title: Ab initio study of cyclobutane: molecular structure, ring-puckering potential, and origin of the inversion barrier.
abstract: The structure and ring-puckering properties of cyclobutane and its perdeuterated isotopomer are studied using high-level ab initio methods and complete basis set extrapolations. Calculations reveal significant coupling between the ring-puckering (theta) and CH(2)-rocking (alpha) motions, with equilibrium angles (theta(eq) = 29.59 degrees and alpha(eq) = 5.67 degrees) that are within the range of experimentally determined values. Our best estimate of the inversion barrier is 482 cm(-1), in excellent agreement with recent experimental determinations. Ring-inversion transition frequencies are evaluated from the eigenstates of the intrinsic reaction coordinate potentials for cyclobutane and cyclobutane-d(8). Natural bond orbital analysis shows that sigma(CC) --> sigma(CH)* and sigma(CH) --> sigma(CH)* hyperconjugative interactions are strengthened as cyclobutane puckers, thereby suggesting that inversion barriers in four-membered ring systems are a consequence of electronic delocalization rather than torsional strain.
authors.affiliation: Department of Chemistry, Indiana State University, Terre Haute, Indiana 47809, USA. glendening@indstate.edu
authors.forename: Eric D
authors.initials: ED
authors.lastname: Glendening
authors.forename: Arthur M
authors.initials: AM
authors.lastname: Halpern