Fundamental model compounds for the Hammett equation, meta- and para-substituted benzoic acids, were investigated by the density functional theory at the B3LYP/6-311+G(d,p) level. Energies of 25 acids and of their anions were calculated in all possible conformations and from them the energies of the assumed mixture of conformers. Relative acidities correlated with the experimental gas-phase acidities almost within the experimental uncertainty, much more precisely than in the case of previous calculations at lower levels. Dissection of the substituent effects into those operating in the acid molecule and in the anion was carried out by means of isodesmic reactions starting from monosubstituted benzenes. Both effects are cooperating in the resulting effect on the acidity; those in the acid molecule are smaller but not negligible. They are also responsible for some deviations from the Hammett equation (through-resonance of para donor substituents) and for the weaker resonance in the acid molecule in meta derivatives; in the anions the inductive and resonance effects are almost equal. On the other hand, the cooperation of effects in the acid and in the anion makes the relative acidity more sensitive to electron withdrawing and is probably one of the reasons why the Hammett equation is so generally valid.

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Electrophilic aromatic substitution in the calix[n]arene series is a well-established procedure leading exclusively to para-substituted derivatives. An unprecedented regioselectivity of the mercuration reaction leading to the meta-substituted calix[4]arenes is described. These compounds represent a new type of substitution pattern in classical calixarene chemistry and open the door for the straightforward synthesis of inherently chiral receptors based on calixarenes.

The classical term ortho effect was quantitatively analyzed and decomposed into its real or supposed components, using the acidity of eleven 2-substituted benzoic acids as a sample. The substituent effects were evaluated by calculations at the B3LYP/6-311þG(d,p) level by means of isodesmic reactions, separately for the acid molecules and for their anions. An intramolecular hydrogen bond, affecting the acidity moderately, was found only in the case of 2-methoxy-and 2-dimethylaminobenzoic acids. For the other acids, the oftenemployed interpretation invoking a hydrogen bond is wrong: the pertinent form was not revealed by calculations or by infrared spectroscopy. Another widespread explanation relying on the steric inhibition of resonance of the COOH group is also not correct in most cases: only some acid molecules are nonplanar (with substituents tert-Bu, CH 2 Cl, CF 3 , NO 2 , Cl) and the steric inhibition exists also in the anions, thus partly cancelling this effect.The main part of the ortho effect consists of polar effects (inductive and resonance) transmitted through the ring and of purely steric effects. Their tentative separation was based on comparison with the 4-substituted isomers. Polar effects are most important for the acidity and are not very different in the ortho and para positions (rough estimate 0.8:1). Steric effects are strong but mostly of similar magnitude in the acid as in the anion; hence they are of restricted importance for the acidity. They may be better described as van der Waals interactions rather than as short-range field effects. The whole analysis leads to the conclusion that the acidity of ortho-substituted benzoic acids cannot be interpreted by a simple universal theory.Benzene ortho derivatives have received less attention compared to their meta and para isomers. While in the latter the dependence of the reactivity and properties on structure is simple and can be approximated by the Hammett equation, effects of ortho substituents are much more complex. 1,2 Mostly they have been assumed to arise from two contributions: one similar as in the meta and para isomers, called inductive effect and resonance, 1,3 the other commonly called steric. The latter was further divided into several tentative components, 2,4,5 which were not always exactly identified: primary steric effect, steric inhibition to resonance (SIR), short-range field effect, hydrogen bonds (HB), steric inhibition to solvation and others. In numerous treatments of this subject, certain of these effects were more or less stressed and sometimes they were expressed quantitatively by an empirical equation with limited success. 4 Note that some of these effects are only simplified models, sometimes poorly defined: for instance, one can hardly distinguish a short-range field effect from a primary steric effect. As an extreme opinion, reactivities of ortho derivatives were explained without referring to any steric effects at all, 3 either by a blend of inductive and resonance effects 3a,b or most simply, by correlation with the...

The interaction of two relatively distant charged or dipolar groups was evaluated on 1,4-disubstituted bicyclo[2.2.2]octane derivatives as the reaction energy of the isodesmic-homodesmotic reaction, in which this derivative is formed from two mono derivatives. Energies of the individual compounds were calculated at the B3LYP/ 6-311þG(d,p) level. All combinations of 12 common substituents were investigated, representing 78 reaction energies in a 12 Â 12 data matrix. The behaviour of charged and dipolar substituents is considerably different. Interaction with a charged group is described precisely (R > 0.997) by one parameter, essentially identical with the standard inductive constant I . Interaction of two dipolar substituents depends also on I , but requires an additional parameter, important particularly in the case of donor substituents (NH 2 , OH, Cl), less important to insignificant with the others. It is significantly correlated with the electronegativity of the first atom. Alternatively, it can be evaluated as a new parameter characterizing the substituent effect, say a new scale of group electronegativity different from the suggested scales. With two parameters, the interaction of polar substituents is expressed with similar high precision (R > 0.99) as with charged substituents and one parameter.

Substituent effects in rigid non‐conjugated systems were followed on the series of 3‐substituted 1‐fluoro‐bicyclo[1.1.1]pentanes and 2‐substituted 1‐fluoroethanes in the fixed ap conformation. Their energies were calculated within the framework of the density functional theory at the B3LYP/6‐311++G(3df,3pd)//B3LYP/6‐311++G(3df,3pd) level and the substituent effects were expressed in terms of isodesmic homodesmotic reactions. The results were confirmed by the energies of 1,4‐disubstituted bicyclo[2.2.2]octanes reported in the literature and calculated at a lower level. Interaction of two common substituents of low or medium polarity cannot be described as the classical inductive effect by one term, proportional in all series, but an additional parameter is necessary, which depends only on the first atom of the substituent and may be identified with its electronegativity. The second term decreases with the distance more steeply than the first term and is always much less important. Nevertheless its statistical significance was proved by several sensitive tests at the highest level used in statistics. When one of the substituents is charged (or at least strongly polar as NO2 or CN), the first term is much increased and the second becomes less significant or insignificant. Therefore, the standard definition of the inductive effect with a uniform, universally valid constant can be retained as far as one treats only the ionization equilibria, both in solution and in the gas phase, or kinetics with a strongly polar transition state.In contrast to the firm statistical proofs, the physical meaning of the electronegativity term was not established. Any relation to various group electronegativities does not exist, similarity to the 13C NMR shifts is merely qualitative. Copyright © 2006 John Wiley & Sons, Ltd.

The infrared (IR) and vibrational circular dichroism (VCD) spectra of guanosine-5'-hydrazide ( G-1), a powerful hydrogelator, have been measured and analyzed on the basis of ab initio modeling. B3LYP/6-31G** DFT calculations predict that G-1, forming a clear solution in deuterated DMSO, is present in monomeric form in this solvent, whereas strong gelation in a phosphate buffer is due to the formation of a guanine-quartet structure, ( G-1)4, in which the four G-1 are linked by hydrogen-bonded guanine moieties and stabilized by an alkali metal cation. The B3LYP/6-31G** IR and VCD spectra of the nearly planar G-quartet, whose structure is slightly distorted from the C4h symmetry, in which the G-bases interact via four Hoogsteen-type hydrogen bonds and a sodium cation is positioned in the middle of the G-quartet, are in very good agreement with the experimental spectra, indicating that this structure is the predominant structure in the gel state. The geometric parameters are discussed. This study is the first to use IR and VCD spectroscopies coupled with DFT calculations to elucidate the structure of a supramolecular species in a gel state and shows the VCD spectroscopy as a powerful method for investigating the structure of complex supramolecular self-assemblies where the use of other structural methods is limited.

A profitable merger: The reaction of K5Bi4 with ZnPh2 yields the 20‐atom cluster [Zn@(Zn8Bi4)@Bi7]5− (see structure; zinc green, bismuth orange), which consists of a central zinc atom within a Zn8Bi4 icosahedron (red) capped by seven bismuth atoms. Extensive amalgamation of the two types of atoms, reminiscent of an alloy, occurs in this intermetalloid cluster.