Department of Chemistry

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Validation of Hammett’s Linear Free Energy Relationship Through an Unconventional Approach
(ACS, 2020) Roy, Ram Kinkar
The present study tries to validate Hammett’s linear free energy relationship through an unconventional approach based on the density functional reactivity theory (DFRT). A kinetic energy component [ΔEB(A)], derived from the DFRT-based comprehensive decomposition analysis of stabilization energy scheme, is used to verify the linear nature of Hammett’s log(kX/kH) versus σ plot. The study shows that the versus σ plot (where −X is the atom or group substituted in place of −H) is linear in nature (with reasonably high correlation coefficient values) for different series of reactions. The slopes of the plots also reveal the electrophilic or nucleophilic nature of the transition states as is obtained from the conventional log(kX/kH) versus σ plot. The study thus establishes that the DFRT-based energy component ΔEB(A) (which is very easy to compute) can be used, instead of k-values, obtained either from the experiment or from computationally intensive conventional thermochemistry calculations to generate reliable Hammett’s plot.
Correlation between Equilibrium Constant and Stabilization Energy: A Combined Approach Based on Chemical Thermodynamics, Statistical Thermodynamics, and Density Functional Reactivity Theory
(ACS, 2020-01-21) Roy, Ram Kinkar
In the present work, an attempt is made to establish the correlation between equilibrium constant and stabilization energy [ΔESE(AB)] generated from density functional reactivity theory (DFRT). The reactions chosen here are of type A + B ⇌ AB (i.e., adduct formation type) between an electron acceptor, A, and an electron donor, B. The representative acceptors are methyltrioxorhenium (MTO) and substituted benzaldehydes whereas donors are 26 mono- and bidentate ligands (having N-donors) and semicarbazide. The trends of experimentally generated equilibrium constant (K) values match with those of ΔESE(AB) in most of the cases, both in gas phase as well as in solvent. Justification of this reliable correlation is provided analytically using the expressions of standard Gibbs free energy of reaction (i.e., ΔrGθ) and the stabilization energy expression generated by DFRT. A further analytical explanation (albeit not very rigorous) is provided through statistical thermodynamics showing how equilibrium constant (K) is related to ΔESE(AB) for reactions of the type A + B ⇌ AB, where either A or B is a common species.
Solvent effect on stabilization energy: An approach based on density functional reactivity theory
(Wiley, 2019-03-02) Roy, Ram Kinkar
In the present article a formalism and the corresponding computational method is developed to take care of the variation of stabilization energy with solvent polarity in the process of adduct formation. For this purpose, a simple but physically insightful definition of “net desolvation energy” is proposed keeping in mind the sequence of events taking place in the process of adduct formation in a solvent. The approach used here is based on density functional reactivity theory (DFRT) and the representative samples chosen are adduct formation between (a) methyltrioxorhenium (MTO) and pyridine and (b) (azidomethyl)benzene and methylpropiolate. The generated data in case (a) is correlated with already known experimental parameter that is, formation constant (Kf). The observed trends claim that with the increase in solvent polarity interaction (or stabilization) energy becomes less negative which means that on increasing the solvent polarity the chances of adduct formation are less. This is further supported by calculating hardness values of adducts in different solvents which goes on decreasing with the increase in solvent polarity. Here, the computed data show that on increasing the polarity (i.e., dielectric constant) of the solvent, the “net desolvation energy” increases. Finally, when “net desolvation energy” is added to the stabilization energy obtained from DFRT the predicted trends are achieved.
Exploring the hidden potential of a benzothiazole-based Schiff-base exhibiting AIE and ESIPT and its activity in pH sensing, intracellular imaging and ultrasensitive & selective detection of aluminium
(RSC, 2018) Roy, Ram Kinkar; Chowdhury, Rajdeep; Laskar, Inamur Rahaman
In this article, we tried to redefine the unexplored potential of a benzothiazole type of Schiff-base (OM), which was identified as an AIE-active molecule that exhibits excited-state intramolecular proton transfer (ESIPT). Interestingly, this compound shows ultra-sensitivity and selectivity in the detection of Al(III) (12 pM; 456 ppt). The OM was capable of pH sensing and was also tested for internalization in cancerous cells for intracellular imaging. Computational modeling was performed and the results were in good agreement with the experimental UV-Vis spectrum and the energy gap obtained in basic and acidic media.
The charge transfer limit of a chemical adduct: the role of perturbation on external potential
(RSC, 2017) Roy, Ram Kinkar
Full profiles of the components (positive and negative) of density functional reactivity theory (DFRT) based stabilization energy with respect to the amount of charge transfer (ΔN) are investigated on three different Diels–Alder pairs and twelve different charge transfer complexes formed by BH3–NH3 and their derivatives. One interesting observation is that the stabilization energy is zero when the charge transfer (ΔN) is either zero (lower limit, L.L.) or two times (higher limit, H.L.) the charge transfer at equilibrium (i.e., when chemical potentials are equalized). However, the existence of zero stabilization energy at the zero charge transfer limit is counter-argued after the inclusion of first and second order effects (due to a perturbing external potential of the partner of a given atom-in-a-molecule) in the individual energy components as well as the overall stabilization energy expressions. It has been shown that even when ΔN is zero (the lower limit), the net energy change is negative (i.e., the combined system is stabilized), highlighting the role of non-bonding interactions, rather than charge-transfer, in stabilizing the combined system at the initial stage of adduct formation. The higher limit (H.L.) of charge transfer is also shifted to a much lower value due to the inclusion of this external potential perturbation.
Encapsulation of multi-stimuli AIE active platinum(ii) complex: a facile and dry approach for luminescent mesoporous silica
(RSC, 2016) Roy, Ram Kinkar; Laskar, Inamur Rahaman
Luminescent materials have great potential in diverse applications in their solid state. Because these materials are subject to the aggregation-caused quenching (ACQ) effect, increasing attention is focused on synthesizing aggregation-induced emission (AIE) active materials to avoid the ACQ effect. Herein a new class of AIE active, excimeric platinum(II) complex, [Pt(C^N)(L1)(Cl)], 3 [C^N = 2-phenylpyridine; L1 = N1-tritylethane-1,2-diamine] is reported. The complex 3 exhibited mechanofluorochromism (MFC) and thereby transformed into an orange-emitting complex, 3a, upon grinding. Crushing of 3 (or 3a) with meso-structured silica produced a luminescent composite material, 3b, and thereby the AIE Pt(II) complex moved into the mesopores and the process signaled with a drastic change of emission color (yellow → green). The solid-state luminescent behaviour of these complexes was thoroughly studied. The photophysical properties were also supported by TD-DFT based theoretical study.
An elusive vinyl radical isolated as an appended unit in a five-coordinate Co(iii)–bis(iminobenzosemiquinone) complex formed via ligand-centered C–S bond cleavage
(RSC, 2016) Roy, Ram Kinkar
Redox-active ligand H4Praedt(AP/AP) experienced C–S bond cleavage during complexation reaction with Co(OAc)2·2H2O in the presence of Et3N in CH3OH in air. Thus, formed complex 1 was composed of two iminobenzosemiquinone radicals in its coordination sphere and an unprecedented stable tethered-vinyl radical. The complex has been characterized by mass, X-ray single crystal, X-band EPR, variable-temperature magnetic moment measurements and DFT based computational study.
Negativity of Fukui function of some isolated s and p block elements: The role of orbital relaxation effect
(Elsiever, 2015-09) Roy, Ram Kinkar
In the present article, we emphasize the correlation between orbital relaxation effect and nature of atomic Fukui functions. While doing so, and values are found to be positive and negative, respectively, for the chosen s block elements. Also, and values for chosen p block atoms are negative. Nodal nature of the highest occupied and lowest unoccupied orbitals, electron–electron repulsion and effective nuclear charge become handy in explaining the observed trends. Out of track results obtained for the inert atoms are also explained.
A density functional reactivity theory (DFRT) based approach to understand the effect of symmetry of fullerenes on the kinetic, thermodynamic and structural aspects of carbon NanoBuds
(Elsiever, 2016-06) Roy, Ram Kinkar
In the present study, we have rationalized the effect of variation in the symmetry of relatively smaller fullerene (C32) on the mode of its interaction with semi-conducting Single-Walled Carbon Nanotubes (SWCNTs) in the process of formation of stable hybrid carbon NanoBuds. Thermodynamic and kinetic parameters, along with the charge transfer values associated with the interaction between fullerene and SWCNTs, have been evaluated using an un-conventional and computationally cost–effective method based on density functional reactivity theory (DFRT). In addition to this, conventional DFT based studies are also performed to substantiate the growth of NanoBud structures formed by the interaction between fullerene and SWCNTs. The findings of the present study suggest that the kinetic, thermodynamic and structural aspects of hybrid carbon NanoBuds are significantly influenced by both the symmetry of C32 fullerene and its site of covalent attachment to the SWCNT.
Effect of Geometrical Distortion on the Electronic Structure: Synthesis and Characterization of Monoradical-Coordinated Mononuclear Cu(II) Complexes
(ACS, 2016) Roy, Ram Kinkar
Ligand H3SamiMixed(tBu) was composed of two different compartments, a redox-active 2-aminophenol and a salen salicylidene. Both compartments were linked via a benzyl linker. The ligand reacted with CuCl2·2H2O under air in the presence of Et3N and provided the corresponding monoradical-coordinated mononuclear Cu(II) complex (1). Complex 1, in solution, reacted with air and provided complex 2 via ligand-centered oxygenation at the benzyl-CH2 position. Both complexes were characterized via IR, mass spectrometry, X-ray single-crystal diffraction, variable-temperature magnetic susceptibility, cyclic voltammograms (CVs), and UV–vis/NIR spectroscopic techniques. X-ray crystallographic analyses clearly showed almost equally distorted square planar geometry around the Cu(II) atom in both complexes. However, the bending of the radical-containing C6 ring compared to the N1–Cu1–O1 plane was different in both complexes. While complex 1 was paramagnetic and showed a ferromagnetic coupling between the dx2–y2 magnetic orbital of Cu(II) ion and the pz orbital of coordinated π-radical, complex 2 was diamagnetic by experiencing a strong antiferromagnetic coupling between the two magnetic orbitals. UV–vis/NIR spectra of the complexes were dominated by charge-transfer transitions. CVs of the complexes showed two reversible one-electron oxidations and one reversible one-electron reduction. E1/2ox2 and E1/2red1 potentials were different in both complexes, while E1/2ox1 values were almost the same and the process corresponded to the formation of phenoxyl radical. Theoretical studies were also performed to understand the magnetic coupling phenomena, and TD-DFT calculations were employed for the assignment of charge-transfer absorption bands.
Highly Selective Detection of H+ and OH– with a Single-Emissive Iridium(III) Complex: A Mild Approach to Conversion of Non-AIEE to AIEE Complex
(ACS, 2015) Roy, Ram Kinkar; Laskar, Inamur Rahaman
A greenish-blue emissive bis-cyclometalated iridium(III) complex with octahedral geometry was synthesized in a convenient route where a bulky substituted ligand, N1-tritylethane-1,2-diamine ligand (trityl-based rotating unit) (L1), was coordinated to iridium(III) in nonchelating mode, [Ir(F2ppy)2(L1)(Cl)], [F2ppy = 2-(2′,4′-difluoro)phenylpyridine; L1 = N1-tritylethane-1,2-diamine], 1. The purpose of introducing a rotor in 1 was anticipated to initiate aggregation-induced emission (AIE) activity in it. The presence of a secondary amine in L1 has attributed to 1 the ability to sense acids. The mechanism of this change in 1 under acidic medium was explored. A bright yellow emissive complex was formed on exposing 1 to hydroxide ion, which was isolated, characterized, and identified as a new aggregation-induced enhanced emission (AIEE) active complex. The detection limit of hydroxide ion was determined to 126 nM. Ground- and excited-state properties of 1 were investigated using DFT- and TD-DFT-based calculations, and several important aspects of the experimental facts were validated.
nteraction between Small Gold Clusters and Nucleobases: A Density Functional Reactivity Theory Based Study
(ACS, 2015-07-17) Roy, Ram Kinkar
The thermodynamic and kinetic aspects associated with the interaction of small gold clusters (Aun, where n = 3–6) with nucleobases are assessed using a density functional reactivity theory based comprehensive decomposition analysis of stabilization energy scheme. It is observed that the trend of interaction between Aun clusters and nucleobases follows the order G > A > C > T > U. Also, the Watson–Crick base pair GC interacts with Aun clusters more preferably than that of the AT pair. The observed trend is further supported by conventional binding energy and transition-state calculations at B3PW91 and MP2 levels.
Interaction between Small Gold Clusters and Nucleobases: A Density Functional Reactivity Theory Based Study
(ACS, 2015) Roy, Ram Kinkar
The thermodynamic and kinetic aspects associated with the interaction of small gold clusters (Aun, where n = 3–6) with nucleobases are assessed using a density functional reactivity theory based comprehensive decomposition analysis of stabilization energy scheme. It is observed that the trend of interaction between Aun clusters and nucleobases follows the order G > A > C > T > U. Also, the Watson–Crick base pair GC interacts with Aun clusters more preferably than that of the AT pair. The observed trend is further supported by conventional binding energy and transition-state calculations at B3PW91 and MP2 levels.
Inter-ligand azo (N[double bond, length as m-dash]N) unit formation and stabilization of a Co(ii)-diradical complex via metal-to-ligand dπ–pπ* back donation: synthesis, characterization, and theoretical study
(ACS, 2015) Roy, Ram Kinkar
An azide (–N3) group attached at the -ortho carbon atom to the aniline moiety of 2-anilino-4,6-di-tert-butylphenol formed a diradical-containing Co(II) complex via inter-ligand azo (N[double bond, length as m-dash]N) bond formation. Metal-to-ligand (azo), dπ-to-pπ* back donation stabilized the metal in its lower oxidation state.
On the trends of Fukui potential and hardness potential derivatives in isolated atoms vs. atoms in molecules
(RSC, 2014) Roy, Ram Kinkar
In the present study, trends of electronic contribution to molecular electrostatic potential [Vel([r with combining macron])r=0], Fukui potential [v+f|r=0 and v−f|r=0] and hardness potential derivatives [Δ+h(k) and Δ−h(k)] for isolated atoms as well as atoms in molecules are investigated. The generated numerical values of these three reactivity descriptors in these two electronically different situations are critically analyzed through the relevant formalism. Values of Vel([r with combining macron]) (when r → 0, i.e., on the nucleus) are higher for atoms in molecules than that of isolated atoms. In contrast, higher values of v+f|r=0 and v−f|r=0 are observed for isolated atoms compared to the values for atoms in a molecule. However, no such regular trend is observed for the Δ+h(k) and Δ−h(k) values, which is attributed to the uncertainty in the Fukui function values of atoms in molecules. The sum of Fukui potential and the sum of hardness potential derivatives in molecules are also critically analyzed, which shows the efficacy of orbital relaxation effects in quantifying the values of these parameters. The chemical consequence of the observed trends of these descriptors in interpreting electron delocalization, electronic relaxation and non-negativity of atomic Fukui function indices is also touched upon. Several commonly used molecules containing carbon as well as heteroatoms are chosen to make the investigation more insightful.
A density functional reactivity theory (DFRT) based approach to understand the interaction of cisplatin analogues with protecting agents
(Springer, 2014-09) Roy, Ram Kinkar
In the present study some new insights are put into one of the major concern of cisplatin therapy and that is on the reduction of various cytotoxic and nephrotoxic side-effects of cisplatin analogues in cancer treatment. A better understanding of the interaction between different cisplatin analogues with various protecting agents can be achieved from the descriptors generated by density functional reactivity theory based comprehensive decomposition analysis of stabilization energy (Bagaria et al. in Phys Chem Chem Phys 11:8306–8315, 2009) scheme. Taking into account of three types of interactions i.e., of (1) Cisplatin analogues with DNA bases and base pairs (2) Cisplatin analogues with protecting agents and (3) Protecting agents with DNA bases, it is possible to develop a strategy (albeit qualitative) that suggests the best possible combinations of these drugs with protecting agents which can cause reduction in the toxic side-effects of cisplatin therapy. The sample set comprises of 96 pairs of cisplatin analogues and rescue agents and the generated data confirms the predictive power of the adopted strategy.
A density functional reactivity theory (DFRT) based approach to understand the interaction of cisplatin analogues with protecting agents
(Springer, 2014-09-03) Roy, Ram Kinkar
In the present study some new insights are put into one of the major concern of cisplatin therapy and that is on the reduction of various cytotoxic and nephrotoxic side-effects of cisplatin analogues in cancer treatment. A better understanding of the interaction between different cisplatin analogues with various protecting agents can be achieved from the descriptors generated by density functional reactivity theory based comprehensive decomposition analysis of stabilization energy (Bagaria et al. in Phys Chem Chem Phys 11:8306–8315, 2009) scheme. Taking into account of three types of interactions i.e., of (1) Cisplatin analogues with DNA bases and base pairs (2) Cisplatin analogues with protecting agents and (3) Protecting agents with DNA bases, it is possible to develop a strategy (albeit qualitative) that suggests the best possible combinations of these drugs with protecting agents which can cause reduction in the toxic side-effects of cisplatin therapy. The sample set comprises of 96 pairs of cisplatin analogues and rescue agents and the generated data confirms the predictive power of the adopted strategy.
ONIOM Studies of Chemical Reactions on Carbon Nanotube Tips: Effects of the Lower Theoretical Level and Mutual Orientation of the Reactants
(ACS, 2003-08-06) Roy, Ram Kinkar
We studied theoretically the interaction of simple aliphatic amines with carboxylated zigzag and armchair single-walled carbon nanotube (SWNT) models. We used single-level MM+ molecular mechanics and the AM1 semiempirical method to study noncovalent interactions. To study the model amidation reaction with methylamine, the two-level ONIOM technique was employed in which the higher level was treated with B3LYP/6-31G(d) DFT, and the lower level was described with either universal force field (UFF) molecular mechanics or AM1. In the single-level calculations, the molecular mechanics strongly overestimated van der Waals interactions of amine molecules with the nanotube walls whereas totally ignored hydrogen bond formation between NH2 and COOH groups. On the contrary, AM1 calculations produced unrealistic hydrogen-bonded structures where no attraction was manifested between the hydrophobic fragments. In the ONIOM calculations at the B3LYP/6-31G(d):UFF level of theory, CH3 group of methylamine was strongly attracted to the nanotube, and its N−C bond was directed toward SWNT's center of mass in reaction complex, transition state, and product. Correspondingly, free rotation around C−C(O)O bond was hampered, which resulted in the existence of two series of isomers, depending on where the methylamine moiety is located, inside or outside the nanotube cavity. At the B3LYP/6-31G(d):AM1 level of theory, attraction between the hydrophobic moieties was very weak or absent, and both “inside” and “outside” starting geometries resulted in very similar reaction complexes, with the N−C bond of methylamine turned outward the nanotube. In addition to that, problems were found in the optimizations requiring force-constant calculations (transition states and vibrational frequencies). In all the ONIOM calculations, the formation of amide derivatives on carboxylated armchair SWNT tips was more energetically preferable than that on the zigzag nanotubes. In addition, in some cases of the “inside” zigzag isomers the reaction was endothermic, whereas it was always exothermic for their armchair models. To study theoretically chemical reactions on carbon nanotube tips by ONIOM technique, where the higher level is treated with B3LYP density functional theory, we recommend UFF molecular mechanics versus the AM1 semiempirical method for the lower-level description. To avoid artifacts associated with wall effects inside the nanotube cavity (such as unrealistically long N−H···O separations, which are supposed to be hydrogen bonds in reaction complexes), the use of nanotube diameters close to the commonly observed SWNT diameters is recommended.
Relative Contribution of Combined Kinetic and Exchange Energy Terms vs the Electronic Component of Molecular Electrostatic Potential in Hardness Potential Derivatives
(ACS, 2013) Roy, Ram Kinkar
The relative contribution of the sum of kinetic [(10/9)CFρ(r̅)2/3] and exchange energy [(4/9)CXρ(r̅)1/3] terms to that of the electronic part of the molecular electrostatic potential [Vel(r̅)] in the variants of hardness potential is investigated to assess the proposed definition of Δ+h(k) = −[VelN+1(k) – VelN(k)] and Δ–h(k) = −[VelN(k) – VelN–1(k)] (Saha; et al. J. Comput. Chem. 2013, 34, 662). Some substituted benzenes and polycyclic aromatic hydrocarbons (PAHs) (undergoing electrophilic aromatic substitution), carboxylic acids, and their derivatives are chosen to carry out the theoretical investigation as stated above. Intra- and intermolecular reactivity trends generated by Δ+h(k) and Δ–h(k) are found to be satisfactory and are correlated reasonably well with experimental results.
Study on photophysical properties and prototropic equilibria of trans-2-[4-(N,N-dimethylaminostyryl)]pyridine
(Elsiever, 2014-03) Roy, Ram Kinkar
Photophysical properties of trans-2-[4-(N,N-dimethylaminostyryl)]pyridine, a “push–pull” molecule, have been studied in different pure solvents. trans-2-[4-(N,N-dimethylaminostyryl)]pyridine exhibits fluorescence from a locally excited state in nonpolar solvents. However, in polar solvents, fluorescence occur from an intramolecular charge transfer state with high dipole moment obtained not only through the rapid relaxation of locally excited state upon a single excitation, but also by the excitation of ground intramolecular charge transfer state. The twisting of the donor moiety with respect to the acceptor moiety results in a complete charge transfer giving highly Stokes-shifted fluorescence in polar solvents. The potential energy surfaces along the donor and acceptor twist coordinates for the ground and first two singlet excited states have been evaluated at density functional theory and time dependent density functional theory levels, respectively. Monocation–neutral and dication–monocation equilibria of trans-2-[4-(N,N-dimethylaminostyryl)]pyridine at both ground and excited states have been studied.