Theoretical study on structures and stability of HCCNN⁺ isomers

The detailed potential energy surface of [HC₂N₂]⁺ is theoretically investigated at the B3LYP/6-311G(d,p) and CCSD(T)/6-311G(3df,2p) (single-point) levels. The linear isomer HCCNN⁺ 5 is predicted to lie just 28.4 kcal/mol higher than the lowest-lying isomer HNCCN⁺ 1 [the neutral species CCNN lies 82.7 kcal/mol above the lowest-energy NCCN (Ding et al., J. Chem. Phys. 2000, 113, 1745)]. Moreover, HCCNN⁺ 5 resides in a very deep potential well stabilized by the barrier of 55.4 kcal/mol either toward dissociation or isomerization (the smallest barrier for CCNN is 42.1 kcal/mol). The calculated C-proton affinity of CCNN is very large as 211.2 kcal/mol. Therefore, CCNN may be significantly stabilized upon protonation at the terminal carbon both thermodynamically and kinetically, a phenomenon characteristic of carbene protonation. In addition, the nitrogen-protonated isomer HNNCC⁺ 6 at 107.3 kcal/mol also possesses high kinetic stability of 51.9 kcal/mol. Our calculations indicate that while the neutral CCNN, the last kinetically stable isomer of the dicyanogen family, has not been characterized yet, its protonated form 5 and even 6 may be promising targets for future laboratory and interstellar detection. The possible formation of the two isomers is briefly discussed.