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 (MCQDPT), and internally contracted multiconfiguration−reference configuration interaction (MRCI) theories in conjunction with a correlation consistent triple-ζ basis set. 4-Dehydrophenylnitrene 3 has a quartet ground state (⁴A₂). The adiabatic excitation energies to the ²A₂, ²B₂, ²A₁, and ²B₁ states are 5, 21, 34, and 62 kcal mol^-1, respectively. The ²B₂ state has pronounced closed-shell carbene/iminyl radical character, while the lowest-energy ²B₁ state is a combination of a planar allene and a 2-iminylpropa-1,3-diyl. The MCQDPT treatment overestimates the excitation energy to ²B₂ significantly as compared to CASSCF and MRCI+Q. Among quartet states, ⁴A₂-3 is the most stable one, while those of 4 and 5 (both ⁴A‘ ‘) are 3 and 1 kcal mol^-1 higher in energy. 5 also has a quartet ground state and a ²A‘ ‘ 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 (²A‘ ‘). Hence, ²A‘ ‘-4 is the most stable dehydrophenylnitrene, 3.5 kcal mol^-1 below ⁴A₂ of 3. The geometry of ²A‘ ‘-4 shows the characteristic features of through-bond interaction between the in-plane molecular orbitals at N and at C3. The ²A‘ state of 4 resembles the ²A₁ state of 3 and lies 32 kcal mol^-1 above ⁴A‘ ‘-4. The lowest-energy ²A‘ state of 5, on the other hand, resembles the ²B₂ state of 3 and lies 22 kcal mol^-1 above ⁴A‘ ‘-5.