dilute solutions of linear poly(2-vinylpyridine) with molecular weights ranging from 3.0×104 to 1.0×106 have been measured under conditions where the polymer can be charged by protonation. A pronounced maximum is exhibited in the reduced moduli occurring at a concentration which is independent of molecular weight and measurement frequency. No change in the polyion hydrodynamic interaction upon dilution is observed, as determined from Zimm model fits of the reduced modulus vs frequency at finite polymer concentrations. The origin of the viscoelasticity maximum is explained in terms of an electrostatically driven polyion coil expansion and contraction resulting in the scaling of the relaxation spectrum by the longest relaxation time τ1.
Hydrophobic hydration of inert gases: Thermodynamic properties, inherent structures, and normal-mode analysis
Authors: Tanaka Hideki , Nakanishi Koichiro
Published: July 2002

Computer simulations on water and aqueous solutions of noble gases have been carried out in order to study the structures of water around a solute. The hydration energy and free energy evaluated for neon (Ne) and xenon (Xe) solutions in the present study were in good agreement with those by experiments. The detailed hydration structures were investigated by means of the so called inherent structures and normal-mode analyses. It was found that the positive excess free energy in the hydration of Xe arises from a decrease in the number of distinct potential-energy minima in configuration space and that the free energy increase in the Ne solution is due partly to the decrease in the number of the potential minima and partly to the anharmonic modes which are harder than those in pure water. The soft anharmonic modes in the Xe solution were almost equivalent to those in pure water. The introduction of a Xe solute gives rise to a change in water structure to a clathrate-like structure and yields an increase in population of the cyclic pentamer connected by hydrogen bonds, which leads to the exothermic hydration.
A photoelectron–photoion coincidence study of H2O, D2O, and (H2O)2
Authors: Norwood K. , Ali A. , Ng C. Y.
Published: July 2002

Photoelectron–photoion coincidence (PEPICO) data for OH+(OD+), H+(D+), and H2O+ (D2O+) from H2O (D2O) have been obtained in the region of 625–700 Å. The PEPICO measurements allow the construction of breakdown diagrams for the unimolecular dissociation of energy-selected H2O+ and D2O+ in the B̃ 2B2 state. The breakdown diagrams for H2O+(B̃ 2B2) and D2O+(B̃ 2B2) in the internal energy range of 129–166 kcal/mol are essentially identical. The branching ratios observed for H+ (D+) are higher than those reported previously. About 3%–5% of stable H2O+ (D2O+) is observed in the time scale of ≊10 μs. These stable H2O+ (D2O+) ions are attributed to ultrafast B̃ 2B2→Ã2A1 nonradiative relaxation followed by the radiative stabilization from H2O+(Ã2A1) [D2O+(Ã2A1)] to H2O+ (X̃2B1) [D2O+(X̃2B1)]. This observation also supports that the formation of H+ (D+) via the H2O+ (Ã 2A1)[D2O+(Ã2A1)] state is a viable process. The relative state-selected cross sections for the reaction H2O+ (X̃2B1,Ã2A1; ν1, ν2)+H2O→H3O+ +H at center-of-mass collision energies ≤1.9 eV have been examined using an effusive beam arrangement. Experimental evidence supports that vibrational and electronic excitations suppress the cross section of the latter reaction. We have also performed PEPICO measurements of the processes, (H2O)2+hν→(H2O)+2+e− and H3O+ +OH +e−, in the region of 1020–1110 Å. The sums of the PEPICO intensities for H3O+ and (H2O)+2 at corresponding photoionization wavelengths yield the photoelectron spectrum for (H2O)2.
An improved H3 potential energy surface
Authors: Boothroyd Arnold I. , Keogh William J. , Martin Peter G. , Peterson Michael R.
Published: July 2002

We report ab initio calculations of the ground state energy for 404 new conformations of H3, supplementing the set of 368 conformations reported previously by others. The entire dataset has been used to constrain an analytical functional form for the potential energy surface, building on that of Truhlar and Horowitz. The new surface extends the Truhlar and Horowitz surface to higher energies and offers some modest improvement at lower energies. In addition, we have eliminated a problem with derivatives of the London equation that was pointed out by Johnson. The new surface matches the 772 ab initio energies with an overall root-mean-square (rms) error of 0.25 mhartree (i.e., 0.16 kcal/mol) and a maximum absolute deviation of 1.93 mhartree (1.21 kcal/mol); for ‘‘noncompact’’ conformations (no interatomic distance smaller than 1.15 bohr), the rms error is 0.17 mhartree (0.11 kcal/mol) and the maximum absolute deviation is 1.10 mhartree (0.69 kcal/mol). The classical barrier height for H+H2→H2+H is estimated to be 15.20±0.15 mhartree (i.e., 9.54±0.09 kcal/mol).
The influence of finite impurity size on heterogeneous nucleation
Authors: Weinberg Michael C.
Published: July 2002

The effects of the finite size of impurities upon the heterogeneous nucleation rate is examined. Simple arguments based upon probability theory are used to find the relative nucleation rate, p( j), on particles containing j nuclei. The expression for p( j) is used in turn to compute the overall nucleation rate and average number of nuclei on an impurity as a function of time.
The near-infrared spectrum of solid CH4
Authors: Calvani P. , Cunsolo S. , Lupi S. , Nucara A.
Published: July 2002

The infrared absorption spectrum of CH4 from 4000 to 12 000 cm−1 has been first studied in the solid phases I and II. More than forty overtone and combination bands, up to the sixth order, have been observed. Some of them are weaker by a factor of ≊105 than the infrared fundamentals. The bands have been assigned on the basis of their frequency, structure, and intensity. Features that can be attributed to propagating vibrons have been observed. The rotovibrational lines of molecules at Oh sites of phase II have been resolved in a few bands, and their relative intensities are in excellent agreement with the predictions of the extended James and Keenan model. The results reported here may help to analyze more complicated spectra, as those of CH4 in planetary atmospheres.
Monte Carlo studies of polymer network formation
Authors: Schulz M. , Sommer J. U.
Published: July 2002

We are presenting a lattice Monte Carlo study of the formation of polymer networks using the bond fluctuating method. Starting from a pure polymer melt (chain length N=50, density Φ=0.3) and two component polymer mixtures (A:B=1:1), we get simple two component or interpenetrated polymer networks. The time evolution of the building process, the static structure factor, and the topology of the network are presented in dependence of the cross-link density.
Molecular Hamiltonians for highly constrained model systems
Authors: Hadder John E. , Frederick John H.
Published: July 2002

We describe a simple approach for constructing the molecular Hamiltonian which is ideal for studying large molecular systems in which many constraints are imposed. The present procedure allows one to evaluate the correct classical G matrix for highly constrained model systems in the absence of total angular momentum, using matrices that are no larger than the number of active vibrations in the model. A straightforward prescription for constructing the appropriate quantum kinetic energy operator for these systems is then introduced. This prescription is a modification of the Podolsky procedure and it allows one to incorporate constraints in the quantum operators without the extensive use of differential calculus. Finally, the extension of the constrained system Hamiltonian to nonzero total angular momentum is made and methods for reducing the effective Hamiltonian to the minimum number of degrees of freedom using the Augustin–Miller canonical transformation are described. The present approach is illustrated with several molecular examples including arbitrary bent equilibrium triatomic molecules, weakly bound van der Waals complexes, and large molecules with low frequency, large amplitude vibrations.
Interaction between overall and internal rotation below, near, and above the summit of a torsional barrier: 1nπ* methylglyoxal
Authors: Huber Alison S. , Gordon Robert D. , Reid Scott A. , McDonald J. D.
Published: July 2002

The effects of rotation–torsion interactions on the rotational structure of torsional vibronic bands in an electronic spectrum are examined, and a computer program to calculate the relevant energies and transition intensities is described. Rotationally resolved type C bands in the 1nπ* spectrum of methylglyoxal are reported and interpreted. Since the torsional conformation of methylglyoxal differs in the two electronic states, a long progression of bands is observed, involving levels well below, near, and above the 190 cm−1 S1 torsional barrier. The rotation–torsion interaction increases rapidly towards the free rotor limit as the barrier is approached. Observed effects include energy shifts and the appearance of transitions forbidden by rigid rotor or torsional vibronic selection rules.
State-specific unimolecular reaction dynamics of HFCO. I. Dissociation rates
Authors: Choi Young S. , Moore C. Bradley
Published: July 2002

Rovibrationally resolved unimolecular reaction rates of highly vibrationally excited HFCO (S0) are measured and found to be strongly state specific in the energy range of 14 500 cm−1–23 000 cm−1. HFCO molecules are excited to single rovibrational levels in the tunneling region by stimulated emission pumping (SEP) and the dissociation rate of each level is measured by monitoring the temporal evolution of its population by laser-induced fluorescence. The dissociation rates increase by a factor of 10 to 100 or more for small increases in rotational quantum number from J=0 up to J=4 and Ka=2. The dependence on Ka is the strongest. At higher energies, where dissociation lifetimes are shorter than the laser pulse duration, dissociation rates are estimated from the linewidths of well-resolved transition lines measured by high-resolution SEP spectroscopy. In this energy region, dissociation rates are also dependent upon rotational state but much less strongly than in the tunneling region. Vibrational mode specificity in the dissociation rates is observed. For states with approximately the same total energy those with higher excitation in the out-of-plane bending mode (ν6) dissociate more slowly than others. For rotationless levels in the tunneling region, the A′ states decay much more rapidly than the A″ states indicating symmetry-induced mode specificity.

The dissociation rates in HFCO almost surely exhibit mode specificity because the ν6 mode is weakly coupled to the reaction coordinate. The enhancement of this coupling by Coriolis forces appears to produce the unprecedentedly large rotational level dependence of the rates. However, the spectroscopically observed coupling of ν6 to the background of vibrationally mixed levels is not significantly increased by rotation. Thus states with nearly all of the excitation energy in the reaction coordinate appear not to be strongly mixed into the background states. The large effect of rotation on the rates thus seems to result from Coriolis coupling of extreme motion in the reaction coordinate to the background levels or directly to ν6. By applying the Rice–Ramsperger–Kassel–Marcus theory to the measured dissociation rates for high rotational states, the barrier height for the molecular dissociation of HFCO to HF+CO is estimated to be 49±4 kcal/mole.
Dielectric relaxation study of oil-continuous microemulsion systems
Authors: Ponton A. , Nozaki R. , Bose T. K.
Published: July 2002

Extensive dielectric measurements in the frequency range 100 kHz–15 GHz have been performed at 25 °C on two oil-continuous microemulsion systems composed of water, n-dodecane, 1-pentanol, sodium dodecylsulfate and water, toluene, 1-butanol, sodium dodecylsulfate. It was found that in the above mentioned systems, the micellar phases display broad dielectric relaxation spectra which imply different dielectric processes. Two processes, occurring in the low frequency region around 1 and 100 MHz seem to be of ionic origin. They can be attributed to Maxwell–Wagner effect for heterogenous systems and the counterion movement in the interfacial layer around the micelles. The relaxation observed in the high frequency range (1 GHz) could possibly be attributed to the presence of water and alcohol in the vicinity of the reverse micellar surface. The strength of the dielectric relaxation process in the very high frequency region of about 16 GHz is dependent on the water content and is possibly related to pure water outside the interfacial layer.
Structure and thermodynamics of mixtures of hard D-dimensional spheres: Overlap volume function approach
Authors: González L. E. , González D. J. , Silbert M.
Published: July 2002

A very simple ansatz for the partial direct correlation functions of binary mixtures of hard D-dimensional spheres, which allows a unified treatment of both the odd and even space dimensionalities D and reduces, for D=1 and D=3, to the Percus–Yevick theory is presented in this paper. A generalized Carnahan–Starling equation of state is proposed, which is in excellent agreement with the available computer simulation results. Finally, two generalized Verlet–Weis procedures for the partial pair distribution functions gij(r) are proposed. The interesting case of hard disks (D=2) is studied in detail leading to excellent results for the gij(r).
Traveling waves in the CO oxidation on Pt(110): Theory
Authors: Falcke M. , Bär M. , Engel H. , Eiswirth M.
Published: July 2002

A dynamic model designed to describe bistability and kinetic oscillations of the reaction rate during the oxidation of CO on a Pt(110) single crystal surface is extended by incorporating surface diffusion of adsorbed CO in order to analyze the properties of traveling waves propagating on the catalytically active surface. In the range of control parameters (partial pressure of oxygen and carbon monoxide and temperature) which corresponds to excitable dynamics, solitary pulses and periodic wave trains can be triggered. Using both asymptotic and numerical methods, the velocity and shape of the pulses as well as the dispersion relation for periodic wave trains are determined and compared to experimental data where available.
Relativistic effects on the bonding and properties of the hydrides of platinum
Authors: Dyall Kenneth G.
Published: July 2002

The ground state of PtH2 and several low-lying states of PtH+ and PtH have been studied at the all-electron self-consistent-field level of theory to examine the importance of relativistic effects. The results of calculations based on Dirac–Hartree–Fock theory, nonrelativistic theory, and the spin-free no-pair relativistic approximation of Hess are compared to separate the effects of the spin-free terms and the spin–orbit terms of the Hamiltonian on the relativistic corrections to the molecular properties. Comparison is also made between first-order perturbation theory including the one-electron spin-free terms and the method of Hess to determine the size of effects beyond first order. It is found that the spin–orbit interaction significantly affects the properties and energetics of these molecules because of the participation of the Pt 5d orbitals in the bonding, and that effects beyond first order in perturbation theory are large. Any treatment of Pt compounds will have to include both the spin-free and spin–orbit interactions for an accurate description.
Zero kinetic energy (ZEKE) photoelectron spectroscopy of ammonia by nonresonant two-photon ionization from the neutral ground state
Authors: Reiser Georg , Habenicht Wieland , Müller-Dethlefs Klaus
Published: July 2002

Rotationally resolved zero kinetic energy-pulsed field ionization (ZEKE-PFI) photoelectron spectra of NH3 were obtained by nonresonant two-photon ionization from the X̃(1A1′) electronic ground state. The ZEKE spectra were recorded up to an internal ion energy of 9500 cm−1 by pulsed field ionization of long-lived Rydberg states of very high principal quantum numbers. Rovibronic bands of the NH3+ cation with v2+=1–9 (umbrella mode), v4+=1, and for the first time the mode ν1 in the combination band v+=1121 of the X̃+(2A2″) electronic ground state are observed in the ZEKE spectra. Rotational constants and band origins are determined in the analysis. The adiabatic ionization energy obtained by a recent ZEKE measurement is confirmed at 82 159±1 cm−1.
The protonation of N2O reexamined: A case study on the reliability of various electron correlation methods for minima and transition states
Authors: Martin J. M. L. , Lee Timothy J.
Published: July 2002

The protonation of N2O and the intramolecular proton transfer in N2OH+ have been studied using large basis sets in conjunction with second-order many-body perturbation theory (MP2), singles and doubles coupled cluster (CCSD), the augmented coupled cluster method [CCSD(T)], and complete active space self-consistent field (CASSCF) methods. It is shown that MP2 is inadequate even for HNNO+, which has a minor nondynamical correlation effect; for the transition state only CCSD(T) produces a reliable geometry due to serious nondynamical correlation effects. Harmonic frequencies accurate to 50 cm−1 or better are predicted for both protonated species. The proton affinity at 298.15 K is found to be 137.6 kcal/mol, in excellent agreement with the recent experimental redetermination of 137.3±1 kcal/mol; the HNNO+ isomer is found to be 4.4 kcal/mol above the HONN+ isomer, with an interconversion barrier of ∼89 kcal/mol, herewith confirming recent experimental evidence that both species occur together with an energy difference of 6±1.5 kcal/mol. Comparison of the traditional double-zeta plus polarization (DZP) basis and the newer correlation consistent polarized valence double zeta (cc-pVDZ) basis set appears to indicate that the latter might lead to more accurate geometries and harmonic frequencies, although a more detailed investigation would be needed before any definitive conclusions.
Force autocorrelation function in Brownian motion theory
Authors: Español Pep , Zúñiga Ignacio
Published: July 2002

The force autocorrelation function of an infinitely massive Brownian particle is studied with a molecular dynamics simulation. The plateau time problem, the calculation of the friction coefficient, and the relationship between the stochastic and real force are discussed.
State-to-state rotational energy transfer and reaction with ketene of highly vibrationally excited b̃ 1B1 CH2 by time-resolved Fourier transform emission spectroscopy
Authors: Hartland Gregory V. , Qin Dong , Dai Hai-Lung
Published: July 2002

Dispersed fluorescence spectra from the CH2 b̃ 1B1→ã 1A1 band were recorded with time-resolution by Fourier transform emission spectroscopy after pulsed excitation of a single rotational level of the b̃ 1B1 (0,160,0) state. Fluorescence observed from the initially excited level and from levels populated by rotational energy changing collisions with the bath gas (ketene) was used to deduce the state-to-state rate constants for rotational energy transfer and the state-resolved rate constants for total collisional removal of b̃ 1B1 CH2. The observed propensity rules for rotational energy transfer—ΔJ=±2, ΔKa=0, and ΔKc=±2—are consistent with a quadrupole–dipole interaction between b̃ 1B1 (0,160,0) CH2 and ketene. The existence of a quadrupole in the intermolecular interaction suggests that the structure of CH2 in the b̃ 1B1 (0,160,0) state, averaged over the time of a collision, must be linear. The state-to-state rotational energy transfer rate constants range from approximately equal to the hard sphere gas kinetic rate to four times the gas kinetic rate, with the largest rate constants between rotational levels with the smallest energy gaps. Examination of fluorescence spectra recorded with polarization analysis shows that rotationally elastic (ΔJ=0)M changing collisions are negligible. State-resolved rate constants for reactive collisions between b̃ 1B1 CH2 and ketene were obtained by subtracting the rotational energy transfer contribution from the total rate constants for collisional removal of b̃ 1B1 CH2 (obtained from a Stern–Volmer analysis). These rate constants vary from one to five times the hard sphere gas kinetic rate, and increase with rotational energy for the levels studied. Their magnitudes show that CH2 is about two times as reactive in its b̃ 1B1 state than its ã 1A1 state.
Relativistic effects in physics and chemistry of element 105. IV. Their influence on the electronic structure and related properties
Authors: Pershina V. , Fricke B.
Published: July 2002

Results of relativistic (Dirac–Slater and Dirac–Fock) and nonrelativistic (Hartree–Fock–Slater) atomic and molecular calculations have been compared for the group 5 elements Nb, Ta, and Ha and their compounds MCl5 to elucidate the influence of relativistic effects on their properties especially in going from the 5d element Ta to the 6d element Ha. The analysis of the radial distribution of the valence electrons of the metals for electronic configurations obtained as a result of the molecular calculations and their overlap with ligands show opposite trends in behavior for ns1/2, np1/2, and (n−1)d5/2 orbitals for Ta and Ha in the relativistic and nonrelativistic cases. Relativistic contraction and energetic stabilization of the ns1/2 and np1/2 wave functions and expansion and destabilization of the (n−1)d5/2 orbitals make hahnium pentahalide more covalent than tantalum pentahalide and increase the bond strength. The nonrelativistic treatment of the wave functions results in an increase in ionicity of the MCl5 molecules in going from Nb to Ha making element Ha an analog of V. Different trends for the relativistic and nonrelativistic cases are also found for ionization potentials, electronic affinities, and energies of charge-transfer transitions as well as the stability of the maximum oxidation state.
Interaction and conformation of polyelectrolyte chains adsorbed on neutral surfaces
Authors: Sjöström Lena , Åkesson Torbjörn , Jönsson Bo
Published: July 2002

Monte Carlo simulations are used to study the conformation of surface bound polyelectrolytes and how two such surfaces interact. The linear polyelectrolyte chains consist of charged monomers connected via simple harmonic bonds, and all electrostatic interactions are treated within a dielectric continuum model. The interaction between the polyelectrolyte covered surfaces is found to be strongly repulsive, significantly stronger than the usual double layer force. The increased repulsion is due to an expansion of the chains perpendicular to the surfaces, which in turn is driven by the counterion entropy. Simple scaling laws are derived, showing the expansion to increase linearly with the number of monomers. The osmotic pressure in a polyelectrolyte double layer can be shown to be related to the pressure in a simple double layer via a relation of corresponding states.
Entropy of electrolytes
Authors: Laird Brian B. , Haymet A. D. J.
Published: July 2002

The entropy of 1–1 and 2–2 model electrolytes is calculated from an expansion in terms of the multiparticle correlation functions. For electrolytes, a simple truncation of this expansion is never sufficient for the accurate calculation of the entropy, even in the limit of low concentration, in marked contrast to the behavior for short-ranged potentials. However, a partial, but infinite-order, summation of the expansion is shown to yield both the correct low-concentration limit and excellent results over a wide range of concentrations for both 1–1 and 2–2 electrolytes. The consequences of this result for some earlier applications of the entropy expansion to electrolytes are discussed.
Jet-cooled fluorescence excitation spectra and carbonyl wagging and ring-puckering potential energy functions of cyclobutanone and its 2,2,4,4-d4 isotopomer in the S1(n,π*) electronic excited state
Authors: Zhang Jian , Chiang Whe-Yi , Laane Jaan
Published: July 2002

The jet-cooled fluorescence excitation spectra of cyclobutanone and its 2,2,4,4-d4 isotopomer have been recorded in the 305–335 nm region. The electronic band origin of the d0 molecule for the S1(n,π*) state of A2 symmetry occurs at 30 292 cm−1 (30 265 cm−1 for the d4 molecule). The observed spectra consisting of more than 50 bands for each isotopomer involve ν7, ν8, and ν9 (the three A1 ring vibrations) as well as ν20(C=O in-plane wag), ν26 (C=O out-of-plane wag), and ν27 (ring puckering). Five bands associated with the excited vibrational states of ν26 in the S1(n,π*) electronic state were observed for each isotopic species, and these were used to determine the one-dimensional potential energy functions for the C=O out-of-plane wagging. The C=O wagging angle was determined to be 39° and the barrier to inversion is 2149 cm−1 (2188 cm−1 for the deuteride). For the ring-puckering in the S1 state the lowest three vibrational energy spacings were found to be 106, 166, and 185 cm−1 as compared to values of 35, 57, and 65 cm−1 in the S0 ground state. Several ring-puckering potential energy functions with varying degrees of asymmetry are capable of reproducing the observed results. In all cases, however, the v=0 puckering state lies above any barrier to planarity. A two-dimensional potential function which fits the observed data was also determined in terms of the wagging and puckering coordinates.
Vibrational energy relaxation dynamics of C–H stretching modes on the hydrogen-terminated H/C(111)1×1 surface
Authors: Sun Ying-Chieh , Gai Huadong , Voth Gregory A.
Published: July 2002

The vibrational energy relaxation rate of an excited C–H stretching mode on the hydrogen-terminated H/C(111)1×1 surface is calculated using Bloch–Redfield theory combined with classical molecular dynamics. The lifetime of an excited state is determined by the strength of the power spectrum of the force on the stretching mode at the resonance frequency. The lifetime of the first excited state is found to be 60 ps at 300 K which is shorter than the Si–H stretching mode lifetime on the H/Si(111)1×1 surface. The lifetime of the v=2 first overtone state is found to be 200 times shorter (0.30 ps). Analysis of the power spectrum of the fluctuating force along the C–H bond suggests that the mechanism of the energy relaxation for the v=1 stretching state on the H/C(111)1×1 surface is due to lower-order interactions than on the H/Si(111)1×1 surface. The predicted fast relaxation of the overtone state may cast some doubt on the observability of that state.
Binary mixture of grafted polymer chains: A Monte Carlo simulation
Authors: Lai Pik-Yin
Published: July 2002

Grafted polymer layer composed of a binary mixture of incompatible chains under good solvent condition is simulated using the bond-fluctuation model. The equilibrium structures of the partially phase separated states are investigated as a function of the incompatibility and relative fraction by monitoring the monomer distribution functions and the chain linear dimensions. For the case of symmetric binary mixtures, we observe the chains of the same type cluster together laterally forming the ‘‘ripple state’’ while the layer state is never observed. For asymmetric mixtures, layer structures with the minority chains staying further away from the grafting plane are observed which is in contrary to previous theoretical predictions. The heights of both the minority and majority chains in the layer state are shown to be proportional to their chain lengths.