Is the Fukui Function a Right Descriptor of Hard−Hard Interactions?

Abstract

To verify whether the maximum or the minimum Fukui function site is better for protonation reactions or an altogether different local reactivity descriptor, viz., the charge is necessary, we calculate the Fukui functions (using a finite-difference approximation as well as a frozen-core approximation) and charges (Mulliken, Hirshfeld, and natural population analysis schemes) of several hydroxylamine derivatives, their sulfur-containing variants, and amino acids using B3LYP/6-311G(d,p) technique. While the Fukui functions provide the wrong selectivity criterion for hard−hard interactions, the charges are found to be more reliable, vindicating Klopman's idea. It is transparent from the present results that the hard−hard interactions are better explained in terms of charges, whereas the Fukui functions can properly account for soft−soft interactions known to be frontier-controlled.

Density functional theory molecular modelling and experimental particle kinetics for CO2–char gasification

“…The function f + (r) is a measure of reactivity toward a donor reagent (nucleophillic attack), f -(r) values measure reactivity toward an acceptor reagent (electrophilic attack) and an average value of both may be taken as a measure of reactivity toward a radical f 0 (r) [82]. Parr and Yang showed that sites in chemical species with the largest f(r) value are those with highest reactivity and are the preferred reaction site(s) [83][84][85]. Research has emphasised that hardness-softness f(r) derived from the DFT are powerful tools to predict active sites of a molecule [84,86,87].…”

“…Parr and Yang showed that sites in chemical species with the largest f(r) value are those with highest reactivity and are the preferred reaction site(s) [83][84][85]. Research has emphasised that hardness-softness f(r) derived from the DFT are powerful tools to predict active sites of a molecule [84,86,87]. The f(r) defines the reactivity of an atom in a molecule and, together with local softness, are suitable to describe reactivity of different substrates [88], including symmetrical char structures.…”

Do the Local Softness and Hardness Indicate the Softest and Hardest Regions of a Molecule?

“…[65] This "minimum Fukui function rule" has been broadly applied, [69][70][71] but also criticized as overly simplistic. [67,72] Klopman asserts that hard-hard interactions are predominately ionic in nature, and so electrostatic effects dominate and frontier-orbital effects are relatively unimportant. The minimum Fukui function principle captures the irrelevance of the frontier orbitals but misses the fact that electrostatic interactions normally dominate hard-hard interactions.…”

“…The minimum Fukui function principle captures the irrelevance of the frontier orbitals but misses the fact that electrostatic interactions normally dominate hard-hard interactions. [67,68,72] In contrast to the well studied local softness, few studies of the local hardness are reported in the literature, mainly by our group using the local hardness as an indicator for charge concentration in studies on zeolite-catalyzed reactions, [73][74][75] noncovalent intermolecular interactions, [76][77][78][79][80] and local HSAB. [81] Then, one of the goals of this work is to see if the local hardness is able to locate the hardest reactive sites in a molecule.…”

Effect of external electric field on ground and singlet excited states of phenylalanine: A theoretical study

“…These parameters, called reactivity descriptors, defined within the framework of density functional theory are global hardness (g) (also called chemical hardness), electrophilicity (x), chemical potential (l) etc [21]. These descriptors have been tested and studied extensively by several research groups and are reported to be very useful in rationalizing the reactivity patterns in the molecular systems [22][23][24][25][26]. Geerlings et al and Chattaraj et al have reviewed the theoretical basis of these descriptors and also their applicability [27,28].…”