Showing papers in "Journal of Physical Chemistry A in 1997"
TL;DR: In this paper, the potential energy surface is transformed into a collection of interpenetrating staircases, and the lowest known structures are located for all Lennard-Jones clusters up to 110 atoms, including a number that have never been found before in unbiased searches.
Abstract: We describe a global optimization technique using “basin-hopping” in which the potential energy surface is transformed into a collection of interpenetrating staircases. This method has been designed to exploit the features that recent work suggests must be present in an energy landscape for efficient relaxation to the global minimum. The transformation associates any point in configuration space with the local minimum obtained by a geometry optimization started from that point, effectively removing transition state regions from the problem. However, unlike other methods based upon hypersurface deformation, this transformation does not change the global minimum. The lowest known structures are located for all Lennard-Jones clusters up to 110 atoms, including a number that have never been found before in unbiased searches.
2,637 citations
TL;DR: In this article, a simple analytical formula for calculating Born radii rapidly and with useful accuracy is presented. But it is based on an atomic pairwise rij-4 treatment and contains several empirically determined parameters that were established by optimization against a data set of >10,000 accurate Born radi computed numerically using the Poisson equation on a diverse group of organic molecules, molecular complexes, oligopeptides, and a small protein.
Abstract: Atomic Born radii (α) are used in the generalized Born (GB) equation to calculate approximations to the electrical polarization component (Gpol) of solvation free energy. We present here a simple analytical formula for calculating Born radii rapidly and with useful accuracy. The new function is based on an atomic pairwise rij-4 treatment and contains several empirically determined parameters that were established by optimization against a data set of >10 000 accurate Born radii computed numerically using the Poisson equation on a diverse group of organic molecules, molecular complexes, oligopeptides, and a small protein. Coupling this new Born radius calculation with the previously described GB/SA solvation treatment provides a fully analytical solvation model that is computationally efficient in comparison with traditional molecular solvent models and also affords first and second derivatives. Tests with the GB/SA model and Born radii calculated with our new analytical function and with the accurate but ...
924 citations
TL;DR: In this paper, a comparison between traditional quantum chemical approaches to the electron correlation problem and the one taken in density functional theory (DFT) is made, and the definitions of exchange and correlation in DFT are compared with the traditional ones.
Abstract: A comparison is made between traditional quantum chemical approaches to the electron correlation problem and the one taken in density functional theory (DFT). Well-known concepts of DFT, such as the exchange−correlation energy Exc = ∫ρ(r) exc(r) dr and the exchange−correlation potential vxc(r) are related to electron correlation as described in terms of density matrices and the conditional amplitude (Fermi and Coulomb holes). The Kohn−Sham one-electron or orbital model of DFT is contrasted with Hartree−Fock, and the definitions of exchange and correlation in DFT are compared with the traditional ones. The exchange−correlation energy density exc(r) is decomposed into kinetic and electron−electron potential energy components, and a practical way of calculating these from accurate wave functions is discussed, which offers a route to systematic improvement. vxc(r) is likewise decomposed, and special features (bond midpoint peak, various types of step behavior) are identified and related to electronic correlation.
563 citations
TL;DR: In this article, the photocatalytic decomposition efficiency of gaseous 2-propanol was studied using a titanium dioxide thin film under very weak UV light; the incident UV light intensity was 36 nW−45 μW·cm-2.
Abstract: The photocatalytic decomposition efficiency of gaseous 2-propanol was studied using a titanium dioxide thin film under very weak UV light; the incident UV light intensity was 36 nW−45 μW·cm-2. Under such low-intensity UV illumination, the value of the quantum yield (QY) increased gradually with decreasing number of absorbed photons and finally saturated (28%) for a number of absorbed photons less than 4 × 1011 quanta·cm-2·s-1 for an initial 2-propanol concentration of 1000 ppmv. Thus, purely light-limited conditions were reached. For lower initial concentrations, the QY values decreased, but the same maximum QY value as that for 1000 ppmv was also approached with decreasing light intensity. We discuss these results in terms of the normalized absorbed photon number (Inorm/s-1), a parameter that we have defined as the ratio of the number of absorbed photons ([photon]ab) to the number of adsorbed 2-propanol molecules ([M]ad). When all of the experimental QY values were plotted as a function of Inorm, all of ...
445 citations
TL;DR: In this article, the effect of nonorthogonality in the broken symmetry approach to magnetic coupling has been explicitly considered for the first time in Hartree−Fock and a variety of DFT methods.
Abstract: The effect of nonorthogonality in the broken symmetry approach to magnetic coupling has been explicitly considered for the first time in Hartree−Fock and a variety of DFT methods. On the basis of the results for three different systems, representative of a variety of physical situations it is shown that the most often quoted trend concerning the much larger degree of delocalization of magnetic orbitals obtained from DFT, as opposed to Hartree−Fock, is not fully justified. A new and simple way to relate the overlap integral entering into the calculation and the spin density is proposed and tested in a variety of model systems.
405 citations
TL;DR: In this paper, the authors performed density functional theory (DFT) calculations of iron−porphyrin and its complexes with O2, CO, NO, and imidazole (Im).
Abstract: We have performed density functional theory (DFT) calculations of iron−porphyrin (FeP) and its complexes with O2, CO, NO, and imidazole (Im). Our fully optimized structures agree well with the available experimental data for synthetic heme models. Comparison with crystallographic data for proteins highlights interesting features of carbon monoxymyoglobin. The diatomic molecule induces a 0.3−0.4 A displacement of the Fe atom out of the porphyrin nitrogen (Np) plane and a doming of the overall porphyrin ring. The energy of the iron−diatomic bond increases in the order Fe−O2 (9 kcal/mol) < Fe−CO (26 kcal/mol) < Fe−NO (35 kcal/mol). The ground state of FeP(O2) is an open shell singlet. The bent Fe−O2 bond can be formally described as FeIII−O2-, and it is characterized by the anti-ferromagnetic coupling between one of the d electrons of Fe and one of the π* electrons of O2. FeP(CO) is a closed shell singlet, with a linear Fe−C−O bond. The complex with NO has a doublet ground state and a Fe−NO geometry intermed...
352 citations
TL;DR: The topological analysis of the electron localization function (ELF) provides a convenient theoretical framework to characterize chemical bonds as mentioned in this paper, which can be applied to exact wave functions as well as to experimental electron densities.
Abstract: The topological analysis of the electron localization function (ELF) provides a convenient theoretical framework to characterize chemical bonds. This method does not rely on the particular approximations that are made in actual quantum chemical calculations of the electronic structure. In principle, it can be applied to exact wave functions as well as to experimental electron densities. Introduction of a control space, such as a set of reaction pathways, allows extension of the analysis to chemical reactions. The study of the bifurcations occurring during such processes is of particular interest for their classification and their qualitative description. This is achieved with the help of Rene Thom's catastrophe theory. The following examples are discussed: the ammonia inversion, the breaking of the ethane C−C bond, and the breaking of the dative bond in NH3BH3. The types of catastrophe and their unfolding have been determined for each of these processes. As by-products, nonempirical definitions of covale...
334 citations
TL;DR: In this article, the second-order quadrupolar line shape for the central transition is split into a comb of sidebands leading to a considerable increase in the sensitivity compared to a conventional QE spectrum.
Abstract: A novel approach to quadrupolar-echo (QE) NMR of half-integer quadrupolar nuclei in static powders is analyzed. By acquisition of the QE spectrum during a Carr−Purcell−Meiboom−Gill (CPMG) train of selective π pulses, the second-order quadrupolar line shape for the central transition is split into a comb of sidebands leading to a considerable increase in the sensitivity compared to a conventional QE spectrum. The applicability of the method for determination of magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors is examined. Through numerical simulation and iterative fitting of experimental 87Rb (RbClO4 and RbVO3) and 59Co spectra (Co(NH3)5 Cl3), it is demonstrated that the quadrupolar CPMG experiment represents a useful method for studying half-integer quadrupolar nuclei exhibiting large quadrupolar coupling combined with anisotropic chemical shielding interactions. Sensitivity enhancements by a factor of up to about 30 are observed for the samples studied.
322 citations
TL;DR: In this paper, the rotational anisotropy decays of coumarin 153 are generally nonexponential as a result of the non-Markovian nature of the friction on its rotational motion.
Abstract: Subpicosecond fluorescence anisotropy measurements are used to characterize the rotational dynamics of coumarin 153 (C153) in 35 common solvents and eight solvent mixtures at room temperature. The rotational anisotropy decays of C153 are generally nonexponential as a result of the non-Markovian nature of the friction on its rotational motion. Rotational correlation times are observed to be larger in polar solvents than in nonpolar solvents of the same viscosity. This difference is examined in the context of theories of dielectric friction, which relate the extra friction in polar solute/solvent systems to long-range dipole -dipole interactions. Since the latter interactions have been thoroughly characterized via dynamic Stokes shift measurements for the same solute/solvent combinations studied here, the present data provide a unique opportunity to test general concepts of dielectric friction. Contrary to expectations, the departures from simple hydrodynamic behavior cannot be modeled using only theories of rotational dielectric friction. More important than dielectric friction is the role that the relative solute/solvent size plays in determining the extent of solute -solvent coupling. Once this size dependence is approximately accounted for, the remaining departures from simple hydrodynamic behavior are relatively small in all solvents. In polar aprotic solvents, solvation data indicate that dielectric friction effects should be rather modest (10-20% of the total friction). In these solvents no clear correlation is found between dielectric friction predictions and the observed solute -solvent coupling. However, in normal alcohol solvents the effects of dielectric friction are predicted to be large and well beyond the scatter in the experimental data. No evidence for such an important dielectric friction contribution is observed in these solvents, in spite of the fact that long-time components of the solvation dynamics do appear to be present in the rotational friction.
321 citations
TL;DR: The current lineup of popular density functional theories, in particular those based on Becke's exchange functionals, fail to predict a correct dissociation behavior in radical ions where charge and spin must be separated (model: H2•+) or where both must be localized on one fragment (Model: He2•+) as discussed by the authors.
Abstract: The current lineup of popular density functional theories, in particular those based on Becke's exchange functionals, fail to predict a correct dissociation behavior in radical ions where charge and spin must be separated (model: H2•+) or where both must be localized on one fragment (model: He2•+). The repercussions of this on the location of certain transition states on radical ion potential energy surfaces are pointed out.
305 citations
TL;DR: Two electron correlation theories, second-order Moller−Plesset perturbation (MP2) and density functional (DFT) methods have been adopted to obtain fully optimized structures of styrene, trans-stil...
Abstract: Two electron correlation theories, second-order Moller−Plesset perturbation (MP2), and density functional (DFT) methods have been adopted to obtain fully optimized structures of styrene, trans-stil...
TL;DR: In this paper, the instantaneous normal mode (INM) approach to liquid state dynamics is presented, and the underlying physical ideas, including the importance of the potential ensembles, are discussed.
Abstract: The instantaneous normal mode (INM) approach to liquid state dynamics is presented. INM is put in historical context, and the underlying physical ideas, including the importance of the potential en...
TL;DR: The fully relaxed single-bond torsional potentials in typical conjugated systems were evaluated with the aid of ab initio self-consistent field and Moller−Plesset second-order calculations and, additionally, with several recently developed variants of the density functional theory as discussed by the authors.
Abstract: The fully relaxed single-bond torsional potentials in typical conjugated systems were evaluated with the aid of ab initio self-consistent-field and Moller−Plesset second-order calculations and, additionally, with several recently developed variants of the density functional theory. For this systematic investigation, 1,3-butadiene, styrene, biphenyl, 2,2‘-bithiophene, 2,2‘-bipyrrole and 2,2‘-bifuran have been selected as model molecules. As representative examples for nonconjugated systems, the molecules n-butane and 1-butene have been treated at the very same calculational levels. For all conjugated molecules, the electron correlation corrections to the self-consistent-field torsional potentials, as obtained with the density functional methods, are dramatically different from those resulting from the more conventional Moller−Plesset second-order approximation. For those cases where experimental data for torsional barriers are available, the self-consistent-field and the Moller−Plesset second-order results...
TL;DR: In this paper, the authors compare the performance of isothermal and nonisothermal experiments for the decomposition of ammonium dinitramide (ADN) and show that the identical dependencies of activation energy on the extent of conversion are not obtained for isothermal or non-isothermal data.
Abstract: Thermogravimetric data for the decomposition of ammonium dinitramide (ADN) have been obtained under isothermal and nonisothermal conditions in order to determine the efficacy of different methods for analyzing the kinetics of solid-state reactions. A widely used model-fitting method gives excellent fits to the experimental data but yields highly uncertain values of the Arrhenius parameters when applied to nonisothermal data because temperature and extent of conversion are not independent variables. Therefore, comparison of model fitting results from isothermal and nonisothermal experiments is practically meaningless. Conversely, model-free isoconversional methods of kinetic analysis yield similar dependencies of the activation energy on the extent of conversion for isothermal and nonisothermal experiments. Analysis of synthetic data generated for a complex kinetic model suggests that, in the general case, the identical dependencies are unlikely to result from experiments obtained under isothermal and noni...
TL;DR: In this paper, an implementation of the g-tensor of electron paramagnetic resonance (EPR) spectroscopy is presented, based on density functional theory (DFT) and the use of gauge-including atomic orbitals (GIAO).
Abstract: An implementation of the g-tensor of electron paramagnetic resonance (EPR) spectroscopy is presented. This implementation is based on density functional theory (DFT) and the use of gauge-including atomic orbitals (GIAO). Contributions from the spin−other-orbit operators are neglected, while all the other relevant perturbation operators are included. The new method is an extension of an existing DFT−GIAO program package for the calculation of the chemical shift of nuclear magnetic resonance spectroscopy; full use is made of the conceptual analogy between the g-tensor and the chemical shift. The new program is applied to various small radicals. The agreement of calculated and experimental g-tensors is good for radicals of first-row elements; experimental trends are generally well reproduced. The quality of calculated results is worse if the scheme is applied to compounds of heavier elements. Possible reasons for these apparent shortcomings of the method are discussed.
TL;DR: In this paper, the authors investigated the reliability of several widely used density functional theory (DFT) functionals and concluded that extreme care must be taken in the choice of the functional since only those that behave properly at large and intermediate values of the reduced density gradient s give relevant results.
Abstract: Although density functional theory (DFT) is more and more commonly used as a very efficient tool for the study of molecules and bulk materials, its applications to weakly bonded systems remain rather sparse in the literature, except studies that consider hydrogen bonding. It is, however, of essential interest to be able to correctly describe weaker van der Waals complexes. This prompted us to investigate more precisely the reliability of several widely-used functionals. The equilibrium geometries and the binding energies of C6H6···X (X = O2, N2, or CO) complexes are determined within the standard Kohn−Sham approach of DFT using different exchange−correlation functionals and at the MP2 level of theory for comparison. It is comprehensively concluded that extreme care must be taken in the choice of the functional since only those that behave properly at large and intermediate values of the reduced density gradient s give relevant results. The PW91 exchange functional, the enhancement factor of which does not diverge at increasing s, appears as the most reliable for the studied systems. It is furthermore demonstrated that the quality of the DFT results is determined by the exchange energy component of the total energy functional.
TL;DR: In this paper, a molecular dynamics simulation of a hydrated phosphatidylcholine bilayer membrane in the liquid crystalline phase was studied using H-bonding between water and DMPC molecules, and it was shown that water bridges are involved in reducing head group mobility and in stabilizing the membrane structure.
Abstract: Hydrogen (H-) bonding between water and phosphatidylcholine was studied using a molecular dynamics simulation of a hydrated phosphatidylcholine bilayer membrane in the liquid crystalline phase. A membrane in the liquid-crystalline phase composed of 72 L-R-dimyristoylphosphatidylcholine (DMPC) and 1622 water molecules was generated, starting from the crystal structure of DMPC. At the beginning of the equilibration process, the temperature of the system was raised to 550 K for 20 ps, which was effective in breaking the initial crystalline structure. The thermodynamic and structural parameters became stable after the equilibration period of 1100 ps, and the trajectory of the system obtained during the following 500 ps agreed well with most of the published experimental data. Each DMPC molecule forms 5.3 H-bonds with water, while only 4.5 water molecules are H-bonded to DMPC. The primary targets of water for the formation of H-bonds are the non-ester phosphate oxygens (4.0 H-bonds) and the carbonyl oxygens (1.0 H-bonds). Of DMPC’s H-bonds, 1.7 are formed with water molecules that are simultaneously H-bonded to two different DMPC oxygens (bridging water). In effect, approximately 70% of the DMPC molecules are linked by water molecules and form clusters of two to seven DMPC molecules. Approximately 70% of the intermolecular water bridges are formed between non-ester phosphate oxygens. The rest are formed between non-ester phosphate and carbonyl oxygens. About half of the intermolecular water bridges are involved in formation of multiple bridges, where two DMPC molecules are linked by more than one parallel bridge. These results suggest a possibility that water bridges are involved in reducing head group mobility and in stabilizing the membrane structure. Non-ester phosphate oxygen of DMPC makes one, two, or three H-bonds with water, but two H-bonds are formed most often (60%). In the case where two H-bonds are formed on non-ester phosphate or carbonyl oxygens, the average geometry of H-bonding is planar trigonal (in the case of water oxygen with two H-bonds, geometry is steric tetragonal). When oxygen atoms form three H-bonds, the geometry of H-bonding is steric tetragonal both for non-ester phosphate and water oxygens. On average, H-bonds make nearly right angles with each other when two or three water molecules are bound to the same DMPC oxygen, but the distribution of the angle is broad.
TL;DR: A detailed analysis of the changes in the electronic structure of CO when a proton or a positive charge approaches the carbon or the oxygen atom is reported using quantum mechanical ab initio calculations and several methods to analyze the theoretical data as mentioned in this paper.
Abstract: A detailed analysis of the changes in the electronic structure of CO when a proton or a positive charge approaches the carbon or the oxygen atom is reported using quantum mechanical ab initio calculations and several methods to analyze the theoretical data. The C−O bond is shortened by nearly the same amount in HCO+ and QCO+ compared to free CO, while the nearly identical C−O bond lengths of COH+ and COQ+ are longer than in CO. H+ and Q+ have a strong electrostatic effect upon the atom to which they are bonded, which leads to an increased electronegativity of carbon and oxygen, respectively. Inspection of the charge distribution and the natural localized orbitals shows clearly that the shorter C−O distances of HCO+ and QCO+ and the longer C−O bond lengths of COH+ and COQ+ are due to the changes in the polarization of the bonding orbitals which are caused by the positive charge of H+ or Q+ that are bonded to the molecule. The bonding orbitals of CO are polarized toward the more electronegative oxygen end. ...
TL;DR: A general-purpose rate-based algorithm, given reliable rate estimation rules, provides a framework for systematically constructing kinetic schemes including all of the numerically significant species, even for systems involving so many reactions that they could not be handled manually.
Abstract: A general-purpose rate-based algorithm for the construction of chemical kinetic models for systems with hundreds or thousands of reacting species is presented. The algorithm comprehensively works out the details of the chemistry implied by given reaction rate estimation rules, identifies the species and reactions that are numerically significant, and solves the resulting system of differential equations to compute the concentrations of the significant species as a function of time. A key innovation is a definition and numerical test for the “completeness” of the kinetic scheme. This approach obviates the need to arbitrarily neglect certain species and reactions in order to keep reaction schemes small enough to be manageable and allows chemical kinetic modelers to focus on the chemistry rather than on the computational details. Examples of hydrocarbon pyrolysis and combustion applications are presented, where the computer evaluates the importance of nearly 100 000 reactions in the process of identifying the few hundred species that are kinetically significant. The new algorithm, given reliable rate estimation rules, provides a framework for systematically constructing kinetic schemes including all of the numerically significant species, even for systems involving so many reactions that they could not be handled manually.
TL;DR: In this article, the complex (electric) permittivity of aqueous solutions of sodium chloride was measured as a function of frequency ν (20 MHz ≤ ν ≤ 40 GHz) and salt content (0.003 ≤ y ≤ 0.035).
Abstract: At 20 °C the complex (electric) permittivity of aqueous solutions of sodium chloride has been measured as a function of frequency ν (20 MHz ≤ ν ≤ 40 GHz) and salt content (0.003 ≤ y ≤ 0.035; y, mas...
TL;DR: The computational study of pericyclic reactions, an important general class of organic reactions, now provides information about the transition structures of these processes with chemical accuracy, as judged by comparisons with experimental data, such as activation energies, substituent effects on rates, and kinetic isotope effects.
Abstract: The computational study of pericyclic reactions, an important general class of organic reactions, now provides information about the transition structures of these processes with chemical accuracy, as judged by comparisons with experimental data, such as activation energies, substituent effects on rates, and kinetic isotope effects. This article introduces the methods used to study these reactions and describes how computational results have contributed to the understanding of transition states and mechanisms of the electrocyclic ring openings of cyclobutenes, Diels−Alder cycloaddition reactions, and [3,3]-sigmatropic shifts such as the Cope rearrangement.
TL;DR: In this paper, the authors used ringdown laser absorption spectroscopy (IR-CRLAS) to determine the absolute concentrations of water dimers, trimers, tetramers, and pentamers in a pulsed supersonic expansion for the first time.
Abstract: The recently developed technique of infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) has been employed in the 3.0 μm region to determine the absolute concentrations of water dimers, trimers, tetramers, and pentamers in a pulsed supersonic expansion for the first time. Additional spectral features are reported, one of which we assign to the bound O−H stretching bands of the hexamer. Additionally, by simple variation of the jet stagnation pressure, the collective O−H stretching absorption from all clusters produced in the expansion was observed to change from that of discrete features of small clusters to band profiles of liquid water and finally to amorphous ice.
TL;DR: In this article, spatial distribution functions are employed to analyze the three-dimensional local structure in water−methanol solutions, and the effects of the alcohol on water structure and water on methanol structure are considered in detail.
Abstract: The diverse properties of hydrogen-bonded liquids and solutions must manifest their unique local structures. An unambiguous three-dimensional picture of the local ordering in these liquid systems is not accessible through radial distribution functions, the usual outputs of computer simulation, or experimental studies. In this work we employ spatial distribution functions to analyze the three-dimensional local structure in water−methanol solutions. Molecular dynamics simulations are performed at room temperature for five water−methanol liquid mixtures scanning the entire range of compositions. The effects of the alcohol on water structure and water on methanol structure are considered in detail. The results are compared to previous simulations and discussed from the point of view of various solvation models. Large structural changes are observed, many of which are not apparent from simple radial analysis. In water-rich solution we confirm a high degree of ordering, characterized by a very strong preference...
TL;DR: In this article, a detailed kinetic analysis of the complex reaction systems arising from the ozonolysis of C 2H4 and (CH3)2CdC(CH 3)2 (TME), respectively, is carried out, using master equations and statistical rate theory.
Abstract: A detailed kinetic analysis of the complex reaction systems arising from the ozonolysis of C 2H4 and (CH3)2CdC(CH3)2 (TME), respectively, is carried out, using master equations and statistical rate theory. The thermochemical as well as the molecular data required are obtained from CCSD(T)/TZ2P and B3LYP/ DZP calculations. It is shown that the primary ozonides are not collisionally stabilized under atmospheric conditions. In the reaction sequence for O3 + TME, the same is true for CH2dC(CH3)OOH formed from (CH3)2COO, which completely dissociates to give OH radicals. However, in this system, a pressure dependence is predicted for the relative branching fractions of the reactions of the Criegee intermediate. Under atmospheric conditions, for both examples, the product yields obtained are in reasonable agreement with experimental results.
TL;DR: In this paper, the ability of two new model potentials constructed using intermolecular perturbation theory methods to reproduce ab initio results at a comparable level of theory was tested.
Abstract: We have tested the ability of two new model potentials constructed using intermolecular perturbation theory methods to reproduce ab initio results at a comparable level of theory. Several configurations of water trimer, tetramer, and pentamer are studied, and in addition to the contributions to the interaction energy, the potential energy surfaces are compared by optimizing the model potential geometries to local stationary points within a rigid-body framework. In general the agreement between the two methods is good, validating the model potentials as suitable candidates for providing starting geometries for further ab initio calculations and for the simulation of larger systems.
TL;DR: In this article, a terahertz laser vibration-rotation-tunneling spectroscopy has been employed to characterize the structure and hydrogen bond network rearrangement dynamics of a cage form of the water hexamer having eight hydrogen bonds.
Abstract: Tunable terahertz laser vibration−rotation-tunneling spectroscopy has been employed to characterize the structure and hydrogen bond network rearrangement dynamics of a cage form of the water hexamer having eight hydrogen bonds. The isolated clusters are produced in a pulsed supersonic slit jet. Striking similarities are found between the structure and the average interoxygen distance RO-O (2.82 A) of the hexamer cage and those of the basic unit of ice VI. The hybrid perpendicular band of b- and c-types is observed near 2.491 THz (83.03 cm-1) and rationalized to originate from the torsional motions of the two single-donor single-acceptor monomers about their donor hydrogen bonds, thereby causing changes in the dipole moments from each monomer to be orthogonal to each other as well as to be perpendicular to the approximate symmetry a-axis. Triplet spectral patterns accompanying each rovibrational transition with line spacings of 1.9 MHz and intensity ratios of 9:6:1 are attributed to the degenerate quantum ...
TL;DR: A critical survey of previously reported van der Waals parameters for alkali metal cations and halide anions is presented in this paper, where a new set of force field parameters is proposed, derived by fitting the experimental lattice constants and lattice energies of 20 ionic alkali halide crystals.
Abstract: A critical survey of previously reported van der Waals parameters for alkali metal cations and halide anions is presented. A new set of force field parameters is proposed, derived by fitting the experimental lattice constants and lattice energies of 20 ionic alkali halide crystals. These parameters are constrained to satisfy two relationships connecting the ions with the isoelectronic noble gasesthe relative van der Waals radii R* and the coefficients of the London dispersion energies C6using the experimentally determined noble gas van der Waals parameters. In addition to reproducing physical trends in common with atoms of isoelectronic species, the present parameters predict more accurate crystal structures and energies and, when combined with a molecular force field for water, also quite accurate gas-phase ion−water interaction energies and aqueous solution structures compared to the computed results previously reported by other authors.
TL;DR: In this article, a distributed-origin coupled Hartree−Fock method is used to compute and map the π and total current densities induced by a magnetic field in the planar monocyclic molecules benzene (C6H6), borazine...
Abstract: A distributed-origin coupled Hartree−Fock method is used to compute and map the π and total current densities induced by a magnetic field in the planar monocyclic molecules benzene (C6H6), borazine...
TL;DR: In this paper, a detailed analysis of hydrogen bonding in supercritical water is presented, based on the recently proposed hybrid distance−energy criterion of H-bonding, which is found with all available experimental and computer simulated results.
Abstract: Monte Carlo computer simulations were performed under thermodynamic conditions corresponding to available X-ray and neutron diffraction measurements of the supercritical water structure. A detailed analysis of hydrogen bonding in supercritical water is presented, based on the recently proposed hybrid distance−energy criterion of H-bonding. Good agreement is found with all available experimental and computer simulated results. With increasing temperature, the average number of H-bonds per a water molecule, 〈nHB〉, decreases with the same slope for both high-density (∼1.0 g/cm3) as well as low-density (∼0.2 g/cm3) supercritical water, asymptotically approaching zero at higher temperatures and lower densities. Over the whole supercritical region, except for the highest density states, 〈nHB〉 is always below the percolation threshold (∼1.6), indicating that the continuous network of hydrogen bonds is broken. Nevertheless, even at the highest temperature and the lowest density simulated, some degree of hydrogen ...
TL;DR: In this paper, a model for bridge-assisted, long-range electron transfer in a molecule interacting with a dissipative external bath is presented, and the effects of the system−bath interaction are included phenomenologically in the evolution of system density matrix as energy dephasings on the bridge sites.
Abstract: A model for bridge-assisted, long-range electron transfer in a molecule interacting with a dissipative external bath is presented. The effects of the system−bath interaction are included phenomenologically in the evolution of the system density matrix as energy dephasings on the bridge sites. When the bridge dephasings are small, the steady state ET rate in this model is found to be the sum of two competing terms; the first is a McConnell-type rate arising from direct tunneling from donor to acceptor, and the second is a dephasing-dependent, length-independent scattering channel through the bridge sites. In the limit of large dephasings, an incoherent channel dominates the dynamics and leads to ET rates that can become only weakly dependent (kET ∝ 1/N) on the number of bridge sites in the system, for multisite bridges.