Single-crystal electron paramagnetic resonance studies of the action of red light on the needle form of humulene nitrosite, C15H24N2O3: the first nitroxide radical formed in the photochemical reaction

E.p.r. studies of the first radical that is produced when a single crystal of the platelet form of humulene nitrosite is irradiated with red light enable the tensor components of its spin-Hamiltonian to be evaluated, and the principal directions of its g, A(14N), and A(1H) tensors to be orientated with respect to the crystallographic axes of the monoclinic unit cell. The radical is a monoalkyl nitroxide to which the structure (2) is assigned: it is a geometric isomer of the corresponding cycloundecatriene derivative that is obtained when the needle form of humulene nitrosite is red-irradiated. Superimposition of relatively minor librational motions in the solid, causes the principal directions within its different nitroxides to become effectively parallel, so that the two crystallographically distinguishable molecules of (2) in the unit cell can not be distinguished by e.p.r. methods at 293 K.