bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure of H+(H2O)n clusters near the magic number n=21
Authors: Kozack R. E. , Jordan P. C.
Published: July 2002

A model potential for proton hydrates, in which the system is treated as a collection of rigid water monomers plus a lone proton, is used to investigate H+(H2O)n clusters for n near the magic number of 21. Various configurations which are local minima in the total energy are generated. Both clathrate-like and amorphous structures are examined. In all cases, the hydroniumlike unit is four coordinated, donating hydrogen bonds to three water molecules and interacting with the fourth through a favorable charge–dipole interaction. Both hydrogen bonding and ion–dipole interactions play a role in determining the stability of a given configuration. These features make the cluster energies sensitive to the input potentials.
Nonlinear optical properties of novel thiophene derivatives: Experimental and ab initio time-dependent coupled perturbed Hartree–Fock studies
Authors: Karna Shashi P. , Zhang Yue , Samoc Marek , Prasad P. N. , Reinhardt Bruce A. , Dillard Ann G.
Published: July 2002

We report in this paper experimental and theoretical studies of the nonlinear optical properties of a new class of compounds which differ from traditional second-order molecules in the sense that a commonly used electron donor chromophore is replaced by a thiophene ring. The molecular second-order nonlinear optical coefficients, β, as determined by the electric field-induced second-harmonic generation technique, for 2-nitro-1-(2-thienyl)ethene and 4-nitro-1-(2-thienyl)-1,3-butadiene are, respectively, 1 and 3 times that of para-nitroaniline. Ab initio time-dependent coupled perturbed Hartree–Fock results are in agreement with the experimental findings. A Mulliken population analysis indicates that the sulfur atom in the thiophene ring acts as an electron donor. Both the experimental and the theoretical results confirm that a thiophene ring acts as an efficient donor, giving rise to highly efficient second-order nonlinear optical properties. We also show that with this type of chromophore one can use a thiophene oligomer, in place of a thiophene monomer, to amplify the nonlinearity.
The H+OCS hot atom reaction: CO state distributions and translational energy from time-resolved infrared absorption spectroscopy
Authors: Nickolaisen Scott L. , Cartland Harry E.
Published: July 2002

Time-resolved infrared diode laser spectroscopy has been used to probe CO internal and translational excitation from the reaction of hot H atoms with OCS. Product distributions should be strongly biased toward the maximum 1.4 eV collision energy obtained from 278 nm pulsed photolysis of HI. Rotations and vibrations are both colder than predicted by statistical density of states theory, as evidenced by large positive surprisal parameters. The bias against rotation is stronger than that against vibration, with measurable population as high as v=4. The average CO internal excitation is 1920 cm−1, accounting for only 13% of the available energy. Of the energy balance, time-resolved sub-Doppler line shape measurements show that more than 38% appears as relative translation of the separating CO and SH fragments. Studies of the relaxation kinetics indicate that some rotational energy transfer occurs on the time scale of our measurements, but the distributions do not relax sufficiently to alter our conclusions. Vibrational distributions are nascent, though vibrational relaxation of excited CO is unusually fast in the OCS bath, with rates approaching 3% of gas kinetic for v=1. At 9720 cm−1 above the activation barrier, energy partitioning is consistent with incomplete IVR in an HSCO intermediate, followed by repulsive release of a large fraction of the available energy into relative translation.
A new solution of the Ornstein–Zernike equation from the perturbation theory
Authors: Tang Yiping , Lu Benjamin C.-Y.
Published: July 2002

A general solution of the Ornstein–Zernike (OZ) equation was obtained by combining the perturbation theory with the application of the Hilbert transform. The development was based on the Percus–Yevick approximation or the mean spherical approximation for potentials consisting of a hard core and an arbitrary tail function. All terms of the solution are analytical and involve only the potential function and the hard sphere quantities.
Vibronic analysis of the Ã→X̃ laser-induced fluorescence of jet-cooled methoxy (CH3O) radical
Authors: Lee Yin-Yu , Wann Gwo-Huei , Lee Yuan-Pern
Published: July 2002

The dispersed fluorescence of the Ã 2A1–X̃ 2E system of CH3O and 13CH3O in a supersonic jet was recorded after excitation of various vibrational levels of the Ã state. Analysis of the spectra yielded ν2″ = 1412, ν3″ = 1047, ν5″ = 1494, and ν6″ = 653 cm−1 for the X̃ state of 12CH3O, and ν2″ = 1408, ν3″ = 1027, ν5″ = 1488, and ν6″ = 649 cm−1 for 13CH3O. The least-squares fitting of the observed 300–306 lines (with spin–orbit splitting removed) yielded ωe=1057±3 cm−1 and ωexe=7.0±0.7 cm−1 for ν3″ of 12CH3O. Several tentative assignments were also made on the basis of the improved fluorescence spectra. The Fermi-resonant levels 1289 and 1319 cm−1 above v=0 of the Ã state were also reassigned; the former is better represented as v2′ = 1, whereas the latter is represented as v3′ = 2.
A refined Monte Carlo study of aqueous urea solutions
Authors: Hernández-Cobos J. , Ortega-Blake I. , Bonilla-Marín M. , Moreno-Bello M.
Published: July 2002

In a Monte Carlo study several concentrations of urea in water were studied with a potential that includes polarization and nonadditive effects in the intermolecular interaction. Very long runs, 20×106 steps, were used to assure that equilibrium was attained. The results obtained reproduce experimental data, in particular the nitrogen and hydrogen centered radial distribution functions, validating the model and also allowing for the assignment of the structure of this function. We can also address the phenomena of the dimerization of urea and the urea water correlation. Energetic and structural analysis show that there is a urea network which has a particular effect on water structure.
Thermodynamical and structural properties of neon in the liquid and supercritical states obtained from ab initio calculations and molecular dynamics simulations
Authors: Eggenberger Rolf , Gerber Stefan , Huber Hanspeter , Searles Debra , Welker Marc
Published: July 2002

Thermodynamical and structural properties including the equation of state, the second virial coefficient, the enthalpy and internal energy, the molar heat capacity, the speed of sound, the thermal expansion and pressure coefficients, the compressibility, and the pair distribution function are calculated in an ab initio approach for supercritical and liquid neon. The neon dimer potential energy curve has been obtained previously from ab initio calculations and is applied in classical molecular dynamics simulations. Care was taken to eliminate all possible errors thus reducing the remaining error in the supercritical state at higher temperatures to two sources, namely, the inaccuracies in the quantum chemical potential curve and the two particle approximation in the simulation. At lower temperatures, there is in addition an error due to the classical simulation. The calculated properties will be used as benchmarks in future work to investigate the influence of an improved potential curve and of an inclusion of the three particle potential in the simulation.
Magnetic field effect in the geminate recombination of radical ion pairs: Possible pitfalls in microsecond measurements of triplet reaction products
Authors: Lavrik N. L. , Molin Yu. N. , Werner U. , Staerk H.
Published: July 2002

In the geminate recombination reaction of radical ion pairs with overall triplet multiplicity, locally excited triplet molecules are generated. The magnetic field effect (of the hyperfine type) on the triplet yield is usually determined by measuring the triplet absorbance both at zero and at higher applied magnetic fields. It is shown that the magnitude of this magnetic field effect decreases with time if the decay of the triplet concentration measured in the microsecond range is not of first order. This has been demonstrated experimentally using the linked acceptor–donor compound pyrene-(CH2)16-DMA as the test molecule in acetonitrile.
Enhanced instability of extreme motion states in propyne: Lifetimes of overtone versus isoenergetic combination states
Authors: Gambogi Joan E. , Timmermans Jozef H. , Lehmann Kevin K. , Scoles Giacinto
Published: July 2002

In contrast to the theoretical and experimental evidence for small polyatomic molecules, we demonstrate that in the low overtone region of propyne (νCH=2–4) pure overtone states are more perturbed by intramolecular vibrational energy redistribution (IVR) than some nearly isoenergetic combination states. We argue that for larger molecules, this is the expected behavior when the combination states have their energy delocalized over modes that interact only weakly by direct low order anharmonic resonances.
Photodissociation of (N2)+n clusters (2≤n≤7): Branching ratios for formation of N+2 and N+4, and N+2 fragment vibrational excitation
Authors: Bieske E. J.
Published: July 2002

Dynamical processes accompanying the photofragmentation of (N2)+n clusters (n=3–6) have been investigated. Branching ratios for the formation of N+2 and N+4 photoproducts have been determined at wavelengths spanning the continuous absorption of the chromophore N+4 (630, 532, 396, 315, and 266 nm). In addition, the fraction of N+2 photofragments in excited vibrational states has been found using the monitor gas technique, whereby vibrationally excited N+2 molecules readily exchange charge with Ar buffer gas, and molecules in the υ=0 state do not. For a given sized cluster, as the photon energy increases, there is a trend towards a larger proportion of N+2 compared to N+4 fragments and a mild increase in the fraction of vibrationally excited N+2 fragments. On the other hand, as the size of the primary cluster grows, there is a growth in the proportion of N+4 fragments and a decrease in the fraction of vibrationally excited N+2 fragments. These features of (N2)+n cluster photodissociation are argued to be consistent with primary absorption by a N+4 chromophore core to form energetic N+2 and N2 fragments followed by efficient intracluster recombination, exchange of charge, and exchange of vibrational quanta. The efficiency of these processes for (N2)+3 and (N2)+4 suggest that in these species the N2 ligand(s) is (are) positioned at the end(s) of the linear N+4 ion core.
High resolution infrared emission spectra of GaH and GaD
Authors: Campbell J. M. , Dulick M. , Klapstein D. , White J. B. , Bernath P. F.
Published: July 2002

The high resolution infrared emission spectra of gallium hydride and gallium deuteride have been recorded with a Fourier transform spectrometer. There were 1045 lines observed including those from the v=1→0 to v=7→6 bands for the 69GaD and 71GaD species and v=1→0 to v=4→3 bands for the 69GaH and 71GaH species. Dunham Yij’s for each isotopomer were obtained by fitting the data set of each isotopomer separately to the Dunham energy levels of the X 1Σ+ electronic ground state. The mass-reduced Dunham Uij’s were determined using two independent methods. In the first fit the Uij’s constants were determined by the traditional method where all the constants were treated as adjustable parameters and determined statistically. In the second fit the Uij’s which satisfied the condition j<2 were treated as adjustable parameters and the remaining constants were fixed by constraints imposed by the Dunham model. In order to predict the positions of transitions with v’s and J’s much higher than those observed the entire data set was fit directly to the eigenvalues of the Schrödinger equation containing a parameterized internuclear potential energy function.
Spectroscopy of XeF in Ar and Ne matrices
Authors: Zerza G. , Sliwinski G. , Schwentner N. , Hoffman G. J. , Imre D. G. , Apkarian V. A.
Published: July 2002

Spectroscopic constants for the B 2Σ+ and C 2Π charge transfer states of XeF in Ar and Ne matrices are derived from vibrational progressions in excitation and emission spectra. Polarization effects on the Te values are discussed and Rittner potentials are fitted to the constants. Emissions at 389 and 411 nm in Ne and Ar, respectively, are attributed to a distorted B state denoted as B* with a strong red shift of Te and an about 30% larger ωe. B* is assigned to a XeF center with an additional F atom which can be a precursor to XeF2 according to a comparison with Xe2F spectra and the concentration dependence of the intensities. The B* vibrational bands display two well resolved fine structure progressions with a common ωe of ∼60 cm−1 in Ar and Ne which is close to the maximum matrix phonon density and one with 30 cm−1 in Ne and 15 cm−1 in Ar. A reversible change in the Ne fine structure pattern with temperature can be correlated to a face-centered-cubic–hexagonal close-packed (fcc–hcp) phase transition.
Determination of absolute polydispersity and molecular weight distribution of high molecular weight polymers from dynamic light scattering
Authors: Jena Sidhartha S. , Bohidar Himadri B.
Published: July 2002

Intensity correlation function obtained from dynamic light scattering (DLS) experiments has been theoretically analyzed within the framework of the Schulz molecular weight distribution. The correlation function has been expanded about the mean molecular weight instead of the average linewidth as is normally done in cumulant analysis. This allows the explicit determination of polydispersity (Z). In addition to this, it has been shown that the molecular weight exponent of diffusion coefficient ν, occurring in the Mark–Houwink equation DM−ν and the average molecular weight M̄, can in principle be determined from DLS data. Finally, from the knowledge of Z and M̄ it is possible to construct the Schulz molecular weight distribution function. Experiments performed on three model systems, polystyrene in cyclohexane at 35 °C (θ system), polystyrene in toluene at 20 °C (good system), and latex beads in a deionized aqueous medium (hard spheres system), provide excellent matching between the experimental data and results obtained using our proposed method of analysis of DLS studies.
Electron stimulated surface migration of CO on Pt(335). First spectroscopic evidence for a new phenomenon
Authors: Jänsch Heinz J. , Xu Jiazhan , Yates John T.
Published: July 2002

The electronic excitation of adsorbed CO molecules has been observed to cause lateral migration. This phenomenon has been observed using a stepped Pt(335) crystal, where the step sites act as traps for migrating terrace-site CO molecules. The possible role of thermal migration has been excluded.
Transient infrared absorption spectrum of the ν1 fundamental of trans-DOCO
Authors: Petty Jeffrey T. , Moore C. Bradley
Published: July 2002

Using infrared (IR) difference-frequency laser spectroscopy and 193 nm photolysis of acrylic acid(OD), the rotation–vibration spectrum of the ν1 O–D stretching fundamental of trans-DOCO radical has been recorded and assigned. The assignment of the spectrum is based on the +0.9% gas phase-argon matrix shift of the band origins, on the H/D isotopic shifts of the frequencies, and on the chemical reaction rate with O2. The rotational constants confirm that the spectrum is due to the planar trans isomer. The band center is ν0=2684.102 cm−1. Perturbations are observed in the Ka=1 and 6 levels, and information is obtained about the interacting states.
Structure of the vapor–liquid interface near the critical point
Authors: Schmidt J. W. , Moldover M. R.
Published: July 2002

We measured the thicknesses of the vapor–liquid interfaces near the critical points of carbon dioxide (CO2), sulfur hexafluoride (SF6), and trifluoromethane (CHF3) using ellipsometry. The data (when scaled by the refractive index difference Δn and the correlation length ξ) are in agreement with other ellipticity data for binary and pseudobinary mixtures at low pressures. Fully constrained theories of the interface correctly predict the temperature dependence and scaling of the thickness but systematically overestimate the thickness itself by 15%–20%. The theory can be brought into agreement with experiment when an intrinsic interfacial stiffness is added to the theory. A novel feature of the present measurements is that the effects from pressure-induced window strain were measured and mitigated by using a cylindrically-symmetric pressure cell with floating seals.
Soluble conjugated polymers with degenerate ground state derivatives of poly(1,6-heptadiyne)
Authors: Pakbaz K. , Wu R. , Wudl F. , Heeger A. J.
Published: July 2002

We report the results of initial measurements on two derivatives of poly(1,6-heptadiyne): a di-ketone (PHDK) and a di-ester (PHDE). Both polymers have the degenerate ground state of poly(1,6-heptadiyne); both are soluble in common organic solvents. Electron spin resonance (ESR) measurements on pristine samples place an upper limit of 10−4 spins per monomer for both polymers. At low doping levels, we observe a single doping-induced absorption band at an energy of approximately half the π−π* gap, Eg/2; the doping-induced spectral features were fully reversible. In situ ESR and absorption measurements during doping show that charge is stored predominantly in spinless charge carriers. Because of the inequivalent carbons on the conjugated backbone, the results are discussed in terms of the (A=B)n polymer. Both PHDK and PHDE are relatively stable in air; the strength of the π−π* absorption band is nearly constant over a period of a week in air, decreasing to 80% of the initial value.
Spectroscopic analysis of jet-cooled AlCu
Authors: Behm Jane M. , Arrington Caleb A. , Langenberg Jon D. , Morse Michael D.
Published: July 2002

Diatomic AlCu has been interrogated using resonant two-photon ionization spectroscopy in a supersonic expansion of helium. The ground state is shown to be X 1Σ+, deriving from the 3sAl23dCu10σ2 configuration, in agreement with theoretical predictions. The closed-shell nature of this molecule results in a low density of electronic states, allowing the chemical bonding and electronic structure to be investigated in detail. Five excited electronic states have been observed and characterized, leading to a potential energy diagram based solely on experimental results. Constants experimentally determined for AlCu include a ground state bond length (r0) of 2.3389±0.0004 Å, a dissociation energy, D0○, of 2.315±0.012 eV, and an ionization potential of 7.065±0.014 eV.
Cumulative reaction probabilities for OH+H2→H2O+H and ClH+Cl→Cl+HCl from a knowledge of the anharmonic force field
Authors: Cohen Michael J. , Willetts Andrew , Handy Nicholas C.
Published: July 2002

The semiclassical transition state theory (STST), introduced by the authors with Miller and Hernandez, for cumulative reaction probabilities and thermal reaction rates, is extended to include the effects of nonzero angular momentum. It is based only on a knowledge of the quartic force field at the transition state. The theory is demonstrated by calculations for the bimolecular reactions H+H2→H2+H, OH+H2→H2O+H, and ClH+Cl→Cl+HCl and isotopic substitutions of these. The reaction rates are compared with experimental values and a variety of quantum mechanical and semiclassical calculations. In all instances STST is a great improvement over the harmonic approximation, and in all cases where experimental rates are known, the STST values lie within (or very close to) given experimental bounds. It is probable that these H containing reactions are the most severe tests of the method. STST can be extended to reactions involving more atoms without difficulty, unlike quantum mechanical methods, and it is very inexpensive.
Computer simulation study of the Rayleigh and Raman spectra of fluid N2
Authors: Mueller Andreas , Steele W. A. , Versmold H.
Published: July 2002

Computer simulations of the Rayleigh and Raman spectral time-correlation functions (TCFs) are reported for N2 at 323 K and densities ranging from 0.48 to 2.5 times the critical value. The results are compared with experiment. Particular attention is paid to the density dependence of the spectral intensity due to the correlated permanent polarizability and to the interaction-induced contributions to the polarizability. The partial cancellations that occur between various two-, three-, and four-body terms in the cross and collision induced (CI) parts of the spectral TCFs are evaluated. It is shown that these terms are significant in both the Raman and the Rayleigh spectra, but cancellation greatly reduces their net contribution at all densities studied. The weak but uncancelled TCFs that are associated with orientational correlations of the molecular polarizability anisotropies are shown to be a significant part of the high density Rayleigh TCFs. It is argued that the long-range nature of this TCF means that its simulated values are poorly known.
The evaluation of microcanonical and canonical rate constants using the stabilization theory of dynamics
Authors: Mandelshtam V. A. , Taylor H. S.
Published: July 2002

A new conceptually simple and computationally economic method of evaluating the spectral density is presented. The spectral density is then used to compute the microcanonical rate constant by a procedure that uses only the eigenfunctions and real eigenvalues of the system in a series of finite enclosures. Absorbing potentials or dilatation analytic methods are not needed. Thermal rates at low temperatures are obtained to high accuracy using very small basis sets. Examples are presented for single symmetric and asymmetric barriers fit to the potential for H+H2→H2+H and Cl+H2→HCl+H 1D reactions. An asymmetric double barrier is also studied so as to include a problem where narrow resonances contribute to the low temperature thermal rate constant. The method presented here should also be of great use in modeling electronic mesoscopic devices.
The absorption spectra of small nickel clusters via photodissociation: NinArm→Nin+mAr
Authors: Knickelbein Mark B.
Published: July 2002

The photodissociation action spectra of NinArm (n=3,4,5, and 7) have been recorded from 306 to 674 nm. The spectra are continuous, with cross sections which increase monotonically into the ultraviolet. It is observed that the photodissociation spectra recorded for Ni3Arm, Ni4Arm, and Ni5Arm are independent on m, indicating a negligible influence of the weakly bound argon atoms on the absorption spectra of the underlying clusters. The oscillator strengths increase monotonically with increasing cluster nuclearity (n) in accord with the Thomas–Reiche–Kuhn sum rule. The results are discussed from both molecular and classical electrodynamic points of view.
A neutron scattering study of shear induced turbidity in polystyrene dissolved in dioctyl phthalate
Authors: Nakatani A. I. , Douglas J. F. , Ban Y.-B. , Han C. C.
Published: July 2002

The influence of shear on the small angle neutron scattering from a semidilute solution of polystyrene (PS) dissolved in dioctyl phthalate (DOP) is examined in the limit of strong shear (γ̇≥100 s−1). These experiments are restricted to room temperature, which is close to the theta point of PS/DOP. The fraction of deuterated PS chains is varied, while the total polymer weight fraction is fixed at 3%. This is near the critical composition of hydrogenated PS in DOP. The increased scattering at high shear rates (‘‘shear induced turbidity’’) is interpreted in terms of a critical temperature shift. Scattering along the flow direction is distorted and the scattering data normal to flow is uninfluenced by shear above a wave vector cutoff q*, which is independent of shear rate. This cutoff is found to correspond to the Debye length, characterizing the average intermolecular potential range.
Effects of rotational, vibrational, and translational energy on the rate constants for the isotope exchange reactions OH−+D2 and OD−+H2
Authors: Viggiano A. A. , Morris Robert A.
Published: July 2002

Rate constants for the isotope exchange reactions of OH− with D2 and OD− with H2 have been measured as a function of average center-of-mass kinetic energy at several temperatures. The reaction of OH− with D2 is slightly exothermic, and the rate constant has a negative temperature dependence. The kinetic energy dependences of the rate constants have minima near 0.1 eV. A strong negative dependence on the D2 rotational temperature was found. The reason for this dependence is unclear at present. In contrast, the reaction of OD− with H2 is slightly endothermic and shows positive dependences on both temperature and kinetic energy. The negative rotational dependence for the reaction of OD− with H2 is not as large as that for OH−+D2, presumably because rotational energy can help overcome the endothermicity in the case of OD−+H2. Vibrational energy is observed to promote reactivity in both reactions.
Lifetime measurements on perturbed levels of NO(B 2Π) and precise term energy of the NO(a 4Π) state
Authors: Ottinger Ch. , Vilesov A. F.
Published: July 2002

Using a pulsed near-UV dye laser, the laser-induced fluorescence (LIF) excitation spectrum of the NO(B 2Π←X 2Π) transition was measured in the (0,9) band on vibrationally hot NO in a molecular beam. Lifetime measurements were made for some of the B,v=0 and v=3 rotational/fine structure levels, including one which was recently shown to exhibit very specific kinetic effects due to a perturbation by the NO(a 4Π) state [Ch. Ottinger and A. F. Vilesov, J. Chem. Phys. 100, 1805 (1994)]. This perturbation manifested itself in the present work by a significantly longer lifetime of the 2Π3/2(10.5) level relative to other B,0 levels. The same effect was observed for the B,v=3 2Π1/2(17.5) level. The S/O interaction matrix element is estimated to be very small (≂10−2–10−3 cm−1). The perturbed level pairs must therefore be in very close, accidental coincidence. This was used for a precise determination of the term energy of the a 4Π state as T0(a 4Π)=38 266.74±0.03 cm−1.
A detailed study of the configuration selected multireference configuration interaction method combined with perturbation theory to correct the wave function
Authors: Engels B.
Published: July 2002

A reliable prediction of the isotropic hyperfine coupling constant Aiso is still a difficult task for ab initio calculations. In previous studies, the configuration selected multireference configuration interaction method in combination with perturbation theory to correct the wave function (MRCI/BK) yielded accurate isotropic hyperfine coupling constants very economically. The present study gives a detailed analysis of the MRCI/BK method based on the X 2Π state of CH as a test case. Furthermore, a comparison to various other methods such as Mo/ller–Plesset perturbation theory and the coupled cluster approach is made. The success of the MRCI/BK method in predicting isotropic hyperfine coupling constants is explained in terms of the influence of higher than double excitations.
Negative ion reactions with PF5 and the electron affinity of PF5
Authors: Miller Thomas M. , Stevens Miller Amy E. , Viggiano A. A. , Morris Robert A. , Paulson John F.
Published: July 2002

Rate coefficients and product branching fractions have been determined for 31 ion–molecule reactions involving PF5 or PF−5. About half of the reactions studied show an ion–molecule association channel. NH−2 and OH− reaction with PF5 yields HF product. F− and electron transfer channels are also observed in many of the reactions studied. Consideration of the efficiency of the electron transfer channel in these reactions leads to the conclusion that the adiabatic electron affinity of PF5 is 0.75±0.15 eV.
The ground state of the Fe2 molecule
Authors: Noro Takeshi , Ballard Charles , Palmer Michael H. , Tatewaki Hiroshi
Published: July 2002

The Fe2 molecule is a typical transition metal dimer which has a rather large dissociation energy and a small bond distance compared with the inter-nuclear distance in the crystalline metal. We have investigated the Fe2 molecule with multireference self-consistent-field (MCSCF) and multireference configuration interaction (CI) calculations. The dissociation energy (De), the equilibrium nuclear distance (Re), and the zero-point frequency (ωe) were calculated (with observed in parentheses) as 1.57 (1.30±0.22) eV, 2.06 (1.87 to 2.02) Å, and 260.9 (299.6) cm−1, respectively. Thus the agreement between experiment and calculation is very satisfactory, and is a marked improvement on previous theoretical studies. The contribution of the d electrons to the bonding is important and a proper description of correlation effects among the d electrons is indispensable.
Activated rate processes: A relation between Hamiltonian and stochastic theories
Authors: Pollak Eli , Berezhkovskii Alexander M. , Schuss Zeev
Published: July 2002

Kramers’ treatment of activated rate processes is based on the Langevin equation of motion for the escaping particle. The stochastic dynamics may be cast equivalently as the dynamics of a particle interacting bilinearly with a bath of harmonic oscillators. This paper explores the connection between the solutions of Kramers’ problem (and its generalization to include memory friction) obtained in the framework of these two approaches. We demonstrate their equivalence for the specific case of a parabolic barrier potential. The Hamiltonian representation is used to construct (a) a nontrivial eigenfunction of the Fokker–Planck equation which is generalized to include time dependent friction; (b) the Kramers’ stationary flux distribution function; (c) the stochastic separatrix.
Frequency-modulation enhanced magnetic rotation spectroscopy: A sensitive and selective absorption scheme for paramagnetic molecules
Authors: McCarthy Michael C. , Bloch Jonathan C. , Field Robert W.
Published: July 2002

The technique of frequency-modulation enhanced magnetic rotation spectroscopy (FM-MRS) is described. An experimental comparison between FM-MRS, frequency-modulation, and magnetic rotation spectroscopies clearly demonstrates that the double null absorption-based FM-MR technique is more sensitive than either FM or MRS separately. The theoretical basis for this sensitivity enhancement can be attributed to a 1/θ (where θ is the small uncrossing angle between two nearly crossed polarizers) improvement in the sensitivity of the combined FM-MR technique relative to that of standard FM spectroscopy. The sensitivity improvement of FM-MRS relative to MRS is even better than that for FM spectroscopy. In practical terms, this enhancement can improve the experimentally achievable sensitivity of FM and magnetic rotation spectroscopies by factors of O(101) to O(102) and O(102) to O(104), respectively, levels that approach the theoretical quantum noise limit. In addition, because FM-MRS is based on the Zeeman effect, Zeeman information is encoded into FM-MR line profiles in the form of relative phase labels and line shapes. The relative phases and line shapes of rotational lines can be used to group features by rotational branch and to differentiate between magnetically active (Λ≠0, Σ≠0) and inactive (Λ=0, Σ=0) features. The theory of FM-MRS, including a discussion of line shapes and a sensitivity analysis, is reviewed here; applications of the FM-MR technique are presented separately in the following paper.
Nickel group cluster anion reactions with carbon monoxide: Rate coefficients and chemisorption efficiency
Authors: Hintz Paul A. , Ervin Kent M.
Published: July 2002

Reactions of Ni−n(n=3–10), Pd−n(n=3–8), and Pt−n(n=3–7) with CO are studied in a flow tube reactor. Bimolecular rate coefficients are measured for the association reaction of CO adsorbing on the cluster surface. The rate coefficients range from about 10% of the collision rate for the trimer anions to near the collision rate for clusters larger than four atoms. The maximum number of CO molecules that bind to each cluster is determined. Whereas the saturation limits for nickel are typical for an 18 electron transition metal, the limits for platinum are lower, reflecting the electron deficient structures observed in condensed phase chemistry. The CO saturated palladium clusters represent the first examples of saturated binary palladium carbonyl compounds. Comparisons are made to similar studies on metal cation and neutral clusters and also to surface scattering studies of nickel group metals.
Gradient fields of potential energy surfaces
Authors: Ruedenberg Klaus , Sun Jun-Qiang
Published: July 2002

While contour plots provide conceptual pictures of potential energy surfaces and exhibit their critical points, quantitative determinations of critical points and reaction paths as well as dynamical calculations require a knowledge of the gradient fields. The corresponding orthogonal trajectory maps are more complex than contour maps, but they provide additional insights. They are found to contain certain frequently occurring structural elements and these patterns are here examined. It is shown that many of them result from local confluences of orthogonal trajectory bundles with gradient extremals. The analysis leads to the distinction between eight different kinds of such gradient extremal channels. The most important ones are the streambeds and ridges, the former being typical conceptual prototypes of reaction channels, the latter being prototypes of reaction barriers. Gradient extremal channels emanate from second order critical points in the directions of all normal modes, but they do not necessarily follow along the entire reaction path of any one reaction. They can also exist unrelated to critical points. The conclusions are exemplified on a number of model potential energy surfaces.
Ab initio equilibrium constants for H2O–H2O and H2O–CO2
Authors: Zhang Nancy Renyou , Shillady Donald D.
Published: July 2002

Ab initio 6-31G** electronic structure calculations have been used to determine the minimum energy geometries and vibrational frequencies of molecular clusters of water and carbon dioxide. Application of statistical thermodynamics leads to theoretical equilibrium constants for gas phase dimerization of water and the formation of an adduct of carbon dioxide with water. The low energy vibrations of the clusters lead to much larger contributions to the vibrational partitioning of the energy than do the fundamental vibrations of the monomeric species. A new ‘‘Harmonic-Morse’’ formula is derived to estimate anharmonicity from optimized harmonic frequencies and two additional values on the potential surface for each vibration. These ab initio calculations of equilibrium constants are very close to recent measurements and fall within the range of values obtained by other methods. This no-parameter treatment gives excellent agreement for the equilibrium of H2O–CO2 near the supercritical fluid range of CO2 and suggests that a ‘‘Theory of Significant Clusters’’ may be extended to a model of supercritical fluids which includes the effects of anharmonicity.
Dynamics of star polymers in a good solvent: A Kramers potential treatment
Authors: Ohno K. , Schulz M. , Binder K. , Frisch H. L.
Published: July 2002

The ‘‘effective’’ relaxation time τ of isolated star polymers with excluded volume interactions in the Rouse model limit (i.e., disregarding hydrodynamic interactions present in real solvents) is studied varying both the number of arms f and the number of monomers per arm l. Here τ is defined from the response of the gyration radius of the star polymer to a Kramers potential that describes the effect of shear flow in lowest order in the shear rate. Monte Carlo simulations are performed with two different techniques (simple sampling with enrichment or dynamic Monte Carlo, respectively) for two different models (simple self-avoiding walks with an extended core or the bond fluctuation model, respectively). It is proposed that the ratio τ(f,l)/τ(1,l) should be a universal function of f for large l, and the Monte Carlo data are used to test this hypothesis.
Ab initio study of the energy difference between the cyclic and linear forms of the C6 molecule
Authors: Pless V. , Suter H. U. , Engels B.
Published: July 2002

The energy difference between the three lowest-lying isomers of C6, the linear 3Σ−g state, and the two ring forms, the benzene structure (1A1g) possessing D6h symmetry and a distorted cyclic form (1A1′, D3h symmetry) have been calculated using various ab initio methods. Variational methods such as multireference configuration interaction (MR-CI) and complete active space second order perturbation treatment (CASPT2) have been applied, as well as perturbational treatments and coupled cluster calculations (CCD). The correlation of all valence shell electrons is found to be important for a balanced description of the isomers of C6. Methods which do not account for higher-order effects appropriately proved to be unsuitable for calculating the energy difference correctly. The results from multireference configuration interaction methods show that the isomers are close in energy with the cyclic forms somewhat lower than the linear form. The ring form possessing D3h symmetry (1A1′) is found to be the lowest-lying structure.
Three-dimensional wave packet studies of ozone photodissociation in the Hartley band: Converged autocorrelation functions and absorption spectra
Authors: Balakrishnan N. , Billing G. D.
Published: July 2002

We report fully converged autocorrelation functions governing the photodissociation of ozone in the Hartley band, by an exact solution of the time-dependent Schrödinger equation. A local-mode representation employing hyperspherical coordinates has been used for describing the dynamics. Two different potential energy surfaces (PESs) have been employed in the present investigation, and the results showed sensitivity to the choice of the PES. Our converged calculation for J=0 on the Sheppard–Walker PES near quantitatively reproduced the characteristic recurrence features in the autocorrelation function obtained by Johnson and Kinsey from the experimental spectrum of Freeman et al. This is in contrast to previously reported three-dimensional calculations which showed a factor of 10 higher recurrence intensities compared to the experimentally derived one. The absorption spectrum, obtained as the Fourier transform of the autocorrelation function, is in excellent agreement with the experimental spectrum and exhibits the unique features seen on top of the experimental spectrum, which has caused considerable theoretical interest over recent years. Calculations on the Yamashita–Morokuma surface showed a factor of 5 higher intensity for the recurrence features, in agreement with existing three-dimensional calculations. The effect of J on the autocorrelation function has also been investigated by carrying out a calculation for J=10 and invoking a planar approximation. The J effect became noticeable only at a later time (t≳150 fs) of the dissociation process and its effect on the absorption spectrum was found to be only marginal.
Pyrolysis jet spectroscopy and ab initio studies of the S1 and T1 states of germanium difluoride
Authors: Karolczak J. , Grev Roger S. , Clouthier Dennis J.
Published: July 2002

Spectra of jet-cooled germanium difluoride were obtained by the reaction of germanium metal and fluorine in the throat of a heated supersonic jet. Two band systems at 331–305 and 231–224 nm were observed by laser-induced emission excitation spectroscopy. On the basis of high quality ab initio predictions of the energies, geometries and vibrational frequencies of the lower electronic states, the two band systems are assigned as ã 3B1–X̃ 1A1 and Ã 1B1–X̃ 1A1, respectively. The T1–S0 spectrum consists of a long, well-resolved progression in the excited state bending frequency with ν2′ = 192.2 cm−1 and T00=30 582.1 cm−1. The S1–S0 spectrum is a complex tangle of vibronic structure accompanied by a rising background. A partial analysis of the vibronic structure gave ν2′ = 159.6 cm−1 and T00=43 860.9 cm−1. The spectra are very similar to the analogous band systems of GeCl2.
13C nuclear magnetic resonance study of the solution-state dynamics of benzene
Authors: Coupry C. , Chenon M.-T. , Werbelow L. G.
Published: July 2002

Nuclear spin relaxation-induced polarization transfer is utilized to investigate the reorientational dynamics of liquid benzene. The second rank reorientational correlation time associated with the overall tumbling of the molecular plane, τ⊥=2.2±0.1 ps, and the motional anisotropy, τ⊥/τ∥=2.4±0.4, were determined at 283 K. From the quantitative measure of relaxation-induced three spin order, the proton shielding anisotropy can be determined, ΔσH[1+(3/5)ηH]=−5.3±1.0 ppm.
Crystal field, orientational order, and lattice contraction in solid C60
Authors: Lamoen D. , Michel K. H.
Published: July 2002

A model of the intermolecular potential in solid C60, which is based on Born–Mayer repulsions, van der Waals attractions, and electrostatic multipoles, is presented. The potential is expanded in terms of multipolar rotator functions. The orientation–orientation interaction and the crystal field are calculated. The orientational phase transition to the Pa3̄ phase is studied with the methods of statistical mechanics. The discontinuity of the order parameter at the transition and the temperature evolution of the order parameter are calculated. The lattice contraction at the phase transition is evaluated. The influence of the lattice contraction on the crystal field and on the orientational order is studied.
Evidence for the formation of clathrate structures of protonated water–methanol clusters at thermal energy
Authors: Zhang X. , Castleman A. W.
Published: July 2002

Gas phase ion–molecule reactions of large protonated water clusters with methanol are studied in a flow reactor under thermal conditions. A distinct feature at H+(H2O)21 corresponding to a stable clathrate structure is observed. Upon the addition of methanol from a reactant gas inlet, products of the general form H+(H2O)n(CH3OH)m (m=1–5) are detected. Distinct features at n+m=21 are observed under different experimental conditions, which are indicative of the formation of clathrate-like structures for mixed water–methanol cluster ions. A switching reaction mechanism rather than an association is found to be operative. Considering the results of the present study and those from previous molecular beam experiments suggests that clathrate structures arise from their inherent stabilities rather than due to kinetic effects.
Quantum effects in the surface penetration of energetic hydrogen atoms
Authors: Yang Kai , Rabitz Herschel
Published: July 2002

The mixed quantum/classical molecular dynamics approach is employed to simulate the process in which an energetic H atom penetrates a Pd(100) surface. The H atom is described by a quantum wave packet, while all the Pd atoms are treated classically. Results from both 2D and 3D wave packet simulations are presented. It is found that the relaxation of the Pd lattice has minimal effect on the observed behavior, due to the lack of efficient energy flow between the light H atom and the heavy Pd atoms. Surprisingly, distinctly different behavior occurred in two and three dimensions, with the increased dimensionality exhibiting less surface penetration. The drastic difference may be traced back to the quantum mechanical nature of the motion of the H atom. Finally, the transport behavior of H in solids is discussed in connection with the simulation results.
Near ultraviolet photolysis of methanethiol studied by H atom photofragment translational spectroscopy
Authors: Wilson Steven H. S. , Ashfold Michael N. R. , Dixon Richard N.
Published: July 2002

The technique of H Rydberg atom photofragment translational spectroscopy has been applied to a high resolution study of the primary photochemistry of methanethiol (CH3SH) following excitation at a wide range of wavelengths in the near ultraviolet. In accord with previous studies of this molecule, excitation within its first (1 1A″−X̃ 1A′) absorption continuum is shown to result in S–H bond fission. Spectral analysis yields a refined value for the bond dissociation energy: D00(CH3S–H)=30 250±100 cm−1. The resulting CH3S(X̃) fragments are deduced to carry only modest vibrational excitation, distributed specifically in the ν3 (C–S stretching) mode and in one other mode having a wave number of ∼1040 cm−1. We associate this latter mode with bending of the CH3 moiety in the plane containing the C and S nuclei and the lobe of the unpaired electron which was originally involved in the S–H bond. Decreasing the excitation wavelength (while remaining within the first absorption continuum) results in an increase in both the vibrational and rotational excitation of the CH3S(X̃) fragments, but a decrease in the relative yield of the upper (2E1/2) spin–orbit component. Excitation at still shorter excitation wavelengths accesses the second (2 1A″−X̃ 1A′) absorption band of CH3SH. The CH3S fragments resulting from S–H bond fission at these excitation wavelengths are observed to carry very much higher levels of vibrational excitation in the above two modes. The observation of H atoms attributable to secondary photolysis of SH(X) fragments indicates increased competition from the alternative C–S bond fission channel at these shorter excitation wavelengths. Additional peaks in the H atom time-of-flight spectrum, most clearly evident following excitation at wavelengths in the range 213–220 nm, are interpretable in terms of secondary photolysis of the primary CH3S(X̃) fragments yielding thioformaldehyde (H2CS), primarily in its Ã 1A2 excited electronic state. Symmetry arguments provide an explanation for this specific electronic branching in the near ultraviolet photolysis of CH3S fragments.
Far-infrared properties of C60 and C70 compacts
Authors: FitzGerald S. A. , Sievers A. J.
Published: July 2002

The far-infrared transmission of polycrystalline C60 and C70 compacts has been measured from 3 to 330 cm−1 as a function of temperature. Both intrinsic and impurity-induced absorption bands are identified in these samples. The low temperature phase of C60 is observed to have two infrared active translational modes at 40.9 and 54.7 cm−1. Samples of C60 exposed to air before cooling to low temperatures contain several additional infrared bands which have been identified with adsorbed H2O vapor. A large isotope shift is observed for some of the bands when H2O is replaced by D2O, consistent with rotational or librational behavior. For C70 one strong intrinsic band is centered at 21 cm−1 and four much weaker bands occur at 28, 32, 52, and 64 cm−1. When doped with H2O vapor, C70 also produces several strong H2O impurity bands, some of which show isotopic shifts. The temperature dependence of five intramolecular C70 modes in the frequency range from 200 to 330 cm−1 is also reported.
Entropy catastrophe and configurational entropies in supercooled and superheated regimes
Authors: Kishore K. , Shobha H. K.
Published: July 2002

Utilizing the well known Kauzmann’s entropy catastrophe paradox, we have developed simple thermodynamic expressions for calculating the Kauzmann’s temperatures in the supercooled liquid (Tk1) and superheated solid (Tk2) regions besides obtaining new Kauzmann’s temperature in the superheated liquid (Tk3) regime. Tk3 is actually the critical temperature of liquids. The configurational entropies in respective regimes can also be obtained from this thermodynamic treatment that leads us to a new ‘‘characteristic entropy constant’’ which also seems to be indicative of configurational entropy. Comparison has also been made for the configurational entropies of supercooled and superheated liquid states. The generality of the above thermodynamic treatment has been successfully tested for alkali metals and Na2K alloy and polymers.