  Rotational transitions of $iso$-propyl cyanide, (CH$_3$)$_2$CHCN, also known
as $iso$-butyronitrile, were recorded using long-path absorption spectroscopy
in selected regions between 37 and 600 GHz. Further measurements were carried
out between 6 and 20 GHz employing Fourier transform microwave (FTMW)
spectroscopy on a pulsed molecular supersonic jet. The observed transitions
reach $J$ and $K_a$ quantum numbers of 103 and 59, respectively, and yield
accurate rotational constants as well as distortion parameters up to eighth
order. The $^{14}$N nuclear hyperfine splitting was resolved in particular by
FTMW spectroscopy yielding spin-rotation parameters as well as very accurate
quadrupole coupling terms. In addition, Stark effect measurements were carried
out in the microwave region to obtain a largely revised $c$-dipole moment
component and to improve the $a$-component. The hyperfine coupling and dipole
moment values are compared with values for related molecules both from
experiment and from quantum chemical calculations.
