Structure and dynamics of the Li+ ion in water, methanol and acetonitrile solvents: ab initio molecular dynamics simulations.

Fundamental understanding of the structure and dynamics of the Li+ ion in solution is of utmost importance in different fields of science and technology, especially in the field of ion batteries. In view of this, ab initio molecular dynamics (AIMD) simulations of the LiCl salt in water, methanol and acetonitrile were performed to elucidate structural parameters such as radial distribution function and coordination number, and dynamical properties like diffusion coefficient, limiting ion conductivity and hydrogen bond correlation function. In the present AIMD simulation, one LiCl in water is equivalent to 0.8 M, which is close to the concentration of the lithium salt used in the Li-ion battery. The first sphere of coordination number of the Li+ ion was reaffirmed to be 4. The radial distribution function for different pairs of atoms is seen to be in good agreement with the experimental results. The calculated potential of mean force indicates the stronger interaction of the Li+ ion with methanol over water followed by acetonitrile. The dynamical parameters convey quite high diffusion and limiting ionic conductivity of the Li+ ion in acetonitrile compared to that in water and methanol which has been attributed to the transport of the Li-Cl ion pair in a non-dissociated form in acetonitrile. The AIMD results were found to be in accordance with the experimental findings, i.e. the limiting ion conductivity was found to follow the order acetonitrile > methanol > water. This study shows the importance of atomistic level simulations in evaluating the structural and dynamical parameters and in implementing the results for predicting and synthesizing better next generation solvents for lithium ion batteries (LIBs).

Silver nanoparticle synthesis and their potency against multidrug-resistant bacteria: a green approach from tissue-cultured Coleus forskohlii.

Drug resistance is a major concern nowadays, and finding alternatives of the well-known antibiotic is necessary. Green nanoparticles are emerging as a tenable alternative to this with a large spectrum of activity. The present manuscript describes an eco-friendly approach for green synthesis of silver nanoparticles from both in vitro and in vivo leaf extract of Coleus forskohlii. Leaf extracts were used in synthesis of nanoparticles which were further analyzed through UV-Vis, dynamic light scattering, energy-dispersive spectroscopy, and transmission electron microscopy. Antimicrobial activity of silver nanoparticles alone, as well as crude extract of the plant itself, was carried out against eight multidrug-resistant respiratory tract infecting pathogenic strains. Satisfactory antimicrobial activities were found with nanoparticles, in vitro and in vivo leaf extracts. However, gradually higher to lower inhibition potential against pathogenic bacterial strains was found in silver nanoparticles, in vitro and in vivo leaf extracts. Seven bioactive compounds were detected in the crude extract through gas chromatography-mass spectroscopy analysis. Results revealed that nanoparticle formation occurred in a wide range of sizes (10-50 nm) and shapes (trigonal, hexagonal, spherical, rod). The diversity in size and shape of the nanoparticles makes them biologically active. Silver nanoparticle exhibits significantly better antimicrobial activities as compared to the plant extract in case of nearly all pathogens with a maximum zone of inhibition of 15.33 ± 0.94 mm where more than 12 well-known antibiotics failed to respond. Because of this broad-spectrum activity of nanoparticles as well as the leaf extracts against life-threatening microbes, it can be used as future generation drugs.

Room-Temperature Amination of Chloroheteroarenes in Water by a Recyclable Copper(II)-Phosphaadamantanium Sulfonate System.

Buchwald-Hartwig amination of chloroheteroarenes has been a challenging synthetic process, with very few protocols promoting this important transformation at ambient temperature. The current report discusses about an efficient copper-based catalytic system (Cu/PTABS) for the amination of chloroheteroarenes at ambient temperature in water as the sole reaction solvent, a combination that is first to be reported. A wide variety of chloroheteroarenes could be coupled efficiently with primary and secondary amines as well as selected amino acid esters under mild reaction conditions. Catalytic efficiency of the developed protocol also promotes late-stage functionalization of active pharmaceutical ingredients (APIs) such as antibiotics (floxacins) and anticancer drugs. The catalytic system also performs efficiently at a very low concentration of 0.0001 mol % (TON = 980,000) and can be recycled 12 times without any appreciable loss in activity. Theoretical calculations reveal that the π-acceptor ability of the ligand PTABS is the main reason for the appreciably high reactivity of the catalytic system. Preliminary characterization of the catalytic species in the reaction was carried out using UV-VIS and ESR spectroscopy, providing evidence for the Cu(II) oxidation state.

Pulse Radiolysis and Computational Studies on a Pyrrolidinium Dicyanamide Ionic Liquid: Detection of the Dimer Radical Anion.

A pulse radiolysis study on pyrrolidinium cation based ionic liquids is presented herein. Time-resolved absorption spectra for 1-methyl-1-propylpyrrolidinium dicyanamide (DCA) at 500 ns after the electron pulse show broad absorption bands at wavelengths below 440 nm and at 640 nm. In pyrrolidinium bis(trifluoromethylsulfonyl)imide (NTf2) and tris(perfluoroethyl)trifluorophosphate (FAP) ILs, the transient absorption below 440 nm is much weaker. The absorption at 500 ns, which increases with wavelength from 500 nm to beyond 800 nm, was assigned to the tail of the solvated electron NIR absorption spectrum, since it disappears in the presence of N2O. In the DCA IL, the presence of a reducing species was confirmed by the formation of pyrene radical anion. The difference in the transient species in the case of the DCA IL compared to other two ILs should be due to the anion, with cations being similar. In pseudohalide ILs such as DCA, radicals are formed by direct hole trapping by the anion (X- + h+ → X•), followed by addition to the parent anion. Prediction of the UV/vis absorption spectra of the dimer radical anion by computational calculation supports the experimental results. The oxidizing efficiency of (DCA)2•- and its reduction potential ( E(DCA)2•-/(2DCA-)) have been determined.

Enhanced fluorescence of aqueous BODIPY by interaction with cavitand cucurbit[7]uril.

One cationic BODIPY chromophore was synthesized and its complexation behaviour with the macrocyclic host cucurbit[7]uril (CB[7]) was studied using different spectroscopy techniques such as UV-vis absorption, steady-state and time-resolved fluorescence, 1H NMR as well as DFT based quantum calculations. The dye showed formation of a 1 : 1 dye-CB[7] complex with improvement in the fluorescence intensity. These new results of the formation of moderate association of aqueous BODIPYs with the nontoxic host CB[7] may lead to promising applications of the dye molecule as a sensitive and efficient off-on mode fluorescent probe in chemical and biological studies.

Supramolecularly Assisted Modulation of Optical Properties of BODIPY-Benzimidazole Conjugates.

Supramolecular host-guest interaction of neutral and cationic (protonated) forms of two boron-dipyromethane (BODIPY)-benzimidazole (mono- and di-benzimidazole) conjugate dyes with the macrocyclic host cucurbit[7]uril (CB7) has been investigated using photophysical and density functional theory studies. Expectedly, cationic forms of the dyes show exceptionally stronger binding than that of the neutral forms with CB7, which can be ascribed to the strong ion-dipole interaction between the positive charge of the dye and the highly polarizable carbonyl portals of the host. The formation of dye-host inclusion complexes is supported by the significant changes in the photophysical properties and longer rotational relaxation times of the dye in the presence of CB7. Job's plot studies indicate the formation of a 1:1 inclusion complex for the mono and a 1:2 inclusion complex for the dibenzimidazole BODIPY dyes. Quantum chemical calculations are in good agreement with the inferences outlined from photophysical measurements. Findings from the studied dye-CB7 systems are of direct relevance to applications such as drug delivery, aqueous dye lasers, sensors, and so on.

A Serendipitous Synthesis of 11a-Hydroxy-11,11a-dihydrobenzo[e]indeno[2,1-b][1,4]diazepine-10,12-dione Derivatives by Condensation of 2-Aminobenzamides with Ninhydrin in Water.

Ninhydrin undergoes an unprecedented condensation reaction with various 2-aminobenzamide derivatives in boiling water to afford 11a-hydroxy-11,11a-dihydrobenzo[e]indeno[2,1-b][1,4]diazepine-10,12-dione derivatives. These hitherto unreported products are easily isolated in high yield by a simple filtration step. An interesting "ortho effect" was observed in the condensation reaction of ninhydrin with 2-amino-N-phenylbenzamide derivatives having an ortho- substituent in the N-phenyl moiety wherein the corresponding expected 3'-phenyl-1'H-spiro[indene-2,2'-quinazoline]-1,3,4'(3'H)-triones were obtained.

On the formation of SFn (-) (n = 1-6) anions via a novel route and their properties.

RATIONALE: Sulfur hexafluoride (SF6 ) being a potential greenhouse gas, coupled with its numerous applications, makes the study of the formation and fragmentation of SF6 -based species important. The formation of SF6 -based anionic species has been studied using the gas feed-sputtering route and the mechanisms at play during the sputter-ejection of guest molecule-derived particles have also been probed. METHODS: Studies of the formation of SFn (-) (n = 1-6) anions were conducted from various surfaces (metal and compound) that were subjected to Cs(+) ion sputtering in the presence of SF6 gas employing the gas feed-cesium sputter technique. The anions generated were mass analyzed using a double-focusing magnetic sector mass spectrometer. Quantum mechanical computations were performed to study the ground state structure and stability of neutral and negatively charged SFn (n = 1-6) systems applying density functional theory (DFT) and ab initio methods (MP2 and CC). RESULTS: This technique readily generated (32) SFn (-) (n = 1-6) anions for all sizes of 'n' with practicable yields. Mass spectrometric measurements of the yield of sputter-ejected (32) SFn (-) (n = 1-6) anions reveal an oscillatory pattern as a function of 'n', with odd values of 'n' being relatively more abundant. The relative yield of (34) SFn (-) (n = 1-6) anions with respect to size was also measured albeit with low signal intensity. Also observed were F(-) , S(-) , F2 (-) , (33) SF5 (-) and (33) SF6 (-) anionic species. The relevant electron affinity and bond dissociation energy (BDE) values were also computed. CONCLUSIONS: Gas-phase SFn (-) (n = 1-6) anions can be effectively generated by using the gas spray-cesium sputter technique. Both experimental measurements and calculations indicate the existence of odd-even oscillations in the stability and electronic structure of the SFn (n = 1-6) systems. The highest yield recorded was for the sputter-ejected SF5 (-) species and this may be attributed to its 'superhalogen' anionic character coupled with the relatively favorable F(0) fragmentation pathway of sputtered SF6 (-) . A signature pertaining to intact SF6 (-) anion ejection is also observed.

C-H···N hydrogen-bonding interaction in 7-azaindole:CHX3 (X=F, Cl) complexes.

The C-H···Y (Y=hydrogen-bond acceptor) interactions are somewhat unconventional in the context of hydrogen-bonding interactions. Typical C-H stretching frequency shifts in the hydrogen-bond donor C-H group are not only small, that is, of the order of a few tens of cm(-1) , but also bidirectional, that is, they can be red or blue shifted depending on the hydrogen-bond acceptor. In this work we examine the C-H···N interaction in complexes of 7-azaindole with CHCl3 and CHF3 that are prepared in the gas phase through supersonic jet expansion using the fluorescence depletion by infra-red (FDIR) method. Although the hydrogen-bond acceptor, 7-azaindole, has multiple sites of interaction, it is found that the C-H···N hydrogen-bonding interaction prevails over the others. The electronic excitation spectra suggest that both complexes are more stabilized in the S1 state than in the S0 state. The C-H stretching frequency is found to be red shifted by 82 cm(-1) in the CHCl3 complex, which is the largest redshift reported so far in gas-phase investigations of 1:1 haloform complexes with various substrates. In the CHF3 complex the observed C-H frequency is blue shifted by 4 cm(-1). This is at variance with the frequency shifts that are predicted using several computational methods; these predict at best a redshift of 8.5 cm(-1). This discrepancy is analogous to that reported for the pyridine-CHF3 complex [W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A- 2005, 109, 3038], in which the blueshift is termed a pseudo blueshift and is shown to be due to the shifting of levels caused by Fermi resonance between the overtones of the C-H bending and stretching modes. The dissociation energies, (D0), of the CHCl3 and CHF3 complexes are computed (MP2/aug-cc-pVDZ level) as 6.46 and 5.06 kcal mol(-1), respectively.

How much water is needed to ionize formic acid?

Structure, molecular properties, energy parameters, and vibrational IR spectra of hydrated clusters of formic acid (FOH), FOH·nH2O (n = 1-8), are presented following first principle based electronic structure theory. Several geometrical arrangements are considered as initial guess structures to look for the minimum energy equilibrium structures applying ωB97X-D density functional and aug-cc-pVDZ set of atomic basis functions. Results on FOH-water clusters suggest that the most stable structure of formic acid exists as a charge-separated ion pair, FO(-δ)---H(+δ)---O(-δ)H2, in hydrated clusters, FOH·nH2O, for cluster size n ≥ 7. The calculated interaction energy between FOH and the solvent water cluster increases significantly by adding the seventh solvent water molecule to the FOH·6H2O cluster whereas the solvent stabilization energy of FOH increases continuously on successive addition of solvent water molecules (n = 1-8). Energy partitioning of the solvent stabilization energy of FOH·nH2O clusters suggests an electrostatic component of energy to play the major role in solvent stability and does depict sudden variation for n = 7. Formation of a charge-separated ion pair for cluster size n ≥ 7 is manifested in simulated IR spectra of FOH·nH2O clusters. Static polarizabilities of hydrated formic acid clusters are calculated and observed to vary linearly with the size of the clusters, suggesting reliability in prediction of polarizability for larger size hydrated clusters of formic acid.

C-H···Y hydrogen bonds in the complexes of p-cresol and p-cyanophenol with fluoroform and chloroform.

In this work, we present spectroscopic investigations of hydrogen bonded complexes of CHF3 and CHCl3 with p-cresol and p-cyanophenol. The systems were chosen as the potential candidates bound by C-H···Y type hydrogen bonds that are known to exhibit unconventional blue shifts in the C-H stretching frequency. The two phenol derivatives chosen offer multiple hydrogen bonding acceptor sites. They also differ from each other in regard to the electron-donating/withdrawing ability of the para substituents which could dictate the global minimum structure in each case. The complexes were formed using the supersonic jet expansion method and were investigated using a variant of the IR-UV double resonance technique, namely fluorescence depletion IR (FDIR) spectroscopy. It was found that in the case of p-cresol the complexes were C-H···π bound in which the C-H stretch was blue-shifted. In the case of p-cyanophenol the complexes were C-H···N bound. In its fluoroform complex the C-H frequency was blue-shifted by 27 cm(-1), whereas the chloroform complex gave an example of zero-shifted hydrogen bond. The ab initio computational studies indicated that for the CHCl3 complexes it is necessary to optimize the structures on the BSSE-corrected PES using the counterpoise method to correctly predict the magnitudes of the C-H frequency shift.

Change of boron substitution improves the lasing performance of Bodipy dyes: a mechanistic rationalisation.

Bodipy laser dyes are highly efficient but degrade rapidly in solution by reacting with in situ generated singlet oxygen ((1)O(2)). To increase the lasing lifetimes of these dyes, we have designed and synthesised two different congeners of the widely studied Pyrromethene 567 (PM567) by substitution at the boron centre and/or at both the boron centre and the meso position. The two new dyes showed high lasing efficiencies with increased photostability. The results of theoretical and pulse radiolysis studies revealed that the substitution at the boron centre reduced the (1)O(2) generation capacity of these dyes as well as their rate of reaction with (1)O(2), thereby enhancing their lifetimes even under lasing conditions.

Supramolecular interaction of coumarin 1 dye with cucurbit[7]uril as host: combined experimental and theoretical study.

Molecules of the coumarin family have fluorescence characteristics that are highly sensitive to their environment, and thus, they have been used as fluorescent sensors in chemical and biological systems. However, the very poor fluorescence yield of most coumarin dyes in aqueous media limits their applications. We have adopted a supramolecular strategy to improve the fluorescence intensity of coumarin dye through its interaction with the relatively new host cucurbit[7]uril (CB[7]). The virtually nonfluorescent coumarin 1 (Φ(f) = 0.04) was converted into a highly fluorescent (Φ(f) = 0.52) entity in water upon addition of the nonfluorescent host CB[7]. Various spectroscopy techniques, namely, UV-vis absorption and steady-state and time-resolved fluorescence spectroscopies, established the formation of a strong 1:1 dye-CB[7] inclusion complex with a high binding constant of (1.2 ± 0.1) × 10(5) M(-1) for the dye. The stable inclusion complex of the neutral molecule was supported by density-functional-theory- (DFT-) based quantum chemical calculations. Energy decomposition analysis of various interaction factors in the host-guest complex revealed that key components providing stability to the complex were electrostatic, polarization, and charge-transfer energies. These new results on the formation of a strong inclusion complex of the versatile fluorophore coumarin 1 with the nontoxic host CB[7] could lead to the design of efficient molecular-scale biological probes, sensors, and photostable aqueous UV dye lasers.

Steric effects on uranyl complexation: synthetic, structural, and theoretical studies of carbamoyl pyrazole compounds of the uranyl(VI) ion.

New bifunctional pyrazole based ligands of the type [C(3)HR(2)N(2)CONR'] (where R = H or CH(3); R' = CH(3), C(2)H(5), or (i)C(3)H(7)) were prepared and characterized. The coordination chemistry of these ligands with uranyl nitrate and uranyl bis(dibenzoyl methanate) was studied with infrared (IR), (1)H NMR, electrospray-mass spectrometry (ES-MS), elemental analysis, and single crystal X-ray diffraction methods. The structure of compound [UO(2)(NO(3))(2)(C(3)H(3)N(2)CON{C(2)H(5)}(2))] (2) shows that the uranium(VI) ion is surrounded by one nitrogen atom and seven oxygen atoms in a hexagonal bipyramidal geometry with the ligand acting as a bidentate chelating ligand and bonds through both the carbamoyl oxygen and pyrazolyl nitrogen atoms. In the structure of [UO(2)(NO(3))(2)(H(2)O)(2)(C(5)H(7)N(2)CON {C(2)H(5)}(2))(2)], (5) the pyrazole ligand acts as a second sphere ligand and hydrogen bonds to the water molecules through carbamoyl oxygen and pyrazolyl nitrogen atoms. The structure of [UO(2)(DBM)(2)C(3)H(3)N(2)CON{C(2)H(5)}(2)] (8) (where DBM = C(6)H(5)COCHCOC(6)H(5)) shows that the pyrazole ligand acts as a monodentate ligand and bonds through the carbamoyl oxygen to the uranyl group. The ES-MS spectra of 2 and 8 show that the ligand is similarly bonded to the metal ion in solution. Ab initio quantum chemical studies show that the steric effect plays the key role in complexation behavior.

Interaction of alcohols with 2-fluoro- and 4-fluorophenylacetylenes: infrared-optical double resonance spectroscopic and computational investigation.

Alcohol complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene were investigated using IR-UV double resonance spectroscopy. Methanol forms a cyclic complex with both the fluorophenylacetylenes incorporating C-H···O and O-H···π hydrogen bonds, the structure of which is similar to that of the corresponding water complex but different from that of a phenylacetylene-methanol complex. The anti conformer of ethanol also binds in a similar fashion to both the fluorophenylacetylenes. Additionally, the gauche conformer of ethanol binds to 2-fluorophenylacetylene in a distinctly different structural motif that incorporates C-H···F and O-H···π hydrogen bonds. The OH group of trifluoroethanol interacts primarily with the π electron density of the C≡C bond. The π electron density of the C≡C bond is the principal point of interaction between the alcohols and both the fluorophenylacetylenes. The present results are indicative of the fact that fluorine substitution on the phenyl ring is sufficient to eliminate the subtle hydrogen bonding behavior of phenylacetylene.

Effect of five membered versus six membered meso-substituents on structure and electronic properties of Mg(II) porphyrins: a combined experimental and theoretical study.

Meso-tetrasubstituted Mg(II) porphyrins containing six membered phenyl groups (MgTPP) and five membered thienyl (MgTThP) and furyl groups (MgTFP) were synthesized and structurally characterized, and the effects of meso-substituents on electronic properties were studied using NMR, absorption, fluorescence spectroscopy, and electrochemical studies. Density functional theory (DFT) calculations were carried out to correlate with experimental observations. The three Mg(II) porphyrins MgTPP, MgTThP, and MgTFP were crystallized as hexa-coordinate systems with Mg(II) ion in the center of the porphyrin plane and having two tetrahydrofuran molecules as axial ligands. The X-ray studies clearly showed that the meso-furyl groups adopt a conformation in which they are more in-plane with the porphyrin plane whereas the thienyl and phenyl groups prefer an orthogonal arrangement with respect to the porphyrin plane. This arrangement of meso-substituents with the porphyrin plane helps in the enhancement of porphyrin π-delocalization in MgTFP compared to MgTThP and MgTPP. The differences in their structures are clearly reflected in their spectral and electrochemical properties. The absorption and fluorescence bands experienced bathochromic shifts on moving from six membered phenyls to five membered thienyl and furyl group, and the maximum effects were observed for meso-tetrafuryl Mg(II) porphyrin. The electrochemical studies indicated that the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases as we move from six membered phenyl groups to five membered thienyl and furyl groups, which explains the bathochromic shifts observed in absorption and fluorescence bands. Results on structure and electronic properties based on DFT studies are in agreement with experimental observations.

Theoretical study on the spectroscopic properties of CO3(*-).nH2O clusters: extrapolation to bulk.

Vertical detachment energies (VDE) and UV/Vis absorption spectra of hydrated carbonate radical anion clusters, CO(3)(*-).nH(2)O (n=1-8), are determined by means of ab initio electronic structure theory. The VDE values of the hydrated clusters are calculated with second-order Moller-Plesset perturbation (MP2) and coupled cluster theory using the 6-311++G(d,p) set of basis functions. The bulk VDE value of an aqueous carbonate radical anion solution is predicted to be 10.6 eV from the calculated weighted average VDE values of the CO(3)(*-).nH(2)O clusters. UV/Vis absorption spectra of the hydrated clusters are calculated by means of time-dependent density functional theory using the Becke three-parameter nonlocal exchange and the Lee-Yang-Parr nonlocal correlation functional (B3LYP). The simulated UV/Vis spectrum of the CO(3)(*-).8H(2)O cluster is in excellent agreement with the reported experimental spectrum for CO(3)(*-) (aq), obtained based on pulse radiolysis experiments.

Infrared-optical double-resonance measurements on O-H...H-Ge dihydrogen-bonded phenol-triethylgermanium hydride complex in the gas phase.

Spectroscopic investigation of a dihydrogen-bonded complex between phenol and triethylgermanium hydride is reported here. Laser-induced fluorescence excitation, fluorescence-detected infrared, and IR-UV hole-burning spectroscopic studies were carried out in supersonic jet to investigate the complex formation between phenol and triethylgermanium hydride. The lowering of the O-H stretching frequency of the phenol moiety in the complex with triethylgermanium hydride clearly establishes the role of phenol as hydrogen bond donor. The experimental results together with the ab-initio calculations unambiguously confirm formation of an O-H...H-Ge dihydrogen-bonded complex between phenol and triethylgermanium hydride.

Role of phenolic O-H and methylene hydrogen on the free radical reactions and antioxidant activity of curcumin.

To understand the relative importance of phenolic O-H and the CH-H hydrogen on the antioxidant activity and the free radical reactions of Curcumin, (1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione), biochemical, physicochemical, and density functional theory (DFT) studies were carried out with curcumin and dimethoxy curcumin (1,7-bis[3, 4-dimethoxy phenyl]-1,6-heptadiene-3,5-dione). The antioxidant activity of these compounds was tested by following radiation-induced lipid peroxidation in rat liver microsomes, and the results suggested that at equal concentration, the efficiency to inhibit lipid peroxidation is changed from 82% with curcumin to 24% with dimethoxy curcumin. Kinetics of reaction of (2,2'-diphenyl-1-picrylhydrazyl) DPPH, a stable hydrogen abstracting free radical was tested with these two compounds using stopped-flow spectrometer and steady state spectrophotometer. The bimolecular rate constant for curcumin was found to be approximately 1800 times greater than that for the dimethoxy derivative. Cyclic voltammetry studies of these two systems indicated two closely lying oxidation peaks at 0.84 and 1.0 V vs. SCE for curcumin, while only one peak at 1.0 V vs. SCE was observed for dimethoxy curcumin. Pulse radiolysis induced one-electron oxidation of curcumin and dimethoxy curcumin was studied at neutral pH using (*)N(3) radicals. This reaction with curcumin produced phenoxyl radicals absorbing at 500 nm, while in the case of dimethoxy curcumin a very weak signal in the UV region was observed. These results suggest that, although the energetics to remove hydrogen from both phenolic OH and the CH(2) group of the beta-diketo structure are very close, the phenolic OH is essential for both antioxidant activity and free radical kinetics. This is further confirmed by DFT calculations where it is shown that the -OH hydrogen is more labile for abstraction compared to the -CH(2) hydrogen in curcumin. Based on various experimental and theoretical results it is definitely concluded that the phenolic OH plays a major role in the activity of curcumin.

Structure, bonding, and spectra of cyclic dithia radical cations: a theoretical study.

Ab initio molecular orbital and hybrid density functional theory methods are employed to characterize the structure, bonding and properties of several cyclic dithia radical cation systems, particularly in the context of intra molecular two-center three-electron (2c-3e) bonding between two sulfur atoms. The calculated results are able to interpret the time-resolved transient optical spectra obtained from pulse radiolysis technique for these positively charged dithia systems in aqueous solution. Visualization of the appropriate molecular orbital (MO) in the systems is able to depict the presence of a 2c-3e bond between two sulfur atoms and its sigma character. Geometry optimizations of these doublet systems are carried out at restricted open shell Becke's half-and-half (BHH) nonlocal exchange and Lee-Yang-Parr (LYP) nonlocal correlation functionals (BHHLYP) with 6-311+G(d,p) basis set including solvent effects adopting Onsager's reaction field model. Hessian calculations are done at the same level to check the nature of the equilibrium geometry. Energy data are further improved by performing MP2/6-311+G(d,p) calculations on these radical cation systems. Excited-state calculations are done following configuration interaction with single-electron excitation (CIS) method and the optical transition wavelength from the highest doubly occupied molecular orbital (HDOMO) to the lowest singly occupied molecular orbital (LSOMO) is seen to correspond and match to the position of the absorption maxima (lambda(max)) obtained from the experimental spectra for all these radical cation systems in aqueous solution. These calculations are able to resolve a long-standing ambiguity in the assignment of intra molecular 2c-3e bonding in the case of the 3-methyl-2,4-dithiapentane radical cation system and to provide new insights into bonding features of this odd electron system as well as of other cyclic dithia systems studied.