If the molecular mass of an unknown compound to the nearest integer value is an odd number, the compound contains an odd number of nitrogens in its molecular formula. Correspondingly, if the molecular mass is an even number, the compound contains zero or an even number of nitrogens in its molecular formula. This rule, illustrated below, results from nitrogen having a valence of three and an even atomic mass. Consistent with the nitrogen rule, a correct molecular formula for a molecule with an odd molecular mass in the nearest integer value will have the sum of the number of hydrogens plus halides as an odd number. Correspondingly, a correct molecular formula for a molecule with an even molecular mass will have the sum of the number of hydrogens plus halides as an even number.
The molecular ion for aminoethane (ethylamine), [CH3CH2NH2]+, is m/z=45 amu, an odd number; the number of hydrogens is five, also an odd number.
The molecular ion for 1,2-diaminoethane, [NH2CH2CH2NH2]+, is m/z= 60 amu, an even number; the number of hydrogens is eight, also an even number.
Next section: Fragment Ions
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