Irradiation of 4-diazo-4H-imidazole (1) in an argon matrix at 10 K at a wavelength of λ = 313 nm leads to rapid loss of nitrogen and the formation of 4H-imidazol-4-ylidene (2). Upon photoexcitation of 4H-imidazol-4-ylidene (2) at wavelengths longer than 570 nm, the ring-opened carbene S-6 can
The fact that no kinetic isotope effect was observed when using mixtures of benzene and deuterated benzene led the authors to conclude that 2 is a highly electrophilic singlet carbene, which preferentially attacks the π electrons of benzene in the rate-determining step and not the CϪH bonds. EPR experiments, carried out in methanol at 77 K, showed no EPR signal upon irradiation of 1, indicating that 2 probably has a singlet ground state. [1]
Results and Discussion
Calculations
Ab initio studies performed by Pasto at the MP2 level of theory using a standard 6-31G(d) basis set predicted a triplet ground state for 4H-imidazol-4-ylidene (2), this lying just 0.4 kcal mol Ϫ1 below the first excited singlet state. [2] According to Pasto, the observed reactivity of 2 may be explained in terms of the co-existence of both species in solution. However, more recent calculations by Whitehead et al. at the HF level of theory using a standard 6-31G(d) basis set gave a larger T/S gap, with the triplet state ( 3 AЈЈ) [a]