Claude Leibovici

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.

On the conformational analysis of (CH3)3M (M = N, P, CH, SiH) molecules: CNDO/2 calculations versus experimental data. LEM or calder conformation?

Abstract CNDO/2 calculations are performed for the homogeneous series of (CH 3 ) 3 M compounds (M = N, P, CH, SiH) in order to determine the molecular equilibrium conformation. In agreement with available experimental data for the amine, the phosphine and the silane, the theoretical energy minimum is found for the (60, 60, 60) - so-called LEM - conformation in which each methyl group is staggered with respect to the two opposite (MC) bonds. The same LEM conformation is predicted to be theoretically preferred for the isobutane molecule, this result being very sensitive to the “C 3V or not” quality of the methyl groups. The values of optimized CMC angles, rotational barriers and dipole moments are well reproduced.

Theoretical conformational analysis of lewis adducts: II. CNDO/2 Calculations versus microwave data for methylphosphine borane, (CH3)H2P·BH3

Abstract The electronic structure and preferred conformations of (CH 3 )H 2 P · BH 3 are investigated within the framework of the CNDO/2 approximation. In complete agreement with microwave data, the (180, 180) conformation. Fig. 1, is predicted to be the only stable conformation. The barriers to internal rotation along the (P-C) and the (P-B) bonds are 3.3 and 3.4 kcal/mole, respectively. Bicentric energy partitioning shows that the variations of the total energy are mainly reflected by variations of the spatial interaction energy between phosphorus and the six hydrogens of the CH 3 and BH 3 groups.

Analyse conformationnelle et structure électronique de la phosphinodifluorophosphine, PH2-PF2

Abstract The electronic structure and conformational analysis of H 2 P-PF 2 are investigated within the framework of the CNDO/2 approximation. Preferred gauche (θ = 65°) and trans (θ = 180°) conformations are predicted, the latter being the only actual one (ΔG° = 4.5 kcal mole −1 ), in perfect agreement with microwave data. Bicentric energy partitioning shows that the variations of the total energy

Analyse conformationnelle theorique des complexes acide-base de lewis: III. Une approche semi-empirique (méthode CNDO/2) des geométries et de l'analyse conformationnelle du borazane et de ses dérives perfluoré

Abstract The optimized geometries and theoretical data from conformational analysis cf borazane and its perfluoro derivatives are investigated within the framework of the CNDO/2 approximation. The effect that fluorination has on the geometrical and electronic parameters of the investigated series of compounds is pointed out and the authors offer an explanation for the anomalous behaviour of H 3 N.BF 3 . Furthermore, it is shown that the assumption of tetragonal hybridization for the N and B atoms of any borazane is incorrect if the F 3 N molecule is used as the Lewis base.

E´tude the´orique de la conformation de la 1,1-dime´thylhydrazine et du complexe (CH3)2N-NH2 · BF3

Abstract CNDO/2 calculations are performed in order to determine the preferred conformations of 1,1-dimethylhydrazine and 1,1-dimethylhydrazine-boron trifluoride complex. In each case, the calculations reveal the existence of an equilibrium between gauche and trans forms (25 % of trans form in the free molecule, 65 % in the complex). In agreement with experiment, the results confirm that BF 3 is attached to the nitrogen atom bearing the methyl groups.

Interprétation quantique des propriétés physicochimiques des alkylamines

Abstract A large set of physicochemical properties of alkylamines is investigated within the framework of the CNDO/2 approximation on the basis of a choice of a standard rigid geometrical model. The preferred theoretical conformations obtained in this way are identical to the experimental ones, when known. The trend of the barriers to rotation and inversion in methylamines is perfectly reproduced as well as the variation of dipole moments and ionization potentials. Finally, the degrees of acidity and basicity in the gas phase (as provided by ion cyclotron resonance) are also reproduced. It is clearly shown that, under such conditions, the CNDO/2 method should be used in concert with the classical physical methods by any chemist, even an experimentalist, who wishes to gain an understanding of the behaviour of electronically localized systems.

Analyse conformationnelle de l'aminodifluorophosphine H2N-PF2 et de la diméthylaminodifluorophosphine (CH3)2N-PF2: Une analyse critique des données expérimentales

Abstract The inconsistency which was recently observed in the conformational analysis experimental data on H 2 N-PF 2 and (CH 3 ) 2 N-PF 2 molecules is investigated within the framework of the CNDO/2 approximation. The X-ray determination made by Nordman and Morris on the latter molecule seems to be the most realistic. However, it was impossible to select the actual set of experimental parameters amongst the various works dealing with the conformational analysis of H 2 N-PF 2 . It seems that, in such conditions, a semi-empirical approach like the CNDO/2 procedure could be considered as a method of solving problems of conformational analysis in highly localized systems.

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.

Theoretical analysis of the structure of monocyclic oxocarbon dianions [CnOn]−2 (n = 3, 4, 5, 6)

Abstract The geometry and electronic structure of monocyclic oxocarbon dianions [C n O n ] −2 are studied on the basis of MINDO/2 wave functions. The optimized CC and CO bond lengths are in very good agreement with available X-ray crystallographic data. The calculations allow an encouraging interpretation of the differences observed between the vibrational spectra of squarate and croconate dianions. The theoretical ultraviolet spectrum (CNDO-CI method) reproduces quantitatively the experimental position and intensity of the first absorption band.