3J(HH)?Pμ,ν correlation for planar 7-membered cyclic π-systems—structural effects on 3J(HH)

It is shown that sterically unperturbed vicinal HH coupling constants in planar 7-membered π-systems correlate linearly with the HMO π-bond order: ³J(HH) = 20.91P_μ,_ν–3.85 (r.m.s. error 0.26 Hz, correlation coefficient =0.988). Systematic deviations from this relationship which most probably originate from valence angle changes are found for fused π-systems containing rings of different size. Model calculations using the CNDO/2 method as well as finite perturbation theory and INDO wave functions support the experimental findings. An improvement of existing ³J(HH)? P_μ,_ν correlations for planar 6-membered rings is possible if CNDO/2 π-bond orders are used instead of HMO or PPP-SCF data.     

A Characterization of Intermediate Weyl Coefficients

 In connection with an indefinite analogue of canonical systems of differential equations some subclasses of the Nevanlinna class come up. We give a criterion for a function to belong to in terms of its poles and residues. Received 5 December 2000; in revised form 31 October 2001     

Photoresponsive Formation of an Intermolecular Minimal G‐Quadruplex Motif

The ability of three different bifunctional azobenzene linkers to enable the photoreversible formation of a defined intermolecular two‐tetrad G‐quadruplex upon UV/Vis irradiation was investigated. Circular dichroism and NMR spectroscopic data showed the formation of G‐quadruplexes with K⁺ ions at room temperature in all three cases with the corresponding azobenzene linker in an E conformation. However, only the para–para‐substituted azobenzene derivative enables photoswitching between a nonpolymorphic, stacked, tetramolecular G‐quadruplex and an unstructured state after E–Z isomerization.