Germaine Robinet

On the electronic origins of barriers to methyl rotation: CNDO/2 calculations on (CH3)2XHn (X = C, Si, N, P, O, S) molecules

Abstract CNDO calculations are performed for the homogeneous series of (CH 3 ) 2 XH n compounds (X = C, Si, N, P, O, S) in order to determine the molecular equilibrium configuration. In agreement with available experimental data, for all investigated molecules, the theoretical energy minimum is found for the (θ = 60°, ψ = 60°) conformation in which one hydrogen of each methyl group is in the X heavy atoms plane but pointing outside the CXC angle. A partitioning of total energy shows that the variations of this quantity are completely reflected by the variations of the interaction energy between non-bonded terms. A more detailed analysis reveals, only for the third-row compounds, the essential role of the interactions between central atom and methyl hydrogens.

Theoretical conformational analysis of lewis adducts, VI. A semi-empirical approach on dimethylsulphoxide-boron trifluoride: An example of rock and roll internal motion

Abstract The preferred conformations and the electronic structure of (CH3)2SO.BF3 are investigated within the framework of the CNDO/2 approximation. The two skew equivalent preferred forms C and C are symmetrical about the bisecting plane of the acute CSC angle. The rotational barrier between C and C is very low (0.45 kcal mole−1) and corresponds to a conformation S in which the BF3 group is twisted through 50° or so relatively to its position in the C conformation. Thus, it appears that the rocking of the OBF3 group around the (S.O) bond from C to C is attended by a rolling of the BF3 group around the (O.B) bond. For this reason, we suggest this peculiar conformational equilibrium be named the rock and roll internal motion.

Une approche théorique de la stabilité relative des métabolites hydroxylés de l'ellipticine, de l'olivacine et du methyl-2 ellipticinium

Abstract A quantum mechanical approach (CNDO/2 approximation) of several possible oxidation pathways of ellipticine in 9-hydroxy-ellipticine shows that the most energetically favoured would pass along an epoxide → diol formation → diol dehydration scheme. Moreover, quantum calculation of adiabatic ionization potentials within the hydroxy-ellipticines, hydroxy-olivacines and hydroxy-ellipticiniums series (including solvation effects) supports experimental evidence for the determinant activity exhibited by 9-hydroxy derivatives as anticancer agents.

Molecular structure of divinyl sulphone as studied by electron diffraction, vibrationai spectroscopy, and semiempirical CNDO/2 molecular-orbital calculations

An electron-diffraction structure analysis has yielded the following bond lengths (ra) for the title compound: SO 1.438(3), S–C 1.769(4), CC 1.332(4), and C–H 1.118(6)A. Of the bond angles, the following two are reliablydetermined: OSO 119.5(12) and S–CC121.5(3)°. CNDO/2 molecular-orbital calculations indicatethe coexistence of three conformers. At least two, possibly more, conformers are present in the vapour according to the electron-diffraction data, which also indicate that the CC bonds tend to eclipse the other bonds. Comparison of a number of bands in the i.r. and Raman spectra for different states of the compound shows the coexistence of at least two, possibly more, conformers in the liquid and in the supercooled liquid, while there is only one form in the crystalline phase. The SO bond lengths and stretching frequencies are consistent with empirical relations found for a relatively large series of sulphones.