Competition between Hückel’s Rule and Jahn–Teller Distortion in Small Carbon Rings: A Quantum Monte Carlo Study

Structural properties and energetics of carbon rings are studied with the diffusion Monte Carlo (DMC) method. Our DMC-based geometry optimization reveals that both polyynic C 4 n and cumulenic C 4 n  + 2 rings exhibit bond length alternations for n ≥ 3, which is understood to be due to Jahn-Teller distortions. The bond length alternation even in a cumulenic (4 n + 2) carbon ring was experimentally observed in a recently synthesized C 18 molecule. From a comparison of the DMC cohesive energies of C 4 n with those of C 4 n  + 2 , we present a comprehensive picture of the competition between Hückel's rule and Jahn-Teller distortion in small carbon rings; the former is more dominant than the latter for n < 5 where C 4 n  + 2 rings are more stable than C 4 n , while C 4 n rings are as stable as C 4 n  + 2 for n < 5 where dimerization effects due to Jahn-Teller distortion are more important.