bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonance Raman spectra of uracil based on Kramers–Kronig relations using time-dependent density functional calculations and multireference perturbation theory
Authors: Neugebauer Johannes , Hess Bernd A.
Published: June 2004

The use of time-dependent density functional calculations for the optimization of excited-state structures and the subsequent calculation of resonance Raman intensities within the transform-theory framework is compared to calculations of Hartree–Fock/configuration interaction singles-type (CIS). The transform theory of resonance Raman scattering is based on Kramers–Kronig relations between polarizability tensor components and the optical absorption. Stationary points for the two lowest excited singlet states of uracil are optimized and characterized by means of numerical differentiation of analytical excited-state gradients. It is shown that the effect of electron correlation leads to substantial modifications of the relative intensities. Calculations of vibrational frequencies for ground and excited states are carried out, which show that the neglect of Duschinsky mixing and the assumption of equal wave numbers for ground and excited state are not in all cases good approximations. We also compare the transform-theory resonance Raman intensities with those obtained within a simple approximation from excited-state gradients at the ground-state equilibrium position, and find that they are in qualitative agreement in the case of CIS, but show some important differences in calculations based on density functional theory. Since the results from CIS calculations are in better agreement with experiment, we also present approximate resonance Raman spectra obtained using excited-state gradients from multireference perturbation theory calculations, which confirm the CIS gradients.
Fragmentation and deformation mechanism of glycine isomers in gas phase: Investigations of charge effect
Authors: Ai Hongqi , Bu Yuxiang , Li Ping , Li Zhiqiang
Published: June 2004

The structural parameters, relative stability, proton transfer energy barriers of four typical and life related isomers and conformers of different charged (n=0,±1,±2) glycine species have been investigated using B3LYP, BHLYP, and CCSD(T) methods. Results indicate that those neutral and (±1)-charged species are stable. For the (+2)-charged cases, all four triplet-state glycine species and only the singlet-state zwitterionic one are stable. On the other hand, only the singlet-state zwtterionic glycine (1GlyZW(-2)) and the corresponding neutral form counterpart (1Gly(-2)) are stable for the (-2)-charged cases. Either of the two stable structures holds a proton lying in the position (2–3 Å) of being separated from its corresponding parental species. Those unstable divalent glycine species are dissociated into different smaller species spontaneously according to the characters of their different structures and electron spins. The presented fragmentation and deformation mechanisms can effectively predict and satisfactorily explain some experimental phenomena, which had been puzzling the mass spectrometry chemists. Also, the mechanisms should be suitable for any other similar molecule systems. Comparisons of the relative energies of the four (+1)-charged glycine species show that doublet-state glycine III (2GlyIII1) is more stable in energy by 12.1 kcal/mol than the (+1)-charged glycine Gly (2Gly1). This is consistent with the energy ordering of their corresponding mono-valence metal ion-bound derivatives. In addition, calculations show that an intramolecular proton transfer of Gly(-1)2 to become its zwitterionic counterpart is preferred due to its least activation energy barrier (5.8 kcal/mol) among four discussed processes.
Toward subchemical accuracy in computational thermochemistry: Focal point analysis of the heat of formation of NCO and [H,N,C,O] isomers
Authors: Schuurman Michael S. , Muir Steven R. , Allen Wesley D. , Schaefer Henry F.
Published: June 2004

In continuing pursuit of thermochemical accuracy to the level of 0.1 kcal mol−1, the heats of formation of NCO, HNCO, HOCN, HCNO, and HONC have been rigorously determined using state-of-the-art ab initio electronic structure theory, including conventional coupled cluster methods [coupled cluster singles and doubles (CCSD), CCSD with perturbative triples (CCSD(T)), and full coupled cluster through triple excitations (CCSDT)] with large basis sets, conjoined in cases with explicitly correlated MP2-R12/A computations. Limits of valence and all-electron correlation energies were extrapolated via focal point analysis using correlation consistent basis sets of the form cc-pVXZ (X=2–6) and cc-pCVXZ (X=2–5), respectively. In order to reach subchemical accuracy targets, core correlation, spin-orbit coupling, special relativity, the diagonal Born–Oppenheimer correction, and anharmonicity in zero-point vibrational energies were accounted for. Various coupled cluster schemes for partially including connected quadruple excitations were also explored, although none of these approaches gave reliable improvements over CCSDT theory. Based on numerous, independent thermochemical paths, each designed to balance residual ab initio errors, our final proposals are ΔHf,0∘(NCO)=+30.5, ΔHf,0∘(HNCO)=−27.6, ΔHf,0∘(HOCN)=−3.1, ΔHf,0∘(HCNO)=+40.9, and ΔHf,0∘(HONC)=+56.3 kcal mol−1. The internal consistency and convergence behavior of the data suggests accuracies of ±0.2 kcal mol−1 in these predictions, except perhaps in the HCNO case. However, the possibility of somewhat larger systematic errors cannot be excluded, and the need for CCSDTQ [full coupled cluster through quadruple excitations] computations to eliminate remaining uncertainties is apparent.
Equilibrium geometries of low-lying isomers of some Li clusters, within Hartree–Fock theory plus bond order or MP2 correlation corrections
Authors: Grassi A. , Lombardo G. M. , Angilella G. G. N. , March N. H. , Pucci R.
Published: June 2004

In a recent study by Kornath et al. [J. Chem. Phys. 118, 6957 (2003)], the Lin clusters with n=2, 4, and 8 have been isolated in argon matrices at 15 K and characterized by Raman spectroscopy. This has prompted us to carry out a theoretical study on such clusters up to n=10, using Hartree–Fock theory, plus low-order Møller–Plesset perturbation corrections. To check against the above study of Kornath et al., as a by-product we have made the same approximations for n=6 and 8 as we have for n=10. This has led us to emphasize trends with n through the Lin clusters for (i) ground-state energy, (ii) HOMO–LUMO energy gap, (iii) dissociation energy, and (iv) Hartree–Fock eigenvalue sum. The role of electron correlation in distinguishing between low-lying isomers is plainly crucial, and will need a combination of experiment and theory to obtain decisive results such as that of Kornath et al. for Li8. In particular, it is shown that Hartree–Fock theory plus bond order correlations does account for the experimentally observed symmetry Td symmetry for Li8.
Theoretical method for full ab initio calculation of DNA/RNA–ligand interaction energy
Authors: Chen Xi H. , Zhang John Z. H.
Published: June 2004

In this paper, we further develop the molecular fractionation with conjugate caps (MFCC) scheme for quantum mechanical computation of DNA–ligand interaction energy. We study three oligonuclear acid interaction systems: dinucleotide dCG/water, trinucleotide dCGT/water, and a Watson–Crick paired DNA segment, dCGT/dGCA. Using the basic MFCC approach, the nucleotide chains are cut at each phosphate group and a pair of conjugate caps (concaps) are inserted. Five cap molecules have been tested among which the dimethyl phosphate anion is proposed to be the standard concap for application. For each system, one-dimensional interaction potential curves are computed using the MFCC method and the calculated interaction energies are found to be in excellent agreement with corresponding results obtained from the full system ab initio calculations. The current study extends the application of the MFCC method to ab initio calculations for DNA– or RNA–ligand interaction energies.
Ab initio investigation of the ground and low-lying states of the diatomic fluorides TiF, VF, CrF, and MnF
Authors: Koukounas Constantine , Kardahakis Stavros , Mavridis Aristides
Published: June 2004

The electronic structure of the ground and low-lying states of the diatomic fluorides TiF, VF, CrF, and MnF was examined by multireference and coupled cluster methods in conjunction with extended basis sets. For a total of 34 states we report binding energies, spectroscopic constants, dipole moments, separation energies, and charge distributions. In addition, for all states we have constructed full potential curves. The suggested ground state binding energies of TiF(X 4Φ), VF(X 5Π), CrF(X 6Σ+), and MnF(X 7Σ+) are 135, 130, 110, and 108 kcal/mol, respectively, with first excited states A 4Σ−, A 5Δ, A 6Π, and a 5Σ+ about 2, 3, 23, and 19 kcal/mol higher. In essence all our numerical findings are in harmony with experimental results. For all molecules and states studied it is clear that the in situ metal atom (M) shows highly ionic character, therefore the binding is described realistically by M+F−.
Adsorption, diffusion, and recombination of hydrogen on pure and boron-doped graphite surfaces
Authors: Ferro Y. , Marinelli F. , Jelea A. , Allouche A.
Published: June 2004

Boron inserted as impurity by substitution of carbon atoms in graphite is known to modify the reactivity of the surface in interaction with hydrogen. Boron induces a better H retention capability in graphite while it makes easier the recombination into molecular hydrogen under heating in thermal-desorption experimental conditions. It has already been calculated that boron modifies the electronic structure of the surface, which results in an increase of the adsorption energy for H. This result seems in good agreement with the better retention for H in doped graphite, but contradictory with the easier recombination observed. The aim of this work is to dismiss this contradiction by elucidating the modifications induced by boron in the recombination mechanism. We studied the diffusion of H on pure and boron-doped graphite in the density functional theory framework. We determined a diffusionlike mechanism leading to molecular hydrogen formation. Finally, we have shown the fundamental modifications induced by boron on the [0001] graphite surface reactivity. From these calculations it stands out that recombination is the result of desorption on pure graphite and diffusion on B-doped surfaces, while the activation energy for the rate limiting step is half reduced by boron. The results are compared to experimental observations. The connection between the cluster and periodic quantum modes for graphite is also discussed.
Electrostatic interactions of charged dipolar proteins in reverse micelles
Authors: Piñero J. , Bhuiyan L. B. , Bratko D.
Published: June 2004

The electrostatic interactions in a reverse micelle containing a small-ionized protein are studied by Monte Carlo simulation. The electrostatic contribution to the potential of mean force of the protein in the reverse micelle is determined for a neutral protein, a uniformly charged protein, and a uniformly charged protein with a dipole moment. The effect of addition of a simple electrolyte is studied. While symmetrically distributed micellar charge exerts no force on enclosed ionic species, the protein is driven to the micellar wall due to interactions with simple ions. Protein binding to the inner wall of the micelle can be regulated by added salt. The presence of a dipole drives the protein further to the wall. These effects are studied for several proteins characterized by different charges and dipole moments. For a weakly charged protein with a strong dipole moment the contribution of dipolar interaction to the free energy can represent a major driving force for protein solubilization in the microemulsion.
New phase for one-component hard spheres
Authors: Wu Guang-Wen , Sadus Richard J.
Published: June 2004

A completely new phase for one-component hard spheres is reported in an unexpected region of the phase diagram. The new phase is observed at compressibility factors intermediate between the solid and the metastable branches. It can be obtained from either Monte Carlo simulations alone or a combination of Monte Carlo and molecular dynamics calculations. An analysis of the intermediate scattering function data shows that the new phase is in a stable equilibrium. Radial distribution function data, configurational snapshots, bond order parameters, and translational order parameters obtained from molecular simulations indicate that the new phase is significantly different from the isotropic liquid, metastable, or crystalline phases traditionally observed in hard sphere systems. This result significantly changes our previous understanding of the behavior of hard spheres.
Stokes/anti-Stokes anomalies under surface enhanced Raman scattering conditions
Authors: Maher R. C. , Cohen L. F. , Etchegoin P. , Hartigan H. J. N. , Brown R. J. C. , Milton M. J. T.
Published: June 2004

The possibility of achieving anti-Stokes stimulation and/or pumping under surface enhanced Raman scattering conditions has been the source of intense controversies in the literature. With the aim of clarifying some of the aspects of this problem, we study theoretically and experimentally the situation in a model system which highlights some of the difficulties in the interpretation of the data. We show that many of the assumptions often presumed in the literature need to be assessed with care in each case. Through careful experiments we show, in particular, that the anti-Stokes/Stokes ratio for a specific mode in the same sample can depend on the chosen laser wavelength. This latter effect is a manifestation of the influence of the internal plasmon resonances (hot spots) in the result. Different possibilities and further research directions are highlighted and discussed.
Liquid–vapor interface of square-well fluids of variable interaction range
Authors: Orea Pedro , Duda Yurko , Weiss Volker C. , Schröer Wolffram , Alejandre José
Published: June 2004

Properties of the liquid–vapor interface of square-well fluids with ranges of interaction λ=1.5, 2.0, and 3.0 are obtained by Monte Carlo simulations and from square-gradient theories that combine the Carnahan–Starling equation of state for hard spheres with the second and third virial coefficients. The predicted surface tensions show good agreement with the simulation results for λ=2 and for λ=3 in a temperature range reasonably close to the critical point, 0.8⩽T/Tc⩽0.95. As expected, the surface tension increases with the range of interaction and decreases monotonically with temperature. A comparison between theory and simulation results is also given for the width of the interface and for the coexistence curves for the different interaction ranges.
Quantum pathways for resonance energy transfer
Authors: Jenkins Robert D. , Daniels Gareth J. , Andrews David L.
Published: June 2004

A quantum electrodynamical calculation is presented that focuses individually on the two quantum pathways or time orderings for resonance energy transfer. Conventional mathematical procedures necessitate summing the quantum pathway amplitudes at an early stage in the calculations. Here it is shown, by the adoption of a different strategy that allows deferral of the amplitude summation, that it is possible to elicit key information regarding the relative significance of the two pathways and their distinct distance dependences. A special function integration method delivers equations that also afford new insights into the behavior of virtual photons. It is explicitly demonstrated that both time-ordered pathways are effective at short distances, while in the far field the dissipation of virtual traits favors one pathway. Hitherto unknown features are exhibited in the oblique asymptotic behavior of the time-ordered contributions and their quantum interference. Consistency with the rate equations of resonance energy transfer is demonstrated and results are presented graphically.
Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers
Authors: Liu Xiao-Juan , Feng Ji-Kang , Ren Ai-Min , Cheng Hong , Zhou Xin
Published: June 2004

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn2+ or Cu+ as coordinate centers and 4,4′-bis(dibutylaminostyryl)-[2,2′]-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn2+ is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu+ only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn2+ as the center with that of Cu+ as the center, it is found that the complex with the Cu+ center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn2+, both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.
An analysis of electronic dephasing in the spin-boson model
Authors: Hwang Hyonseok , Rossky Peter J.
Published: June 2004

In order to develop a more complete understanding of the limitations of mixed quantum-classical simulation methods, the origins of electronic dephasing are analyzed in a simple model of the condensed phase, namely, the spin-boson model with an ohmic spectral density. We focus on the decay of the thermally averaged nuclear overlap/phase function (NOPF). Considering the strong coupling/high temperature limit, a relationship is obtained at short time between the rate of electronic coherence loss and the electronic dephasing rate characteristic of a classical bath. Using this relationship, we clarify the origin of the decay of the NOPF. In the same limit, we also reproduce an earlier relationship between the electronic decoherence time and a solvation relaxation time. Finally, we point out that, for the spin-boson model, the exact quantum mechanical description of electronic dephasing is reproduced by mixed quantum/classical methods if a Gaussian distribution of quantum fluctuations around each classical phase space point is introduced. That spatial distribution of quantum fluctuations is functionally the same as that appearing in the Feynman–Kleinert variational local harmonic approximation, and also that implemented in existing classical trajectory-based estimates of coherence dissipation times.
Characteristics and relaxation dynamics of van der Waals complexes between p-difluorobenzene and Ne
Authors: Jayasekharan Thankan , Parmenter Charles S.
Published: June 2004

Characteristics of the single and double Ne van der Waals complexes of p-difluorobenzene (pDFB) have been explored with ultraviolet fluorescence excitation and dispersed fluorescence spectroscopy. Eight S1–S0 fluorescence excitation bands involving six ring modes of pDFB–Ne and two bands of pDFB–Ne2 have been identified. Band assignments are confirmed by dispersed fluorescence from the pumped band. Shifts of the complex bands from the analogous monomer bands are generally 4 cm−1 to the red for pDFB–Ne and 8 cm−1 for pDFB–Ne2. None of the observed ring modes is significantly perturbed by complexation in either the S1 or S0 states. The pDFB–Ne S1 van der Waals binding energy D0′⩽120 cm−1 is inferred from fluorescence band assignments with D0′−D0″=4 cm−1. Vibrational predissociation of pDFB–Ne to produce the S1 monomer is observed after pumping several levels, but the dissociation process is generally slow compared to fluorescence decay of the complex. Dissociation of the double complex pDFB–Ne2 occurs from one level to produce S1 pDFB–Ne in its zero point level. Comparisons are made with the relaxation dynamics of the S1 complexes pDFB–Ar and pDFB–N2.
Monte Carlo simulations on the effect of substrate geometry on adsorption and compression
Authors: Wetzel T. E. , Erickson J. S. , Donohue P. S. , Charniak C. L. , Aranovich G. L. , Donohue M. D.
Published: June 2004

Canonical Monte Carlo simulations were used to study the adsorption and compression of fluid layers on model substrates with cubic, (111) fcc, and graphite geometries. The effect of the relative size of the fluid and substrate molecules on adsorption was considered for strong molecule–surface interactions. In the case of monolayer formation, it was found that the surface geometry and the size of the adsorbate molecules had a significant effect on the structure of the adsorbed layer. These structures varied from well-ordered, commensurate layers to liquid-like structures. Lateral compression was observed for certain fluid to substrate molecule sizes. For the interactions studied in this work, it was found that maximum lateral compression occurred on the cubic surface when adsorbate molecules had a diameter approximately 15% larger than the substrate diameter. In the case of multilayer formation, it was found that second and higher adsorbed layers could compress into the adsorbed layers below them. For cubic substrates, the interlayer compression was predicted analytically with reasonable accuracy, with maximum interlayer compression found for fluid diameters approximately 90% the size of substrate molecule diameters.
Proton transfer dynamics via high resolution spectroscopy in the gas phase and instanton calculations
Authors: Roscioli Joseph R. , Pratt David W. , Smedarchina Zorka , Siebrand Willem , Fernández-Ramos Antonio
Published: June 2004

Tunneling splittings have been observed in the eigenstate-resolved electronic spectrum of the 2-hydroxypyridine/2-pyridone dimer in the gas phase. Deuterium substitution experiments show that these splittings are caused by a concerted double proton transfer reaction along the O–H⋯O and N⋯H–N hydrogen bonds that hold the dimer together, substitution of the weaker and longer N⋯H–N bond having the larger effect. Tunneling splittings calculated by the instanton method for the zero-point level of the ground state are in good agreement with experiment for all observed isotopomers, showing that the dynamics occurs in this state, rather than in the electronically excited state.
Collective contributions to the dielectric relaxation of hydrogen-bonded liquids
Authors: Weingärtner Hermann , Nadolny Holger , Oleinikova Alla , Ludwig Ralf
Published: June 2004

Dielectric relaxation times are often interpreted in terms of the reorientation of dipolar species or aggregates. The relevant time correlation function contains, however, cross terms between dipole moments of different particles. In the static case, these cross terms are accounted for by the Kirkwood factor gK. Theories and molecular dynamics simulations suggest that such cross correlations may also affect the time-dependent properties, as reflected in the dielectric spectra. We present an experimental method for detecting effects of such cross correlations in dielectric spectra by a comparative analysis of dielectric and magnetic relaxation data. We demonstrate that such collective contributions can substantially affect dielectric relaxation. Experiments for n-pentanol (gK=3.06 at 298 K) and 2,2-dimethyl-3-ethyl-pentane-3-ol (gK=0.59) and their solutions in carbon tetrachloride show that in systems with gK>1, the cross correlations slow down dielectric relaxation. In systems with gK<1, dielectric relaxation is enhanced. The results conform to theoretical predictions by Madden and Kivelson [Adv. Chem. Phys. 56, 467 (1984)] and to results of molecular dynamics simulations. The relaxation enhancement by cross terms in the case of gK<1 is difficult to rationalize by conventional models of dielectric relaxation.
Homogeneous nucleation rates of higher
n
-alcohols measured in a laminar flow diffusion chamber
Authors: Hyvärinen Antti-Pekka , Lihavainen Heikki , Viisanen Yrjö , Kulmala Markku
Published: June 2004

Nucleation rate isotherms of n-butanol, n-pentanol, n-hexanol, n-heptanol, and n-octanol were measured in a laminar flow diffusion chamber using helium as carrier gas. The measurements were made at 250–310 K, corresponding to reduced temperatures of 0.43–0.50, and at atmospheric pressure. Experimental nucleation rate range was from 103 to 107 cm−3 s−1. The expression and accuracy of thermodynamic parameters, in particular equilibrium vapor pressure, were found to have a significant effect on calculated nucleation rates. The results were compared to the classical nucleation theory (CNT), the self-consistency corrected classical theory (SCC) and the Hale’s scaled model of the CNT. The average ratio between the experimental and theoretical nucleation rates for all alcohols used was 1.5×103 when the CNT was used, and 0.2×10−1 when the SCC was used and 0.7×10−1 when the Hale’s scaled theory was used. The average values represent all the alcohols used at the same reduced temperatures. The average ratio was about the same throughout the temperature range, although Jexp/Jthe calculated with the Hale’s scaled theory increased slightly with increasing temperature. The saturation ratio dependency was predicted closest to experiment with the classical nucleation theory. The nucleation rates were compared to those found in the literature. The measurements were in reasonable agreement with each other. The molecular content of critical alcohol clusters was between 35 and 80 molecules. At a fixed reduced temperature, the number of molecules in a critical cluster decreased as a function of alcohol carbon chain length. The number of molecules in critical clusters was compared to those predicted by the Kelvin equation. The theory predicted the critical cluster sizes well.
Intermediate state representation approach to physical properties of electronically excited molecules
Authors: Schirmer J. , Trofimov A. B.
Published: June 2004

Propagator methods provide a direct approach to energies and transition moments for (generalized) electronic excitations from the ground state, but they do not usually allow one to determine excited state wave functions and properties. Using a specific intermediate state representation (ISR) concept, we here show how this restriction can be overcome in the case of the algebraic–diagrammatic construction (ADC) propagator approach. In the ISR reformulation of the theory the basic ADC secular matrix is written as a representation of the Hamiltonian (or the shifted Hamiltonian) in terms of explicitly constructable states, referred to as intermediate (or ADC) states. Similar intermediate state representations can be derived for operators other than the Hamiltonian. Together with the ADC eigenvectors, the intermediate states give rise to an explicit formulation of the excited wave functions and allow one to calculate physical properties of excited states as well as transition moments for transitions between different excited states. As for the ground-state excitation energies and transition moments, the ADC excited state properties are size consistent so that the theory is suitable for applications to large systems. The established hierarchy of higher-order [ADC(n)] approximations, corresponding to systematic truncations of the IS configuration space and the perturbation–theoretical expansions of the ISR matrix elements, can readily be extended to the excited state properties. Explicit ISR matrix elements for arbitrary one-particle operators have been derived and coded at the second-order [ADC(2)] level of theory. As a first computational test of the method we have carried out ADC(2) calculations for singlet and triplet excited state dipole moments in H2O and HF, where comparison to full CI results can be made. The potential of the ADC(2) method is further demonstrated in an exploratory study of the excitation energies and dipole moments of the low-lying excited states of paranitroaniline. We find that four triplet states, T1–T4, and two singlet states, S1 and S2, lie (vertically) below the prominent charge transfer (CT) excitation, S3. The dipole moment of the S3 state (17.0D) is distinctly larger than that of the corresponding T3 triplet state (11.7D).
Magnetic enhancement and magnetic reduction in binary clusters of transition metal atoms
Authors: Andriotis Antonis N. , Mpourmpakis Giannis , Froudakis George E. , Menon Madhu
Published: June 2004

Electronic and magnetic properties of small binary clusters containing one or two transition metal atoms are investigated using ab initio calculations with a view to explain the experimentally observed magnetic enhancement/reduction in these systems. As the present investigations do not rely on spin–orbit effects, our results reveal the enhancement or reduction in the magnetic moment to depend on two main factors; namely geometry and, most importantly, the d-band filling. The results can be used as a guide in the experimental synthesis of high density magnetic grains.
Distributed polarizability analysis for para-nitroaniline and meta-nitroaniline: Functional group and charge-transfer contributions
Authors: in het Panhuis M. , Munn R. W. , Popelier P. L. A.
Published: June 2004

Topological partitioning of electronic properties is used to investigate the polarizability of para-nitroaniline and meta-nitroaniline. The distributed polarizabilities for atoms are combined into total local or generalized distributed contributions for the amino, ring, and nitro functional groups; generalized distributed group contributions have not been calculated before. The local group contributions are transferable between the two molecules only when charge transfer is suppressed, but the generalized distributed contributions prove surprisingly similar in the two molecules, apparently because they treat charge-transfer contributions explicitly.
An efficient atomic orbital based second-order Møller–Plesset gradient program
Authors: Saebø Svein , Baker Jon , Wolinski Krzysztof , Pulay Peter
Published: June 2004

Based on the orbital-invariant atomic orbital formulation of the MP2 (Møller–Plesset second-order perturbation theory) energy and gradient [P. Pulay and S. Saebø, Theor. Chim. Acta 69, 357 (1986)], we have derived and programmed detailed working equations for closed-shell MP2 gradients. The orbital-invariant form avoids the difficulties of other formulations with frozen orbitals, and allows the use of arbitrary occupied orbitals, an important consideration for local correlation theories, although the present program uses canonical molecular orbitals. The atomic orbital formulation offers savings both in storage and computer time. Test calculations on systems containing up to ∼100 atoms and ∼1000 basis functions, performed on a single personal computer, are reported. Parallelization of the code is underway.
Real time observation of the photo-Fries rearrangement
Authors: Lochbrunner S. , Zissler M. , Piel J. , Riedle E. , Spiegel A. , Bach T.
Published: June 2004

The photo-Fries rearrangement of 4-tert-butylphenyl acetate dissolved in cyclohexane is investigated by two-color femtosecond pump probe spectroscopy. The spectral transmission changes are characterized in the visible and ultraviolet spectral region and allow for the first time to temporally resolve the primary reaction steps. We find that the photoinduced homolytic cleavage of the CO bond occurs within 2 ps and that the geminate recombination of the generated radical pair to the intermediate substituted cyclohexadienone takes 13 ps. The experimental results support a model in which the initial reaction proceeds from the originally excited ππ* state via a barrier to a dissociative πσ* state.
Molecular ordering and phase transitions in alkanol monolayers at the water–hexane interface
Authors: Tikhonov Aleksey M. , Pingali Sai Venkatesh , Schlossman Mark L.
Published: June 2004

The interface between bulk water and bulk hexane solutions of n-alkanols (H(CH2)mOH, where m=20, 22, 24, or 30) is studied with x-ray reflectivity, x-ray off-specular diffuse scattering, and interfacial tension measurements. The alkanols adsorb to the interface to form a monolayer. The highest density, lowest temperature monolayers contain alkanol molecules with progressive disordering of the chain from the -CH2OH to the -CH3 group. In the terminal half of the chain that includes the -CH3 group the chain density is similar to that observed in bulk liquid alkanes just above their freezing temperature. The density in the alkanol headgroup region is 10% greater than either bulk water or the ordered headgroup region found in alkanol monolayers at the water–vapor interface. We conjecture that this higher density is a result of water penetration into the headgroup region of the disordered monolayer. A ratio of 1:3 water to alkanol molecules is consistent with our data. We also place an upper limit of one hexane to five or six alkanol molecules mixed into the alkyl chain region of the monolayer. In contrast, H(CH2)30OH at the water–vapor interface forms a close-packed, ordered phase of nearly rigid rods. Interfacial tension measurements as a function of temperature reveal a phase transition at the water–hexane interface with a significant change in interfacial excess entropy. This transition is between a low temperature interface that is nearly fully covered with alkanols to a higher temperature interface with a much lower density of alkanols. The transition for the shorter alkanols appears to be first order whereas the transition for the longer alkanols appears to be weakly first order or second order. The x-ray data are consistent with the presence of monolayer domains at the interface and determine the domain coverage (fraction of interface covered by alkanol domains) as a function of temperature. This temperature dependence is consistent with a theoretical model for a second order phase transition that accounts for the domain stabilization as a balance between line tension and long range dipole forces. Several aspects of our measurements indicate that the presence of domains represents the appearance of a spatially inhomogeneous phase rather than the coexistence of two homogeneous phases.
Analysis and simulation of the structure of nanoparticles that undergo a surface-driven structural transformation
Authors: Gilbert Benjamin , Zhang Hengzhong , Huang Feng , Banfield Jillian F. , Ren Yang , Haskel Daniel , Lang J. C. , Srajer George , Jürgensen Astrid , Waychunas Glenn A.
Published: June 2004

A room temperature solid-state structural transformation was observed in 3 nm ZnS nanoparticles in methanol following the addition of water (Zhang et al., Nature 424, 1025, 2003). Experimental wide angle x-ray scattering (WAXS), x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy measurements show a large increase in crystallinity associated with water addition, in agreement with molecular dynamics (MD) predictions. Here we perform first-shell EXAFS and pair distribution function analysis and whole-nanoparticle calculations of WAXS, EXAFS and XANES to compare structural data with the MD predictions. The predicted WAXS patterns give excellent agreement with data, while the predicted EXAFS and XANES spectra give poor agreement. Relative to WAXS, XANES and EXAFS spectra contain additional structural information related to the distribution of disorder. The discrepancy between the x-ray diffraction and x-ray absorption results indicates that structural disorder is partitioned between interior and surface regions more strongly than predicted in the MD simulations.
Coherent spin control of matrix isolated molecules by IR+UV laser pulses: Quantum simulations for ClF in Ar
Authors: Korolkov M. V. , Manz J.
Published: June 2004

Two coherent sequential IR+UV laser pulses may be used to generate two time-dependent nuclear wave functions in electronic excited triplet and singlet states via single (UV) and two photon (IR+UV) excitation pathways, exploiting spin–orbit coupling and vibrational pre-excitation, respectively. These wave functions evolve from different Franck–Condon domains until they overlap in a domain of bond stretching with efficient intersystem crossing. Here, the coherence of the laser pulses is turned into optimal interferences of the wave packets, yielding the total wave packet at the target place, time, and with dominant target spin. The time resolution of spin control is few femtoseconds. The mechanism is demonstrated by means of quantum model simulations for ClF in an Ar matrix.
An adaptive immune optimization algorithm for energy minimization problems
Authors: Shao Xueguang , Cheng Longjiu , Cai Wensheng
Published: June 2004

Based on the immune theory of biology, a novel evolutionary algorithm, adaptive immune optimization algorithm (AIOA), is proposed. In AIOA, density regulation and immune selection is adopted to control the individual diversity and the convergence adaptively. By an application of the algorithm to the optimization of test functions, it is shown that the algorithm is a highly efficient optimization method compared with other stochastic optimization methods. The algorithm was also applied to the optimization of Lennard-Jones clusters, and the results show that the method can find the optimal structure of N⩽80 with a very high efficiency. The proposed algorithm may be a good tool for fast global optimization in chemical or biological molecular simulations.
Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of hexafluorobenzene: The Jahn-Teller effect and vibrational analysis
Authors: Kwon Chan Ho , Kim Myung Soo
Published: June 2004

One-photon mass-analyzed threshold ionization (MATI) spectrum of hexafluorobenzene was obtained by using vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The ionization energy of hexafluorobenzene determined from the position of the 0–0 band was 9.9108±0.0006 eV. To aid the spectral analysis, the Jahn-Teller coupling parameters for four e2g modes of C6F6+ in the ground electronic state were calculated from the topographical data of the potential energy surface obtained at the density functional theory (DFT) level. These were used in the initial calculation of the energies of the Jahn-Teller states and upgraded through the multimode fit to the experimental data. Excellent agreement between the experimental and calculated frequencies was achieved. The vibrations which are not linear Jahn-Teller active were observed and could be assigned by referring to the frequencies obtained at the DFT level.
Efficient penetration of the basal plane (0001) face of ice Ih by HF at Ts=150 K: Dependence on incidence energy, incidence angle, and rotational energy
Authors: Niel Gardner Devon O. , Al-Halabi Ayman , Kroes Geert-Jan
Published: June 2004

Classical trajectory simulations are carried out to investigate the influence of incidence energy, incidence angle, and rotational energy on the penetration of the basal plane (0001) face of ice Ih by HF at a surface temperature (Ts) of 150 K. The interaction of HF with ice is modelled by pair interactions, with the pair potential fitted to ab initio (Hartree–Fock+MP2) calculations. The penetration of ice by HF occurs already at very low incidence energies, viz., Ei⩾20 kJ mol−1. This is much lower than the threshold incidence energy obtained for penetration of ice by HCl (Ei≈96.5 kJ mol−1); the calculated average barrier to penetration of ice by HF is 16.0 kJ mol−1 and is much lower than that previously reported for HCl. As was the case for HCl, penetration of ice by HF decreases with decreasing incidence energy and increasing incidence angle. Though in general, the penetration probability is independent of the molecule’s initial rotational energy, penetration beyond the second bilayer (deep penetration) is suppressed by initial rotation. This suggests that, like was found for HCl, the steering operative in deep penetration is inhibited by initial rotation. Finally, because HF is a weak acid experimental observation of HF penetrated into ice may well be possible using infrared spectroscopy, and we suggest experiments along this line.
Molecular isomerization induced by ultrashort infrared pulses. I. Few-cycle to sub-one-cycle Gaussian pulses and the role of the carrier-envelope phase
Authors: Uiberacker Christoph , Jakubetz Werner
Published: June 2004

Using 550 previously calculated vibrational energy levels and dipole moments we performed simulations of the HCN→HNC isomerization dynamics induced by sub-one-cycle and few-cycle IR pulses, which we represent as Gaussian pulses with 0.25–2 optical cycles in the pulse width. Starting from vibrationally pre-excited states, isomerization probabilities of up to 50% are obtained for optimized pulses. With decreasing number of optical cycles a strong dependence on the carrier-envelope phase (CEP) emerges. Although the optimized pulse parameters change significantly with the number of optical cycles, the distortion by the Gaussian envelope produces nearly equal fields, with a positive lobe followed by a negative one. The positions and areas of the lobes are also almost unchanged, irrespective of the number of cycles in the half-width. Isomerization proceeds via a pump–dumplike mechanism induced by the sequential lobes. The first lobe prepares a wave packet incorporating many delocalized states above the barrier. It is the motion of this wave packet across the barrier, which determines the timing of the pump and dump lobes. The role of the pulse parameters, and in particular of the CEP, is to produce the correct lobe sequence, size and timing within a continuous pulse.
Molecular isomerization induced by ultrashort infrared pulses. II. Pump–dump isomerization using pairs of time-delayed half-cycle pulses
Authors: Uiberacker Christoph , Jakubetz Werner
Published: June 2004

We investigate population transfer across the barrier in a double-well potential, induced by a pair of time-delayed single-lobe half-cycle pulses. We apply this setup both to a one-dimensional (1D) quartic model potential and to a three-dimensional potential representing HCN→HNC isomerization. Overall the results for the two systems are similar, although in the 3D system some additional features appear not seen in the 1D case. The generic mechanism of population transfer is the preparation by the pump pulse of a wave packet involving delocalized states above the barrier, followed by the essentially 1D motion of the delocalized part of wave packet across the barrier, and the eventual de-excitation by the dump pulse to localized states in the other well. The correct timing is given by the well-to-well passage time of the wave packet and its recurrence properties, and by the signs of the field lobes which determine the direction and acceleration or deceleration of the wave packet motion. In the 3D system an additional pump–pump–dump mechanism linked to wave packet motion in the reagent well can mediate isomerization. Since the transfer time and the pulse durations are of the same order of magnitude, there is also a marked dependence of the dynamics and the transfer yield on the pulse duration. Our analysis also sheds light on the pronounced carrier envelope phase dependence previously observed for isomerization and molecular dissociation with one-cycle and sub-one-cycle pulses.
Double hydrogen tunneling revisited: The breakdown of experimental tunneling criteria
Authors: Tautermann Christofer S. , Loferer Markus J. , Voegele Andreas F. , Liedl Klaus R.
Published: June 2004

Formic acid dimer was chosen as a model system to investigate synchronous double proton transfer by means of variational transition state theory (VTST) for various isotopically modified hydrogen species. The electronic barrier for the double proton transfer was evaluated to be 7.9 kcal/mol, thus being significantly lower than it was determined in previous studies. The tunneling probabilities were evaluated at temperatures from 100 up to 400 K and typical Arrhenius behavior with enhancement by tunneling is observed. When comparing the transmission factors κ in dependence of the mass of the tunneling hydrogen, it was found that there are two maxima, one at very low masses (e.g., 0.114 amu, corresponding to the muonium entity) and one maximum at around 2 amu (corresponding to deuterium). With the knowledge of the VTST-hydrogen transfer rates and the corresponding tunneling corrections, various tunneling criteria were tested (e.g., Swain–Schaad exponents) and were shown to fail in this reaction in predicting the extent of tunneling. This finding adds another aspect in the ongoing “Tunneling-Enhancement by Enzymes” discussion, as the used tunneling criteria based on experimental reaction rates may fail to predict tunneling behavior correctly.
Theory of the two step enantiomeric purification of 1,3 dimethylallene
Authors: Gerbasi David , Brumer Paul , Thanopulos Ioannis , Král Petr , Shapiro Moshe
Published: June 2004

An application of a recently proposed [P. Král et al., Phys. Rev. Lett. 90, 033001 (2003)] two step optical control scenario to the purification of a racemic mixture of 1,3 dimethylallene is presented. Both steps combine adiabatic and diabatic passage phenomena. In the first step, three laser pulses of mutually perpendicular linear polarizations, applied in a “cyclic adiabatic passage” scheme, are shown to be able to distinguish between the L and D enantiomers due to their difference in matter-radiation phase. In the second step, which immediately follows the first, a sequence of pulses is used to convert one enantiomer to its mirror-imaged form. This scenario, which only negligibly populates the first excited electronic state, proves extremely useful for systems such as dimethylallene, which can suffer losses from dissociation and internal conversion upon electronic excitation. We computationally observe conversion of a racemic mixture of dimethylallene to a sample containing ≈95% of the enantiomer of choice.
Theoretical study of the UV photodissociation of Cl2: Potentials, transition moments, extinction coefficients, and Cl*/Cl branching ratio
Authors: Kokh Daria B. , Alekseyev Aleksey B. , Buenker Robert J.
Published: June 2004

Potential energy curves for the X 1Σg+ ground state and Ω=0u+, 1u valence states and dipole moments for the 0u+, 1u−X transitions are obtained in an ab initio configuration interaction study of Cl2 including spin-orbit coupling. In contrast to common assumptions, it is found that the B 3Π0+u−X transition moment strongly depends on internuclear distance, which has an important influence on the Cl2 photodissociation. Computed energy curves and transition moments are employed to calculate the A, B, C←X extinction coefficients, the total spectrum for the first absorption band, and the Cl*(2P1/2)/Cl(2P3/2) branching ratio as a function of excitation wavelength. The calculated data are shown to be in good agreement with available experimental results.
Metallophilic attractions between d8–d10 heterometallic compounds trans-[Pt(PH3)2(CN)2] and M(PH3)2+ (M=Ag or Cu): Ab initio study
Authors: Xia Bao-Hui , Zhang Hong-Xing , Jiao Yu-Qiu , Pan Qing-Jiang , Li Ze-Sheng , Sun Chia-Chung
Published: June 2004

The weak metal–metal interactions of Pt(II)–Ag(I)/Cu(I) have been investigated by ab initio method at MP2 level through the model complexes [trans-Pt(PH3)2(CN)2-M(PH3)2+] (M=Ag,Cu). The calculated interaction energy of 12.9 and 11.5 kcal mol−1 for [trans-Pt(PH3)2(CN)2-Ag(PH3)2+] and [trans-Pt(PH3)2(CN)2–Cu(PH3)2+] respectively, are in the middle of the van der Waals force and the strong hydrogen bond. The estimated equilibrium separations between Pt and M, req(Pt–M) (3.32 Å for M=Ag and 3.23 Å for M=Cu), lie within the region expected for weak metal–metal interaction. The electronic dispersive contributions dominate the weak interaction.