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Low rotational energy spacings can be excited in cold interstellar gas when electronic and most vibrational excitations are frozen out. A polar molecule such as carbon monosulfide has allowed electric dipole transition lines whose frequencies depend on its moment of inertia, providing a distinctive pattern for rotational spectral line assignment. Several matching spectral lines and their isotope shifts give a much stronger identification than one coincidence. The method is especially effective for polar species; a molecule without a permanent electric dipole moment lacks an ordinary pure electric-dipole rotational spectrum.

A set of spectral lines associated with transitions between molecular rotational energy levels. A polar linear rigid rotor has electric-dipole transitions with $\Delta J=\pm 1$; in the ideal model adjacent absorption frequencies are separated by twice the rotational constant in frequency units.