Van Der Waals Molecules

Coexistence of solvated electron and benzene-centered valence anion in the negatively charged benzene-water clusters

We present a combined M06 functional calculation and ab initio molecular dynamics simulation study of an excess electron (EE) in a microhydrated aromatic complex (modeled by benzene (Bz)-water binary clusters, Bz(H(2)O)(n)). Calculated results illustrate that Bz ring and water clusters are indeed linked through the π···HO interactions in the neutral Bz(H(2)O)(n) (n = 1-8) clusters, and the size of the water cluster does not influence the nature of its interaction with the π system for the oligo-hydrated complexes. The states and the dynamics of an EE trapped in such Bz-water clusters were also determined. All of possible localized states for the EE can be roughly classified into two types: (i) single, ring-localized states (the Bz-centered valence anions) in which an EE occupies the LUMO of the complexes originating from the LUMO (π*) of the Bz ring, and the π···HO interactions are enhanced for increase of electron density of the Bz ring. In this mode, the carbon skeleton of the Bz part is significantly deformed due to increase of electron density and nonsymmetric distribution of electron density induced by the interacting H-O bonds; (ii) solvated states, in which an EE is trapped directly as a surface state by the dangling hydrogen atoms of water molecules or as a solvated state in a mixed cavity formed by Bz and water cluster. In the latter case, Bz may also participate in capturing an EE using its C-H bonds in the side edge of the aromatic ring as a part of the cavity. In general, a small water cluster is favorable to the Bz-centered valence anion state, while a large one prefers a solvated electron state. Fluctuations and rearrangement of water molecules can sufficiently modify the relative energies of the EE states to permit facile conversion from the Bz-centered to the water cluster-centered state. This indicates that aromatic Bz can be identified as a stepping stone in electron transfer and the weak π···HO interaction plays an important role as the driving force in conversion of the two states.

Theoretical investigation of the He-I2(E3Πg) ion-pair state: ab initio intermolecular potential and vibrational levels

We present a theoretical study on the potential energy surface and vibrational bound states of the E electronic excited state of the HeI(2) van der Waals system. The interaction energies are computed using accurate ab initio methods and large basis sets. Relativistic small-core effective core potentials in conjunction with a quintuple-zeta quality basis set are employed for the heavy iodine atoms in multireference configuration interaction calculations for the (3)A' and (3)A" states. For the representation of the potential energy surface we used a general interpolation technique for constructing potential surfaces from ab initio data based on the reproducing kernel Hilbert space method. The surface presents global and local minima for T-shaped configurations with well-depths of 33.2 and 4.6 cm(-1), respectively. Vibrational energies and states are computed through variational quantum mechanical calculations. We found that the binding energy of the HeI(2)(E) T-shaped isomer is 16.85 cm(-1), in excellent agreement with recent experimental measurements. In lieu of more experimental data we also report our predictions on higher vibrational levels and we analyze the influence of the underlying surface on them. This is the first attempt to represent the potential surface of such a highly excited electronic state of a van der Waals complex, and it demonstrates the capability of the ab initio technology to provide accurate results for carrying out reliable studies to model experimental data.

Theoretical investigation of the He4Br2 conformers

Full dimensional quantum dynamics calculations of the three lowest isomers of the He(4)Br(2) van der Waals molecule in its ground electronic state are reported. The calculations are performed using the multiconfiguration time-dependent Hartree (MCTDH) method and a realistic potential form that includes the sum of three body ab initio coupled-cluster single double triple [CCSD(T)] He-Br(2) interactions plus the He-He and Br-Br interactions. This potential exhibits several multiple minima, with the three lowest ones lying very close in energy, just within 2 cm(-1). Such small differences are also found in the calculated binding energies of the three most stable conformers, indicating the floppiness of the system and, thus, the need of accurate potential forms and quantum full dynamics methods to treat this kind of complexes. The 12 dimensional results reported in this work present benchmark data and, thus, can serve to evaluate approximate methods aiming to describe higher order rare gas-dihalogen (N > 4) complexes. A comparison with previous studies using different potential forms and approaches to the energetics for the He(4)Br(2) cluster is also presented.

Communication: Quantum Zeno-based control mechanism for molecular fragmentation

A quantum control mechanism is proposed for molecular fragmentation processes within a scenario grounded on the quantum Zeno effect. In particular, we focus on the van der Waals Ne-Br(2) complex, which displays two competing dissociation channels via vibrational and electronic predissociation. Accordingly, realistic three-dimensional wave packet simulations are carried out by using ab initio interaction potentials recently obtained to reproduce available experimental data. Two numerical models to simulate the repeated measurements are reported and analyzed. It is found that the otherwise fast vibrational predissociation is slowed down in favor of the slow electronic (double fragmentation) predissociation, which is enhanced by several orders of magnitude. Based on these theoretical predictions, some hints to experimentalists to confirm their validity are also proposed.

Gaussian approximation for the structure function in semiclassical forward-backward initial value representations of time correlation functions

Initial value representations (IVRs) of semiclassical (SC) theory provide a general approach for adding quantum mechanical effects to classical molecular dynamics simulations of large molecular systems. Of the various versions of SC-IVR methodology for evaluating time correlation functions, the Fourier transform forward-backward (FB) approach is the simplest one that is able to describe true quantum coherence effects, so it is of considerable importance to find efficient and systematic ways for implementing it. It is shown in this paper that a Gaussian approximation for the "structure function"-the dependence of the correlation function on the (typically) momentum jump parameter-provides an efficient and accurate way for doing so. The approach is illustrated by an application to the time-dependent radial distribution function of I(2) (after photoexcitation) in a cluster of (up to 16) argon atoms.

Experimental and theoretical investigations of the He⋯I2 rovibronic spectra in the I2 B–X, 20–0 region

The laser-induced fluorescence and action spectra of I2 in a helium supersonic expansion have been recorded in the I2 B-X, 20-0 region. Two features are identified within the spectra. The lower-energy feature arises from transitions between states that are localized in a T-shaped conformation on both the X- and B-state potentials. The higher-energy feature reflects transitions from states that are localized in a linear conformation on the X state to states that have energies that are larger than the barrier for free rotation of the rare gas atom about the I2 molecule on the B-state potential. Ground-state binding energies of 16.6(6) and 16.3(6) cm-1 were determined for the T-shaped and linear conformers, respectively. These spectra are compared to those calculated using the experimentally determined rotational temperatures. Based on the agreement between the experimental and calculated spectra, the binding energies of the J'=0 states with 0 and 2-6 quanta of excitation in the He...I2 bending mode on the B state were determined. Several models for the B-state potential were used to investigate the origins of the shape of the contour of the higher-energy feature in the spectra for He...I2 and He...Br2. The shape of the contours was found to be relatively insensitive to the choice of potential. This leads us to believe that the spectra of these systems are relatively insensitive to the parameters of the B-state potential energy surface and are more sensitive to properties of the halogen molecule.

A theoretical study of He2ICl van der Waals cluster

The structure, energetics, and dynamics of He2ICl complex in its ground state are studied by means of ab initio electronic structure and quantum-mechanical calculations. Interaction energies for selected He2ICl configurations are calculated at the coupled-cluster [CCSD(T)] level of theory using a large-core pseudopotential for the I atom and the aug-cc-pVTZ and aug-cc-pV5Z basis sets for the Cl and He atoms, respectively. The surface is characterized around its lower five minima and the minimum energy pathways through them. The global minimum of the potential corresponds to a "police-nightstick (1)" configuration, the second one to a linear, the next one to tetrahedral configuration, and the following two to "bifork" and "police-nightstick (2)" structures, with well depths of -99.12, -97.42, -88.32, -85.84, and -78.54 cm(-1), respectively. An analytical form based on the sum of the three-body parametrized HeICl interactions plus the He-He interaction is found to represent very well the tetra-atomic CSSD(T) results. The present potential expression is employed to perform variational five-dimensional quantum-mechanical calculations to study the vibrational bound states of the van der Waals He2ICl complex. Results for total angular momentum J = 0 provide the binding energy D0 and the corresponding vibrationally averaged structure for different isomers of the cluster. Comparison of these results with recent experimental observations further justifies the potential used in this work.

Stabilization and rovibronic spectra of the T-shaped and linear ground-state conformers of a weakly bound rare-gas–homonuclear dihalogen complex: He⋯Br2

Laser-induced fluorescence spectra of Br(2) entrained in a He supersonic expansion have been recorded in the Br(2) B-X, 8-0, 12-0, and 21-0 spectral regions at varying downstream distances, and thus different temperature regimes. Features associated with transitions of the T-shaped and linear He...Br(2)(X,nu(") = 0) complexes are identified. The changes in the relative intensities of the T-shaped and linear features with cooling in the expansion indicate that the linear conformer is energetically more stable than the T-shaped conformer. A He + Br(2)(X,nu(") = 0) ab initio potential-energy surface, computed at the coupled cluster level of theory with a large, flexible basis set, is used to calculate the binding energies of the two conformers, 15.8 and 16.5 cm(-1) for the T-shaped and linear complexes, respectively. This potential and an excited-state potential [M. P. de Lara-Castells, A. A. Buchachenko, G. Delgado-Barrio, and P. Villareal, J. Chem. Phys. 120, 2182 (2004)] are used to calculate the excitation spectra of He...(79)Br(2)(X,nu(") = 0) in the Br(2) B-X, 12-0 region. The calculated spectra are used to make spectral assignments and to determine the energies of the excited-state intermolecular vibrational levels accessed in the observed transitions. Temperature-dependent laser-induced fluorescence spectra and a simple thermodynamic model [D. S. Boucher, J. P. Darr, M. D. Bradke, R. A. Loomis, and A. B. McCoy, Phys. Chem. Chem. Phys. 6, 5275 (2004)] are used to estimate that the linear conformer is 0.4(2) cm(-1) more strongly bound than the T-shaped conformer. Two-laser action spectroscopy experiments reveal that the binding energy of the linear He...(79)Br(2)(X,nu(") = 0) conformer is 17.0(8) cm(-1), and that of the T-shaped He...(79)Br(2)(X,nu(") = 0) conformer is then 16.6(8) cm(-1), in good agreement with the calculated values.

The dissociation dynamics of He⋯I 35Cl(B,v′=2,3) complexes with varying amounts of internal energy

The He...I (35)Cl intermolecular vibrational levels with n'=0-6 that are bound within the He+ICl(B,v'=3) potential [A. B. McCoy, J. P. Darr, D. S. Boucher, P. R. Winter, M. D. Bradke, and R. A. Loomis, J. Chem. Phys. 120, 2677 (2004)] are identified in laser-induced fluorescence experiments performed at very low temperatures within a supersonic expansion. Comparisons of the positions and intensities of these lines with the excitation spectra, calculated using potential surfaces to describe the interactions between the helium atom and ICl in its ground and excited state, assist in the assignments. Based on these comparisons the excited state potential was rescaled so that the experimental and calculated J'=0 energies agree to within the experimental uncertainties for all but the lowest, n'=0, intermolecular level. Two-laser, action, and pump-probe spectroscopy experiments indicate that the bound He...I (35)Cl(B,v'=3) intermolecular vibrational levels undergo vibrational predissociation forming rotationally excited I (35)Cl(B,v'=2,j') products with distributions that depend upon the initial intermolecular vibrational level excited. Action spectra recorded in the ICl B-X, 2-0 region while monitoring the Deltav=0, I (35)Cl(B,v'=2) channel reveal two additional dissociation mechanisms for the He...I (35)Cl(B,v') excited state complexes: rotational predissociation of discrete metastable states lying slightly above the He+I (35)Cl(B,v'=2) asymptote and direct dissociation that occurs when the linear conformer is excited to the continuum of states above the same asymptote. The rotational predissociation pathway forms I (35)Cl(B,v'=2,j') products in all of the rotational states energetically accessible. The direct dissociation mechanism yields very cold rotational product state distributions; for instance, the average rotational energy in the product state distribution measured when the linear complexes are prepared 20 cm(-1) above the dissociation limit is only 1.51 cm(-1), representing only 7.6% of the available energy.

Combined experimental/theoretical investigation of the He+ICl interactions. I. Rovibronic spectrum of He⋯ICl complexes in the ICl B–X, 3–0 region

Transitions of two different stereoisomers of the He...ICl(X,v" = 0) weakly bound complex, one with a T-shaped orientation and another that is most likely linear, have been observed in laser-induced fluorescence experiments performed in the ICl B-X region. Here we present experimental and theoretical results aimed at confirming the previous assignments and at gaining additional insights into the He+ICl interactions. High resolution action spectra were recorded in the same region to identify those features that could be attributed to transitions of the He...I35Cl(X,v" = 0) isomers and not to higher-order complexes, Hen...I35Cl, where n > or = 2, or I37Cl containing species. Calculations of the rovibronic spectra of the He...I35Cl complexes in the ICl B-X, 2-0 and 3-0 regions were performed using an ab initio potential energy surface for the He+ICl(X,v" = 0) ground state and two different pairwise additive potentials for the He+ICl(B,v' = 2,3) excited states. The rotation-vibration energies and wave functions for the He cdots, three dots, centered I35Cl complexes were obtained for all bound states with total angular momentum J < 10 using both of these potentials. Electronic spectra were generated using these results, assuming that the transition moment lies along the ICl bond and is not perturbed by the presence of the helium atom. The calculations qualitatively reproduce the He cdots, three dots, centered I35Cl action spectrum and strongly support the previous assignments. The calculations also indicate that some of the spectral congestion observed near the linear band may be attributed to transitions of the linear isomer to multiple intermolecular levels in the excited state. Coriolis coupling strongly mixes He cdots, three dots, centered ICl(B,v') states with rotational excitation, making simulations and assignments of the linear band observed in the experimental spectrum difficult.

The open-shell interaction of He with the B 3Πu(0+) state of Br2: Anab initiostudy and its comparison with a diatomics-in-molecule perturbation model

The interaction of He with Br2 in electronically excited B 3Piu state is investigated using spin-unrestricted single and double coupled-cluster approach with noniterative perturbative treatment of triple excitations. Internal electrons of the Br atom are described by effective core pseudopotentials. The validity of this approach is analyzed by comparing the lowest 2Sigma+ and 2Pi electronic states of the HeBr molecule with those obtained in all electron calculations [J. Chem. Phys. 115, 10438 (2001)]. In this context, we examine the performance of different basis sets and saturation with bond functions. The comparison of theoretical blue-shifts with the experiment provides confidence about the present ab initio calculations. In addition, He-Br results of ab initio calculations at the same level are used to obtain approximate He-Br2 (3Piu) interactions in the framework of the diatomics-in-molecule first order perturbation theory (IDIM-PT1) [J. Chem. Phys. 104, 9913 (1996)]. Overall, the IDIM-PT1 model results show a good agreement with the ab initio ones, being the main difference the sensitivity to the elongation of the Br-Br bond.

HIGH RESOLUTION SPECTROSCOPY IN THE GAS PHASE: Even Large Molecules Have Well-Defined Shapes

▪ Abstract  A review of recent high-resolution microwave, infrared, and optical spectroscopy experiments demonstrates that remarkable progress has been made in the past 20 years in determining the equilibrium geometries of large polyatomic molecules and their clusters in the gas phase, and how these geometries change when the photon is absorbed. A special focus is on the dynamical information that can be obtained from such studies, particularly of electronically excited states.