Studies of translational diffusion in the smectic A phase of a Gay-Berne mesogen using molecular dynamics computer simulation

Molecular dynamics computer simulations are used to determine the self-diffusion coefficients for a Gay-Berne model mesogen GB (4.4,20,1,1) in the isotropic, nematic and smectic A phases along two isobars. The values of the parallel and perpendicular diffusion coefficients, D(parallel) and D(perpendicular), are calculated and compared in the different phases. For the phase sequence isotropic-smectic A, D(perpendicular)*> or =D(parallel)* over the whole smectic A range with the ratio D(parallel)*/D(perpendicular)* decreasing with decreasing temperature. At a higher pressure, a nematic phase is observed between these two phases and we find that D(parallel)*>D(perpendicular)* throughout the nematic region and the inequality D(parallel)*>D(perpendicular)* remains on entering the smectic A phase. However, the ratio D(parallel)*/D(perpendicular)* decreases with decreasing temperature within the smectic A range and eventually this ratio inverts such that D(perpendicular)*>D(parallel)* at low temperatures. The temperature dependence of the parallel diffusion coefficient in the smectic A phase for this model mesogen is compared to that predicted by a theoretical model for diffusion subject to a cosine potential.     

The guanine cation radical: investigation of deprotonation states by ESR and DFT

This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G*+) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D2O solutions of 2'-deoxyguanosine (dGuo) at low temperatures at various pHs at which the guanine cation radical, G*+ (pH 3-5), singly deprotonated species, G(-H)* (pH 7-9), and doubly deprotonated species, G(-2H)*- (pH > 11), are found. C-8-deuteration of dGuo to give 8-D-dGuo removes the major proton hyperfine coupling at C-8. This isolates the anisotropic nitrogen couplings for each of the three species and aids our analyses. These anisotropic nitrogen couplings were assigned to specific nitrogen sites by use of 15N-substituted derivatives at N1, N2, and N3 atoms in dGuo. Both ESR and UV-visible spectra are reported for each of the species: G*+, G(-H)*, and G(-2H)*-. The experimental anisotropic ESR hyperfine couplings are compared to those obtained from DFT calculations for the various tautomers of G(-H)*. Using the B3LYP/6-31G(d) method, the geometries and energies of G*+ and its singly deprotonated state in its two tautomeric forms, G(N1-H)* and G(N2-H)*, were investigated. In a nonhydrated state, G(N2-H)* is found to be more stable than G(N1-H)*, but on hydration with seven water molecules G(N1-H)* is found to be more stable than G(N2-H)*. The theoretically calculated hyperfine coupling constants (HFCCs) of G*+, G(N1-H)*, and G(-2H)*- match the experimentally observed HFCCs best on hydration with seven or more waters. For G(-2H)*-, the hyperfine coupling constant (HFCC) at the exocyclic nitrogen atom (N2) is especially sensitive to the number of hydrating water molecules; good agreement with experiment is not obtained until nine or 10 waters of hydration are included.     

Conformational changes of the S2YZ* intermediate of the S2 to S3 transition in photosystem II

The paper extends earlier studies on the S(2)Y(Z)* intermediate that is trapped by illumination in the temperature range 77 K to 190 K of untreated samples poised in the S(2)...Q(A) state. X-band EPR experiments on untreated and glycerol (50% v/v) treated samples at 10 K indicate that the intermediate consists of two components. A wide one with a splitting of ca 170 G, and a narrow one characterized by a splitting of ca 120 G (untreated), or 124 G (glycerol-treated samples). Lower temperatures of illumination in the above temperature range favor the wide component, which at 10 K decays faster than the narrow one. Re-illumination at 10 K after decay of the signal trapped at 77-190 K induces only the narrow component. Rapid scan experiments in the temperature range 77-190 K reveal high resolution spectra of the isolated tyz Z* radical and no evidence of alternative radicals. The two split signals are accordingly assigned to different conformations of the S(2)Y(Z)* intermediate A point-dipole simulation of the spectra yields "effective distances" between the spin densities of Y(Z)* and the Mn(4)Ca center of 5.7 ? for the wide and 6.4 ? for the narrow component. The results are discussed on the basis of a molecular model assuming two sequential proton transfers during oxidation of tyr Z. The wide component is assigned to a transient S(2)Y(Z)* conformation, that forms during the primary proton transfer.     

Structure of V(2)O(5)*nH(2)O xerogel solved by the atomic pair distribution function technique

A long-standing issue regarding the local and long-range structure of V(2)O(5)*nH(2)O xerogel has been successfully addressed. The full three-dimensional structure of the lamellar turbostratic V(2)O(5)*nH(2)O xerogel was determined by the atomic pair distribution function technique. We show that on the atomic scale the slabs of the xerogel can be described well as almost perfect pairs (i.e., bilayers) of single V(2)O(5) layers made of square pyramidal VO(5) units. These slabs are separated by water molecules and stack along the z-axis of a monoclinic unit cell (space group C2/m) with parameters a = 11.722(3) A, b = 3.570(3) A, c = 11.520(3) A, and beta = 88.65 degrees. The stacking sequence shows signatures of turbostratic disorder and a structural coherence limited to 50 A.     

A theoretical study on fullerene-dizincocene hybrids

Using the density functional theory, we investigated the possible formation of fullerene-dizincocene hybrids, specifically C(60)*-Zn-Zn-Cp*, C(60)*-Zn-Zn-C(60)*, C(70)*-Zn-Zn-Cp*, and C(70)*-Zn-Zn-C(70)*, where C(60)*, Cp*, and C(70)* represent C(60)(CH(3))(5), C(5)(CH(3))(5), and C(70)(CH(3))(5) radicals. Our calculation shows that these hybrids have HOMO-LUMO gaps which are larger than has been experimentally identified for C(60)*-Fe-Cp. In addition, the strength of the Zn--Zn bonds is similar to that in Cp*-Zn-Zn-Cp* which was also synthesized recently. Furthermore, heterohybrids, C(60)*-Zn-Zn-Cp* and C(70)*-Zn-Zn-Cp* are expected to exist in equilibria with homohybrids, C(60)*-Zn-Zn-C(60)* and C(70)*-Zn-Zn-C(70)*, in which heterohybrids are much more favored. On the other hand, another hybrid involving Sc(3)N@C(68) as a fullerene unit is not highly probable.     

Cyclometallated gold(III) hydroxides as versatile synthons for Au-N, Au-C complexes and luminescent compounds

The gold(III) hydroxide κ(3)-(C^N^C)*Au(OH) reacts with C-H and N-H compounds and arylboronic acids to produce a range of perfluoroaryls, N-heterocyclic and alkynyl compounds in high yields; some of which show unexpectedly strong modulation of their photoluminescence from yellow to blue [(C^N^C)* = 2,6-(C(6)H(3)Bu(t))(2)pyridine].     

Intramolecular electron transfer in cofacially pi-stacked fluorenes: evidence of tunneling

The one-electron reduction of neutral pi-stacked di- and trifluorenes (F-2 and F-3) in HMPA, where ion association is absent, results in the formation of anion radicals in which the odd electron resides predominantly on just one of the external fluorene moieties, as established by EPR spectroscopy. However, in the case of tetrafluorene, introduction of a single electron leads to a kinetically controlled anion radical F-4(int)*- in which the odd electron undergoes rapid exchange between two central fluorene rings, where the anionic charge is partially shielded from solvation due to the presence of external fluorene rings. On a time scale of minutes, anion radical F-4(int)*- converts to a thermodynamically stabilized anion radical F-4(ext)*-, with the electron exhibiting coupling from the protons on an external fluorene moiety. The charge and spin residing on an external moiety allow efficient solvation of the anionic charge. A similar fast exchange of a single electron (probably with the involvement of quantum mechanical tunneling) among three and four internal fluorene moieties is initially observed via EPR spectroscopy in the penta- and hexafluorene derivatives, F-5 and F-6, respectively.     

Polyelectrolytes in multivalent salt solutions

We study conformational and electrophoretic properties of polyelectrolytes (PEs) in tetravalent salt solutions under the action of electric fields by means of molecular dynamics simulations. Chain conformations are found to have a sensitive dependence on the salt concentration C(s). As C(s) is increased, the chains first shrink to a globular structure and subsequently re-expand above a critical concentration C(s)*. An external electric field can further alter the chain conformation. If the field strength E is larger than a critical value E*, the chains are elongated. E* is shown to be a function of C(s) by using two estimators E(I)* and E(II)* through the study of the polarization energy and the onset point of chain unfolding, respectively. The electrophoretic mobility of the chains depends strongly on C(s), and the magnitude increases significantly, accompanying the chain unfolding, when E>E(II)*. We study the condensed ion distributions modified by electric fields and discuss the connection of the modification with the change of chain morphology and mobility. Finally, E* is studied by varying the chain length N. The inflection point is used as a third estimator E(III)*. E(III)* scales as N(-0.63(4)) and N(-0.76(2)) at C(s) =0.0 and C(s)*, respectively. E(II)* follows a similar scaling law to E(III)* but a crossover appears at C(s) =C(s)* when N is small. The E(I)* estimator fails to predict the critical field, which is due to oversimplifying the critical polarization energy to the thermal energy. Our results provide valuable information to understand the electrokinetics of PE solutions at the molecular level and could be helpful in micro/nanofluidic applications.     

Ultrafast excited state dynamics and spectroscopy of 13,13'-diphenyl-β-carotene

Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newly-synthesized artificial β-carotene derivative 13,13'-diphenyl-β-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S(2) state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S(2)→ S(1) and S(1)→ S(0)* with results for β-carotene. Global analysis provides IC time constants and also time-dependent S(1) spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the (1)H NMR spectrum, a small red-shift of the S(0)→ S(2)(0-0) transition energy in the steady-state absorption spectrum relative to β-carotene, and almost the same S(1)→ S(0)* IC time constant as in β-carotene, suggesting a similar S(1)-S(0) energy gap. The oscillator strength of the S(0)→ S(2) transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an "S* state". Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S(0)* superimposed on the bleach of room temperature molecules S(0). The S(0)* population is formed by IC from S(1). These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties.     

Naphthalene diols: a new class of antioxidants intramolecular hydrogen bonding in catechols,naphthalene diols,and their aryloxyl radicals

1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H-atom transfer to the 2,2-di(4-t-octylphenyl)-1-picrylhydrazyl radical (DOPPH*), k(ArOH/DOPPH)*, or as antioxidants during inhibited styrene autoxidation, k(ArOH/ROO)*, initiated with AIBN. The rate constants showed that 5 and 6 are more active HAT compounds than the ortho-diols, catechol, 1, 2,3-naphthalenediol, 2, and 3,5-di-tert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, k(ArOH/ROO)* = 6.0 x 10(6) M(-)(1) s(-1), of that of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, 4. Calculations of the O-H bond dissociation enthalpies compared to those of phenols, (deltaBDEs), of 1-6 predict a HAT order of reactivity of 2 < 1 < 3 approximately 4 < 5 < 6 in general agreement with kinetic results. Calculations on the diols show that intramolecular H-bonding stabilizes the radicals formed on H-atom transfer more than it does the parent diols, and this effect contributes to the increased HAT activity of 5 and 6 compared to the activities of the catechols. For example, the increased stabilization due to the intramolecular H-bond of 5 radical over 5 parent of 8.6 kcal/mol was about double that of 2 radical over 2 parent of 4.6 kcal/mol. Linear free energy plots of log k(ArOH/DOPPH)* and log k(ArOH/ROO)* versus deltaBDEs for compounds 1-6 along with available literature values for nonsterically hindered monophenols placed the compounds on common scales. The derived Evans-Polanyi constants from the plots for the two reactions, alpha(DOPPH)* = 0.48 > alpha(ROO)* = 0.32, gave the expected order, since the ROO* reaction is more exothermic than the DOPPH* reaction. Compound 6 is sufficiently reactive to react directly with oxygen, and it lies off the log k(ArOH/ROO)* versus deltaBDE plot.     

Ultrafast intersystem crossing and spin dynamics of zinc meso-tetraphenylporphyrin covalently bound to stable radicals

tert-Butylphenylnitroxide (BPNO(?)) and α,γ-bisdiphenylene-β-phenylallyl (BDPA(?)) stable radicals are each attached to zinc meso-tetraphenylporphyrin (ZnTPP) at a fixed distance using one of the ZnTPP phenyl groups. BPNO(?) and BDPA(?) are oriented para (1 and 3, respectively) or meta (2 and 4, respectively) relative to the porphyrin macrocycle. Following photoexcitation of 1-4, transient optical absorption spectroscopy is used to observe excited state quenching of (1)*ZnTPP by the radicals and time-resolved electron paramagnetic resonance (TREPR) spectroscopy is used to monitor the spin dynamics of the paramagnetic product states. The presence of BPNO(?) or BDPA(?) accelerates the intersystem crossing rate of (1)*ZnTPP about 10- to 500-fold in 1-4 depending on the structure compared to that of (1)*ZnTPP itself. In addition, the lifetime of (3)*ZnTPP in 1 is shorter than that of (3)*ZnTPP itself as a result of enhanced intersystem crossing (EISC) from (3)*ZnTPP to the ground state. The TREPR spectra of the three unpaired spins produced within 1 and 2 show spin-polarized excited doublet (D(1)) and quartet (Q) states and subsequent formation of a spin-polarized ground state radical (D(0)). All three signals are absorptive for 1 and emissive for 2. Polarization inversion of the Q state is observed on a tens of nanoseconds time scale in 2, while no polarization inversion is observed for 1. The lack of polarization inversion in 1 is attributed to the short lifetime of the doublet-quartet manifold as a result of the very large exchange interaction. The TREPR spectra of 3 and 4 show ground state radical polarization at X-band (9.5 GHz) at room temperature, but not at 85 K, and similarly no polarization is observed at W-band (94 GHz). No evidence of excited doublet or quartet states is observed, indicating that the exchange interaction is both weak and temperature dependent. These results show that although ultrafast EISC produces (3)*ZnTPP within 1-4, the magnitude of the exchange interactions between the three relevant spins in the resulting (3)*ZnTPP-BPNO(?) and (3)*ZnTPP-BDPA(?) systems dramatically alters their spin dynamics.     

半晶聚对苯二甲酸乙二酯中的受限非晶相

当半晶聚对苯二甲酸乙二酯 (PET)的结晶度 (Xwc)处于一定范围内时 ,其物理老化后在差示扫描量热(DSC)曲线上的玻璃化转变区有吸热双峰出现 .通过对此吸热双峰分别与完全非晶试样和具有相当高Xwc 的半晶试样物理老化后在DSC曲线上出现的吸热单峰的比较 ,表明半晶PET中存在两种性质极为不同的非晶区 ,即自由非晶区和受限非晶区 .动态力学热分析 (DMTA)曲线上显示的损耗正切 (tanδ)双峰进一步证实了这两种不同非晶区的存在 .这两种不同非晶区的产生是由于试样中晶粒对非晶相中高分子链段活动性的不同限制作用所致 .研究发现 ,对于由冷结晶得到的半晶试样来说 ,出现两种不同非晶区所需的Xwc 上下限都随结晶温度 (Tc)的升高而增高 .还发现 ,在物理老化过程中 ,虽然非晶相的总量基本保持不变 ,但部分自由非晶区却逐渐转变为受限非晶区 .上述实验结果很好地符合Struik的“扩展玻璃化转变”模型 .     

Resonant dissociative electron attachments to cysteine and cystine

Shape-resonant electron attachments to cysteine and cystine and the subsequent dissociation dynamics are investigated with the single-center expansion potential scattering calculations. Selectivity of the direct bond cleavage at a given resonant state or by the specific resonant state coupling is demonstrated with the one-dimensional complex potential energy curves of the temporary anion (cysteine)(-). The wave function of the lowest shape resonant state of the temporary anion (cystine)(-) distinctly shows the localized anti-bond (S-S)* character, implying that this disulfide bond can be easily broken due to the low-energy electron resonant attachment.     

6-Hydroxyindole-based borondipyrromethene: synthesis and spectroscopic studies

A 6-hydroxyindole-based BODIPY, named BODIPY-OH, with distinct spectroscopic characteristics is reported. Through a systematic study of the spectroscopic characteristics of BODIPY-OH and BODIPY-O(-) in various solvents containing an organic base, we found that the light-color of the fluorophore can be tuned over a wide range by changing the polarity of solvent/base combinations. The absorption color of the solution can be tuned over a range of 100 nm and the emission color within a wide range from 571 to 681 nm by simply converting the phenol form of BODIPY-OH to the phenolate form. Fluorescence of BODIPY-O(-) with high quantum yield shows relatively large Stokes shift in solvent/base combinations, which are ascribed to the excited state deprotonation from (BODIPY-OH)* to (BODIPY-O(-))*, followed by emission from the ion form.     

Chemiluminescence accompanying solid-phase interaction of terbium(III) sulfate with xenon difluoride

It has been found that solid-phase interaction of anhydrous terbium(III) sulfate powder with xenon difluoride at room temperature is accompanied by chemiluminescence (CL). The formation of terbium( IV) and ozone in the course of the reaction has been observed spectrophotometrically. Two CL emitters, xenon in the Xe(I)* oxidation state (540, 580 nm) and Tb(III)* (490, 545, 590 nm), have been revealed. Emission by the former is observed at the beginning of the reaction. After the consumption of xenon difluoride, the emission is due to excited Tb(III) generated as a result of reduction of the Tb(IV) intermediate.     

Fragmentation of protonated oligoalanines: Amide bond cleavage and beyond

The fragmentation reactions of the singly-protonated oligoalanines trialanine to hexaalanine have been studied using energy-resolved mass spectrometry in MS(2) and MS(3) experiments. The primary fragmentation reactions are rationalized in terms of the b(x)-y(z) pathway of amide bond cleavage which results in formation of a proton-bound complex of an oxazolone and a peptide/amino acid; on decomposition of this complex the species of higher proton affinity preferentially retains the proton. For protonated pentaalanine and protonated hexaalanine the major primary fragmentation reaction involves cleavage of the C-terminal amide bond to form the appropriate b ion. The lower mass b ions originate largely, if not completely, by further fragmentation of the initially formed b ion. MS(3) energy-resolved experiments clearly show the fragmentation sequence b(n) --> b(n-1) --> b(n-2). A more minor pathway for the alanines involves the sequence b(n) --> a(n) --> b(n-1) --> b(n-2). The a(5) ion formed from hexaalanine loses, in part, NH(3) to begin the sequence of fragmentation reactions a(5) --> a(5)* --> a(4)* --> a(3)* where a(n)* = a(n) - NH(3). The a(3)* ion also is formed from the b(3) ion by the sequence b(3) --> a(3) --> a(3)* with the final step being sufficiently facile that the a(3) ion is not observed with significant intensity in CID mass spectra. A cyclic structure is proposed for the a(3)* ion.     

CO2在Cu表面还原成碳氢化合物的DFT计算研究

应用密度泛函理论（DFT）反应能计算及最小能量路径分析研究了CO2在气相和电化学环境中于Cu(111)单晶表面的还原过程。气相中,CO2还原为碳氢化合物的反应路径可能为：CO2(g) + H* → COOH* → (CO +OH)* → CHO*;CHO + H* → CH2O* → (CH2 + O)*;CH2* + 2H* → CH4或2CH2* → C2H4。整个反应由CO2(g) + H* → COOH* → (CO +OH)*,(CO + H)* → CHO*和CH2O* → (CH2 + O)*等几个步骤联合控制。在-0.50V (vs RHE) 以正的电势下,CO2在Cu(111)表面电化学还原主要形成HCOO-和CO吸附物;随着电势逐渐负移,CO2加氢解离形成CO的反应越来越容易,CO成为主要产物;随电势进一步变负,形成碳氢化合物的趋势逐渐变强。与CO2的气相化学还原不同的是,电化学环境下CO质子化形成的CHO中间体倾向于解离形成CH,而在气相中CHO中间体则倾向于进一步质子化形成CH2O中间体。     

Evaluation of size-exclusion chromatography and size-exclusion electrochromatography calibration curves

Size-exclusion chromatography (SEC) and size-exclusion electrochromatography (SEEC) are chromatographic techniques used to determine molecular mass (weight) distributions (MWD) of polymers. One important step in the data treatment to derive MWD parameters is the modelling of the calibration curves. The calibration curves applied in SEC and SEEC are generally not linear. In this study the modelling of calibration curves is being examined. Different polynomial models have been evaluated and compared, not only for model fit but also for their predictive properties. It was found that sometimes a straight line and sometimes a third-order polynomial model were best. The best model across the effective range (also called linear range) is not always found to be a straight line. The SEEC curves were found to have considerably higher prediction errors than the SEC ones. Reduction of the number of calibration standards to five or six did not greatly affect the predictive properties of the calibration curves, neither in SEC nor in SEEC.     

Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH.H+ and H3O+ cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s(C[double bond]O/OH) electrons into pi(CO)* orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s(C[double bond]O)-->pi(CO)* transition by approximately 0.2 eV and a redshift of the O1s(OH)-->pi(CO)* by approximately 0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer andor trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO+ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD2O+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD.D+ (protonated HCOOH.H+) cations within the clusters.