Conformation and ring inversion in gamma-butyrolactone- Part 2. Ab initio and flexible model computations

The conformation and ring inversion of ?-butyrolactone have been studied using ab initio and flexible model computations. The potential energy surface has been computed using STO-3-21G orbitals and complete geometry optimization as a function of the ring puckering coordinates. The computed equilibrium geometry has a ring in which all of the atoms with the exception of the ? carbon are nearly coplanar. The computed inversion barrier is 6.1 kJ mol-1. Analytical expressions, as functions of the correct symmetry of the ring puckering coordinates, have been obtained for the potential energy surface and internal coordinates by least-squares fits to the computed molecular energies and geometries. These data have been used as the input for flexible model computations of the vibrational frequencies, inversion splittings and rotational constants. By using a simple uniform scaling of the potential energy surface good agreement with the observed vibrational frequencies and inversion splittings is obtained for a barrier to ring inversion of 7.9 kJ mol-1. The changes in the rotational constants are reasonably well reproduced for excitation of the vibration which is associated with a double minimum potential and less successfully for the other ring puckering vibration.