Carbene Formation in Its Lower Singlet State from Photoexcited 3H-Diazirine or Diazomethane. A Combined CASPT2 and ab Initio Direct Dynamics Trajectory Study

Abstract

The potential energy surfaces of the ground and valence excited states of both 3H-diazirine and diazomethane have been studied computationally by mean of the CASSCF method in conjunction with the cc-pVTZ basis set. The energies of the critical points found on such surfaces have been recomputed at the CASPT2/cc-pVTZ level. Additionally, ab initio direct dynamic trajectory calculations have been carried out on the S(1) and S(2) surfaces, starting each trajectory run at the region dominated by the conformational molecular rearrangement of diazomethane. It is found that both isomers are interconnected along a C(s)() reaction coordinate on each potential surface. Radiationless deactivation of the corresponding S(1) state of each isomer occurs through the same point on the surface, an S(1)/S(0) conical intersection. Thereafter, the system has enough energy to surmount the barrier which leads to dissociation products (CH(2) + N(2)) on S(0) state. Therefore, photoexcitation to S(1) state of either diazirine of diazomethane produces methylene in its lower singlet state on a very short time scale (ca. 100 fs). Furthermore, both isomers can generate excited singlet carbene when they are excited onto the S(2) surface; in this case, they lose the activation energy passing through another common S(2)/S(1) conical intersection and then proceed to dissociation into carbene and N(2) on the S(1) surface. For the special case of methylene, it rapidly experiences deexcitation to S(0) state.

Valence Isomerization between Diazo Compounds and Diazirines

“…A significant number of calculations utilizing different approaches have been performed to explain the formation or lack thereof of these or other intermediates during thermal and photochemical isomerization. [20,87,98,102,106,115,121] However, the following phrases still apply often enough: "... there are still controversies about the photolytic and thermolytic reaction mechanisms of diazirines ...", [121e] "... experimental information has never been successfully rationalized from a mechanistic point of view", [121j] "... which is an artefact of the calculations…", [20] "These complexities have made a comprehensive understanding of diazirine photochemistry an elusive goal", [122] "... no rules exist to predict the relative efficiency of the formation of the diazo intermediate competes with the production of carbene". [67] Nevertheless, considerable progress in chemical theory should form a good basis for further discoveries of new fundamental principles of mutual isomerization between diazirines and the diazo compounds as well as their use.…”

“…The computed barrier energy heights clearly testify that methylene formation should be the priority process with respect to isomerization, [20] however this prediction needs to be checked.…”

Photoinduced Ultrafast Wolff Rearrangement: A Non‐Adiabatic Dynamics Perspective

“…[1][2][3][4][5][6][7][8] Experimentally, both concerted and stepwise mechanisms have been proposed. [1][2][3][4][5][6][7][8][9][10][11][12] On the theoretical side, previous electronic structure calculations [13,14] have focused on diazo compounds that do not undergo the Wolff rearrangement. Recently, Blancafort and co-workers [15,16] simulated the photochemical Wolff rearrangement in 2-diazo-1-naphthoquinone (DNQ) using high-level electronic structure calculations and quantum dynamics based on a fitted threedimensional two-state model surface.…”

Photoinduced Ultrafast Wolff Rearrangement: A Non‐Adiabatic Dynamics Perspective

“…Experimentally, both concerted and stepwise mechanisms have been proposed 1–12. On the theoretical side, previous electronic structure calculations13, 14 have focused on diazo compounds that do not undergo the Wolff rearrangement. Recently, Blancafort and co‐workers15, 16 simulated the photochemical Wolff rearrangement in 2‐diazo‐1‐naphthoquinone (DNQ) using high‐level electronic structure calculations and quantum dynamics based on a fitted three‐dimensional two‐state model surface.…”