Theoretical study on photophysical properties of phenolpyridyl boron complexes

Organoboron complexes have potential application in organic light-emitting devices (OLEDs). Our group has synthesized four phenolpyridyl boron complexes (Inorg. Chem. 2006, 45, 2788), which can function as an electron transport materials (ETM), white and blue emitters, and exhibit high efficiency and stability. To reveal the relationship between the properties and structures of these functional materials, theoretical analysis of spectral properties and electronic structures of these complexes was systematically characterized with the B3LYP and 6-31G* basis set. The calculated absorption and emission spectra of these systems are in good agreement with the experimental ones. It is clear seen that these transitions are charge transferred along 2,6-bis(2-hydroxyphenyl)pyridyl boron moiety, and the contribution of boron atom in these compounds to the main transition orbitals is vanishingly small. The substitution of methyl and methoxyl for hydrogen does not change the absorption wavelengths and transition natures, but influences the radioactive efficiencies and electron transport properties, which are observed and discussed in detail. Furthermore, large red shifts of fluorescence are caused by replacing the hydrogen with CN or NO2 groups, which indicates that they are potential candidates as green-light-emitting materials. These results are favorable to further understanding the photophysical properties of this kind of complexes.     

Substitution kinetics of chelate complexes as studied by isotopic exchange

The fact that isotopic exchange reactions take place at chemical equilibirum makes this type of reaction useful for kinetic studies. In the case of chelate complexes [M(chel)_n] isotopic exchange reactions can be classified as metal exchange reactions. [M(chel)_n]+^*M⇆[^*M(chel)_n]+M and ligand exchange reactions [M(chel)_n]+^*chel⇆[M(chel)_n−1 ^*chel]+chel The literature on type (a) and type (b) reactions of mainly transition metal complexes is reviewed with respect to the kinetic information obtained. The value and the limitations of isotopic exchange studies as a kinetic technique are discussed.     

QCT study of isotopic effects in H + HBr abstraction and exchange reactions

The method of quasi-classical trajectories on an LEPS hypersurface was used for studying the influence of the exchange of one or both of the hydrogen atoms for deuterium in the reaction H¹ + H²Br. As expected, the reaction cross sections of the exchange and abstraction reactions were found to increase if H¹ was replaced by D and decrease if H² was replaced by deuterium. A similar change in the reaction cross sections have also been observed for vibrationally excited reactants. The distribution of vibrational (rotational) energy is related to the ω_e (B_e) values of the respective reactants and products.     

Theoretical study of addition reactions of heavy carbenes to carbon and boron nitride nanotubes

In an effort to gain insight into the activation energies and reaction enthalpies of the chemical functionalization of carbon and boron nitride nanotubes, calculations using density functional theory have been carried out for the cycloaddition of a heavy carbene to a single-walled carbon (SWCNT; C(130)H(20)) and a boron nitride (SWBNNT; B(65)N(65)H(20)) nanotube. The (CH(3))(2)X + SWCNT and (CH(3))(2)X + SWBNNT (X = C, Si, Ge, Sn, and Pb) reactions are the subject of the present study. All the stationary points were determined at the B3LYP/LANL2DZ level of theory. The major conclusions that can be drawn from this work are as follows: (i) Considering both the activation barrier and reaction enthalpy based on the model calculations presented here, it is found that the order of (CH(3))(2)X reactivity is X = C > Si > Ge > Sn > Pb, irrespective of whether cycloaddition is to a SWCNT or a SWBNNT sidewall. That is to say, (CH(3))(2)C and (CH(3))(2)Si can readily add to the sidewalls of SWCNT and SWBNNT, whereas (CH(3))(2)Ge, (CH(3))(2)Sn, and (CH(3))(2)Pb are unreactive. (ii) Since the chemical reactivities of SWCNT and SWBNNT sidewalls closely resemble those of the small C(16)H(10) and B(8)N(8)H(10) molecules, at least in a qualitative sense, the use of the above small molecules as models is sufficient to provide qualitatively correct results. (iii) Our theoretical observations indicate that all the (5,5) SWCNT and SWBNNT cycloadducts favor opened rather than closed three-membered ring structures. (iv) The theoretical investigations demonstrate that the singlet-triplet splitting of the carbene species (R(2)X) as well as that of the small model molecules can be used as a diagnostic tool to predict the addition reactivities of carbene analogues and sidewalls of various nanotubes, respectively. Moreover, the results obtained in this work allow a number of predictions to be made.     

A G2 study of SH+ exchange reactions involving lone-pair donors and unsaturated hydrocarbons

The ligand exchange reactions between mono-adducts of the sulfenium ion ([SH-X]-) and either unsaturated hydrocarbons or lone-pair donors have been investigated computationally at the G2 level. The mono-adducts react with acetylene or ethylene to form a thiiranium or a thiirenium ion, in most cases without an overall barrier. In the reactions involving lone-pair donors, the original lone-pair donor is expelled from the [SH-X]- mono-adduct with the formation of a new mono-adduct. The reaction proceeds in this case via an intermediate di-adduct. Both the hydrocarbon and the lone-pair donor attack the mono-adduct with the relevant orbitals aligned in a near-collinear fashion, as was also the case for previously investigated reactions involving PH2+ and Cl+. The reaction energies and the binding energies of the intermediate complexes in the exchange reactions are primarily determined by the electronegativities of the lone-pair donors. The thermochemical data can be rationalized within the framework of qualitative molecular orbital theory, and the results are compared with our previous findings for the corresponding reactions involving PH2+ and Cl+.     

Theoretical study of complexes and fluoride cation transfer between N2F+ and electron donors

A theoretical study of the complexes formed by the N2F cation (fluorodiazonium ion) and a series of small molecules containing nitrogen atoms have been carried out at the MP2 computational level. In addition, fluorine transfer has been studied. The electron density, NMR shielding and indirect coupling constants of the complexes have been evaluated. The covalent or halogen bonding characteristics of the N...F interactions observed in the complexes are defined by the interatomic distance. It has been determined that the limiting value is 1.6 A.     

Organic fluorines as halogen bond donors: Theoretical study and crystallographic evidence

Organic fluorines usually cannot act as halogen bond donors, as a result of the extreme electronegativity and least polarizability of the fluorine atom. However, when the electronegative ability of the substituents bound to the carbon atom is very strong, organic fluorines do show positive electrostatic potentials (ESPs) along the C? F bond and thus can form halogen bonds with electron donors. In this work, the effects of six different substituents, i.e., NO₂, CF₃, CN, COOH, CHO, and CCH, on the ESPs of the F atom were studied using the M06‐2x method. When two or three substituents with very strong electron‐withdrawing ability, such as NO₂ and CN, are linked to the C atom, positive ESPs take place on the outermost portion of the F atom. However, the ESPs remain negative along the C? F bond with the introduction of relatively weak electron‐withdrawing substituents, irrespective of the number of the subsituents. In addition, some complexes between fluorinated molecules with positive ESPs on the F atom and NH₃ were calculated, to characterize the structural and energetic features of these specific halogen bonds. Finally, some crystal structures extracted from the Cambridge Structural Database were selected to provide experimental evidence of these interactions involving the C? F bond. The results presented in this work are very important for the modern definition of halogen bonding as well as for the development of pharmaceuticals and a wide range of functional material. © 2015 Wiley Periodicals, Inc.     

Metal exchange reactions in cadmium complexes with porphyrins of various structures

The reaction kinetics of the metal exchange Cd(II)-Cu(II) and Cd(II)-Zn(II) in the cadmium complexes (CdP) with porphyrin ligands (H₂P) differing by degree of stiffness (tetraphenylporphin, N-methyltetraphenylporphin, and tetraphenyltetrabenzoporphin) in the DMSO medium, was studied using spectrophotometric method. The rate of metal exchange reaction depends on the nature of the non-planatrity of H₂P in the structure of CdP complexes, as well as on the additional screening of the reaction center MN₄ by the extra-ligands and substituents. The reduction of the coordinating ability of the anion X⁻ in the structure of the solvate-salt of incoming metal M’X₂(Solv) _n−2 in a series: acetylacetonate > acetate > chloride > nitrate favors the metal exchange. In the most studied cases the reaction of CdP proceeds along a combined associative-dissociative mechanism. The order of the metal exchange reaction is found to be depending on temperature indicating a change in the contributions of associative and combined routes. The “pure” associative reaction mechanism in a medium of DMSO was for the first time found for the labile complex CdTPTBP with the saddle-type nonplanar ligand.     

Quantum statistical study of O + O2 isotopic exchange reactions: cross sections and rate constants

Using a wave packet based statistical model, we compute cross sections and thermal rate constants for various isotopic variants of the O + O2 exchange reaction on a recently modified ab initio potential energy surface. The calculation predicts a highly excited rotational distribution and relatively cold vibrational distribution for the diatomic product. A small but important threshold effect was identified for the (16)O + 18O2 reaction, which is suggested to contribute to the experimentally observed negative temperature dependence of the rate ratio, k(18O + 16O2)/k(16O + 18O2). Despite reasonable agreement with quasiclassical trajectory results, however, the calculated thermal rate constants are smaller than experimental measurements by a factor from 2 to 5. The experimentally observed negative temperature dependence of the rate constants is not reproduced. Possible reasons for the theory-experiment discrepancies are discussed.     

A novel series of iridium complexes with alkenylquinoline ligands: Theoretical study on electronic structure and spectroscopic property

The ground and the lowest-lying triplet excited state geometries, electronic structures, and spectroscopic properties of a novel series of neutral iridium(III) complexes with cyclometalated alkenylquinoline ligands [(C^N)₂Ir(acac)] (acac = acetoylacetonate; C^N = 2-[(E)-2-phenyl-1-ethenyl]pyridine (pep) 1; 2-[(E)-2-phenyl-1-ethenyl]quinoline (peq) 2; 1-[(E)-2-phenyl-1-ethenyl]isoquinoline (peiq) 3; 2-[(E)-1-propenyl]pyridine (pp) 4; 2-[(E)-1-fluoro-1-ethenyl]pyridine (fpp) 5) were investigated by DFT and CIS methods. The highest occupied molecular orbital is composed of d(Ir) and π(C^N) orbital, while the lowest unoccupied molecular orbital is dominantly localized on C^N ligand. Under the TD-DFT with PCM model level, the absorption and phosphorescence in CH₂Cl₂ media were calculated based on the optimized ground and triplet excited state geometries, respectively. The calculated lowest-lying absorptions at 437 nm (1), 481 nm (2), 487 nm (3), 422 nm (4), and 389 nm (5) are attributed to a {[d_x²-y²(Ir) + d_xz(Ir) + π(C^N)] → [π∗(C^N)]} transition with metal-to-ligand/intra-ligand charge transfer (MLCT/ILCT) characters, and the calculated phosphorescence at 582 nm (1), 607 nm (2), 634 nm (3), 515 nm (4), and 491 nm (5) can be described as originating from the ³{[d_x²-y²(Ir) + d_xz(Ir) + π (C^N)] [π∗(C^N)]} excited state with the ³MLCT/³ILCT characters. The calculated results revealed that the phosphorescent color of these new Ir(III) complexes can be tuned by changing the π-conjugation effect strength of the C^N ligand.     

Theoretical study on the intramolecular proton transfer reactions of 3-methyl-5-hydroxyisoxazole and its water complexes

The optimized structures and proton transfer reactions of 3-methyl-5-hydroxyisoxazole and its water complexes (3-M-5-HIO · (H₂O)_n · (n = 0–3)) were computed at B3LYP and MP2 theoretical level. The results indicates that 3-M-5-HIO has four isomers (Ecis, Etrans, K1 and K2), and the keto tautomer, and K2 is the most stable isomer in the gas phase. Hydrogen bonding between 3-M-5-HIO and the water molecules can dramatically lower the barrier by the concerted transfer mechanism. Ecis · (H₂O)₃ → K1 · (H₂O)₃ and Ecis · (H₂O)₂ → K2 · (H₂O)₂ is found to be very efficient. Comparing with the proton transfer mechanism of 5-HIO shows that the methyl substitution prevents the intramolecular proton transfer.     

Theoretical study of exchange coupling in 3d-Gd complexes: large magnetocaloric effect systems

Polynuclear 3d transition metal-Gd complexes are good candidates to present large magnetocaloric effect. This effect is favored by the presence of weak ferromagnetic exchange interactions that have been investigated using methods based on Density Functional Theory. The first part of the study is devoted to dinuclear complexes, focusing on the nature and mechanism of such exchange interactions. The presence of two bridging ligands is found more favorable for ferromagnetic coupling than a triple-bridged assembly, especially for complexes with small M-O···O-Gd hinge angles. Our results show the crucial role of the Gd 5d orbitals in the exchange interaction while the 6s orbital seems to have a negligible participation. The analysis of the atomic and orbital spin populations reveals that the presence of spin density in the Gd 5d orbital is mainly due to a spin polarization effect, while a delocalization mechanism from the 3d orbitals of the transition metal can be ruled out. We propose a numerical DFT approach using pseudopotentials to calculate the exchange coupling constants in four polynuclear first-row transition metal-Gd complexes. Despite the complexity of the studied systems, the numerical approach gives coupling constants in excellent agreement with the available experimental data and, in conjunction with exact diagonalization methods (or Monte Carlo simulations), it makes it possible to obtain theoretical estimates of the entropy change due to the magnetization/demagnetization process of the molecule.     

Theoretical study on the reactions of trimethylsilane with chlorine and bromine atoms

Theoretical studies are carried out on the multi-channel reactions of SiH(CH₃)₃ with Cl (reaction 1, R1) and Br atoms (R2) by direct dynamics method. The minimum energy path is calculated at the MP2/6-31+G(d,p) level, and energetic information is further refined by the MC-QCISD (single-point) method. The rate constants for individual reaction channels, R1a, R1b-in, R1b-out, R1c, R1d, R2a, R2b-in, R2b-out, R2c, and R2d, are calculated by the improved canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200–1,500 K. The theoretical three-parameter expressions k ₁ (T) = 6.30 × 10⁻¹⁵ T ^1.36exp(704.94/T) and k ₂ (T) = 9.41 × 10⁻²⁶ T ^4.89exp(−1,033.80/T) cm³ molecule⁻¹ s⁻¹ are given. Our calculations indicate that reaction channels R1c and R2c are the major channel.     

Theoretical study of boron nitride nanotubes with defects in nitrogen-rich synthesis

On the basis of calculations using the density functional theory, it is shown that BNNT synthesis could produce tubes deprived of one (B1 hole) or two (B2 hole) boron atoms under the condition where nitrogen atoms exist in excess throughout this study. The relative populations of various isomers of defective tubes will depend on the chirality of the tube. Interestingly, calculations show that B2 holes are much more favored than B1 holes, particularly in armchair tubes. Electronic properties are modified in such a way that the band gap is decreased through the introduction of defect states inside the gap. Magnetic properties will also be dependent on the chirality. The majority of armchair tubes with B2 holes will be nonmagnetic, while the majority of zigzag tubes with defects will exhibit magnetism. Contrary to the case of defect-free BNNT, the defective tubes are expected to be easily subject to reduction by accommodating excess electrons in the presence of Li atoms. In addition, the defect sites will show a higher affinity toward hydrogenation than the defect-free sites.     

A Preliminary experimental study of the boron concentration in vapor and the isotopic A preliminary experimental study of the boron concentrationin vapor and the isotopic fractionation of boron betweenseawater and vapor during evaporation of seawater

A laboratory experiment was undertaken to investigate the behaviour of boron at theseawater-air interface under air flow conditions. Dried air at 25 and 35℃ was passed over or bub-bled through seawater at the same temperature. A combination of ice-chilled condensers and KOHimpregnated cellulose fibre filters was used to collect boron from the reacted air. When air strippedof boron was passed over the seawater, boron was found in the reacted air, and its concentrationwas higher in the higher temperature test. In the tests where air was bubbled through seawater theconcentration of boron in the reacted air was directly proportional to the air flow rate. In this situa-tion the boron in the reacted air was mainly introduced as a spray of microdroplets. Isotopic analy-sis of the collected boron in the non-bubbled tests yields fractionation factors which demonstratethat the lighter isotope, 10B, is enriched in the reacted air. The size of the fractionation changeswith temperature, ruling out a purely kinetic effect.     

A comparative study of the reactions of fluorinated oxi- and thiiranes with acyl chlorides

The CoCl₂-catalyzed reactions of fluorinated 1,2-oxi- and thiiranes with acyl chlorides were studied. It was found that a regioselective heterocycle opening reaction resulted in two isomers having normal and abnormal structure in a ratio predetermined by the substituents in both the starting heterocycles and acyl chlorides.