Unifying Conceptual Density Functional and Valence Bond Theory: The Hardness-Softness Conundrum Associated with Protonation Reactions and Uncovering Complementary Reactivity Modes.

Thijs Stuyver,Sason Shaik

doi:10.1021/jacs.0c09041

Thijs Stuyver, Sason Shaik

Open Access

https://doi.org/10.1021/jacs.0c09041

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Abstract

In this study, we address the long-standing issue—arising prominently from conceptual density functional theory (CDFT)—of the relative importance of electrostatic, i.e., “hard–hard”, versus spin-pairing, i.e., “soft–soft”, interactions in determining regiochemical preferences. We do so from a valence bond (VB) perspective and demonstrate that VB theory readily enables a clear-cut resolution of both of these contributions to the bond formation/breaking process. Our calculations indicate that appropriate local reactivity descriptors can be used to gauge the magnitude of both interactions individually, e.g., Fukui functions or HOMO/LUMO orbitals for the spin-pairing/(frontier) orbital interactions and molecular electrostatic potentials (and/or partial charges) for the electrostatic interactions. In contrast to previous reports, we find that protonation reactions cannot generally be classified as either charge- or frontier orbital-controlled; instead, our results indicate that these two bonding contributions generally interplay in more subtle patterns, only giving the impression of a clear-cut dichotomy. Finally, we demonstrate that important covalent, i.e., spin pairing, reactivity modes can be missed when only a single spin-pairing/orbital interaction descriptor is considered. This study constitutes an important step in the unification of CDFT and VB theory.

Highlights

The systematic and reliable prediction of regioselective preferences associated with chemical reactions remains an outstanding challenge in computational chemistry.[1−5] Molecules often contain a multitude of potentially reactive sites, so that manual investigation of the relative energetics of each individual transition and product state, associated with a pair of reaction partners, quickly becomes unfeasible as the size of the investigated system increases
In valence bond (VB) theory, the electronic wave function associated with a chemical system is represented as a superposition of
Focusing on a selection of protonation reactions, we demonstrated that these two interaction types contributing to chemical bonding can be readily resolved with the help of VB theory

Summary

■ INTRODUCTION

The systematic and reliable prediction of regioselective preferences associated with chemical reactions remains an outstanding challenge in computational chemistry.[1−5] Molecules often contain a multitude of potentially reactive sites, so that manual investigation of the relative energetics of each individual transition and product state, associated with a pair of reaction partners, quickly becomes unfeasible as the size of the investigated system increases. Most general-purpose reactivity descriptors proposed so far can roughly speaking be classified into one of two main classes: the descriptor focuses on either so-called “hard−hard” or “soft−soft” interactions This dichotomous approach (which emerges, among others, from the Klopman−Salem equation[14] and constitutes a cornerstone of the “hard−soft acid base” (HSAB) principle,[15,16] embedded within the conceptual density functional theory (CDFT) framework17−21) has been demonstrated to work reasonably well in the limiting situations of either charge-controlled reactivity, i.e., reactions leading to formation of a purely ionic bond, or (frontier) orbital-controlled reactivity, i.e., reactions leading to formation of a purely covalent bond. The energy spacings between the HL and the ionic structures in the dissociation limit were corrected by the deviation from the corresponding DFT values (calculated at (U)B3LYP/6-31++G***//(U)B3LYP/def2-TZVP)

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES