| Abstract
| - Computations find that o-phenylene(halo)carbenonitrenes 2-XN, X = F, Cl, Br, have quinoidal singletbiradical ground states such as the parent o-phenylenecarbenonitrene (2-HN). Compared to theparent 2-HN, halogen substitution stabilizes the A‘ ‘ states relative to the A‘ ones. Halogensubstitution also affects the barrier and exothermicity of the ring-opening reaction (to formunsaturated nitriles 4-XN, X = F, Cl, Br), but it has a smaller effect on the ring-closing reaction(to form benzo(aza)cyclobutadiene 3-XN, X = F, Cl, Br). Attempts to generate and observe theo-phenylene(halo)carbenonitrenes 2-XN, X = F, Cl, Br, using matrix isolation spectroscopy underconditions similar to those of the successful observation of 2-HN failed. Instead, the observedphotoproducts were a mixture of 3-XN and 4-XN. In each case, the major product of the mixtureappears to be the thermodynamically more stable one. In the case of X = Br, the observed mixturecontains an additional component that is postulated to be Z-6-BrN. o-Phenylenechlorocarbenocarbene is also computed to have a quinoidal singlet biradical ground state and relatively stabilizedA‘ ‘ excited states. Attempts to generate the biscarbene under matrix isolation conditions led to thedetection of benzochlorocyclobutadiene (3-ClC), small amounts of the ring-open product (dienediyne4-ClC), and cycloalkyne 5-ClC. Computations suggest that the formation of 5-ClC implies thegeneration of Z-6-ClC, which is analogous to the formation of Z-6-BrN from 2-BrN.
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