Debasish Mandal

Interplay of Experiment and Theory in Elucidating Mechanisms of Oxidation Reactions by a Nonheme RuIVO Complex

A comprehensive experimental and theoretical study of the reactivity patterns and reaction mechanisms in alkane hydroxylation, olefin epoxidation, cyclohexene oxidation, and sulfoxidation reactions by a mononuclear nonheme ruthenium(IV)-oxo complex, [Ru(IV)(O)(terpy)(bpm)](2+) (1), has been conducted. In alkane hydroxylation (i.e., oxygen rebound vs oxygen non-rebound mechanisms), both the experimental and theoretical results show that the substrate radical formed via a rate-determining H atom abstraction of alkanes by 1 prefers dissociation over oxygen rebound and desaturation processes. In the oxidation of olefins by 1, the observations of a kinetic isotope effect (KIE) value of 1 and styrene oxide formation lead us to conclude that an epoxidation reaction via oxygen atom transfer (OAT) from the Ru(IV)O complex to the C═C double bond is the dominant pathway. Density functional theory (DFT) calculations show that the epoxidation reaction is a two-step, two-spin-state process. In contrast, the oxidation of cyclohexene by 1 affords products derived from allylic C-H bond oxidation, with a high KIE value of 38(3). The preference for H atom abstraction over C═C double bond epoxidation in the oxidation of cyclohexene by 1 is elucidated by DFT calculations, which show that the energy barrier for C-H activation is 4.5 kcal mol(-1) lower than the energy barrier for epoxidation. In the oxidation of sulfides, sulfoxidation by the electrophilic Ru-oxo group of 1 occurs via a direct OAT mechanism, and DFT calculations show that this is a two-spin-state reaction in which the transition state is the lowest in the S = 0 state.

Determination of Spin Inversion Probability, H-Tunneling Correction, and Regioselectivity in the Two-State Reactivity of Nonheme Iron(IV)-Oxo Complexes

We show by experiments that nonheme Fe(IV)O species react with cyclohexene to yield selective hydrogen atom transfer (HAT) reactions with virtually no C═C epoxidation. Straightforward DFT calculations reveal, however, that C═C epoxidation on the S = 2 state possesses a low-energy barrier and should contribute substantially to the oxidation of cyclohexene by the nonheme Fe(IV)O species. By modeling the selectivity of this two-site reactivity, we show that an interplay of tunneling and spin inversion probability (SIP) reverses the apparent barriers and prefers exclusive S = 1 HAT over mixed HAT and C═C epoxidation on S = 2. The model enables us to derive a SIP value by combining experimental and theoretical results.

Kinetics and Mechanism of the Tropospheric Oxidation of Vinyl Acetate Initiated by OH Radical: A Theoretical Study

Vinyl acetate [VA (CH3COOC2H3)] is an important unsaturated and oxygenated volatile organic compound responsible for atmospheric pollution. In this work, possible reaction mechanisms for the degradation of OH-initiated atmospheric oxidation of VA are investigated. The potential energy surfaces (PESs) for the reaction of OH radical with VA in the presence of O2 and NO have been studied using the M06-2X/6-311++G(d,p) method. The initial addition reactions of more and less substituted ethylenic C-atoms of VA are treated separately, followed by a conventional transition state theory (TST) calculation for reaction rates. The direct H-abstraction mechanism and kinetics have also been studied. The initial OH addition occurs through a prereactive complex, and the calculated rate constants in the temperature range 250-350 K for both the addition reactions are found to have negative temperature dependence. The calculation indicates that the reaction proceeds predominantly via the addition of OH radical to the double bond rather than the direct abstraction of H-atoms in VA. IM1 [CH3C(O)O(•)CHCH2OH] and IM2 [CH3C(O)OCH(OH)(•)CH2], the OH adduct complexes formed initially, react with ubiquitous O2 followed by NO before their rearrangement. The formation of the prereactive complex plays an important role in reaction mechanism and kinetics. The calculated rate constant, k298K = 1.61 × 10(-11) cm(3) molecule(-1) s(-1), is well harmonized with the previous experimental data, k298K = (2.48 ± 0.61) × 10(-11) cm(3) molecule(-1) s(-1) (Blanco et al.) and k298K = (2.3 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1) (Picquet-Varrult et al.). Additionally, consistent and reliable enthalpies of formation at 298.15 K (ΔfH°298.15) have been computed for all the species involved in the title reaction using the composite CBS-QB3 method. The theoretical results confirm that the major products are formic acetic anhydride, acetic acid, and formaldehyde in the OH-initiated oxidation of VA in the presence of O2 and NO, which are in excellent agreement with the experimental findings.

Optoelectronic and photovoltaic properties of graphene quantum dot–polyaniline nanostructures

In aqueous dispersions of graphene quantum dots (GQDs) produced by a sono-Fenton method, aniline is in situ polymerized to produce different polyaniline (PANI)–GQD hybrids (PAGD) without using external dopant. FTIR studies indicate that the carboxylic acid groups of the GQDs dope PANI well. The UV-Vis spectra exhibit a π to polaron band transition of the PAGD hybrids and show a gradual red shift with increasing intensity for increasing amounts of GQDs due to the gradual uncoiling and increase of polarons in the doped PANI chains. The fluorescence intensity of the GQDs is drastically quenched in the PAGD hybrids suggesting effective charge transfer between the GQDs and PANI chains. The X-ray diffraction study suggests the presence of a lamellar structure with a lamellar thickness of 13.57 A. The morphologies of the PAGD hybrids studied using field emission scanning electron microscopy exhibit a change from flakes to rods with increasing GQD concentration, which has been attributed to the change from a flat to cylindrical lamella formation. The thermogravimetric analysis result indicates that, in comparison to HCl-doped PANI, the PAGD hybrids exhibit better thermal stability. In the PAGD composites the dc conductivity increases by three orders compared to that of the GQDs due to polaron formation in the PANI chains. The current–voltage (I–V) characteristics of the PAGD composites indicate semiconducting behaviour and on irradiation with light an almost reversible photoresponse occurs. Dye-sensitized solar cells (DSSCs) fabricated with the PAGD hybrids and N719 dye indicate a highest power conversion efficiency (PCE) of 3.12%. Impedance data of the PAGD hybrids exhibit semicircular Cole–Cole plots indicating the presence of a resistance (R)–capacitance (C) circuit where the capacitance is in parallel to the bulk resistance which increases with increasing GQD concentration. The Debye plot and the dielectric permittivity values also support the variation of the photovoltaic properties of the PAGD hybrids. The impedance spectra of the DSSCs indicate the presence of three semicircles exhibiting a complex equivalent circuit composed of three R–C circuits, and analysis of the data yields the lifetime values of photo-injected electrons supporting the PCE variation of the PAGD hybrids.

Interplay of Tunneling, Two-State Reactivity, and Bell–Evans–Polanyi Effects in C–H Activation by Nonheme Fe(IV)O Oxidants

The study of C-H bond activation reactions by nonheme Fe(IV)O species with nine hydrocarbons shows that the kinetic isotope effect (KIE) involves strong tunneling and is a signature of the reactive spin states. Theory reproduces the observed spike-like appearance of plots of KIE(exp) against the C-H bond dissociation energy, and its origins are discussed. The experimentally observed Bell-Evans-Polanyi correlations, in the presence of strong tunneling, are reproduced, and the pattern is rationalized.

Colorimetric and ratiometric fluorescent chemodosimeter for selective sensing of fluoride and cyanide ions: tuning selectivity in proton transfer and C–Si bond cleavage

A simple innovative anthraimidazolyldione (LHSi) based colorimetric and ratiometric fluorescent chemodosimeter was designed and synthesized for fluoride and cyanide ion sensing. Upon reaction with the F− and CN− anions in THF solution, probe LHSi shows dramatic color changes from light yellow to red and remarkable ratiometric fluorescence enhancement signals. These properties are mechanistically ascribed to a fluoride/cyanide-triggered deprotonation and C–Si bond cleavage that resulted in a green to red fluorescence.

A BODIPY/pyrene-based chemodosimetric fluorescent chemosensor for selective sensing of hydrazine in the gas and aqueous solution state and its imaging in living cells

A BODIPY-based pyrenebutyrate-linked (BPB) chromogenic and fluorogenic probe was synthesized and characterized for the specific detection of hydrazine. In the presence of hydrazine, BODIPY-based pyrenebutyrate was selectively deprotected, producing switch off meso-phenoxyBODIPY along with a color change from yellow to brown, allowing colorimetric detection of hydrazine by the naked eye. Selectivity experiments proved BPB has excellent selectivity to hydrazine over other environmentally abundant ions and common amine-containing species. Probe BPB was also successfully applied in vapor hydrazine detection into a solid state over other interfering volatile analytes. Furthermore, the probe BPB coated with silica gel TLC plates could act as a visual and fluorimetric probe for hydrazine vapor detection. The probe (BPB) has been shown to detect hydrazine up to 1.87 μM at pH 7.4. DFT and TDDFT calculations were performed in order to demonstrate the sensing mechanism and the electronic properties of the probe and hydrazinolysis product. BPB can also be used for the detection of hydrazine in Vero cells without appreciable interference from other biologically abundant analytes.

Emergence of Function in P450-Proteins: A Combined Quantum Mechanical/Molecular Mechanical and Molecular Dynamics Study of the Reactive Species in the H2O2-Dependent Cytochrome P450SPα and Its Regio- and Enantioselective Hydroxylation of Fatty Acids

This work uses combined quantum mechanical/molecular mechanical and molecular dynamics simulations to investigate the mechanism and selectivity of H2O2-dependent hydroxylation of fatty acids by the P450SPα class of enzymes. H2O2 is found to serve as the surrogate oxidant for generating the principal oxidant, Compound I (Cpd I), in a mechanism that involves homolytic O-O bond cleavage followed by H-abstraction from the Fe-OH moiety. Our results rule out a substrate-assisted heterolytic cleavage of H2O2 en route to Cpd I. We show, however, that substrate binding stabilizes the resultant Fe-H2O2 complex, which is crucial for the formation of Cpd I in the homolytic pathway. A network of hydrogen bonds locks the HO· radical, formed by the O-O homolysis, thus directing it to exclusively abstract the hydrogen atom from Fe-OH, thereby forming Cpd I, while preventing the autoxoidative reaction, with the porphyrin ligand, and the substrate oxidation. The so formed Cpd I subsequently hydroxylates fatty acids at their α-position with S-enantioselectivity. These selectivity patterns are controlled by the active site: substrates binding by Arg241 determines the α-regioselectivity, while the Pro242 residue locks the prochiral α-CH2, thereby leading to hydroxylation of the pro-S C-H bond. Our study of the mutant Pro242Ala sheds light on potential modifications of the enzymes active site in order to modify reaction selectivity. Comparisons of P450SPα to P450BM3 and to P450BSβ reveal that function has evolved in these related metalloenzymes by strategically placing very few residues in the active site.

Nucleophilic Degradation of Fenitrothion Insecticide and Performance of Nucleophiles: A Computational Study

Ab initio and density functional theory (DFT) calculations have been performed to understand the destruction chemistry of an important organophosphorus insecticide O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate, fenitrothion (FN), toward nucleophilic attack. Breaking of the P-OAr linkages through nucleophilic attack is considered to be the major degradation pathway for FN. One simple nucleophile, hydroxide (OH(-)), and two different α-nucleophiles, hydroperoxide (OOH(-)) and hydroxylamine anion (NH(2)O(-)), have been considered for this study. Nucleophilic attack at the two different centers, S(N)2@P and S(N)2@C, has been monitored, and the computed reaction energetics confirms that the S(N)2@P reactions are favorable over the S(N)2@C reactions for all the nucleophiles. All electronic structure calculations for the reaction are performed at DFT-B3LYP/6-31+G(d) level of theory followed by a refinement of energy at ab initio MP2/6-311++G(2d,2p) level. The effect of aqueous polarization on both the S(N)2 reactions is taken into account employing the conductor-like screening model (COSMO) as well as polarization continuum model (PCM) at B3LYP/6-31+G(d) level of theory. Relative performance of the two α-nucleophiles, OOH(-) and NH(2)O(-), at the P center has further been clarified using natural bond orbital (NBO), conceptual DFT, and atoms in molecules (AIM) approaches. The strength of the intermolecular hydrogen bonding in the transition states and topological properties of the electron density distribution for -X-H···S (X = O, N) intermolecular hydrogen bonds are the subject of NBO and AIM analysis, respectively. Our calculated reaction energetics and electronic properties suggest that the relative order of nucleophilicity for the nucleophiles is OOH(-) > NH(2)O(-) > OH(-) for the S(N)2@P, whereas for the S(N)2@C the order, which gets little altered, is NH(2)O(-) > OOH(-) > OH(-).

Isomerization and decomposition of a model nerve agent: a computational analysis of the reaction energetics and kinetics of dimethyl ethylphosphonate.

The gas-phase isomerization and decomposition reactions of dimethyl ethylphosphonate (DMEP) are investigated using the CBS-QB3 method followed by the calculation of rate constant for all reaction pathways using Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Three conformational isomers C1, C2, and C3 are identified for DMEP having stability order C1 (0.00) < C2 (0.11) < C3 (1.87). Each conformer can isomerize via H-transfer reaction and can decompose via CH(3)OH, CH(2), and H(2) elimination at higher temperatures. The conformers C1, C2, and C3 can isomerize to IM1, IM2, and IM3, respectively, via different pathways. Decomposition requires much higher activation energy and therefore higher temperatures than the corresponding isomerization. For instance, the most stable conformer C1 isomerizes to IM1 twice as fast as decomposing to P1 + CH(2) at 1000 K whereas the least stable conformer C3 isomerizes to IM3 10(4) times faster than decomposing to P5 + CH(3)OH. Only one decomposition channel is identified for C1 and two different decomposition channels are identified for C2 as well as for C3. The decompositions of C2 and C3 to P2 + CH(3)OH and P5 + CH(3)OH, respectively, are predicted to be more favorable thermodynamically as well as kinetically over the other decomposition channels within the temperature range 1000-3000 K. For the lack of experimental data, we have calculated the low as well as high-pressure limit rate constants for the decomposition reactions of DMEP. In addition, consistent and reliable enthalpies of formation at 298.15 K (Δ(f)H°(298.15)) have been computed for all the species involved in the isomerization and decomposition reaction of DMEP. The results obtained for DMEP for the reaction mechanism and energetics are compared with that for DMHP and DMMP.