Screening the Band Shape of Molecules by Optimal Tuning of Range-Separated Hybrid Functional with TD-DFT: A Molecular Designing Approach.

In the present article we have demonstrated the effectiveness of optimally tuned range-separated hybrid (RSH) functional to determine the electronic transitions from two fluorophore moieties (blue and yellow/orange) within a single white light emitter (WLE). First, the optimally tuned range separation parameter (ω) is calculated for two white emitting fluorophores (W1 and W2) already reported in the literature. The success of the optimally tuned RSH functional ω*B97XD, used in the TDDFT study, encouraged the authors to design eight new single organic white light emitters with frustrated energy transfer between the two individual fluorophore moieties (blue and yellow/orange). The simulated spectra (the band shapes, to be more specific) generated by TDDFT study and outcomes through natural transition orbital (NTO) and natural bond orbital (NBO) studies clearly demonstrate that all the designed eight organic molecules are potential white light emitters and can be synthesized in future.

Validation of Hammett's Linear Free Energy Relationship Through an Unconventional Approach.

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 [Formula: see text] 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.

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.

The charge transfer limit of a chemical adduct: the role of perturbation on external potential.

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.

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.

Redox-active ligand H4Pra(edt(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.

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.

Inter-ligand azo (N=N) unit formation and stabilization of a Co(II)-diradical complex via metal-to-ligand dπ-pπ* back donation: synthesis, characterization, and theoretical study.

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=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.

In the present study, trends of electronic contribution to molecular electrostatic potential [Vel(r¯)(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¯) (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(+)|(r=0) and v(-)|(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.

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.

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 Δ­h(k) = −[VelN(k) ­ VelN­1(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 Δ­h(k) are found to be satisfactory and are correlated reasonably well with experimental results.

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.

CDASE--a reliable scheme to explain the reactivity sequence between Diels-Alder pairs.

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 (DeltaN). 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.

A comprehensive decomposition analysis of stabilization energy (CDASE) and its application in locating the rate-determining step of multi-step reactions.

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 (w(A)), to locate the rate-determining step in multi-step reactions.

N-dependence problem of local hardness parameter.

The broader applicability of local hardness, [eta(r)] as a reliable intermolecular reactivity descriptor primarily depends on the removal of its 1/N (N, total number of electrons of the system) dependence. In this article an attempt is made to illustrate the problem of evaluating eta(r) by using rho(r) (the electron density) or Nf(r) (Fukui function multiplied by N) as composite functions and a scheme is proposed to evaluate the local hardness values.

Correlation of global electrophilicity with the activation energy in single-step concerted reactions.

The experimental energy of activation (Ea) of the single-step concerted oxidation process of aliphatic primary alcohols by quinolinium bromochromate (QBC) are correlated with the theoretically evaluated global electrophilicity values (w) [as proposed by Parr et al. (J. Am. Chem. Soc. 1999, 121, 1922)]. Conceptual justification in favor of correlating w of the substrate with Ea involved in a single-step concerted reaction is also discussed. The evaluated w values at HF/cc-pVTZ and MP2/6-31G(d,p) methods are found to be as expected (when we consider structural aspects), although there are some inconsistencies in other methods [e.g., HF/6-31G(d,p), B3LYP/cc-pVTZ, BLYP/dnp, PW91/dnp, PWC/dnp, VWN/dnp]. The reasons for the inconsistencies, even with a superior B3LYP/cc-pVTZ method, are discussed thoroughly. It is observed that the higher the value of w, the more the value of Ea involved in the process of oxidation of primary alcohols by QBC. The present study also reveals that the apparent success of insignificant (i.e., much smaller) local electrophilicity values (S+O(OH)), evaluated using Hirshfeld population analysis (HPA), in explaining observed trend of experimental Ea values turns out to be ambiguous when more significant (i.e., much larger) local nucleophilicity values (S-O(OH)) are also compared. This is evident from the corresponding correlation coefficient values.

"One-into-many" model: an approach on DFT based reactivity descriptor to predict the regioselectivity of large systems.

The present work consists of the development of a new model (named "one-into-many") to predict the regioselectivity of large chemical and biological systems. Large chemical and biological systems with multiple reactive sites are proposed to be broken into small fragments having at least one reactive site in each fragment. The environment around each reactive site is mimicked by incorporating a buffer zone. Local reactivity descriptor (i.e., local hardness), originally proposed by Berkowitz et al. (J. Am.Chem. Soc. 1985, 107, 6811) and later implemented by Langenaeker et al. (J. Phys. Chem. 1995, 99, 6424), is evaluated for each reactive site adopting a new modified approach (i.e., without neglecting kinetic energy and exchange energy parts). When the model is applied to predict the regioselectivity (toward an electrophilic attack) of the base pairs in DNA (PDB ID: 1BNA) (Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 2179) the generated results are found to be satisfactory in most cases.

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. 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. 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 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.

Chemoselectivities in acetalization, thioacetalization, oxathioacetalization and azathioacetalization.

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) approximately (S,S) > (S,O) > (O,O). It is argued that the attack on C(CHO) (i.e., C-atom of the CHO group) in p-(NO2)C6H4CHO by O(OH) (i.e., O-atom of OH group) or N(NH2) (i.e., N-atom of NH2 group) is mainly charge-controlled but the attack on C(CHO) in p-(OH)C6H4CHO) by S(SH) (i.e., S-atom of SH group) is orbital-controlled.

Are the local electrophilicity descriptors reliable indicators of global electrophilicity trends?

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.