Complex sentence: Each electronic transition may show vibrational course structure and, if the molecule is in the gas phase, rotational fine structure even when the molecule has a zero dipole moment and therefore has no vibration-rotation spectrum in the infrared or pure rotational microwave spectrum.
Split into simpler sentences: Each electronic transition may show vibrational coarse structure, and for molecules in the gas phase, rotational fine structure. This is true even when the molecule has a zero dipole moment and therefore has no vibration-rotation infrared spectrum or pure rotational microwave spectrum.