We propose a new scale of group electronegativities, derived from benzene ring deformations in Ph−X molecules. A recent analysis of such deformations (Campanelli, A. R.; Domenicano, A.; Ramondo, F. J. Phys. Chem. A 2003, 107, 6429) has shown that two orthogonal linear combinations of the internal ring angles, termed S E and S R, are directly related to the electronegativity and resonance effects of the substituent, respectively. In the present paper, we show that S E increases linearly with the electronegativity of X within each of the first two rows of the periodic table, acting as a sensitive indicator of the polarity of the Ph−X bond. By using S E values from ab initio quantum chemical calculations, we have derived the electronegativities of 100 organic and inorganic groups. Nonbonded interactions with the ortho hydrogens and carbons may fictitiously alter the electronegativity of a group; in most cases, however, they are easily eluded by changing the conformation of the substituent with respect to the benzene ring. Although the atom directly linked to the ring tends to dominate the electronegativity of a group, the role of its adjacent atoms is also important. Their effect depends markedly on the nature of chemical bonding and electron density distribution within the group.
The deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from the structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry of the substituted ring is shown to contain valuable information on the electronegativity, resonance, and steric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the C ipso -C ortho bonds. The results obtained substantially augment previous knowledge from the analysis of experimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983, 39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the ipso, ortho, and para positions, coupled with a change in the C ipso -C ortho bond length. The values of the ipso angle span a remarkably wide range, 113-126°. Enhancing the resonance interaction between a substituent and the ring causes a complex pattern of angular distortions, arising from the superposition of two separate effects. The first originates from the decreased length of the C-X bond, and consists primarily in a concerted change of the ipso and ortho angles. It occurs irrespective of whether the substituent is a π donor or a π acceptor. The second effect is associated with π-charge alternation on the ring carbons. It involves all the internal ring angles, and depends on the substituent being a π donor or a π acceptor. These angular changes are generally accompanied by changes in all C-C bond lengths, as expected from an enhanced contribution of polar canonical forms to the electronic structure of the molecule. By using symmetry coordinates, we have derived two orthogonal linear combinations of the internal ring angles, S E and S R , measuring the electronegativity and resonance effects of a substituent, respectively, as seen from their impact on the ring geometry. S E and S R values are affected in a typical way by steric effects.
supported on alumina. The appearance of all of the products and the disappearance of one of the reactants (OH-) can be simultaneously followed by FTIR.RhC13 has been shown to react with surface O H groups on alumina at 180 "C. The product of this reaction is believed to be Rh3+ bonded to a surface oxygen on alumina. This species is an intermediate in the room-temperature oxidation of CO listed above.Removal of the surface O H groups by reaction with RhC1, dramatically suppresses the reaction of COz with alumina.Acknowledgment. This work has been funded in part by the Office of Naval Research, and the University of Richmond Faculty Research Committee. We thank Dr. John Yates for his helpful comments and for his assistance in designing the experimental setup and Hampton Rexrode for his work on the apparatus construction. Previously a helical model was satisfactorily verified for sodium (NaDC) and rubidium (RbDC) deoxycholate micelles in aqueous solutions by means of SAXS, EXAFS, NMR, ESR, and CD measurements. Here we report the beginning of an analogous study carried out on sodium glycodeoxycholate (NaGDC) and taurodeoxycholate (NaTDC) following the strategy applied to NaDC and RbDC. The crystal structure of NaGDC sesquihydrate, solved by X-ray analysis, provides again a helical model that is compared with those of NaDC, RbDC, and NaTDC. Since it was previously observed that bilirubin-IXa (BR) exhibits a bisignate CD Cotton effect in NaDC aqueous solutions, and it was suggested that the chiral micelles of NaDC interact preferentially or exclusively with one of the two enantiomeric conformers of BR, we have recorded CD spectra of BR in aqueous micellar solutions of the bile salts in order to check the helical models. The spectra show in all cases two large and proximate bands of opposite sign between 400 and 500 nm, which support both our chiral models and the selection of the BR left-handed enantiomer. Moreover, we have accomplished van der Waals energy calculations for the system formed by a NaGDC helix and the left-or right-handed BR molecule to test if the interaction energy is lower for one of the two BR enantiomeric conformers. The results of the calculations seem to indicate that the NaGDC helix binds preferentially the BR molecule with left-handed chirality. Interaction models are proposed. IntroductionStudies carried out on sodium and rubidium deoxycholate (NaDC and RbDC, respectively) following a strategy previously reported' showed that a helical model, observed in the crystal structures of NaDC and RbDC,2v3 describes very satisfactorily the behavior of their micellar aggregates in aqueous solutions.'*e7 The helical structure of the NaDC and RbDC micelles was verified unambiguously by nuclear magnetic resonan~e,'.~ circular dichroism! electron spin resonance! small-angle X-ray scattering,6 and extended X-ray absorption fine structure7 measurements.Subsequently, the investigation of the structure of the sodium taurodeoxycholate (NaTDC) micellar aggregates was undert a k e~? +~ and other helical models were...
The transmission of polar effects through the bicyclo[2.2.2]octane framework has been investigated by ascertaining how the geometry of a phenyl group at a bridgehead position is affected by a variable substituent at the opposite bridgehead position. We have determined the molecular structure of several Ph-C(CH(2)-CH(2))(3)C-X molecules (where X is a charged or dipolar substituent) from HF/6-31G and B3LYP/6-311++G molecular orbital calculations and have progressively replaced each of the three -CH(2)-CH(2)- bridges by a pair of hydrogen atoms. Thus the bicyclo[2.2.2]octane derivatives were changed first into cyclohexane derivatives in the boat conformation, then into n-butane derivatives in the anti-syn-anti conformation, and eventually into assemblies of two molecules, Ph-CH(3) and CH(3)-X, appropriately oriented and kept at a fixed distance. For each variable substituent the deformation of the benzene ring relative to X = H remains substantially the same even when the substituent and the phenyl group are no longer connected by covalent bonds. This provides unequivocal evidence that long-range polar effects in bicyclo[2.2.2]octane derivatives are actually field effects, being transmitted through space rather than through bonds. Varying the substituent X in a series of Ph-C(CH(2)-CH(2))(3)C-X molecules gives rise to geometrical variation (relative to X = H) not only in the benzene ring but also in the bicyclo[2.2.2]octane cage. The two deformations are poorly correlated. The rather small deformation of the benzene ring correlates well with traditional measures of long-range polar effects in bicyclo[2.2.2]octane derivatives, such as sigma(F) or sigma(I) values. The much larger deformation of the bicyclo[2.2.2]octane cage is controlled primarily by the electronegativity of X, similar to deformation of the benzene ring in Ph-X molecules. Thus the field and electronegativity effects of the substituent are well separated and can be studied simultaneously, as they act on different parts of the molecular skeleton.
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