Volume 119, Issue 5, 01 August 2003
Index of content:
- Theoretical Methods and Algorithms
119(2003); http://dx.doi.org/10.1063/1.1586913View Description Hide Description
This paper demonstrates the excellent temperature control and rapid equipartioning of the kinetic energy of the massive generalized Gaussian moment thermostat (MGGMT, one thermostat is coupled to each degree of freedom) in isothermal density functionalmolecular dynamics (MD) simulations on the Born–Oppenheimer potential energy surface. The MGGMT is implemented in the approach and, as far as we know, it is the first time in literature that the MGGMT is combined with density functional methods. The performance of the MGGMT approach is illustrated with MD simulations of the iron porphyrin–imidazole–carbon monoxide [FeP(Im)(CO)] complex and compared with constant energy MD simulations on the same system. Both MD approaches lead to similar average structures of the FeP(Im)(CO) complex. The examination of the frequency distribution functions reveals that the structural dynamics are not seriously affected by the dynamics of the parameters introduced by the MGGMT. The equipartitioning rates in the MGGMT simulations are significantly faster than in the constant energy simulation. We recommend the MGGMT approach as an very efficient equilibration technique in MD simulations and it emerges as a useful technique for, e.g., simulated annealing and nonequilibrium MD simulations.
119(2003); http://dx.doi.org/10.1063/1.1587124View Description Hide Description
A quantum-classical approximation, capable of describing the evolution of open quantum systems well beyond the applicability limits of Redfield theory is suggested. The theory is based on the short lifetime of the quantum correlations between the quantum and the classical subsystem, caused by energy dispersion (per degree of freedom) of the canonical bath. The resulting quantum-classical approximation has the form of two auxiliary differential equations and fully accounts both for the arbitrary long memory of the heat reservoir and detailed balance. These equations allow direct solution in the time domain without constructing/diagonalizing Liouville space operators, and, in combination with molecular dynamics techniques to simulate bath dynamics, may be applied to quantum subsystems with a fairly large number of levels. A simple example of a two-level system, coupled to a single correlation time canonical bath, was considered to demonstrate different regimes of approaching the canonical equilibrium state.
Molecular dynamics approach to vibrational energy relaxation: Quantum-classical versus purely classical nonequilibrium simulations119(2003); http://dx.doi.org/10.1063/1.1587125View Description Hide Description
We present an efficient method for the direct solution in the time domain of the equations of a novel recently proposed non-Markovian quantum-classical approximation, valid well beyond the applicability limits of both Redfield theory and Fermi’s Golden Rule formula. The method is based on an ab initiomolecular dynamics description of the classical bath and is suitable for applications to systems with a fairly large number of quantum levels. A simple model of the breathing sphere in a Lennard-Jones fluid was used to compare the results of the quantum-classical and purely classical treatments of vibrational energy relaxation.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
119(2003); http://dx.doi.org/10.1063/1.1584671View Description Hide Description
We compare theoretical results on statistical electron emission from electronically hot but vibrationally cold with recent experimental results involving excitation with ultrashort laser pulses. Both photoelectron spectra and ion yields of as well as fragment ions are compared with the predictions of the statistical electron emission model. Quantitative agreement is obtained between the calculations and the experimentally measuredphotoelectron spectra, total ion yields and singly-doubly charged branching ratio. The electron-phonon coupling time is fitted to a few hundred femtoseconds, consistent with independent measurements. The data allow a determination of the thermal properties of the electron system, which are consistent with the theoretical input. The data also allow a fit of the averaged photon absorption cross section.
Conformations of indan and 2-indanol: A combined study by UV laser spectroscopy and quantum chemistry calculation119(2003); http://dx.doi.org/10.1063/1.1587111View Description Hide Description
Three conformational isomers of 2-indanol are identified by use of resonance enhanced two-photonionization (R2PI) and single vibronic level dispersed fluorescence spectroscopy in a supersonic jet expansion. By combining the experimental results with the predictions of the ab initio quantum chemistry calculations at the level of theory, the major species is identified as a conformational isomer in which the hydroxyl hydrogen is involved in an intramolecular hydrogen bonding with the π-electrons of the aromatic ring. The theoretical estimate of the hydrogen bond energy is ∼6.5 kJ/mol. A comparative investigation with indan reveals that this weak hydrogen bonding in the former significantly affects the puckering potential of the five-member side ring. The dispersed fluorescence data indicate for a much higher ring-puckering barrier in the ground state than what has been suggested recently by measuring rotational spectra of the unsubstituted indan.
119(2003); http://dx.doi.org/10.1063/1.1587115View Description Hide Description
Presented in this work are the results of a quantum chemical study of oxygen adsorption on small and clusters. Density functional theory(DFT), second order perturbation theory (MP2), and singles and doubles coupled clustertheory with perturbative triples [CCSD(T)] methods have been used to determine the geometry and the binding energy of oxygen to The multireference character of the wave functions has been studied using the complete active space self-consistent field method. There is considerable disagreement between the oxygen binding energies provided by CCSD(T) calculations and those obtained with DFT. The disagreement is often qualitative, with DFT predicting strong bonds where CCSD(T) predicts no bonds or structures that are bonded but have energies that exceed those of the separated components. The CCSD(T) results are consistent with experimental measurements, while DFT calculations show, at best, a qualitative agreement. Finally, the lack of a regular pattern in the size and the sign of the errors [as compared to CCSD(T)] is a disappointing feature of the DFT results for the present system: it is not possible to give a simple rule for correcting the DFT predictions (e.g., a useful rule would be that DFT predicts stronger binding of by about 0.3 eV). It is likely that the errors in DFT appear not because of gold, but because oxygen binding to a metal cluster is a particularly difficult problem.
119(2003); http://dx.doi.org/10.1063/1.1587112View Description Hide Description
The title reaction was investigated in a crossed-beam experiment. A resonance-enhanced multiphon ionization technique was used to interrogate the internal-state distributions of the product at three different collision energies. Only the (umbrella) mode excitation was observed. Its distribution changes from a monotonically declined distribution at low energy to a slightly inverted one at higher collision energy. Although the rotational excitations of were small, a strong preference for was found, indicative of the dominance of the tumbling rotation motion of the product. The vibration-resolved excitation functions were also measured for A reaction barrier of 0.5 kcal/mol was deduced.
Ab initio derived analytical fits of the two lowest triplet potential energy surfaces and theoretical rate constants for the system119(2003); http://dx.doi.org/10.1063/1.1586251View Description Hide Description
This work presents two new analytical fits of the ground potential energy surface (PES) and the first excited PES involved in the title reaction, considering the N-abstraction (1) and the O-abstraction (2) reaction channels, and the reverse reaction (−1). The PESs are derived from ab initio electronic structure calculations by means of second-order perturbation theory on a complete active-space self-consistent-field wave function (CASPT2 method). Stationary points and extensive grids of ab initio points (about 5600 points for the PES and 4900 points for the PES) were fitted along with some diatomic spectroscopic data to better account for the experimental exoergicity. Thermal rate constants were calculated (200–5000 K) for all mentioned reaction processes by means of the variational transition-state theory with the inclusion of a semiclassical tunneling correction. Excellent agreement with the experimental data was observed for reaction (1) and its reverse, within all the temperature range, substantially improving the results derived from previous analytical PESs. The contribution of the PES to the reaction rate constant was small even at high temperatures (e.g., only 10.8% at 2500 K). Moreover, the main contribution to reaction rate constant was due to the PES, differing from what happens for reaction (1). The O-abstraction reaction channel accounts for a 3.0% of the total reaction at 5000 K, consistent with the very limited experimental information available.
Laser-induced fluorescence, electronic absorption, infrared and Raman spectra, and ab initio calculations of 1,2-dihydronaphthalene: Investigation of the out-of-plane ring modes for the ground and excited states119(2003); http://dx.doi.org/10.1063/1.1586697View Description Hide Description
The laser-induced fluorescencespectra and dispersed fluorescence spectra of jet-cooled 1,2-dihydronaphthalene have been analyzed to investigate the ring inversion process in both the and excited states. Ultraviolet absorption, infrared, and Raman spectra were also recorded to complement the analyses.Ab initio calculations predict the inversion process to involve four out-of-plane ring motions, and linear combinations of these were made to model the inversion process. The data show the barrier to inversion in the ground state to be (the triple-zeta ab initio value is The experimental data indicate that the barrier increases substantially in the excited state, for which the calculated barrier is with a basis set.
Instantaneous nonvertical electronic transitions with shaped femtosecond laser pulses: Is it possible?119(2003); http://dx.doi.org/10.1063/1.1586701View Description Hide Description
In molecular electronic transitions, a vertical transition can be induced by an ultrashort laser pulse. That is, a replica of the initial nuclear state—times the transitiondipole moment of the electronic transition—can be created instantaneously (on the time scale of nuclear motion) in the excited electronic state. Now, applying pulse shaping via the modulation of the phases of each spectral component of an ultrashort pulse, it is tempting to ask whether it is also possible to induce instantaneous nonverticaltransitions to bound electronic states, provided that the phases of each spectral component of the pulse are set to appropriate values at the discrete frequencies corresponding to the energy levels of the potential. We analyze the problem in the weak-field limit, and show that such a phase requirement cannot be encoded into an ultrashort pulse. This result is equivalent to the statement that it is not possible to move matter faster than the time associated with the natural (field-free) dynamics of the system.
119(2003); http://dx.doi.org/10.1063/1.1587113View Description Hide Description
A clear explanation for an anomalous isotope effect in ozone formation is given in terms of the energy transfer mechanism, where the metastable states of ozone are formed first, and then stabilized by collisions with other atoms. Unusual nonstatistical properties of metastable states spectra discovered earlier [J. Chem. Phys. 118, 6298 (2003)] are incorporated into the kinetics model, where different metastable states are treated as different species, and the stabilization step is treated approximately. The population of the ozone metastable states builds up and decays through three possible channels. When different isotopes of oxygen are involved the three channels become open at different energies because of the differences in the quantum zero-point-energies (ΔZPE) of the different molecules. The spectrum of metastable states is anomalously dense below the ΔZPE threshold and these states are accessible only from the lower entrance channel. Also, these low-lying metastable states are stabilized very efficiently (by collisions with third body) because they are energetically close to the bound states. Such processes significantly enhance the formation rates of ozone isotopologues through the lower channels over the formation rates through the upper channels. Numerical results obtained for give isotope effects in the right direction and of the right order of magnitude. Consideration of should improve the comparison with experiment.
First determination of the stretching frequencies by jet cooled intracavity laser absorption spectroscopy around119(2003); http://dx.doi.org/10.1063/1.1587116View Description Hide Description
Using intracavity laser absorption spectroscopy combined with a supersonic slit jet, we have observed and analyzed seven vibronic cold bands of between 10 800 and The vibronic energies, relative intensities, and rotational constants, and the spin-rotation constants have been determined. The rotational constants play a crucial role in the vibronic assignments. The seven observed states are vibronically mixed. However, three of them have a dominant electronic character while the four others have a dominant character. The vibrational assignments and energies of the three levels are: (1, 0, 0) at (0, 2, 0) at and (0, 0, 2) at The frequencies of the symmetric stretch, and bending, modes derived from the (1, 0, 0) and (0, 2, 0) levels are in agreement with ab initio calculations. In contrast, the frequency of the antisymmetric stretch, derived from the observed (0, 0, 2) level, agrees only with the ab initio value obtained by Kaldor while other ab initio values ranging from 390 to were reported. The four levels with a dominant character have been tentatively vibrationally assigned. The present experimental data allows for a first experimental determination of the two stretch vibrational frequencies which are necessary to model the vibronic interactions, i.e., the conical intersection between and potential energy surfaces.
A realistic multi-sheeted potential energy surface for from the double many-body expansion method and a novel multiple energy-switching scheme119(2003); http://dx.doi.org/10.1063/1.1586911View Description Hide Description
We report a new multi-sheeted double many-body expansion potential energy surface that reproduces most known topological features of the title system. Near spectroscopic accuracy is conveyed to the ground-state sheet of in symmetry in the vicinity of the minimum by merging it with a spectroscopically determined Taylor-series-expansion-type form via a novel multiple energy-switching scheme. A high energy ridge for insertion of into has also been imposed to mimic the result of accurate ab initio complete active space self-consistent field and second-order perturbation theory on CAS wave function calculations carried out for such geometries. This ridge decreases for geometries yielding a minimum barrier height for the reaction of 0.273 eV at a bent N–O–O structure defined by and Both the location and height of this barrier are in good agreement with existing ab initio calculations and the recommended values. Another salient feature is a shallow minimum on the potential energy surface that is separated from the absolute minimum by a conical intersection. Such a feature is accurately predicted by the newly reported ab initio calculations and well mimicked by the global double many-body expansion/energy-switching potential energy surface here reported. This is therefore commended both for spectroscopic and reactive dynamics studies on the title system. A final comment in relation to the conical intersection and the energy-switching scheme goes to the expected accuracy of current approaches for spectroscopically determined effective single-valued forms.
119(2003); http://dx.doi.org/10.1063/1.1580112View Description Hide Description
We report new calculations and measurements of fluorescenceproperties of the title system, which is important in astrochemical processes. Dispersed fluorescence spectra show extensive vibrational progressions that depend on the initial state. Observed and calculated lifetimes are in good accord, save for (bent molecule notation), and calculated lifetimes are longer than the ones. The calculated laser-induced fluorescencespectrum is compared with experimental absorption data and with previous calculations, finding that the present treatment underestimates the intensity of the (0,4,0)Σ band.
Examination of the correlation energy and second virial coefficients from accurate ab initio calculations of rare-gas dimers119(2003); http://dx.doi.org/10.1063/1.1587684View Description Hide Description
Calculations of rare-gas dimers (He–He, Ne–Ne, Ar–Ar, He–Ne, He–Ar, and Ne–Ar) at the coupled-cluster single double (triple) level of theory with large basis sets including bond functions and counterpoise corrections are reported over a wide range of 100 internuclear separations. These results are compared to experimental curves obtained from fitting to rovibrational spectra, and to second virial coefficients and Boyle temperatures. Accurate analytic potentials are developed for the total interactionenergy, Hartree–Fock (exchange) energy, and correlation(dispersion)energy; the transferability of the latter is demonstrated to very high accuracy even in the region of considerable wave function overlap. These calculations represent an important set of benchmarks that can be used to develop improved empirical molecular mechanical force fields and new quantum models.
Perturbational ab initio calculations of relativistic contributions to nuclear magnetic resonance shielding tensors119(2003); http://dx.doi.org/10.1063/1.1586912View Description Hide Description
We present perturbational ab initio calculations of the leading-order one-electron relativistic contributions to the nuclear magnetic resonance shielding tensor based on the Pauli Hamiltonian. The scalar relativistic and spin–orbit interaction effects, including both relativistic corrections to the wave function (“passive” relativistic effects) and relativistic magnetic perturbation operators (“active” effects), are considered for (X=O, S, Se, Te, Po), HX (X=F, Cl, Br, I, At), and noble gas (Ne, Ar, Kr, Xe, Rn) systems. The perturbational corrections are calculated using linear and quadratic response theory applied to nonrelativistic reference states. We use the uncorrelated self-consistent field as well as correlated, multiconfigurational complete active space self-consistent field models. Results for the and heavy-atom shielding constants and anisotropies are compared with Dirac–Hartree–Fock and quasirelativistic data.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
119(2003); http://dx.doi.org/10.1063/1.1585017View Description Hide Description
Integral equationtheories have been quite successful at providing structural information for isotropic fluids. In particular, the expansion method originally proposed by Blum and Torruella [J. Chem. Phys. 56, 303 (1972)] has been pivotal in making these theories feasible for molecular fluids. Recently, integral equationtheories have also been applied to nematic phases for simple one-component fluids. In this article, we derive the formalism appropriate for fully, and partially, orientationally anisotropic fluids. Appropriate expansions of the pair and singlet functions are employed to obtain tractable expressions for the three coupled equations which must be solved. The numerical solution of these equations is discussed and simplifications resulting from symmetry are considered in detail. Numerical results are presented for fluids of dipolar ellipsoidal molecules in an external field.
Condensed-phase effects on absolute cross sections for dissociative electron attachment to CFCs and HCFCs adsorbed on Kr119(2003); http://dx.doi.org/10.1063/1.1587688View Description Hide Description
We present measurements of absolute dissociative electron attachment (DEA) cross sections to and adsorbed on the surface of Kr as a function of electron energy (0–10 eV). The DEA cross sections are measured to be at ∼0 eV, at 0.65 eV, and at 0.89 eV for and respectively. This cross section is similar to the gas-phase value for while for the latter two molecules, it is orders of magnitude higher than the gaseous values. These results can be explained by considering the changes in the survival probability of the anion resonance and in the electron capture probability due to the decrease of the nuclear wave function overlap in the Franck–Condon region.
119(2003); http://dx.doi.org/10.1063/1.1584665View Description Hide Description
Magic angle sample spinning (MASS) averages the inhomogeneous magnetic field due to the susceptibility contrast in porous media by modulating the local magnetic field(frequency modulation). Molecular diffusion introduces a homogeneous broadening, which modulates the amplitude of the signal (amplitude modulation). The depth of amplitude modulation is determined by the interplay of molecular diffusion and MASS averaging and contains rich information on local magnetic fields, through which the spatial structure of the sample may be obtained. In this paper, we present two methods to quantify the amplitude modulation: a phase suppressed method and a more conventional two-dimensional (2D)-exchange method. The phase suppressed method directly observes the amplitude modulation in the time domain. The approximate equations are derived to extract the physical information from the time domain data. The conventional 2D-exchange spectrum contains both the frequency modulation and the amplitude modulation terms. The amplitude modulation introduces cross peaks in the 2D spectrum. The integration of the 2D spectrum along lines parallel to the main diagonal line will give a one-dimensional spectrum, that is the Fourier transform of the amplitude modulation.
119(2003); http://dx.doi.org/10.1063/1.1587130View Description Hide Description
We use a fast Fourier transform block Lanczos diagonalization algorithm to study the electronic states of excess electrons in fluid alkanes (methane, ethane, and propane) and in a molecular model of amorphous polyethylene (PE) relevant to studies of space charge in insulating polymers. We obtain a new pseudopotential for electron–PE interactions by fitting to the electronic properties of fluid alkanes and use this to obtain new results for electron transport in amorphous PE. From our simulations, while the electronic states in fluid methane are extended throughout the whole sample, in amorphous PE there is a transition between localized and delocalized states slightly above the vacuum level (∼+0.06 eV). The localized states in our amorphous PE model extend to −0.33 eV below this level. Using the Kubo–Greenwood equation we compute the zero-field electron mobility in pure amorphous PE to be Our results highlight the importance of electron transport through extended states in amorphous regions to an understanding of electron transport in PE.