bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational relaxation of CO (n=1) in collisions with H2. I. Potential energy surface and test of dynamical approximations
Authors: Bačić Z. , Schinke R. , Diercksen G. H. F.
Published: July 2002

We present a fully quantal dynamical study of vibrational relaxation of CO in collisions with H2 treating H2 as a structureless projectile. The potential energy surface consists of a SCF part, including explicitly the variation with the CO bond distance, and a damped long range dispersion part. This potential model contains only two free parameters in the damping function which have been determined previously by fitting rotationally inelastic beam data for D2–CO. The dynamical calculations are performed within the infinite order sudden approximation (IOSA) and the coupled states approximation (CSA). The IOSA relaxation cross sections are generally a factor of ∼1.5 smaller than the CSA cross sections independent on the initial rotational state of CO for j≲4–6. The relaxation rate constants are very sensitive to small changes of the interaction potential. After readjusting one of the damping parameters (within a range still acceptable with the beam data) the IOSA rate constants agree very well with the experimental ones for ortho H2. It is demonstrated that on the same level of approximation the present rate constants are about three times larger than those obtained from the potential energy surface of Poulsen.
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3–5D4 cross relaxation in Tb3+ pairs in CsCdBr3 crystals
Authors: Berdowski P. A. M. , Lammers M. J. J. , Blasse G.
Published: July 2002

The 5D3–5D4 cross relaxation process of Tb3+ in CsCdBr3 has been evaluated by analysis of the decay curves of the 5D3 and 5D4 emission. The temperature dependence of the cross relaxation rate can be explained assuming phonon-assisted processes. The results show that the larger amount of Tb3+ ions is incorporated in pairs into the CsCdBr3 lattice.
Molecular Hessians for large-scale MCSCF wave functions
Authors: Helgaker Trygve U. , Almlöf Jan , Jensen Hans Jo/rgen Aa. , Jo/rgensen Poul
Published: July 2002

The calculation of molecular Hessians for large-scale multiconfiguration self-consistent-field (MCSCF) functions is described. The formalism is based on exponential parametrization of the wave function and symmetric orthonormalization of the molecular orbitals. Extensive use is made of one-index transformations of the molecular integrals, both to construct the gradient vectors that appear in the linear MCSCF response equations, and to perform the multiplication of the trial vectors on the electronic Hessian in the iterative, direct solution of the response equations. No element of the electronic Hessian is ever calculated explicitly, allowing for use of large configuration expansions. Efficient methods are developed for obtaining the solution vectors of the linear response equations. The accuracy of the molecular Hessian is analyzed in terms of the accuracy of these solution vectors. To allow for large basis sets Fock matrices are used to minimize transformations and integrals are recalculated to minimize storage requirements. Integral derivatives are calculated following the McMurchie–Davidson scheme. A simplified algorithm for calculation of derivatives of integrals involving one-center overlap distributions is described. Sample calculations involving several thousand configurations are reported.
Theoretical study of an unusual reactive collision Cs(7p)+H2→CsH+H. Diabatic approach of the collinear collision potential energy surfaces
Authors: Gadea F. X. , Spiegelmann F. , Pelissier M. , Malrieu J. P.
Published: July 2002

The Cs(7p)+(X 1∑+g, v=0) H2→(X 2∑+, v=0) CsH+H reactive collision was recently experimentally observed from a crossed beam experiment by Crépin et al. This reaction is rather unusual since it starts from a highly excited state of the system (11th potential surface) and must reach the ground state surface in the product channel without energy loss. Accurate nonempirical CI calculations of the potential energy surface for the collinear collision are reported, using large basis sets and a nonempirical relativistic pseudopotential for the Cs atom. The adiabatic potential surfaces, which exhibit irregular behavior, are reinterpreted in terms of an ab initio nearly diabatic effective Hamiltonian spanned by five neutral repulsive channels Cs(6s)×H2, Cs(6p)×H2,..., Cs(7p)×H2 and an ionic very flat Cs+H−2 channel which tends to the product wave function (Cs+H−)×H. The intersection of these diabatic potential surfaces are accessible from the entrance energy and this picture supports a harpooning mechanism, the 7p electron jumping on the H2 system at RCsH ≊6 bohr, the reactive systems being those which remain diabatically on this ionic surface through the avoided crossings with the lower Cs(7s)×H2,...,Cs(6s)×H2 neutral surfaces. The electronic couplings between the diabatic states are given as functions of the RCsH and RHH coordinates and their behavior is qualitatively understood. The existence of accidental conical intersections between some neutral diabatic states and the ionic state potential surfaces is demonstrated; these conical intersections should increase the reaction cross section. This diabatic study confirms, therefore, the harpooning mechanism previously proposed to interpret this paradoxical photochemical reaction.
Efimov states for three 4He atoms?
Authors: Cornelius Th. , Glöckle W.
Published: July 2002

The system of three 4He atoms interacting through the intermolecular potential HFDHE2 is solved exactly within the Faddeev scheme. We find that the trimer has two bound states at E=−0.11 K and E=−0.16(−2) K, where the excited state shows the property of an Efimov state. It is overrun by the two-body fragmentation threshold for an artificially enhanced overall interaction strength. Uncertainties in the potential parameters, possibly combined with solvent effects, may allow a slightly weaker force, which supports a second Efimov state. In any case dynamical effects of the Efimov states will be the same whether it lies close to E=0 on the physical or unphysical sheet.
Phenomenological theory of equilibrium thermodynamic properties and phase separation of micellar solutions
Authors: Blankschtein Daniel , Thurston George M. , Benedek George B.
Published: July 2002

A detailed description and generalization of a recently developed theory, which provides analytic representations of the distribution of micellar species and the equilibrium thermodynamic properties of amphiphile–water solutions that exhibit phase separation and critical phenomena, is presented. We propose a form for the structure of the Gibbs free energy which accurately describes the essential physical factors responsible for micellization and phase separation. These are: the free-energy advantage associated with the formation of individual micellar species,the entropy of mixing of the extended micelles and the water molecules, and the free energy of interaction between each member of the micellar size distribution. By applying to this Gibbs free energy the conditions of multiple chemical equilibrium and thermodynamic stability, all the relevant statistical and thermodynamic equilibrium properties of the micellar solution can be calculated. These properties include the location of the critical concentration and temperature, the shape of the coexistence curve and the spinodal line, the micellar size distribution, and the osmotic compressibility. The proposed Gibbs free energy provides an excellent description of a wide range of experimental findings in two quite different micellar solutions. The first is constituted of the zwitterionic amphiphile dioctanoyl phosphatidylcholine (C8-lecithin) and water, which exhibits an upper consolute point. The second is constituted of the nonionic amphiphile n-dodecyl hexaoxyethylene glycol monoether (C12E6) and water, which exhibits a lower consolute point. In addition, we use our theoretical approach to analyze a model of amphiphile–water solutions containing monodisperse micelles in equilibrium with monomeric amphiphiles. We also implement our theoretical framework with a Flory–Huggins type model of the Gibbs free energy and compare its predictions with experimental findings in the C12E6 and water micellar system.
Microwave anticrossing spectrum of ammonia
Authors: Ozier Irving , Meerts W. Leo
Published: July 2002

A microwave spectrometer designed to make precision Stark measurements at high electric fields has been used to study the inversion spectrum of ammonia. An avoided crossing has been measured between the lower member of the inversion doublet for (J=13, K=11) and the upper member of the (14,14) doublet. The two parameters that characterize the anticrossing are the zero-field splitting Δ0 between the levels and the distortion dipole moment μD that provides the (ΔJ=±1, ΔK=±3) mixing. For a number of other (J,K) values, a detailed study of the Stark effect has been carried out to determine the dipole distortion constants μJ and μK which appear in the effective dipole moment μQ(J,K) that provides the (ΔJ=0, ΔK=0) mixing between members of the same doublet: μQ=μ0 +μJJ(J+1) +μKK2. From a simultaneous fit to the anticrossing and conventional measurements, it has been determined that: Δ0=678.70(20) MHz; ‖μD‖ =124.1(7.0) μD; μJ =177.5(3.0) μD; and μK =−348.5(3.0) μD. The signs of μJ and μK are specified relative to that of μ0. The values of μD, μJ, and μK are discussed in the light of centrifugal distortion theory; the current measurements of μJ and μK are compared to the recent laser-Stark determinations of Ueda and Iwahori. By using the molecular beam results of Marshall and Muenter, values have been obtained for μ0 and for the static anisotropy in the polarizability.
On the evaluation of nonadiabatic coupling matrix elements for MCSCF/CI wave functions. IV. Second derivative terms using analytic gradient methods
Authors: Saxe Paul , Yarkony David R.
Published: July 2002

A recently proposed methodology for determining second derivative nonadiabatic coupling matrix elements h(J,I,Rα,R) ≡〈ΨJ(r;R)‖(∂2/∂R2α )ΨI(r;R)〉r based on analytic gradient methods is implemented and discussed. Here r denotes the electronic coordinates, R the nuclear coordinates, and the ΨJ (r;R) are eigenfunctions of the nonrelativistic Born–Oppenheimer Hamiltonian at the state averaged MCSCF/CI level. The region of a conical intersection of the 1,2 2A′ potential energy surfaces of the Li–H2 system is considered in order to illustrate the potential of this approach. The relation between h(J,I,Rα,R) and the first derivative matrix elements g(J,I,Rα,R) ≡〈ΨJ(r;R)‖(∂/∂Rα)ΨI (r;R)〉r is considered and the role of symmetry discussed. The h(J,I,Rα,R) are analyzed in terms of contributions from molecular orbital and CI coefficient derivatives and the importance of the various nuclear degree of freedom, Rα, is considered. It is concluded that for the case considered a flexible multiconfiguration wave function is desirable for characterizing h(J,I,Rα,R). This methodology complements recent advances in treating nonadiabatic processes for diatomic and triatomic systems starting with adiabatic states, including the work of Mead, Truhlar, and co-workers on conical (Jahn–Teller) intersections in X3 systems, by providing an essential computational step for the ab initio characterization the relevant electronic structure parameters.
Measurement and rate law analysis of D2 Q-branch line broadening coefficients for collisions with D2, He, Ar, H2, and CH4
Authors: Smyth Kermit C. , Rosasco Gregory J. , Hurst Wilbur S.
Published: July 2002

Continuous-wave stimulated Raman spectroscopy has been used to obtain high resolution vibrational Q-branch spectra at room temperature for pure D2 and D2:He, D2:H2, D2:Ar, and D2:CH4 mixtures. Measurements have been made for J=0–5 in the density region of 0.5–20.0 amagat, from which line broadening coefficients have been determined. These coefficients have been analyzed using a modified exponential energy gap rate law to identify the contributions of rotationally inelastic collisions and vibrational dephasing collisions to the linewidth. This analysis has assumed that vibrational dephasing is independent of rotational state, in accord with available theoretical studies. Results are compared with experimental and theoretical work on H2, HD, and D2, thereby characterizing the contributions of rotationally inelastic and vibrational dephasing collisions to the line broadening coefficients as a function of both rotational level and collision partner.
Inherent structure formalism for quantum systems
Authors: Stillinger Frank H.
Published: July 2002

A theoretical procedure in classical statistical mechanics, using configurational mapping to distinguish particle packing effects from vibrational smearing effects, has previously been developed for understanding short-range order in condensed phases. The present paper exhibits the generalization to quantum systems with arbitrary spin, but with spin-independent Hamiltonians. Remarks are included indicating how the popular path-integral Monte Carlo method for simulating quantal many-body systems via digital computer can be adapted to determine inherent packing structure. It is predicted that quantum effects in both liquid and solid phases tend to produce inherent structures that are more porous, defective, and higher in energy than their classical counterparts.
Two-dimensional exchange NMR of powder samples. II. The dynamic evolution of two-time distribution functions
Authors: Wefing S. , Kaufmann S. , Spiess H. W.
Published: July 2002

Theoretical as well as experimental examples concerning the evolution of the two-time distribution S2‖0 (ω1,ω2 ;tm) as a function of the mixing time tm are presented, where S2‖0 is identical with the two-dimensional (2D) absorption spectrum rendered by 2D exchange NMR spectroscopy of static powder samples. The model calculations comprise standard models like isotropic rotational diffusion or overall isotropic reorientation combined with discrete internal rotational jumps to simulate the chain dynamics of polymers. In any case, the 2D spectrum directly reflects the main aspects of the underlying motional mechanism. An axially symmetric coupling (η=0) between spin and lattice is assumed throughout. Thus, the angular information contained in a 2D spectrum is completely specified by a one-dimensional jump angle distribution supplied with each spectrum. In connection with the simulations the numerical mapping of a discrete distribution function into a space of new variables is discussed. In the experimental section 2H NMR spectra of chain deuterated linear polystyrene above its glass transition temperature are shown, which are compared with the model calculations.
Turing instabilities with nearly equal diffusion coefficients
Authors: Pearson John E. , Horsthemke Werner
Published: July 2002

We show that if a Turing instability occurs in a reaction–diffusion system with a nearly scalar diffusion matrix, then the parameters of the corresponding well-mixed system are necessarily such that the well-mixed system has at least two eigenvalues near zero. Conversely, if the corresponding well-mixed system is sufficiently close to a coalescence point of Hopf and saddle-node bifurcations (two eigenvalues are zero at such a point), and if the spatial domain is sufficiently large, then there exists a nearly scalar diffusion matrix such that a Turing instability occurs. These results imply that information on bifurcation loci from experiments in continuous-flow stirred tank reactor suffices to locate regions of parameter space where Turing instabilities are likely to occur; no knowledge of the reaction mechanism and rate constants is needed. In order to illustrate these results, we have analyzed a six-step model of the Belousov–Zhabotinskii reaction due to Showalter, Noyes, and Bar-Eli. In this model, the sixth step describes the reduction of the metal ion catalyst by the organic substrate. Depending on the rate constant of this step, we find that Turing instabilities can occur when the diffusion coefficient of either Ce4+ or of HOBr is very near, but slightly larger than that of Br−.
High orbital angular momentum states in H2 and D2. II. The 6h–5g and 6g–5f transitions
Authors: Jungen Ch. , Dabrowski I. , Herzberg G. , Kendall D. J. W.
Published: July 2002

A group of lines accompanying the first line of the Pfund series of the H atom has been observed by Fourier transform infrared spectrometry. The lines are due to transitions in molecular hydrogen of a nonpenetrating Rydberg electron possessing a high-orbital angular momentum, which is coupled only loosely to the vibrations and rotations of the H+2 core. Lines belonging to the 6h–5g and 6g–5f (v=0–3) transitions of H2 have been identified. The identifications are based on a calculation of the spectrum from first principles by multichannel quantum defect theory. The interaction between the nonpenetrating electron and the core was evaluated in terms of the permanent and induced molecular moments of H+2 as calculated by Bishop and collaborators. The analogous transitions in D2 have also been observed and assigned.
Photoacoustic measurement of differential broadening of the Λ doublets in NO(X 2Π1/2,v=2–0) by Ar
Authors: Pine A. S.
Published: July 2002

A differential broadening of the Λ doublets in the v=2–0 overtone band of the 2Π1/2 ground electronic state of NO in an Ar buffer gas has been observed by photoacoustic spectroscopy using a tunable color-center laser. The broadening coefficients for the f symmetry components are larger than for the e symmetry components by up to ∼6% for J∼16.5. This differential depends on J and vanishes at low J, implicating the anisotropy of the unpaired electron Π orbital in the plane of rotation. The 2Π3/2 transitions are slightly broader than the 2Π1/2 as a result of spin-flipping collisional relaxation. The observed line shapes also exhibit collisional or Dicke narrowing due to velocity-changing collisions.
Nuclear magnetic resonance chemical shifts using optimized geometries
Authors: Chesnut D. B. , Phung C. G.
Published: July 2002

Isotropic chemical shifts and shift anisotropies for carbon, nitrogen, oxygen, and fluorine in first-row-atom molecules have been calculated in the perturbed Hartree–Fock gauge including atomic orbital scheme for both experimental and optimized molecular structures using a 6-311G* basis for heavy atoms and a scaled 4-31G basis for hydrogen. Structure optimization leads to the expected shortening of bond lengths, which is accompanied by an increase in the isotropic chemical shifts. The increased shifts show much improved agreement with gas phase experimental values for nitrogen and oxygen, while the results for carbon are only mildly affected and remain good; shift anisotropies for all species tend to decrease in magnitude and also generally improve. Fluorine is anomalous, its increasing shifts upon structure optimization moving further away from experiment at this level of basis set. The trend in the optimized isotropic shifts is explained in terms of the general tendency for atoms in the right-hand portion of the first row of the periodic table to have negative shift derivatives with bond extension.
Calculated thermodynamic properties and phase transitions of solid N2 at temperatures 0≤T≤300 K and pressures 0≤P≤100 GPa
Authors: Belak J. , LeSar R. , Etters R. D.
Published: July 2002

Thermodynamic properties of solid nitrogen are calculated over a variety of isotherms and isobars using a constant pressure Monte Carlo method with deformable, periodic boundary conditions. Vibron frequencies are calculated using a simple perturbation theory. In addition, pressure–volume relations, thermal expansion coefficients, structures, and phase transition pressures and temperatures are determined. In particular, the nature of the orientational disorder in the plastic crystal phases is examined by calculating a variety of orientational order parameters.
Packing, correlation function, defect structures, and order parameters of liquid-crystal side-chain polymers by x-ray-diffraction investigations
Authors: Fan Z. X. , Buchner S. , Haase W. , Zachmann H. G.
Published: July 2002

X-ray investigations were carried out on well-oriented liquid-crystal side-chain polymers of two series: (1) a polyacrylate-methylacrylate copolymer series with polar –CN end group and (2) the polyacrylate or polymethylacrylate with –OCH3 end group. To explain the diffraction phenomena a new fan-shaped model is suggested. Using this model the packings in the SmA phase were discussed and the correlation function G(r) was determined from the inner reflections. The order parameters were calculated on the basis of the intensity simulation of the outer reflections.
Theoretical characterization of the potential energy surface for H+N2→HN2. II. Computed points to define a global potential
Authors: Walch Stephen P.
Published: July 2002

A previous calculation for H+N2 [Walch, Duchovic, and Rohlfing, J. Chem. Phys. 90, 3230 (1989)] focused on the minimum energy path (MEP) region of the potential energy surface and on estimates of the lifetime of the HN2 species. In this paper, we report energies computed at geometries selected to permit a global representation of the potential energy surface (PES). As in the previous work, the calculations were performed using the complete active space self-consistent field/externally contracted configuration interaction (CASSCF/CCI) method. The surface was characterized using the same basis set as in the previous paper except that an improved contraction of the H s basis is used. Calculations with a larger basis set were carried out along an approximate MEP obtained with the smaller basis set. The new PES exhibits a sharp curvature, which was not present in the previous calculations, and has a slightly narrower and smaller barrier to dissociation. Saddle points for H atom exchange via collinear and T-shaped HN2 complexes are also reported.
Potential energy surfaces for the Pt2+H2 reaction
Authors: Balasubramanian K.
Published: July 2002

Potential energy surfaces for the Pt2+H2 reaction are obtained using a complete active space multiconfiguration self-consistent field (CAS-MCSCF) method followed by multireference singles+double CI (MRSDCI) calculations. Several approaches of H2 such as parallel, perpendicular, collinear, end-on with respect to Pt2 are considered. In addition, out-of-plane twist motions of hydrogens relative to the Pt–Pt bond are considered. The parallel approach was found to be most reactive in the 1A1 electronic state, which forms a cis Pt2H2 saddle point after surmounting a barrier of ∼20 kcal/mol. The saddle point thus formed spontaneously transforms to a trans Pt2H2, 1Ag ground state through an out-of-plane twist motion. The dissociation of H2 in the parallel mode of collision was found to be brought about primarily through the interaction of the d(δ) orbitals of the two Pt atoms with the H2 1σg and 1σ*u orbitals. The spin–orbit effects were studied using a relativistic CI (RCI) method and found to be significant for Pt2H2. Spin–orbit coupling was found to induce an avoided crossing. This destabilizes the Pt2H2(1Ag) molecular state with respect to the dissociated Pt2+H2. The energy separation between the Pt2H2 1Ag trans minimum and the cis saddle point was calculated at the MRSDCI level as 3 kcal/mol. We find that the reactivity of Pt2 with H2 varies as a function of electronic state and orientation.
Photofragment spectroscopy of HF+
Authors: Cosby P. C. , Helm H. , Larzilliere M.
Published: July 2002

Electronic absorptions leading to predissociation of the HF+ ion are observed by coaxial laser-ion beam photofragment spectroscopy. The absorptions occur in the A 2Σ+←X 2Π system and terminate in the N=4, v=3 level of the A state which predissociates into H++F. The (3,4) band of this system is observed for the first time and the observations extend earlier emission measurements in the (3,0) and (3,2) bands to higher rotational levels. In addition, the nuclear hyperfine structure of the ion is observed. Extended molecular constants are derived for the X and A states and the first values are given for the hyperfine structure constants of the A state.
Is Au6 a circular ring?
Authors: Balasubramanian K. , Liao Dai-Wei
Published: July 2002

Complete active space multiconfiguration self-consistent field followed by higher-order configuration interaction calculations reveal that the ground state of Au6 is of 1A1 symmetry with a capped pentagonal structure. Similar calculations on Au−6 are also made. The 1A1g state of the neutral Au6 with a ring structure (D6h) is found to be 1.36 eV above the capped-pentagonal structure while the 3T2g state with an octahedral geometry is 1.5 eV above the capped pentagon. We conclude that Taylor et al.’s photodetachment spectra of Au−6 results from detachment of an electron from the 2E1u excited state of Au−6 with a ring structure to yield the 1A1g state of neutral Au6 with the same structure, although this is not the ground state of Au6.
Thermophysical properties of the lanthanide sesquisulfides. II. Schottky contributions and magnetic and electronic properties of γ-phase Pr2S3, Tb2S3, and Dy2S3
Authors: Gruber John B. , Burriel Ramón , Westrum Edgar F. , Plautz W. , Metz G. , Ma Xiao-Xia , Beaudry B. J. , Palmer P. E.
Published: July 2002

Heat-capacity measurements by adiabatic equilibrium calorimetry are reported for γ-phase Pr2S3, Tb2S3, and Dy2S3 between 5 and 350 K. Highly purified samples were prepared and their composition verified by chemical analysis. Precision lattice parameters were determined for each compound and are compared with literature values. The total heat capacity has been resolved into lattice, magnetic, and Schottky components by a volumetric approach. The experimental Schottky contributions accord with the calculated curves based on the crystal-field splitting of the 2S+1LJ ground state of the lanthanide ions occupying sites of S4 symmetry in the Th3P4 lattice. The individual crystal-field electronic energy levels have been obtained in part from an analysis of the hot-band data observed in the absorption spectra of Pr2S3, Tb2S3, and Dy2S3, and from a calculated splitting in which the crystal-field parameters Bkm, were determined from a lattice-sum calculation. Molar thermodynamic properties are reported for all three compounds. The entropy at 298.15 K {S0−S0 (7 K)}, is 22.78R, 22.93R, and 23.36R, for γ-phase Pr2S3, Tb2S3, and Dy2S3, respectively.
Rotationally resolved laser spectroscopy of the jet-cooled methylcyclopentadienyl radical (CH3–C5H4 and CD3–C5H4)
Authors: Yu Lian , Cullin David W. , Williamson James M. , Miller Terry A.
Published: July 2002

The rotationally resolved, electronic, excitation spectra of supersonically cooled methylcyclopentadienyl radicals, CH3–C5H4 and CD3–C5H4, have been obtained. Analysis of these spectra characterizes both the ground and electronically excited states involved in this transition. The ground state barrier to internal rotation of the methyl group is very high compared to similar closed-shell species. Additionally it shows a strong isotopic dependence and increases in magnitude further in the excited state. The doubly degenerate 2E″1 ground state of C5H5 is split into two components, X̃ 2B2 and Ã 2A2. The rotational analysis shows that the cyclopentadienyl ring undergoes a significant, static distortion from a regular pentagon. The measured distortion is consistent with and related to the dynamic Jahn–Teller distortion in C5H5 itself.
The microwave spectrum, spin-rotation coupling constants, and structure of thiocarbonyl fluoride, SCF2, observed with a cavity microwave Fourier transform spectrometer
Authors: Xu Yunjie , Gerry M. C. L. , Joo D. L. , Clouthier Dennis J.
Published: July 2002

The microwave spectrum of thiocarbonyl fluoride, SCF2, has been investigated in the frequency range of 6.7–18.4 GHz using a pulsed molecular-beam microwave Fourier transform spectrometer. Twelve a-type transitions of the 32S12C19F2 isotopomer with J between 0 and 9, including ΔKa=2 transitions, as well as eight transitions of the 34S12C19F2 isotopomer with J between 0 and 9, have been assigned. The rotational constants and quartic centrifugal distortion constants have been determined. A 19F nuclear spin-rotation interaction, as well as a rather large direct 19F nuclear spin–spin interaction, have been observed. The three diagonal elements of the spin-rotation tensor at the 19F nuclei have been determined to be Maa=22.9(9) kHz, Mbb=10.4(4) kHz, and Mcc=5.4(3) kHz for 32S12C19F2 and Maa=20.9(9) kHz, Mbb=10.7(4) kHz, Mcc=6.4(4) kHz for 34S12C19F2, respectively. For 32S13C19F2, seven transitions with J between 0 and 6 have been assigned and rotational constants and two quartic centrifugal distortion constants have been derived. The splitting patterns due to the two 19F nuclei and the 13C nucleus have been observed but not completely resolved. A ground-state average structure has been determined for 32S12C19F2 as rz(CS)=1.5870(20) Å, rz(CF)=1.3182(13) Å, and ∢(FCF)=106.89(11)°.
The thermal decomposition of CH3Cl using the Cl-atom absorption method and the bimolecular rate constant for O+CH3 (1609–2002 K) with a pyrolysis photolysis-shock tube technique
Authors: Lim K. P. , Michael J. V.
Published: July 2002

Rate constants for the thermal decomposition of CH3Cl and the atom–radical reaction O+CH3 have been measured in shock tube experiments. The decomposition of CH3Cl, at three loading pressures, has been carried out between 1663–2059 K with mixtures that varied from 1.25 to 24.9 ppm in Ar. The first-order rate constant k1=1.21×1010 ×exp(−27 838 K/T) s−1 describes the experimental results to within ±29% at the one standard deviation level. The bimolecular rate experiment has then been carried out over the temperature range 1609–2002 K using mixtures of SO2 (49.9 ppm) and CH3Cl (5.14 and 8.2 ppm) in Ar. The technique first involves allowing the thermal decomposition to proceed forming CH3 radicals, and this is then followed by delayed photolysis of SO2 forming the O-atom species. This new method is called the pyrolysis photolysis-shock tube (PyPh-ST) technique. A reaction mechanism had to be used to simulate the measured O-atom profiles for the various experimental conditions, and the bimolecular rate constant was found to be temperature independent with a value of k2=1.4×10−10 cm3 molecule−1 s−1. Reactions (1) and (2) are both theoretically discussed.
Experimental and theoretical studies of the reaction of atomic oxygen with silane
Authors: Ding Luying , Marshall Paul
Published: July 2002

The flash-photolysis resonance-fluorescence technique has been employed to measure the rate constant for O+SiH4→products from 295–565 K, and yielded k1=1.23×10−10 exp(−14.6 kJ mol−1/RT) cm3 s−1 with an accuracy of about ±15%. The transition state for direct H-atom abstraction has been characterized at up to the Gaussian-2 ab initio level of theory. With small adjustments it is possible to model kinetic data for O+SiH4 in terms of an abstraction channel leading to OH+SiH3. This agreement does not rule out minor participation by addition or insertion channels, but there is no theoretical evidence for bound triplet intermediates in the potential energy surface. A transition state theory analysis suggests that k1 at 1000 K is 16 times larger than previously thought.
Infrared diode laser spectroscopy of the ν3 fundamental and ν3+ν2−ν2 sequence bands of 13C3 and of the ν3 fundamental band of 12C 12C 13C
Authors: Moazzen-Ahmadi N. , McKellar A. R. W.
Published: July 2002

Gas-phase infrared spectra of 13C-substituted C3 molecules have been studied in the region of the ν3 vibration around 5 μm, using a tunable diode laser spectrometer and a hollow-cathode discharge in a flowing mixture of acetylene and helium. Three bands were observed and assigned: the ν3 fundamentals of 13C3 and of 12C 12C 13C, and the ν3+ν2−ν2 sequence band of 13C3 . The observed band origins are consistent with previous matrix-isolation spectroscopy of substituted C3 . The rotational and centrifugal distortion parameters are consistent with previous gas-phase work on 12C3 , and they also reflect the highly anharmonic nature of the bending vibration in this molecule. The ν3 band of 12C 12C 13C, measured here for the first time, provides a sensitive means to measure 13C abundances in astrophysical sources containing C3 .
Many-body theory of exchange effects in intermolecular interactions. Second-quantization approach and comparison with full configuration interaction results
Authors: Moszynski Robert , Jeziorski Bogumil/ , Szalewicz Krzysztof
Published: July 2002

Explicitly connected many-body perturbation expansion for the energy of the first-order exchange interaction between closed-shell atoms or molecules is derived. The influence of the intramonomer electron correlation is accounted for by a perturbation expansion in terms of the Mo/ller–Plesset fluctuation potentials WA and WB of the monomers or by a nonperturbative coupled-cluster type procedure. Detailed orbital expressions for the intramonomer correlation corrections of the first and second order in WA+WB are given. Our method leads to novel expressions for the exchange energies in which the exchange and hybrid integrals do not appear. These expressions, involving only the Coulomb and overlap integrals, are structurally similar to the standard many-body perturbation theory expressions for the polarization energies. Thus, the exchange corrections can be easily coded by suitably modifying the existing induction and dispersion energy codes. As a test of our method we have performed calculations of the first-order exchange energy for the He2, (H2)2, and He–H2 complexes. The results of the perturbative calculations are compared with the full configuration interaction data computed using the same basis sets. It is shown that the Mo/ller–Plesset expansion of the first-order exchange energy converges moderately fast, whereas the nonperturbative coupled-cluster type approximations reproduce the full configuration interaction results very accurately.
High resolution electronic spectroscopy of p-toluidine. A precessing rotor model for G12 molecules
Authors: Tan Xue-Qing , Pratt David W.
Published: July 2002

Based on a study of the high resolution S1←S0 fluorescence excitation spectrum of p-toluidine (p-methylaniline) and related G12 molecules, we propose that the threefold axis of the methyl group is tilted slightly with respect to the symmetry axis of the molecular frame, and exhibits a kind of precessional motion in the course of its hindered internal rotation. We derive a new Hamiltonian to describe this motion and show that it is consistent with previous modifications of the traditional torsion–rotation Hamiltonian first proposed by Wilson, Lin, and Lide [J. Chem. Phys. 23, 136 (1955)]. Applying the new Hamiltonian to the S1←S0 spectrum of p-toluidine, we have determined the sixfold barrier heights V6(S0) = ( − ) 5.6 and V6(S1) = ( − ) 43.9 cm−1, values that are similar to those of toluene and other 4-substituted toluenes.
Time-resolved frequency modulation spectroscopy of photochemical transients
Authors: Bloch Jonathan C. , Field Robert W. , Hall Gregory E. , Sears Trevor J.
Published: July 2002

We report the use of time-resolved frequency modulation (FM) spectroscopy for the measurement of photochemically generated radicals. CN radicals from the 193 nm photodissociation of cyanogen (NCCN) have been detected using a phase-modulated cw Ti:sapphire ring laser, probing single rotational lines of the A 2Π←X 2Σ system. The combination of sensitivity with time and frequency resolution is more than adequate to record Doppler-broadened line shapes of collisionless photofragments. Significant signal-to-noise enhancement is demonstrated compared to a dual-beam transient absorption technique.
Rate expressions for excitation transfer I. Radiationless transition theory perspective
Authors: Scholes Gregory D. , Ghiggino Kenneth P.
Published: July 2002

The ideas concerning preparation of an initial state and its subsequent temporal evolution are discussed with reference to interchromophore electronic excitation (energy) transfer (EET), particularly the ‘‘resonance’’ case. The cases of strong and weak coupling, and their consequences, are discussed. It is shown that when the interactions between two identical chromophores lie in the weak coupling regime, then a rate of excitation transfer may be defined on an experimental time scale and is given by a Fermi golden rule expression; the quasicontinuous final state being provided primarily by nonradiative line broadening. In general, such a rate expression applies so long as the donor–acceptor electronic coupling is less than the vibronic band width. The strong coupling limit is shown to result in excimer or exciplex-type emission. A rate equation formalizing donor–acceptor EET in general is determined by consideration of the Green’s function methods of scattering theory. The role of nuclear coordinates, including the possibility of Herzberg–Teller coupling, and the influence of nonradiative processes are examined.
Theory of vibrational relaxation of polyatomic molecules in liquids
Authors: Kenkre V. M. , Tokmakoff A. , Fayer M. D.
Published: July 2002

A simple tractable theory of vibrational relaxation of polyatomic molecules in polyatomic solvents, which is also applicable to solid solutions, is presented. The theory takes as its starting point Fermi’s golden rule, avoids additional assumptions such as the rotating wave or random phase approximations, and treats both the internal degrees of freedom of the relaxing molecule and the bath degrees of freedom in a fully quantum mechanical manner. The results yield intuitively understandable expressions for the relaxation rates. The treatment of the annihilation as well as the creation of all participating bosons allows the theory to go beyond earlier analyses which treated only cascade processes. New predicted features include temperature effects and asymmetry effects in the frequency dependence. The theory is constructed in a manner which facilitates the use of recent developments in the analysis of instantaneous normal modes of liquids.
The structure of a rotational isomeric state alkane melt near a hard wall
Authors: Sen Sudeepto , Cohen Jennifer M. , McCoy John D. , Curro John G.
Published: July 2002

Polyatomic density functional theory was used to model tridecane chains near a hard wall under melt conditions. Polymer reference interaction site model (PRISM) liquid state theory provided the bulk structure input for the density functional. The density profile, the fractional distribution of sites, and the variation of the end-to-end separation of the chains as a function of distance from wall contact were calculated, and excellent agreement with the results of full multichain simulation was found.
The structure of the CF−4 anion and the electron affinity of the CF4 molecule
Authors: Gutsev Gennady L. , Adamowicz Ludwik
Published: July 2002

The electronic and geometrical structure of the CF−4 anion and its neutral parent, CF4, are calculated with the second-order Moller–Plesset perturbation theory. Several diffuse sp shells were added to the standard 6-31+G* basis when calculating the potential energy surface of the CF4+e− system. It was found that the CF4 molecule does not attach an additional electron in the ground state, i.e., the molecule possesses a zero vertical electron affinity under the Born–Oppenheimer approximation. The optimized C3v and C2v configurations of the anion are transition states, whereas its Cs configuration corresponds to a local minimum and is thermodynamically stable by 20 kcal/mol. The CF4 molecule has the negative adiabatic electron affinity of −1.22 eV with respect to this configuration of the anion.
Electron paramagnetic resonance lineshape analysis of the photoexcited triplet state of C60 in frozen solution. Exchange narrowing and dynamic Jahn–Teller effect
Authors: Bennati Marina , Grupp Arthur , Mehring Michael
Published: July 2002

The EPR lineshape of the photoinduced triplet state of C60 in frozen toluene solution was studied by pulsed EPR spectroscopy. Lineshape calculations of the triplet spectra were performed including dynamical exchange effects. The observed spectra in a glassy matrix are compatible with zero field splitting parameters ‖D1‖ = 0.0114 cm−1 (12.2 mT) and ‖E1‖ = 0.0005 cm−1 (0.5 mT). The temperature dependence of the powder lineshape was simulated using a dynamical exchange model, where the triplet principal axis jumps between all equivalent sites allowed by the D5d symmetry for the lowest excited triplet state. The determined exchange rate turned out to be only weakly temperature dependent and suggests that the dynamic process is due to tunneling between Jahn–Teller distorted states rather than to real molecular jumps. In addition we have observed a different triplet state with zero field splitting parameters ‖D2‖ = 0.0100 cm−1 (10.6 mT), ‖E2‖ = 0.0015 cm−1 (1.6 mT) after annealing of the matrix. We attribute this to a C60 dimer or, alternatively, to crystal field effects.
Electronic spectroscopy of the Mg+–N2 complex: Evidence for photoinduced activation of N2
Authors: Robbins D. L. , Brock L. R. , Pilgrim J. S. , Duncan M. A.
Published: July 2002

The ion–molecule complex, Mg+–N2 is formed in a supersonic expansion and studied with mass-selected photodissociation spectroscopy. The lowest energy bands observed in the electronic excitation spectrum are redshifted more than 12 000 cm−1 from the Mg+ (2P←2S) atomic transition at 280 nm. The red-shift, resulting from differential bonding in the ground and excited states of the complex, is much larger than the shifts observed in previously studied Mg+–ligand complexes. Resolved vibronic structure is observed extending for more than 5000 cm−1. The observation of spin–orbit multiplet structure indicates that the complex is linear and that the electronic transition is 2Π←X 2Σ+. The spin–orbit splitting of 46 cm−1 is significantly less than that observed for other Mg+–L complexes. Vibronic intervals of about 1000 and 500 cm−1 are assigned respectively to a stretching mode and to double quanta in a bending mode. The study of isotopically substituted complexes indicates that the best assignment for the stretch progression is the N–N mode, with a frequency far below that in free N2. The vibrational activity, frequency shift, and spin–orbit splitting are all signatures for N2 activation by the excited metal ion. However, the degree of this interaction is greater than that predicted by ab initio calculations.
Pseudospectral multireference single and double excitation configuration interaction
Authors: Martinez Todd J. , Carter Emily A.
Published: July 2002

We present a pseudospectral formulation of the multireference single and double excitation configuration interaction method. A formal scaling advantage is achieved and practical timings are presented. The accuracy of the pseudospectral approximation within this method is probed for a variety of test cases. The method is typically accurate to within 1 mhartree while being up to six times faster than conventional codes.
Nuclear coherence-transfer echoes in pulsed EPR
Authors: Ponti Alessandro , Schweiger Arthur
Published: July 2002

Nuclear coherence-transfer echoes created in disordered electron–nuclear spin systems by microwave pulses are described. Once nuclear coherence is generated, a nonselective microwave π pulse inverts the electron spins thereby interchanging nuclear coherence between the two electron spin manifolds. This exchange refocuses the inhomogeneity of the secular part of the hyperfine interaction. The mechanisms of echo formation for weak and strong hyperfine couplings are explained by using simple vector diagrams. Various methods to create and detect nuclear coherence by microwave pulses are discussed with special attention directed to the sequence π/2–τ–π/2–t1–π–t2–π/2–τ. In this four-pulse experiment the nuclear coherence-transfer echo is observed as an amplitude modulation of the electron spin echo intensity. The nuclear coherence-transfer echo can be shifted to a time interval fully separated from the one covered by the instrumental deadtime. The time evolution of an S=1/2, I=1/2 model spin system during the four-pulse sequence is described using the density operator formalism. Several one- and two-dimensional four-pulse experiments are discussed on this basis and their spectra are analyzed in terms of nuclear coherence-transfer pathways. A new one-dimensional experiment based on nuclear coherence-transfer echoes, deadtime free ESEEM by nuclear coherence-transfer echoes (DEFENCE) is proposed to exploit undistorted electron spin echo envelope modulation (ESEEM) spectra. DEFENCE is a fast pulsed EPR technique that is unpretentious with respect to the required microwave equipment. The predicted features of the nuclear coherence-transfer echoes and of the DEFENCE approach are verified experimentally.
Symmetry breaking and localization in resonant photon emission
Authors: Cederbaum L. S.
Published: July 2002

The photon emission following the excitation to an intermediate decaying state which interacts with other electronic states is discussed in some detail. Particular attention is paid to resonant x-ray emission in symmetric polyatomic systems. The decaying core-excited state always possesses partner states to which it couples through the nontotally symmetric nuclear motion. The resulting dynamical symmetry breaking induces localization of the core holes and may have considerable impact on the intensity and selection rules. The effect of vibronic coupling, lifetime of the decaying state and of temperature are analyzed and interpreted.
Characterization of the minimum energy paths for the reactions of CH(X 2Π) and 1CH2 with C2H2
Authors: Walch Stephen P.
Published: July 2002

The reactions of CH(2Π) and singlet methylene (1CH2) with acetylene lead to intermediates which may be important in soot formation. CH(2Π)+acetylene leads to CHCHCH (C3H3), CHCCH, CH2CC, and propargyl (CH2CCH). 1CH2+acetylene leads to cyclopropene, allene, and propyne. Except for CHCCH, which involves a barrier of 7.5 kcal/mol with respect to reactants, all of these reaction products are formed with no barrier. Miller and Melius have previously discussed the dimerization of propargyl to give benzene. C3H3 and CHCCH are shown to dimerize with no barrier to give benzene and para-benzyne, respectively. C3H3, CHCCH, and CH2CC can also add to smaller polycyclic aromatic hydrocarbons (PAH), and may be important species in forming larger PAH or fullerenes.
Fast process of amorphous polystyrene below and above the glass transition temperature T
g as studied by quasielastic neutron scattering
Authors: Kanaya T. , Kawaguchi T. , Kaji K.
Published: July 2002

The dynamics of atactic polystyrene (PS) has been studied by a quasielastic neutron scattering technique in a temperature range of 21–475 K covering the glass transition temperature Tg (=373 K). The so-called fast process in picosecond order is observed similarly to other glass-forming materials in supercooled states so far reported. However, the onset temperature of the fast process is ∼170 K below Tg in PS, which is far below Tg in contrast to the previous observations. This result implies that the fast process is not necessarily the β process predicted by the recently noted mode coupling theory. By comparing the result of hydrogenated PS with that of partially deuterated polystyrene (PS-d5) where only phenyl rings were preferentially deuterated, the origin of the fast process occurring far below Tg has been assigned to librational motion of phenyl rings, which is coupled with main chain motion near Tg. Analysis of the spectra by curve fit has suggested that the nature of the fast process changes at a temperature ∼30 K above Tg. This change also involves the appearance of a slow relaxation process in the energy range of several tens μeV. The origin of the slow process is discussed in terms of conformational transitions of the main chain.