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Volume 102, Issue 3, 15 January 1995

Nonadiabatic dynamical studies of the rotational Raman spectrum of H_{2} in water
View Description Hide DescriptionThe rotational Raman spectrum of H_{2} in liquid H_{2}O and D_{2}O have been calculated using our nonadiabatic time correlation function results presented in earlier work [L. Xiao and D. F. Coker, J. Chem. Phys. 100, 8646 (1994)]. The rotational potential experienced by the H_{2} molecule in this solvent environment is so anisotropic that classical solventfluctuations not only drive transitions between orientational basis states, but mixing of states from different total angular momentum levels is also considerable. Our nonadiabatic calculations are able to quantitatively reproduce the experimental rotational Raman line shapes and their trends with solvent isotopic substitution. We demonstrate that nonadiabatic transitions between adiabatic rotor states play a key role in smoothing out artificial structures predicted in the static and adiabatic limit spectra.

Donor–acceptor interchange tunneling in HDO–DOH and the higher energy HDO–HOD isotopomer
View Description Hide DescriptionThe microwave and submillimeter spectra of the a‐type K=0←0 and K=1←1, c‐type K=1←0, and isotopically allowed b‐type K=1←0 bands of the O–D bonded HDO–DOH water dimer isotopomer and the higher energy O–H bonded HDO–HOD isotopomer have been measured using molecular‐beam electric resonance optothermal and pulsed‐nozzle Fourier‐transform microwave spectrometers. The present results obtained in He and He/Ne seeded molecular beams give the first evidence for the presence of the higher energy O–H bonded mixed protonated‐deuterated water dimers. These species were not reported previously in studies using seeded Ar molecular beams. The donor–acceptor interchange tunneling splittings are found to be 1322.1019(43) and 5004.059(20) MHz for the HDO–DOH and the metastable HDO–HOD dimers, respectively. For both isotopomers, the donor‐accepter interchange tunneling‐state selections rules for the b‐ and c‐type bands are consistent with tunneling pathways corresponding to geared partial internal rotation of the two subunits in double‐minima potentials. The larger tunneling splitting in HDO–HOD is primarily the consequence of the smaller effective reduced mass for tunneling in this system compared to that in HDO–DOH. The presence of both b‐ and c‐type K=1←0 bands allows the direct measurement of the largest tunneling splitting, that associated with the internal rotation about the O–H–O or O–D–O bond of the nonbonded proton/deuteron on the proton donating unit. For the K=0 state of HDO–DOH this splitting is 214 208.38(23) MHz, while for the K=0 state of HDO–HOD it is 117 440.97(17) MHz. A strong b‐type Coriolis interaction is observed between the upper K=0 and lower K=1 states in HDO–DOH, similar to that observed previously in (H_{2}O)_{2}.

Ammonia rotations in ytterbiumhexaammine
View Description Hide DescriptionThe results of inelastic neutron scattering experiments on ytterbiumhexaammine are presented. At temperatures below 35 K, the dominant dynamical process is a uniaxial rotation of the ammonia in hindering potentials of about 35 meV. At higher temperatures, another type of ammonia motion is observed, which is described as reorientations of entire Yb(NH_{3})_{ x } complexes (x≊6).

Interatomic potentials of singlet s‐Rydberg series of a HgNe van der Waals dimer: Evidence for stabilization by superexchange interaction
View Description Hide DescriptionThe interatomic potentials of the singletRydberg series of the HgNe dimer ^{1}Σ^{+}Hg(n ^{1} S _{0})Ne (n=7–9) were determined over a wide range of interatomic distance by the analysis of the optical–optical double resonance (OODR) spectra measured in the present study via the A ^{3}Π_{0+ } and B ^{3}Π_{1} states. The interatomic potential for n=7 consists of one bound vibrational level (v=0), three quasibound levels (v=1–3) trapped inside a potential barrier, and one weakly trapped quasibound level just above the potential barrier. The dissociation energy (D _{ e }) of this potential is 50(3) cm^{−1} and the large potential barrier with a height of 133(2) cm^{−1} is located at 4.20(3) Å, while the interatomic potentials for n=8 and n=9 consist of a deep bound well, whose dissociation energies (D _{ e }) are 240(3) and 297(3) cm^{−1}, respectively. From these potentials, it was shown that the principal quantum number dependence (n) of the interatomic potential originates mainly from that of the exchange repulsion between the Rydberg electron of Hg and the attached Ne atom in a similar manner as the tripletRydberg series of HgNe, ^{3}Σ^{+}Hg(n ^{3} S _{1})Ne (n=7–10) [K. Onda et al. J. Chem. Phys. 101, 7290 (1994)]. By comparing the potentials of the singlet with the tripletRydberg series, it was found that an interatomic potential of the singlet state is always deeper than that of the triplet state for the same n. This difference between singlet and triplet was interpreted by a superexchange interaction model [P. N. Anderson, Phys. Rev. 79, 350 (1950)], in which a singlet Rydberg state becomes more stable due to small spin density on the Ne atom induced by the kinetic exchange interaction between the Hg^{+} ion and the Ne atom.

Spectroscopy of benzene dimers in the algebraic model
View Description Hide DescriptionWe study the spectroscopy of benzene dimers within the algebraic model. We report results of calculations of the infrared and Raman spectrum in the region of the CH stretching fundamentals ν_{7}, ν_{13}, ν_{20} and ν_{2}. On the basis of these calculations, we suggest that the precise geometric form of the dimer can be inferred from detailed vibrational spectroscopy.

Self‐collisional coupling and broadening in the asymmetric rotor CHF_{2}Cl
View Description Hide DescriptionThe line shape of 15 selected CHF_{2}Cl rotational transitions is studied in the 59–1049 GHz frequency region. Collisional broadening parameters are measured and compared to semiclassical calculations. For some pairs of lines a large collisional coupling effect is observed and analyzed. A semiclassical theoretical model is presented allowing calculations of the coupling parameters in good agreement with measurements.

High‐pressure Raman study of liquid and crystalline CH_{3}F up to 12 GPa
View Description Hide DescriptionHigh‐pressure Raman spectra of liquid and crystalline CH_{3}F were measured up to 12 GPa at 300 K in a gasketed diamond‐anvil cell. Two solid phases have been found; the transition pressures of liquid to solid phase I and phase I to phase II were determined to be 2.75 and 3.63 GPa, respectively. Among these, the solid phase I is an orientationally disordered (plastic) phase, while the solid phases II is an orientationally ordered phase. The frequency of CF stretching ν_{3}(A _{1}) vibration shows a large red shift with a slope dν/dP of about −2.6 cm^{−1}/GPa in the liquid phase, and it splits into the TO and LO modes in the two solid phases. The Fermi resonances between the same symmetry vibrations of ν_{1}(A _{1}) and 2ν_{5}(A _{1}), and of ν_{4}(E) and 2ν_{5}(E) have been observed and their behaviors have been analyzed by the Fermi resonance theory. The pressure dependence of the CH stretching mode is compared with those of the other fluorinated methanes CH_{2}F_{2}, CHF_{2}Cl, and CHF_{3}.

The electronic structure of Cr^{2+,4+} in fluoride host materials
View Description Hide DescriptionThe electronic absorption spectra of Cr^{4+} in Rb_{2}CrF_{6} and Cr^{2+} in KCrF_{3} have been studied using ab initio molecular orbital methods. Near‐degeneracy effects within the d ^{ n } (n=2,4) manifold are treated using the complete active space (CAS) self‐consistent field method, while dynamical correlation is dealt with using both the averaged coupled‐pair functional (ACPF) method or second‐order perturbation theory (CASPT2). The ground stateelectronic structure of the CrF^{ n−} _{6} (n=2,3,4) clusters in the ionic crystals Rb_{2}CrF_{6}, K_{2}NaCrF_{6}, KCrF_{3} was analyzed first using Bader’s theory of atoms in molecules. The topological analysis of the charge density is consistent with an ionic picture of the chemical bond in all three compounds, although the contribution of covalent effects clearly increases in the series CrF^{4−} _{6}<CrF^{3−} _{6}<CrF^{2−} _{6}. The ligand field strength exerted by the first coordination sphere of fluorine ligands increases within the same series. The present results indicate that the ligand field strength in the d ^{2} system CrF^{2−} _{6} has always been underestimated in the past. Indeed, our calculations do not confirm the earlier assignment of a band at 21 000 cm^{−1} as the 10Dq transition. Both the ligand field strength and the Jahn–Teller splitting of CrF^{4−} _{6} in KCrF_{3} are reasonably well reproduced by the calculations. The composition of the excited triplet states was used as a guideline for the analysis of the complex quintet–triplet spectrum of this system.

Concentration dependence of the vibrational band shape and frequency in binary mixtures
View Description Hide DescriptionWe propose a stochastic model which describes vibrational broadening of Raman lines in binary mixtures. The line broadening by a dynamic environment is included by considering two‐particle exchange processes in the nearest neighbor shell of the reference molecule. Different environment states influence the vibration of the reference molecule. A finite interaction time between the molecules leads to finite correlation times in the vibrational relaxation. As a result the symmetric band shapes in the neat liquids deviate from Lorentzian profiles. Experimentally this is observed for the symmetric C–H stretching vibration in neat CH_{2}I_{2}. The model represents an extension of the model proposed by Knapp and Fischer, which reproduces Lorentzian shapes in the neat liquids. A quantitative comparison of both models with experimental data for the C–H stretching vibration in CH_{2}I_{2}/CCl_{4} mixtures is performed at several mole fractions.

Rotational spectra and van der Waals potentials of Ne–Ar
View Description Hide DescriptionThe high sensitivity and resolution of Fourier‐transform microwave spectroscopy using a pulsed jet coaxial to a Fabry–Perot resonator have been exploited to measure pure rotational transitions of several isotopomers of the weakly polar Ne–Ar van der Waals dimer in natural abundance. Transitions of the most abundant isotopomer, ^{20}Ne–^{40}Ar, could be observed with an excellent signal‐to‐noise ratio with a single polarization pulse. The ground‐state rotational constants for this species yield a zero‐point separation of R _{0}≂360.7 pm. Simple model van der Waals potentials have been fit to the microwave transitions for the various isotopomers, providing estimates of the equilibrium spacing at the well minimum of R _{ e }=348.0(2) pm. More elaborate potentials based on ab initio calculations or on molecular‐beam scattering cross sections and thermodynamic and transport properties have also been tested. The induced electric dipole moment is estimated to be μ_{0}=7.3(1.6)×10^{−33} C m [0.0022(5) D] by comparison of π/2 polarization pulses with a reference molecule (Ar–CO_{2}) whose dipole moment is known from Stark effect splitting measurements. Uncertainties in parentheses are one standard deviation.

Vibrational mode and collision energy effects on a highly constrained reaction: OCS^{+}(ν)+OCS→CS^{+} _{2}+CO_{2} and S^{+} _{2}+2 CO
View Description Hide DescriptionWe report the effects of collision energy and OCS^{+}vibrational state (ν_{1}, ν_{2}, and ν_{3}) on the reaction of OCS^{+} with OCS. Production of CS^{+} _{2}+CO_{2} is exoergic and the cross section shows no evidence of an activation barrier. Nonetheless, the cross section is only ∼0.1% of the collision cross section, even at low collision energies where formation of an intermediate complex is facile. There appears to be a severe phase‐space (steric) bottleneck for this rearrangement reaction. CS^{+} _{2} production is weakly inhibited by collision energy, and enhanced by all three modes of OCS^{+} reactant vibrational excitation. Production of S^{+} _{2} is endoergic and is enhanced by collision energy and by ν_{2} (bend) and ν_{3} (CS stretch) excitation. Excitation of ν_{1} (CO stretch) does not enhance this channel, even though it is the highest energy mode. At high collision energies, S^{+} _{2} production becomes relatively efficient, suggesting that the reaction mechanism for this channel is direct with no significant bottleneck.

Vibrational energy transfer between isotopes of CO and isotopes of CO_{2} in the gas phase and in liquid Kr solution
View Description Hide DescriptionRate constants are presented for (VV) energy transfer between CO(ν=1) and CO_{2}(00^{0}1) in the gas phase down to 115 K and in liquid Kr solution at 118 and 130 K. Four isotopically substituted systems were investigated for which the energy mismatches varied between 104 and 306 cm^{−1}. The gas and liquid phase data show several systematic effects with changing energy mismatch. In particular it was found that the ratio of the liquid and gas phase rate constants at the same temperature, k _{ L }/k _{ G }, increased with decreasing energy mismatch. This is not predicted by current theories of liquid phase energy transfer and is in contrast to previous work using liquid Kr as the solvent.

Vibrational mode effects, scattering dynamics, and energy disposal in reaction of C_{2}H^{+} _{2} with methane
View Description Hide DescriptionThe effects of collision energy and mode‐selective vibrational excitation on the reaction of C_{2}H^{+} _{2} with CH_{4} and CD_{4} have been measured, along with the corresponding product velocity distributions. Two distinct reaction mechanisms are active in the energy range below 5 eV. At low energies, a long‐lived C_{3}H^{+} _{6} complex forms efficiently, then decomposes primarily to C_{3}H^{+} _{5}+H and C_{3}H^{+} _{4}+H_{2}. The RRKM lifetime of this complex is estimated to range between ∼10 ns and ∼10 ps over the experimental energy range, and this is sufficient time to allow substantial H‐atom scrambling. Complex formation is strongly inhibited by collision energy, weakly inhibited by CC stretching, and enhanced by bending excitation. Competing with the complex‐mediated mechanism is a direct H‐atom abstraction reaction, producing C_{2}H^{+} _{3}+CH_{3} with little atom scrambling. This reaction is shown to have a ∼150 meV activation barrier and is strongly enhanced by collision energy, becoming the dominant channel above 0.4 eV. CC stretching provides a weaker enhancement than collision energy, while bending enhances the reaction ∼10 times more efficiently. As collision energies increase, the C_{2}H^{+} _{3} product is increasingly forward scattered with an increasing fraction of the available energy going into recoil. Energy put into reactant vibration mostly is retained as internal energy of the products. Over the collision energy range from 0.4 to 2.8 eV, the collision time in the direct reaction varies from ≥1.3 ps to ≤70 fs.

Kinetic study of the 308 and 345 nm emissions of the molecule XeCl
View Description Hide DescriptionThe light pulses of two emissions of the molecule XeCl, centered at 308 and 345 nm, both have decay time constants that behave in a very similar way with respect to variations in the partial pressures of xenon and HCl; this is explained by collisional coupling between the states B and C of XeCl which are at the origin of the two emissions. The identification of the different collision processes and the determination of the reaction rate constants particularly underline the occurrence of a three‐body collision process between XeCl (B,C), Xe, and HCl; the rate constant is k _{ M }=(5.85±2.70)×10^{−5} Torr^{−2} ns^{−1}. Measurements were also made of the coupling constants B→C and C→B which were found to be k _{ BC }=(2.56±1.05)×10^{−4} Torr^{−1} ns^{−1} and k _{ CB }=(1.45±1.05)×10^{−4} Torr^{−1} ns^{−1}. The deduced energy gap between the two states (E _{ B }−E _{ C }=119 cm^{−1}) is in very good agreement with the values in the literature.

Control of long time dependence of CO binding to heme systems by inhomogeneous spread in electronic prefactor
View Description Hide DescriptionRecent work has emphasized that the rebinding kinetics of ligands to heme systems can exhibit nonexponential kinetics below the glass transition due to inhomogeneous spreads in factors other than enthalpy. We show that an expected distribution in the nonadiabatic electronic prefactor will lead to a t ^{−1/2} dependence for the rebinding rate at long times. This will occur at temperatures below the glass transition but for T≳1/2αR, where α characterizes the enthalpy spread and R is the gas constant. We show that this spread in prefactor explains the long time behavior in the appropriate temperature range for Mb–CO and Mb3–CO.

Calculation of the differential two‐photon ionization cross section of H_{2}S
View Description Hide DescriptionDifferential cross section of nonresonant two‐photon ionization of the outer valence shell of H_{2}S is computed using ab initio methods for several choices of photonpolarization and experimental setup. The theoretical model employed is based on the second‐order perturbation theory in the dipole and vertical transition approximations and the calculations are performed adopting a large basis set of L ^{2} integrable functions. The excitations to intermediate states are described in the random phase approximation, while the final states are described in the static‐exchange approximation adopting for the continuum orbitals a K‐matrix technique that allows a proper description of the angular distribution of the emitted electrons.

A detailed three‐dimensional quantum study of the Li+FH reaction
View Description Hide DescriptionAccurate quantum reactive scattering calculations in the full three‐dimensional physical space have been carried out for the Li+FH reaction at zero total angular momentum using the adiabatically adjusting principal axis of inertia hyperspherical coordinate formalism. The procedures for fitting the potential energy surface, calculating the surface functions, and propagating the solutions in a coupled channel treatment are given and discussed. Features of the resulting reactive probability plots are analyzed, and simple explanations of a number of the quantum resonance and oscillatory features are found.

Orbital‐based direct inversion in the iterative subspace for the generalized valence bond method
View Description Hide DescriptionWe present an algorithm that is a new combination of the direct inversion in the iterative subspace (DIIS) and the generalized valence bond (GVB) methods. The proposed algorithm is based on applying the DIIS directly to the orbitals updated via the GVB scheme as opposed to the conventional approach of applying DIIS to a series of composite Fock matrices (CFMs). The new method results in GVB convergence in situations where the CFM‐based GVB‐DIIS cannot be applied at all, e.g., when the original GVB method diverges. When both the new and the conventional methods converge, the former achieves the same reduction in the number of self‐consistent field (SCF) iterations as the latter, but using considerably less storage and DIIS‐related CPU time. Also, the orbital‐based GVB‐DIIS is less sensitive to the proximity of an initial guess to the exact wave function, and it does not depend on empirical criteria used in the CFM‐based GVB‐DIIS. Finally, the orbital‐based DIIS formulation is not limited to GVB; it can be easily incorporated into any SCF approach that involves an iterative updating of the orbitals, such as, e.g., multiconfiguration SCF or Kohn–Sham density‐functional theory.

Three‐dimensional Cartesian finite element method for the time dependent Schrödinger equation of molecules in laser fields
View Description Hide DescriptionA finite element(FE) method in three‐dimensional Cartesian coordinates is described to solve the time dependent Schrödinger equation for H^{+} _{2}, H_{2}, and H^{+} _{3} in the presence of time dependent electromagnetic fields. The ionization rates and harmonic generationspectra have been calculated for these molecules for field directions parallel or perpendicular to the molecular axis. Nonlinear optical susceptibilities of H^{+} _{2} have been also obtained for different laser field directions. The time dependent Hartree–Fock results are compared to frozen core calculations for H_{2}. Comparisons of present FE numerical results with previously published calculations show the FE method reproduces perturbative results and can also treat nonperturbatively the effect of intense short laser pulses as the method includes both bound and continuum electronic states.

Chemical bonding in water clusters
View Description Hide DescriptionDifferent sizes of water clusters from a dimer to twenty water molecules are studied using density functional theory. The binding energies of water clusters are calculated, and a relationship in terms of a simple function has been found between binding energy and the size of the water clusters. The interpolation of this correlation function reproduces the binding energies for the other water clusters to an accuracy within 1 kcal/mol. The extrapolation of the function gives the binding energy, −11.38 kcal/mol, which agrees very well with the experimental binding energy of ice, −11.35 kcal/mol. We also find small water clusters composed of mainly planar four membered rings to be more stable, implying the existence of magic numbers for water clusters with sizes of 4, 8, and 12.