= Solution
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>.
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