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Author: search.filters.author.Roy, Ram KinkarSubject: search.filters.subject.Reactivity

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    A Simple Model to Predict Preferable Aldol Products from Unsymmetrical Ketones Using Local Hard−Soft Acid−Base Concept
    (ACS, 2001) Roy, Ram Kinkar
    Unsymmetrical ketones produce two types of enolate intermediates upon deprotonation, which are known as “less substituted” and “more substituted” ones. The major final aldol product will depend on which one of these two enolate intermediates is more reactive toward the incoming reactant (i.e., aldehyde in the case of aldol condensation reaction) in the next stage of the reaction. As here the active sites belong to two different chemical systems, “local hardness” values are reported to be more reliable (Langenaeker et al. J. Phys. Chem. 1995, 99, 6424, and Roy et al. J. Phys. Chem. 1998, 102, 3746) to predict the intermolecular reactivity order of these two intermediates. But in this article we have proposed a very simple model which simultaneously represents both kinds of enolate intermediates. So the present study can be considered as a first one in which an intermolecular problem has been recasted as an intramolecular one, and thus “local softness” and related reactivity descriptors have been used, instead of the “local hardness”, to predict the intermolecular reactivity orders. The generated results show that the model works at satisfactory level.
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    Nucleophilic Substitution Reaction of Alkyl Halides:  A Case Study on Density Functional Theory (DFT) Based Local Reactivity Descriptors
    (ACS, 2002-12-28) Roy, Ram Kinkar
    Density functional theory (DFT) based local reactivity descriptors, e.g., condensed Fukui function (FF) indices (or condensed local softness) have been tested on the nucleophilic substitution reaction of alkyl halides. As the carbon atom of the C−X (X = Cl, Br, and I) bond (hereafter denoted as CC-X) is the center for nucleophilic substitution, it should emerge as the most preferable site for an attack by a nucleophile (hereafter denoted as Nu-). It was found out when local reactivity descriptors are evaluated from the atomic charges derived from Mulliken population analysis (MPA), the CC-X did not emerge out to be the strongest electrophilic center in majority of cases. However, when these local reactivity descriptors were evaluated by a newly proposed way in which the Mulliken charges on the H-atoms are summed up to those of the heavy atoms to which they are bonded, the results improved significantly. When the reactivity descriptors are evaluated employing the later method and at elongated C−X distances (thus mimicking the situation of the nucleophilic substitution of alkyl halides, in which case the C−X bond is gradually broken), the results show significant improvement. In addition, the present study demonstrates that as the C−X bonds are elongated the global softness values of the systems increase in nearly all cases, thus confirming the validity of maximum hardness principle (PMH).
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    Stockholders Charge Partitioning Technique. A Reliable Electron Population Analysis Scheme to Predict Intramolecular Reactivity Sequence
    (ACS, 2003-11-11) Roy, Ram Kinkar
    Stockholders charge partitioning (i.e., Hirshfeld population analysis) is used to evaluate the intramolecular reactivity sequence (i.e., site selectivity) of some chosen alkyl halides. It is shown that the local reactivity descriptors, e.g., condensed local softness (or condensed Fukui function indices) in general and “relative electrophilicity” and “relative nucleophilicity” in particular, when evaluated by Hirshfeld population analysis, correctly reproduce the strongest electrophilic center in all 18 systems chosen in the present study. No manipulation of the charge summation scheme, i.e., addition of H-charges on the carbon atoms to which they are bonded, is required here as was done in a previous study (Roy, R. K. J. Phys. Chem. 2003, 107, 397) on the same systems using Mulliken population analysis. Extension of the study to polyfunctional systems (e.g., m-chloroaniline and m-anisidine) also shows that HPA correctly predicts the gas phase protonation site although MPA fails. This clearly indicates that Hirshfeld population analysis is superior to Mulliken population analysis as a charge-partitioning scheme (i.e., Hirshfeld population analysis provides a more reliable definition of atoms in molecules) as long as the intramolecular reactivity trend is concerned.
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    On the Reliability of Global and Local Electrophilicity Descriptors
    (ACS, 2004) Roy, Ram Kinkar
    The global electrophilicity index w (as defined by Parr et al. J. Am. Chem. Soc.1999, 121, 1992) and local “philicity” index, (as defined by Chattaraj et al. J. Phys. Chem. A2003, 107, 4973) of some carbonyl compounds are evaluated on the basis of Mulliken population analysis (MPA) and Hirshfeld population analysis (HPA) schemes. It is observed that the local electrophilicity indices (i.e., ), extracted from both HPA and MPA based charge, produce reliable intermolecular electrophilicity trends except in one case each. However, the reliability is lost in some cases when w is used. It is also shown both through numerical demonstrations and analytical proof that for generating intramolecular reactivity trends “philicity” index does not provide any extra reliability over local softness or Fukui function values. “Relative electrophilicity” and “relative nucleophilicity”, as defined by Roy et al. (J. Phys. Chem. A 1998, 102, 3746), generate the most reliable intramolecular reactivity (i.e., site selectivity) sequences.
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    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.
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    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.
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    “One-into-Many” Model:  An Approach on DFT Based Reactivity Descriptor to Predict the Regioselectivity of Large Systems
    (ACS, 2017-07-21) Roy, Ram Kinkar
    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.