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In this paper, the author performed two centrifuge model tests at the University of Science and Technology of Hong Kong for two cases where the tunnel was located at Cover over Diameter (C/D) of 1.5 and 3.3 to investigate the passive failure and deformation mechanism due to tunnelling in two-layer soils: sand and clay. The tests were carried out on a 1/100 scale miniature model. The experimental process simulated the progress of the tunnel face with a speed of 0.2 mm/s (equivalent to 15 m per day in practice), displaced to 40 mm and then stopped. Linear variable differential transformers are used to measure the ground transitions during the testing process. Particle image velocimetry technology measures the movement of soil in front of the tunnel face. The load cells were attached to one end of the tunnel face in order to obtain the passive pressure exerted on the tunnel face. As the tunnel face displaces, the ground in front of the tunnel face is shifted forward, while the ground that is far from the surface of the tunnel is pushed outward; the effect on the ground thus causes the ground to emerge, and so form a breakout. The partial failure mechanism in front of the tunnel face is similar to the localised cutting mechanism. When the observation failure mechanisms are idealised by continuous lines, the failure mechanism of the ground in front of the tunnel face is shaped like a funnel.

Les analogies entre certaines fragmentations en spectrometrie de masse et des reactions photochimiques ont deja signalees par de nombreux auteurs. Il parait donc tentant d'appliquer aux fragmentations des methodes qui ont fait leurs preuves en photochimie.La methode des diagrammes de correlation de Salem est presentee et illustree par deux exemples (Norrish I et II). La meme technique est ensuite appliquee a la coupure en α du cation radical acetaldehyde.Les avantages des diagrammes de Salem, leurs limitations, ainsi que leurs differences avec les diagrammes de Woodward-Hoffmann et de Longuet-Higgins/Abrahamson seront brievement passes en revue.

SCF+CI calculations are performed to investigate the potential energy surface associated with the thermal C-C ring opening of oxirane (CH 2 OCH 2 ). Conrotatory, disrotatory and non-synchronous processes are studied, and the results discussed with respect to experimental data (Huisgen and toll.) which indicate a loss of stereoselectivity in the ring opening reactions of some substituted oxirane molecules.

Resume L'influence des substituants alkyles sur l'acidite des alcools et des amines est traitee par une methode perturbationnelle. L'effet d'un substituant alkyle peut se decomposer en deux termes : un terme repulsif qui est plus grand dans les molecules neutres (alcools, amines) que dans les bases conjuguees correspondantes (RO − , RNH − ), un effet attractif qui est important uniquement dans les anions. On montre de plus que, meme si l'on ne tient pas compte des effets steriques, la solvatation augmente l'acidite des molecules non substituees plus que celle des molecules substituees. On peut ainsi rationaliser l'ordre d'acidite en phase gazeuse et en solution. “L'effet inductif” des groupes alkyles est egalement discute. On montre qu'un groupe alkyle peut etre attracteur d'electrons meme attache a un atome electronegatif. Un raisonnement analogue permet de rationaliser les ordres de basicites des alcools et des amines en phase gazeuse et en solution. Le traitement perturbationnel est confirme par des calculs ab initio (STO-3G).

Organic and organometallic haptotropic shifts in cyclopentadienyl compounds CpX are analyzed by studying the motion of an X+ across the face of a C5H5- ring. The ligand in transit, X+, may be H+, CH3+, XH3+ (X = Si, Ge, Sn), Mn(CO)3+, Li+, CUR+, and CH22+ and CH2, the latter serving as models for BR2+, AIR2+ and NR2+, PR2+ as well. The Cp- offers donor orbitals of a + e symmetry. The interactions of the ligand may be dominated by an acceptor orbital of symmetry type a (H+, CH3+) in which case 7' and q2 geometries are favored, or a set of acceptor orbitals of a + e symmetry (MII(CO)~+, Li+, CUR+), the e component of which greatly stabilizes the most symmetrical q5 coordination. The lower the energy of the e acceptor set and the better the overlap with its Cp counterpart, the more are q2 and q5 stabilized relative to 7'. In the case of XH3+ (X = C, Si, Ge, Sn) an e acceptor orbital moves to lower energy as one proceeds down the group, and this is responsible for the decreasing barrier for sigmatropic shifts in CpXH3. The XH2 case, which yields a number of interesting collapse struc- tures, is analyzed in detail for X, a main group center.

Resume It is shown that, given a diene substituted with a donor D and a dienophile substituted with an attractor A. the reaction rate will generally decrease when the addends exchange their substituents, i.e. A going to the diene and D to the dienophile. Donor substituents are more effective in raising an HOMO than an attractor in lowering a LUMO of a conjugated substrate. It follows that for thermal symmetry-allowed cycloadditions between acyclic systems, the greatest rate increase is expected when the donor substituent is put on the compound with the higher HOMO (usually the more conjugated compound) and the attractor on the compound with the lower HOMO. This does not mean “put the donor on the donor and the attractor on the attractor”: butadiene is simultaneously a better donor and a better acceptor than ethylene. Some kinetic effects of bridging groups in cyclic systems are discussed.

Germyl- and silyl-phosphines R3MPR′2(M = Ge, Si) generally add in 1-4 position to α-ethylenic aldehydes and ketones with formation of phosphorus alkenoxy-germanes or -silanes. The α-ethylenic aldehydes always lead to 1-4 Z and E adducts (Z preponderant), α-ethylenic ketones exclusively to the Z isomer. This particular stereochemistry is explained by a mechanism involving initial nucleophilic attack of the metal phosphine on carbon 4, steric hindrance of the reaction intermediate and participation of a 6-center pseudo-cyclic complex. On the contrary 1-2 addition on the carbonyl group of the 9-fluorenone is exculsively observed. The orientation of the additions (1–2 or 1–4) is interpreted in terms of Pearson (HSAB) theory.

The most usual difficulties encountered in the study of frontier-controlled reactions by approximate methods (aromaticity rules, frontier-orbital approximation and correlation diagram) are reviewed.

The »overlap graph«, where vertices represent atomic orbitals and edges, their in-phase or out-of-phase overlaps, suggests that, . in the thermal electrocyclic opening of oxirane, the second oxygen lone pair has little effect on the allowed conrotatory mode but lowers the activation energy of the forbidden disrotatory mode. This effect does not exist in the electrocyclic opening of aziridine, which is one reason why the energy gap between the allowed and forbidden processes should remain substantial. In the course of an aziridine ring opening, the nitrogen's hydrogen may remain in the plane of the heavy atoms or may move out into either of the two half-spaces defined by this plane. Three different disrotatory pathways are thus conceivable. They are compared by a perturbational treatment using simple Hiickel theory. Localization of the transition states on these three reaction paths - i. e. determination of their »early« or »late« character, as measured by the magnitude of the rotation of the two methylene groups, for example - can be made with the help of »natural correlation diagrams«. This provides a simple criterion for predicting the breakdown of the perturbational treatment, when pathways of similar energies are compared: if one transition state is much earlier (later) than the other(s), no safe conclusion can be reached. These qualitative arguments are fully supported by ab initio calculations (ST0-3G + (3 X 3) CI and 4-31G + extensive CI). Substituent effects and possible cases of violation of orbital symmetry requirements are discussed.

Es werden die Gruppen Wasser-Methanol und Ammoniak-Methylamin sowie ihre entsprechenden Anionen gegenubergestellt.