Ram Kinkar Roy

Variational principles for describing chemical reactions: Condensed reactivity indices

Two recent papers [P. W. Ayers and R. G. Parr, J. Am. Chem. Soc. 122, 2010 (2000); 123, 2007 (2001)] have shown how variational principles for the energy may be used to derive and elucidate the significance of the chemical reactivity indices of density-functional theory. Here, similar ideas are applied, yielding a systematic, mathematically rigorous, and physically sound approach to condensed reactivity indices. First, we use the variational principle for the energy to derive an expression for the condensed Fukui function index in terms of the condensed hardness kernel. Next, we address an important open problem pertaining to condensed reactivity indices: when (if ever) is the condensed Fukui function for an atom in a molecule negative? In particular, our analysis confirms the observation, hitherto based only on computational evidence, that the Hirshfeld partitioning is optimal for obtaining non-negative Fukui functions. We also hypothesize that the strong diagonal dominance of the condensed hardness kern...

On non-negativity of Fukui function indices

In this paper we have analyzed the factors which cause Fukui function (FF) indices to be negative, when evaluated in condensed form through crude finite difference approximation. Inability to take care of the relaxation effect and improper charge partitioning techniques have been cited to be the probable reasons. For the first time, we have shown that the “stockholders” charge partitioning technique (i.e., Hirshfeld’s analysis) produces non-negative FF values which, when evaluated through other kinds of charge partitioning techniques, become negative in some cases. Advantages of “stockholders” charge partitioning over other kinds of partitioning techniques are also discussed, particularly in case of evaluation of condensed FF.

Mulliken population analysis based evaluation of condensed Fukui function indices using fractional molecular charge

A thorough study on the nature of Mulliken population analysis (MPA) based condensed Fukui function (FF) indices has been performed. It is claimed analytically that nothing can be predicted about the sign of condensed FF indices, even when evaluated by using very small fractional molecular charge (i.e., approximately following the analytical definition of FF indices). The corresponding numerical demonstrations on different chemical systems confirm our claim.

Studies of regioselectivity of large molecular systems using DFT based reactivity descriptors

This report describes the recent works on Conceptual Density Functional Theory (DFT) based reactivity descriptors used to predict the regioselectivity of large systems, biomolecular systems, in particular. The challenges of bio-systems, the large number of atoms and high structural flexibility, made the way to a routine application of DFT more laborious. To cope with extended systems, fragmentation based method is developed recently (given the name ‘One-into-Many’ model) for a reliable determination of the regioselectivity of biomolecular systems. Thus, our main motivation to embark on the endeavor of this report is to provide a brief introduction of Conceptual DFT and fragmentation approaches based on these reactivity descriptors for predicting the regioselectivity of large biomolecular systems.

Acetalization and thioacetalization of cabonyl compounds: A case study based on global and local electrophilicity descriptors

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 ( J Am Chem Soc 1999, 121, 1922 ). 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 ( J Org Chem 2002, 67, 5842 ). 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 ( Org Biomol Chem 2004, 2, 1670 ) 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. ( Proc Natl Acad Sci USA 1998, 95, 11578 ).

Understanding the preferential binding interaction of aqua-cisplatins with nucleobase guanine over adenine: a density functional reactivity theory based approach

The binding interaction of hydrolyzed cisplatin with the nucleobases guanine and adenine has been investigated using density functional reactivity theory (DFRT). The energy changes involved in the interaction of both mono-aqua and di-aqua cisplatins with these two purine bases are explored on the basis of different reactivity parameters of DFRT. As observed by earlier experimental and theoretical studies, the present approach also reveals that interaction of the cisplatin di-aqua complex is stronger than the corresponding mono-aqua complex and, more importantly, interactions of both mono and di-aqua complexes of cisplatin are stronger with guanine than with adenine. Interestingly, all these observations are based on the energy components and charge transfer quantities which depend solely on the electronic properties of the isolated aqua-cisplatins and nucleobases (i.e., not on the electronic properties of the adducts). Moreover, the sign of the energy components and charge transfer values clearly demonstrate the electron donor and acceptor nature of the purine bases and aqua cisplatins, respectively. The results are also consistent across all three methods (both ab initio and DFT) adopted in this study.

Hardness potential derivatives and their relation to Fukui indices

A simple as well as easy to compute formalism of hardness potential (originally defined by Parr and Gazquez, J. Phys. Chem., 1993, 97, 3939) is presented. Use of hardness potential formally resolves the N‐dependence problem of local hardness. However, the hardness potential cannot describe the intra as well as intermolecular reactivity sequence satisfactorily of some chemical systems. The corresponding electrophilic [Δ+h(k)] and nucleophilic [Δ−h(k)] variants of the hardness potential are also developed, which measure the reactivity toward a nucleophilic (i.e., Nu−) and an electrophilic (i.e., El+) reagent, respectively. Interestingly, these two variants of the hardness potential lead to the right and left derivatives of Fukui potential. The proposed reactivity descriptors correctly predict the expected reactivity trends in the chosen systems. It has also been illustrated that the values of the variants of hardness potential (or Fukui potential) at the atomic nucleus have the ability to explain the intramolecular reactivity of biologically active indole derivatives. The future scope of applications as well as limitations of the proposed descriptors is also highlighted.

Effect of Geometrical Distortion on the Electronic Structure: Synthesis and Characterization of Monoradical-Coordinated Mononuclear Cu(II) Complexes

Ligand H3Sami(Mixed(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 d(x(2)-y(2)) magnetic orbital of Cu(II) ion and the p(z) 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. E(1/2)(ox2) and E(1/2)(red1) potentials were different in both complexes, while E(1/2)(ox1) 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.

Hardness as a function of polarizability in a reaction profile

Abstract In this article, for the first time, we have correlated chemical hardness with polarizability when a single bond in a complex polyatomic molecule is distorted. A predominantly linear relation has been observed between the cube root of polarizability and hardness when various types of bonds are distorted. The molecules CH 3 Cl, CH 3 F and CH 4 are chosen as typical example systems.

Relative Contribution of Combined Kinetic and Exchange Energy Terms vs the Electronic Component of Molecular Electrostatic Potential in Hardness Potential Derivatives

The relative contribution of the sum of kinetic [(10/9)CFρ(r)2/3] and exchange energy [(4/9)CXρ(r)1/3] terms to that of the electronic part of the molecular electrostatic potential [Vel(r)] 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.