Theoretical Investigation On Photochemistry Of5-diazo Meldrum’s Acid: Wolff Rearrangement And Isomerization

5-Diazo meldrum’s acid (DMA) as a kind of a-diazocarbonyl compound plays animportant role in organic synthesis, drug delivery, photolithography, biochemistry, andmany other industrial fields. DMA undergoes photo-induced Wolff rearrangement (WR)and is widely used as photoresists in lithography.The ground-state and excited-state potential energy surfaces (PES) of DMA have beeninvestigated at the MS-CASPT2//CASSCF level to assign the wavelength dependentphotochemistry. The PES is featured with a conical intersection seam between the groundstate (S0) and the first excited state (S1), which plays the key role in the internal conversion,the WR, and the isomerization reaction.Our MS-CASPT2//CASSCF calculations reveal that photoexcitation at266nm populatesDMA molecule on the bright S2state, which has ππ*character. Our result corrects theprevious report that the S2state of DMA comes from the excitation from the nonbondingorbital of the carbonyl oxygens (nO) to the in-plane diazo π*NNorbital. The relaxation pathgoes through the (S2/S1)Xwithout any barrier and leads to an intersection seam between S1and S0. The (S1/S0)Xseam mainly lies along the C5-N8stretching coordinate and iscomposed of different parts associated with different reactivity. Decay at regions with shortC5-N8distance leads to the regeneration of ground-state DMA. This is consistent with thelow quantum yield of WR and isomerization that indicates the majority of excited DMAmay go back to ground state through internal conversion. Decay at regions with longC5-N8distance overcomes a barrier of less than0.1eV and consequently leads to theformation of singlet carbene, which instantaneously converts to the final WR productketene. This decay path provides strong evidence for the concerted WR mechanism.Carbene is the key intermediate for the WR mechanism. We have optimized threeconfigurations of the singlet carbene. Three singlet carbene conformers are the boatlike(closed-shell), the chairlike (closed-shell), and the planar one (open-shell). Our calculationspredict that the T1state of DMA may be populated via the S1→T2→T1mechanism at the(S1/T2)Xvicinity. Afterward, the reaction path leads to the formation of triplet carbene.There is no heavy atom in DMA, so the strong spin-orbital interaction at (S1/T2)Xis not dueto heavy atom effect. In fact, the reason for the large spin-orbital coupling (SOC) value at(S1/T2)Xof DMA lies in the difference of S1and T2nature.In competition with the WR, the DMA undergoes isomerization to diazirine on S1state. This process has the low quantum yield for the formation of diazirine upon overcoming abarrier of0.51eV.Our calculations have rationalized the experimental observations and expected to provideinsights into the WR mechanism of related diazo compounds.