On irradiation in solution with ultraviolet (UV) light, cis ‐stilbene undergoes reversible photocyclization to give trans ‐4a,4b‐dihydrophenanthrene, an intermediate that can be trapped oxidatively with hydrogen acceptors such as iodine, oxygen, or tetracyanoethylene to give phenanthrene in high yield. This type of photoreaction occurs with a wide range of substituted stilbenes and related molecules, including various polycyclic and heterocyclic analogs; in addition, certain systems with a single heteroatom (nitrogen, oxygen, or sulfur) in place of the central π bond undergo photocyclization, creating a new five‐membered heterocyclic ring. In some of these photocyclizing systems, the transient dihydroaromatic intermediates are trapped without added oxidants by the elimination of suitable leaving groups or by various hydrogen shifts. Photocyclization is the preferred method for the synthesis of many different polynuclear aromatic systems. The discovery and early development of stilbene photocyclization has been surveyed, and several other general reviews have appeared.

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The 19 F 19 F nuclear spin-spin coupling constants J FF for a set of eighteen compounds related structurally to 1,8-difluoronaphthalene were measured by 19 F NMR spectroscopy. The FF distances d FF in these compounds were determined by ab initio 3-21G* molecular orbital calculations. Consistent with the lone-pair overlap theory of the origins of through-space 19 F 19 F coupling, an exponential relationship is found between J FF and d FF (regression coefficient r 2 ) 0.991), and a linear relationship is found between J FF and the extent of the overlap interaction between the in-plane fluorine 2p lone-pair orbitals (regression coefficient r 2 ) 0.993). The magnitudes of these lone-pair interactions were estimated from molecular orbital energies obtained by ab initio 6-31G* calculations for a model consisting of a pair of HF molecules separated by various distances.

A theoretical description of the mechanism of "through-space" fluorine-fluorine nuclear spin-spin coupling is presented, based on the concept that two lone-pair orbitals from spatially proximate fluorines can interact by direct overlap to form one bonding and one antibonding molecular orbital delocalized over both fluorines. Positive coupling constants are explained in terms of spin-correlation effects (Hund's rule behavior) of the electrons in these two-center molecular orbitals and Fermi contact interactions of the spin-polarized fluorine core electrons. The pronounced dependence of the magnitude of this type of coupling on the distance between the two fluorines is accounted for in terms of the Pauli exclusion principle and the extent to which the bonding and antibonding orbitals differ in their electron density distributions.he existence of a "through-space" mode of spin-