What is the Preferred Structure of the Singlet Cyclopentadienyl Cation?

Pseudorotation of the singlet cyclopentadienyl cation (a C_2v, ′ethylene′-type structure 2) proceeds through the ′allylic′-type C_2v singlet 3 as the transition state with an extremely low barrier [0.09] kcal mol^–1 at MP4SDTQ/6–31G(2d,p)//MP2(full)/6–13G*] and the C5H5+ nuclear configuration oscillates among the degenerate C_2v minima 2,2a,2b, etc.