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Molecular ion

IMPORTANT

When calculating molecular weights to match to a molecular ion you need to use integer atomic weights.
This is because relative atomic weights are based on the natural abundance of isotopes and since MS separates the isotopes, the peaks appear at different m/z values.

The nitrogen rule

Organic molecules composed of C, H, O and halogens have even molecular weights
Examples : CH4 = 16 g/mol, CH3CH3 = 30 g/mol, CH3OH = 32 g/mol, CH3F = 34 g/mol, CH3CO2H = 60 g/mol

Since N forms 3 bonds, compounds with an odd number of N atoms, have odd molecular weights
Examples : NH3 = 17 g/mol, CH3NH2 = 31 g/mol, C5H5N = 79 g/mol

Therefore, the implication is the nitrogen rule.... "an odd molecular weight implies the presence of an odd number of N atoms" .
This can be a useful piece of information and prompt you to look for N containing functional groups in the IR spectrum.


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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary