Structure 3.2.8

Mass Spectrometry (MS)

IHD = 1: One Double Bond OR One Ring.
IHD = 4: Suggests Benzene Ring (3 π \pi π bonds + 1 ring).

Using fragmentation patterns to deduce molecular structure.

1. Index of Hydrogen Deficiency (IHD)

Calculates the "Degree of Unsaturation". First step in any analysis.

IHD = \frac{2C + 2 + N - H - X}{2}

High energy electrons bombard the molecule, knocking off an electron to form the molecular ion $M^+$ . This ion is unstable and fragments.

m/z Lost	Fragment Lost	Indicates
M - 15	$CH_3$	Loss of Methyl group
M - 17	$OH$	Loss of Hydroxyl (from alcohols)
M - 29	$C_2H_5$ or $CHO$	Loss of Ethyl or Aldehyde group
M - 31	$CH_2OH$	Loss of $CH_2OH$ (Primary Alcohol)
M - 45	$COOH$	Loss of Carboxyl