Ethynamine - Ketenimine - Acetonitrile - Rearrangements: A computational Study of Flash Vacuum Pyrolysis Processes

The rearrangement of ethynamine 3 (H-CoC-NH2) to ketenimine 4 (CH2=C=NH) and acetonitrile 5 (CH3CN) has been investigated by ab initio theory up to the MP4(SDTQ)/631G*//MP2(FU)/6-31G* level. Multiconfigurational calculations at the CASPT2/CASSCF(6,5) (2s + 3p) level were performed in selected cases. The most direct rearrangements 3 à 4 à 5 involve two consecutive 1,3-H shifts, which are formally forbidden by the Woodward-Hoffmann rules. The ab initio barrier for the concerted first step 3 à 4 via TS1 is very high, 74 kcal/mol at the MP4 level, and the structure of the transition state TS1 very unusual with a linearized CCN angle (169.4), which suggest that the observed reaction may not proceed in this way. In fact, a lower barrier of about 60 kcal/mol via aminovinylidene 2 and imidoylcarbene 15 has been found. Thus, the direct 1,3-shift via TS1 is discredited. The ab initio barrier for the concerted second step, 4 à 5 via TS2, is 61 kcal/mol, and the transition state structure is again very unusual with a virtually linear CCN backbone. In TS2 the migrating H atom is bonded to the central sp-type carbon atom with C-H distances of 1.5 Å. However, this transition state does not appear to correspond to physical reality, as it cannot be confirmed by CASPT2/CASSCF calculations. Instead, the CASPT2/CASSCF calculations predict reaction via vinylnitrene 9, but the calculated barrier of 78 kcal/mol for 4 à 9 is seriously higher than the experimental shock-tube activation energy of 70 ± 3 kcal/mol for the 4 à 5 rearrangement. The activation energy for formation of the radical pair •CH2CN + H• (11) from 4 is estimated as