Volume 117, Issue 11, 15 September 2002
 COMMUNICATIONS


Predicting water’s phase diagram and liquidstate anomalies
View Description Hide DescriptionWater expands upon freezing, has minima in its volume, heat capacity, and isothermal compressibility with temperature, and shows signs of a firstorder phase transition when supercooled. We present an analytical molecular theory that can account for these behaviors. It suggests that local network formation and hydrogenbonding cooperativity between triplets of neighboring molecules are keys to understanding water’s thermodynamics.

Stability of a chemically bound helium compound in highpressure solid helium
View Description Hide DescriptionHHeF, a chemicallybound helium compound, has been predicted to be metastable in the gas phase. It decays by tunneling through energy barriers in picosecond timescales into He+HF and H+He+F. This paper studies the stability of HHeF in pressurized solid helium. Using realistic potentials for the HHeF/He interaction, the potential energy along the minimum energy paths for decomposition is evaluated, and tunneling decay times are computed by the WKB approximation. It is found that for pressures above 500 MPa, decomposition into H+He+F is completely suppressed. At 23 GPa, the highest pressure studied, the timescale for HHeF→He+HF is in the millisecond range. At pressures well above 23 GPa, HHeF is thus expected to remain stable indefinitely.

Optical and transport properties of an alkalidoped methanofullerene
View Description Hide DescriptionWe examine the effects of monoderivatization on the electronic properties of For this we chose the acid cholesteryl ester, [6,6]PCOCr, whose nonlinear optical properties have been investigated in the past. While the optical absorptionspectrum of this methano fullerene is similar to that of substantial differences are observed upon doping with potassium. Similarly, the dopingdependent conductivity of the functionalized fullerene shows two maxima as opposed to the single maximum for The experimental observations are consistent with the dopinginduced degeneracy removal of the parent LUMO orbital, which in potassiumdoped methanofullerene splits into two components separated by about 0.5 eV. We provide experimental evidence that the doping of [6,6]PCOCr proceeds, as in with six consecutive reduction (electron transfer) steps, yielding stoichiometry at the end. The transport in partially doped [6,6]PCOCr thin films occurs by thermally activated hopping of the charge carriers with activation energy and hopping probability proportional to the number of unpaired electrons in the reduced molecule.
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 ARTICLES

 Theoretical Methods and Algorithms

Electrons weakly bound to hydrogen bonded clusters: A pseudopotential model including dispersion interactions
View Description Hide DescriptionA pseudopotential model for the description of binding of an excess electron to polar clusters or molecules is presented. In addition to Coulomb, short range repulsion, and polarization interactions between the excess electron and the neutral core, the model also accounts for dispersion within a second order perturbation treatment. The pseudopotential, which should enable future dynamical calculations coupling the excess electron with nuclear motions, is successfully tested against accurate ab initio results for a whole set of geometries of hydrogen fluoride dimer anion. Predictions are made for an electron bound to a collinear hydrogen fluoride trimer for different values of the intermonomer separations. For the optimal and shorter values of this separation two bound states of the excess electron in are predicted to exist.

Symmetryforcing procedure and convergence behavior of perturbation expansions for molecular interaction energies
View Description Hide DescriptionSymmetryadapted perturbation theory (SAPT) expansions corresponding to several symmetryforcing procedures are applied through large order to study the interaction of lithium and hydrogen atoms. The interactionenergies predicted by the perturbation theory are compared with the results obtained using the full configuration interaction (FCI) method. Since the ground state of the LiH molecule is submerged in the continuum of Pauliforbidden states, these calculations are a demanding test for the SAPT approach in which the electrons from different monomers are treated as distinguishable particles. We show that if the symmetry is forced in a rather weak way, characteristic of the Murrell–Shaw–Musher–Amos theory, a divergent perturbation series is obtained. When the symmetry is forced in a strong way, as is done in the Eisenschitz–London–Hirschfelder–van der Avoird theory, one obtains a convergent series, but the interactionenergy computed through any finite order exhibits wrong asymptotic behavior at large interatomic distancesR. We show that by forcing the symmetry in an appropriate, intermediate way one obtains perturbation series which correctly predict leading terms in the asymptotic expansion of the interactionenergy and, despite the presence of the Pauliforbidden continuum, converge quickly to the FCI value of the interactionenergy.

Effective procedure for energy optimizing antisymmetrized geminal power states
View Description Hide DescriptionA procedure for energy optimizing an antisymmetrized geminal power state, which contains a description of correlation effects in an unbiased fashion, is presented. This procedure overcomes difficulties and shortcomings of past optimization procedures for antisymmetrized geminal power states. It is shown that the computational cost scales as where r is the size of the spin orbital atomic basis set, which is superior to the scaling costs of multiexcited complete active space selfconsistent field calculations, but unlike such calculations their is no preselection of configurations or excitation level. The variational parameters are the geminal coefficients with respect to a fixed atomic orbital basis, which allows one to classify these variables into chemically significant and non significant ones according to their magnitudes and thus reduce size of calculations.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

The ground electronic state by Fourier transform spectroscopy: Dispersion coefficients
View Description Hide DescriptionThis study presents the derivation of an accurate potential energy curve for the ground electronic state of the molecule. High resolution laser induced emission spectra data involving vibrational levels of the ground state up to wave numbers) have been determined. The ground state potential energy curve is constructed by combining the inverted perturbation approach for internuclear distances up to 11 Å, with an analytical expression for longer internuclear distances. This potential curve allows an improved derivation of the dissociation energy and of the Coulombic parameters governing the interaction in the electronic ground state, compared with values derived either by calculations or by recent photoassociative spectroscopy measurements. The main constants are a.u.=32.945 and

Rotationally resolved laserinduced fluorescence of biacetyl in
View Description Hide DescriptionLaserinduced fluorescence of jetcooled biacetyl was recorded in the region of 22 177–22 277 cm^{−1} with a resolution of 0.012 cm^{−1}. Three bands, and with transitions were observed and analyzed. For the band, and subbands were observed. However, only ctype transitions, were observed. Another kind of transition, dtype which is allowed by selection rules, was not observed. For the band, two subbands and were observed. The subband can be described by the rigid rotor model, and it contains only transitions. On the other hand, the subband cannot be even approximately described by the rigid rotor model. It is dominated by ctype transitions. In addition, type and dtype transitions were all observed, though the intensities are small. For the band, and subbands were observed. The subbands contain all the possible types of transitions, i.e., and type transitions. However, the subband has transitions only. The band origin, rotational constants, and the splitting due to the tunneling in these three bands was accurately measured. Relatively large vibronic transition intensities were observed in both the and bands.

Chebyshev real wave packet propagation: statetostate reactive scattering calculations
View Description Hide DescriptionIn this paper we explore the relative performance of two recently developed wave packet methodologies for reactive scattering, namely the real wave packet Chebyshev domain propagation of Gray and BalintKurti [J. Chem. Phys. 108, 950 (1998)] and the Lanczos subspace wave packet approach of Smith et al. [J. Chem. Phys. 116, 2354 (2002); Chem. Phys. Lett. 336, 149 (2001)]. In the former method, a modified Schrödinger equation is employed to propagate the real part of the wave packet via the wellknown Chebyshev iteration. While the timedependent wave packet from the modified Schrödinger equation is different from that obtained using the standard Schrödinger equation, timetoenergy Fourier transformation yields wave functions which differ only trivially by normalization. In the Lanczos subspace approach the linear system of equations defining the action of the Green operator may be solved via either timedependent or timeindependent methods, both of which are extremely efficient due to the simple tridiagonal structure of the Hamiltonian in the Lanczos representation. The two different wave packet methods are applied to three dimensional reactive scattering of (total J=0). Statetostate reaction probabilities, product state distributions, as well as initialstateresolved cumulative reaction probabilities are examined.

Fulldimensional quantum wave packet study of rotationally inelastic transitions in collision
View Description Hide DescriptionWe report fulldimensional accurate quantum dynamical calculations of the rotationally inelastic collision: using a wave packet approach based on the Chebyshev polynomial expansion of Green’s operator. The sixdimensional Hamiltonian within the coupledstates approximation is discretized in a mixed grid/basis representation and its action is computed in appropriate representations facilitated by a series of onedimensional pseudospectral transformations. Both the parity and diatomic exchange symmetry are adapted. The Smatrix elements for the rotational transitions are obtained at all energies by the Fourier transform of Chebyshev correlation functions and used to compute transition probabilities, differential and integral cross sections, and stateresolved thermal rate constants. Results are compared for two recently proposed ab initio based potential energy surfaces and with previous quantum results.

Ab initio dipole moment functions of and intensity anomalies in rovibrational spectra
View Description Hide DescriptionIntensities of fundamental and of twoquanta overtone and combination rovibrational bands of have been evaluated from electric dipole moment functions calculated using ab initiocoupledclusters methods. Particular features and derivatives of the dipole moment are discussed in relation with observed intensity anomalies in infrared spectra of this molecule. Rovibrational energy levels and corresponding eigenfunctions have been obtained variationally using an empirical potential energy function. Calculated absolute line intensities are compared with available experimental data. The generatedelectric dipole moment functions have allowed to considerably improve upon previous ab initio intensity calculations for fundamental bands, and to describe for the first time major observed anomalies in intensity distributions of rovibrational bands. Discrepancies between ab initio and empirical values of the integrated band intensities are 12%, 8%, and 10% for the first triad bands and 4% and 6% for the strongest bands and

Intracluster multiple trimeric cyclization of acrylonitrile clusters initiated by electron transfer from a potassium atom: Sizedependent pathways in metastable dissociation of photoions
View Description Hide DescriptionSizedependent stabilities and intracluster reactions of potassium atom and acrylonitrile molecules (AN; clusters were investigated. Previously reported magic numbers (intensity anomalies) of n=3k (k=1–4) using photoionization mass spectrum of and sizespecific elimination reactions (HCN elimination from clusters of and elimination from n=3 and 6 clusters) were explained by a cyclohexane derivative formation in an intracluster trimeric cyclization (anionic oligomerization) initiated by electron transfer from a K atom in To elucidate larger structures, unimolecular metastable dissociations of photoions were observed using a reflectron timeofflight mass spectrometer. A metastable dissociation pathway of n→n−1 (ANloss) was predominantly observed for all parent sizes; furthermore, for parent ions with n=6, 9, and 12, pathway of n→n−3 loss] was also observed. These sizedependent dissociation pathways of photoions are related to structures of neutral clusters since intramolecular bonds are expected to be formed in the oligomerization reactions in neutrals and to be conserved in the photoionization process. Parent clusters that cause the n→n−1 dissociations have structures in which at least one AN monomer can coordinate without forming any chemical bonds. The observation of n→n−3 pathways corresponds to the existence of isomers of n= 3k (k=2–4) clusters having k cyclohexane derivatives, which are formed by intracluster multiple trimeric cyclization reactions with 3k AN molecules in neutral clusters. The existence of at least two types of structural isomers (including reacted AN or unreacted AN) in these clusters is shown from these experimental results, and is further supported by calculations of the microcanonical dissociation rate constants for each pathway based on the Rice–Ramsperger–Kassel–Marcus theory.

Methylation effects in stateresolved quenching of highly vibrationally excited azabenzenes II. Collisions with carbon dioxide
View Description Hide DescriptionTo explore the role of molecular structure in collisions that relax highly excited polyatomic molecules, we have studied collisional deactivation of a series of highly vibrationally excited methylated pyridines in a bath. Complementary studies that investigated quenching by have been presented in Part I of this series [M. S. Elioff, M. Fang, and A. S. Mullin, J. Chem. Phys. 115, 6990 (2001)]. We have used highresolution transient infrared absorption probing to measure rotational and translational energy gain in individual quantum levels of that are populated via collisions with vibrationally excited picoline (2methylpyridine) and lutidine (2,6dimethylpyridine). Vibrationally excited picoline and lutidine were prepared by absorption of pulsed light and fast internal decay to the ground electronic state. The nascent distribution of rotational states was measured for Translational energy gain distributions were determined for the states of using Dopplerbroadened linewidthmeasurements.Energy transfer probabilities were determined by measuring absolute energy transfer rate constants for energy gain into specific quantum states. These results are compared to previous singlecollision energy transfer studies on hot pyridine [M. C. Wall, B. Stewart, and A. S. Mullin, J. Chem. Phys. 108, 9658 (1998)] and hot pyrazine [M. C. Wall and A. S. Mullin, J. Chem. Phys. 108, 9658 (1998)] initially excited with 266 nm light and quenched via collisions with We find that donor methylation reduces the amount of translational and rotational energy imparted to the highJ states, but that the cross section for exciting the highJ states of increases upon donor methylation. Fermi’s golden rule is used to describe the relaxation process, and the energy transfer distribution functions for are found to correlate remarkably well to the energy dependence of the density of states of the hot donor molecule. This analysis is also successfully applied to earlier quenching data for vibrationally excited [C. A. Michaels et al., J. Chem. Phys. 106, 7055 (1997)], suggesting that this may be a general approach for describing relaxation of highly excited molecules.

Spectroscopy of anion complexes
View Description Hide DescriptionTotal energy calculations based on the generalized gradient approximation to the density functional theory reveal that the Ni(benzene) and anions are unstable against autodetachment of the additional electron while other anion complexes containing more than one Ni atom are stable. Although the adiabatic electron affinities increase with Ni content, they are significantly smaller than those in pure Ni clusters containing the same number of Ni atoms. The difference between adiabatic electron affinities and vertical detachment energies are around 0.2 eV in most cases, indicating that the equilibrium geometries of are not significantly altered from their corresponding neutral geometries. The vertical transitions from the anion to the neutral provide new insight into the magnetic moment of these organometallic complexes.

Reassignment of the spectrum just below dissociation threshold based on ab initio calculations
View Description Hide DescriptionVibrational Herzberg bands of the molecule just below its first dissociation limit are since longknown to be perturbed. Jenouvrier et al. [J. Mol. Spectrosc. 198, 136 (1999)] assigned the cause of the perturbations to five vibrational levels supported by the shallow minimum in the potential energy curve around Using ab initio potential energy curves and spin–orbit couplings from previous work [J. Chem. Phys. 116, 1954 (2002)] we present a full quantum calculation of all ungerade rotation–vibration–electronic states of oxygen just below the dissociation threshold, through a total angular momentum quantum number of This calculation shows that the original assignment, based on a Hund’s case (a) model of a regular multiplet was not correct. Based on our calculation we present a new assignment of the perturbing states: and in order of ascending term values. We show the new assignment to be consistent with experimental data and we also propose new spectroscopic parameters for the perturbing states.

Tunable diode laser study of the infrared band of fluoroketene: Confirmation of anomalously low stretching frequency
View Description Hide DescriptionThe shortlived molecule fluoroketene (FHCCO) was generated by laser pyrolysis of fluoroacetyl chloride in a static multipass cell to record the first highresolution infrared spectrum of its fundamental band. More than 400 transitions between 2143 and 2160 cm^{−1} were assigned and analyzed to yield very accurate rotational and centrifugal distortion constants for the upper vibrational state. The band origin of fluoroketene lies at 2147.826 93 cm^{−1}, which is surprisingly low compared to the band origins of other ketenes. This result is confirmed by matrix isolation Fourier transform infrared spectroscopy of all four monohaloketenes.

Structures, energetics, and spectra of and clusters, Ab initio study
View Description Hide Descriptionand clusters have been studied using ab initio calculations. The relative stabilities of various isomers of are quite different from those of the corresponding isomers of at 0 K, and are almost the same as those of the corresponding isomers of at 298 K. That is, the relative stabilities of various isomers of and at 298 K are quite different from those at 0 K due to the entropy effect. The ionization potential, chargetransfertosolvent energy, and OH stretching vibrational spectra are reported to facilitate future experimental work.

Dissociative recombination of water cluster ions with free electrons
View Description Hide DescriptionDissociative recombination (DR) of the water cluster ion has been studied at the heavyion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Cluster ions were injected into the ring and accelerated to an energy of 2.28 MeV. The stored ion beam was merged with an almost monoenergetic electron beam, and neutral fragments produced by DR were detected by an energysensitive surface barrierdetector. The first experimental determinations of the absolute DR cross section and branching ratios for a cluster ion are reported. The cross section for the process is large and reaches at a low centerofmass collision energy of 0.001 eV. The cross section has an dependence in the energy range 0.001–0.0052 eV, and a steeper slope with an dependence for eV. The general trends are similar to the results for previously studied molecular ions, but the cross section is higher in absolute numbers for the cluster ion. Thermal rate coefficients for electron temperatures of 50–2000 K are deduced from the cross section data and the rate coefficients are consequently also large. Branching ratios for the product channels are determined with a grid technique. Breakup into is the dominating dissociation channel with a probability of 0.94±0.04. The channel resulting in the fragments has a probability of 0.04±0.02, and the probability for formation of is 0.02±0.03. The results are compared with data for molecular ions, and the cluster dissociationdynamics are discussed.

Relationship between the thermal properties and optical emission spectra of the polyatomic exciplexes
View Description Hide DescriptionThe geometricalstructures and optical emission spectra of the polyatomic exciplexes are calculated as functions of temperature in the range from 1 to 200 K. The relationships between the emission spectra and the thermal properties are investigated by use of the Metropolis Monte Carlo method. The peak energy values and the linewidths of the transition energy of the calculated emission light spectra of individual exciplexes reflect the structure and the thermal properties of each polyatomic exciplex. As the result of a detailed examination of relationships between the geometricalstructure and the optical emission spectra of the and polyatomic exciplexes, a change in the structure of a given polyatomic exciplex, or the dissociation into a smaller cluster, can be detected by the discontinuities in the peak energy of the emission light and the linewidth as functions of temperature.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Proton transfer reaction of 4methyl2,6dicarbomethoxyphenol in nonpolar and weakly polar solvents
View Description Hide DescriptionProton transfer processes in the ground and excited state of 4methyl2,6dicarbomethoxyphenol (CMOH) have been investigated by means of steadystate and nanosecond transient spectroscopy in some nonpolar and weakly polar solvents at room temperature and 77 K. The results obtained for CMOH were compared with that of 4methyl2,6diformylphenol (MFOH) reported previously. Unlike MFOH the fluorescence spectra of CMOH show dual emission. This was explained as due to the presence of both the enol tautomer and intramolecularly hydrogenbonded closed conformer as the fluorescing species. At 77 K the emission spectra show phosphorescence only in the presence of a base like triethylamine. The fluorescence decay rates of CMOH are relatively slower than that of MFOH and the nonradiative rates are always found higher than the radiative rates. A theoretical calculation at the Austin model 1 level of approximation revealed that the excitedstate intramolecular proton transfer barrier in the ground singlet and excited triplet states is rather large compared to the excited singlet state in the respective potential energy surfaces.