The previously discussed principles involved in electrocyclic reactions of openshell ions are exemplified for a type C system, the radical cation couple quadricyclane+ (Q+)/norbornadiene+ (NBD+). Exhaustive calculations (MINDO/3) for the various states of the ions support the general predictions for such types of systems. The barrier of interconversion Q+ → NBD+ is estimated to be significantly lower than that for the corresponding neutrals. Experimental support to this prediction is obtained by γ‐irradiation of Q or NBD in an electron scavenging matrix. In both cases only NBD+ is observed, indicating that Q+ is not stable at 77 K. Previous observations and propositions referring to the system Q+/NBD+ are discussed in view of the present findings.

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SummaryElectrocyclic reactions connecting radical cations are compared with those of the parent neutrals. Principles governing over-all reactivity and stereoselectivity of the open-shell processes are discussed.

Summary?-Irradiation of tris (methy1idene)-cyclopropane ("31radialene') 1 in a rigid electron scavenging matrix (butylchloride/i-pentane, 1 : 1) at 77 K leads to formation of its molecular cation 1 ' . Slight softening of the matrix by a temperature increase of 3-5 K results in formation of a newly absorbing species, tentatively assigned as arising from structural relaxation of 1 by x-bond rotation: 1'Introduction. -A UV.-photoelectron (PE.) spectrum of a neutral closed shell molecule M can be considered as the state diagram of its radical cation M' in which the difference between I,(n> 1) and I, (I=ionization energy) corresponds to the energy of an electronically excited state of M' relative to its ground state. Another way to obtain such a state diagram consists in recording the electronic spectrum of M t , or, in principle, the emission spectrum of excited M'. At first sight one might suspect that such studies would yield only redundant information, but a closer

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