Giorgio Favini

Molecular conformation of cyclenes: Part VI. MINDO/2' study of 1,3-cycloheptadiene, cis- and trans-cyclooctene

Abstract Quantum mechanical calculations indicate that the most stable conformation for 1,3-cycloheptadiene is a C s semi-planar form, for trans -cyclooctene a C 2 twist form and for cis -cyclooctene a form without symmetry. An equilibrium between two C 1 forms and the C s form can be suggested as a consequence of the negligible rotational barrier in 1,3-cycloheptadiene; no obviously preferred conformations exist in cis -cyclooctene, where the molecule is quite flexible. Theoretical results are consistent with the experimental data available.

Modes of interconversion in the cycloheptene ring

The possible modes of interconversion for cycloheptene have been studied by carrying out calculations in the CNDO/2 and MINDO/2 approximations.The transition state of lower energy is found along an asymmetrical pathway from the chair form, which is the most stable conformation, to the twist-boat form; the calculated activation energy agrees with the experimental data obtained from DNMR measures. The possibility of a change in the inversion mechanism for benzocycloheptene is also discussed.

Conformation of bicyclo[n.1.0]-compounds: Part I. Bicyclo[5.1.0] octane and cycloheptene epoxide

Abstract MINDO/3 quantum-mechanical calculations indicate that the most stable conformations of cis-bicyclo[5.1.0]octane and cycloheptene epoxide are the chair—chair (CC) and boat—chair (BC) conformations, respectively. The possible pathways of interconversion have been studied and are discussed for each molecule. The geometry of trans-bicyclo[5.1.0]-octane has been calculated and a heat of isomerization of 3.8 kcal mol−1 has been obtained. Theoretical results are compared with the experimental data available.

A theoretical conformational study of push–pull ethylenes. Part 1. Substituted methyleneimidazolidines

Geometries, polarisations, torsional barriers, ionisation potentials, and dipole moments have been computed by MNDO-type calculations for a representative group of ‘push–pull’ ethylenes, formed by an imidazoline ring and a series of acceptor substituents to the double bond, such as cyano, acetyl, and phenyl groups. A relevant number of geometrical parameters was included in the conformational energy optimisation procedure. The comparison with the set of available experimental data allows a rationalisation of the role of π- and strain-energy contributions to the conformational stabilities, and the effect of the withdrawing power of substituents on the height of the barrier.

Conformation of bicyclo[n.1.0] derivatives

Abstract The conformation al geometries and the possible interconversion paths for bicyclo[4.1.0] heptane (norcarane) and cyclohexene epoxide have been studied by both quantum mechanical and molecular mechanics calculations. The half-chair conformation is the most stable in the two compounds even if conformational equilibria with other forms cannot be excluded. Theoretical results are compared with the experimental data of molecular geometry, heat of formation, rotational constants, dipole moment, molar Kerr constant and proton coupling constants. The results obtained by the force-field method are satisfactory and consequently the extension of the method to derivatives of this kind seems possible.

CONFORMATIONAL ANALYSIS OF TRIMETHYLPHOSPHITE AND ITS METAL COMPLEXES

Abstract Trimethylphosphite ligands in about 40 different molecular environments in the solid state adopt five distinct conformations, apart from those related by chirality. The ligand symmetry is usually C 1, occasionally C 3 or C s, but never the maximum attainable C 3v. Average experimental P[sbnd]O and O[sbnd]C bond lengths in the complexes are 1.583 and 1.440 A; O[sbnd]P[sbnd]O bond angles (average 101.4°) vary considerably from conformer to conformer, whereas the mean P[sbnd]O[sbnd]C angle is 124.0°. Conformational analysis, using the MNDO method, indicates that the steric requirements of the metal rule out a C 3v, several C 3, C s and lower symmetry conformations for the ligand which are energetically favorable in case of the isolated molecule. Except for the experimentally observed C 3 symmetry conformer, none of the other ligand geometries (of C s and C 1 symmetry) is amongst the set of low energy conformations of the free molecule. This stands in relation to the steric requirement of the metal, ...

Empirical force field calculations for bridged annulenes

A systematic study of a number of bridged annulenes by empirical force field calculations is described. The theoretical results obtained for the isolated molecules confirm the different geometrical situations found for the compounds in the crystalline state. The packing forces do not modify the geometry of the rigid molecules with respect to the gas phase.

Conformation of bicyclo [n.1.0] Derivatives: Part 3. Dioxa- and trioxa-bicyclo[5.1.0] octanes

Abstract The conformational geometries and possible interconversion paths for some oxa derivatives of bicyclo[5.1.0] octane have been studied by the molecular mechanics method. The theoretical results are compared with the experimental data for the molecular geometry of bicyclo[5.1.0] octane and 3,5,8-trioxabicyclo[5.1.0] octane, the free energy of activation for cycloheptene epoxide and 3,5-dioxabicyclo[5.1.0] octane, the dipole moments and molar Kerr constants in solution for cycloheptene epoxide, 3,5-dioxa- and 3,5,8-trioxabicyclo[5.1.0] octane.

Empirical force field calculations for bridged annulenes. II. 1,6‐ethano‐8,13‐methano‐ and 1,6:7,12‐bismethano‐‖14‖annulenes

A study of some new bridged ‖14‖annulenes by empirical force field calculations is described. A comparison between the theoretical results obtained for the isolated molecules and few experimental results allows us to aim towards unambiguous interpretations. Some considerations are also performed on the aromaticity character of the studied ‖14‖annulenes.

Molecular structure of strained polycyclic hydrocarbons. A MINDO/3 study of some bicyclo- and tricyclo-derivatives

Abstract The molecular geometries of bicyclo|2.1.1|hex-2-ene, bicyclo|2.1.1|hexane, benzvalene and tricyclo|3.1.0.0 2,6 |hexane have been calculated by the semi-empirical MINDO/3 method. The values obtained for the structural parameters of the molecular skeletons are in good agreement with the experimental ones. The calculated heats of formation have been used to evaluate the strain energies of each molecule in the minimum energy configuration. It was found that the tricyclic compounds have more strain energy than the bicyclic derivatives; the heat of hydrogenation of benzvalene and bicyclo|2.1.1|hex-2-ene should be about 11 and 15 kcal mol −1 , respectively, above the normal values. Other physical quantities (dipole moments, rotational constants and ionization potentials) are discussed.