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ABSTRACT:The effects of substituents on the pK a s of a set of 16 substituted benzoic acids have been examined using density functional theory [B3LYP/6-311G(d, p)] calculations. A variety of quantum chemical parameters were examined as indicators for the variations observed in the experimental pK a s, including the Löwdin, Mulliken, AIM, and natural population analysis charges (Q L , Q M , Q A , and Q n ) on atoms of the dissociating carboxylic acid group, the energy difference (⌬E prot ) between the carboxylic acid and its conjugate base, and the Wiberg bond index. Several of these calculated quantities yielded excellent correlations with the experimental pK a : ⌬H prot , r 2 ϭ 0.958; Q L (H), r 2 ϭ 0.963; Q n (CO 2 H), r 2 ϭ 0.969; Q L (CO 2 Ϫ ), r 2 ϭ 0.970; and Q L (CO 2 H), r 2 ϭ 0.978. The best correlation observed with the pK a , as expected, was from the empirical Hammett constants designed for this purpose (r 2 ϭ 0.999).

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to P, within the VP scheme). The conclusions derived from the empirical analysis reported at the beginning of this section, therefore, have a sounder physical basis.The changes in VP nitrogen charges following deprotonation agree perfectly with the observed charge redistributions. QTAM allows one to investigate quantitatively these mechanisms, providing also a self-explanatory chemical description of them. Deprotonation of a planar tricoordinated nitrogen increases its u charge, thereby allowing an efficient release of its A charge to the *-conjugated ring system. So the "increase in u electron density can be associated with the observed ISN low-field shift"12 mainly because a greater amount of A charge is pushed away. In fact, the u electron density made available upon deprotonation at nitrogen is to a great extent transferred to the whole molecular u framework. Pushing away A charge enables the amino nitrogen to effectively conjugate to the ring A system or, in the case of 1 and 5c systems, to set up A conjugation throughout all the heterocyclic ring. This is clearly indicated by the differences in the portrait of the Laplacian of the charge density for the protonated and the deprotonated forms.This study demonstrates that useful and enlightening information is obtained by a combined analysis of changes in nuclear magnetic shielding tensors and charge density rearrangement mechanisms, despite the lack of a firm physical basis relating the two. Moreover, the study shows that apparently intriguing experimental behavior of chemical shift changes upon deprotonation can be rationalized in terms of plain concepts. Acknowledgment.We thank a referee for his very helpful and stimulating comments. Thanks are also due to Dr. P. J. MacDougall for suggesting that we explore correlations involving the charge density quadrupole moment tensor. The help of Mr. M. Bandera in preparing the drawings is gratefully acknowledged. Registry No.Abstract: The effect of substituents on the properties of acyl derivatives has been examined. Geometry optimizations were carried out at the MP2/6-31G* theoretical level, followed by calculation of energies at the MP3/6-311++G** level. The energies were studied via isodesmic reactions with ethane leading to acetone and a methyl derivative. The calculated energy changes were in g o d agreement with the available experimental data and showed that groups more electronegative than carbon stabilize a carbonyl group more than methyl, whereas the opposite is true with the more electropositive groups. The reasons for the energy changes were studied by examining bond dissociation energies for the acyl derivatives and for the corresponding methyl derivatives. The changes in electron populations, bond orders, and atom energies that result from changes in substituents were also studied. It is concluded that the stabilization of carbonyl groups by the substituents NH2, HO, and F results from a combination of two interactions. The first is the interaction of the substituent lone pair with the electron deficien...