The spacing between successive vibrational energy levels is h omega = h v, where v is the vibrational frequency, v = omega/(2 pi). Vibrational frequencies are commonly reported as wavenumbers, v/c, in units of cm^-1.^1H^35CI absorbs at 2991 cm^-1. The vibrational frequencies are ordered as follows:^14n_2 >^16O_2>^1H^19F>^1H^35CI The vibrational frequencies of the isotopologues of HCI are ordered as follows:^1H^35CI >^1H^37CI >^2H^35CI >^2H^37CI Vibrational transitions of N2 and 02 are dipole forbidden, and consequently very weak. The following table lists the force constants for some diatomic molecules, and the masses of the associated isotopes: Diatomic molecule kl (N m^-1)