Showing papers in "Journal of Physical Chemistry A in 2013"
TL;DR: A comparative assessment of the performance of the M06 suite of density functionals against an MP2 benchmark for calculating the relative energies and geometric structures of the Cl(-)·arginine and Br(-)-amino acid clusters is presented, providing insight into the types of physical systems where each of these functionals should perform best.
Abstract: We present a comparative assessment of the performance of the M06 suite of density functionals (M06, M06-2X, and M06-HF) against an MP2 benchmark for calculating the relative energies and geometric structures of the Cl(-)·arginine and Br(-)·arginine halide ion-amino acid clusters. Additional results are presented for the popular B3LYP density functional. The Cl(-)·arginine and Br(-)·arginine complexes are important prototypes for the phenomenon of anion-induced zwitterion formation. Results are presented for the canonical (noncharge separated) and zwitterionic (charge separated) tautomers of the clusters, as well as the numerous conformational isomers of the clusters. We find that all of the M06 functions perform well in terms of predicting the general trends in the conformer relative energies and identifying the global minimum conformer. This is in contrast to the B3LYP functional, which performed significantly less well for the canonical tautomers of the clusters where dispersion interactions contribute more significantly to the conformer energetics. We find that the M06 functional gave the lowest mean unsigned error for the relative energies of the canonical conformers (2.10 and 2.36 kJ/mol for Br(-)·arginine and Cl(-)·arginine), while M06-2X gave the lowest mean unsigned error for the zwitterionic conformers (0.85 and 1.23 kJ/mol for Br(-)·arginine and Cl(-)·arginine), thus providing insight into the types of physical systems where each of these functionals should perform best.
501 citations
TL;DR: This work considers an alternative formulation of the master equation for complex-forming chemical reactions with multiple wells and bimolecular products and considers the situation where some of the chemical eigenvalues approach the energy relaxation time scale and how to modify the phenomenological rate coefficients.
Abstract: We consider an alternative formulation of the master equation for complex-forming chemical reactions with multiple wells and bimolecular products. Within this formulation the dynamical phase space consists of only the microscopic populations of the various isomers making up the reactive complex, while the bimolecular reactants and products are treated equally as sources and sinks. This reformulation yields compact expressions for the phenomenological rate coefficients describing all chemical processes, i.e., internal isomerization reactions, bimolecular-to-bimolecular reactions, isomer-to-bimolecular reactions, and bimolecular-to-isomer reactions. The applicability of the detailed balance condition is discussed and confirmed. We also consider the situation where some of the chemical eigenvalues approach the energy relaxation time scale and show how to modify the phenomenological rate coefficients so that they retain their validity.
425 citations
TL;DR: Using density functional calculations, this work systematically investigated the hydrogen storage properties of the two-dimensional Ti2C phase, which is a representative of the recently synthesized MXene materials and confirmed the hydrogen molecules bound by Kubas-type interaction can be adsorbed and released reversibly under ambient conditions.
Abstract: Searching for reversible hydrogen storage materials operated under ambient conditions is a big challenge for material scientists and chemists. In this work, using density functional calculations, we systematically investigated the hydrogen storage properties of the two-dimensional (2D) Ti2C phase, which is a representative of the recently synthesized MXene materials ( ACS Nano 2012 , 6 , 1322 ). As a constituent element of 2D Ti2C phase, the Ti atoms are fastened tightly by the strong Ti-C covalent bonds, and thus the long-standing clustering problem of transition metal does not exist. Combining with the calculated binding energy of 0.272 eV, ab initio molecular dynamic simulations confirmed the hydrogen molecules (3.4 wt % hydrogen storage capacity) bound by Kubas-type interaction can be adsorbed and released reversibly under ambient conditions. Meanwhile, the hydrogen storage properties of the other two MXene phases (Sc2C and V2C) were also evaluated, and the results were similar to those of Ti2C. Therefore, the MXene family including more than 20 members was expected to be a good candidate for reversible hydrogen storage materials under ambient conditions.
365 citations
TL;DR: A novel definition of bond order is proposed, called the Laplacian bond order (LBO), which is defined as a scaled integral of negative parts of the LaPlacian of electron density in fuzzy overlap space, which has a direct correlation with the bond polarity, the bond dissociation energy, and the bond vibrational frequency.
Abstract: Bond order is an important concept for understanding the nature of a chemical bond. In this work, we propose a novel definition of bond order, called the Laplacian bond order (LBO), which is defined as a scaled integral of negative parts of the Laplacian of electron density in fuzzy overlap space. Many remarkable features of LBO are exemplified by numerous structurally diverse molecules. It is shown that LBO has a direct correlation with the bond polarity, the bond dissociation energy, and the bond vibrational frequency. The dissociation behavior of LBO of the N–N bond in N2 has been studied. Effects of the basis sets, theoretic methods, and geometrical conformations on LBO have also been investigated. Through comparisons, we discussed in details similarities and discrepancies among LBO, Mayer bond order, natural localized molecular orbital bond order, fuzzy overlap population, and electron density at bond critical points.
327 citations
TL;DR: In this paper, a kinetic-based structure-activity relationship for quercetin is analyzed by quantum chemical calculations, which support the knowledge acquired from experimental studies, in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom-and electron-transfer efficiency.
Abstract: Polyphenols (synthetically modified or directly provided by human diet) scavenge free radicals by H-atom transfer and may thus decrease noxious effects due to oxidative stress. Free radical scavenging by polyphenols has been widely theoretically studied from the thermodynamic point of view whereas the kinetic point of view has been much less addressed. The present study describes kinetic-based structure–activity relationship for quercetin. This compound is very characteristic of the wide flavonoid subclass of polyphenols. H-atom transfer is a mechanism based on either atom or electron transfer. This is analyzed here by quantum chemical calculations, which support the knowledge acquired from experimental studies. The competition between the different processes is discussed in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom- and electron-transfer efficiency. The role of the catechol moiety and the 3-OH group of quercetin as scavengers of ...
217 citations
TL;DR: Compounds showing this mechanism are highly attractive for applications in OLEDs or LEECs as, in principle, it is possible to harvest all singlet and triplet excitons for the generation of light in the lowest excited singlet state.
Abstract: With the chelating aminophosphane ligands Ph2P-(o-C6H4)-N(CH3)2 (PNMe2) and Ph2P-(o-C6H4)-NC4H8 (PNpy), the four halide (Cl, Br, I)-bridged copper coordination compounds [Cu(μ-Cl)(PNMe2)]2 (1), [Cu(μ-Br)(PNMe2)]2 (2), [Cu(μ-I)(PNMe2)]2 (3), and [Cu(μ-I)(PNpy)]2 (4) were synthesized and structurally characterized. Their photophysical properties were studied in detail. The complexes exhibit strong blue (λmax = 464 (3) and 465 nm (4)) and green (λmax = 506 (1) and 490 nm (2)) luminescence as powders with quantum yields of up to 65% at decay times as short as 4.1 μs. An investigation of the emission decay behavior between 1.3 and 300 K gives insight into the nature of the emitting states. At temperatures below T ≈ 60 K, the decay times of the studied compounds are several hundred microseconds long, which indicates that the emission originates from a triplet state (T1 state). DFT calculations show that this state is of (metal+halide)-to-ligand charge transfer 3(M+X)LCT character. Investigations at 1.3 K allow ...
214 citations
TL;DR: Temperature dependent Raman spectra suggest that at low temperature the softening of the C-H stretching frequencies is due to the decrease in steric hindrance between the methyl groups of methyl imidazole, and opens the window for increased nitrogen and methane uptake at temperatures below 153 K.
Abstract: Here we have used Raman spectroscopy to investigate molecular level changes in the zeolitic imidazolate framework ZIF-8 (a prototypical zeolite-like porous metal organic framework) as a function of temperature. Temperature dependent Raman spectra suggest that at low temperature the softening of the C–H stretching frequencies is due to the decrease in steric hindrance between the methyl groups of methyl imidazole. The larger separation between the methyl groups opens the window for increased nitrogen and methane uptake at temperatures below 153 K. The appearance of Raman bands at 2323 cm–1 and 2904 cm–1 at or below 153 K in ZIF-8 are characteristic signatures of the adsorbed nitrogen and methane gases respectively. Nanoscale ZIF-8 uptakes more molecules than bulk ZIF-8, and as a result we could provide evidence for encaged CO2 at 203 K yielding its Raman mode at 1379 cm–1.
199 citations
TL;DR: Both non-specular, inelastic scattering rules lead to excellent agreement between predictions and experimental mobility measurements and that polarization potentials must be considered to make correct predictions for high-mobility particles/ions.
Abstract: Ion/electrical mobility measurements of nanoparticles and polyatomic ions are typically linked to particle/ion physical properties through either application of the Stokes–Millikan relationship or comparison to mobilities predicted from polyatomic models, which assume that gas molecules scatter specularly and elastically from rigid structural models. However, there is a discrepancy between these approaches; when specular, elastic scattering models (i.e., elastic-hard-sphere scattering, EHSS) are applied to polyatomic models of nanometer-scale ions with finite-sized impinging gas molecules, predictions are in substantial disagreement with the Stokes–Millikan equation. To rectify this discrepancy, we developed and tested a new approach for mobility calculations using polyatomic models in which non-specular (diffuse) and inelastic gas-molecule scattering is considered. Two distinct semiempirical models of gas-molecule scattering from particle surfaces were considered. In the first, which has been traditional...
184 citations
TL;DR: The optical properties of gadolinium gallium aluminum garnet, Gd3(Ga,Al)5O12, doped with Ce(3+) are investigated as a function of the Ga/Al ratio to improve understanding of the energy flow and luminescence quenching in these materials.
Abstract: The optical properties of gadolinium gallium aluminum garnet, Gd3(Ga,Al)5O12, doped with Ce(3+) are investigated as a function of the Ga/Al ratio, aimed at an improved understanding of the energy flow and luminescence quenching in these materials. A decrease of both the crystal field strength and band gap with increasing content of Ga(3+) is observed and explained by the geometrical influence of Ga(3+) on the crystal field splitting of the 5d level in line with theoretical work of Munoz-Garcia et al. ( unoz-Garcia, A. B.; Seijo, L. Phys. Rev. B 2010, 82, 184118 ). Thermal quenching results in shorter decay times as well as reduced emission intensities for all samples in the temperature range from 100 to 500 K. An activation energy for emission quenching is calculated from the data. The band gap of the host is measured upon Ga substitution and the decrease in band gap is related to Ga(3+) substitution into tetrahedral sites after all octahedral sites are occupied in the garnet material. Based on the change in band gap and crystal field splitting, band diagrams can be constructed explaining the low thermal quenching temperatures in the samples with high Ga content. The highest luminescence intensity is found for Gd3(Ga,Al)5O12 with 40% of Al(3+) replaced by Ga(3+).
183 citations
TL;DR: In this paper, the authors reported femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2′-bipyridine) in water.
Abstract: X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (∼1012 photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2′-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is ∼160 fs.
167 citations
TL;DR: The present study may be helpful in understanding how the position of NO2 group affects the decomposition kinetics of substituted imidazoles.
Abstract: We have investigated the decomposition kinetics of imidazole, 2-nitroimidazole, and 4-nitroimidazole using TG-DTA technique under nitrogen atmosphere. Isoconversional methods were used for the evaluation of kinetic parameters from the kinetic data of different heating temperatures. The Friedman method provided comparably higher values of activation energy than the Flynn–Wall–Ozawa method. Imidazole, 2-nitroimidazole, and 4-nitroimidazole were decomposed by the multistep reaction mechanism evident from the nonlinear relationship of activation energy and the conversion rate. The NO2 elimination and nitro–nitrite isomerization are expected to be competitive reactions in the decomposition of 2-nitroimidazole and 4-nitroimidazole. The present study may be helpful in understanding how the position of NO2 group affects the decomposition kinetics of substituted imidazoles.
TL;DR: This work presents a high-dimensional neural network (NN) potential for water clusters based on density-functional theory (DFT) calculations, which is constructed using clusters containing up to 10 monomers and is in principle able to meet all these requirements.
Abstract: The fundamental importance of water for many chemical processes has motivated the development of countless efficient but approximate water potentials for large-scale molecular dynamics simulations, from simple empirical force fields to very sophisticated flexible water models. Accurate and generally applicable water potentials should fulfill a number of requirements. They should have a quality close to quantum chemical methods, they should explicitly depend on all degrees of freedom including all relevant many-body interactions, and they should be able to describe molecular dissociation and recombination. In this work, we present a high-dimensional neural network (NN) potential for water clusters based on density-functional theory (DFT) calculations, which is constructed using clusters containing up to 10 monomers and is in principle able to meet all these requirements. We investigate the reliability of specific parametrizations employing two frequently used generalized gradient approximation (GGA) exchan...
TL;DR: A negative correlation between ΔEst and efficiency of the delayed fluorescence was observed, ascribed to a lower intersystem crossing rate, kISC, and increased nonradiative decay from S1, k(s)nrs, in polar solvents.
Abstract: Thermally activated delayed fluorescence (TADF) is fluorescence arising from a reverse intersystem crossing (RISC) from the lowest triplet (T1) to the singlet excited state (S1), where these states are separated by a small energy gap (ΔEst), followed by a radiative transition to the ground state (S0). Rate constants relating TADF processes in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) were determined at four different solvent polarities (toluene, dichloromethane, ethanol, and acetonitrile). We revealed that the rate constant of RISC, kRISC, which is the most important factor for TADF, was significantly enhanced by a reduced ΔEst in more polar solvents. The smaller ΔEst was mainly attributable to a stabilization of the S1 state. This stabilization also induced a Stokes shift in fluorescence through a relatively large change of the dipole moment between S1 and S0 states (17 D). Despite of this factor, we observed a negative correlation between ΔEst and efficiency of the delayed fluorescence (φd). This was ascribed to a lower intersystem crossing rate, kISC, and increased nonradiative decay from S1, k(s)nrs, in polar solvents.
TL;DR: The formation mechanism of polycyclic aromatic hydrocarbons with three fused aromatic rings starting from naphthalene has been studied using accurate ab initio G3(MP2,CC)//B3LYP/6-311G** calculations followed by the kinetic analysis of various reaction pathways and computations of relative product yields.
Abstract: The formation mechanism of polycyclic aromatic hydrocarbons (PAH) with three fused aromatic rings starting from naphthalene has been studied using accurate ab initio G3(MP2,CC)//B3LYP/6-311G** calculations followed by the kinetic analysis of various reaction pathways and computations of relative product yields. The results reveal new insights into the classical hydrogen abstraction–C2H2 addition (HACA) scheme of PAH growth. The HACA mechanism has been shown to produce mostly cyclopentafused PAHs instead of PAHs with six-member rings only, in contrast to the generally accepted view on this mechanism. Considering naphthalene as the initial reactant, the HACA-type synthesis of higher PAHs with all six-member rings, anthracene and phenanthrene, accounts only for 3–6% of the total product yield at temperatures relevant to combustion (1000–2000 K), whereas cyclopentafused PAHs, including acenaphthalene (41–48%), 4-ethynylacenaphthalene (∼14%), 3-ethynylacenaphthalene (∼7.5%), 1-methylene-1H-cyclopenta[b]naphtha...
TL;DR: It is observed that maximum hardness principle is more likely to fail in the cases of very hard species like F(-), H(2), CH(4), N(2, and OH appearing in the reactant side and in most cases of the association reactions.
Abstract: Hardness and electrophilicity values for several molecules involved in different chemical reactions are calculated at various levels of theory and by using different basis sets. Effects of these aspects as well as different approximations to the calculation of those values vis-a-vis the validity of the maximum hardness and minimum electrophilicity principles are analyzed in the cases of some representative reactions. Among 101 studied exothermic reactions, 61.4% and 69.3% of the reactions are found to obey the maximum hardness and minimum electrophilicity principles, respectively, when hardness of products and reactants is expressed in terms of their geometric means. However, when we use arithmetic mean, the percentage reduces to some extent. When we express the hardness in terms of scaled hardness, the percentage obeying maximum hardness principle improves. We have observed that maximum hardness principle is more likely to fail in the cases of very hard species like F–, H2, CH4, N2, and OH appearing in t...
TL;DR: A procedure to prepare Au(67)(SR)(35) nanomolecule, which is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles, allowing high-yield isolation of the title compound.
Abstract: The preparation of gold nanomolecules with sizes other than Au25(SR)18 ,A u 38(SR)24 ,A u 102(SR)44, and Au144(SR)60 has been hampered by stability issues and low yields. Here we report a procedure to prepare Au67(SR)35, for either R = −SCH2CH2Ph or -SC6H13, allowing high-yield isolation (34%, ∼10-mg quantities) of the title compound. Product high purity is assessed at each synthesis stage by rapid MALDI−TOF mass-spectrometry (MS), and high-resolution electrospray-ionization MS confirms the Au67(SR)35 composition. Electronic properties were explored using optical absorption spectroscopy (UV− visible−NIR regions) and electrochemistry (0.74 V spacing in differential-pulsed-voltammetry), modes of ligand binding were studied by NMR spectroscopy ( 13 C and 1 H), and structural characteristics of the metal atom core were determined by powder X- ray measurements. Models featuring a Au17 truncated-decahedral inner core encapsulated by the 30 anchoring atoms of 15 staple- motif units have been investigated with first-principles electronic structure calculations. This resulted in identification of a structure consistent with the experiments, particularly, the opening of a large gap (∼0.75 eV) in the (2−) charge-state of the nanomolecule. The electronic structure is analyzed within the framework of a superatom shell model. Structurally, the Au67(SR)35 nanomolecule is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles. Its electronic HOMO−LUMO gap (∼0.75 eV) is nearly double that of the larger Au102 compound and it is much smaller than that of the Au38 one. The intermediary status of the Au67(SR)35 nanomolecule is also reflected in both its optical and electrochemical characteristics.
TL;DR: A deeper understanding of the effect that temperature has on the Raman spectral characteristics of a metal oxide such as WO3 has helped to extend the knowledge regarding the behavior of metal oxide-gas interactions for sensing applications.
Abstract: Metal oxides are suitable for detecting, through conductive measurements, a variety of reducing and oxidizing gases in environmental and sensing applications. Metal-oxide gas sensors can be developed with the goal of sensing gases under specific conditions and, as a whole, are heavily dependent on the manufacturing process. Tungsten oxide (WO3) is a promising metal-oxide material for gas-sensing applications. The purpose of this paper is to determine the existence of a correlation between thermal effects and the changes in the Raman spectra for multiple WO3 structures. We have obtained results utilizing Raman spectroscopy for three different structures of WO3 (monoclinic WO3 on Si substrate, nanopowder, and nanowires) that have been subjected to temperatures in the range of 30-160 °C. The major vibrational modes of the WO3:Si and the nanopowder samples, located at ~807, ~716, and ~271 cm(-1), correspond to the stretching of O-W-O bonds, the stretching of W-O, and the bending of O-W-O, respectively; these are consistent with a monoclinic WO3 structure. However in the nanowires sample only asymmetric stretching of the W-O bonds occurs, resulting in a 750 cm(-1) band, and the bending of the O-W-O mode (271 cm(-1)) is a stretching mode (239 cm(-1)) instead, suggesting the nanowires are not strictly monoclinic. The most notable effect of increasing the temperature of the samples is the appearance of the bending mode of W-OH bonds in the approximate range of 1550-1150 cm(-1), which is related to O-H bonding caused by humidity effects. In addition, features such as those at 750 cm(-1) for nanowires and at 492 and 670 cm(-1) for WO3:Si disappear as the temperature increases. A deeper understanding of the effect that temperature has on the Raman spectral characteristics of a metal oxide such as WO3 has helped to extend our knowledge regarding the behavior of metal oxide-gas interactions for sensing applications. This, in turn, will help to develop theoretical models for the identification of specific metal oxide-gas relationships.
TL;DR: It was demonstrated for collinear H + H2 reactive scattering on the BKMP2 potential energy surface that reaction probabilities computed via the SQC methodology using a Gaussian window function of 1/2 unit width produces good agreement with quantum mechanical results over the 0.4-0.6 eV energy range.
Abstract: A microscopically reversible approach toward computing reaction probabilities via classical trajectory simulation has been developed that bins trajectories symmetrically on the basis of their initial and final classical actions. The symmetrical quasi-classical (SQC) approach involves defining a classical action window function centered at integer quantum values of the action, choosing a width parameter that is less than unit quantum width, and applying the window function to both initial reactant and final product vibrational states. Calculations were performed using flat histogram windows and Gaussian windows over a range of width parameters. Use of the Wigner distribution function was also investigated as a possible choice. It was demonstrated for collinear H + H2 reactive scattering on the BKMP2 potential energy surface that reaction probabilities computed via the SQC methodology using a Gaussian window function of 1/2 unit width produces good agreement with quantum mechanical results over the 0.4–0.6 ...
TL;DR: An ab initio MP2/aug'-cc-pVTZ study has been carried out on complexes formed between PO2X (X = F and Cl) as the Lewis acids and a series of nitrogen bases ZN, including NH3, H2C═NH, NH2F, NP, NCH, NCF, NF3, and N2.
Abstract: An ab initio MP2/aug′-cc-pVTZ study has been carried out on complexes formed between PO2X (X = F and Cl) as the Lewis acids and a series of nitrogen bases ZN, including NH3, H2C═NH, NH2F, NP, NCH, NCF, NF3, and N2. Binding energies of these complexes vary from −10 to −150 kJ/mol, and P—N distances from 1.88 to 2.72 A. Complexes ZN:PO2F have stronger P...N bonds and shorter P—N distances than the corresponding complexes ZN:PO2Cl. Charge transfer from the N lone pair through the π-hole to the P—X and P—O σ* orbitals leads to stabilization of these complexes, although charge-transfer energies can be evaluated only for complexes with binding energies less than −71 kJ/mol. Complexation of PO2X with the strongest bases leads to P···N bonds with a significant degree of covalency, and P—N distances that approach the P—N distances in the molecules PO2NC and PO2NH2. In these complexes, the PO2X molecules distort from planarity. Changes in 31P absolute chemical shieldings upon complexation do not correlate with chan...
TL;DR: The strongest bonds found in chemistry involve protonated species of hydrogen cyanide, carbon monoxide, and dinitrogen and CCSD(T)/CBS calculations reveal that bond dissociation energies are misleading strength descriptors.
Abstract: Increasing the effective electronegativity of two atoms forming a triple bond can increase the strength of the latter. The strongest bonds found in chemistry involve protonated species of hydrogen cyanide, carbon monoxide, and dinitrogen. CCSD(T)/CBS (complete basis set) and G4 calculations reveal that bond dissociation energies are misleading strength descriptors. The strength of the bond is assessed via the local stretching force constants, which suggest relative bond strength orders (RBSO) between 2.9 and 3.4 for heavy atom bonding (relative to the CO bond strength in methanol (RBSO = 1) and formaldehyde (RBSO = 2)) in [HCNH]+(1Σ+), [HCO]+(1Σ+), [HNN]+(1Σ+), and [HNNH]2+(1Σg+). The increase in strength is caused by protonation, which increases the electronegativity of the heavy atom and thereby decreases the energy of the bonding AB orbitals (A, B: C, N, O). A similar effect can be achieved by ionization of a nonbonding or antibonding electron in CO or NO. The strongest bond with a RBSO value of 3.38 i...
TL;DR: This work investigates the structure and energetics of the various accessible oxidation states of ExBox(4+) using a combination of spectroscopy and computation, and examines photoinitiated electron transfer from perylene bound within ExBox (4+) (ExBox( 4+)⊂perylene) using visible and near-infrared femtosecond transient absorption (fsTA) spectroscope.
Abstract: Multielectron acceptors are essential components for artificial photosynthetic systems that must deliver multiple electrons to catalysts for solar fuels applications. The recently developed boxlike cyclophane incorporating two extended viologen units joined end-to-end by two p-phenylene linkers—namely, ExBox4+—has a potential to be integrated into light-driven systems on account of its ability to complex with π-electron-rich guests such as perylene, which has been utilized to great extent in many light-harvesting applications. Photodriven electron transfer to ExBox4+ has not previously been investigated, however, and so its properties, following photoreduction, are largely unknown. Here, we investigate the structure and energetics of the various accessible oxidation states of ExBox4+ using a combination of spectroscopy and computation. In particular, we examine photoinitiated electron transfer from perylene bound within ExBox4+ (ExBox4+⊂perylene) using visible and near-infrared femtosecond transient absor...
TL;DR: It is shown that a combination of DFT energies with small atom-centered basis sets, the D3 dispersion correction, and the gCP correction can accurately describe van der Waals and hydrogen-bonded crystals.
Abstract: We extend the previously developed geometrical correction for the inter- and intramolecular basis set superposition error (gCP) to periodic density functional theory (DFT) calculations. We report g...
TL;DR: The combination of polarization-resolved mid-infrared spectroscopy of marker modes and time-dependent density functional theory (TD-DFT) provides key insights into the transient structures of the molecular chromophore during ultrafast isomerization dynamics.
Abstract: We characterize the structural and electronic changes during the photoinduced enol-keto tautomerization of 2-(2'-hydroxyphenyl)-benzothiazole (HBT) in a nonpolar solvent (tetrachloroethene). We quantify the redistribution of electronic charge and intramolecular proton translocation in real time by combining UV-pump/IR-probe spectroscopy and quantum chemical modeling. We find that the photophysics of this prototypical molecule involves proton coupled electron transfer (PCET), from the hydroxyphenyl to the benzothiazole rings, resulting from excited state intramolecular proton transfer (ESIPT) coupled to electron transfer through the conjugated double bond linking the two rings. The combination of polarization-resolved mid-infrared spectroscopy of marker modes and time-dependent density functional theory (TD-DFT) provides key insights into the transient structures of the molecular chromophore during ultrafast isomerization dynamics.
TL;DR: Neutral silver clusters, Ag(n), were studied using density functional theory (DFT) followed by high level coupled cluster CCSD(T) calculations to determine the low energy isomers for each cluster size for small clusters to predict that the normalized atomization energies start to converge slowly to the bulk at n = 55.
Abstract: Neutral silver clusters, Ag(n), were studied using density functional theory (DFT) followed by high level coupled cluster CCSD(T) calculations to determine the low energy isomers for each cluster size for small clusters. The normalized atomization energy, heats of formation, and average bond lengths were calculated for each of the different isomeric forms of the silver clusters. For n = 2-6, the preferred geometry is planar, and the larger n = 7-8 clusters prefer higher symmetry, three-dimensional geometries. The low spin state is predicted to be the ground state for every cluster size. A number of new low energy isomers for the heptamer and octamer were found. Additional larger Ag(n) structures, n < 100, were initially optimized using a tree growth-hybrid genetic algorithm with an embedded atom method (EAM) potential. For n ≤ 20, DFT was used to optimize the geometries. DFT with benchmarked functionals were used to predict that the normalized atomization energies ((AE)s) for Ag(n) start to converge slowly to the bulk at n = 55. The (AE) for Ag99 is predicted to be ~50 kcal/mol.
TL;DR: The Random First Order Transition Theory and the Coupling Model successfully predict the large surface-enhancement of mobility and its increase on cooling, but disagree with the experimental observation of the faster surface diffusion of Nifedipine.
Abstract: Surface self-diffusion coefficients have been determined for the organic glass Nifedipine using the method of surface grating decay. The flattening of 1000 nm surface gratings occurs by viscous flow at 12 K or more above the glass transition temperature and by surface diffusion at lower temperatures. Surface diffusion is at least 107 times faster than bulk diffusion, indicating a highly mobile surface. Nifedipine glasses have faster surface diffusion than the previously studied Indomethacin glasses, despite their similar bulk relaxation times. Both glasses exhibit fast surface crystal growth, and its rate scales with surface diffusivity. The observed rate of surface diffusion implies substantial surface rearrangement during the preparation of low-energy glasses by vapor deposition. The Random First Order Transition Theory and the Coupling Model successfully predict the large surface-enhancement of mobility and its increase on cooling, but disagree with the experimental observation of the faster surface di...
TL;DR: To obtain insight into the physical nature of these bonds, Bader's Quantum Theory of Atoms-in-Molecules (QTAIM) is used and Pendás' Interacting Quantum Atoms (IQA) scheme is applied, which enables rigorous and physical study of each interaction at work in the formation of the halogen-bonded complexes.
Abstract: In this article, we report a detailed study on halogen bonds in complexes of CHCBr, CHCCl, CH2CHBr, FBr, FCl, and ClBr with a set of Lewis bases (NH3, OH2, SH2, OCH2, OH–, Br–). To obtain insight into the physical nature of these bonds, we extensively used Bader’s Quantum Theory of Atoms-in-Molecules (QTAIM). With this aim, in addition to the examination of the bond critical points properties, we apply Pendas’ Interacting Quantum Atoms (IQA) scheme, which enables rigorous and physical study of each interaction at work in the formation of the halogen-bonded complexes. In particular, the influence of primary and secondary interactions on the stability of the complexes is analyzed, and the roles of electrostatics and exchange are notably discussed and compared. Finally, relationships between QTAIM descriptors and binding energies are inspected.
TL;DR: CH2OO plays a critical role in the formation of oligomeric hydroperoxides followed by SOA formation in ethylene ozonolysis, and a new formation mechanism for the oligomers, which includes sequential addition of CH2OO to hydro peroxides is proposed.
Abstract: Ethylene ozonolysis was investigated in laboratory experiments using a Teflon bag reactor. A negative ion chemical ionization mass spectrometer (NI-CIMS) using SO2Cl– and Cl– as reagent ions was used for product analysis. In addition to the expected gas-phase products, such as formic acid and hydroperoxymethyl formate, oligomeric hydroperoxides composed of the Criegee intermediate (CH2OO) as a chain unit were observed. Furthermore, we observed secondary organic aerosol (SOA) formation from the ethylene ozonolysis, and the particle-phase products were also analyzed by NI-CIMS. The CH2OO oligomers were also observed as particle-phase components, suggesting that the oligomeric hydroperoxides formed in the gas phase partition into the particle phase. By adding methanol as a stabilized Criegee intermediate scavenger, both the gas-phase oligomer formation and SOA formation were strongly suppressed. This indicates that CH2OO plays a critical role in the formation of oligomeric hydroperoxides followed by SOA form...
TL;DR: Comparisons of measured and model predicted pyrolysis speciation provides validation of theoretically calculated channels for the oxidation of DMF and Sensitivity and reaction flux analyses highlight important reactions as well as the primary reaction pathways responsible for the decomposition ofDMF and formation and destruction of key intermediate and product species.
Abstract: A detailed kinetic model describing the oxidation of 2,5-dimethylfuran (DMF), a potential second-generation biofuel, is proposed. The kinetic model is based upon quantum chemical calculations for the initial DMF consumption reactions and important reactions of intermediates. The model is validated by comparison to new DMF shock tube ignition delay time measurements (over the temperature range 1300 – 1831 K and at nominal pressures of 1 and 4 bar) and the DMF pyrolysis speciation measurements of Lifshitz et al. [J. Phys. Chem. A 102 (52) (1998) 10655-10670]. Globally, modeling predictions are in good agreement with the considered experimental targets. In particular, ignition delay times are predicted well by the new model, with model-experiment deviations of at most a factor of two, and DMF pyrolysis conversion is predicted well, to within experimental scatter of the Lifshitz et al. data. Additionally, comparisons of measured and model predicted pyrolysis speciation provides validation of theoretically calculated channels for the oxidation of DMF. Sensitivity and reaction flux analyses highlight important reactions as well as the primary reaction pathways responsible for the decomposition of DMF and formation and destruction of key intermediate and product species.
TL;DR: The present study underlines the importance of solvent polarization effects in hydration of ions with high charge density and implemented the electronic continuum correction by implementing the electronic polarizability of the medium.
Abstract: Molecular dynamics simulations of concentrated aqueous solutions of LiCl and Li2SO4 were conducted in order to provide molecular insight into recent neutron scattering data. The structures predicted from the molecular dynamics simulations using standard nonpolarizable force fields provided a very poor fit to the experiment; therefore, refinement was needed. The electronic polarizability of the medium was effectively accounted for by implementing the electronic continuum correction, which practically means rescaling the ionic charges. Consistent with previous studies, we found that this approach in each case provided a significantly improved fit to the experimental data, which was further enhanced by slightly adjusting the radius of the lithium ion. The polarization effect was particularly pronounced in the Li2SO4 solution where the ions in the nonpolarizable simulations tended to cluster unphysically. With the above alterations, the employed force field displayed an excellent fit to the neutron scattering...
TL;DR: A new spectroscopic model is developed for theoretical predictions of vibration-rotation line positions and line intensities of the methane molecule and allows generating a spectral line list for the Dyad and Pentad bands with the accuracy ~10(-3) cm(-1) for line positions combined with ab initio predictions for line intensity.
Abstract: A new spectroscopic model is developed for theoretical predictions of vibration-rotation line positions and line intensities of the methane molecule. Resonance coupling parameters of the effective polyad Hamiltionians were obtained via high-order contact transformations (CT) from ab initio potential energy surface. This allows converging vibrational and rotational levels to the accuracy of best variational calculations. Average discrepancy with centers of 100 reliably assigned experimental bands up to the triacontad range was 0.74 cm(-1) and 0.001 cm(-1) for GS rotational levels up to J = 17 in direct CT calculations without adjustable parameters. A subsequent "fine tuning" of the diagonal parameters allows achieving experimental accuracy for about 5600 Dyad and Pentad line positions, whereas all resonance coupling parameters were held fixed to ab initio values. Dipole transition moment parameters were determined from selected ab initio line strengths previously computed from a dipole moment surface by variational method. New polyad model allows generating a spectral line list for the Dyad and Pentad bands with the accuracy ~10(-3) cm(-1) for line positions combined with ab initio predictions for line intensities. The overall integrated intensity agreement with Hitran-2008 empirical database is of 4.4% for the Dyad and of 1.8% for the Pentad range.