Structural parameter relaxation and pucker mode—internal mode coupling in the four-ring molecules cyclobutanol, cyclobutylfluoride and cyclobutylchloride

Abstract

The present work is directed towards the investigation of the pucker—internal rotation modes of cyclobutanol and cyclobutanol-OD by the aid of a semirigid two-dimensional model. Based on spherical four-ring coordinates and general geometrical relations expressing wagging, twisting and rocking coordinates for CH 2 and CHX groups, experimental rotational constants of cyclobutanol, cyclobutylfluoride and cyclobutylchloride in the ground and excited pucker states are analyzed in terms of a two-dimensional four- ring pucker model with structural relaxation of CH 2 and CHX groups concerted with the ring puckering angles. The rotational data are found to be satisfactorily described by a one-dimensional ring pucker model including structural relaxation of the rocking angles of the CHX and the CH 2 groups. Then results of extended SCF computations (4-31G level) of the puckering—internal rotation potential of cyclobutanol are presented. These emphasize again the importance of structural relaxation of bond length and rocking type coordinates of the corner groups concerted with the puckering angle and serve as a basis for numerical solution of the two-dimensional pucker—internal rotation problem within the framework of a semirigid model. Predicted transitions and energy eigenstates will be correlated with far i.r. spectral data of cyclobutanol and cyclobutanol-OD observed in the 30–300 cm −1 range and will be interpreted in terms of an alternative assignment, whose relation to an earlier analysis will finally be discussed.