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The NH 4 + ( NH 3 ) n [n=0–6] clusters have been studied using ab initio calculations. For n=0 and 1, the geometry of clusters are optimized at B3LYP, MP2, CCD and QCISD levels with several basis sets, and the binding energies are compared to experimental results to find the reliable and less computationally demanding methods for the calculations of larger clusters. For n=2–6, the geometry optimizations and NH stretching vibrational spectra are performed at B3LYP and MP2 levels with 6-31+G* basis set. The binding energies are corrected by basis set superposition errors (BSSE) and zero-point vibrational energies (ZPVE). These two approaches that correspondingly predict the filled first solvation shell are the lowest in energies at n=1–4. The vibrational frequency shift of ammonium molecules have been investigated along with the frequency characteristics depending on the presence/absence of outer-shell ammonia molecules. In this study, the calculated binding energies and the characteristic NH stretching vibrational frequency shift are in good agreement with experimental data. In addition, the barriers of proton transfer between two heavy atoms and the internal rotation of ammonia molecular along the NH axis in NH4+(NH3)n are estimated with several levels.

The motion of a spherical Brownian particle in an asymmetric periodic channel is considered. Under an external periodic stimulus, the particle switches between two states with different particle radius, every half-period. Using Brownian dynamics simulations, we show that the particle size oscillation, combined with the asymmetry of the channel, induces a drift along the channel axis, directed towards the steeper wall of the channel. The oscillation of the particle size is accompanied by a time variation of the space accessible to the particle and by an oscillation of its diffusion coefficient. The former underlies the drift inducing mechanism of purely entropic nature. The latter, combined with the former, leads to a significant amplification of the effect. The drift velocity vanishes when interconversion between the states occurs either very slow or very fast, having a maximum in between. The position and magnitude of the maximum are discussed by providing an analytical approach based on intuitively appealing assumptions.

The results of a theoretical investigation of coherent -electron dynamics for nonplanar (P)2,2’-biphenol induced by ultrashort linearly polarized UV pulses are presented. The coherent ring currents originate from an excitation of a pair of quasi-degenerate electronic states by an ultrashort linearly polarized UV laser pulse. The magnitudes of the electronic ring currents are expressed as the sum of expectation values of the corresponding operators in the two phenol rings (L and R rings). Here, L (R) denotes the phenol ring in the left (right)-hand side of (P)2,2’-biphenol. For electrons in (P)-2,2’-biphenol, there are four possible rotational patterns. The bond current of the bridge bond linking the L and R rings is zero for the symmetric coherent state, while it is nonzero for the antisymmetric coherent state.

The pressure-dependent interactions between the ionic liquid mixture ([MPI][I1.5]) and nano-TiO2 surfaces have been studied up to 2.5 GPa. The results of infrared spectroscopic profiles of [MPI][I1.5] and [MPI][I1.5]-nano-TiO2 indicated that no appreciable changes in the C-H stretching bands with the addition of nano-TiO2 were observed under ambient pressure. As the pressure was elevated to 0.7 GPa, the C-H stretching absorption of [MPI][I1.5] underwent band-narrowing and red-shifts in frequency. In contrast to the results of [MPI][I1.5], the spectra of [MPI][I1.5]-nano-TiO2 do not show dramatic changes under high pressures. A possible explanation for this observation is the formation of certain pressure-enhanced C-H···nano-TiO2 interactions around the imidazolium C-H and alkyl C-H groups. As imidazolium C-H···I(-) is replaced by the weaker imidazolium C-H···polyiodide, the splitting of the imidazolium C-H stretching bands was observed. The experimental results indicate that both nano-TiO2 and polyiodides are capable of disturbing the self-assembly of ionic liquids. This study suggests the possibility to tune the efficiency of dye-sensitized solar cells via a high pressure method.

Damped harmonic oscillators are utilized to calculate Franck-Condon factors within displaced harmonic oscillator approximation. This is practically done by scaling unperturbed Hessian matrix that represents local modes of force constants for molecule in gaseous phase, and then by diagonalizing perturbed Hessian matrix it results in direct modification of Huang-Rhys factors which represent normal modes of solute molecule perturbed by solvent environment. Scaling parameters are empirically introduced for simulating absorption and fluorescence spectra of an isolated solute molecule in solution. The present method is especially useful for simulating vibronic spectra of polycyclic aromatic hydrocarbon molecules in which hydrogen atom vibrations in solution can be scaled equally, namely the same scaling factor being applied to all hydrogen atoms in polycyclic aromatic hydrocarbons. The present method is demonstrated in simulating solvent enhanced X (1)Ag ↔ A(1)B1u absorption and fluorescence spectra of perylene (medium-sized polycyclic aromatic hydrocarbon) in benzene solution. It is found that one of six active normal modes v10 is actually responsible to the solvent enhancement of spectra observed in experiment. Simulations from all functionals (TD) B3LYP, (TD) B3LYP35, (TD) B3LYP50, and (TD) B3LYP100 draw the same conclusion. Hence, the present method is able to adequately reproduce experimental absorption and fluorescence spectra in both gas and solution phases.

The photoisomerization of cis- and trans-stilbene through conical intersections (CI) is mainly governed by four dihedral angles around central C═C double bonds. The two of them are C-C═C-C and H-C═C-H dihedral angles that are found to form a mirror rotation coordinate, and the mirror plane appears at the two dihedral angles equal to zeros with which the middle state is defined through partial optimization. There exist the first-type of hula-twist-CI enantiomers, the second-type of hula-twist-CI enantiomers, the first-type of one-bond-flip-CI enantiomers, and the second type of one-bond-flip-CI enantiomers as well as cis-enantiomers and trans-enantiomers with respect to this mirror plane. The complete active space self-consistent field method is employed to calculate minimum potential energy profile along the mirror rotation coordinate for each enantiomers, and it is found that the left-hand manifold and the right-hand manifold of potential energy surfaces can be energetically transferred via photoisomerization. Furthermore, two-dimensional potential energy surfaces in terms of the branching plane g-h coordinates are constructed at vicinity of each conical intersection, and the landscapes of conical intersections show distinct feature, and in excited-state four potential wells separated in different section of g-h plane related to different conical intersections which indicate different photoisomerization pathways.

Although several reaction models have been proposed in the literature to explain matrix-assisted laser desorption/ionization (MALDI), further study is still necessary to explore the important ionization pathways that occur under the high-temperature environment of MALDI. 2,4,6-Trihydroxyacetophenone (THAP) is an ideal compound for evaluating the contribution of thermal energy to an initial reaction with minimum side reactions.Desorbed neutral THAP and ions were measured using a crossed-molecular beam machine and commercial MALDI-TOF instrument, respectively. A quantitative model incorporating an Arrhenius-type desorption rate derived from transition state theory was proposed. Reaction enthalpy was calculated using GAUSSIAN 03 software with dielectric effect. Additional evidence of thermal-induced proton disproportionation was given by the indirect ionization of THAP embedded in excess fullerene molecules excited by a 450 nm laser.The quantitative model predicted that proton disproportionation of THAP would be achieved by thermal energy converted from a commonly used single UV laser photon. The dielectric effect reduced the reaction Gibbs free energy considerably even when the dielectric constant was reduced under high-temperature MALDI conditions. With minimum fitting parameters, observations of pure THAP and THAP mixed with fullerene both agreed with predictions.Proton disproportionation of solid THAP was energetically favorable with a single UV laser photon. The quantitative model revealed an important initial ionization pathway induced by the abrupt heating of matrix crystals. In the matrix crystals, the dielectric effect reduced reaction Gibbs free energy under typical MALDI conditions. The result suggested that thermal energy plays an important role in the initial ionization reaction of THAP.

In the second-order approximation, the Markovian generalised master equation is applied to the spin-boson Hamiltonian to find the dynamics of a dissipative two-state subsystem (TSS). The results are compared with those obtained by the path-integral formalism and considered to evaluate the possibility of apparition of electronic coherence for the electron transfer reaction in certain photosynthetic reaction centers.

The infrared spectroscopic profiles of HCOOD/D2O mixtures were measured as a function of pressure and concentration. The C–H bond of HCOOD shortens as the pressure is elevated, while the increase in C–H bond length upon diluting HCOOD with D2O was observed. Based on the experimental results, the shift in frequency of C–H stretching band is concluded to relate to the mechanism of the hydration of the C–H group and the water structure in the vicinity of the C–H group. The pressure-dependent results can be attributed to the strengthening of C–H---O electrostatic/dispersion interaction upon increasing pressure. The observations are in accord with ab initio calculation forecasting a blueshift of the C–H stretching mode via C–H---O interaction in HCOOD-water/(HCOOD)2-(D2O) complexes relative to the noninteracting monomer/dimer. Hydrogen-bonding nonadditivity and the size of water clusters are suggested to be responsible to cause the redshift in C–H stretching mode upon dilution HCOOD with D2O.

The surface vibrational spectroscopy of ethanol (C2H5OH, ethyl alcohol): water binary mixture, or the aqueous solution of ethanol, is investigated via sum frequency generation (SFG) in the OH region (3000 to 3900 cm−1). The bulk molar ratio of the ethanol is varied from x = 0.0024 to 0.20. Our concentration dependence results support the statement that the surface density of water molecules with a free dangling OH bond is about 25% in the air/water interface. It also indicates that the ethyl group of the surface ethanol molecules sticks out above the nominal surface, while the hydroxyl group alone is immersed in the mixture solution. And the surface layer is a mixed monolayer. We have demonstrated here that a LiNbO3 crystal containing -OH can be sufficient for the SFG spectroscopy measurement in the -OH region regardless of its absorption dip.

The C-H—O interactions of formic acid and acetone aqueous solutions were studied under various concentrations. The infrared spectra of acetone/D2O were also measured as a function of pressure. The C-H stretching frequency of formic acid reveals a red-shift with dilution. On the other hand, upon diluting acetone with D2O, the blue-shift with dilution in C-H stretching frequency was observed. Further high-pressure study provides the experimental evidence for the enhancement of hydrophobic hydration as the sample was compressed to high-pressure ices, while C-H vibration of acetone, being trans formed from liquid to high-pressure ice VII, are altered into a band located at ∼2950 cm−1. The C-H—O inter action may be a distinct possibility to understand the band-shape and frequency-shift of the C-H stretching modes of formic acid/D2O and acetone/D2O.

The nanocrystalline cubic Phase of zirconia was found to be thermally stabilized by the addition of 2.56 to 17.65 mol % Y2O3 (5.0 to 30.0 mol % Y, 95.0 to 70.0 mol % Zr cation content). The cubic phase of yttria stabilized zirconia was prepared by thermal decomposition of the hydroxides at 400°C for 1 hr. 2.56 mol % Y2O3-ZrO2 was stable up to 800°C in an argon atmosphere. The samples with 4.17 to 17.65 mol % Y2O3 were stable to 1200°C and higher. All samples at temperatures between 1450°C to 1700°C were cubic except the sample with 2.56 mol % Y2O3 which was tetragonal. The crystallite sizes observed for the cubic phase ranged from 50 to 150 A at temperatures below 900°C and varied from 600 to 800 nm between 1450°C and 1700°C. Control of furnace atmosphere is the main factor for obtaining the cubic phase of Y-SZ at higher temperature. Nanocrystalline cubic Fe-SZ (Iron Stabilized Zirconia) with crystallite sizes from 70 to 137 A was also prepared at 400°C. It transformed isothermally at temperatures above 800°C to the tetragonal Fe-SZ and ultimately to the monoclinic phase at 900°C. The addition of up to 30 mol % Fe(III) thermally stabilized the cubic phase above 800°C in argon. Higher mol % resulted in a separation of Fe2O3. The nanocrystalline cubic Fe-SZ containing a minimum 20 mol % Fe (III) was found to have the greatest thermal stability. The particle size was a primary factor in determining cubic or tetragonal formation. The oxidation state of Fe in zirconia remained Fe3+. Fe-SZ lattice parameters and rate of particle growth were observed to decrease with higher iron content. The thermal stability of Fe-SZ is comparable with that of Ca-SZ, Mg-SZ and Mn-SZ prepared by this method.

The nanocrystalline cu bic phase of zir co nia was found to be ther mally sta bi lized by the ad di tion of 3 to 40 mol % man ga nese. The nanocrystalline cu bic, tetragonal and monoclinic phases of zir co nia sta bi lized with man ga nese (III)ox ide (Mn-Stabilized Zir co nia) were pre pared by ther mal de com po si tion of car bon ate and hy drox ide pre cur sors. Both the crys tal li za tion and iso ther mal phase tran si tions as so ci ated with Mn-SZ were stud ied us ing high tem per a ture x-ray dif frac tion and x-ray dif frac tion of quenched sam ples. Cu bic Mn-SZ ini tially crys tal lized and pro gres sively trans formed to tetragonal, and monoclinic struc tures above 700 °C. The nanocrystalline cu bic Mn-SZ con tain ing 25 mol % Mn was found to have the great est ther mal sta bil ity, re tain ing its cu bic form at tem per a tures as high as 800 °C for pe ri ods up to 25 hours. Higher than 40 mol %, cu bic Mn2O3 was found to co ex ist with cu bic Mn-SZ. The crys tal lite sizes ob served for the cu bic, tetragonal and monoclinic Mn-SZ phases ranged from 50 to 137, 130 to 220, and 195 to 450 A re spec tively, in di c at ing, for ZrO2, that par ti cle size was a pri mary fac tor in de ter min ing its poly morphs. The clas si cal Avrami equa tion for nu cle ation and growth was ap plied to the ob served phase trans for ma tions.

The photoluminescence (PL) at 300 and 90 K from the mesoporous silicas (MSs) [ordered (MCM-41) and disordered (DMSs)] of variously sized pores induced either by ArF laser light (λexc = 193 nm) or by Nd:YAG (yttrium−aluminum−garnet) laser light (λexc = 266 nm) was studied. The spectra were measured in both conventional and time-resolved detection modes. It has been shown that PL induced by 266-nm light results from a direct one-photon excitation of hydrogen-related species (the green band) situated on the internal pore surfaces and nonbridging oxygen hole centers (NBOHCs, the red bands) located both on the surface and inside ∼1-nm sized bulk fragments (pore walls). Alternatively, an indirect excitation of the light emitters due to the energy transfer through two-photon-produced free excitons (FEs) is responsible for PL in the case of 193-nm excitation. In addition, the self-trapping of FEs followed by radiative relaxation results in the blue band, which can only be induced by 193-nm light. The PL propertie...

In this work, we construct a microscopic model with which one can reconstruct all the experimentally observed characteristics of the RCs including temperature effects. For this purpose, we analyze the absorption spectra of Rb. sphaeroides RCs (R26 and R26.Phe-a mutants). We apply the excitonic-vibronic model with the displaced harmonic potential surface model for the nuclear freedoms and determine microscopic properties such as the electronic coupling constants, the vibrational frequencies, and the displacements among the potential surfaces of these electronic states. We also take into account inhomogeneity of RCs in our theoretical analysis. We introduce a thermal expansion model to treat temperature dependent peak shifts observed in the P band of the absorption spectra of the RCs. With the resulting information, we also calculate the rate constants of the dynamic processes occurring in RCs, determining the electronic coupling constants among the electronic states involved in electron transfer and electronic energy transfer of the RCs. Based on the excitonic-vibronic model, we also investigate the mechanism of the direct path PB * H → P + B - H recently observed in YM210W mutant.

The photoluminescence (PL) from 7- and 15-nm silica $({\mathrm{SiO}}_{2})$ nanoparticles induced both by ArF laser light $[{\ensuremath{\lambda}}_{\mathrm{exc}}=193\mathrm{nm}(6.4\mathrm{eV}),$ ${\ensuremath{\tau}}_{L}=15\mathrm{ns}]$ and by Nd:YAG (yttrium-aluminum-garnet) laser light $[{\ensuremath{\lambda}}_{\mathrm{exc}}=266\mathrm{nm}(4.66\mathrm{eV}),$ ${\ensuremath{\tau}}_{L}=8\mathrm{ns}]$ was studied. The laser light intensity dependencies of the PL yields reveal the two-photon (TP) process of the PL excitation in the case of ArF laser light. The PL results from the radiative relaxation of self-trapped excitons (STE- the blue band), also from the surface hydrogen-related species (the green band), and the bulk nonbridging oxygen hole centers (NBOHC's- the red band) excited by a radiationless relaxation of TP-produced free excitons (FE's). The main point is focused on the effect of the nanoparticle surface condition on the FE dynamics. The dynamics includes either an elastic scattering or quenching by the nanoparticle boundary, the laser heating of FE's up to energies in excess of the STE barrier, the FE energy transfer to the surface and bulk NBOHC's and hydrogen-related centers, the saturation of the FE density, and the biexciton process in the formation of Frenkel defects with their subsequent transformation into NBOHC's.

Heats of formation of spiropentadiene, spiropentane and bicyclo[1.1.0]but-1(3)-ene have been calculated employing the G2M(RCC,MP2) method. The values obtained using 27, 8 and 10 different working reactions are 150.5, 43.0 and 133.0 kcal/mol for spiropentadiene, spiropentane and bicyclo[1.1.0]but-1(3)-ene, respectively, with the error bars estimated as ±2 kcal/mol. The results are in close agreement with available experimental data and allow to resolve the persisting uncertainties in the heats of formation of the simplest bicyclic hydrocarbons.

The optical line-shape of electronic transitions due to the polar electron-LO-phonon interaction in semiconductor quantum dots is studied. The mechanism of the finite optical line-width is based on the multiple electron-LO-phonon scattering. The influence of the higher order corrections beyond the self-consistent Hartree-Fock approximation to the electron-phonon part of the electronic self-energy is estimated quantitatively. The problem is studied within the two-level model of a single electron interacting with the system of longitudinal optical phonons of the whole bulk sample of the polar semiconductor. The influence of the higher order corrections is demonstrated on the line-shape of the optical transitions, the temperature dependence of the line-width and on the dependence of the line-width on the size of the quantum dot.

In this paper, we present a microscopic model to describe the spectroscopy and dynamics for photosynthetic bacterial reaction centers (RCs). In this model, we propose eight vibrational modes and their Huang-Rhys factors for different electronic energy states, and the couplings between the electronic states. As applications, we have constructed steady state and ultrafast time-resolved spectra for the mutant RC of Rhodobacter (Rb.)sphaeroides R26. Our model can also treat the temperature effect on the spectroscopy and dynamics of RCs.

Solid-state NMR methods and ab initio calculations have been employed to investigate the structure of the trimethylphosphine (TMP)−Bronsted acid site complex in zeolite HY. 27Al/31P and 27Al/1H rotational echo double-resonance NMR experiments performed at −150 °C were utilized to measure Al−P and Al−HB distances for the acid site−TMP complex of 3.95 (±0.05) and 2.8−3.1 A, respectively, where HB is the Bronsted acid site proton. A more accurate measurement of the Al−HB distance was not possible since models that assume the presence of isolated Al−H spin pairs are not valid in this case. A P−HB distance of 1.40 (±0.02) A was obtained by fitting the spinning sidebands in the 1H magic angle spinning (MAS) NMR spectrum. These internuclear distances are within the range of the Al−P, Al−HB, and P−HB distances obtained from ab initio calculations for the ion pair (IP) TMPH+−zeolite complex that is formed by transferring a Bronsted acid proton to TMP. In contrast to the 31P MAS NMR spectra, which indicated that th...

Effects of a centrally disposed symmetric cyclic bridge on the absorption spectra of polymethine compounds are discussed using Green's functions in the vicinity of the Fermi level. Simple expressions are derived which relate bridge-induced changes in the absorption maximum position and the transition dipole moment to topological parameters additively contributed by all constituents of the molecules concerned. As shown, the polymethine compounds containing four-membered cyclic bridge with oxygen substituents (squaraines) should have slightly changed spectral characteristics of the first electronic transitions as compared to parent unbridged molecules; in contrast, compounds with the analogous five-membered cycle incorporated into the polymethine chain (crocoines) are expected to manifest drastically red-shifted absorption. The trends revealed are corroborated by the experimental data available in the literature.

The Smoluchowski theory describes the kinetics of trapping of Brownian particles by absorbers randomly placed without correlations between their positions. We generalize this theory to take trap correlations into account when traps occur in spherical clusters distributed in space in a noncorrelated manner. A cluster contains n traps uniformly distributed within the cluster. An effective medium treatment is used to handle trap-correlation effects. Explicit expressions are obtained for the time-dependent rate coefficient and the particle survival probability valid for the entire range of n and cluster radius R. We analyze how the trap clustering manifests itself in the kinetics. In particular, we show that there exists a domain of the parameters n and R, where the kinetics is well fitted by a stretched exponential function for more than 99% of the decay. Such behavior should be contrasted to the essentially exponential kinetics predicted by Smoluchowski theory for noncorrelated traps.

Time-resolved pump–probe spectra of 1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′indotricarbocyanine (HDITC), a cyanine dye, in ethylene glycol are obtained using 11 fs and 90 fs duration pulses and analyzed in order to study its potential energy surfaces and vibrational dynamics. Ten oscillatory frequencies ranging from 30 cm−1 to 1400 cm−1 are observed in the 11 fs duration wavelength-resolved pump–probe measurements. They are assigned as fundamental vibrational frequencies of HDITC. The relative displacements of the equilibrium position between electronic excited and ground states along the resolved ten vibrational modes are determined through the wavelength dependence of the oscillatory amplitude. After considering the contributions of the ten vibrational modes, it is found that most of the Stokes shift and the early fast decays of the pump–probe signals are due to relaxation along the low frequency overdamped modes of the chromophore. The overdamped modes are characterized by the 90 fs pump–probe signals with the excitation at the red edge of the absorption band.

Wavelength-resolved pump-probe measurements using 11 fs duration pulses were performed to study the vibronic structures of HDITC, a cyanine dye, in ethylene glycol. Ten vibrational modes were observed in the forms of quantum beats. The frequencies of these ten vibrational modes ranged from 135 cm−1 to 1300 cm−1. The relative potential displacements along these ten vibrational coordinates were estimated by comparison with theoretical calculations.

The dynamics of a macroscopic dissipative two-state system are approached in the weak coupling limit by using a resonance treatment developed within the framework of the spin–boson model. The longitudinal and transversal relaxation times are obtained by applying the quantum relaxation theory. The model is tested to find the rate constants for non-adiabatic electron transfer reactions in photosynthetic reaction centers in the case of weak coupling between the microscopic two-state subsystem and its environment.

Brownian particle survival is studied in the case where traps are gathered in clusters. If the number of traps n in a cluster is small, the trap clustering is only manifested at the final stage of the process. In the opposite limit, the clusters are perfectly absorbing and trapping proceeds considerably slower than that by noncorrelated traps from the very beginning. We treat the intermediate case, where n is neither small nor large, and present an approximate expression for the rate constant that connects the two limiting cases. The results are in agreement with computer simulations.

In this paper, the studies of the dynamics of the high Rydberg states for n = 10 and n = 100 of hydrogen atom under the influence of an electric field will be reported. It will be shown from our calculated results that the two time constants of τslow ≈ 1 – 10 μs and τfast ≈ 1 – 10 ns obseved in ZEKE experiments are obtained in the stray field condition, i.e., F = 10 – 100 mV/cm. To treat the ZEKE states of complex systems like many-electron atoms, molecules, clusters etc., the inverse Born-Oppenheimer approximation can be used and it will be discussed.

We have introduced the satellite hole spectral method to examine the binding sites of 3,3‘-diethyloxadicarbocyanine cation (DODC+) to various guanine-rich oligonucleotides. The satellite hole patterns along with normal mode calculations allow us to determine the interaction of DODC+ with a dimeric hairpin quadruplex formed by sequence d(G4T4G4). Our results are consistent with the groove binding model and eliminate the possibility of intercalation in the base pairs of guanine quartet.

Both exact analytical formula and numerical calculations are performed to study the l-mixing population decay of the n = 10 and n = 100 high Rydberg states of hydrogen atoms in an electric field. The two time constants of and observed in zero-electron-kinetic-energy (ZEKE) experiments in the common stray field condition, i.e. - can be obtained by 2-exponential fitting of the exact decay curve. The principle of high-resolution ZEKE spctroscopy based on the two time constants is then elucidated. The inverse Born-Oppenheimer approximation will be introduced to treat the ZEKE states of complex systems like many-electron atoms, molecules, clusters etc.

2,2‘-Bis(trifluoromethyl)-4,4‘,5,5‘-biphenyltetracarboxylic dianhydride (6FBPDA) was synthesized from 1-iodo-4,5-dimethyl-2-nitrobenzene via a six-step synthetic route. The dianhydride was polymerized with eight different substituted 4,4‘-diaminobiphenyls in refluxing m-cresol containing isoquinoline to afford a series of fluorinated aromatic polyimides. The polyimides were soluble in polar aprotic, ether and ketone solvents. They had intrinsic viscosities that ranged from 1.70 to 6.72 dL/g in N-methyl-2-pyrrolidinone or m-cresol at 30 °C. The polymers underwent 5% weight losses when subjected to thermal gravimetric analysis between 440 and 570 °C in air and nitrogen atmospheres. The polymers could be solution cast into water-white, flexible, tough films which exhibited “in-plane” structural anisotropy. Their glass transition temperatures along the directions parallel to the film surface (in-plane) ranged from 327 to 345 °C (thermal mechanical analysis). Multiple relaxation processes associated with segme...

We study how the kinetics of diffusion-influenced reactions is modified when the reactivity of species fluctuates in time (stochastically gated) with emphasis on the many-particle aspect of the problem. Because of the fact that the dynamics of ligand binding to proteins originally motivated the problem, it is considered in that context. Recently, Zhou and Szabo [J. Phys. Chem. 100, 2597 (1996)] have demonstrated many-particle effects in the problem and found that the kinetics of reaction between a gated protein with a large number of ligands significantly differs from that between a protein and gated ligands. With our approach, the difference between the kinetics of ligand-gated and protein-gated reactions appears formally the same as the difference between the target and trapping problems despite the origin of the corresponding effects and their manifestations are distinctly different. A simple approximate method to treat the many-particle effects is proposed. The theory is applied to a particular two-st...

Kinetic studies of protein aggregation processes based on the nucleation-polymerization (NP) mechanism and/or its variants are often formulated in terms of the law of mass action. Although these works have greatly enhanced our knowledge in the microscopic descriptions of fibril/aggregate formation (e.g., fragmentation and other secondary pathways), they have to inevitably deal with a huge number of filament species Fn (n=1 to 10∧3 ∼ 10∧4 or even more) as well as a number of kinetic parameters. Very often some kinetic assumptions for the species with a wide distribution of sizes and shapes would be needed in order to simplify the calculation. In contrast, starting from a phase-transition perspective, one is able to provide thermodynamic insight into the nucleation-controlled kinetic behavior. In this work, we propose to apply the kinetic Ising model to investigate the thermodynamics and kinetics of protein aggregation. At mean-field (MF) level, our model can offer thermodynamic interpretation to the lag time; in particular, it can explain the effect of temperature, concentration, and seeding on the change of kinetic profiles. Using four real proteins as examples, we show that the calculated fibril stability is consistent with experimental measurements. In addition, their kinetics can be classified according to a generalized scheme of the model. Our results suggest that protein aggregation could be studied within the framework of the Ginzberg-Landau phase transition theory.

The structure, energetics, and vibrational spectra of the (CH3)2CO·(H2O)n (n = 1−3) complexes have been studied using density functional and ab initio B3LYP, MP2, and CCSD(T) methods. The excitation energies and the oscillator strength for the n−π* electronic transition in acetone and acetone−water complexes have been calculated using the CIS, CASSCF, and CASPT2 approaches. The results show that the first water molecule is coordinated to the carbonyl group of acetone, while the oxygen atom of H2O forms a weak hydrogen bond with a methyl hydrogen. The second H2O occupies a position between the first water and a methyl group, and the third H2O occupies a position between the second H2O and the methyl hydrogen of acetone. The energies of the coordination of the first, second, and third water molecules to the complexes are 3.7, 5.7, and 6.7 kcal/mol, respectively. The formation of the (CH3)2CO·(H2O)n complexes results in the shift of vibrational frequencies for acetone and water, particularly, red shifts for ...

Two high-molecular-weight organosoluble aromatic polyimides having the same diamine, 2,2′-(trifluoromethyl)-4,4′-diaminobiphenyl diamine (PFMB), but different dianhydrides, 2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride (HFBPDA) and 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), were prepared via one-step polycondensation reactions. Molecular weight fractionation of these two polymers was carried out and the polyimide fractions possess successively lower weight-average molecular weights. Intrinsic viscosity data from both sets of polyimide fractions gave the Mark-Houwink-Sakurada constants of these two polyimides, which reflect differing chain rigidity in tetrahydrofuran (THF) solutions. The MHS relations are [η] = 5.24 × 10−4 M0.65 and [η] = 3.77 × 10−5 M0.97 for 6FDA-PFMB and HFBPDA-PFMB, respectively. The physical meanings of these constants are discussed using the intrinsic viscosity data of these two polyimides in varying solvents. The persistence...

Recently, Tsai, Jiang, and Lin have found a plausible $(2\ifmmode\times\else\texttimes\fi{}2)$ \ensuremath{\pi}-bonded-trimer structure for the clean diamond C(111) surface. Using the local-orbital density-functional molecular-dynamics method, this $(2\ifmmode\times\else\texttimes\fi{}2)$ structure is found to undergo phase transition into the Pandey's $(2\ifmmode\times\else\texttimes\fi{}1)$ \ensuremath{\pi}-bonded chain structure with the adsorption of H or Li. However, this surface does not undergo phase transition with the adsorption of F or O for the same adsorption site considered. These results indicate that $s$ electrons play an important role in disrupting the ${\mathrm{sp}}^{2}$ bonding of this $(2\ifmmode\times\else\texttimes\fi{}2)$ structure.

Potential energy surfaces (PES) of the ground and excited states of allene C3H4 and vinylidenecarbene C3H2 have been studied by ab initio CCSD(T) and MRCI methods. The three lowest singlet excited states of allene, 1A2, 1B1, and 1E, are calculated to have the vertical excitation energies of 6.10, 6.55, and 6.94 eV, respectively. Three local minima are found on the excited S1 surface, 2b (1Ag, D2h), 5 (1A‘‘, Cs), and 10 (1B2, C2v‘), and their adiabatic excitation energies are 3.02, 3.05, and 4.70 eV, respectively. The PES of the ground and excited states are shown to cross when the geometry of allene changes by twisting the CH2 groups and bending the CCC angle or along the pathway that leads to H2 detachment. For vinylidenecarbene the lowest singlet excited states are 1A2 and 1B1 with the respective vertical excitation energies of 1.88 and 2.44 eV and the adiabatic excitation energies of 1.77 and 2.05 eV. The endothermicity of the C3H4 → C3H2 + H2 reaction is predicted to be ∼83 kcal/mol with the barrier o...

A novel class of organo-soluble polyimides has been developed based on monomers containing twisted-biphenyl structures. Thus, 2,2'-disubstituted-4,4'-diaminobiphenyls and 2,2'-disubstituted-4,4',5,5'-biphenyltetracarboxylic dianhydrides have been prepared and polymerized in refluxing phenolic solvents. The polyimides, some of which are soluble in ketone and ether solvents, can be readily solution cast into colorless thin films. The rigid polymer chains align parallel to the substrate surface (in-plane orientation) during the casting process, which results in anisotropy in the film properties. The anisotropy in the refractive index has been used in the development of compensation layers for liquid crystal displays. The polymers have also been dry jet-wet spun from phenolic solutions to afford fibers that display tensile strengths of 3.5 GPa and moduli of 130 GPa.

Eight excited doublet electronic states of C 2 H 3 have been studied using multireference configuration interaction calculations. Close agreement of the excitation energies with experiment is found for the A 2 A″, 5 2 A′ (σ-n), and 3 2 A″ (π-R) states where the experimental data are available. The undiscovered B 2 2 A″ (n - π ∗ ) state is predicted to have the adiabatic and vertical excitation energies of 33530 and 38546 cm −1 , respectively. The π-π ∗ C 2 2 A′ state with the vertical energy of 45001 cm −1 is predicted to be difficult to observe because of the small oscillator strength.

The Smoluchowski–Collins–Kimball theory of irreversible diffusion-influenced reactions with one of the reactants in excess is generalized to the case of stochastic gating when one of the reactants can be in one of M states. Distinct states are characterized by various efficiencies of the reaction of contacting partners. General expressions are derived for the rate constant and for the survival probability of the reactant which is in deficiency. We present these quantities in terms of the solution of the isolated pair problem. The difference between the cases when gating is due to the reactant, which is in excess, and one, which is in deficiency, is explicitly demonstrated. General relationships between the rate constants and the survival probabilities in the two cases are established. We show that in the former case the reaction goes faster compared to the latter one. To make the problem treatable analytically in the case when gating is due to the reactant which is in deficiency, a partial mean-field approximation is introduced. General theory is applied to a particular case of the two-state gating model. Explicit analytical solutions for the time-dependent rate constant and the survival probability are obtained in one dimension. They illustrate the general theory as well as show how the kinetics depends on the jump rate between the two states of the gate in both cases when gating is due to the reactant, which is in excess, and one, which is in deficiency.

We investigate herein the propagation and tunneling of monochromatic light through a tapered fiber tip, modeled as a tapered perfectly conducting waveguide. The opening aperture of the waveguide is much smaller than the wavelength of light. The transmission rate through the tip as well as the profiles of electric fields are obtained. A detailed formulation for calculating the fluorescent spectra of a single molecule under near-field excitation is also presented. In addition, the limitations of fluorescence measurement for obtaining near-field pattern is discussed on the basis of the present formulation.

The dephasing and energy relaxation contributions to the line width in infrared (IR) and sum-frequency generation (SFG) spectra of adsorbates are derived from the generalized master equation approach. Expression for the line shift is also obtained. The anharmonic interaction between the adsorbate and the substrate is expanded in a polynomial in terms of the adsorbate and phonon coordinates, and the dephasing is shown to be mainly due to two-phonon processes, while two-phonon, three-phonon or four-phonon processes can contribute to energy relaxation, depending on the relative values of the adsorbate vibrational and the phonon frequencies. The temperature-dependence data of the IR absorption for C(111):H is found to be consistent with the theory, and the large line width for C(111):D can be accounted for by the efficient two-phonon energy relaxation process which is not available for C(111):H due to the higher adsorbate vibrational frequency for C(111):H.

The geometries, vibrational frequencies and vertical and adiabatic excitation energies of the excited valence and Rydberg 3s, 3p, 3d, and 4s electronic states of CH 3 have been studied using ab initio molecular orbital multiconfigurational SCF (CASSCF), internally contracted multireference configuration interaction (MRCI) and equation-of-motion coupled cluster (EOM-CCSD) methods. The vibronic spectra are determined through the calculation of Franck-Condon factors. Close agreement between theory and experiment has been found for the excitation energies, vibrational frequencies and vibronic spectra. The adiabatic excitation energies of the Rydberg 3s B 2 A′ 1 and 3p 2 2 A″ 2 states are calculated to be 46435 and 60065 cm −1 compared to the experimental values of 46300 and 59972 cm −1 , respectively. The valence 2 A″ excited state of CH 3 has been found to have a pyramidal geometry within C s symmetry and to be adiabatically by 97 kcal/mol higher in energy than the ground state. The 2 A″ state is predicted to be stable by 9 and 13 kcal/mol with respect to H 2 and H elimination.

The numerical results of the transition rate for spontaneous emission from highly excited states ( n > 50) have been calculated with the corresponding lifetimes. These results can provide a helpful reference to understand the extraordinary long lifetimes found in ZEKE states, which are responsible for the high resolution ZEKE spectroscopy.

The vibrational modes of the excited electronic state of 9-aminoacridine (9-AA) doped in amorphous glasses of glycerol and water mixtures have been examined via the satellite holes in the persistent hole-burned spectra. The satellite holes with high resolution were used to characterize tautomer structures of 9-AA at different pH values and provide information on the vibrationalmode coupling for the study of guest–host interactions. More fundamental modes were observed for monoprotonated 9-AA, butstronger couplings among the 390, 1162, and 1338 cm -1 modes are reflected in the combination modes for neutral 9-AA. Normal mode calculations of vibrational frequencies were performed to help mode assignments. Deuteriation of the amino group protons is carried out to distinguish between the vibrational modes of the acridine ring and the amino group. Franck–Condon factors were obtained from hole-burned spectra to assist in understanding the intensity distributions of the burned holes. Proton transfer between the chromophore and matrix, triggered by charge density migration along the short axis of 9-AA via electronic excitation, is proposed for the hole-burning mechanism.

We study analytically the effect of a small inertial correction on the properties of adiabatically driven flashing ratchets. Parrondo's lemma [J. M. R. Parrondo, Phys. Rev. E 57, 7297 (1998)] is generalized to include the inertial term so as to establish the symmetry conditions allowing directed motion (other than in the overdamped massless case) and to obtain a high-temperature expansion of the motion velocity for arbitrary potential profiles. The inertial correction is thus shown to enhance the ratchet effect at all temperatures for sawtooth potentials and at high temperatures for simple potentials described by the first two harmonics. With the special choice of potentials represented by at least the first three harmonics, the correction gives rise to the motion reversal in the high-temperature region. In the low-temperature region, inertia weakens the ratchet effect, with the exception of the on-off model, where diffusion is important. The directed motion adiabatically driven by potential sign fluctuations, though forbidden in the overdamped limit, becomes possible due to purely inertial effects in neither symmetric nor antisymmetric potentials, i.e., not for commonly used sawtooth and two-sinusoid profiles.

Multi-reference configuration interaction with single and double excitation method has been utilized to calculate the potential energy surfaces of the five low-lying electronic states (1)A1, (1)A2, (3)A2, (1)B2, and (3)B2 of carbon dioxide molecule. Topology of intersections among these five states has been fully analyzed and is associated with double-well potential energy structure for every electronic state. The analytical potential energy surfaces based on the reproducing kernel Hilbert space method have been utilized for illustrating topology of surface crossings. Double surface seam lines between (1)A1 and (3)B2 states have been found inside which the (3)B2 state is always lower in potential energy than the (1)A1 state, and thus it leads to an angle bias collision dynamics. Several conical∕surface intersections among these five low-lying states have been found to enrich dissociation pathways, and predissociation can even prefer bent-geometry channels. Especially, the dissociation of O((3)P) + CO can take place through the intersection between (3)B2 and (1)B2 states, and the intersection between (3)A2 and (1)B2 states.