ortho-Phenylene-bisnitrene (2) and -carbenonitrene (3) were generated in an inert matrix at low temperature, characterized by IR spectroscopy, and identified with the help of DFT calculations. Their thermal and photochemical reactivity was examined in the matrix, and it was found that both 2 and 3 ring-open to 1,4-substituted butadienes. Calculations (DFT and MCSCF) on the lowest singlet, triplet, and quintet states of 2, 3, and the all-carbon analogue ortho-phenylene-biscarbene (4) are also reported. All three species are found to have singlet ground states. The S−T splitting is small (2−3 kcal mol-1), but the quintet states lie siginificantly higher in energy (20−25 kcal mol-1). The ring-opening reactions of 2−4 (to give 1,4-substituted butadienes) as well as the ring-closure reactions (to give benzocyclobutadiene derivatives) were also investigated computationally. The calculated barriers for the ring-opening reactions are in reasonable agreement with the experimentally obtained activation energies for 2 and 3. Calculations also suggest that 4, unlike its heteroanalogues 2 and 3, has a stronger preference for the ring-closure reaction (to form benzocyclobutadiene) rather than for the ring-opening reaction.

Thermal reactivity of the ortho isomers of the titled compounds has been investigated by using the fulloptimized reaction space multiconfiguration self-consistent field (MCSCF) method with the STO-3G and 6-31G(d) basis sets, followed by multireference second-order Moller-Plesset perturbation calculation (MCSCF+MP2). The MCSCF+MP2 calculations lead to the prediction that all the ortho isomers have a singlet ground state but the lowest triplet state lies above the singlet ground state by less than 3 kcal/mol. Fortunately, since the spin-orbit coupling between the two states of interest is small and the singlet ground state is more likely to be produced in the photolysis, the thermal behavior of the isomers has been analyzed on the singlet energy surface. In o-phenylenebis(methylene), the E,E stereoisomer undergoes ring-closure reaction with no energy barrier to give a bicyclic compound, while the Z,Z isomer exhibits a situation of competition between the ring-opening reaction and the CH inversion followed by the ring-closure reaction. In (o-nitrenophenyl)methylene, the E isomer exhibits a competitive feature between the ring-opening and ring-closure reactions, whereas the Z isomer reacts into a linear compound with no energy barrier. In o-phenylenebis(nitrene), a spontaneous ring-opening reaction takes place definitely with the formation of a linear compound. These calculated results are in good agreement with the available experimental evidences. Methods of CalculationRing-Closure Reactions. Since static electron correlation strongly affects the relative stability of diradical systems, the full-optimized reaction space (FORS) MCSCF method 17 with the STO-3G basis set proposed by Pople et al. [18][19] was employed for the qualitative study on the ring-closure reactions of CC, CN, and NN systems. The MCSCF active spaces include all valence π orbitals and a 2pσ (lone-pair) orbital of carbene and two 2pσ (lone-pair) orbitals of nitrene. Accordingly, the MCSCF active space includes 10 electrons and 10 orbitals for the CC system, 12 electrons and 11 orbitals for the CN system, and 14 electrons and 12 orbitals for the NN system. All geometries were assumed to be planar and optimized at this computational level. The relative energies of the stationary structures were re-estimated by using a larger basis set (6-31G-(d)) 20-22 and the MCSCF method with the same active space, followed by multireference second-order Moller-Plesset perturbation calculation (MCSCF+MP2). 23 Ring-Opening Reactions. The MCSCF calculations were also carried out on the ring-opening reactions of the three systems. The MCSCF active spaces include the bonding and antibonding σ orbitals of the C 1 -C 6 bond (see Figure 1) and the bonding and antibonding π orbitals of substituents connected with a benzene nucleus, as well as a 2pσ (lone-pair) orbital of