K. Senthilkumar

Optical Absorption and Emission Properties of Fluoranthene, Benzo[k]fluoranthene, and Their Derivatives. A DFT Study

Fluoranthene and benzo[k]fluoranthene-based oligoarenes are good candidates for organic light-emitting diodes (OLEDs). In this work, the electronic structure and optical properties of fluoranthene, benzo[k]fluoranthene, and their derivatives have been studied using quantum chemical methods. The ground-state structures were optimized using the density functional theory (DFT) methods. The lowest singlet excited state was optimized using time-dependent density functional theory (TD-B3LYP) and configuration interaction singles (CIS) methods. On the basis of ground- and excited-state geometries, the absorption and emission spectra have been calculated using the TD-DFT method with a variety of exchange-correlation functionals. All the calculations were carried out in chloroform medium. The results show that the absorption and emission spectra calculated using the B3LYP functional is in good agreement with the available experimental results. Unlikely, the meta hybrid functionals such as M06HF and M062X underestimate the absorption and emission spectra of all the studied molecules. The calculated absorption and emission wavelength are more or less basis set independent. It has been observed that the substitution of an aromatic ring significantly alters the absorption and emission spectra.

Structural and Spectral Properties of 4-Bromo-1-naphthyl Chalcones: A Quantum Chemical Study

The structural and optical properties of 4-bromo-1-naphthyl chalcones (BNC) have been studied by using quantum chemical methods. The density functional theory (DFT) and the singly excited configuration interaction (CIS) methods were employed to optimize the ground and excited state geometries of unsubstituted and substituted BNC with different electron withdrawing and donating groups in both gas and solvent phases. Based on the ground and excited state geometries, the absorption and emission spectra of BNC molecules were calculated using the time-dependent density functional theory (TDDFT) method. The solvent phase calculations were performed using the polarizable continuum model (PCM). The geometrical parameters, vibrational frequencies, and relative stability of cis- and trans-isomers of unsubstituted and substituted BNC molecules have been studied. The results from the TDDFT calculations reveal that the substitution of electron withdrawing and electron donating groups affects the absorption and emission spectra of BNC.

Analysis of polarization in QM/MM modelling of biologically relevant hydrogen bonds

Combined quantum mechanics/molecular mechanics (QM/MM) methods are increasingly important for the study of chemical reactions and systems in condensed phases. Here, we have tested the accuracy of a density functional theory-based QM/MM implementation (B3LYP/6-311+G(d,p)/CHARMM27) on a set of biologically relevant interactions by comparison with full QM calculations. Intermolecular charge transfer due to hydrogen bond formation is studied to assess the severity of spurious polarization of QM atoms by MM point charges close to the QM/MM boundary. The changes in total electron density and natural bond orbital atomic charges due to hydrogen bond formation in selected complexes obtained at the QM/MM level are compared with full QM results. It is found that charge leakage from the QM atoms to MM atomic point charges close to the QM/MM boundary is not a serious problem, at least with limited basis sets. The results are encouraging in showing that important properties of key biomolecular interactions can be treated well at the QM/MM level employing good-quality levels of QM theory.

Copper ion mediated selective cleavage of C-S bond in ferrocenylthiosemicarbazone forming mixed geometrical [(PPh3)Cu(μ-S)2Cu(PPh3)2] having Cu2S2 core: toward a new avenue in copper-sulfur chemistry.

Unprecedented selective cleavage of the carbon-sulfur bond of the ferrocenylthiosemicarbazone moiety has been observed for the first time, resulting in the formation of mixed geometrical binuclear copper complex [(PPh(3))Cu(μ-S)(2)Cu(PPh(3))(2)]. Upon trying direct synthesis of the title complex, an unusual tetranuclear [Cu(4)(μ(3)-Cl)(4)(PPh(3))(4)] cubane resulted.

Mechanism and Kinetics of the Atmospheric Oxidative Degradation of Dimethylphenol Isomers Initiated by OH Radical

Dimethylphenols are highly reactive in the atmosphere, and their oxidation plays a vital role in the autoignition and combustion processes. The dominant oxidation process for dimethylphenols is by gas-phase reaction with OH radical. In the present study, the reaction of OH radical with dimethylphenol isomers is studied using density functional theory methods, B3LYP, M06-2X, and MPW1K, and also at the MP2 level of theory using 6-31G(d,p) and 6-31+G(d,p) basis sets. The activation energy values have also been calculated using the CCSD(T) method with 6-31G(d,p) and 6-311+G(d,p) basis sets using the geometries optimized at the M06-2X/6-31G(d,p) level of theory. The reactions subsequent to the principal oxidation steps are studied, and the different reaction pathways are modeled. The positions of the OH and CH3 substituents in the aromatic ring have a great influence on the reactivity of dimethylphenol toward OH radical. Accordingly, the reaction is initiated in four different ways: H-atom abstraction from the phenol group, H-atom abstraction from a methyl group, H-atom abstraction from the aromatic ring by OH radical, or electrophilic addition of OH radical to the aromatic ring. Aromatic peroxy radicals arising from initial H-atom abstraction and subsequent O2 addition lead to the formation of hydroperoxide adducts and alkoxy radicals. The O2 additions to dimethylphenol-OH adduct results in the formation of epoxide and bicyclic radicals. The rate constants for the most favorable reaction pathways are calculated using canonical variational transition state theory with small curvature tunneling corrections. This study provides thermochemical and kinetic data for the oxidation of dimethylphenol in the atmosphere and demonstrates the mechanism for the conversion of peroxy radical into aldehydes, hydroperoxides, epoxides, and bicyclic radicals, and their lifetimes in the atmosphere.

Tautomerization and solvent effects on the absorption and emission properties of the Schiff base N,N′-bis(salicylidene)-p-phenylenediamine – A TDDFT study

Time-dependent density functional theory combined with a polarized continuum model has been applied to study solvent effects on the geometrical and energetic properties, as well as the absorption and emission properties, of three tautomeric forms of N,N′-bis(salicylidene)-p-phenylenediamine (BSP). The calculated properties are in agreement with the available experimental data. It was observed that the solvent environment does not affect the vertical excitation energies significantly, whereas tautomerization strongly affects both the absorption and emission spectra of BSP.

Absorption and emission properties of phenylene ethynylene oligomers: effect of substitution and π-conjugation length

The optical properties of a series of π-conjugated phenylene-ethynylene oligomers (OPEs) have been studied by advanced quantum chemical methods. The ground state and lowest singlet excited state geometries of unsubstituted and different electron donor and acceptor groups substituted OPEs are optimized by density functional theory and configuration interaction singles methods. The absorption and emission spectra of unsubstituted and substituted OPEs have been calculated using the time-dependent density functional theory (TDDFT) method. The results of theoretical calculations are in good agreement with the available experimental results. It has been found that the substitution of electron donating and withdrawing groups in the phenyl ring and conjugation length of the OPEs has significantly affect both the absorption and emission spectra

Lennard−Jones Parameters for B3LYP/CHARMM27 QM/MM Modeling of Nucleic Acid Bases

Combined quantum mechanics/molecular mechanics (QM/MM) methods allow computations on chemical events in large molecular systems. Here, we have tested the suitability of the standard CHARMM27 forcefield Lennard-Jones van der Waals (vdW) parameters for the treatment of nucleic acid bases in QM/MM calculations at the B3LYP/6-311+G(d,p)-CHARMM27 level. Alternative parameters were also tested by comparing the QM/MM hydrogen bond lengths and interaction energies with full QM [B3LYP/6-311+G(d,p)] results. The optimization of vdW parameters for nucleic acid bases is challenging because of the likelihood of multiple hydrogen bonds between the nucleic acid base and a water molecule. Two sets of optimized atomic vdW parameters for polar hydrogen, carbonyl carbon, and aromatic nitrogen atoms for nucleic acid bases are reported: base-dependent and base-independent. The results indicate that, for QM/MM investigations of nucleic acids, the standard forcefield vdW parameters may not be appropriate for atoms treated by QM. QM/MM interaction energies calculated with standard CHARMM27 parameters are found to be too large, by around 3 kcal/mol. This is because of overestimation of electrostatic interactions. Interaction energies closer to the full QM results are found using the optimized vdW parameters developed here. The optimized vdW parameters [developed by reference to B3LYP/6-311+G(d,p) results] were also tested at the B3LYP/6-31G(d) QM/MM level and were found to be transferable to the lower level. The optimized parameters also model the interaction energies of charged nucleic acid bases and deprotonation energies reasonably well.

Structural diversity in aroylthiourea copper complexes – formation and biological evaluation of [Cu(I)(μ-S)SCl]2, cis-Cu(II)S2O2, trans-Cu(II)S2O2 and Cu(I)S3 cores

Copper complexes of types [Cu(HL1-{μ-S})(HL1-S)Cl]2 (1), cis-[Cu(L1-O,S)2] (2), [Cu(L2-S)3]Cl (3) and trans-[Cu(L2-O,S)2] (4) were synthesized and characterized by analytical, spectroscopic (UV-vis, FT-IR, and NMR/EPR) and single crystal X-ray diffraction techniques. In order to understand the structure, bonding, and band gap of complex 1, DFT calculations were performed. The computed results revealed that the absorption was associated with 1MLCT transitions while the emission had 3MLCT character. The calculated geometrical parameters agreed well with the experimental values. The binding affinity and binding mode of these copper complexes toward calf thymus (CT) DNA were determined by using UV-vis spectroscopic titrations and the fluorescent indicator displacement (FID) method. These studies showed that the complexes bind in the order of 1 > 2 > 3 > 4 to CT DNA. The protein binding ability has been monitored by the quenching of tryptophan emission in the presence of the complexes using bovine serum albumin (BSA) as a model protein. These copper complexes displayed high cytotoxicity against two cancer cell lines (A549 and MCF7).

A theoretical study of structural and electronic properties of pentacene/Al(100) interface.

The first principle calculations within the framework of density functional theory have been performed for the pentacene molecule deposited on the aluminum Al(100) substrate to study the structural and electronic properties of the pentacene/Al(100) interface. The most stable configuration was found at bridge site with 45° rotation of the pentacene molecule on Al(100) surface with a vertical distance of 3.4 Å within LDA and 3.8 Å within GGA functionals. The calculated adsorption energy reveals that the adsorption of pentacene molecule on Al(100) surface is physisorption. For the stable adsorption geometry the electronic properties such as density of states (DOS), partial density of states (PDOS), Mulliken population analysis and Schottky barrier height are studied. The analysis of atomic charge, DOS and PDOS show that the charge is transferred from the Al(100) surface to pentacene molecule, and the transferred charge is about -0.05 electrons. For the adsorbed system, the calculated Schottky barrier height for hole and electron transport is 0.27 and 1.55 eV, respectively.