An ab Initio Study of Ionised Cyclobutanone and Cyclopentanone. Comparison of the Thermodynamic and Kinetic Stabilities of the Distonic Isomers ĊH 2 (CH 2 ) n + C = O

Ab initio molecular orbital theory has been used to examine ionised cyclobutanone and ionised cyclopentanone, together with their distonic and ionised ketene isomers. The results are compared with previous findings for the analogous C₃H₄O^•+ ions. Ionised cyclopropanone and ionised cyclobutanone are predicted not to be equilibrium species, but rather transition structures for the exchange of methylene groups in the ĊH₂(CH₂) _n ⁺C = O (n = 1 and 2) distonic ions, respectively. Ionised cyclopentanone, on the other hand, is found to be a stable isomer that lies 61 kJ mol⁻¹ lower in energy than ĊH₂CH₂CH₂CH₂⁺C = O. This provides a rationalisation for the different reactivity and spectra observed for ionised cyclopropanone and ionised cyclobutanone compared with ionised cyclopentanone. The transition structures for hydrogen migration in the distonic ions to form the corresponding alkylketene ions were also calculated.