Verbrückte und unverbrückte norbornyl-kationen in der gasphase. Berechnungen zur stabilität isomerer norbornyl-kationen mit hilfe quantenmechanischer verfahren

The geometry and the relative stability of bicyclic compounds 1–20 have been calculated by standard quantum mechanics methods.MINDO/3 yields the following stability order of isomeric norbornyl cations (relative energies in $\text{kcal}\text{mole}$): 1-norbornyl cation 9 (0.0); 1.7 σ-bridged cation 6 (0.7); 7-norbornyl cation (nonplanar) 7 (1.1); 2-norbornyl cation (classical) 2 (4.2); 7-norbornyl cation (planar) 8 (4.3); 2-norbornyl cation (bridged) 1 (6.1). The stability of the same ions calculated by ab initio methods (STO-3G, MINDO/3-geometry) leads to an order more nearly consistent with experimental results: 2-norbornyl cation (classical) 2 (0.0); 2-norbornyl cation (bridged) 1 (5.9); 7-norbornyl cation (planar) 8 (11.1); 1-norbornyl cation 9 (14.6); 7-norbornyl cation (nonplanar) 7 (21.2). For the secondary 7-norbornyl cation, MINDO/3 gives a pyramidal configuration, 3.2 $\text{kcal}\text{mole}$ more stable than the planar form. In contrast, the ab initio results (complete optimization of all geometrical parameters) indicate the planar cation to be the most stable form. The bridged structure of 2-norbornyl cation 1 is calculated (STO-3G, partly optimized) to be 4.3 $\text{kcal}\text{mole}$ less stable than the classical counterpart, 2. For the lower homologues 12 and 13 (STO-3G, complete geometry optimization), this difference is 6.4 $\text{kcal}\text{mole}$. However, more extended basis sets should favour the bridged structures. The hydrogen bridged norbornyl cations 3, 4, and 5 have been calculated (STO-3G, partly optimized) to be 14.4, 23.6 and 29.9 $\text{kcal}\text{mole}$ less stable than 2. The stability differences between the corresponding tertiary bicyclic ions 10 vs 11, and 14 vs 15 are calculated (ab initio) to be 15.3 and 19.0 kcal/mole, respectively, in favour of classical structures. The influence of methyl substitution at positions C₁ and C₆ (exo) on bridged and unbridged structure of 2-norbornyl cation is calculated. Pyramidal secondary and tertiary 2-norbornyl cations 19 (a; R=H, b; R=CH₃) and 20 (a; R=H, b; r=CH₃) have been used to model the electrical effects in the solvolysis transition states of epimeric 2-norbornyl esters. Due to more efficient hyperconjugation the pyramidal exo cation is stabilized more than the endo cation by 5.2 $\text{kcal}\text{mole}$ for the secondary series and 3.5 $\text{kcal}\text{mole}$ for the tertiary series. Bonding of endo cation 20 with a nucleophile should be stronger than bonding of exo cation 19 due to more efficient HOMO-LUMO interaction.     

Recent improvements of the ion analyzer and typical examples of applications

Improvements of a CAMECA IMS 300 ion microanalyzer are described: ion source, resolution, ion detector and counting system. Comparison with the performances of an electron microprobe is presented. Examples of application are given.     

Effect of non-planarity on the spin distribution in the radical anions containing the stilbene or the azobenzene π-system: An ESR and ENDOR Study

The radical anions of 5H-dibenzo[a,d]cycloheptene ( 9 ), 5 H-dibenzo[c,f][1,2]diazepine ( 10 ), 5,6-dihydrodibenzo[a,e]cyclooctene ( 11 ), 5,6-dihydrodibenzo[c,g][1,2]diazocine ( 12 ), and (E)-2,2,5,5-tetramethyl-3,4-diphenylhex-3-ene ( 13 ) were characterized by ESR and ENDOR spectroscopy. Their hyperfine data were compared with those previously reported for radical anions also containing the stilbene or the azobenzene π-system. Whereas the π-spin distribution in the radical anions of the stilbene series is only moderately sensitive to deviations of the π-system from planarity, the radical anions of the azobenzene series respond to steric strain by shifting the π-spin population from the benzene rings to the azo group. This finding is impressively demonstrated by the similar hyperfine data for 9 ^{· ?} and 11 ^{· ?} which contrast with the strongly highly hindered 13 ^{· ?}. A. plausible interpretation is readily provided by the electron affinities of the constituent π-moieties in stilbene and azobenzene. While those of benzene and ethene are both comparatively low, the azo group has a considerably higher electron affinity.     

Polarizability effects and dispersion interactions in alkene-Br2 pi-complexes

Weakly bound molecular complexes play an important role in chemistry, physics, and biodisciplines. The preequilibrium pi-complexes of various alkenes with bromine have been examined quantitatively, and a direct relationship between association constants (KF) of these pi-complexes and polarizability of the olefins was found. The stability of the Br2-olefin pi complexes is affected by both the donor ionization potential and the polarizability of the olefin, and an equation able to take into account both effects is proposed.     

Laser fragmentation in liquid medium: a new way for the synthesis of PbTe nanoparticles

Synthesis of PbTe nanoparticles has been performed by pulsed laser fragmentation of PbTe micron-sized powders in distilled water with a Nd:YAG laser. The influence of various experimental parameters (wavelength, treatment duration, and output energy) on the yield of fragmentation, the size and the crystallographic structure of the nanoparticles produced has been investigated. Characterization of the nanopowders was performed by X-ray diffraction analyses and transmission electron microscopy observations. Thanks to a careful control of the synthesis parameters, PbTe nanoparticles with an average diameter close to 6 nm and exhibiting a sharp distribution in size have been produced.     

Band-pass filtered velocity statistics in decaying turbulent box

For homogeneous isotropic turbulence study, the acquisition of band-pass filtered velocity increments (FVI) in a non-forced turbulent box is still a challenge both experimentally and numerically. Turbulence and associated physical processes, at a given instant, are permanently contaminated by a forcing process which can seldom be universal. The situation tends to be the origin of intermittency and the non-Gaussian probability density distribution for acceleration and velocity gradients. To reveal implied mechanism, grid turbulence is adapted to observe non-perturbed homogeneous isotropic turbulence. The velocity increments (VI) can be obtained following Comte-Bellot and Corrsin (GCBC) by means of two point-two time shifted velocity measurements. It is difficult to obtain decaying turbulence (DT) at large turbulent Reynolds number without pollution coming from walls. Nevertheless it is also significant to investigate DT in low Reynolds number regimes to determine non-polluted tendencies. The similarity of DT between particle image velocimetry (PIV) and hot wire anemometry measurements by GCBC are presented. Here we focus our tendency on VI and FVI probability density function (PDF) shapes in this letter. In conclusion, the tendency to Gaussian shape in inertial zone wavenumbers, demonstrates that there will be no intermittency if turbulent cascade is not perturbed.