C54 N6, a potentially aromatic molecule

Relative energies for the D3d and D3 symmetric isomers of C54N6 are ≈ 76 (1), 100 (2), 44 (3), 0 (4), 43 (5), and 30 kcal/mol (6) at the SCF/3–21G//SCF/3–21G level. D3d symmetric isomers 1 and 2 exhibit endohedral He NMR chemical shifts of ≈ −63 ppm (GIRO-SCF/tzp(He)dzp(C, N)//SCF/3–21G level), reminiscent of the highly aromatic, isoelectronic C606− (−67 ppm). Although more stable than 1 and 2, D3 symmetric isomers 3–6 are indicated to be less aromatic, according to the computed endohedral chemical shifts, δ(He) ≈ −12 to −28 ppm. Formation of 4 from trimerization of NC(CC)8CN is highly exothermic, but recent experiments suggest large barriers for such a process.