Rahul Kar

A THEORETICAL STUDY ON THE ANTIOXIDANT PROPERTY OF GALLIC ACID AND ITS DERIVATIVES

We analyze the antioxidant property of four phenolic compounds i.e., gallic acid, methyl gallate, ethyl gallate, and 4, 5-dihydroxy 3-methoxy benzoic acid through the determination of bond dissociation enthalpy (BDE), vertical ionization potential (IPv) in gas phase as well as in six different solvent medium. The theoretical trends of these phenolic compounds, based on BDE and IP, is compared with the scavenging activity towards HOCl. In addition, we compute and analyze the values of the density-based reactivity descriptors such as chemical potential, hardness, electrophilicity, and local softness.

Experimental and theoretical study of urea and thiourea based new colorimetric chemosensor for fluoride and acetate ions.

Two new anion receptors 1,1-(4-nitro-1,2-phenylene) bis(3-phenylurea) (1) and 1,1-(4-nitro-1,2-phenylene) bis(3-phenylthiourea) (2) have been reported here. The binding and colorimetric sensing properties of receptors 1 and 2 with different anions were investigated by naked-eye, (1)H-NMR and UV-Vis spectroscopy. They showed effective and selective binding with two biologically important anions F(-) and CH3COO(-), in presence of other anions, such as Cl(-), Br(-), I(-), NO2(-), ClO4(-), HSO4(-), H2PO4(-), N3(-), CN(-) in acetonitrile. The relative binding mode of fluoride and acetate anions towards receptors 1 and 2 were studied using density functional theory (DFT), in gas phase and in acetonitrile solvent. Computational studies revealed that receptor 1 formed complexes by two intermolecular hydrogen bonds while receptor 2 by three intermolecular hydrogen bonds. In addition, time dependent DFT (TD-DFT) calculations qualitatively match the experimental UV-Vis spectra.

Structural, electronic and reactivity studies on group 15 analogues of N-heterocyclic carbene

The present article reports density functional studies on the Group 15 analogues of N-heterocyclic carbene (NHC) on their structure, reactivity, stability and ligating properties. Long-range corrected density functionals have been used due to its recent success in predicting orbital energies. These ligands are found to have greater π-accepting ability than NHC. Electron-donating substituents have a dramatic effect on their stability as well as ligating properties. Furthermore, natural resonance theory (NRT) calculations have been performed to determine the percentage weighting of resonance contributing structures. In addition, density-based global reactivity descriptors such as chemical potential, hardness, electrophilicity index and softness are calculated using four different density functional methods, and compared with CCSD(T) results. Moreover, the density-based local reactivity descriptors are employed to study the reactivity of Group 15 analogues. From the plot of dual descriptors, it is found that the “ene” centre of the Group 15 analogues of NHC is pseudodual.

Assessment of range-separated functionals in the presence of implicit solvent: Computation of oxidation energy, reduction energy, and orbital energy

Recently, we have investigated the ionization potential (IP) theorem for some small molecules in the presence of external electric field [M. P. Borpuzari et al., J. Chem. Phys. 144, 164113 (2016)]. In this article, we assess the performance of some density functionals, local density approximation, generalized-gradient approximation (GGA), hybrid, meta-GGA hybrid, and range-separated functionals in the presence of two different solvent dielectrics, water and cyclohexane, in reproducing the vertical oxidation energy, reduction energy, and the frontier orbital energies. We also study the accessibility of different computational solvent models like the polarizable continuum model (PCM) and non-equilibrium PCM (NEPCM) in reproducing the desired properties. In general, the range-separated functionals do not perform well in reproducing orbital energies in the PCM. Range separation with the NEPCM is better. It is found that CAM-B3LYP, M06-2X, and ωB97XD functionals reproduce highest occupied molecular orbital energy in solvents, which may be due to the cancellation of PCM and density functional theory errors. Finally, we have tested the validity of the IP theorem in the solvent environment.

Antioxidant properties can be tuned in the presence of an external electric field: accurate computation of O–H BDE with range-separated density functionals

In this article, we have studied the effect of the external electric field on the antioxidant properties of gallic and caffeic acids. The variation of the O–H bond dissociation enthalpy (BDE), adiabatic ionization energy, proton dissociation enthalpy, proton affinity and electron transfer enthalpy, in the presence of an external electric field is studied. It is found that the antioxidant properties can be tuned by the application of an external electric field. In addition, the O–H bond strength, qualitatively measured from the O–H bond length, is found to depend on the external electric field. Furthermore, the performance of seven range-separated density functionals in the presence of the external electric field is reported for the first time. On comparing with the composite model CBS-QB3, it is found that the LC-BLYP functional reproduces the O–H BDE with high accuracy.

Long-range corrected density functionals combined with local response dispersion: A promising method for weak interactions.

Density functional theory, in general, is considered to underestimate the weak van der Waals type of intermolecular interactions. We optimized parameters of the local response dispersion (LRD) method applied to the long‐range corrected exchange‐correlation functionals (LC‐BOP12+LRD and LCgau‐BOP+LRD) on the interaction energy for the complexes in the recently compiled S66 database and found to be comparable with the high‐level wave function‐based methods reported in Řezáč et al. (J. Chem. Theory Comput. 2011, 7, 2427). Our calculations with the S66 intermolecular complexes at equilibrium geometries suggests that the LC‐BOP12+LRD and LCgau‐BOP+LRD are well‐balanced and lower cost alternatives to the methods reported in the database. Further, test on the S66X8 database (with eight nonequilibrium points) and the HBC6 and NBC10 database shows LC+LRD method with newly optimized parameters is a promising candidate for dealing such weak interactions. Finally, the new parameterized LC+LRD method was tested on X40 benchmark halogenated complexes.Copyright

Ionisation potential theorem in the presence of the electric field: Assessment of range-separated functional in the reproduction of orbital and excitation energies

Recently, the range-separated density functionals have been reported to reproduce gas phase orbital and excitation energies with good accuracy. In this article, we have revisited the ionisation potential theorem in the presence of external electric field. Numerical results on six linear molecules are presented and the performance of the range-separated density functionals in reproducing highest occupied molecular orbital (HOMO) energies, LUMO energies, HOMO-LUMO gaps in the presence of the external electric field is assessed. In addition, valence and Rydberg excitation energies in the presence of the external electric field are presented. It is found that the range-separated density functionals reproduce orbital and excitation energies accurately in the presence of the electric field. Moreover, we have performed fractional occupation calculation using cubic spline equation and tried to explain the performance of the functional.

Molecules relevant for organic photovoltaics: a range-separated density functional study

Accurate determination of both fundamental and optical gap is necessary for designing molecules relevant for organic photovoltaics. Here, we study how range-separated density functionals reproduce frontier orbital energies, HOMO (highest occupied molecular orbital)–LUMO (lowest unoccupied molecular orbital) gaps, and optical gaps for molecules relevant for organic photovoltaics. In this study, we consider 12 different range-separated density functional for computing HOMO energy, HOMO–LUMO gap, and optical gap which are compared with available experimental and reported GW values. We found that the reproduction of desired photovoltaic properties primarily depend on range separation parameter. Moreover, the tested functionals are comparable with OT-BNL functional.

Microhydration of 2-Naphthol at Ground, First Excited Triplet, and First Excited Singlet States: A Case Study on Photo Acids

Molecular interactions of 2-naphthol (nap) with water molecules are studied at the ground, first excited triplet and first excited singlet states, applying DFT and TD-DFT methods. The minimum energy structure of hydrated clusters of 2-naphthol up to four water molecules are selected from several possible input geometries. It is observed that the minimum energy conformer of the tetra-hydrate of 2-naphthol has proton transfer occurring from nap to solvent water molecules, in its first excited singlet state. This is however not observed in case of its ground or first excited triplet state. It is consistent with the fact that the pKa of nap in the first excited singlet state is very much lower compared to the ground and first excited triplet state. This is also reflected in the O-H potential energy profile of tetrahydrate of nap, obtained by performing a rigid potential energy scan of the dissociating O-H bond of nap at ground, first excited triplet and first excited singlet states. Frequency of O-H stretching vibration of 2-napthol and its hydrated clusters in the ground (S0) as well as in the first excited singlet (S1) state are calculated and compared with the available experimental data. The performance of macroscopic solvation model is also examined in the ground and these excited states.

How does the presence of an oxyradical influence the behavior of polyphenolic antioxidant? A case study on gallic acid

AbstractPolyphenolic antioxidants are an integral component of the present-day lifestyle due to growing environmental and pollution-related issues. Gallic acid (GA), a popular polyphenolic antioxidant, is widely distributed in different fruits and vegetables. It scavenges free radicals mostly through the hydrogen atom transfer (HAT) mechanism. The present work reports a fundamental understanding of the atom transfer involved in the antioxidant activity of GA in the presence of prototype oxyradicals. In this article, the behavior of GA in the presence of three prototype oxyradicals, based on the availability in biological systems, viz. hydroxyl (•OH), hydroperoxyl (•OOH), and methyl peroxyl (•OOMe) is studied. On radical scavenging, a sharp change in the structural, electronic, and thermodynamic properties of GA is observed. Finally, a new descriptor based on chemical hardness, which describes the stability of the antioxidant molecule during a HAT reaction, is also proposed. Graphical abstractThe present work reports a fundamental understanding of the atom transfer involved in antioxidant activity of gallic acid in presence of prototype oxyradicals