Browsing by Author "Roy, Ram Kinkar"

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Acetalization and thioacetalization of cabonyl compounds: A case study based on global and local electrophilicity descriptors
(Wiley, 2006-03-08) Roy, Ram Kinkar
Acetalization of benzaldehyde and substituted benzaldehydes (containing both electron-donating and electron-withdrawing groups) is explained qualitatively on the basis of global electrophilicity descriptor, w, as proposed by Parr and coworkers (11). The generated values of w can explain qualitatively the preferential electrophilic addition, and hence, the yield of acetalization obtained in an earlier experimental study carried by Patel and coworkers (17). The present study also reveals that although both steric and electronic factors affect the yield, only later can be taken care of by w. In the case of a competitive formation of cyclic acetals and cyclic thioacetals from a reaction mixture containing p-hydroxybenzaldehyde, p-nitrobenzaldehyde, 1,2-ethanediol (i.e., glycol), and 1,2-ethanedithiol, the relative experimental yields (18) could be explained from the difference of the global electrophilicity values between aldehydes and acetalizing agents in the same line of arguments of Maynard et al.
Aggregation-induced emission mechanism of styrene derivative: a theoretical study
(RSC, 2025-05) Roy, Ram Kinkar
The aggregation-induced emission (AIE) mechanism of the fluorescent styrene derivative 4-dimethylamino-2-benzylidene malonic acid dimethyl ester (BIM) in methanol solution is theoretically investigated using spin–flip long-range corrected time-dependent density functional theory (SF-LC-TDDFT). The potential energy surfaces (PESs) for the ground (S0) and first singlet excited (S1) states of BIM were calculated along the rotation of the aryl main axis (α angle rotation), consistent with experimental observations. For the monomer, our findings reveal a significant reduction in oscillator strength, approaching zero at the optimized geometry in the S1 state. As this state corresponds to a charge transfer state, it suggests that the BIM monomer operates as a twisted intramolecular charge transfer (TICT) system, undergoing quenching through α angle rotation. The restriction of TICT, and consequently the inhibition of fluorescence quenching in the aggregate state, is also investigated by extracting the coordinates of 13 monomers from the crystal structure of BIM. The α-torsional angle of the central monomer was manually rotated in both clockwise and anti-clockwise directions to assess the intramolecular restrictions within the constrained environment. This analysis reveals that even a 10° rotation of the α-torsional angle, in either direction, causes the atoms of the central monomer to come into close contact with the atoms of the neighboring monomers. These short contacts effectively inhibit the TICT process, thereby leading to aggregation-induced emission.
Are the Hirshfeld and Mulliken population analysis schemes consistent with chemical intuition?
(Wiley, 2008-11-25) Roy, Ram Kinkar
In the present article, we report a comparative study between the Hirshfeld and Mulliken population analysis schemes (abbreviated as HPA and MPA, respectively). Trends of atomic charges derived from these two population analysis schemes are compared with those expected from other commonly used chemical concepts like electronegativity, inductive effects, and resonance effects. Although previous studies on intramolecular reactivity sequences demonstrated that HPA generates reliable and non-negative (and thus physically more realistic) condensed Fukui function (FF) values, the present study reveals problems with the HPA charge partitioning technique. Specifically, HPA fails to reproduce reliable intermolecular and intramolecular charge trends in several systems. Reasons for the success and failure of HPA are discussed and a method for improving the Hirshfeld charge partitioning is proposed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Are the Local Electrophilicity Descriptors Reliable Indicators of Global Electrophilicity Trends?
(ACS, 2005) Roy, Ram Kinkar
Density functional theory based global and local electrophilicity descriptors are used to study the reliability of local electrophilicity values of the strongest electrophilic sites in generating global intermolecular electrophilicity trends. The evaluated values on 15 different organic chlorides show that, for systems having more than one comparatively strong electrophilic site, the local electrophilicity value of the strongest site does not produce a reliable global intermolecular electrophilicity trend. But for systems having one distinctly strong electrophilic site it does. The analytical explanation in favor of the above observation is also provided. Thus, what was argued in an earlier study (Roy, R. K. J. Phys. Chem. 2004, 108, 4934) is established strongly by numerical demonstrations as well as analytical reasoning in the present one.
Basicity of the framework oxygen atom of alkali and alkaline earth-exchanged zeolites: a hard–soft acid–base approach
(Elsiever, 2000-12) Roy, Ram Kinkar
The basicity of framework oxygen atoms of alkali and alkaline earth-exchanged zeolites has been studied using reactivity descriptors based on a local hard–soft acid–base (HSAB) concept. We have calculated the `local softness' and the `relative nucleophilicity' values of the framework oxygen atoms of zeolite clusters as the measure of basicity. The local softness and relative nucleophilicity appear to be more reliable descriptors to predict the experimental basicity trend, compared to the negative charge on the oxygen atom.
Bimolecular quenching rate of electron transfer reactions from aromatic amines to coumarin dyes: a conceptual density functional theory-based approach
(ACS, 2025-07) Roy, Ram Kinkar
In the current work, a qualitative trend of the quenching rate of a series of bimolecular electron transfer reactions is computed through an alternative and cost-effective approach. The electron transfer reactions from eight aromatic amines in their ground state to a series of six substituted coumarin dyes in their singlet excited states (S1) are chosen as representative quenching processes. The acetonitrile solvent is used as the reaction medium. While the reaction free energy values are evaluated through conceptual density functional theory (CDFT) [or CDASE-scheme, to be more specific]─based stabilization energy, the reorganization energy values are calculated using a conventional method already prescribed in the literature. The reactions, being diffusion-controlled ones, only outer-sphere reorganization energy (or, solvent reorganization energy) is considered, neglecting the intramolecular reorganization energy (as suggested by the corresponding experimental study available in the literature). The generated data demonstrate that as the absolute values of the CDFT-based stabilization energy become closer to the corresponding reorganization energy, the experimental quenching rate constant values of the chosen bimolecular electron transfer reactions increase. This observation exactly correlates with the Marcus theory of electron transfer reaction. Thus, the authors would like to claim that the Marcus theory is validated through an unconventional approach based on conceptual density functional theory (CDFT).
CDASE - A reliable scheme to explain the reactivity sequence between Diels–Alder pairs
(RSC, 2010) Roy, Ram Kinkar
The reliability of the Comprehensive Decomposition Analysis of Stabilization Energy (CDASE) scheme, proposed recently (Phys. Chem. Chem. Phys., 2009, 11, 8306), has been demonstrated in the present study. Reactivity sequence among more than 100 pairs, taking part in Diels–Alder (DA) reaction, is successfully generated by this scheme. The diene series consisted mainly of cis-1,3-butadiene and different substituted butadienes whereas dienophiles are mainly ethylene and its different substitutions. Both the positive energy component (i.e., the energy parameter defined as ‘internal assistance’) and the negative energy component could generate the expected reactivity trend among the chosen DA pairs, which is also supported by the global electrophilicities of dienes and dienophiles and the corresponding charge-transfer values (ΔN). The numerical values of these components are capable of predicting even the ‘normal electron demand’ (NED) and ‘inverse electron demand’ (IED) nature of the corresponding DA reaction. The method is also capable of reproducing the lower reactivity of acetylene as dienophile when compared to that of ethylene. The reason for the success of CDASE-scheme in explaining intermolecular reactivity sequence is also analysed.
Change of Hardness and Chemical Potential in Chemical Binding: A Quantitative Model
(ACS, 1994) Roy, Ram Kinkar
In this paper we relate the change of chemical hardness and potential with the binding energies as the chemical binding takes place. We use a simple model to demonstrate that the hardness change is not only related to the binding energy of the molecule being formed, but also to the binding energies of the corresponding molecular cations and anions. Similar expressions are also derived for chemical potential. We have shown that our analysis can explain the existing data. The separated limit values of hardness and chemical potential are also derived. Two demonstrative calculations on the molecules H2 and HF are presented. The future potential of this model is discussed.chemical potential
Change of Hardness and Chemical Potential in Chemical Binding: A Quantitative Model
(ACS, 1994) Roy, Ram Kinkar
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.
Chemical Potential and Hardness for Open Shell Radicals: Model for the Corresponding Anions
(ACS, 1995) Roy, Ram Kinkar
In this article, we have obtained theoretical values of chemical potential and hardness for open shell free radicals using a wave function approach. The calculated values of the hardness are useful particularly for rank ordering of the corresponding anions. A ΔSCF procedure is used to obtain these values, and the results are compared with corresponding experimental values. The procedure also leads us to the reliability of the ΔSCF procedure for these quantities.
Chemoselectivities in Acetalization, Thioacetalization, Oxathioacetalization and Azathioacetalization
(ACS, 2006) Roy, Ram Kinkar
In the present article (experimental as well theoretical) the relative yields of cyclic (O,O), (S,S), (S,O), and (S,N) acetals, formed from p-(NO2)C6H4CHO and p-(OH)C6H4CHO, are compared. Atomic charges, global electrophilicity descriptor (w) [as proposed by Parr et al., J. Am. Chem. Soc.1999, 121, 1922] and hard−soft acid−base concept of Pearson (J. Am. Chem. Soc.1963, 85, 3533) are used to explain the experimental observations. Although the w values can explain the yields, charge and local softness values of the interacting sites explain the plausible reaction mechanism. The bisnucleophiles chosen for acetalization are CH2(OH)−CH2(OH) (glycol), CH2(SH)−CH2(SH) (dithiol), CH2(OH)−CH2(SH) (oxathiol) and CH2(SH)−CH2(NH2) (azathiol). For p-(NO2)C6H4CHO, the experimental yield of cyclic acetals were found to follow the trend as (S,N) > (S,O) > (O,O) > (S,S), which is also supported by theoretical explanation based on the w values and applying the concept of hard−hard (i.e., charge-controlled) and soft−soft (i.e., orbital-controlled) interaction between the interacting sites of the substrates (i.e., aldehydes) and the reactants (bisnucleophiles). Similarly, for p-(OH)C6H4CHO the relative yields of cyclic acetals follow the trend (S,N) ≈ (S,S) > (S,O) > (O,O). It is argued that the attack on CCHO (i.e., C-atom of the CHO group) in p-(NO2)C6H4CHO by OOH (i.e., O-atom of OH group) or NNH2 (i.e., N-atom of NH2 group) is mainly charge-controlled but the attack on CCHO in p-(OH)C6H4CHO) by SSH (i.e., S-atom of SH group) is orbital-controlled.
Chiral Interaction in Enantiomeric and Racemic Dipalmitoyl Phosphatidylcholine Langmuir Monolayer
(ISSST, 2002) Roy, Ram Kinkar
An Effective Pair Potential (EPP) theory is used to study relation between the molecular chiral structure of a phospholipid (Dipalmitoyl phosphatidyl choline; DPPC) and the shape of the mesoscopic domain composed of DPPC. It is experimentally known that the monolayers of DPPC in the condensed phase exhibit the formation of triskellion shaped domains. The curvature of the arms of the triskellions has specific handedness depending on the chirality of the constituent molecule. The study uses an equivalent description of the groups attached to the chiral center and calculates the mutual orientation of a pair of molecules (enantiomeric and racemic pairs). The pair potential is calculated using Lennard Jones potential. Also, the electrial interaction is computed using semi-empirical approach. Based on the minimum energy arrangement, the handedness of the aggregates and their pair potential are predicted. Theoretically obtained results are compared with the experimental results and also with the previously obtained coarse-grained description of the molecule. The study uses no adjustable parameter and uses the microscopic information obtained from X-ray diffraction data. It shows that the equivalent sphere description of the molecule can fairly describe the mesoscopic features of the domains such as the handedness based on the pair potential calculation.
Components of density functional reactivity theory-based stabilization energy: descriptors for thermodynamic and kinetic reactivity
(Elsevier, 2023) Roy, Ram Kinkar
Density Functional Reactivity Theory (DFRT) based global (i.e., for the whole chemical system) descriptors are used to explain various types of chemical interactions in this chapter. Some formal developments as well as applications of DFRT based CDASE (Comprehensive Decomposition of Stabilization Energy) scheme e.g., (i) Full profiles of the kinetic and thermodynamic energy components of the DFRT based stabilization energy and the role of perturbation on external potential; (ii) Effect of solvent on stabilization energy; (iii) Correlation of equilibrium constant with stabilization energy; (iv) Validation of Hammett's Linear Free Energy Relationship (LFER) through DFRT are discussed in details to gain some insight in chemical reactivity.
A comprehensive decomposition analysis of stabilization energy (CDASE) and its application in locating the rate-determining step of multi-step reactions
(RSC, 2009) Roy, Ram Kinkar
Stabilization energy, as proposed by Parr and Pearson (J. Am. Chem. Soc., 1983, 105, 7512) is decomposed into fragments. When the donor is not a perfect one and both the donor and the acceptor are ordinary organic molecules this decomposition is shown to provide energy fragments which, individually, can be correlated to the reaction rate of that particular step. It is shown how these different energy fragments can be used, together with the global electrophilicity value of the acceptor (wA), to locate the rate-determining step in multi-step reactions.
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.
Correlation of Polarizability, Hardness, and Electronegativity: Polyatomic Molecules
(ACS, 1994) Roy, Ram Kinkar
In this paper, for the first time we have correlated the hardness (η), softness (s), and polarizability (P) with the electronegativity and the distortion (ΔR) of a single bond in a complex polyatomic system. The study also shows some interesting: trends as different types of bonds are distorted. We have suggested a new concept, "local bond hardness", in a complex polyatomic system. The molecules H2O, CH4, CH3Cl, and CH3F are chosen as primary test cases.
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.
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.
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.