In this work, an algorithm was developed to study the potential energy surfaces in the coordinate spaces of molecules by a nonlocal way, in contrast to classic energy minimizers as the BFGS or the DFP method. This algorithm, based on the specificities of semiempirical methods, mixes simulated annealing and local searches to reduce computation costs. By this technique, the global energy minimum can be localized. Moreover, local minima that are close in energy to the global minimum are also obtained. If the search is not only for minima but for all stationary points (minima, saddle points.. . ), then the energy is replaced by the gradient norm, which reaches its minimum values at stationary points. The annealing process is stopped before having accurately reached the global minimum and generates a list of geometries whose energies (respectively, whose gradients) are optimized by local minimizers. This list of geometries is shortened from the nearly equivalent geometries by a dynamic single-clustering analysis. The energy/gradient local minimizers act on the clustered list to produce a set of minimahtationary points. A targeted search of these points and reduction of the costs are reached by the way of several penalty functions. They eliminatewithout energy calculation-most of the points generated by random walks on the potential energy surface. These penalty functions (on the total moment of inertia or on interatomic distances) are specific to the class of problem studied. They account for the nonrupture of either specific chemical bonds or rings in cyclic molecules, they assure that molecular systems are kept bonded, and they avoid the collapsing of atoms. 0 1992 John Wiley & Sons, Inc.
Four semiempirical methods (AM1, MNDO, PM3, and MIND0/3) are used to calculate the deformation angles of [n]paracyclophanes and their Dewar benzene isomers for n = 3 . . . 10. The results obtained by all these methods are in good agreement with data from X-ray studies. We have determined the strain energies that, in both series of compounds, are due to two components: (1) the strain energy of deformation of the cycle (aromatic or Dewar benzene skeletons) and (2) the strain energy of the oligomethylene chain. In (61paracy-clophane, the strain energy [SEring(MNDO) = 32.9 kcal/mol] almost compensates the resonance energy (Eresonance = 36 kcal/mol) so that its chemical properties are closer to alkenes than to benzenic compounds. To better reproduce the enthalpy of the valence isomerization [n]Dewar benzene + [nlparacyclophane, which is poorly calculated with these methods, a correction is proposed and the reaction enthalpy of [6]paracyclophane is estimated to be about AH, = 15 +-15 kcal/mol. It is found that MNDO and MIND0/3 need the smallest corrections, but MNDO leads to better geometries than MIND0/3. In conclusion, MNDO seems to be the best technique for further studies of these compounds.
Activation energyIrradiation in the wavelength range of h = 270 -320 nm of
8,9-bis(N-cyclohexylcarbamoyloxymethyl)-[6]paracyclophane(1 b) in dilute fluid solutions produces the 1,4-Dewar isomer 2b in a quantum yield of 2.8% while the quantum yield of the rearomatization is 19% in heptane at room temperature. A photostationary equilibrium may be attained which depends strongly on the wavelength of irradiation. The aromatic system of anthracene may be deformed from planarity by introducing steric strain by large substituents in position 9. This deformation has a great influence on the photochemical properties of these molecules. For example, irradiation of 9-tert-butylanthracene results in the formation of its 9,lO-Dewar isomer, while e. g. 9-trimethylsilylanthracene and 9-methylanthracene are photochemically stable at infinite dilution2). An alternative way of deforming a benzene ring is by bridging its two opposite positions with an n-membered methylene chain. Synthesis, structure and chemical reactivity of these [nlparacyclophanes constitute a very active field of research3).The angles of deformation a (Scheme 1) are known to increase with decreasing length of the chain. Thus, for fn3-paracyclophanes with long methylene chains (n 2 7) the aromatic isomer is thermodynamically more stable than the Dewar isomer, while for a short chain the equilibrium is Scheme 1
Das Nitrat ist sowohl in Wasser ah auch in Alkohol leicht 10s-Ech. Wird die waBrige -LZisung rnit Eisenchlorid versetzt, so erhiilt man beim Stehenlassen der Losung federartige, zu Buscheln vereiuigte Krystalle. Schwefelsaure gibt ein kleinkrystdinisches Pulver von hellgriiner Farbe. Pyridin und Bromwasserstoff verandern die Nitratlosung nicht. Zur Darstellung des Jodids wird die LBsung des Nitrats mit festem Jodkalium versetzt, wobei das Jodid als griines, glhzendes, in kleinen Bliittchen krystallisierendes Salz ausfiillt. Es lBst sich leicht in Wasser, Aceton, Alkohol und Methylalkohol, ist aber unl6slich in Essigkther, Chloroform, Li-grOib etc. Um es in mohlausgebildeten Krystallen zu erhalten, iiberschichtet man es mit Essigiither und versetzt dann tropfenweise mit der zur LBsung notwendigen Menge Methylalkohol. Beim, Verdunsten krystallisiert es in kleinen, seidenglhzenden Schiippchen von hellgriiner Farbe. Die Analyse des lufttrocknen Salzes ergab folgende Resultate: 0.0950 g Sbst.: 0.0299 g CrsOs. -0.1022 g Sbst.: 0.0322 g CrsOa.-0.1195 g Sbst.: 0.0379 g Ag J. -0.1234 g Sbst.: 0.0401 g AgJ. -0.1023 g Sbst.: 0.0737 g COI. -0.1257 g Sbst.: 0.0865 g COs. -0.1034 g Sbst.: 1.93 ccm N (20°, 724 mm).
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