Tuning Properties of Layered Materials Based on Hexagonal Boron Nitride by Methylation: A Density Functional Theory Study.

In this work, the rational design of optoelectronic properties of two-dimensional materials based on hexagonal boron nitride (h-BN) by functionalization by methyl (CH3) groups is proposed. Using density functional theory, we examine the functionalization of single- or double-layer systems with either CH3 radicals alone or with both CH3 (cations) and chlorine (anions), i. e., under conditions of homolytic or heterolytic splitting of CH3Cl precursor molecules, respectively. Different degrees of methylation (coverages) are considered. The methylation of pure h-BN leads to a reduction of the band gap, while in h-BN/G heterostructures (with methylated graphene layer), methylation increases the band gap. As a consequence, h-BN/G heterostructures offer a high tunability of their optoelectronic properties. To guide possible experiments, vibrational properties and spectra of methylated h-BN and methylated h-BN/G are determined.

TEMPO and a co-reductant mediated aerobic epoxidation of olefins using a new magnetically recoverable iron(III) bis(phenol)diamine complex: experimental and computational studies.

A magnetically recoverable catalyst of an iron(III) bis(phenol) diamine complex immobilized onto amine functionalized silica-coated magnetic nanoparticles has been synthesized. The catalyst was characterized using FESEM, TEM and XRD which confirmed the nano structure of the catalyst. The physicochemical techniques of ICP, FT-IR, XPS, EDS and TGA proved the loading of the ligand and metal complex on silica-coated magnetic nanoparticles. Using the prepared heterogeneous catalyst, aerobic epoxidation reactions of different alkenes have been investigated in the presence of SO32- as a reducing agent. Moreover, using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) to discover the mechanism of the aerobic epoxidation of olefins, a new TEMPO-assisted route has been explored. Both of the reaction pathways led to a moderate to high percentage yield of epoxides in water at room temperature. For further understanding mechanistic aspects, density functional theory (DFT) computational studies have been performed. The DFT calculations confirm the suggested mechanism for the title reaction and show the electron density in the vicinity of Fe(II) in the presence of TEMPO as a co-catalyst was more than that in the presence of SO32-.

Band Gap Engineering in Two-Dimensional Materials by Functionalization: Methylation of Graphene and Graphene Bilayers.

Graphene is well-known for its unique combination of electrical and mechanical properties. However, its vanishing band gap limits the use of graphene in microelectronics. Covalent functionalization of graphene has been a common approach to address this critical issue and introduce a band gap. In this Article, we systematically analyze the functionalization of single-layer graphene (SLG) and bilayer graphene (BLG) with methyl (CH3) using periodic density functional theory (DFT) at the PBE+D3 level of theory. We also include a comparison of methylated single-layer and bilayer graphene, as well as a discussion of different methylation options (radicalic, cationic, and anionic). For SLG, methyl coverages ranging from 1/8 to 1/1, (i.e., the fully methylated analogue of graphane) are considered. We find that up to a coverage θ of 1/2, graphene readily accepts CH3, with neighbor CH3 groups preferring trans positions. Above θ = 1/2, the tendency to accept further CH3 weakens and the lattice constant increases. The band gap behaves less regularly, but overall it increases with increasing methyl coverage. Thus, methylated graphene shows potential for developing band gap-tuned microelectronics devices and may offer further functionalization options. To guide in the interpretation of methylation experiments, vibrational signatures of various species are characterized by normal-mode analysis (NMA), their vibrational density of states (VDOS), and infrared (IR) spectra, the latter two are obtained from ab initio molecular dynamics (AIMD) in combination with a velocity-velocity autocorrelation function (VVAF) approach.

CH2 + O2: reaction mechanism, biradical and zwitterionic character, and formation of CH2OO, the simplest Criegee intermediate.

The singlet and triplet potential surfaces for the title reaction were investigated using the CBS-QB3 level of theory. The wave functions for some species exhibited multireference character and required the CASPT2/6-31+G(d,p) and CASPT2/aug-cc-pVTZ levels of theory to obtain accurate relative energies. A Natural Bond Orbital Analysis showed that triplet 3CH2OO (the simplest Criegee intermediate) and 3CH2O2 (dioxirane) have mostly polar biradical character, while singlet 1CH2OO has some zwitterionic character and a planar structure. Canonical variational transition state theory (CVTST) and master equation simulations were used to analyze the reaction system. CVTST predicts that the rate constant for reaction of 1CH2 + 3O2 is more than ten times as fast as the reaction of 3CH2 (X3B1) + 3O2 and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the 1CH2O + 3O product set is dominant at all temperatures and the primary yield of OH radicals is negligible below 600 K, due to competition with other primary reactions in this complex system.

Closer Investigation of the Kinetics and Mechanism of Spirovinylcyclopropyl Oxindole Reaction with 3Σ-g-O2 by Topological Approaches and Unraveling the Role of the I2 Catalyst.

In this investigation at the MN15L/Def2-TZVP level of theory, we present computational evidence indicating that the reaction of 3Σ-g-O2 with spirovinylcyclopropyl oxindole (2) leads to a product called spiro-1,2-dioxolane (2) in its singlet state; this reaction occurs via a stepwise mechanism and its rate-determining step is catalyzed by iodine radicals, which promotes opening of the three-membered ring under dark conditions. The conversion of 2 to 1-benzylindoline-2,3-dione (3) and 2-vinyloxirane (4) takes place via a concerted and slightly asynchronous reaction. Both electron localization function and AIM topological analysis reveal that the step associated with the attack of the 3Σ-g-O2 molecule on the intermediate 3MC characterizes the formation of the only new O2-C3 single bond, which occurs in a stepwise mechanism, in contrast to the Δg-O2 reaction with 15 species.

A new whole-cell biocatalyst for sulfur dioxide filtering and degradation.

In this study, the interaction of the magnetotactic bacterium with sulfite compounds and their potential to degrade SO2 was investigated using cyclic voltammetry (CV), molecular emission cavity analysis (MECA) and ion-exchange chromatography (IEC). This biofilter was able to degrade SO2 up to 22281 mg m-3 by disproportionation reaction and the formation of S2- and SO42- with ≥99% efficiency. Designed biofilter was able to restart the initial performance at least after seven cycles if it was used at 14-day intervals. According to theoretical studies, the value of mean free energy (E) obtained using the Dubinin-Radushkevich isotherm model was 0.02 kJ mol-1, which is in the range expected for physical adsorption. Designed biofilter can be considered as a powerful tool to degrade SO2 in diverse urban and industrial centers.

Unraveling the kinetics and molecular mechanism of gas phase pyrolysis of cubane to [8]annulene.

The kinetic and electron density flows are studied theoretically for the gas phase pyrolysis of cubane via its cage opening to reach bicyclooctatriene and then thermal rearrangement of bicyclooctatriene to produce [8]annulene which is the experimentally observed major product. The observed kinetic data at the MN15-L/maug-cc-pVTZ level of theory were in good agreement with the experimental results as compared to the CBS-QB3 method. The cage opening and the thermal rearrangement steps at the experimentally employed temperature of 520 K were exergonic and exothermic. The atmospheric rate constants calculated by means of the RRKM theory show that the cage opening is the rate-determining step. The temperature dependence of the rate constant for the cage opening step at the MN15-L level can be expressed as log(k/s-1)1bar MN15-L = (15.63) - (48.99 kcal mol-1)/RT ln 10. The molecular mechanism of the reactions has been investigated by means of the bonding evolution theory (BET) at the B3LYP/6-311G (d,p) level of theory. The cage opening course is described topologically by cleaving of C1-C2, C4-C8, and C5-C6 single bonds and electron saturation of the C1-C4, C2-C6, and C5-C8 bonds, while the rearrangement of bicyclooctatriene is described by C3-C7 bond rupture, depopulation of C1-C4 and C5-C8 double bonds, and electron saturation of C1-C5, C3-C4, and C7-C8 bonds. Electron density rearrangement along the two successive steps are asynchronous and the sequence of catastrophes can be represented as: η-1-13-C†C†FFFC†C†FFFC†C†-2-6-[C]2C†[F]2[C†]2C†-0.

From Kinetics of OH Reaction with Glutamic Acid to Oxidative Damage to Proteins.

Hydroxyl (OH) is a radical that is distributed all over the universe including the human body, where it can be readily produced, results in oxidative damage to cellular components, and leads to several health implications. While the reaction of OH with proteins is usually addressed as the gas-phase reaction of free amino acids with OH in theoretical studies, this study questions the efficiency of such calculations by focusing on the reaction of glutamic acid (Glu) with OH. According to the results, the reaction profile alters significantly by shifting from gas-phase calculations to solvent-phase due to the zwitterionic nature of this amino acid. So that the barrier height of the N-terminal reaction path and the relative energies of the species vary considerably. Furthermore, the interconnection of all collisional energy transfer events and chemical changes through solving the reaction's master equation and inclusion of the effect of tunneling suggests that the major product changes from vdw1-N to R-CA, R-CB, and R-CG by considering the effect of solvent on the gas-phase reaction. Despite these changes, the rate constants of both gas- and solvent-phase reactions (1.23 × 1010 and 7.32 × 109 L mol-1 s-1, respectively, at 310 K and 1 atm) demonstrate positive temperature dependency. With respect to the rate coefficients reported in the literature, Glu, Ser, and Met are the most vulnerable amino acids to oxidative attack by OH among the Ser, Cys, Asn, ß-Ala, Ala, Gly, Met, and Glu amino acids.

Theoretical Study on the Dynamics and Kinetics of the Reaction of CH2OH with OH.

The stochastic one-dimensional chemical master equation (CME) simulation method was used to investigate the dynamics of the reaction of CH2OH with OH. A multiwell multichannel potential energy surface (PES) was constructed at the CCSD(T)/Aug-cc-pVTZ//CBS-QB3 and QCISD(T)/Aug-cc-pVTZ//CBS-QB3 levels of theory. The constructed PES consisted of three chemically activated intermediates and two van der Waals complexes. The fractional population analysis unraveled the role of the energized intermediates and van der Waals complexes in the early stages of this complex reaction. The CME calculations provided the phenomenological rate constants through analysis of the eigenvalues and eigenvectors of collision matrices while Leonard-Jones potential was used to model the collisions. The CME results indicated that CH2O and H2O were the major products, in accordance with the literature. Also, the findings declared the temperature and pressure independence of the reaction over a wide range of temperature (250 to 2400 K) and pressure (0.1 to 7 atm). Furthermore, the efficiency of tunneling on the hydrogen transfer isomerization reaction of trans-HCOH to CH2O was confirmed over the temperature range of 250 to 3000 K. The rate constants for different reaction channels are reported.

A Theoretical Study on the Dynamics of the Reaction of CH Radicals with Water.

Quasi-classical trajectory (QCT) and RRKM-SSA calculations are carried out to gain insight into the dynamics of the title reaction. The barrier-less initiation step in this system is governed by the capture probability in the entrance channel to form an energized adduct once the centrifugal barrier is surmounted. The dynamics of the title reaction on its lowest doublet electronic state is studied at the DFT level of MPWB1K/6-31++g(2df,2p). An iteratively modified Shepard interpolation technique implemented in the GROW program suite was used to construct a global potential energy surface for the title reaction. The total and individual cross sections for the main products and corresponding reaction probabilities as a function of initial collision energy ranging from 0.4 kJ mol-1 to 52.5 kJ mol-1 are calculated. These data are used to calculate the total rate constant for the title reaction by means of collision theory. Our calculated QCT rate constant is compared with the calculated rate constant from RRKM-SSA method at the CCSD(T)/Aug-cc-pVTZ//MP2/6-31++g(2df,2p) level. The energy partitioning for the main products CH2O + H and HCOH + H and also for the reactants after nonreactive collisions as a function of initial collision energy are discussed.

Study skills and habits in Shiraz dental students; strengths and weaknesses.

INTRODUCTION: The dental students, the same as other students, during their academic courses are required to learn a wide range of scientific subjects. Obviously, choosing the inappropriate method of study leads to confuse and disenchantment of students and it causes wasting of their energy. The purpose of this study was to assess the existing strengths and weaknesses of the skills and study habits in Dental Students of Shiraz University of Medical Sciences in 2009-10. MATERIALS AND METHODS: In this cross-sectional study, all of the dental students (n = 274), who studied at the time of study at all levels in the academic year of 2009-10, were selected by the census. Data were collected by using the Huston University questionnaire consisted of two parts of demographic questions and 64 specific areas of study skills in eight domains of time management, concentration/memory, study aids/note taking, test strategies, information processing, motivation, self-assessment/reading, and writing skills. Following the retranslation of the questionnaire, the validity was confirmed by using the content validity method. The reliability was obtained by using the Cronbach's Alpha of 0.92. The data were analyzed with SPSS software version 17 and using analytical statistic tests. RESULTS: Students who have previously participated in the study skills workshops had stronger skills in comparison with the students who had not participated in these workshops. Time management skills (P = 0.04), motivation (P = 0.0001) and information processing (P = 0.03) in students with professional status were in a more favorable position and showed significant differences in terms of educational levels. The study skills mean score of the students living in student housings in comparison with the other students were significantly higher (P = 0.04). Marital status showed no significant differences in reading skills. CONCLUSION AND RECOMMENDATIONS: The review of study skills in the undergraduate and post-graduate dental students indicated that the residents had higher reading skills. By recognizing the existing strengths and weaknesses and holding programs through counseling centers can develop the study skills in the students.