Average local ionization energies on the molecular surfaces of aromatic systems as guides to chemical reactivity

Abstract

. The average ionization energy, j(r), is introduced and is demonstrated to be useful as a guide to chemical reactivity in aromatic systems. I(r) is rigorously defined within the framework of self-consistent-field molecular orbital (SCF-MO) theory and can be interpreted as the average energy needed to ionize an electron at any point in the space of a molecu_le. An a b initio SCF-MO approach has been used to calculate I(r) at the 6-31G* level, using STO-3G optimized geometries. I(r) has been computed on molecular surfaces defined by the contour of constant electronic density equal to 0.002 electrons/bohr3, for a series of aromatic systems. This surface I(r) provides site specific predictions for preferred positions of electrophilic aromatic substitution. Relative reactivity toward electrophiles increases as the magnitudes of the smallest I(r) values (Imin) for these systems decr5ase. An excellent relationship, with a correlation coefficient of 0.99, has been found between the Hamrnett constants and Imin; this allowed us to predict the values of these constants for the substituents NHF and NF2, for which they were previously not known.Key words: average local ionizations energy, chemical reactivity, electrophilic aromatic substitution, molecular surfaces, Hammett constants.PER SJOBERG, JANE S. MURRAY, TORE BRINCK et PETER POLITZER. Can. J. Chem. 68, 1440 (1990). On introduit le concept d'tnergie rnoyenne d'ionisation locale, j(r), et on dtmontre qu'il est un guide utile pour prtdire la rtactivitt chimique dans les systernes aromatiques. On dtfinitI(r) d'une f a~o n rigoureuse dans le cadre de la thtorie des orbitales moltculaires en champ auto-cohtrent (OM-CAC) et on peut l'interprtter c o m e l'tnergie moyenne ntcessaire pour ioniser un tlectron dans un point quelconque de l'espace d'une moltcule. On a utilist une approche ab initio d'OM-CAC pour calculeri(r) au nivequ 6-3 IG*, en utilisant des gtomttries optimistes au niveau STO-3G. Pour une strie de systemes aromatiques, on a aussi calcul< I(r) sur les surfaces moltculaires dtfinies par le contour d'une densitt Clectronique constante de 0,002 tlectrons/bohr3. Cette I(r) de surface fournit des prtdictions sptcifiques au site pour les positions prtftrtes par les substitutions arornatiques Clectrophiles, La rtactivitt relative vis-i-vis des tlectrophiles augmente avec une diminution des amplitudes des valeurs I(r) les plus faibles Imin de ces systemes. On a ob_tenu une excellente corrtlation, avec un coefficient de corrtlation de 0,99, entre les constantes de Hammett et les valeurs de Imin; cette corrtlation nous permet de prtdire les valeurs de ces constantes pour les substituants NHF et NF2 pour lesquels ils n'ttaient pas encore connus.Mots clis : tnergie moyenne d'ionisation locale, rtactivitt chimique, substitution arornatique tlectrophile, surfaces rnoltculaires, constantes de Harnmett.[Traduit par la revue] Introduction derivatives, I-IX, on three-dimensional surfaces encompassing The interpretation and prediction of molecular reactive the molecules. These r...

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