Puckering structure in the infra-red spectrum of cyclobutane

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Abstract
The theory of rotational-pucker-vibrational transitions in the vibrational spectrum of cyclobutane is reviewed. Puckering sideband structure on the 1453 cm-1 v 14 infra-red fundamental of C4H8 has been observed and analysed, in terms of two slightly different puckering potential functions for the ground and the excited vibrational states. The results have been fitted to quartic-quadratic potential functions in the puckering coordinate, with a barrier to inversion of 503 cm-1 (1·44 kcal mole-1 = 6·02 kJ mole-1) in the ground state and 491 cm-1 in the excited state ν 14 = 1. For reasonable assumptions about the reduced mass, the equilibrium dihedral angle of the C4 ring is determined to be about 35°, in agreement with previous estimates. Ueda and Shimanouchi's observations on the 2878 cm-1 C4H8 band have been re-analysed, and puckering sidebands have also been observed and analysed for the 1083 cm-1 v 14 infra-red fundamental of C4D8. Pure puckering transitions have been observed in the Raman spectrum of C4H8 vapour. All of these observations are shown to be consistent with the same ground state puckering potential function.