The location of energy minima on the conformational energy surface of molecules by computational methods (conformational searching) continues to play a key role in computer-assisted molecular modeling. Although a number of conformational search procedures have been devised over the past several years, new more efficient methods are urgently needed if molecules with increased complexity are to be treated in a quantitative manner. In this paper we describe a method, termed low-mode search (LMOD), which is based on eigenvector following (or mode following), for the exhaustive exploration of the potential energy hypersurface of molecules. It is particularly efficient at searching the conformational space of both cyclic and acyclic molecules, and we describe its effectiveness for a number of conformational search problems including acyclic, monocyclic, and bicyclic hydrocarbons and cyclic pentapeptides. No special treatment of rings in cyclic molecules is necessary, nor is it necessary to define rotatable bonds. LMOD generates structures “automatically” with minimum input from the user. We demonstrate that LMOD is one of the most efficient procedures yet devised for conformational searching of small- to medium-sized molecules.

Ionic transport in 0.2[XNa 2 OÁ(1ÀX)Rb 2 O]Á0.8B 2 O 3 mixed-alkali glasses A ´rpa ´d W. Imre, Stephan Voss and Helmut Mehrer 3225 Diffusion of calcium and barium in alkali alkaline-earth silicate glasses

A range of 6-oxoverdazyls 2b–h and 6-thioxoverdazyls 3a–c and 3e–h has been directly prepared by dehydrogenation of the corresponding 1, 4, 5, 6-tetrahydro-1, 2, 4, 5-tetrazin-3(2H)-ones 5b–h and thiones 7a–c and 7e–hwith lead dioxide, potassium hexacyanoferrate (III), or bis(4-methylphenyl)-aminyl. X-Ray analyses reveal a nearly planar verdazyl framework for the oxoverdazyl 2a, whereas the thioxoverdazyl 3ftakes on a flat boat conformation. In the latter, owing to the bulky sulfur the N-phenyl groups are considerably twisted out of the plane of the verdazyl ring. Electronic absorption spectra of the deeply coloured radicals exhibit characteristic bands in the visible region with λmax, 1 ranging from 445 to 608 nm. EPR, ENDOR and 2H NMR studies have led to a complete analysis and full assignment of all hyperfine coupling constants. The π-SOMO of 6-oxo-2 and 6-thioxo-verdazyls 3, having nodes at C(3) and C(6), is mainly confined to the nitrogens of the verdazyl ring. Spin delocalization into the N-phenyl groups is reduced, particularly in the 6-thioxoverdazyls, owing to the large torsion angle about the N–C(phenyl) bond

The rate of charge separation in a series of cyclophane-bridged Zn-porphyrin-quinone systems has been investigated in nonpolar and polar solvents by means of fluorescence upconversion. In all systems with driving forces in the range 0.3-1.3 eV, ultrafast charge separation occurs with a rate constant of (2-5) × 10 12 s -1 . In combination with previous investigations on free-base porphyrin-quinone systems the driving force dependence can be probed from the (slightly endoergic) normal to the moderately inverted region for the rate of charge separation alone. The (more limited) data for charge recombination in these systems are reasonably well reproduced by the same reorganization energies and electronic couplings that result from the analysis of the charge separation. The data allow, for the first time, a satisfactory quantum mechanical analysis of the driving force dependence in porphyrin-quinone systems of the charge separation alone and, consequently, the testing of the assumption on the comparability of charge separation and recombination by experimental means.

No abstract

The molecular structure of kekulene (1) was determined by X‐ray structure analysis. From the bond lengths of 1 a remarkable localisation of aromatic sextets and double bonds is concluded (cf. formula 1b). – The problem of annulenoid versus benzenoid diatropicity in 1 is discussed on the basis of 1H NMR absorptions. These data, in agreement with recent theoretical calculations, support a predominant ring‐current induction in the benzenoid subunits of 1 and rule out a significant contribution of annulenoid structures like 1a. – Absorption and emission spectra of 1 are discussed as are the zero‐field splitting parameters of the excited triplet state of 1.

Kekulene (1), the synthesis of which was recently reported!2 1 , is the first example of a new class of aromatic compounds in which the annellation of six-membered rings leads to a cyclic system enclosing a cavity lined with hydrogen atoms. Compound (1), for which 200 Kekule structures with different arrangements of double and single bonds can be formulated[3J, was of interest with regard to 'IT-electron delocalization and the related problem of the diatropicity in the macrocyclic system for which, as long ago as 1951, different theoretical approaches were shown to lead to contradictory predictions in the specific case of (1]141. Experimentally, the 'H-NMR absorption of the internal hydrogens did not show any evidence for a diatropicity in the macrocyclic system!21, in qualitative agreement with MO calculations of chemical shifts!4.51. In this context, and in connection with some spectroscopic properties of (1), the determination of the molecu-... In the crystal lattice of (1) the molecules are stacked along the b axis with the stacking axis forming an angle of 42.9° with the molecular planes. Neighboring molecules in such a stack have an interplanar distance of 336 pm and are parallel-shifted by 312 pm. The molecular planes of adjacent stacks are inclined by 86° to each other, resulting in the "herringbone pattern" shown in Figure 1 as a side view along the a axis.lar structure of kekulene by X-ray analysis was of special interest. (1) crystallized from pyrene (purified by zone-melting) in a tube sealed under high-vacuum on slow cooling from 450With a mean deviation of carbon atoms from the mean plane through the 48 carbon atoms of only 3 pm and a maximum deviation of 7 pm, (1) has an almost perfectly planar structure. This includes, with deviations of 5 and 10 pm, respectively, even the six internal hydrogens, although the non-bonding distances of 196(2) pm between adjacent hydrogen atoms are unusually short.

Based on X-ray analyses the molecular structures of the electron donor-acceptor C2. 21-and [3.3]paracyclophanes 1,2,4,6,10, and 11 are discussed in terms of steric and electronic effects. The with in correlation to the variation of the strength of the electron Elektron-Donor-Acceptor-Verbindungen, 46'). In the preceding paper') the syntheses of the 4,5,7,8-tetracyano[2.2]paracyclophanes 1 -4, of the 5,6,8,9-tetracyano[3.3]paracyclophanes 5 -7 and of the 6,7,9,10-tetracyano[4.4]paracyclophanes 8 and 9 have been described. In this series of compounds the tetracyanobenzene (TCNB) unit as the common electron acceptor is facing various electron donors of different ionisation potential and with graduated donor-acceptor distances. In the present paper we deal with the X-ray structure analyses of some typical representatives of this series and with investigations concerning charge-transfer (CT) absorptions which we try to correlate with the molecular structures. Molecular Structures of TetracyanoparacyclophanesX-Ray Structure Analysis2': Crystal and data collection parameters for 1, 2, 4, 6, 10, and 11 are listed in Table 1. Intensity data were collected by using graphite-monochromated Mo-K, radiation and applying 0 / 2 0 scan technique. The structures of 1, 4, 6, 10, and 11 were solved by direct methods (MULTAN) and were refined by full-matrix least-squares technique using anisotropic temperature factors for non-hydrogen atoms and isotropic temperature factors for hydrogen atoms. In the case of 2 the solution of the structure was not possible by direct method. Based on a Patterson synthesis and considering the structures of 1 and of pseudoyem-tetramethoxy-[2.2]paracyclophane3) a molecular model was constructed and introduced into the elementary cell. By variation of the x and z components for the centre of the molecule and by variation of the three Euler angles the location and orientation of the molecule of 2 was refined using anisotropic temperature factors for non-hydrogen and isotropic temperature factors for hydrogen atoms. The R values are listed in Table 1 Figure 1 A shows the molecular structure of 1 in a top-view perpendicular to the planes of the aromatic rings. The bond lengths given in this figure do not deviate significantly from normal values with the exception of the central bonds in the bridges which, obviously due to transanular 7c-K repulsion, are considerably elongated. There is only a slight deviation from an ecliptic arrangement of the two rings, the axes through the bridge-head atoms of which form an angle of 4" with each other. In Figure 1 B a side-view of 1 is presented which shows the boat-type deformation of the aromatic rings which is similar to that of C2.2lparacyclo-phane itself4'. The deviation from planarity is somewhat stronger for the acceptor ring whereas the bridge bonds to the bridge-head atoms deviate from the corresponding tnangle planes stronger on the unsubstituted side. The exocyclic bonds on the ring atoms deviate from the sp2 plane of these atoms into the direction towards the...