If a molecule has a temporary dipole moment, is it infrared-inactive or infrared-active? (A molecule is infrared-inactive if the vibration does not change the dipole moment. All molecules undergo polarization and have very short-lived dipole moments. I have not heard that it affects their IR-behavior. But never say never, some ultrafast instrumentation might detect those fluctuations if they occur slower than IR-induced vibrations. - GS).