The importance of molecular vibration: the sign change of the optical rotationof methyloxirane

We demonstrate that the experimentally observed sign change of the optical rotation of (S)-methyloxirane when going from 589 nm to 355 nm is due to zero-point vibrational corrections. The previous agreement between DFT/B3LYP calculations at a wavelength of 355 nm is shown to be fortuitous due to a too low excitation energy predicted by DFT/B3LYP. Excellent agreement between theory and experiment is obtained when CC3 equilibrium optical rotations are combined with the zeropoint vibrational contributions calculated in this communication.