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Many data, collected from the literature, support the postulation of two general rules which describe the static and dynamic results of having in a molecule or generating in an intermediate adjacent electron pairs and/or polar bonds.In disagreement with current chemical intuition, structures which contain the maximum number of gauche-interactions between lone electron pairs or polar bonds represent energy minima and, frequently, the lowest minima. The stereochemical implications of the phenomenon are discussed, along with possible physical explanations. It is suggested that partitioning of the total energy of the system into attractive-dominant and repulsive-dominant interactions provides the most helpful framework for the construction of a physical picture of the phenomenon. The total energy and its components can be obtained by ab initio molecular quantum mechanical calculations.The only apparent exception to the phenomenon that has been found corresponds to a polar bond adjacent to two lone pairs (as in the case of two hetero-atoms attached to the same carbon atom). This exception, here termed the Edward-Lemieux effect, has been examined theoretically by an ab initio (Hartree-Fock) calculation using fluoromethanol as a model compound. The calculation has reproduced the Edward-Lemieux effect; the stable conformation has the C-F bond trans to one ' electron pair ' and gauche to another, and the conformation in which the C-F bond bisects the ' electron pairs ' is the energy maximum. The partitioning of the total energy into its components of attraction and repulsion and comparison of the results with other systems for which barriers to internal rotation have been obtained by ab initio methods reveals a similarity between fluoromethanol, hydrazine, hydroxylamine, and hydrogen peroxide. An interpretation of this result is provided in which an early suggestion by Lemieux and Chu is supported and the concept of ' rabbit-ears ' is not.

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The static and dynamic structures of the amide derivatives of racemic and (R)-(+)-2-methoxy-2-phenyl-3,3,3-trifluoropropanoic acid (MPTA) with diisopropyl amine (1), meso-2,6-dimethylpiperidine (2), (R)-(−)- and (S)-(+)-2-methylpiperidine (3 and 4, respectively) were investigated by proton nmr spectroscopy. The piperidine rings adopt a single dominant conformation in which the substituent methyl groups occupy axial positions. One of the isopropyl groups of 1 is oriented in the same way as the corresponding side of the piperidine ring of 2–4 but the other (syn to carbonyl oxygen) is turned so that the methyl groups are toward the carbonyl oxygen and the oxygen lies in the plane bisecting the C—C—C angle. Rotation about the amide C(O)—N bond of 1 is accompanied by concurrent rotation about each of the N—iPr single bonds. The free energies of activation of the hindered rotation about the C(O)—N bond in 1–4, 81 kJ/mol, 77 kJ/mol, 77 kJ/mol, and 79 kJ/mol, respectively, are higher than found in most other hindered amides, suggesting that buttressing due to the steric crowding around the amide functionality is felt throughout the rotational pathway.

The electronic structure and bonding in the hypothetical PH5 are examined by nonempirical molecular orbital calculations. Computations on a model system derived from PH5 are used to discuss the stereochemistry and intramolecular ligand exchange processes of phosphoranes. On the basis of calculations on PH5, it is suggested that the lowest frequency fundamental of trigonal-bipyramidal phosphoranes corresponds to the equatorial in-plane bending motion. Pentacoordinatecompounds of phosphorus, in which the ligands are aryl groups or highly electronegative groups, are well known,3 and some fluorohydrides of phosphorus have been prepared.4-6 However, the parent compound, PH6, has not been observed. It is thought that the high H-H bond energy, coupled with the low electronegativity of H, precludes its formation.7Until now, theoretical investigations of pentacoordinate phosphorus appear to have been restricted to semiempirical (EHMO,8-11 modified EHMO,12 CNDO/ 2,13 VESCF-MO14) calculations on PF5,8,[12][13][14]8,10 pentaoxyphosphoranes,11 and the series PF"C1(6_"), 0 < < 5.9We have carried out nonempirical LCAO-MO-SCF (Hartree-Fock) calculations on PH5 and model systems derived from PH5 in order to probe, with a minimal amount of experimental input, the electronic structure of phosphorus with an "expanded valence shell" and to explore the nature of the polytopal rearrangements that these compounds can undergo. A moderately large basis set of Gaussian-type functions (Gtf)15 was used. The calculations were performed with and(1) (a) This work was supported by the Chemistry Division of the Natural Science Foundation, Grant No.

Ab initio computations (B3LYP/6-31G(D), coupled with isodesmic reactions) were used to predict αC→H bond dissociation energies (BDEs) for proline as a residue in a model peptide, intended to mimic the environment in proteins. The environment was further constrained to mimic common proline positions in turns of different types. The BDEs were found to be very dependent on the structural constraints imposed by the turn type, implying different structure-mediated susceptibilities to free radical damage to proline residues. Unnatural repair of proline (inversion of chirality) was found to be thermodynamically unfavourable. The predicted BDEs for the proline αC→H bond, in kJ mol-1, to an estimated accuracy of ±10 kJ mol-1 are as follows: fully optimized trans rotamer, 368.6; fully optimized cis rotamer, 357.7; ß turn type I, 380.7; ß turn type II, 397.8; ß turn type II', 385.4; ß turn type VIa, 374.0; ß turn type VIb, 355.0. Key words: proline, ß -turns, free radical, bond dissociation energy, molecular structure, oxidative damage.<

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