[Show abstract][Hide abstract] ABSTRACT: An efficient edge-functionalization strategy with high specificity was employed to study the effects of conjugated structures on photoluminescence (PL) properties of graphene quantum dots (GQDs). Both the experimental results and density functional theory (DFT)-based calculations suggested the mechanism for conjugated structures in GQDs to tune the band gap of GQDs.
[Show abstract][Hide abstract] ABSTRACT: Two-dimensional covalent organic frameworks (2D COFs) provide a unique platform for the molecular design of electronic and optoelectronic materials. Here, the synthesis and characterization of an electroactive COF containing the well-known tetrathiafulvalene (TTF) unit is reported. The TTF-COF crystallizes into 2D sheets with an eclipsed AA stacking motif, and shows high thermal stability and permanent porosity. The presence of TTF units endows the TTF-COF with electron-donating ability, which is characterized by cyclic voltammetry. In addition, the open frameworks of TTF-COF are amenable to doping with electron acceptors (e.g., iodine), and the conductivity of TTF-COF bulk samples can be improved by doping. Our results open up a reliable route for the preparation of well-ordered conjugated TTF polymers, which hold great potential for applications in fields from molecular electronics to energy storage.
Full-text · Article · Nov 2014 · Chemistry - A European Journal
[Show abstract][Hide abstract] ABSTRACT: The reactions of hydroperoxy radicals with hydroxymethylperoxy and methoxymethylperoxy radicals were studied using the hybrid density functional theory and the coupled-cluster theory with complete basis set extrapolation. In contrast with the unsubstituted alkylperoxy reactions, it was found that OH-substitution has a significant effect on the reaction mechanism. Several hydrogen bonding reaction precursors exist at the start of the reaction. The reaction pathways show a strongly anisotropic character. The preferred transition states are four-, five-, six-, or seven-membered cyclic structures. The predicted rate coefficients are expressed as k(T) = 8.48 × 10−24T3.55e2164/T + 2.37 × 10−29T4.70e3954/T cm3 molecule−1 s−1. Based on the available experimental data in the temperature range 275–333 K, the theoretical and experimental results are in agreement with a relative average deviation of only 8%. The nascent products at low and high temperatures are hydroperoxide molecules and hydroxyl radicals, respectively. A potential source has been found for the production of formic acid and new insights into the experimental observations are presented.
No preview · Article · Sep 2014 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Formic acid was used as the model of lauric acid to investigate the microscopic mechanism of the anti-icing behavior and was checked to find out if it can be catalyzed to produce H-2 for fuel cells by the alpha-Al2O3(0001) 2 x 2 supercell slab model. The density functional theory with the all-electron double numerical polarized basis sets was employed. The results show that when it involves the carboxyl O and hydroxyl H atom the 1,2-dissociated adsorbate is the most stable intermediate on the dry Al2O3(0001) surface and is energetic barrier free to form the fairly stable ROCO- and HO-covered surface with the binding energy of 59.5 kcal/mol, and this dissociation mode has the lowest energy barrier of 14.9 kcal/mol to form the HOCO- and H2O-covered surface after the surface is fully hydroxylated. The energetic barrier of the HCOOH dehydrogenation and dehydration reactions on the alumina surface decreased considerably from 65.3 to 30.6 kcal/mol and from 62.1 to 26.8 kcal/mol, respectively, in comparison with the gaseous decomposition. The dissociated configuration of lauric acid was tested, and it was found that it dissociated with free energy barrier through 1,2-hydrogen migration into the C11H23OCO- and HO-covered surface with a binding energy of 60.7 kcal/mol. The present theoretical work is useful to gain some new insights on the microscopic interaction mechanism of the superhydrophobic alumina surface fabrication procedure and on the heterogeneous catalysis reactions of the H-2 production.
Full-text · Article · Sep 2014 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The epoxy spacers in SF6 Gas Insulated Switchgear (GIS) are vulnerable to discharge. Once the surface of the epoxy spacers is deteriorated under discharge, insulation break down may occur subsequently. From a number of experiments carried out on 110 kV GIS, a new gas CS2 was detected when creeping discharges occur on the epoxy spacer surface. In order to verify the correlation between CS2 and the degradation of epoxy spacers, the quantum chemistry analysis was used to investigate the mechanism of CS2 development. Based on the results, the relation between CS2 and spacer insulation degradation was determined.
No preview · Article · Dec 2013 · IEEE Transactions on Dielectrics and Electrical Insulation
[Show abstract][Hide abstract] ABSTRACT: Electronic structures, vibrational properties, and dissociation energies of the elusive HO3(X2A″) radical have been calculated using various benchmark ab initio methods, including multireference RS2, MRCI+Q, MR-AQCC, NEVPT2, and the explicitly correlated RCCSD(T)-F12, RS2-F12, and MRCI-F12. The RS2 results strongly depend on the semiempirical level shifts applied to modify the zeroth-order Hamiltonian and the basis sets. The sizes of active spaces are crucial to the MRCI+Q data. The calculated dissociation energy (De) of HO3 into OH + O2 ranges from 4.6 to 6.2 kcal/mol. Theoretical intermolecular vibrational frequencies are in excellent agreement with the experimental measurements. The zero-point energy correction to De is estimated to be 2.6 kcal/mol. The formation of HO3 from OH(X2Π) and O2(X3Σg-) is determined to be a barrierless process. Temperature and pressure dependent kinetics for the association reaction are calculated variationally using the Master equations. It is revealed that the rate constants exhibit steep negative temperature dependence and typical non-Arrhenius behavior. The experimental low-temperature rate constants have been well reproduced theoretically.
No preview · Article · Dec 2013 · Computational and Theoretical Chemistry
[Show abstract][Hide abstract] ABSTRACT: The reaction between sulfur tetrafluorine (SF4) and H2O was computationally investigated using the G4//MP2/6-311G(d, p) theoretical model. Among four reactive channels in the initial attack process, the primary path, which occurred via a S(N)2 displacement reaction to produce SF3OH, is the rate-determining step in the complete hydrolysis of SE4, resulting in the formation of SO2 (H2SO3). The energy barrier (i.e., 22.48 kcal mol(-1)) in the gas phase will be substantially reduced when catalyzed by H2O and/or HF, and the Berry pseudorotation reaction (BPR) process of SF3OH becomes the rate-controlling step. The hydrolysis mechanism of SF4 in aqueous solution and the activation energy of the rate-determining step using the PCM model do not substantially change compared to the gas phase results. The hydrolysis of SF4 in an aqueous solution has a higher enthalpy. For secondary reactions in aqueous solution, the intermediates are prone to dehydrofluorination instead of hydrolysis. Although the activation energy of SOF2 hydrolysis is higher than that of SF4, the relative energy of the former is lower than that of the latter in aqueous solution. Therefore, SOF2 might be an important intermediate for the formation of SO2.
No preview · Article · Sep 2013 · Journal of Fluorine Chemistry
[Show abstract][Hide abstract] ABSTRACT: The anionic mechanisms for the elementary dimerization reaction of monosilicic acid in basic aqueous solution have been characterized comprehensively using various ab initio methods. Many new insights into the silicate oligomerization reaction, which is fundamentally important in sol-gel chemistry, zeolite synthesis, and cement hydration, are presented in this work. Conformational dependence of the dimerization reaction is proposed in view of hundreds of conformations with various inter- and intramolecular hydrogen bonding patterns along the reaction routes. An alternative water cleavage route from the five-coordinated silicon intermediate is revealed. The detour involves a six-center cyclic transition state, which is more preferable energetically than the well-known four-center water removal step. By including explicit water molecules, the activation barrier of the four-center water cleavage path can be reduced considerably to be even lower than the first barrier of the Si-O bond formation. In contrast, the six-center detour is less affected by the additional water molecules due to the unfavorable geometric distortion. The new understanding of the dimerization mechanism could have considerable impact on the initial stages of silica nucleation.
No preview · Article · Aug 2013 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The B3LYP hybrid density functional has been used for decades but its three empirical parameters were copied straightforwardly from the B3PW91 functional. We found that the serious flaw of B3LYP for the enthalpies of formation of large organic molecules and the qualitative failure of B3LYP for organic chemical reactivity are caused by these arbitrary parameters. Using the rigorously optimized empirical parameters, namely, a0 = 0.20, ax = 0.67, and ac = 0.84, B3LYP does outperform the old formula significantly with the comparable or even better performance than the state-of-the-art density functionals. It is suggested that the B3LYP method with the optimized semiempirical parameters might continue to be a good density functional in chemistry especially in the calculations of thermochemistry and reactivity of large organic molecules.
[Show abstract][Hide abstract] ABSTRACT: It has been expected that superhydrophobic (SHP) surfaces could have potential anti-icing applications due to their excellent water-repellence properties. However, a thorough understanding on the anti-icing performance of such surfaces has never been reported; even systematic characterizations on icing behavior of various surfaces are still rare because of the lack of powerful instrumentations. In this study, we employed the electrochemical anodic oxidation and chemical etching methods to simplify the fabrication procedures for SHP surfaces on the aluminum alloy substrates, aiming at the anti-icing properties of SHP surfaces of various engineering materials. We found that the one-step chemical etching with FeCl3 and HCl as the etchants was the most effective for ideal SHP surfaces with a large contact angle (CA, 159.1°) and a small contact angle hysteresis (CAH, 4.0°). To systematically investigate the anti-icing behavior of the prepared SHP surfaces, we designed a robust apparatus with a real-time control system based on the two stage refrigerating method. This system can monitor the humidity, pressure, and temperature during the icing process on the surfaces. We demonstrated that the SHP surfaces exhibited excellent anti-icing properties, i.e., from the room temperature of 16.0 °C, the icing time on SHP surfaces can be postponed from 406s to 676s compared to the normal aluminum alloy surface if the surfaces were put horizontally, and the icing temperature can be decreased from −2.2 °C to −6.1 °C. If such surfaces were tilted, the sprayed water droplets on the normal surfaces iced up at the temperature of −3.9 °C, but bounced off the SHP surface even as the temperature reached as low as −8.0 °C. The present study therefore suggests a general, simple, and low-cost methodology for the promising anti-icing applications in various engineering materials and different fields (e.g., power lines and aircrafts).
[Show abstract][Hide abstract] ABSTRACT: TIP4P/2005 force-field-based classical molecular dynamics simulations were employed to investigate the microscopic mechanism for the ice growth from supercooled water when the external electric (0–109 V/m) and magnetic fields (0–10 T) are applied simultaneously. Using the direct coexistence ice/water interface, the anisotropic effect of electric and magnetic fields on the basal, primary prismatic, and the secondary prismatic planes of ice Ih has been calculated. It was revealed for the first time that the solvation shells of supercooled water could be affected by the cooperative electric and magnetic fields. Meanwhile, the self-diffusion coefficient is lowered, and the shear viscosity increases considerably. The critical electric and magnetic fields to accelerate ice growth on the prismatic plane are fairly low (ca. 106 V/m and 0.01 T). In contrast, the basal plane is hardly affected unless the fields increase to the order 109 V/m and 10 T. Rotational dynamics of water molecules might play an important role in ice growth with the applied external fields. Density functional theory with the triple numerical all-electron basis set was used to reveal the electronic structures of the basal and primary prismatic planes of ice Ih with respect to the anisotropic effect of ice growth.
No preview · Article · Sep 2012 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Many methods have been proposed to develop the fabrication techniques for superhydrophobic surfaces. However, such techniques are still at their infant stage and suffer many shortcomings. In this paper, the superhydrophobic surfaces on an Al substrate were prepared by a simple etching method. Effects of etching time, modifiers, and modification concentration and time were investigated, and optimal conditions for the best superhydrophobicity were studied. It was demonstrated that for etching the aluminum plate in Beck's dislocation, if the etching time was 15 s, modifier was Lauric acid–ethanol solution, and modification concentration and time was 5% and 1.5 h, respectively, the surface exhibited a water contact angle as high as 167.5° and a contact angle hysteresis as low as 2.3°.
[Show abstract][Hide abstract] ABSTRACT: The mechanism and kinetics of the reaction of acrylonitrile (CH(2)=CHCN) with hydroxyl (OH) has been investigated theoretically. This reaction is revealed to be one of the most significant loss processes of acrylonitrile. BHandHLYP and M05-2X methods are employed to obtain initial geometries. The reaction mechanism conforms that OH addition to C[double bond, length as m-dash]C double bond or C atom of -CN group to form the chemically activated adducts, 1-IM1(HOCH(2)=CHCN), 2-IM1(CH(2)=HOCHCN), and 3-IM1(CH(2)=CHCOHN) via low barriers, and direct hydrogen abstraction paths may also occur. Temperature- and pressure-dependent rate constants have been evaluated using the Rice-Ramsperger-Kassel-Marcus theory. The calculated rate constants are in good agreement with the experimental data. At atmospheric pressure with N(2) as bath gas, 1-IM1(OHCH(2)=CHCN) formed by collisional stabilization is the major product in the temperature range of 200-1200 K. The production of CH(2)CCN and CHCHCN via hydrogen abstractions becomes dominant at high temperatures (1200-3000 K).
No preview · Article · Aug 2011 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The microscopic reaction mechanism for the water adsorption/dissociation processes on the α-Al2O3(0001) surface was calculated using density functional theory with the all-electron triple numerical polarized basis sets. Both unit-cell and 2 × 2 supercell slab models were employed to investigate the coverage-dependent hydroxylation of the surface. Geometries of the molecular adsorbed intermediates, transition states, and the hydroxylated products were fully optimized, and the energetic reaction routes were clarified. The hydroxylation occurs predominantly via the low-barrier 1,4-hydrogen migration path, and the 1,2-dissociation path is competitive. The 1,2-hydroxylated surface is more preferable thermodynamically in the consideration of reaction exothermicity. It was found that the in-plane hydrogen atoms can roam between the surface oxygen atoms, resulting in isomerization between the 1,2- and 1,4-hydroxylated products. Calculations for the multiple layer adsorption confirm that the hydroxylated surface is relatively inert to further hydroxylation by water. Further added water molecules prefer to form multilayered hexagonal ice-like arrangements through a hydrogen-bonding network. The electric field might not play a significant role in either surface reconstruction or the hydroxylation process until it exceeds 108 V/m. The present theoretical work is useful to gain some new insights on the ice accumulation of high-voltage power lines under high humidity and supercooled environment.
No preview · Article · Jun 2011 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Scorpion toxins are invaluable tools for ion channel research and are potential drugs for human channelopathies. However, it is still an open task to determine the molecular basis underlying the diverse interactions between toxin peptides and ion channels. The inhibitory peptide Maurotoxin (MTX) recognized the distantly related IK(Ca) and Kv1.2 channel with approximately the same potency and using the same functional residues, their differential binding mechanism remain elusive. In this study, we applied computational methods to explore the differential binding modes of MTX to Kv1.2 and IK(Ca) channels, which would help to understand the diversity of channel-toxin interactions and accelerate the toxin-based drug design.
A reasonably stable MTX-IK(Ca) complex was obtained by combining various computational methods and by in-depth comparison with the previous model of the MTX-Kv1.2 complex. Similarly, MTX adopted the β-sheet structure as the interacting surface for binding both channels, with Lys23 occluding the pore. In contrast, the other critical residues Lys27, Lys30, and Tyr32 of MTX adopted distinct interactions when associating with the IK(Ca) channel. In addition, the residues Gln229, Ala230, Ala233, and Thr234 on the IK(Ca) channel turret formed polar and non-polar interactions with MTX, whereas the turret of Kv1.2 was almost not involved in recognizing MTX. In all, the pairs of interacting residues on MTX and the IK(Ca) channel of the bound complex indicated that electrostatic and Van der Waal interactions contributed equally to the formation of a stable MTX-IK(Ca) complex, in contrast to the MTX-Kv1.2 binding that is dominantly mediated by electrostatic forces.
Despite sharing similar pharmacological profiles toward both IK(Ca) and Kv1.2 channels, MTX adopted totally diverging modes in the two association processes. All the molecular information unveiled here could not only offer a better understanding about the structural differences between the IK(Ca) and Kv1.2 channels, but also provide novel structural clues that will help in the designing of more selective molecular probes to discriminate between these two channels.
Preview · Article · Jan 2011 · BMC Structural Biology
[Show abstract][Hide abstract] ABSTRACT: Kinetics of OH formation for the reaction of C2H5CO radicals with O2 have been studied using the low-pressure discharge flow technique coupled with resonance fluorescence monitoring of OH radicals at room temperature in He buffer gas. The OH yields are close to unity at the lowest pressures studied, but decrease rapidly with increasing pressure. The experimental OH yields are reproduced well using multichannel variational RRKM theory.
No preview · Article · Aug 2010 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: [1,2,3,4]Tetrazino-[5,6-e]-[1,2,3,4]tetrazine-[1,3,5,7]tetraoxide (TTTO, C2N8O4) was suggested to be a possible candidate of high energy density materials (HEDM). The most stable conformation of TTTO is a planar structure with C2h symmetry. Using various high-level ab initio methods including Gaussian-n, Complete Basis Set, Full Coupled Cluster, and W1U models of chemistry together with density functional theory based models, the enthalpies of formation of TTTO at 0 K and standard state were calculated precisely. Moreover, the rovibrational and nuclear magnetic resonance properties were predicted as well. The solid state TTTO was studied using the crystal packing models with the Dreiding force-filed and the plane-wave periodic local-density approximation density functional theory. Three stable polymorphous cells of TTTO have been found with either P212121 or P21/C symmetry. The high heat of formation (>200 kcal/mol), the high density (>2.0 g/cm3), the planar nonpolar electronic structure, and the perfect oxygen balance lead TTTO be a very promising HEDM with exceptional performance. This work provides the first theoretical support for further experimental synthesis and testing.
No preview · Article · Sep 2009 · Journal of Computational Chemistry
[Show abstract][Hide abstract] ABSTRACT: The electrochemical polymerizations of a series of ortho-disubstituted benzenes were investigated by experimental and theoretical approaches. The results indicated that o -dihydroxybenzene (o HDB), o - dimethoxybenzene (oDMOB) and 1-hydroxy-2-methoxybenzene (HMOB) in pure boron trifluoride diethyl etherate (BFEE) can be succefully electropolymerized. The 'H'3 C-NMR spectra of these monomers and their corresponding polymers proved that the polymerization occurred mainly at C4 and C5 positions. However the others such as o - xylene, 1 , 2-benzenedithiol, 1, 2-dibromobenzene, 1 , 2-dichlorobenzene, 1 , 2-dicyanobenzene and 1 , 2-dinitrobenzene can not be electropolymerized under similar conditions. These intrinsic characteristics have been revealed by the experimental investigation on these o-disubstituted benzenes together with the theoretical calculations. The cyclic voltammetry and linear sweep voltanunetry were used to determine oxidation potential onset of the monomers. Optimizing the ground-state geometries of the monomer and the radical cation of HOMO, LUMO and electron spin density were calculated by using the density functional theory (DFr) at the B3LYP/6-3 1CC d, p) level. From the experimental point of view, the oxidation potential onset of the monomer is not the control factor for the electropolymerization. According to theoretical studies, the monomers substituted by electron-donating groups have more negative electronics than the monomers substituted by electron-withdrawing groups. These monomers o DHB, oDMOB and HMOB with negative electronic charges of (C4, C5 ) Ca. - 0.093 - - 0.103 can be electropolymerized in BFEE. The others with the electronic changes of (C4 , C5) Ca. - 0.078 - - 0.085 cannot be electropolymerized in the same med. Therefore, we can extrapolate that the negative electronic of reaction site maybe has a critical value (probobly I 0. 09 ) , which is one of the condition for electropolymerization of the monomer. From the frontier molecule orbitals theory, the reactions between the active molecules mainly happens on the highest frontier electron density involving the two frontier orbitals having the smallest energy gap and obey orbital symmetry rules. Therefore,the orbital symmetry of HOMO and LUMO of the polymerization site should be matching. oHDB, oDMOB,and HMOB via a-HOMO and p-LUMO interaction having the smallest energy gap are satisfied with symmetry rules, they can be electrochemically polymerized. Whereas the others such as o-xylene, 1, 2- dibromobenzene, 1, 2-dichlorobenzene, 1, 2-dicyanobenzene and 1, 2-dinitrobenzene via i9-HOMO and a-LUMO interaction having the smallest energy were anti-symmetrical, which cannot be electropolymerized. Therefore, the HOMO-LUMO orbital symmetry matching is another condition for the electrochemical polymerization of odisubstituted benzenes.
No preview · Article · Mar 2009 · Acta Polymerica Sinica
[Show abstract][Hide abstract] ABSTRACT: Ab intitio molecular dynamics simulation of the electronic structure of the aqueous superoxide anion (O2(-)) has been carried out using the Car-Parrinello density functional theory at 298 and 310 K. The modeling system consists of one O2(-) solvated in 31 water molecules. On the basis of our 40 ps production run, the novel mechanism and the nature of the hydration of the superoxide anion in a relatively big aqueous environment have been revealed by using various radial distribution functions. The averaged coordinated water number was estimated to be 4.5. The calculated microscopic configurations of the first solvation shell are in good agreement with the experimental results. The vibrational frequency of the solvated O2(-) anion was red-shifted significantly in comparison with that of the free radical anion in the gas phase. The diffusion coefficient of O2(-) was estimated to be about 8 x 10(-5) cm2/s at 298 K. Comparisons with the previous force-field-based classical molecular dynamics simulations have been made, and the differences were discussed.
No preview · Article · Mar 2009 · The Journal of Physical Chemistry A