Volume 115, Issue 23, 15 December 2001
 ARTICLES

 Theoretical Methods and Algorithms

Torsional barrier, ionization potential, and electron affinity of biphenyl—A theoretical study
View Description Hide DescriptionThe structure and torsional barriers at 0° and 90° for biphenyl were studied by both ab initio and density functional methods by using various levels of theory with different basis sets. The torsional angle (φ) calculated at the level was 42.1°, while φ calculated using various density functionals with different basis sets was close to 40°. In contrast with the ab initio results, the torsional barrier at 0° obtained using various density functionals coincided well with experimental values. The torsional barrier ratio obtained at the B3LYP/ccpVTZ level, 1.0988, agreed well with the experimental value, 1.0833, whereas it was 0.416 at the level. Structural studies on biphenyl ions showed that the biphenyl cation has a nonplanar structure whereas its anionic counterpart has a planar structure. The ionization potential obtained at the level was 7.86 eV. Contrary to an earlier study, a positive electron affinity (EA) was obtained, in accordance with experimental predictions. EA values of 0.021 and 0.076 eV were obtained at the and B3LYP/augccpVDZ levels, respectively.

Properties of atoms in molecules: Construction of onedensity matrix from functional group densities
View Description Hide DescriptionThe demonstrated transferability of functional groups defined as proper open systems within the theory of atoms in molecules is used to iteratively construct a oneelectron density matrix P and its derived electron density distribution. The initial guess at the density used in the fitting procedure is obtained from the addition of the density distributions of groups defined in parent molecules by the maximal matching of their interatomic surfaces. The method thus takes advantage of the observation that the “zeroflux” boundary condition defining a proper open system maximizes the transferability of the density distribution of a given group between molecules, one that is accompanied by a paralleling transferability in all of its properties. The construction is subject to the constraints that P be idempotent and normalized. The method is applied to the construction of P for the molecules with OH, and F, where the vertical bar denotes the new C–C interatomic surface, the new zeroflux boundary. The densities for the groups and are defined in their dimer molecules, and

Phases and amplitudes of recurrences in autocorrelation function by a simple classical trajectory method
View Description Hide DescriptionThe interference between timedependent recurrences in the quantum autocorrelation function is eliminated by carrying out orthogonal transformations in the timeenergy domain. The timedependent phases and amplitudes of the individual recurrences are compared with the results obtained from simple classical trajectory calculations. Using classical trajectories we calculate a twodimensional survival probability which is defined in the time and energy domain. The twodimensional survival probability provides the phase and enables to distinguish between overlapping recurrences. Remarkable agreement between the quantum and classical results is obtained for the initial Gaussian wave packet which is preferentially located either in the regular or in the chaotic regimes in the classical phase space of the Pullen–Edmonds Hamiltonian (nonlinearly coupled two harmonic oscillators). A novel method which enables to determine the molecular potential energy surfaces from a measuredabsorption or emission spectra is proposed. The method employs the matching of Wigner transforms of individual quantum recurrences with the twodimensional classical survival probability.

An intermediate state representation approach to Kshell ionization in molecules. I. Theory
View Description Hide DescriptionThe general intermediate state representation (ISR) for singleelectron ionization is adapted to the case of Kshell (or corelevel) ionization in molecules. The development is based on the socalled core–valence separation (CVS) approximation leading to a considerable simplification of the ISR secular equations. Using the CVS approximation the corelevel ISR can be formulated entirely in terms of the intermediate states of the valence electron excitation problem, which allows one to construct consistent nthorder approximation schemes for the (singlehole) ionizationenergies by a specific extension of the nd order ISR approximation for electronic excitation. In particular, the CVSISR concept is used to derive a consistent fourthorder approximation for corelevel ionization based on the existing secondorder algebraicdiagrammatic construction [ADC(2)] approximation to electron excitation. The computational scheme combines the diagonalization of a Hermitian secular matrix with finite perturbation expansions for the secular matrix elements. The explicit configuration space is spanned by onehole twoholeoneparticle and ionic states with exactly one hole in the corelevel shell of interest, while the configurations considered implicitly via perturbation theory extend to the class of states. A characteristic of the method is that the dominant valence electron relaxation effect is accounted for at the postHartree–Fock (HF) level. This calls for the relatively high order of perturbationtheoretical consistency, but avoids, on the other hand, the necessity of a localized (symmetry breaking) oneparticle representation in the case of molecules with equivalent 1s orbitals. The method is size consistent and thus suitable for applications to large systems.

Making use of Connolly’s molecular surface program in the isodensity adapted polarizable continuum model
View Description Hide DescriptionWe present a general method to obtain well shaped cavities for treating solvation within the polarizable continuum model (PCM). The cavities are built with Connolly’s molecular surface program MSROLL, that allows easy evaluation of all the critical parameters involved in present day PCM technology. The procedure basically works in adapting the isodensity of the solutes at different threshold values. The important features to realize are, that on the one hand there is no uniform threshold value applicable for a reliable description of the solvation effect, and that on the other hand, there are two types of characteristic points to be found when plotting versus isodensity threshold value of the solute, which are, a minimum for neutral and anionic molecules, or a shoulder for cationic molecules. Taking the respective cavity dimensions at these characteristic points, the experimental values of are modeled within chemical accuracy and thus a true ab initio treatment of any arbitrary chemical system should be conceivable within this socalled isodensity adapted polarizable continuum model (IDAPCM).
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Generation of fulldimensional potential energy surface of intramolecular hydrogen atom transfer in malonaldehyde and tunneling dynamics
View Description Hide DescriptionThe potential energy surface (PES) for the malonaldehyde intramolecular hydrogen atom transfer has been generated with full dimensionality by the modified Shepard interpolation method at the computational level of the secondorder Møller–Plesset perturbation theory. The reference points have been set along the reaction path of H atom transfer (51 points), in a threedimensional reaction space determined by geometrical features of the reaction path (219 points), and in the region of cis and transenol isomerizationreaction paths (428 points), so the resultant PES was generated in terms of ab initio data (energies, gradients, and Hessian matrices) of 698 reference points. Following trajectory simulations on the fulldimensional PES, the energy splitting of vibrational ground states due to tunneling was estimated by the semiclassical method of Makri and Miller [J. Chem. Phys. 91, 4026 (1989)]. The tunneling splitting was evaluated as 13.9 cm^{−1}, which is in good agreement with the experimental value of 21.6 cm^{−1}.

Femtosecond probing of photodissociation dynamics in acyl cyanides
View Description Hide DescriptionThe photodissociation of two acyl cyanide compounds, R–C(O)–CN, where R=methyl and tertbutyl groups, has been investigated using femtosecondtimeresolvedlaserinduced fluorescence(LIF)spectroscopy. Both compounds were excited by twophoton excitation at a total energy of ∼6.4 eV and the formation of the free radical products was probed in real time by monitoring the LIF signal. The results revealed that the temporal evolution of the formation can be well characterized by delayed biexponential rise functions with time constants in the picosecond time scale, indicating that the dissociation occurs via a complexmode mechanism. We proposed a dissociation mechanism involving two discernable stages to account for the observed temporal behaviors as well as previous photofragment translational spectroscopic results reported by other groups. Our analyses suggested that the selectivity between the C–CN and C–R bond cleavage is determined by the competition between the adiabatic and nonadiabatic dynamics of the state. The results also indicated that the adiabatic dissociation process occurring on the surface is not statistical. We speculate that this nonstatistical dissociation behavior is due to an initial nonuniform phase space distribution and a slow intramolecular vibrational energy redistribution process that prevents the system from sampling the entire phase space before the reaction completes.

Electronimpact detachment and dissociation of ions
View Description Hide DescriptionCRYRING was used to study collision processes between an electron and a negative ion cluster The total detachment cross sections for the production of the neutral 4C, 3C, 2C, and C fragments were measured. The cross sections for pure detachment, and for detachment plus dissociation leading to the production of and were extracted using a grid. It was found that the pure detachment process overwhelmingly dominates all other fragmentation processes. The threshold location for the detachment channel is found to be around 6.0 eV. Although the doubly charged negative ion has received little previous attention, a defined nearthreshold resonance observed in the detachment cross section curve, has been associated with the shortlived state (0.7 fs lifetime).

Theoretical ab initio study of the water trimer anion: Ground and excited state
View Description Hide DescriptionThe ground and excited state of the water trimer anion, are studied via highlevel ab initio calculations. A systematic search for the stable configuration is carried out, based on the configuration formed by the stable plus one additional water molecule. Four isomers, including three surface structures and one interior structure, are located on the potential energy surface. While the linear chainlike structure is determined to be the major species detected in supersonic expansion experiments, consistent with previous theoretical studies, the other three isomers may also exist in molecular beams as evidenced by the matches of their vertical detachment energies with highenergy shoulders observed in photoelectron spectra. A significant geometric distortion of water molecules directly interacting with the excess electron indicates that the Franck–Condon effects are the major cause for the experimental observation of the excitation of water vibrational modes upon the photodetachment of This is in contrast to the vibronic effects for the case of In addition, the excited states of are also investigated. The results show the existence of electronically bound excited states in surface structures of These states possess considerable p character, and therefore, can be regarded as a precursor of three plike excited states of bulk hydrated electrons.

Ab initio calculations of electric field effects on the tensor of a nitroxide radical
View Description Hide DescriptionThe effects of applied electric fields on the magnetic parameters of a nitroxide spin label have been calculated using an intermediate level of Rayleigh–Schrödinger perturbation theory based on unrestricted Hartree–Fock ab initio calculations. The theory is tested for selected simple model organic radicals and gives electronic gtensors that compare well with previous calculations and experimental values. The method is then applied to calculate effects of a local electrostatic field on the larger nitroxide radical, 2,2,5,5tetramethyl3,4dehydro pyrrolidine1oxyl (TMDP) using a finite field approach. The method slightly underestimates the and of TMDP, but the predicted shifts in these quantities with field are in excellent agreement with recent experimental observations using highfrequency (220 GHz) electron paramagnetic resonance. Results are expressed in terms of the phenomenological formalism given earlier by Mims to characterize linear electric fieldeffects in metal centers.

Isomers of Production and infrared spectra of cis and transOSNO from irradiated inert matrices containing OCS and
View Description Hide DescriptionNew species cis and transOSNO, designated and respectively, are produced and identified with infrared absorption spectra when an argon or nitrogen matrix containing OCS and is irradiated with laser emission at 248 nm. Lines at 1156.1 and 1454.4 cm^{−1} are assigned to and those at 1178.0 and 1459.0 cm^{−1} are assigned to in solid Lines at 1154.9 and 1450.8 cm^{−1} are assigned to and those at 1181.2 and 1456.0 cm^{−1} are assigned to in solid Ar; further lines associated with minor matrix sites are identified. Assignments of spectral lines are based on results of both experiments with  and isotopic substitution and theoretical calculations using densityfunctional theories, B3LYP with an augccpVTZ basis set; these calculations predict the geometry, energy, vibrational frequencies, and infrared intensities of as four isomers: and in increasing order of stability. Mechanisms are proposed to rationalize that and rather than or are produced from irradiated matrices containing OCS and and that no reaction product is observed in an Ar matrix containing and after irradiation at 193 nm.

A study of the reaction: Global potential energy surface and quantum dynamics
View Description Hide DescriptionThe adiabatic global potential energy surface of the system for the first singlet state of symmetry has been computed. Ab initio, multireference, single and double configuration interaction calculations have been used to characterize this state. This potential energy surface has a calculated well depth of 99.7 kcal/mol relative to the asymptote. The surface has no barrier for the perpendicular geometry, but presents a large barrier (12.35 kcal/mol) for the collinear geometry. The ab initio calculations were carried out over 1748 geometries and the resulting energies were fitted to a many body expansion. Based on this surface, we have performed the first quantum reactive scattering calculations for the reaction and total angular momentum The hyperspherical coordinates timeindependent method has been used. We note that the statetostate reaction probabilities as a function of the collision energy show a dense resonance structure which is unusual for this type of atom+diatom reaction. We present also rotational distributions.

The binding energies of pdifluorobenzene–Ar,–Kr measured by velocity map imaging: Limitations of dispersed fluorescence in determining binding energies
View Description Hide DescriptionThe technique of velocity map imaging has been used to determine the dissociation energies of the van der Waals complexes pdifluorobenzene–Ar and pdifluorobenzene–Kr. The values determined for the and states, respectively, are and for pdifluorobenzene–Ar and and for pdifluorobenzene–Kr. An ionization potential of for pdifluorobenzene–Kr has been determined by velocity map imaging of photoelectrons. The dissociation energies determined here are inconsistent with dispersed fluorescence spectra of the complexes when these are assigned in the usual way. The issue is that spectra for levels below dissociation show bands where free pdifluorobenzene emits, suggesting that dissociation is occurring from these levels. For the dispersed fluorescence and velocity map imaging results to be consistent, these fluorescence bands must arise from transitions of the van der Waals complexes shifted such that they appear at the free pdifluorobenzene wavelengths. It is proposed that these bands are due to emission from highly excited van der Waals modes populated by intramolecular vibrational redistribution from the initially excited level. From calculations performed for the related benzene–Ar system [B. Fernandez, H. Koch, and J. Makarewicz, J. Chem. Phys. 111, 5922 (1999)], the emitting levels are most likely above the barrier separating different pdifluorobenzene–partner configurations. The fluorescence observations are consistent with those of other techniques if the pdifluorobenzene–partner interaction is the same in the ground and excited electronic states for such highly excited levels. Emission then occurs at the pdifluorobenzene monomer position since the energy shift is the same for the initial and final states. Deducing van der Waals binding energies from the observation of spectral transitions at the free chromophore position following excitation of the complex can be confounded by such an effect. The dispersed fluorescence spectra reveal that the rate of intramolecular vibrational redistribution is reduced for the Kr complex compared with the Ar complex.

Argon predissociation and electron autodetachment spectroscopy of sizeselected clusters
View Description Hide DescriptionPhotodetachmentspectra of clusters in the midIR are dominated by three strong resonances. These are assigned to autodetaching (AD) C–H stretching vibrational transitions in the valence (as opposed to dipolebound) form of the molecular anion on the basis of a H/D isotopic substitution study and their solvation dependence. The AD resonances disappear promptly upon addition of the third argon atom, while the resonant structure appears in the action spectrum for formation of photoproducts for The strong argon solvation dependence of the photoproducts is traced to the rapidly changing endoergicity of the electron loss channel due to the differential solvation behavior of the valence anion relative to the neutral. We discuss a statistical limit for this competition, and introduce an intramolecular vibrational energy redistribution mediated AD mechanism unique to polyatomic anions.

Postpulse alignment of molecules robust to thermal averaging
View Description Hide DescriptionAlignment of molecules under field free conditions with negligible vibrational or electronic excitation is created by a short offresonant low frequency laser pulse. Typically the global maximum in postpulse alignment occurs at a rotational wave packet revival close to half a rotational period after the short pulse. The alignment effect is robust to thermal averaging at the revivals, but averaging cancels the alignment in between. The permanent dipole–field interaction can be efficient for alignment with offresonant frequencies between the rotational and the vibrational frequencies of the molecule.

An excess electron bound to urea oligomers. II. Chains and ribbons
View Description Hide DescriptionThe possibility of electron binding to chain and ribbonlike urea oligomers was studied at the secondorder Møller–Plesset perturbation theory level as well as at the coupled cluster level with single, double, and noniterative triple excitations. It was found that all the chains form stable dipolebound anions whose electron binding energies grow rapidly with chain length, while ribbontype oligomers bind an excess electron only when they contain an odd number of urea monomer units. Moreover, the chain oligomers support bound excited anionic states of Σ and Π symmetry.

Electronic spectroscopy of jetcooled vinyl radical
View Description Hide DescriptionThe spectra of the electronic transition of jetcooled vinyl radical, and its partially deuterated isotopomer, have been obtained via action spectroscopy. The appearance of the H or D fragment was monitored via resonance enhanced multiphoton ionization through the Lymanα transition. The vibrational structure of the state was fully resolved and analyzed. The experimental vibrational frequencies agree well with ab initio calculations. The simulation of the rotational structure of several bands reinforces vibrational assignments.

Photodissociation dynamics of at 234 nm: An implication of symmetry reduction during photodissociation
View Description Hide DescriptionThe photodissociationdynamics of in the Aband has been investigated utilizing a twodimensional photofragment ionimaging technique coupled with a stateselective resonanceenhanced multiphoton ionization scheme. The total translational energy distributions for the and channels are well characterized by Gaussian functions with average translational energies of 183 and 151 kJ/mol, respectively. The recoil anisotropies were measured to be β=0.66 for Br and 1.83 for Br^{*}. It was found that Br^{*} production is preferred, with a relative quantum yield of 0.80. The reduction in the recoil anisotropy for Br results from nonadiabatic coupling between the and states. The fraction of molecules that dissociate via a distorted pathway induced by symmetry reduction from to is estimated to be 0.11.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Vibrational spectroscopy of shockcompressed nitromethane
View Description Hide DescriptionRealtime vibrational spectra of shockcompressed nitromethane up to 10 GPa have been measured using a singlepulse laser Raman spectrometer in conjunction with a propellant gun and vibrational modedependent behavior has been examined. The stretching mode shows small frequency shift compared to other stretching modes, which may be attributed to increased intermolecular interaction under pressure. Pressureinduced Raman frequency shift of all the investigated stretching modes shows monotonic increase up to ∼5.0 GPa. Above 5 GPa, however, an abrupt drop in Raman frequency shift is observed for the CN and the stretching modes. At higher pressures, Raman frequencies of these two modes increase again until all Raman bands disappear at above ∼8.5 GPa, where a strong background emerges over the whole spectral range (500–2600 cm^{−1}). This strong emission appears to indicate an onset of a single shock induced chemical reaction at ∼8.5 GPa.

Quantum, intramolecular flexibility, and polarizability effects on the reproduction of the density anomaly of liquid water by simple potential functions
View Description Hide DescriptionThe ability of simple potential functions to describe the properties of liquid water at a range of thermodynamic state points has been explored. These simple potential function models represent a water molecule by a set of sites, either rigid or flexible relative to each other, that interact with a simple, generally classical, Hamiltonian, which has parameters that are empirically determined. Calculations on several models that include intramolecular flexibility, electronic polarization or quantum mechanical effects have been performed. The consequences of altering these parameters have been systematically examined to determine factors of importance in reproducing properties of pure liquid water. It is found that simple foursite models that incorporate classical intramolecular flexibility or electronic polarization do not improve the description of the density anomaly of liquid water. Quantum statistical mechanical path integral calculations on the classical rigid nonpolarizable TIP5P model [J. Chem. Phys. 112, 8910 (2000)] and the classical flexible nonpolarizable TIP4F model indicate that although quantum mechanical effects destructure the rigid model, they improve the radial distribution and energy distribution properties of the flexible model. In addition, although quantum effects make the density behavior of the rigid model worse, they improve the density behavior of the flexible model. Path integral calculations have also been performed on quantum TIP5P water; this leads to a temperature of maximum density that is higher and to a more structured liquid than results from calculations on quantum TIP5P water. A similar effect is seen with calculations on a fivesite rigid model, TIP5P(PIMC), which was parameterized using path integral rather than classical Monte Carlo calculations.