Anna Rita Campanelli

From crystal to micelle: A new approach to the micellar structure

The structure of sodium and rubidium deoxycholate micellar aggregates in aqueous solutions was found to be helical and to be stabilized mainly by polar interactions. Astonishingly, the lateral surface of the helix is covered by nonpolar groups and the interior part is filled with cations surrounded by water molecules, as in the case of an inverted micelle. This helical model was inferred from the crystal structures of sodium and rubidium deoxycholates and proved by spectroscopic and diffractometric experimental data. The strategy of the approach to the determination of the micellar structure and the comparison with another model, previously proposed for the bile salt micelles, are reported. On the basis of some results obtained for sodium tauro- and glyco-deoxycholates, micellar models are suggested which could account for the biological function of these important conjugated bile salts.

Toward a more accurate silicon stereochemistry: An electron diffraction study of the molecular structure of tetramethylsilane

The present electron diffraction study of the molecular structure of tetramethylsilane, augmented with theoretical calculations, answers the need for accurate and detailed information on the most fundamental molecules containing silicon. The Si—C bond length is rg = 1.877 ± 0.004 Å, in perfect agreement with a previous study (Beagley, B.; Monaghan, J. J.; Hewitt, T. G. J. Mol. Struct.19718 401). The C—H bond length is rg= 1.110 ± 0.003 Å and the Si—C—H angle is 111.0 ± 0.2°. The experimental data are consistent with a model of Td symmetry and staggered methyl conformation. The barrier to methyl rotation is estimated to be 5.7 ± 2.0 kJ mol−1 on the basis of the experimentally observed average torsion of the methyl groups.

Electronic Substituent Effects in Bicyclo[1.1.1]pentane and [n]Staffane Derivatives: A Quantum Chemical Study Based on Structural Variation

The transmission of electronic substituent effects through one or more bicyclo[1.1.1]pentane units has been investigated by ascertaining how a variable substituent at a bridgehead position perturbs the geometry of a phenyl group at the opposite end of the molecule. We have analyzed the molecular structures of many bicyclo[1.1.1]pentane and [n]staffane derivatives of general formula Ph-[C(CH(2))(3)C](n)-X (n = 1-5), as obtained from molecular orbital calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. When n = 1, the structural variation of the benzene ring is controlled primarily by the long-range polar effect of X, with significant contributions from electronegativity and pi-transfer effects. The capability of the bicyclo[1.1.1]pentane framework to transmit these short-range effects originates from the rather high electron density inside the cage and the hyperconjugative interactions occurring between substituent and framework. A set of at least two bicyclo[1.1.1]pentane units appears to be necessary to remove most of the electronegativity and pi-transfer effects. In higher [n]staffanes (n >or= 3), the very small variation of the benzene ring geometry is controlled entirely by the long-range polar effect of X. With charged groups and for n >or= 2, the potential energy of the ring deformation decreases linearly with n(-3). In Ph-C(CH(2))(3)C-X molecules, the relatively large deformation of the bicyclo[1.1.1]pentane cage is determined primarily by the electronegativity of X, similar to the electronegativity distortion of the benzene ring in Ph-X molecules. Transfer of pi electrons from substituent to cage or vice versa also plays a role in determining the cage deformation.

Transmission of electronic substituent effects through a benzene framework: a computational study of 4-substituted biphenyls based on structural variation.

The transmission of substituent effects through a benzene framework has been studied by a novel approach, based on the structural variation of the Ph group in p-Ph-C(6)H(4)-X molecules. The molecular structures of many 4-substituted biphenyls were determined from MO calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. The twist angle between the phenyl probe (ring B) and the benzene framework carrying the substituent (ring A) was set at 90° to prevent π-electron transfer from one ring to the other and at 0° to maximize it. The structural variation of the probe is best represented by a linear combination of the internal ring angles, termed S(F)(BIPH(o)) and S(F)(BIPH(c)) for the orthogonal and coplanar conformations of the molecules, respectively. Regression analysis of these parameters using appropriate explanatory variables reveals a composite field effect, a substantial proportion of which is originated by resonance-induced π-charges on the carbon atoms of ring A. Field-induced polarization of the π-system of ring A also contributes to the structural variation of the probe. Thus, the S(F)(BIPH(o)) parameter is very well reproduced by a linear combination of the π-charges on the ortho, meta, and para carbons of ring A, an uncommon example of a quantitative relationship between molecular geometry and electron density distribution. Comparison of S(F)(BIPH(o)) with the gas-phase acidities of para-substituted benzoic acids shows that, while the deprotonating carboxylic probe is more sensitive to π-electron withdrawal than donation, the phenyl probe is equally sensitive to both. While the ability of the orthogonal biphenyl system to exchange π-electrons with the para substituent is the same as that of the benzene ring in Ph-X molecules, an increase by about 18% occurs when the conformation is changed from orthogonal to coplanar. The structural variation of the probe becomes more complicated, however. This is due to π-electron transfer from one ring to the other, which is shown to introduce quadratic terms in the regressions.

Transmission of electronic substituent effects across the 1,12-dicarba-closo-dodecaborane cage: a computational study based on structural variation, atomic charges, and ¹³C NMR chemical shifts.

The ability of the 1,12-dicarba-closo-dodecaborane cage to transmit long-range substituent effects has been investigated by analyzing the structural variation of a phenyl probe bonded to C1, as caused by a remote substituent X at C12. The geometries of 41 Ph-CB10H10C-X molecules, including 11 charged species, have been determined by MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed SF(CARB). Multiple regression analysis of SF(CARB), using appropriate explanatory variables, reveals the presence of resonance effects, superimposed onto the field effect of the remote substituent. The ability of the para-carborane cage to transmit resonance effects is, on average, about one-half of that of the para-phenylene frame in coplanar para-substituted biphenyls. Analysis of the π-charge variation of the phenyl probe confirms that the para-carborane frame is less capable than the coplanar para-phenylene frame of transmitting π-electrons from the remote substituent to the phenyl probe, or vice versa. The para-carborane cage is a better π-acceptor than π-donor; this makes π-donor substituents less effective than π-acceptors in exchanging π-electrons with the phenyl probe across the cage. When the remote substituent is an uncharged group, the para-carborane cage acts as a very weak π-acceptor toward the phenyl probe. The structural variation of the para-carborane cage has also been investigated. It consists primarily of a change of the C1···C12 nonbonded separation, coupled with a change of the five B-C-B angles at C12. This concerted geometrical change is controlled by the electronegativity of the substituent and the resonance interactions occurring between substituent and cage. These, however, appear to be important only when π-donor substituents are involved. The (13)C NMR chemical shifts of the para-carbon of the phenyl probe correlate nicely with SF(CARB), pointing to the reliability of these quantities as measures of long-range substituent effects. On the contrary, the (11)B and (13)C chemical shifts of the cage atoms do not convey information on electronic substituent effects.

Crystal structures of bile salts: Sodium taurocholate

Crystals of sodium taurocholate (NaC26H44NO7S · 2.5 H2O) belonging to the triclinic space groupP1 have unit cell parametersa = 12.731 (2),b = 16.104 (2),c = 7.628 (1) ⫗A, α =83.40 (1),β = 101.20 (1), γ = 105.35 (1)°, and two molecules in the asymmetric unit. The refinement, carried out on 4424 observed reflections, gaveR = 0.059 andRw = 0.066. The packing is characterized by bilayers, formed by antiparallel monolayers and with nonpolar outermost surfaces, held together by van der Waals interactions. Inside the bilayers there are channels, lined with polar groups, and filled by sodium ions and water molecules. A structural unit has been identified that could provide a reasonable model for the micellar aggregates of this bile salt.

Molecular structure and conformation of trimethylsilylbenzene: A study by gas-phase electron diffraction and theoretical calculations

Abstract The molecular structure and conformation of trimethylsilylbenzene have been investigated by gas-phase electron diffraction, molecular mechanics (MM3 force field), and ab initio MO calculations at the HF/6–31G * and MP2(f.c.)/6–31G * levels. The theoretical calculations show that the coplanar conformation of the molecule, with an Si-Me bond in the plane of the benzene ring, is a potential energy minimum. The perpendicular conformation, with an Si-Me bond in a plane orthogonal to the ring plane, is 0.2–0.5 kJ mol −1 higher in energy and corresponds to a rotational transition state. This low barrier makes the conformational space of the molecule almost evenly populated at the temperature of the electron diffraction experiment (305 K). A model approximating a freely rotating SiMe 3 group is consistent with the experimental data. Important geometrical parameters from electron diffraction are 〈 r g (C-C)〉 = 1.402 ± 0.003 A, 〈 r g (Si-C)〉 = 1.880 ± 0.004 A, and ∢C ortho -C ipso -C ortho = 117.2 ± 0.2°. The corresponding r e values from MP2 calculations are 1.400 A, 1.887 A, and 117.4°. The MO calculations also show that the C ipso -C ortho bonds are 0.011 A longer than the other C-C bonds. The MM3 and MO calculations indicate that the lengths of the Si-Me and Si-Ph bonds differ by only a few thousandths of an angstrom. This is less than what chemical expectation would suggest, but is in agreement with electron diffraction results from molecules containing either Si-Me or Si-Ph bonds.

A possible helical model for sodium glycocholate micellar aggregates

Sodium glycocholate crystallizes in the tetragonal space group14 witha =b = 27.793(4),c = 7.937(1) Å andZ = 8. Refinement based on 2290 observed reflections led to a conventionalR = 0.10. The bile salt molecules are arranged in a helix with 21 symmetry stabilized mainly by polar interactions. Four helices are held together by hydrogen bonds involving water molecules, giving rise to hydrophilic channels, with a small cross section, which can include water molecules. The packing of these tetramers form hydrophobic channels containing some disordered acetone and water molecules. The helices will be checked as a model for the micellar aggregates of this important conjugated bile salt, following the same strategy successfully applied to sodium deoxycholate.

Molecular Structure and Benzene Ring Deformation of Three Cyanobenzenes from Gas-Phase Electron Diffraction and Quantum Chemical Calculations

The molecular structures of cyanobenzene, p-dicyanobenzene, and 1,2,4,5-tetracyanobenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations. The equilibrium structures of these molecules are planar, but their average geometries in the gaseous phase are nonplanar because of large-amplitude vibrational motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in electron diffraction analysis is necessary to yield ring angles consistent with the results of MO calculations. The angular deformation of the benzene ring in the three molecules is found to be much smaller than obtained from previous electron diffraction studies, as well as from microwave spectroscopy studies of cyanobenzene. While the deformation of the ring CC bonds and CCC angles in p-dicyanobenzene is well interpreted as arising from the superposition of independent effects from each substituent, considerable deviation from additivity occurs in 1,2,4,5-tetracyanobenzene. The changes in the ring geometry and C ipso-C cyano bond lengths in this molecule indicate an enhanced ability of the cyano group to withdraw pi-electrons from the benzene ring, compared with cyanobenzene and p-dicyanobenzene. In particular, gas-phase electron diffraction and MP2 or B3LYP calculations show a small but consistent increase in the mean length of the ring CC bonds for each cyano group and a further increase in 1,2,4,5-tetracyanobenzene. Comparison with accurate results from X-ray and neutron crystallography indicates that in p-dicyanobenzene the internal ring angle at the place of substitution opens slightly as the molecule is frozen in the crystal. The small geometrical change, about 0.6 degrees , is shown to be real and to originate from intermolecular C identical withN...HC interactions in the solid state.

Molecular Structure and Conformation of p-Bis(trimethylsilyl)benzene: A Study by Gas-Phase Electron Diffraction and Theoretical Calculations

AbstractThe molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two