p-phenylenecarbenonitrene and its halogen derivatives: How does resonance interaction between a nitrene and a carbene center affect the overall electronic configuration?

A series of para-conjugatively coupled phenylenecarbenonitrenes {(4-nitreno phenyl)methylene (3a), (4-nitrenophenyl)fluoromethylene (3b), (4-nitrenophe nyl)chloromethylene (3c), and (4-nitrenophenyl)bromo-methylene (3d)} were g enerated in argon matrix at low temperature (10 or 13 K) and characterized by IR and UV/vis spectroscopy. Density functional theory (B3LYP/6-31G(d)) a nd ab initio (MCSCF, CASPT2) methods were used to study the ground- and som e low-lying excited states of 3a-d. The experimental and computational data suggest that 3a-d have singlet ground states (S-0) and can be thought of a s*quinonoidal biradicals: In all cases, the lowest triplet (T-1) and quinte t (Q(1)) states lie about 2 kcal mol(-1) and 28-29 kcal mol(-1), respective ly, higher in energy than S-0. On the other hand the substituent is found t o have a significant effect on the relative energy of the second excited tr iplet (T-2) state. This state tends to become relatively more stable as the ability of the substituent to enforce a closed-shell configuration at the carbene subunit increases, Interestingly, the energy difference between the T-2 and S-0 states in 3a-d is found to depend linearly on the S-T gap of t he corresponding phenylcarbenes 7a-d. This relationship is; helpful in pred icting when a substituted p-phenylenecarbenonitrene may have a triplet grou nd state instead of a singlet one.