Chemical reactivity, electron density

C. Gatti and A. Famulari Interaction Energies and Densities. A Quantum Theory of Atom in Molecules insight on the Effect of Basis Set Superposition Error Removal , P.G. Mezey and B. Rohertson (Eds.), Understanding Chemical Reactivity Electron, Spin and Momentum Densities and Chemical Reactivity, Vol. 2, Kluwerhook series (1999). In press. 

The Fukui function is a three dimensional function calculated quantum chemically from electron densities. It exhibits maxima in regions of space, where a molecule prefers to be attacked by a nucleophile/electrophile or radical. It can be shown (21) that local maxima of the electrophilic Fukui function relate to preferred sites of oxidative metabolic attack in many cases. One has to keep in mind that the Fukui function describes reactivity against an isotropic, abstract "reactivity bath". Any enzyme specific effects are not accounted for within the Fukui framework. 

The electrostatic potential map of benzene (Figure 11 3c) shows regions of high electron density above and below the plane of the ring which is where we expect the most loosely held electrons (the rr electrons) to be In Chapter 12 we will see how this region of high electron density is responsible for the characteristic chemical reactivity of benzene and its relatives... 

The orbital and resonance models for bonding in arylamines are simply alternative ways of describing the same phenomenon Delocalization of the nitrogen lone pair decreases the electron density at nitrogen while increasing it m the rr system of the aro matic ring We ve already seen one chemical consequence of this m the high level of reactivity of aniline m electrophilic aromatic substitution reactions (Section 12 12) Other ways m which electron delocalization affects the properties of arylamines are described m later sections of this chapter... 

Extended Huckel provides the approximate shape and energy ordering of molecular orbitals. It also yields the approximate form of an electron density map. This is the only requirement for many qualitative applications of quantum mechanics calculations, such as Frontier Orbital estimates of chemical reactivity (see Frontier Molecular Orbitals on page 141). 

The order of enolate reactivity also depends on the metal cation which is present. The general order is BrMg < Li < Na < K. This order, too, is in the order of greater dissociation of the enolate-cation ion pairs and ion aggregates. Carbon-13 chemical shift data provide an indication of electron density at the nucleophilic caibon in enolates. These shifts have been found to be both cation-dependent and solvent-dependent. Apparent electron density increases in the order > Na > Li and THF/HMPA > DME > THF >ether. There is a good correlation with observed reactivity under the corresponding conditions. 

These concepts play an important role in the Hard and Soft Acid and Base (HSAB) principle, which states that hard acids prefer to react with hard bases, and vice versa. By means of Koopmann s theorem (Section 3.4) the hardness is related to the HOMO-LUMO energy difference, i.e. a small gap indicates a soft molecule. From second-order perturbation theory it also follows that a small gap between occupied and unoccupied orbitals will give a large contribution to the polarizability (Section 10.6), i.e. softness is a measure of how easily the electron density can be distorted by external fields, for example those generated by another molecule. In terms of the perturbation equation (15.1), a hard-hard interaction is primarily charge controlled, while a soft-soft interaction is orbital controlled. Both FMO and HSAB theories may be considered as being limiting cases of chemical reactivity described by the Fukui ftinction. 

Electron, Spin and Momentum Densities and Chemical Reactivity... 

ELECTRON, SPIN AND MOMENTUM DENSITIES AND CHEMICAL REACTIVITY... 

If the functional form of a molecular electron density is known, then various molecular properties affecting reactivity can be determined by quantum chemical computational techniques or alternative approximate methods. 

Spin densities determine many properties of radical species, and have an important effect on the chemical reactivity within the family of the most reactive substances containing free radicals. Momentum densities represent an alternative description of a microscopic many-particle system with emphasis placed on aspects different from those in the more conventional position space particle density model. In particular, momentum densities provide a description of molecules that, in some sense, turns the usual position space electron density model inside out , by reversing the relative emphasis of the peripheral and core regions of atomic neighborhoods. 

This book contains a selection of chapter topics based on papers given at the 12th conference of the Commission on Charge, Spin and Momentum Density of the International Union for Crystallography, held in Waskiesiu, Prince Albert National Park, SK, Canada, July 27-August 1, 1997. The choice of topics represents some of the latest advances in the field of electron, spin, and momemtum densities and the analysis of these densities with respect to their roles in determining chemical reactivity. 

It is the hope of the editors that this book will provide our readers with an exciting collection of accounts of the latest advances, and also will provide further motivation for new research to address some of the challenging, unsolved problems of the fascinating interrelations between electron, spin, and momemtum densities, and the complex subject of chemical reactivity. 

Paul G. Mezey and Beverly E. Robertson (eds.), Electron, Spin andMomentum Densities and Chemical Reactivity, 45-69 0 2000 Kluwer Academic Publishers. Printed in Great Britain... 

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