Mass Spec

 
Index of hydrogen deficiency

With a molecular formula determined by high resolution mass spec, U can be calculated.

U = C + 1 - 0.5(H + X - N)

C is number of carbon atoms
H is number of hydrogen atoms
X is number of heteroatoms with valence 1
N is number of heteroatoms with valence 3

U is the sum of the number of rings and multiple bonds present in the molecule
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