Dehydrophenylnitrenes: Quartet versus doublet states

The geometries and energies of 4-, 3-, and 2-dehydrophenylnitrenes (3, 4, and 5) are investigated using complete active space self-consistent field (CASSCF), multiconfiguration quasi-degenerate second-order perturbation (MCODPT), and internally contracted multiconfiguration-reference configuration interaction (MRCI) theories in conjunction with a correlation consistent triple-zeta basis set. 4-Dehydrophenylnitrene 3 has a quartet ground state ((4)A(2)). The adiabatic excitation energies to the (2)A(2), B-2(2) (2)A(1), and B-2(1) states are 5, 21, 34, and 62 kcal mol(-1), respectively. The B-2(2) state has pronounced closed-shell carbene/iminyl radical character, while the lowest-energy B-2(1) state is a combination of a planar allene and a 2-iminylpropa-1,3-diyl. The MCQDPT treatment overestimates the excitation energy to B-2(2) significantly as compared to CASSCF and MRCI+Q. Among quartet states, (4)A(2)-3 is the most stable one, while those of 4 and 5 (both (4)A) are 3 and 1 kcal mol-1 higher in energy. 5 also has a quartet ground state and a (2)A state 7 kcal mol(-1) higher in energy. On the other hand, the doublet-quartet energy splitting is -6 kcal mol(-1) for 4 in favor of the doublet state ((2)A). Hence, (2)A-4 is the most stable dehydrophenylnitrene, 3.5 kcal mol(-1) below (4)A(2) of 3. The geometry of (2)A-4 shows the characteristic features of through-bond interaction between the in-plane molecular orbitals at N and at C3. The (2)A' state of 4 resembles the (2)A(1) state of 3 and lies 32 kcal mol(-1) above (4)A-4. The lowest-energy (2)A' state of 5, on the other hand, resembles the B-2(2) state of 3 and lies 22 kcal mol(-1) above (4)A-5.