Ring-puckering potential energy functions for cyclobutane and related molecules based on refined kinetic energy expansions and theoretical calculations

Abstract With the aid of ab initio calculations, the one-dimensional representations of the ring-puckering vibrations of several four-membered ring molecules have been refined. These one-dimensional models include contributions from MH2 (M= C, Si, Ge) rocking motions and utilize the computed parameter ω which reflects the relative amounts of ring angle bendings. This model leads to much more accurate determinations of the coordinate dependent kinetic energy expressions g 44 (x) which in turn allow much more accurate calculations of the ring-puckering potential energy functions (PEFs) based on experimental data. The resulting experimental PEFs agree remarkably well with the theoretical functions determined from ab initio calculations. For each molecule spectroscopic data were fit very well utilizing the refined kinetic energy functions and two term PEFs. They were fit even better using three term PEFs including x6 terms. The PEFs, barrier heights, and energy minima are presented for each of these molecules and their isotopic species.