Conformation and ring inversion in gamma-butyrolactone- Part 1. Microvawe spectrum

The microwave spectrum of gamma-butyrolactone has been reinvestigated in order to determine small vibrational splittings associated with the ring inversion. For the ground vibrational state the inversion splitting has been measured directly as 30.3 MHz by the observation of the µc rotation-inversion transitions. The inversion splitting in the first excited state of the two ring puckering vibrations has been determined through analysis of a type Coriolis interactions. In the first excited state of the 148 cm-1 vibration the inversion splitting is 609 MHz while for the 225 cm-1 it is 2930 MHz. These inversion splittings indicate that the 148 cm-1 vibration is associated with a single minimum potential function and the 225 cm-1 vibration with a double minimum potential function but that there is some interaction between the two vibrations. A one-dimensional reduced potential function has been derived for the 225 cm-1 vibration and gives a barrier to ring inversion of 8.0 kJ mol-1(670 cm-1).