The is inversely proportional to the wavelength and reported in reciprocal centimeters (cm−1).a technique used for measuring the molecular weight and determining the molecular formula of an organic molecule is .a compound that contains an odd number of atoms gives a molecular ion an odd mass. when the molecular ion consists of two peaks (m and m 2) in a 1: 1 ratio, a atom is present in the molecule. for a bond to absorb in the ir, there must be a change in the dipole moment during the .organic chemists use to identify the functional groups in a compound. when the molecular ion consists of two peaks (m and m 2) in a 3: 1 ratio, a atom is present in the molecule. the higher the percent s-character, the strong the bond, and the the wavenumber of an ir absorption. hydrocarbons and compounds that only contain c, h, and o atoms always have a molecular ion with an mass.