Aliette Cossé-Barbi

Anomeric effects in carbohydrates: non equivalence of endocyclic oxygen lone pairs

Abstract In furanoses, a high selectivity of the axial orientation of substituents at the anomeric center is established statistically through crystallographic data studies of anomeric substructures and related hydrocarbons. This fact constitutes conclusive evidence of non equivalence of the endo oxygen lone pairs (O = sp 2 ).

Infrared study of complexes between iron, salicylaldehyde benzoyl hydrazone and seven analogous derivatives

SummaryAn i.r. study of Fe complexes prepared from salicylaldehyde benzoyl hydrazone and seven similar chelating agents allowed the assignment of some iron-ligand bands. The amide I and Φ—O band shifts indicate that the ligand O atoms are directly linked to Fe. The analogy between FeII and FeIII complexes in the mid-i.r. range and the chiefly identical frequencies in the far-i.r. range provide evidence that oxidation of FeIIhas occurred. The bands directly involved in complexation are shifted compared to the free hydrazones, and all are medium or weak.

When local crowding reinforces an anomeric effect

Abstract In furanose-pyranose structures, a bulky substituent at the anomeric center of the furanose ring induces an axial conformation of the pyranosic part. The relative positions of the two rings are also modified: syn instead of anti with regard to the C-3/O-4/C-5 plane. These conformational adaptations are monitored by a subtle balance between anomeric effects and non bonded atom interactions.

Influence de la structure des cetones et de l'acidite des phenols sur leur mode de complexation: Partie I. Comportement global et échelles d'acidité et de basicité

Abstract Associations between methyl or halogenated phenols and saturated aliphatic ketones of varied hindrance in positions α or β to the carbonyl group are studied with regard to the search for behavioral similarities and determination of the influence of phenol chelation or steric hindrance of one of the association partners. From a statistical treatment by Principal Component Analysis on 147 couples of ketone-phenol dimers it can be inferred that overall the association constants K pc and the associated OH bandwidth (Δν 1 2 ) OH can be factorized into a product of reduced characteristics, which expresses the association aptitude of each phenol or ketone. Thus phenol donor characteristics K p (Δν 1 2 ) p are linearly related to their p K a values at least in the absence of bisubstitution at ortho positions. For ortho substituted phenols, reduced bandwidths (Δν 1 2 ) p do not depend on intramolecular auto association, and so allow for determination of actual ketone-phenol association constants (and estimation of chelation constants). Comparison of ketone reduced characteristics with the usual basicity scales (association constant with parafluorophenol or iodine, solvation parameters, ν CO frequencies) assess the scope of our proportionality model. The specific behavior of saturated aliphatic ketones when compared to other carbonyl derivatives is interpreted through conformational effects acting differently on accessibility or stretching frequency ν CO of the carbonyl.

Rotational barriers adjacent to the CN double bond in methyl and ethylimines

Abstract The rotational barrier about the C-C bond adjacent to the CN group in methyl and ethylimines was evaluated by MO calculations at the RHF level and by considering electron-correlation effects. Structural regularities (unequal lengths for in plane and out-of-plane CH bonds, changes in C-C bond lengths or NC-C angles) were used as a test for hyperconjugative interactions. For trans situations (alkyl group facing the nitrogen lone pair) n /π ∗ CH 3 , hyperconjugation is effective only for the eclipsed conformations (CH or CC bonds eclipsing the double bond), and leads to stabilization. For cis situations (alkyl group facing the NH bond) π ∗ CN /π CH 3 hyperconjugation is observed only when an alkyl group bisects the double bond and destabilizes this conformation. In both cases, the expected interaction ( −1 ) is too small to account for the barrier heights. These are mainly controlled by bond-bond exchange repulsions. In most cases, direct or reverse methyl tilts do not result from hyperconjugative interactions as is commonly affirmed.

Conformational dynamics: Association of correlated and non-correlated rotations of methoxy groups in anomeric structures. Convergence of a theoretical study of 2,2-dimethoxypropane and of some crystallographic data of acyclic analogues of pyranoses.

Abstract For anomeric acyclic structures, a peripheral interconversion pathway in several steps with correlated and non correlated relative rotations of the two alkoxy groups of the 2,2-dimethoxy propane-like fragments is favoured. One cannot oversimplify the evaluation of interconversion energy of such complex processes. The final geometry and the dynamics of the system depend on competition or cooperative contributions due to non bonded steric terms and the usual stereodependent anomeric effect.

IR Study of Complex Between Salicylalbenzoylhydrazone (SBH) and Iron III: Normal Mode Assignment Assisted by Quantum Mechanical Calculation

Abstract A theoretical investigation of SBH and a bicomplex ion, [(Fe III (SBH)2)]+ is used as a basis to control and clarify previous vibrational assignments for complexes formed between iron and ligands related to SBH. Main conclusions are the following: 1. Complexation shifts the vC=0 vibration by about 100 cm−1 towards low wavenumbers. 2. vC=N and δHC=N modes are enhanced by complexation. 3. In the complex, the enhanced vC=N mode hides the amide II vibration. 4. The vφ-O- stretching mode is expected to be near 1400 cm−1 in the complex. 5. Point 1 provides a criterion for carbonyl involvement in complexation. Points 2 and 4 are possible probes for hydrazide nitrogen and phenate involvement. 6. Despite ab initio calculations, the vibration at 1360 cm−1 is, as previously, assigned to the amide III mode.

Discrete pattern recognition by fitting onto a continuous function

This article outlines an original method for matching discrete structures when atom correspondences are unknown. This method avoids the current atom‐by‐atom treatment (and its inherent combinatorial problems) and considers the structures to be compared in their totality. The basic idea is to first obtain the atom correspondences by fitting one of the two discrete structures onto a spline approximation of the other, rather than optimizing in discrete space, and, second, to overlap the two discrete structures on the basis of the proposed assignment. As starting data, the method requires only the Cartesian coordinates of the two structures. No connectivity information, neither atom labeling nor matching tolerance is required. This method can readily handle matches of molecules with a few hundred atoms. It is able to search for a given 3D pattern as well as for a pattern common to two structures. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1875–1892, 1997

Conformational identity change of 3-methylbutanone from gaseous phase to solutions: An IR vibrational spectroscopy study

Infrared vibrational spectroscopy of 3-methylbutanone [Me(CO)iPr] leads to two conclusions: (1) The conformational identity is different in the gas phase and in various solvents. (2) In the gas phase, type B rovibrational structures are observed. Thus, the molecular symmetry isC5. The following interpretation is based upon a model which implicitly takes the solvent into account in the framework of an empirical calculation. The solvent increases the interconversion barrier between two enantiomers. As a consequence, the molecule changes conformation, moving from the stable conformation with bisected carbonyl seen in the gas phase, to a conformation with an eclipsed carbonyl in solutions.

Infrared Study of Seven Potential Siderophores Analogous to Salicylaldehyde Benzoyl Hydrazone (SBH)

Abstract Seven siderophores which differ from salicylaldehyde benzoyl hydrazone by only one substituent on the benzoyl moiety are studied with infrared Spectroscopy between 2000 and 1100 cm−1. The shift of the amide-I band and the intensity variations of cycle stretching bands are specially discussed according to the nature of the substituents.