  Spectra of doubly deuterated propylene oxide, CH$_3$CHCD$_2$O, were recorded
in the millimeter-wave spectral region up to 330 GHz utilizing a 2f frequency
modulated Terahertz spectrometer. Rotational and centrifugal distortion
constants and tunneling parameters for the description of the internal rotation
of the methyl group were retrieved. The software XIAM was used to describe the
A-E tunneling splitting with a frequency uncertainty of 82 kHz. Molecular
parameters derived from our measurements are complemented by quantum chemical
anharmonic frequency calculations on the B3LYP/augcc-pVTZ level of theory. In
addition, the barrier height to internal rotation of the methyl group,V3, was
derived experimentally and found to slightly differ from the value reported for
the main isotopologue. In view of astrophysical observations we provide
accurate line lists to search for doubly deuterated propylene oxide in space.
Furthermore, we deliver improved molecular parameters to further investigate
the properties of a simple chiral molecule possessing internal large amplitude
motions.
