Volume 114, Issue 16, 22 April 2001
Index of content:
- Theoretical Methods and Algorithms
114(2001); http://dx.doi.org/10.1063/1.1358864View Description Hide Description
The isotope effects on the bifurcating reaction path of the isomerizationreaction, (stereoisomers of P and have been investigated recently [J. Chem. Phys. 113, 477 (2000)], and it was found that the valley-ridge inflection point appears in the totally symmetric direction relative to the intrinsic reaction coordinate in the deuterium-substituted system. In the present report, the kinetic isotope effect on this reaction has been studied using an ab initio direct-trajectory method at the computational level of the complete active space self-consistent field method with basis sets. The branching ratio to the respective stereoisomers has been investigated in terms of distributions of several coordinates along the trajectories. The energy landscape for is also described in terms of several structures and the respective permutational isomers.
114(2001); http://dx.doi.org/10.1063/1.1358866View Description Hide Description
Using the time-dependent Lagrangian response approach, the recently revived orbital optimized coupled cluster (OCC) model is reformulated using nonorthogonal orbital rotations in a manner that conserves the commutativity of the cluster excitation operators. The gauge invariance and the simple pole structure of the OCC linear response function are retained, while the dimension of the eigenvalue problem is reduced by a factor of 2. Restricting the cluster operator to double excitations, we have carried out the first implementation of gauge invariant coupled cluster response theory. Test calculations of the excitation energy, and length and velocity gauge oscillator strengths are presented for the lowest electric dipole allowed transitions of the molecular ion and the Ne atom. Additionally, the excitation energies to the four lowest-lying states of water are calculated.
114(2001); http://dx.doi.org/10.1063/1.1358861View Description Hide Description
The “Smart Walking” Monte Carlo algorithm is examined. In general, due to a bias imposed by the interbasin trial move, the algorithm does not satisfy detailed balance. While it has been shown that it can provide good estimates of equilibrium averages for certain potentials, for other potentials the estimates are poor. A modified version of the algorithm, Smart Darting Monte Carlo, which obeys the detailed balance condition, is proposed. Calculations on a one-dimensional model potential, on a Lennard-Jones cluster and on the alanine dipeptide demonstrate the accuracy and promise of the method for deeply quenched systems.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
Intermolecular bending levels in an open-shell diatom–diatom complex: Infrared spectroscopy and model calculations of the complex114(2001); http://dx.doi.org/10.1063/1.1357790View Description Hide Description
A theoretical framework has been developed to describe the bending levels associated with an intermolecular potential of moderate anisotropy between an open-shell diatom and a diatom partner, such as OH–CO or The model explicitly allows for coupling between the electronic and spin angular momenta of the open-shell OH radical and the vibrational angular momentum arising from intermolecular bending motion of the complex. The energies and wave functions of the intermolecular bending levels for the complex have been computed based on a dipole–quadrupole interaction. The model is used to interpret the infrared spectrum of the linear complex in the OH overtone region, which has been recorded by detecting the OH fragments from vibrational predissociation. The pure OH overtone band at 6973.54(2) cm−1 and several combination bands, which involve the simultaneous excitation of OH stretching and geared bending modes, have been observed, analyzed, and assigned within the context of the model. In addition, the time evolution and quantum state distribution of the OH fragments yield the lifetime for vibrationally activated of 30±4 ns and an upper limit for the ground state binding energy of
114(2001); http://dx.doi.org/10.1063/1.1359447View Description Hide Description
The semirigid vibrating rotor target (SVRT) model is applied to study the branching reaction on the Schatz–Elgersma potential energy surface. Using the SVRT model, the time-dependent wave packet calculation is carried out in four-mathematical dimensions with the two additional internal coordinates fixed at/near transition state geometries. The reaction probabilities for producing two product branches are calculated from two separate dynamics calculations. Comparison with results from the six-dimensional dynamics calculation shows that the SVRT reaction probabilities and cross sections for both branching products are accurate within a wide range of collision energy. This shows that the SVRT model is capable of giving quantitatively accurate dynamics information for polyatomic reactions.
The stretching vibrational overtone spectra of Local mode vibrational analysis, dipole moment surfaces from density functional theory and band intensities114(2001); http://dx.doi.org/10.1063/1.1352038View Description Hide Description
The infrared spectra of molecule were recorded on a Bruker IFS 120HR Fourier transform spectrometer from 4000 to 9500 cm−1. The P–H stretching vibrational frequencies and intensities were derived from the experimental data. The Morse oscillator parameters and α in the anharmonically coupled anharmonic oscillator local mode model were determined by the least-squares fitting with the observed vibrational band centers. The ab initio three-dimensional P–H stretching dipole moment surfaces were calculated by the density functional theory method. The dipole moment vectors were projected to three kinds of molecule-fixed reference systems. The corresponding dipole moment components were fitted to polynomial functions in terms of the P–H bond length displacements with the molecular symmetry taken into account. The absolute band intensities were obtained and then compared with the experimental data. The results showed that a proposed improved bond dipole model can predict the absolute band intensities within a factor of 2 for most of the observed transitions, indicating a reasonably good agreement.
Dissociation dynamics of Rotational population distributions of fragments from the T-shaped and linear complexes114(2001); http://dx.doi.org/10.1063/1.1359771View Description Hide Description
Optical-optical double resonance techniques have been used to examine the dissociationdynamics of Rotational population distributions were characterized for the fragments. Vibrational predissociation of the T-shaped complex yielded fragments with smooth rotational distributions. The high-energy limits of the distributions were consistent with events that channeled almost all of the available energy into product rotation. These data indicate a dissociation energy for of Most initial states of the complex produced bimodal rotational population distributions, but a few gave Boltzmann-type product distributions. The dependence of the character of the distribution on the initially excited state suggests that predissociation is mediated by intramolecular vibrational energy redistribution. Dissociation of linear yielded fragments with Boltzmann type rotational population distributions. Excitation of the complex within the bound regions of the transition gave rotationally cold fragments, consistent with direct dissociation from a near-linear geometry. Excitation above the B state dissociation limit produced fragments via caged recombination. The rotational distributions of these fragments were cold, supporting earlier studies that attribute the one atom cage recombination to the linear isomer.
Surface solvation of halogen anions in water clusters: An ab initio molecular dynamics study of the complex114(2001); http://dx.doi.org/10.1063/1.1360200View Description Hide Description
The structure and dynamics of has been studied by ab initiomolecular dynamics using the Car–Parrinello approach, and compared to results of ab initio quantum chemical calculations, molecular dynamics based on both polarizable and nonpolarizable empirical potentials, and vibrational spectroscopy. The electronic structure methodology (density functional theory with the gradient-corrected BLYP exchange-correlation functional) used in the Car–Parrinello dynamics has been shown to give good agreement with second-order Møller–Plesset results for the structures and energies of clusters. The configurational sampling during the 5 ps ab initiomolecular dynamics simulation at 250 K was sufficient to demonstrate that the chloride anion preferred a location on the surface of the cluster which was significantly extended compared to the minimum energy geometry. The structure of the cluster predicted by the polarizable force field simulation is in agreement with the ab initio simulation, while the nonpolarizable force field calculation was in qualitative disagreement, predicting an interior location for the anion. The time evolution of the electronic structure during the ab initio simulation was analyzed in terms of maximally localized orbitals (Wannier functions). Calculation of the dipole moments from the centers of the Wannier orbitals revealed that the chloride anion is significantly polarized, and that the extent of water polarization depends on location in the cluster, thus underscoring the importance of electronic polarization in halogen ion solvation. The infrared absorptionspectrum was computed from the dipole–dipole correlation function, including both nuclear and electronic contributions. Aside from a systematic redshift by 3%–5% in the frequencies, the computed spectrum was in quantitative agreement with vibrational predissociation data on Our analysis suggests that accounting for anharmonicity and couplings between modes is more important than the fine tuning of the electronic structure method for the quantitative prediction of hydrogen bond dynamics in aqueous clusters at elevated temperatures.
Alkali metal cation-ligand affinities: Basis set superposition correction for the Gaussian protocols114(2001); http://dx.doi.org/10.1063/1.1360196View Description Hide Description
The effect of basis set superposition error (BSSE) on Gaussian-2 and Gaussian-3 calculated alkali metal cation-ligand affinities has been studied. For these systems, we found that the standard Boys–Bernadi full counterpoise (CP) method often leads to correction terms that are physically incorrect. This problem may be rectified by using the geometry corrected counterpoise (GCP) method. The relationship between CP, GCP corrections, and deformation energy is discussed. In order to yield good agreement with existing experimental and ligand affinities, we recommend the adoption of either the G3 (with GCP correction) or the G2(MP2,SVP)-FC (without GCP correction) protocols. In the case of the GCP correction is of negligible magnitude, and hence GCP corrections may be omitted in the G2(MP2,SVP)-ASC affinity calculations for these complexes.
114(2001); http://dx.doi.org/10.1063/1.1359774View Description Hide Description
We did laser pump–probe far-wing scattering experiments to study the photochemical reactions We show in this work that the reaction takes place when the kinetic energy of the reactants are high enough. We think that the collisions take place preferentially in bent geometry and that the intermediate plays a major role where wide-amplitude internal vibrations eventually result in losing a hydrogen atom from the complex. We also have observed the reaction. The rotational state distribution of the LiH product for excitation in the red wing is identical to that found in the blue wing region, it does not show any preference to high or low rotational states, and no asymmetry is observed in the reactive to nonreactive branching ratio. We show different possibilities for the reactive and nonreactive collisions and provide a tentative explanation of the reaction mechanism using the highly accurate ab initiopotential energy surfaces. We also show that the harpooning model cannot be used to explain the reaction mechanism in the collision for all the atomic states from to and that no long range electron transfer occurs from the metal atom to the hydrogen molecule.
Photodissociation spectroscopy of benzene cluster ions in ultraviolet and infrared regions: Static and dynamic behavior of positive charge in cluster ions114(2001); http://dx.doi.org/10.1063/1.1359446View Description Hide Description
Photodissociationspectroscopy is applied to benzene cluster ions in ultraviolet and infrared regions. In the ultraviolet photodissociationspectrum of a characteristic broad band emerges at 255 nm. This band is assigned to a transition of a solvent benzene molecule that exists in the trimer. This is in accordance with the previous model of the ion cluster with a dimer ion core and a solvent benzene molecule. The infrared photodissociationspectra of show a sharp band at The band is attributed to a C–H stretching vibration of the dimer ion core. The infrared spectra of are fitted to the model spectra reproduced by combining the C–H stretching bands of the dimer ion core and the solvent benzene molecule. The infrared photodissociationspectra of mixed benzene trimer ions with one or two molecules demonstrate that there is no correlation between the excited dimer ion core site in the trimer and the photofragment dimer ion species. This implies that a dimer ion core switching occurs in photoexcited vibrational states prior to the dissociation.
114(2001); http://dx.doi.org/10.1063/1.1360198View Description Hide Description
Ab initio calculations were performed for the clusters including complete optimization of several isomers of the cluster. Binding energies, enthalpies, and ionization potentials are calculated. Well defined patterns of chromism are predicted for the collective vibrations and for the breathing vibrations. The calculations for the clusters allow us to understand their shell structure in terms of concentric spheres of molecules. The first and second shells have occupation numbers equal to 3 and 6, respectively, while for the third shell, this number is within the range 12–15.
114(2001); http://dx.doi.org/10.1063/1.1360786View Description Hide Description
The rovibrational state distributions for the HCl product of the 193 nm photodissociation of have been measured. The HCl dimer is prepared in a supersonic expansion of HCl in Ar, and its photoproduct detected by resonant multiphoton ionization under collisionless conditions. The state distributions are extremely “cold,” with very little of the available energy deposited in either rotation or vibration of the surviving HCl molecule. Only product is observed, and linear rotational surprisal analysis yields an extremely large surprisal parameter, of 95(8). The results are in excellent agreement with theoretical predictions.
Microsolvation of the water cation in neon: Infrared spectra and potential energy surface of the open-shell ionic complex114(2001); http://dx.doi.org/10.1063/1.1359770View Description Hide Description
The intermolecular potential of the open-shell ionic dimer in its doublet electronic ground state has been investigated by infrared spectroscopy in the vicinity of the O–H stretch vibrations and and ab initio calculations at the unrestricted Møller–Plesset second-order (MP2) level with a basis set of aug-cc-pVTZ quality. The rovibrational structure of the photodissociationspectrum is consistent with a proton-bound planar H–O–H–Ne structure and a Ne–H separation of The complexation-induced redshifts are and respectively. Tunneling splittings observed in the perpendicular component of the hybrid band of are attributed to hindered internal rotation between the two equivalent proton-bound equilibrium structures. The interpretation of the spectrum is supported by the spectrum of the monodeuterated species, for which both the proton-bound and the deuteron-bound isomers are observed The equilibrium structure of the calculated potential energy surface of has a slightly translinear proton bond, which is characterized by a Ne–H separation of a bond angle of and dissociation energies of and According to the calculated potential, the exchange tunneling between the two equivalent minima occurs via the planar bridged transition state with symmetry and a barrier of 340 cm−1. In general, the calculated properties of show good agreement with the experimental data. Initial steps in the microsolvation of the water cation in neon are discussed by comparing the calculated and experimental properties of with neon matrix isolation data
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
Application of artificial neural networks and genetic algorithms to modeling molecular electronic spectra in solution114(2001); http://dx.doi.org/10.1063/1.1358835View Description Hide Description
A novel approach is presented for finding the vibrational frequencies, Franck–Condon factors, and vibronic linewidths that best reproduce typical, poorly resolved electronic absorption (or fluorescence)spectra of molecules in condensed phases. While calculation of the theoretical spectrum from the molecular parameters is straightforward within the harmonic oscillator approximation for the vibrations, “inversion” of an experimental spectrum to deduce these parameters is not. Standard nonlinear least-squares fitting methods such as Levenberg–Marquardt are highly susceptible to becoming trapped in local minima in the error function unless very good initial guesses for the molecular parameters are made. Here we employ a genetic algorithm to force a broad search through parameter space and couple it with the Levenberg–Marquardt method to speed convergence to each local minimum. In addition, a neural network trained on a large set of synthetic spectra is used to provide an initial guess for the fitting parameters and to narrow the range searched by the genetic algorithm. The combined algorithm provides excellent fits to a variety of single-mode absorption spectra with experimentally negligible errors in the parameters. It converges more rapidly than the genetic algorithm alone and more reliably than the Levenberg–Marquardt method alone, and is robust in the presence of spectral noise. Extensions to multimode systems, and/or to include other spectroscopic data such as resonance Raman intensities, are straightforward.
Dispersion of the first hyperpolarizability of a strongly charge-transfer chromophore investigated by tunable wavelength hyper-Rayleigh scattering114(2001); http://dx.doi.org/10.1063/1.1357789View Description Hide Description
The first hyperpolarizability (β) of a strongly charge-transfer (azulenic-barbituric) chromophore is measured by hyper-Rayleigh scattering (HRS) experiment using 11 excitation wavelengths, ranging from 900 nm to 1907 nm. The measured wavelength-dependence of the β value is used to investigate the dispersion relation of the first hyperpolarizability of the charge-transfer chromophore. When excited by the 1064 nm wavelength laser radiation, the experimental HRS spectrum shows a two-photon resonance (TPR) peak, which does not coincide with the maximum of the linear absorptionspectrum. The wavelength dependence of the first hyperpolarizability cannot be explained by an undamped or damped two level model. However, incorporating vibrational motion in the damped two-level model results in a good fit between theoretical and experimentally measured β, especially in the vicinity of TPR region.
114(2001); http://dx.doi.org/10.1063/1.1359182View Description Hide Description
A new self-consistent closure is formulated for the additive hard sphere mixtures at high densities within the Ornstein–Zernike integral equation approach. Diameter ratios from 0.3 to 0.9 and several compositions are examined. The consistencies include the thermodynamic ones (e.g., pressure consistency, and Gibbs–Duhem relation), and the structural ones (e.g., the zero-separation theorems). The bridge functions have built-in “flexibility” that can be adapted to the consistency requirements. Comparison with Monte Carlo simulation shows that the present closure yields highly accurate results. The contact values and zero-separation values are more accurate than those obtained from the conventional closures, such as the Percus–Yevick and Martynov–Sarkisov closures. A structural theory for hard sphere mixtures has been formulated that is accurate and consistent at the same time.
114(2001); http://dx.doi.org/10.1063/1.1358877View Description Hide Description
Wide band dielectric spectroscopy has been used to study the bulk dynamics of the simple supercooled liquidm-toluidine. Following the temperature and frequency dependence of the dielectric response we find a stretched relaxation process, the stretching parameter of which is temperature dependent. The observed decoupling between rotational and translational diffusion parameters at low temperatures may be the result of heterogeneous dynamics.
The process in simple glass forming liquid II. The temperature dependence of the mechanical response114(2001); http://dx.doi.org/10.1063/1.1358878View Description Hide Description
We have measured, as a function of temperature and frequency, the velocity and attenuation of sound down to the glass transition of the fragile glass-forming molecular liquidm-toluidine. The mechanical relaxation process is compared with the dielectric one recently studied on the same substance. The Arrhenius plot of vs shows that the relaxation time associated with ultrasonic measurements is the same as that previously found by the dielectric spectroscopy in the same temperature region.
Semiclassical molecular dynamics computation of spontaneous light emission in the condensed phase: Resonance Raman spectra114(2001); http://dx.doi.org/10.1063/1.1357205View Description Hide Description
The three-time correlation function that describes resonance Raman (RR) spectra is computed directly using the Herman–Kluk semiclassical propagator. The trace expression for this correlation function allows forward–backward time propagation of trajectories over closed time-circuits, leading to efficient convergence in multidimensional systems. A local harmonic approximation is used to derive an expression for the density operator in the coherent state representation This allows efficient sampling of phase space as well as simulations at arbitrary temperatures and in arbitrary coordinates. The resulting method is first analyzed for a one-dimensional problem, where the results are shown to be in excellent agreement with exact quantum calculations. The method is then applied to the problem of RR scattering of iodine in the condensed phase. The RR spectrum of an molecule in a xenon fluid at 230 K is calculated and also found to be in excellent agreement with experiment.