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Showing papers in "Journal of Physical Chemistry A in 2000"


Journal ArticleDOI
TL;DR: The Next Reaction Method is presented, an exact algorithm to simulate coupled chemical reactions that uses only a single random number per simulation event, and is also efficient.
Abstract: There are two fundamental ways to view coupled systems of chemical equations: as continuous, represented by differential equations whose variables are concentrations, or as discrete, represented by stochastic processes whose variables are numbers of molecules. Although the former is by far more common, systems with very small numbers of molecules are important in some applications (e.g., in small biological cells or in surface processes). In both views, most complicated systems with multiple reaction channels and multiple chemical species cannot be solved analytically. There are exact numerical simulation methods to simulate trajectories of discrete, stochastic systems, (methods that are rigorously equivalent to the Master Equation approach) but these do not scale well to systems with many reaction pathways. This paper presents the Next Reaction Method, an exact algorithm to simulate coupled chemical reactions that is also efficient: it (a) uses only a single random number per simulation event, and (b) ...

1,823 citations


Journal ArticleDOI
TL;DR: The field of femtochemistry has been studied from a personal perspective, encompassing our research at Caltech and focusing on the evolution of techniques, concepts, and new discoveries.
Abstract: This anthology, which is adapted from the Nobel Lecture, gives an overview of the field of Femtochemistry from a personal perspective, encompassing our research at Caltech and focusing on the evolution of techniques, concepts, and new discoveries. In developing femtochemistrythe study of molecular motions in the ephemeral transition states of physical, chemical, and biological changeswe have harnessed the powerful concept of molecular coherence and developed ultrafast-laser techniques for observing these motions. Femtosecond resolution (1 fs = 10-15 s) is the ultimate achievement for studies of the dynamics of the chemical bond at the atomic level. On this time scale, matter wave packets (particle-type) can be created and their coherent evolution as a single-molecule trajectory can be observed. The field began with simple systems of a few atoms and has reached the realm of the very complex in isolated, mesoscopic, and condensed phases and in biological systems such as proteins and DNA. It also offers new ...

1,570 citations


Journal ArticleDOI
TL;DR: In this paper, a hybrid Hartree−Fock−density functional (HF-DF) model called the modified Perdew−Wang 1-parameter model for kinetics (MPW1K) was optimized against a database of 20 forward barrier heights.
Abstract: A new hybrid Hartree−Fock−density functional (HF-DF) model called the modified Perdew−Wang 1-parameter model for kinetics (MPW1K) is optimized against a database of 20 forward barrier heights, 20 r...

1,418 citations


Journal ArticleDOI
TL;DR: The ab initio multiple spawning (AIMS) method is a time-dependent formulation of quantum chemistry, whereby the nuclear dynamics and electronic structure problems are solved simultaneously as mentioned in this paper. But it does not consider the nonadiabatic effects which are crucial in modeling dynamics on multiple electronic states.
Abstract: The ab initio multiple spawning (AIMS) method is a time-dependent formulation of quantum chemistry, whereby the nuclear dynamics and electronic structure problems are solved simultaneously. Quantum mechanical effects in the nuclear dynamics are included, especially the nonadiabatic effects which are crucial in modeling dynamics on multiple electronic states. The AIMS method makes it possible to describe photochemistry from first principles molecular dynamics, with no empirical parameters. We describe the method and present the application to two molecules of interest in organic photochemistryethylene and cyclobutene. We show that the photodynamics of ethylene involves both covalent and ionic electronic excited states and the return to the ground state proceeds through a pyramidalized geometry. For the photoinduced ring opening of cyclobutene, we show that the disrotatory motion predicted by the Woodward−Hoffmann rules is established within the first 50 fs after optical excitation.

724 citations


Journal ArticleDOI
TL;DR: The properties of the π* excited doublet states of these organic radical anions remain obscure as mentioned in this paper, however, they have been implicated recently in a wide variety of photochemical electron transfer reactions.
Abstract: The radical anions of aromatic diimides have been implicated recently in a wide variety of photochemical electron transfer reactions. Photoexcitation of these radical anions produces powerfully reducing species. Yet, the properties of the π* excited doublet states of these organic radical anions remain obscure. The radical anions of three aromatic imides with increasingly larger π systems, N-(2,5-di-tert-butylphenyl)phthalimide, 1, N-(2,5-di-tert-butylphenyl)-1,8-naphthalimide, 2, and N-(2,5-di-tert-butylphenyl)perylene-3,4-dicarboximide, 3, as well as the three corresponding aromatic diimides, N,N‘-bis(2,5-di-tert-butylphenyl)pyromellitimide, 4a, N,N‘-bis(2,5-di-tert-butylphenyl)-naphthalene-1,8:4,5-tetracarboxydiimide, 5a, and N,N‘-bis(2,5-di-tert-butylphenyl)perylene-3,4:9,10-tetracarboxydiimide, 6, were produced by electrochemical reduction of the neutral molecules in an optically transparent thin layer electrochemical cell. The radical anions of these imides and diimides all exhibit intense visible a...

535 citations


Journal ArticleDOI
TL;DR: In this paper, the concept of generalized time, introduced by Ozawa, has been used to define master plots for the analysis of solid-state reactions, regardless of the type of temperature program used for recording the experimental data.
Abstract: Master plot methods based on the integral and/or the differential forms of the kinetic equation describing solid-state reactions have been redefined by using the concept of the generalized time, θ, introduced by Ozawa. This redefinition permits the application of these master plots to the kinetic analysis of solid-state reactions, whatever the type of temperature program used for recording the experimental data. In isothermal conditions, a single curve is enough to construct the experimental master plots. In nonisothermal conditions, the knowledge of both α as a function of temperature and activation energy is required for calculating the master plot curves from the experimental data. Practical usefulness of the present master plot methods is examined, and exemplified by being applied to the thermal decomposition of ZnCO3 under isothermal, linear nonisothermal, and nonlinear nonisothermal conditions.

506 citations


Journal ArticleDOI
TL;DR: In this article, a method to evaluate ab initio energy, wave function, and gradient of a solvated molecule in an electronically excited state is presented, which extends the Polarizable Continuum.
Abstract: We present a method to evaluate ab initio energy, wave function, and gradient of a solvated molecule in an electronically excited state. In particular, this paper extends the Polarizable Continuum ...

504 citations


Journal ArticleDOI
TL;DR: In this article, an accurate method for solving the electronic Schrodinger equation that is applicable to a broad range of moleculesthe CCSD(T) method and families of basis sets that systematically converge to the complete basis set limitthe correlation consistent basis sets is presented.
Abstract: During the past decade dramatic progress has been made in calculating the binding energies of molecules. This is the result of two advances reported in 1989: an accurate method for solving the electronic Schrodinger equation that is applicable to a broad range of moleculesthe CCSD(T) methodand families of basis sets that systematically converge to the complete basis set limitthe correlation consistent basis sets. The former provides unprecedented accuracy for the prediction of a broad range of molecular properties, including molecular binding energies. The latter provides a means to systematically approach the complete basis set limit, i.e., the exact solutions of approximations to the Schrodinger equation. These two advances combined with a thorough analysis of the errors involved in electronic structure calculations lead to clear guidelines for ab initio calculations of binding energies, ranging from the strong bonds derived from chemical interactions to the extremely weak binding due to dispersion int...

495 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the C functional has negligible effect on the calculated dipole moment (μ), α, β, and γ whereas the X-part is responsible for the large property overestimations when the size of the system increases.
Abstract: DFT schemes based on conventional exchange-correlation (XC) functionals have been employed to determine the dipole moment (μ), polarizability (α), and first (β) and second (γ) hyperpolarizabilities of push−pull π-conjugated systems. In addition to the failures already pointed out for α and γ in a recent study on polyacetylene chains [J. Chem. Phys. 1998, 109, 10489; Phys. Rev. Lett. 1999, 83, 694], these functionals are also unsuitable for the evaluation of μ and β. In the case of β, in particular, an almost catastrophic behavior with respect to increasing chain length is found. We show that the C functional has a negligible effect on the calculated μ, α, β, and γ whereas the X-part is responsible for the large property overestimations when the size of the system increases. The overly large μ values are associated with an overestimation of the charge transfer between the donor and the acceptor whereas for α, β, and γ, incomplete screening of the external electric field is responsible for the large discrep...

488 citations


Journal ArticleDOI
TL;DR: In this article, the C2H5• + O2 reaction has been examined in detail via highly sophisticated electronic structure methods, including the geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products.
Abstract: The C2H5• + O2 reaction, central to ethane oxidation and thus of fundamental importance to hydrocarbon combustion chemistry, has been examined in detail via highly sophisticated electronic structure methods. The geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products for the reaction of the ethyl radical (X 2A‘) with O2 (X 3 , a 1Δg) have been investigated using the CCSD and CCSD(T) ab initio methods with basis sets ranging in quality from double-zeta plus polarization (DZP) to triple-zeta plus double polarization with f functions (TZ2Pf). Five mechanisms (M1−M5) involving the ground-state reactants are introduced within the context of previous experimental and theoretical studies. In this work, each mechanism is systematically explored, giving the following overall 0 K activation energies with respect to ground-state reactants, Ea(0 K), at our best level of theory: (M1) direct hydrogen abstraction from the ethyl radical by O2 to give e...

480 citations


Journal ArticleDOI
TL;DR: In this article, the fluorescence fluctuations of Cy5 have been investigated under steady-state excitation conditions by fluorescence correlation spectroscopy (FCS), which can be used to characterize the photoinduced isomerization and back-isomerization, as well as the transitions between the singlet and triplet states.
Abstract: Cy5 is one of a few commercially available dyes in the near-infrared wavelength range. In this study, the fluorescence fluctuations of Cy5 have been investigated under steady-state excitation conditions by fluorescence correlation spectroscopy (FCS). The fluctuations in fluorescence are compatible with and can be used to characterize the photoinduced isomerization and back-isomerization, as well as the transitions between the singlet and triplet states of the dye. By employing a simple kinetic model, the rate constants of these processes can be determined. The model was used over a broad range of experimental conditions, where the influence on the isomerization properties of solvent viscosity, polarity, and temperature, excitation intensity and wavelength, and the presence of different side groups was investigated. We propose FCS as a useful and simple complementary approach to study isomerization processes of cyanine dyes yielding information about the rates of both the photoinduced isomerization and the...

Journal ArticleDOI
TL;DR: In this article, the authors compared the link atom and local self-consistent field (LSCF) formalism for hybrid QM/MM methods and found that the LSCF formalism is generally of similar accuracy if care is taken in the choic...
Abstract: A major complication in hybrid QM/MM methods is the treatment of the frontier between the quantum part, describing the reactive region, and the classical part, describing the environment. Two approaches to this problem, the “link atom” method and the “local self-consistent field” (LSCF) formalism, are compared in this paper. For this purpose, the LSCF formalism has been introduced into the CHARMM program. A detailed description of the two approaches is presented. The results of semiempirical calculations of deprotonation enthalpies and proton affinities of propanol and a tripeptide with different treatments of the frontier bond are compared. Particular emphasis is placed on the effect of an external charge. It is shown that the choice of the QM/MM electronic interactions included in the frontier region is of considerable importance in determining the electron distribution of the QM region and the overall energy. The link atom and LSCF methods are generally of similar accuracy if care is taken in the choic...

Journal ArticleDOI
TL;DR: In this article, the bimodal response of confined water is studied, with one bulk water-like subpicosecond component and a much slower component in a time scale of hundreds or thousands of picoseconds.
Abstract: Although water is often hailed as the lubricant of life, a detailed understanding of its role in many chemical and biological processes still eludes us. In many natural systems, water is confined in an environment where its free movement is restricted and its three-dimensional hydrogen-bonded network is disrupted. Very recently, several groups applied ultrafast laser spectroscopy to study the dynamics of the constrained water molecules. It is observed that the dynamic behavior of the confined water molecules is markedly different from that of the ordinary water molecules. The most striking result is the bimodal response of confined water, with one bulk water-like subpicosecond component and a much slower component in a time scale of hundreds or thousands of picoseconds. This slow second component constitutes 10−30% of the total response and is crucial in the understanding of the role of water in complex chemical and biological processes. The origin of the slow component has been a subject of intense recen...

Journal ArticleDOI
TL;DR: In this paper, the non-bonded interactions in the dimeric complexes were analyzed as a function of theory level (HF, DFT(B3LYP), MP2, LMP2, MP3, MP4, CCSD(T), and others) and basis set (6-31G, cc-pVXZ, X = D, T, Q, 5).
Abstract: We present calculations for the nonbonded interactions in the dimeric complexes: methane dimer, ammonia dimer, water dimer, H2O·(NH3), CH4·(NH3), and (FHF)- as a function of theory level (HF, DFT(B3LYP), MP2, LMP2, MP3, MP4, CCSD(T), and others) and basis set (6-31G**, cc-pVXZ, X = D, T, Q, 5). Dimer minimum energy structures are determined at the MP2 theory level for the cc-pVTZ basis set employing analytical second derivatives. For HF and DFT levels of theory, methane dimer and one structure of CH4·(NH3) are not bound. The basis set superposition error (BSSE) begins to converge (becomes systematically small) for basis sets larger than cc-pVTZ. For hydrogen-bonded systems, most levels of theory seem to give reasonable estimates of the experimentally known binding energies, but here, too, the BSSE overwhelms the reliability of the binding energies for the smaller basis sets. The CH4·(NH3) dimer has two minimum energy conformations with similar binding energies, but very different BSSE values especially f...

Journal ArticleDOI
TL;DR: In this paper, the lifetime of terphenyl-based lanthanide complexes with a triphenylene antenna chromophore (Ln) has been investigated and the processes preceding the luminescence in the sensitization process have been studied in detail.
Abstract: m-Terphenyl-based lanthanide complexes functionalized with a triphenylene antenna chromophore ((Ln)1) exhibit sensitized visible and near-infrared emission upon photoexcitation of the triphenylene antenna at 310 nm. Luminescence lifetime measurements of the (Eu)1 and (Tb)1 complexes in methanol-h1 and methanol-d1 revealed that one methanol molecule is coordinated to the lanthanide ion, indicating that all eight donor atoms provided by the ligand are involved in the encapsulation of the lanthanide ion. The luminescence lifetimes of the near-IR-emitting complexes (Er)1, (Nd)1, and (Yb)1 in DMSO-h6 and DMSO-d6 are in the microsecond range, and are dominated by nonradiative deactivation of the luminescent state. The processes preceding the lanthanide luminescence in the sensitization process have been studied in detail. The complexed lanthanide ion reduces the antenna fluorescence and increases the intersystem crossing rate via an external heavy atom effect. The subsequent energy-transfer process was found to take place via the antenna triplet state in all complexes. Luminescence quantum yield measurements and transient absorption spectroscopy indicated that in solution two conformational isomers of the complexes exist: one in which no energy transfer takes place, and one in which the energy transfer does take place, resulting in the lanthanide luminescence. The intramolecular energy-transfer rate is higher in the (Eu)1 and (Tb)1 complexes than in the near-infrared-emitting complexes. In methanol the energy-transfer rate is 3.8 × 107 s-1 for (Eu)1 and (Tb)1. In DMSO-d6 the intramolecular energy-transfer rate is higher in the (Nd)1 complex (1.3 × 107 s-1) than in the (Er)1 (3.8 × 106 s-1) and (Yb)1 (4.9 × 106 s-1) complexes.

Journal ArticleDOI
TL;DR: In this paper, a sonochemical degradation of aqueous solution of azobenzene and related azo dyes (methyl orange, omethyl red, and p-methyl red) was performed at 500 kHz and 50 W, under air, O2, or Ar saturation at 288 K. Reaction products and intermediates were identified by HPLC-ES-MS.
Abstract: The sonochemical degradation of aqueous solutions of azobenzene and related azo dyes (methyl orange, o-methyl red, and p-methyl red) was performed at 500 kHz and 50 W, under air, O2, or Ar saturation at 288 K. Reaction products and intermediates were identified by HPLC-ES-MS. Total organic carbon (TOC) was also determined as a function of reaction time. We propose a reaction mechanism based on the observed species and the extent and rate of TOC depletion. The addition of OH radicals to the azo double bond is considered to be the first step of the sequence of oxidative bond cleavages leading to the production of carboxylic acids, quinones, carbon dioxide, and nitrate ions as the main degradation products. The effects of the dye structures and of the background gas on the sonochemical bleaching rates were also investigated. The reaction rates for o-methyl red were approximately 30−40% faster than those for the other compounds. This appears to be a strong influence by a carboxylic group ortho to the azo grou...

Journal ArticleDOI
TL;DR: In this article, single-molecule spectroscopy (SMS) has been used for a variety of applications including molecular photophysics, quantum optics, and molecular dynamics.
Abstract: Single-molecule spectroscopy (SMS) combines some of the advantages of local probe microscopies with those of optics. Since this field came into being 10 years ago, it has expanded at a breathtaking pace. From the first cryogenic experiments up to the recent studies of basic processes in molecular biology, single-molecule methods have found their way into an ever broadening range of applications. Their common feature is the complete elimination of ensemble averaging. By exposing individual variations as well as dynamical fluctuations, SMS provides new insights into any system with spatial or temporal inhomogeneity. The present article illustrates single molecule spectroscopic experiments at cryogenic temperatures, mainly from the authors' group. The results reviewed here range from molecular photophysics, to the dynamics of the solid matrix around the molecule, and to the interactions between a single molecule and electromagnetic fields, i.e., quantum optics. SMS is now ripe for a variety of applications i...

Journal ArticleDOI
TL;DR: The participation of 4f electrons in the bonding of the lanthanide complexes Ln(NR2)3; R = H, SiH3, has been investigated at the DFT level as mentioned in this paper.
Abstract: The participation of 4f electrons in the bonding of the lanthanide complexes Ln(NR2)3; R = H, SiH3, has been investigated at the DFT level. Structural parameters obtained with small core (f electrons in the valence) and large core (f electrons in the core) effective core potentials (ECPs) suggest the nonparticipation of the f electrons to the Ln−N bonding. A methodological study has been carried out on the lanthanide contraction with various ab initio methods using large core ECPs. The calculated lanthanide contraction (0.180 A) is in excellent agreement with the experimental value (0.179 A). Comparison of calculated structural parameters with available X-ray data shows that calculations with large core ECPs and density functional methods quantitatively reproduce the bonding at the lanthanide.

Journal ArticleDOI
TL;DR: In this paper, a series of monoaxially chloro- and aryl-substituted indium(III) phthalocyanines are described and the absorption cross sections and temporal evolution of the low-lying excited states are also reported.
Abstract: Nonlinear absorption, refraction, and optical limiting by a series of monoaxially chloro- and aryl-substituted indium(III) phthalocyanines are described. The absorption cross sections and temporal evolution of the low-lying excited states are also reported. A large nonlinear absorption that increased with wavelength between 500 and 590 nm was observed in each material. The nanosecond nonlinear absorption and the optical limiting are shown to be dominated by a strong excited state absorption from an orientationally averaged triplet state. We derive and experimentally confirm the relation between the molecular absorption cross sections and the fluence-dependent nonlinear absorption coefficients. The effective nonlinear refraction on the nanosecond time scale was reduced because the electronic contribution to the nonlinear refractive index was of the opposite sign from the thermal contribution. An optical limiter using the new material, p-(trifluoromethyl)phenylindium(III) tetra-tert-butylphthalocyanine [(t-...

Journal ArticleDOI
TL;DR: A buffered tris(2,2,bipyridyl)ruthenium complex-colloidal IrO2 system was studied as a photocatalyst for the production of O2 from water as discussed by the authors.
Abstract: A buffered tris(2,2‘-bipyridyl)ruthenium complex-colloidal IrO2 system was studied as a photocatalyst for the production of O2 from water. Phosphate buffer, which has historically been used to control the pH in this system, accelerates the decomposition of the photosensitizer and inhibits O2 evolution, whereas sodium hexafluorosilicate (Na2SiF6)−base solutions are ideal buffers for the reaction. Na2SiF6-containing buffers poise the solution under visible light irradiation at ca. pH 5, preventing the pH drop that accompanies oxidation of water in unbuffered solutions. Decomposition of the photosensitizer is not kinetically competitive with oxygen evolution in these buffers. In particular, the Na2SiF6−NaHCO3 buffer greatly improves the turnover number of the photosensitizer, relative to previously used phosphate buffers, without any decrease in activity. Photocatalytic reactions studied under various conditions suggest that adsorbed carbonate or bicarbonate on the surface of the colloidal IrO2 particles con...

Journal ArticleDOI
TL;DR: In this article, Hartree−Fock (HF) and density functional theory (DFT) calculations of the electric dipole−magnetic dipole polarizability βαβ are reported for the chiral molecules methyloxirane (1) and tr...
Abstract: Ab initio Hartree−Fock (HF) and density functional theory (DFT) calculations of the electric dipole−magnetic dipole polarizability βαβ are reported for the chiral molecules methyloxirane (1) and tr...

Journal ArticleDOI
TL;DR: In this paper, time-resolved infrared measurements indicate ultrafast, <350 fs, electron injection from (4,4‘dcb)2Ru(NCS)2 (1) and (5,5'dcb)-nodes to nanostructured TiO2 electrodes (where 4,4''dcb = 4, 4''-(COOH)2-2,2''-bipyridine).
Abstract: Time-resolved infrared measurements indicate ultrafast, <350 fs, electron injection from (4,4‘dcb)2Ru(NCS)2 (1) and (5,5‘dcb)2Ru(NCS)2 (2) to nanostructured TiO2 electrodes (where 4,4‘dcb = 4,4‘-(COOH)2-2,2‘-bipyridine). Although rapid, the injection from 2 apparently occurs with a lower quantum yield than that from 1, explaining a lower overall photon-to-current efficiency for 2/TiO2 solar cells. Transient visible spectroscopy reveals similar rates of both halide oxidation and injected electron-oxidized dye recombination for the two sensitizers. Substituting SnO2 for TiO2 increases the electron injection yield from 2 in the case of transparent metal oxide films and improves the photon-to-current efficiency. Results indicate a wavelength-dependent electron injection yield.

Journal ArticleDOI
TL;DR: In this article, the authors studied the lifetime of bending vibrations in the broad water stretching band (2900−3700 cm-1), frequency-dependent vibrational energy relaxation (VER), and spectral diffusion both occur on the time scale of a few picoseconds.
Abstract: In the broad water stretching band (2900−3700 cm-1), frequency-dependent vibrational energy relaxation (VER), and spectral diffusion both occur on the time scale of a few picoseconds. Ultrafast IR−Raman spectroscopy of water is used to study both processes. VER is also studied in solutions of HDO in D2O (HDO/D2O). The OH stretch (νOH) lifetime for water and HDO is ∼1 ps. The OD stretch (νOD) lifetime for D2O is ∼2 ps. Stretch decay generates substantial excitation of the bending modes. The lifetimes of bending vibrations (δ) in H2O, HDO, and D2O can be estimated to be in the 0.6 ps ≤ T1 ≤ 1.2 ps range. νOH decay in water produces δH2O with a quantum yield 1.0 ≤ φ ≤ 2.0. In HDO/D2O solutions, νOH(HDO) decay generates νOD(D2O), δHDO, and δD2O. The quantum yield for generating νOD(D2O) is φ ≈ 0.1. The quantum yield for generating both δHDO and δD2O is φ ≥ 0.6. Thus, each νOH(HDO) decay generates at minimum 1.2 quanta of bending excitation. After narrow-band pumping, the distribution of excitations within the...

Journal ArticleDOI
TL;DR: In this article, the quenching rate of three model ketone triplets by nine phenols bearing various substituents, from electron-donating alkyl and alkoxy groups to the electron-withdrawing cyano group, were measured by nanosecond laser flash photolysis.
Abstract: Aromatic ketones efficiently mediate the photo-oxidative degradation of phenols in aerated aqueous solution, a process likely to be relevant in sunlit natural waters. Absolute bimolecular rate constants for the quenching of three model ketone triplets by nine phenols bearing various substituents, from electron-donating alkyl and alkoxy groups to the electron-withdrawing cyano group, were measured by nanosecond laser flash photolysis. Triplet benzophenone (BP) is quenched at nearly diffusion-controlled rates (2.6−5.6 × 109 M-1 s-1). Triplet state quenching of 3‘-methoxyacetophenone (3‘-MAP) and 2-acetonaphthone (2-AN) by the same set of phenols occurs more selectively, with rate constants spanning a range of 1 and more than 2 orders of magnitude, respectively. Quenching rate constants obey a Rehm−Weller relationship to the free energy of electron transfer from the phenol to the ketone triplet. By comparison of the quenching constants with overall photo-oxidation rates obtained by stationary irradiation in ...

Journal ArticleDOI
TL;DR: In this article, Monte Carlo statistical mechanical computer simulations of the electric field poling of second-order nonlinear optical chromophores, characterized by large dipole moments, polarizabilities, and hyperpolarizabilities are presented.
Abstract: Monte Carlo statistical mechanical computer simulations of the electric-field poling of second-order nonlinear optical chromophores, characterized by large dipole moments, polarizabilities, and hyperpolarizabilities, are presented. Such theoretical analysis is critical to defining the structure/function relationships that permit maximization of electro-optic activity for π-electron chromophore-containing polymeric materials. Polymeric electro-optic materials may, in turn, be important for high-bandwidth telecommunications, new forms of radar, and high-speed data processing. The experimentally observed maxima in plots of electro-optic activity versus chromophore number density (loading) in polymer matrices are theoretically reproduced, as are the shifts of the maxima to lower loading with increasing chromophore dipole moment. Modification of the chromophore shape to realize the maximum achievable electro-optic activity for a given π-electron structure is discussed, as is the role of polymer electrical perm...

Journal ArticleDOI
TL;DR: In this paper, the structures and vibrational frequencies of UO2(H2O)52+, NpO 2(H 2O) 52+ and PuO2Q+ corresponding to An(VI) oxidation states were calculated using density functional theory (DFT) and relativistic effective core potentials (RECPs).
Abstract: The structures and vibrational frequencies of UO2(H2O)52+, NpO2(H2O)52+, and PuO2(H2O)52+ corresponding to An(VI) oxidation states and UO2(H2O)5+, Np(H2O)5+, and Pu(H2O)5+ corresponding to An(V) have been calculated using density functional theory (DFT) and relativistic effective core potentials (RECPs) The resulting structures are compared to EXAFS solution studies, and the Raman and IR vibrational frequencies of the actinyl unit are compared to experimental studies in solution Free energies for reactions in solution are computed by combining thermodynamic free energies in the gas phase with a dielectric continuum model to treat solvent effects The hydrolysis reaction of UO2(H2O)52+ to form UO2(H2O)4(OH)+ and the reactions for removing or adding a water to the first shell in UO2(H2O)52+ are examined using this approach Multiplet and spin−orbit effects not included in a single-configuration DFT wave function are incorporated by model spin−orbit CI calculations PuO2q+ is used as a model for the aquo c

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the gas-phase unimolecular decomposition of RDX and showed that the concerted pathway is dominant while theoretical calculations have suggested that the homolytic pathway might require significantly less energy.
Abstract: Gas phase hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a relatively stable molecule which releases a large amount of energy upon decomposition. Although gas-phase unimolecular decomposition experiments suggest at least two major pathways, there is no mechanistic understanding of the reactions involving RDX or other energetic molecules (such as HMX and TATB), used in applications ranging from automobile air bags to rocket propellants. For the unimolecular decomposition of RDX, we find three pathways: (i) concerted decomposition of the ring to form three CH2NNO2 (M = 74) molecules, and (ii) homolytic cleavage of an NN bond to form NO2 (M = 46) plus RDR (M = 176), which subsequently decomposes to form various products. Experimental studies suggest that the concerted pathway is dominant while theoretical calculations have suggested that the homolytic pathway might require significantly less energy. We report here a third pathway: (iii) successive HONO elimination to form 3 HONO (M = 47) plus stable 1,3,...

Journal ArticleDOI
TL;DR: In this paper, the effects of electric fields on vibrational spectra were measured for the C-N stretch mode of several small nitriles, including unconjugated and conjugated compounds, and mono-and dinitriles.
Abstract: Vibrational Stark effects, which are the effects of electric fields on vibrational spectra, were measured for the C-N stretch mode of several small nitriles. Samples included unconjugated and conjugated nitriles, and mono- and dinitriles. They were immobilized in frozen 2-methyl-tetrahydrofuran glass and analyzed in externally applied electric fields using an FTIR; details of the methodology are presented. Difference dipole moments, ¢I, equivalent to the linear Stark tuning rate, range from 0.01/ f to 0.04/f Debye (0.2/f to 0.7/f cm -1 /(MV/cm)) for most samples, with aromatic compounds toward the high end and symmetric dinitriles toward the low end (the local field correction factor, f, is expected to be similar for all these samples). Most quadratic Stark effects are small and negative, while transition polarizabilities are positive and have a significant effect on Stark line shapes. For aromatic nitriles, transition dipoles and ¢I values correlate with Hammett numbers. Symmetric dinitrile ¢I values decrease monotonically with increasing conjugation of the connecting bridge, likely due to improved mechanical coupling and, to a lesser extent, an increased population of inversion symmetric conformations. ¢I values are close to those expected from bond anharmonicity and ab initio predictions.

Journal ArticleDOI
TL;DR: In this article, a threshold collision-induced dissociation of Na+−L with xenon was studied using guided ion beam mass spectrometry, and the results showed that the primary product formed corresponds to endothermic loss of the neutral ligand and the only other product observed is the result of ligand exchange processes to form NaXe+.
Abstract: Threshold collision-induced dissociation of Na+(L) with xenon is studied using guided ion beam mass spectrometry. The ligand L includes ethene, benzene, phenol, ammonia, acetaldehyde, acetone, and N,N-dimethylformamide. In all cases, the primary product formed corresponds to endothermic loss of the neutral ligand and the only other product observed is the result of ligand exchange processes to form NaXe+. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for Na+−L after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at several levels of theory compare favorably to the experimentally determined bond energies for these and previously studied systems, L = Ar, CO, dimethyl ether, H2O, methylamine, imidazole, dimethoxyethane, and several alcohols. Combined, these ligands cover a very wide range in binding energies, and thereby help to establish an absolute scale for sodium cat...

Journal ArticleDOI
TL;DR: In this paper, the authors provided a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyne 1 provided the following are considered: Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT calculations at those locations where the former description becomes unstable.
Abstract: DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyne 1 provided the following are considered. (a) Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT (UDFT) calculations at those locations where the former description becomes unstable. This is the case in the region of the p-didehydrobenzene biradical 2, which possesses significant multireference character. (b) LSD and pure GGA functionals are more stable than hybrid functionals, which can be directly related to the composition of these functionals. With increasing instability, RDFT calculations lead to increasing errors in the S−T splitting and the geometry of 2 as well as in the energetics of the Bergman reaction. (c) LSD and GGA functionals underestimate the energy barrier of the Bergman reaction of 1. This becomes obvious when the correct experimental barrier is considered, which was not done in previous DFT investigations. (d) The best description of the Berg...