bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical analysis of the multicopy sampling method in molecular modeling
Authors: Zheng Qiang , Rosenfeld Rakefet , Kyle Donald J.
Published: July 2002

Langevin Monte Carlo dynamics and equilibrium properties of the multicopy sampling approximation in the modeling of ligand–protein complexes are studied. A perturbation form is derived based on the normal mode analysis, and can be used as a quantitative means to estimate the approximation prior to running a multicopy simulation. The multicopy dynamics is shown to induce an average dynamics of the protein and all the ligand copies, resulting in a well-defined equilibrium with uniform temperature. The induced average dynamics and the equilibrium represent the desired physics of the multicopy approximation, and are extended to include heterogeneous ligand copies. Such an extension presents a natural way to incorporate biologically important information on evolution into energy-based molecular modeling of homologous proteins.
Ab initio study of the electronic structures of lithium containing diatomic molecules and ions
Authors: Boldyrev Alexander I. , Simons Jack , Schleyer Paul von R.
Published: July 2002

Ab initio calculations are used to provide bond lengths, harmonic frequencies, and dissociation energies of low-lying electronic states for LiX, LiX+, and LiX− (with X=Li through F and Na through Cl). Most of these species represent hitherto experimentally unknown molecules or ions, which provides the focus of the work presented here. All of these species are stable to dissociation and the anions are stable to loss of an electron. Differences among the electronic structures of the valence isoelectronic LiX; and HX, LiX+, and HX+; and LiX− and HX− species are analyzed. Optimized geometries, dissociation energies, ionization potentials, and electron affinities were calculated for the following ground states of the respective species: 1Σ+ for Li2(1Σ+g) LiNa, LiBe+, LiBe−, LiMg+, LiMg−, LiF, LiAl, LiS−, and LiCl; 2Σ+ for Li+2(2Σ+g), Li−2(2Σ+u) LiBe, LiB+, LiF−, LiNa+, LiNa−, LiMg, LiAl+, and LiCl−; 2Πr for LiB−, LiAl−; 2Πi for LiO, LiF+, LiS, and LiCl+; 3Πr for LiB, LiC+, and LiSi+; 3Σ− for LiN, LiO+, LiSi−, LiP, and LiS+; 4Σ− for LiC, LiN+, LiN−, LiSi, LiP+, and LiP−; and 5Σ− for LiC−.
Atomization enthalpy and enthalpy of formation of gaseous Si4 from mass spectrometric equilibrium measurements
Authors: Schmude R. W. , Ran Q. , Gingerich K. A.
Published: July 2002

Recent spectroscopic and theoretical investigations have allowed us to calculate new thermal functions for the Si4 molecule. These thermal functions have been combined with new and with reevaluated mass spectrometric data in obtaining an atomization enthalpy of ΔHa,0=1160±22 kJ mol−1 and an enthalpy of formation of ΔHf,0=643±22 kJ mol−1. The experimental results are compared to MP4/6–31G* and G2 predictions.
The ν1+ν3 combination mode of C3 in Ar and Kr matrices: Evidence for a bent structure
Authors: Szczepanski Jan , Vala Martin
Published: July 2002

The ν1+ν3 combination mode of laser-ablated C3 has been observed in Ar and Kr matrices for all six possible 12C/13C isotopomers. The ν1 totally symmetric mode frequencies have been determined by difference with the known ν3 asymmetric stretching frequencies, corrected for anharmonicity effects. A comparison of certain isotopomer pairs reveals that C3 is bent in these matrices. A normal coordinate analysis using four force constants resulted in an excellent fit of the 12 observed frequencies (ν1 and ν3 for six isotopomers). From the fit, a value for the ν2 bending mode frequency of 82 cm−1 was obtained. This is shown to be in good agreement with the value obtained from the C3 phosphorescence spectrum. Finally, the molecular bending angle which was determined to be 160.0° (in Ar and Kr) is shown to match very well with the predicted value from ab initio theory (161.6°) and from a fitting of a Morse oscillator–rigid bender potential to gas phase vibrational frequencies (162.5°). The influence of the matrix environment on the vibrational force constants and frequencies of C3 is discussed.
Electron spin resonance studies of P2H+6 and PH+3 radical cations in rare gas matrices at 4 K. Comparison with theoretical calculations
Authors: Knight Lon B. , Tyler Donald J. , Kudelko Paul , Lyon John B. , McKinley Allan J.
Published: July 2002

Detailed experimental and theoretical electron spin resonance (ESR) studies of the cation radicals, PH+3 and P2H+6, isolated in neon and argon matrices are reported. These cations were generated by photoionization (17 eV) and x-irradiation (80 keV) methods. The observed nuclear hyperfine interactions (A tensors) showed reasonable agreement with Aiso and Adipolar values calculated at the ab initio configuration interaction (CI) level. These ESR results for PH+3 clearly indicate its nonplanarity; for P2H+6, an earlier theoretical prediction of a C2h distortion from C3v was confirmed by these new experimental results which reveal two different H atom environments. The neon magnetic parameters for PH+3 are g∥=2.002(1), g⊥=2.007(1), A⊥=920(3), and A∥=1638(15) MHz for 31P and A(H)≤6 MHz. The neon parameters for P2H+6 are g∥=2.001(1), g⊥=2.003(1), A⊥=1527(2), and A∥=1909(4) MHz for 31P and A(2H)=182(2) and A(4H)=±22(2) MHz.
State-to-state ozone relaxation rates for O3–N2 collisions
Authors: Flannery C. C. , Steinfeld J. I. , Gamache R. R.
Published: July 2002

State-to-state relaxation rates have been determined for ozone in collisions with itself and with nitrogen by two different methods. A theoretical model of collisional broadening of ozone lines, the quantum Fourier transform theory with improved dynamics, was adapted to compute state-to-state inelastic contributions to the pressure-broadened halfwidths for ozone–nitrogen collisions. These quantities were then determined experimentally using time-resolved infrared double resonance spectroscopy and a kinetic model for relaxation. Comparisons of kinetic model simulations and experimental double-resonance signals confirmed a propensity for first-order dipolar transitions, but also clearly demonstrated the importance of higher-order interactions leading to large changes in J in single collisions. Simple energy and angular-momentum-scaling laws were found to be inadequate to represent the experimental data.
Ion-pair formation in the photodissociation of HCl and DCl
Authors: Yencha Andrew J. , Kaur Devinder , Donovan Robert J. , Kvaran Agust , Hopkirk Andrew , Lefebvre-Brion H. , Keller F.
Published: July 2002

The vibrationally resolved excitation functions for Cl− formation from jet-cooled HCl and DCl, following photoabsorption in the region 75–86 nm (HCl/DCl+hν→H+/D++Cl−), are reported and compared with ab initio calculations. This completes our understanding of the various possible decay channels available to superexcited states of HCl and DCl that lie above the first ionization limit. Our experimental and theoretical results suggest that the Rydberg states responsible for ion-pair formation in HCl and DCl via predissociation by the V 1Σ+ ion-pair state correspond to 1Σ+ states that are only weakly autoionized. These same Rydberg states appear to act as, at least, one class of precursor states for neutral dissociation to yield excited H* and Cl* atoms.
Zero-kinetic-energy photoelectron spectrum of carbon dioxide
Authors: Merkt F. , Mackenzie S. R. , Rednall R. J. , Softley T. P.
Published: July 2002

The zero-kinetic-energy (ZEKE) photoelectron spectrum of carbon dioxide has been measured between 111 000 and 112 000 cm−1 at a resolution of 1.5 cm−1 using a coherent source of XUV radiation based on four-wave mixing in krypton. The spectrum consists of six bands corresponding to transitions from the ground X 1Σ+g(v1,v2,v3=000) state of the neutral to the two spin–orbit components of the (000) vibrational level and the four Renner–Teller states associated with the (010) vibrational level of the ground electronic state (X 2Πg) of the ion. The analysis of the partially resolved rotational structure of the various bands leads to a detailed picture of the photoionization process. The propensity rules for angular momentum transfer during photoionization are strongly dependent on the symmetry (2Πg,3/2, 2Πg,1/2, 2Δu,5/2, 2Δu,3/2, 2Σ+u, and 2Σ−u) of the different ionic states probed and on the Hund’s coupling case they follow [case (a) for the Π and Δ states and case (b) for the Σ states]. A comparison of the experimental ZEKE line intensities with theoretical predictions and conventional photoelectron spectra reveals a series of anomalies which are discussed in terms of final state interactions. The ionization potential of CO2 is estimated to be 111 111.0±3 cm−1, somewhat lower than the value of 111 121±2 cm−1 determined from extrapolation of the Rydberg series by Cossart-Magos et al. [Mol. Phys. 61, 1077 (1987)].
Measurement of the deactivation of Br* by atomic iodine
Authors: Pastel Robert L. , McIver J. K. , Miller Harold C. , Hager G. D.
Published: July 2002

Laser photodissociation of IBr is used to observe the deactivation of spin–orbit excited Br(4 2P1/2) (Br*) by ground state atomic I(5 2P3/2). The measured quenching rate constant is kI=1.9±0.2×10−11 cm3/s. A total Br* quenching rate due to both ground state Br(4 2P3/2) and self-quenching by Br* is observed following photodissociation of Br2. The effective rate constant is more than 20 times smaller than kI. The results are compared with Landau, Zener, Stueckelberg (LZS) calculations and other experiments. The consequence of the fast I atom quenching on photodissociation-pumped Br* lasers is discussed.
The structures of CO–CH3CCH and N2–CH3CCH: Spectroscopic measurements and modeling
Authors: Lovas Frank J. , Fowler P. W. , Kisiel Z. , Tseng Shao-Hui , Beck Rainer D. , Eggers D. F. , Blake Thomas A. , Watts R. O.
Published: July 2002

Microwave and infrared measurements have been made for the van der Waals complexes N2–CH3CCH and CO–CH3CCH, and their isotopomers. The infrared spectra are complicated by the effects of a slightly hindered internal rotation of the propyne unit. Tunneling motions arising from the N2 or CO molecules are not apparent in the infrared spectra, but in the case of N2 are seen in the microwave spectra. Analysis of the spectral data shows that the dimers are near slipped parallel in structure. Their dipole moments are almost parallel to the dimer b axis, with center-of-mass separations of 3.756(1) Å for CO–CH3CCH and 3.708(2) Å for N2–CH3CCH. (The uncertainties shown in parentheses are two standard deviations.) A simple model based on electrostatic interactions between rigid monomers gives dimer structures in agreement with the experimental assignments, and in conjunction with an approximate model of dispersion energy allows estimation of barriers to internal rotation.
Ion-molecule recombination and other activationless processes in solution: Foundations of a capture model
Authors: Ben-Nun M. , Levine R. D.
Published: July 2002

A three dimensional model Hamiltonian is used to mimic and interpret the results of full molecular dynamics simulations of an ion-molecule activationless recombination process in a solvent of structureless atoms. By making an adiabatic separation of variables it is shown that the gas phase capture model, suitably modified to incorporate the dynamical role of the solvent motion, can be used also in solution. Specifically, a motion along one uncoupled coordinate describes the capture process. The angular momentum for this coordinate is constant during the approach motion and thereby it provides a suitable criterion for capture. The motion of the approaching reactants is shown to be in the strong coupling adiabatic limit. In this limit there is a combination of two effects: A weak ion-molecule attractive interaction at large separations and a substantial solvation of the ion by the liquid. Thus the solvent is able to follow the motion along the reaction coordinate and to take part in the crossing of the centrifugal barrier. A second implication of the model is the efficient deactivation of the ion pair as a result of nonadiabatic V-T transitions. These transitions are confined to the ion-pair polarization well region, i.e., to the left of the adiabatic region of the centrifugal barrier. If a ‘‘solvent-separated’’ ion pair is formed the recombination process is delayed and the reorganization of the solvent is required to facilitate a successful capture. To model this effect a nonlinear, space dependent, coupling term is used in the model Hamiltonian. Comparison is made throughout between the results of full molecular dynamics simulations, computational results for the model Hamiltonian, and the predications of the adiabatic separation. The role of strong solvation in activationless recombination reactions is discussed in terms of the adiabatic separation and its breakdown. The conclusions are compared, and contrasted, with the case of activated bimolecular reactions.
An extended x-ray absorption fine structure study of aqueous solutions by employing molecular dynamics simulations
Authors: D’Angelo P. , Di Nola A. , Filipponi A. , Pavel N. V. , Roccatano D.
Published: July 2002

Bromine–oxygen radial distribution functions [g(r)] have been calculated by means of molecular dynamics simulations for aqueous solutions of rubidium bromide, 2-bromopropane and bromoethane. X-ray absorption spectra at the bromine K edge have been recorded for these solutions. The water contribution to the extended x-ray absorption fine structure spectra has been calculated starting from the gBr,O(r) distribution function. Fits of the x-ray absorption spectra have been performed directly on the raw experimental data, allowing the reliability of the g(r) distribution functions to be verified. The agreement between theoretical and experimental spectra is satisfactory. A procedure to improve model g(r) functions on the basis of the short-range structural information provided by extended x-ray absorption fine structure data is proposed.
Near-resonant versus nonresonant chemiluminescent charge-transfer reactions of atomic ions with HCl
Authors: Glenewinkel-Meyer Th. , Ottinger Ch.
Published: July 2002

Charge-transfer reactions of C+, O+, F+, Ar+ and some other atomic ions with hydrogen chloride were investigated at collision energies between <1 eV and 1 keV. The electronically excited products HCl+ (A 2Σ+) were detected by means of the A 2Σ+→X 2Πi optical emission. In some cases the spectra showed, at low collision energies, an enhanced excitation of specific vibrational HCl+(A,v′) levels: for C+, v′=1; for O+, v′=3 as well as v′=1; and for F+, v′=6. These levels are populated in near-resonant, slightly exothermic processes. Their rotational temperature was on the order of 600–700 K. For the other vibrational levels the excitation is off-resonance, mostly endothermic, and here the rotational temperature was 1000–4000 K. Corresponding data are also given for DCl. The selectivity for certain vibrational states is explained by crossings between the vibronic entrance and exit state energy surfaces, calculated from classical electrostatic multipole potentials. The cross sections for the near-resonant reactions decrease monotonically with increasing collision energy, while for the endothermic channels they rise steeply from threshold to a plateau. With argon ions the excitation function exhibits an unusual shape. Here the charge-transfer cross sections for all vibrational levels go through a maximum just above threshold, which is followed by a distinct minimum at about 10 eVc.m.. This may be due to formation of a long-lived collision complex (Ar–HCl)+.
Construction principle for stable multiply-negative charged molecular systems. Part I. Doubly-negative charged systems
Authors: Scheller M. K. , Cederbaum L. S.
Published: July 2002

The existence of free dianionic M2X2−4 alkali halides which are designed according to a recently introduced construction principle is discussed. The systems are found to be highly ionic. Considerable evidence for their stability to fragmentation and to electron autodetachment is predicted at various levels of theory.
An analytic equation of state and structural properties of nonadditive hard sphere mixtures
Authors: Jung Jinkyung , Jhon Mu Shik , Ree Francis H.
Published: July 2002

We present a new analytic fluid-phase equation of state (EOS) of binary nonadditive hard sphere mixtures having equal collision diameters (d11=d22=d) between like species, but having an unequal collision diameter (d12) between unlike species. For this purpose, we have generated EOS data by Monte Carlo simulations over a wide range of density, composition, and d12. The analytic EOS produces reliable results for d12≥d and d12<d. These results are used to test the van der Waals one-fluid model of mixtures and the modified Martynov and Sarkisov integral equation [Mol. Phys. 59, 275 (1986)]. Short-range structures at high density reveal a broad peak in heterocoordinated packing and a minimum in homocoordination as d12 is reduced below d. The present analysis also strongly indicates, in agreement with available data, a fluid–fluid phase change at high density when d12≳d.
Fourier transform infrared and tunable diode laser spectra of the 13C12CH6 ν12 torsion–vibration–rotation band: Frequencies, intensities, and barriers to internal rotation
Authors: Weber Mark , Reuter Dennis C. , Sirota J. Marcos , Blass William E. , Hillman John J.
Published: July 2002

High resolution spectra in the 12.2 μm region of the ν12 fundamental of the minor isotopomer 13C12CH6 have been recorded. Torsional Coriolis interaction of the excited v12=1 state with the close lying excited torsional v6=3 state leads to observed splittings of up to several tenths of cm−1 in ν12 near the crossing region at KΔK=−18. Torsional splittings measured with a high resolution tunable diode spectrometer (TDL) and more than 2200 lines measured with a Fourier transform spectrometer (FTS) have been analyzed in a global nonlinear least squares fit to a symmetric top Hamiltonian. The standard deviation of the overall fit is 5.5×10−4 cm−1. From 17 intensity measurements of individual lines, a square dipole moment of R2v=7.45(29)×10−4 D2 has been derived for the ν12 fundamental.
Nonadiabatic perturbations and fine structure splittings in the 1,2 3Πg states of B2: An analysis based on adiabatic and rigorous diabatic states
Authors: Manaa M. Riad , Yarkony David R.
Published: July 2002

Nonadiabatic effects and fine structure splittings in the 1,2 3Πg states of B2 were studied. Adiabatic electronic wave functions Ψa(J 3Πg) were computed at the multireference configuration interaction level. The interstate derivative couplings 〈Ψa(1 3Πg)‖(d/dR)Ψa(2 3Πg)〉 were computed and used to construct ‘‘rigorous’’ diabatic electronic states. The spin–orbit interactions responsible for the fine structure splitting in the 1,2 3Πg states were determined, as were the 1,2 3Πg∼1 1Πg spin–orbit interactions. These electronic structure data were used to determine the vibrational structure of the 1,2 3Πg states in both coupled adiabatic and coupled diabatic states bases. As a result of an avoided crossing of the 1,2 3Πg states the fine structure splitting constants exhibit a marked geometry dependence. The fine structure splitting constants for the (1 3Πg, v=0–4) levels were found to be −4.2(−4.4), −3.7, −2.8, −1.8, and −0.83 cm−1, respectively. The (1 3Πg, v=0) value is in good agreement with a recent experimental determination given parenthetically. While the diabatic basis provides illuminating qualitative insights into the electronic structure of the states in question the adiabatic basis is preferred computationally.
Anomalous complex shift of low-frequency out-of-plane vibrations in aniline-M van der Waals complexes (M=He, Ne, Ar)
Authors: Coutant Bruno , Brechignac Philippe
Published: July 2002

Fluorescence excitation vibronic spectra of aniline-M (M=He, Ne, Ar) van der Waals (vdW) complexes have been measured in the spectral region from 300 to 500 cm−1 above S1−S0 origin. Anomalous behavior of the vdW shifts are observed for two out-of-plane vibrational modes. The resulting vdW vibrational shifts for the ν16a vibration are as large as 26.5 cm−1. Similarity with shifts which have already been reported for other aromatic molecules, like pyrimidine [H. Abe, Y. Ohyanagi, M. Ichijo, N. Mikami, and M. Ito, J. Phys. Chem. 89, 3512 (1985)] and tetrazine [D. V. Brumbaugh, J. E. Kenney, and D. H. J. Levy, J. Chem. Phys. 78, 3415 (1983); P. M. Weber and S. A. Rice, J. Chem. Phys. 88, 6120 (1988)] are pointed out.
Dipole-induced dipole light scattering in supercooled liquids near the liquid–glass transition
Authors: Bykhovskii Alexis D. , Pick Robert M.
Published: July 2002

The integrated intensity produced by the dipole-induced dipole (DID) light scattering mechanism in a Lennard-Jones fluid is investigated for different points of the phase diagram corresponding to the normal and undercooled liquid at zero pressure. The exactly computed intensity is compared to the full Kirkwood superposition approximation (KSA) and its so-called Stephen approximation. The latter gives one or two orders of magnitude too large results, while the former is in much better agreement with the exact computation up to large reduced density values. On the basis of the mode coupling theory of the glass transition, it is argued that, in the vicinity of this transition, in real glass forming isotropic molecular liquids for which DID is the only light scattering mechanism, both a dynamical extension of the KSA and its Stephen approximation should yield dynamical spectra proportional to the real ones.
Density functional transition states of organic and organometallic reactions
Authors: Stanton Robert V. , Merz Kenneth M.
Published: July 2002

Baker’s transition state (TS) locating algorithm was incorporated into the density functional (DF) program deMon. Using this TS locating procedure TSs for several model organic and organometallic reactions were located using both the local density approximation (LDA) and nonlocal (NL) gradient corrected methods. The predicted geometries and relative energetics for the reactions were examined and compared to those of Hartree–Fock HF, post HF, and semiempirical calculations. These calculations indicate that the LDA level of DF theory is not reliable in estimating some reaction barriers and TS structures. However, the NL level of DF theory gave results of the same quality as post HF calculations. Our results clearly demonstrate that DF methods can be used to obtain information as accurate as that obtained using similar quality HF based methods for organic and organometallic systems.
Calculation of the electric field gradients, generalized Sternheimer shielding constants, and electric-field-gradient polarizabilities for ten small molecules
Authors: Bishop David M. , Cybulski Sl/awomir M.
Published: July 2002

Electric field gradients, generalized Sternheimer shielding constants, and electric-field-gradient polarizabilities are calculated for H2, N2, F2, HF, HCl, CO, HCN, HNC, H2O, and NH3. The calculations are performed at both the Hartree–Fock and second order Mo/ller–Plesset levels of approximation using large basis sets. For most of these molecules this is the first time that the shielding constants and electric field gradient polarizabilities have been determined. Electron correlation is generally found to be a significant factor.
Diode laser spectroscopy and coupled analysis of the ν2 and ν4 fundamental bands of SiH+3
Authors: Davies P. B. , Smith D. M.
Published: July 2002

The gas phase infrared spectra of the ν2 and ν4 fundamentals of SiH+3 have been measured between 730 and 1015 cm−1. The ion was produced in an ac glow discharge in silane and hydrogen mixtures. Vibration-rotation transitions were detected using diode laser velocity modulation spectroscopy. 112 transitions were included in a combined fit of both bands which yielded B0=5.214 51(27) and C0=2.585 20(36) cm−1. The ν2 [838.0669(24) cm−1] and ν4 [938.3969(36) cm−1] bands are coupled by a Coriolis x,y resonance for which ξ24=−3.8339(22) based on a calculated value of ξ4=−0.051 27. Experimental band origins and rotational and quartic distortion parameters are compared with recent ab initio calculations.
Two-photon wave-packet interferometry
Authors: Engel V. , Metiu H.
Published: July 2002

The excitation of a molecule by a short pulse creates a wave packet whose motion depends on the nuclear forces of the excited electronic state. The properties of such a packet can be studied interferometrically. This is done by exposing the molecule to two identical pulses delayed from each other. Two packets are thus created and the excited state population has a term which depends on the interference between them. This interference population is a quantum effect and its dependence on the delay time reflects the coherence properties of the packet and is very sensitive to the potential energy surface. In this paper, we examine the use of wave-packet interferometry to study the dynamic properties of packets created by two-photon absorption. We find several interference populations since the packets created by two-photon absorption interfere with each other and with packets created by absorption of one photon from each pulse. The single interference terms contain different physical information and we discuss how each can be measured separately. This method provides a sensitive way for studying the dynamic properties of high energy electronic states that can be reached by two-photon absorption.
Low frequency depolarized Raman spectra in water: Results from normal mode analysis
Authors: Sastry Srikanth , Stanley H. Eugene , Sciortino Francesco
Published: July 2002

The Raman spectrum of water in the translational frequency regime has been interpreted in terms of localized vibrational density of states and, in seeming contradiction, in terms of contributions of long-range dipole induced dipole (DID) reactions. We show that these interpretations can be consistently understood by obtaining the Raman spectrum from the normal modes of the inherent liquid structures. We calculate the DID contributions to the Raman spectra for each individual mode, and show that the aggregate spectrum obtained agrees well with both the DID spectrum obtained directly from a molecular dynamics simulation and the spectrum obtained by simulating harmonic dynamics (i.e., exciting all the modes at once and calculating the DID spectrum from the resulting dynamical trajectory of the system).
Detection and characterization of gas-phase InCl using resonance enhanced multiphoton ionization
Authors: Johnson Russell D. , Dearden David V. , Hudgens Jeffrey W.
Published: July 2002

We report resonance enhanced multiphoton ionization (REMPI) spectra of gas-phase indium monochloride (InCl) between 330 and 490 nm. The spectra originate from one-, two-, and three-photon resonances with electronic states that reside between 27 000 and 75 000 cm−1. We assign 12 new Rydberg states. From these Rydberg origins we can extrapolate the adiabatic ionization potential IPa=77 460±600 cm−1 (9.60±0.08 eV). The Rydberg states exhibit vibrational frequencies ranging between 307 and 355 cm−1.
An ab initio molecular orbital study of the unimolecular dissociation reactions of di- and trichloroethylene
Authors: Riehl Jean-Frédéric , Musaev Djamaladdin G. , Morokuma Keiji
Published: July 2002

The potential energy surfaces for the unimolecular ground state elimination reactions of dichloroethylene (DCE) and trichloroethylene (TCE) are studied with ab initio molecular orbital calculations. By the gradient optimization with the second-order Mo/ller–Plesset perturbation (MP2) method and single point calculations with the fourth-order Mo/ller–Plesset perturbation (MP4) and the quadratic single and double configuration interaction including a triple contribution [QCISD(T)], many molecular elimination channels including three- and four-center HCl, H2, and Cl2 elimination and H and Cl migration reactions are systematically examined. For cis- and trans-DCE, the three-center HCl elimination with subsequent chlorovinylidene rearrangement has the lowest overall barrier, whereas for 1,1-DCE for which the three-center path is not available, the four-center elimination has a rather low barrier. Another path starting with the rearrangement of DCE isomers to 1,2-dichloroethylidene followed by HCl elimination is not far in energy from these paths, complicating the overall mechanism of HCl elimination. The H2 elimination from DCE isomers also can take either the three-center path or the 1,2-dichloroethylidene path. For TCE, though the overall barrier to produce HCl+dichloroacetylene is the lowest for the four-center HCl elimination pathway, the production of HCl itself is most easily accomplished by the three-center HCl elimination, and the easiest path for disappearance of TCE is its rearrangement via H or Cl migration to give the 1,2,2-trichloroethylidene intermediate. Thus the overall mechanism of HCl elimination reactions from TCE can also be complicated.
Structure and energetics of van der Waals complexes of carbon monoxide with rare gases. He–CO and Ar–CO
Authors: Kukawska-Tarnawska B. , Chal/asiński G. , Olszewski K.
Published: July 2002

The potential energy surfaces for Ar–CO and He–CO were calculated at the fourth order Mo/ller–Plesset perturbation theory and analyzed using perturbation theory of intermolecular forces. Both the complexes reveal only one minimum related to the approximately T-shaped geometry. For Ar–CO, our best ab initio estimates of Re and De are 3.70 Å and 496 μhartrees, respectively, and the optimal angle Rg–com–O is about 80°. For He–CO, our best Re and De are 3.4 Å and 100 μhartrees, respectively, at the optimal angle Rg–com–O of 70°. Our geometrical parameters agree very well with the experimental data. Our ab initio well depths are estimated to be within ±5% in error and are expected to be the most accurate in the literature so far. The De values were obtained with extended basis sets which included bond functions. Basis set effects on the dispersion and electrostatic correlation terms that are caused by bond functions were also analyzed. Both complexes are bound by dispersion forces, but the anisotropy of the interaction is determined by the exchange repulsion component. This anisotropy may be interpreted in terms of the relative concentrations and depletions in the diffuse region of the CO charge density. The electrostatic and induction effects proved to be negligible.
Rovibrationally resolved, Fourier-transform near infrared spectroscopy of the ν1 and ν2 vibrations of the HCl dimer in a supersonic jet
Authors: Meads Roger F. , McIntosh Avery L. , Arnó Josep I. , Hartz Christopher L. , Lucchese Robert R. , Bevan John W.
Published: July 2002

We have recorded the high-resolution (0.007 cm−1) Fourier-transform near infrared absorption spectrum of the symmetric 35Cl–35Cl, and the mixed 35Cl–37Cl isotopomers of the hydrogen chloride dimer in a supersonic jet. The spectrum was recorded under a low effective rotational temperature (≊20 K). Rovibrational analysis of the observed spectra has been performed for the K″a=0 subbands of the ν+1 and ν−1 perpendicular-type bands centered at 2890 and 2879 cm−1, respectively. The K″a=0 and 1 parallel-type subbands of the ν−2 at 2839 cm−1 and ν+2 at 2857 cm−1 have also been analyzed. Furthermore, we have observed and investigated the (H 35Cl)2ν+1, K″a=1, ΔK=−1 rovibrational transition centered near 2869 cm−1. The results are compared with spectra previously recorded using difference frequency tunable laser spectrometers both in a supersonic slit jet and in an equilibrium gas phase mixture to demonstrate the advantages and limitations of the FTIR technique.
Optimal treatment of diffraction coordinates in wave packet scattering from surfaces
Authors: Lemoine Didier
Published: July 2002

In the context of wave packet methodology we show how to take advantage of the diffractive scattering symmetry arising when the incident beam is normal to the surface or to a surface principal axis. This may lead to a reduction in dimensionality being up to a factor of 8. The Fourier transformation is applied to evaluate the translational kinetic energy operator. Two alternative treatments are possible depending on whether the transformation is utilized to calculate the kinetic energy matrix elements in coordinate space, or whether it is applied to the wave function itself to switch between coordinate and momentum representations. The first approach is similar to the discrete variable representation treatment in the spirit of Light and co-workers whereas the second one enables the use of the fast Fourier transform (FFT) scheme of Kosloff and Kosloff. We provide a detailed comparison between the two approaches as a function of the size of the grid, with and without the presence of symmetry in the diffractive scattering.
The doubly excited 1 3Σ−g state of 7Li2
Authors: Yiannopoulou A. , Ji B. , Li Li , Li M. , Urbanski K. , Lyyra A. M. , Stwalley W. C. , Jeung G.-H.
Published: July 2002

This paper reports the first experimental observation of the doubly excited valence (2p+2p)3Σ−g state of 7Li2. We used cw perturbation-facilitated optical–optical double resonance (PFOODR) fluorescence excitation and resolved fluorescence spectroscopic techniques. All the observed levels have been detected through perturbations by the 2 3Πg state. The deperturbed primary molecular constants of this 1 3Σ−g state are Te=34 045.354(43) cm−1, ωe=216.820(37) cm−1, Be=0.673 69(47) cm−1, Re=2.670 81(94) Å, and De=4279.306(43) cm−1. The equilibrium internuclear distance of the 1 3Σ−g state is smaller than that of the X 1Σ+g ground state.
Rotation–vibration state resolved unimolecular dynamics of highly vibrationally excited CH3O (X̃ 2E). I. Observed stimulated emission pumping spectra
Authors: Geers A. , Kappert J. , Temps F. , Wiebrecht J. W.
Published: July 2002

Using the technique of stimulated emission pumping (SEP) spectroscopy, highly excited vibration–rotation states of the CH3O (X̃ 2E) molecule were probed up to energies of E≤10 000 cm−1. The highest excitation energies exceed the asymptotic H–H2CO dissociation limit of the molecule [ΔrH00(H–H2CO)≊6900 cm−1]. Work was carried out at different experimental resolutions. First, low resolution survey SEP spectra were found to exhibit persistent vibrational structure up to energies far above the dissociation limit. The observed main features were found to be assignable, in a zero-order picture that leaves aside possible mode-to-mode couplings, to the progression of the excited C–O stretch vibration states (ν3). The widths of the respective features correspond to localized short-time vibrational motion for times of ≥0.3 ps (≥10 C–O vibrational periods). Second, in high resolution scans over the coarse vibrational features, characteristic clumps of individual vibration–rotation eigenstates were revealed. These clumps are ascribed to distinctive Franck–Condon active bright zero-order levels which are mixed with the large number of Franck–Condon inactive dark bath states. Under carefully selected conditions, the clumps could be attributed to states with defined and well known values of the total angular momentum quantum number J, which remains as a good quantum number in different coupling cases. These clump spectra will be analyzed quantitatively in the following paper with respect to their bearing for the intramolecular vibrational dynamics of highly excited CH3O (X̃) as a function of vibrational and rotational excitation. From the observed spectra, quantitative data can be obtained on the rate and extent of collision-free intramolecular vibrational and rovibrational energy redistribution (IVR and IRVR) processes, which would result after coherent ultrashort pulse excitation of the molecules.
Detailed thermal study of the rotator phase of pure heptacosane (n-C27H56) by alternating current calorimetry
Authors: Saruyama Y. , Takamizawa K. , Urabe Y.
Published: July 2002

The dynamic heat capacity of heptacosane was measured by ac calorimetry. Samples with a homologous purity of 99.9% were used. The characteristic feature of ac calorimetry of excluding latent heat made it possible to study details of the dynamic heat capacity of the rotator phase. In both the heating and cooling processes at a constant rate of 0.5 K/min, slow motion with a correlation time comparable to 1/3 s was observed. At constant temperatures of the rotator phase significant and slow annealing effects were observed. Experimental results showed that the equilibrium state at temperatures equal to or higher than 329.9 K was apparently different from the equilibrium state at temperatures equal to or lower than 329.5 K. It was shown that the observed experimental results could be explained by assuming existence of a parameter which changes only very slowly.
Rotational spectra and structures of Rg–C6H6–H2O trimers and the Ne–C6H6 dimer (Rg=Ne, Ar, or Kr)
Authors: Arunan E. , Emilsson T. , Gutowsky H. S.
Published: July 2002

Rotational spectra of Rg–C6H6–H2O isotopomers, where Rg=Ne, Ar, or Kr, have been observed with a Balle–Flygare Fourier transform microwave spectrometer. In these trimers the benzene is sandwiched between the rare gas and H2O. Isotopic substitution and inertial analyses show that the Rg–C6H6 distance in the trimer is reduced by about 0.01 Å for Ne, Ar, and Kr compared to the corresponding distance in the Rg–C6H6 dimer. On the other hand, the c.m. (C6H6) to c.m. (H2O) distance in the trimers is increased by only about 0.003 Å from its distance in the dimer. Symmetric top spectra of 20Ne–C6H6 and 22Ne–C6H6 were observed as an aid in the comparison. Hyperfine structure (hfs) and substitution analyses with HDO/D2O containing isotopomers reveal that the m=0 and 1 states of H2O in the trimer are virtually unchanged from those in the C6H6–H2O dimer, including essentially free rotation. In addition, analyses are made of the root-mean square (rms) deviation for the benzene C6 axis from the a axis in the dimers (∼19 deg) and trimers (∼21 deg) and of the displacement of the C2 axis of the water from the a axis in the trimers (∼37 deg).
Exact quantum and time-dependent Hartree studies of the HBr/LiF(001) photodissociation dynamics
Authors: Fang Jian-Yun , Guo Hua
Published: July 2002

Photodissociation dynamics of HBr adsorbed on a LiF(001) surface are investigated using both exact and time-dependent Hartree (TDH) methods on realistic potential energy surfaces. The dissociation dynamics are restricted in two dimensions and two coupled dissociative states of the adsorbate are included. The wave packets are propagated on numerical grids, and fast Fourier transform (FFT) and discrete variable representation (DVR) are used to calculate the action of the Hamiltonian. In the TDH treatment, each excited electronic state is represented by a single nuclear configuration. Final radial, angular, and momentum distributions of the H fragment are calculated. Comparisons between the exact and TDH results reveal that the agreement between the two is generally reasonable and is better for highly averaged quantities. Results also show that nonadiabatic transition dynamics are correctly reproduced by the TDH approximation. Finally, the calculated results are found consistent with the experimental observations.
Photodissociation of molecules oriented by dc electric fields: Determining photofragment angular distributions
Authors: Wu M. , Bemish R. J. , Miller R. E.
Published: July 2002

A quantum mechanical model has been derived for the photofragmentation of a molecule that is preoriented by a moderate to large dc electric field. The hybridization of the rotational wave functions in the dc electric field gives rise to net orientation and the resulting state mixing breaks the symmetry that is normally responsible for the fact that many laboratory measurements are only sensitive to a single anisotropy parameter (β). Consequently, the laboratory photofragment angular distributions resulting from molecules dissociated from ‘‘pendular’’ type states are sensitive to all of the terms in the Legendre expansion of the center-of-mass angular distribution, up to a value determined by the magnitude of the electric field. In essence, the electric field ‘‘locks’’ the molecule fixed frame onto the space fixed frame determined by the field direction. Additional advantages of this approach include the fact that the anisotropy is not quenched by rotation or hyperfine depolarization, owing to the fact that the orientation of the molecule is controlled by the electric field. The application of this method to angularly resolved photofragment experiments and Doppler spectroscopy is discussed.
Appearance and ionization energies of Cz+60-2m and Cz+70-2m ions (with z and m up to 4) produced by electron impact ionization of C60 and C70, respectively
Authors: Wörgötter R. , Dünser B. , Scheier P. , Märk T. D.
Published: July 2002

Electron impact ionization of C60 and C70 has been studied as a function of electron energy in the threshold region using a crossed molecular beam/electron beam double focusing sector field mass spectrometer system. Appearance energies have been determined (using a special threshold extrapolating procedure) for Cz+60-2m and Cz+70-2m ions with z ranging from 1 to 4 and m from 0 to 4, respectively. Whereas the different charge states of the parent ions of C60 and C70 have the same appearance energies within the experimental error bars, the fragment ions of C70 show slightly higher values than their C60 counterparts due to an increased kinetic shift. The presently deduced ionization energies for the C60 parent ions are in excellent agreement with previously reported reliable data for Cz+60 ions (previous data are available only up to z is 3). In contrast, these experimental values disagree strongly with a theoretical calculation by Yannouleas and Landman predicting an approximately linear dependence of the ionization energy on the charge state z, i.e., the experimental values deviate strongly from linearity with increasing charge.
Frequency-dependent viscosity of a ferrofluid
Authors: Salueña C. , Rubí J. M.
Published: July 2002

We have analyzed the viscoelastic behavior of a suspension of ferromagnetic particles. The dynamics of the correlations of the magnetic moment of the particle follows from the corresponding Smoluchowski equation, which generates a hierarchy of equations coupling the consecutive moments. Different decoupling approximations are proposed and studied. The viscosity has been calculated and found to depend on frequency and magnetic field. Our results have been compared to experiments carried out for a suspension with certain polidispersity, reproducing the decay of this coefficient when increasing the frequency.
State resolved rotational relaxation of CS2 in argon free jet
Authors: Liu Shilin , Zhang Qing , Chen Congxiang , Zhang Zhiping , Dai Jinghua , Ma Xingxiao
Published: July 2002

Rotational relaxation of CS2 in argon free jet was studied using laser-induced fluorescence (LIF). The rotational populations were obtained from LIF detection at 19 different points along the jet axis from 3 to 15 nozzle diameters. On the basis of the energy gap power law, the master equation was solved to fit the experiment results, and the two temperature-independent parameters in the law were determined. The results show that the state-to-state relaxation cross sections vary as the inverse 1.7 power of the energy gap between the initial and the final states. The expressions for the state-to-state relaxation cross sections and the rate constants were written. The relaxation cross sections decrease with increasing temperature over the range of 2–60 K for the CS2+Ar collision.
Re-examination of the hyperfine structure of 14NH2
Authors: Kong Jing , Boyd Russell J. , Eriksson Leif A.
Published: July 2002

The hyperfine structure of the 14NH2 radical is investigated by means of multireference single and double configuration interaction (MRCI) techniques. Particular attention is paid to the dependence of the coupling constants on the basis set, reference space, and configuration selection energy threshold. It is found that convergence can be obtained only if more than 83 reference configurations are included with an energy threshold of at least 10−7 hartree. With up to 126 reference configurations, an energy threshold smaller than 10−8 hartree and an uncontracted (13s8p2d/8s2p) basis set, the MRCI isotropic couplings (27.44 and −68.47 MHz for N and H, respectively) are in very good agreement with experimental data (27.9 and −67.2 MHz, respectively).
On the thermodynamics of fluids adsorbed in porous media
Authors: Given James A.
Published: July 2002

We develop thermodynamics for partly quenched systems, i.e., systems in which some of the particles are quenched, or frozen in place, and some of which are annealed, or allowed to equilibrate. In particular, we focus on a class of models for fluids adsorbed in microporous media, in which the quenched particles constitute a microporous matrix, while the annealed particles constitute a fluid adsorbed in that matrix. The replica method is used to relate the matrix-averaged quantities describing such a model to the thermodynamic quantities of a corresponding fully equilibrated model, called the replicated model. For these models, we present averaging methods that give the matrix-averaged thermodynamic quantities of the fluid. We show that there are two natural definitions for the average pressure and three natural definitions for the chemical potential of these systems. We provide both operational definitions and Mayer expansions of these quantities. We establish the Gibbs–Duhem relations for these quantities. We also present new exact relations that express the thermodynamic quantities of partly quenched media in terms of the correlation functions in such media. These include a set of compressibility relations and a virial relation.
Unusual inverse secondary isotope effects: A statistical study of CD2HNC and CH3NC isomerization
Authors: Marks Alison J.
Published: July 2002

Two formulations of microcanonical transition-state theory are used to compute isomerization rate constants k(E,J=0) for CD2HNC and CH3NC. The first of these uses a Monte Carlo method to approximate the classical flux integral expression for k(E,J=0). This method allows a direct comparison to be made with classical trajectory results. The second approach uses a quantum, harmonic state counting method to evaluate k(E,J=0) within the familiar RRKM formalism. The results of the transition-state theory calculations differ at low energies, but at high energies an unusual inverse secondary isotope effect is observed in both. This is analyzed in terms of the RRKM formalism and it is concluded that the presence of a methyl free-rotor mode at the transition state is responsible for the onset of the inverse isotope effect. A similar, but somewhat larger inverse isotope effect has been observed in high-energy classical trajectory calculations.
Density functional calculation of nuclear magnetic resonance chemical shifts
Authors: van Wüllen Christoph
Published: July 2002

A current-density functional theory for the calculation of nuclear magnetic resonance chemical shifts is presented. If the Kohn–Sham orbitals are expanded in a finite basis set, one of the main problems is the strong dependency of the results with respect to a shift of the gauge origin of the vector potential which describes the external magnetic field. Two computational schemes implementing both the individual gauge for localized orbitals (IGLO) and gauge including atomic orbitals (GIAO) concepts, which overcome this problem by introducing distributed gauge origins, are presented in detail. A comparison of the density functional IGLO and GIAO schemes shows that IGLO is much more efficient if one neglects the current-dependent part of the density functional (as is done in ‘‘uncoupled’’ density functional theory), but that this advantage is less pronounced in the full current-density functional treatment.