Unimolecular ion decomposition

The green trace in the lower part of the diagram indicates the rate of the rearrangement reaction given by and the blue trace indicates the direct cleavage reaction A rate constant of 106 s−1 is sufficiently fast for ion decomposition within the ion source of a typical mass spectrometer.

When the internal energy of ABCD+ is greater than Em(AD+), the ions are metastable (indicated by m*); this occurs near log k > 5.

A metastable ion has sufficient internal energy to dissociate prior to detection.

Like all chemical reactions, the unimolecular decomposition of ions is subject to thermodynamic versus kinetic reaction control: the kinetic product forms faster, whereas the thermodynamic product is more stable.

However, the steric effect makes it more difficult for the molecule to achieve the rearrangement transition state and form AD+.

Wahrhaftig diagram showing the relationship between internal energy and unimolecular ion decomposition for a hypothetical ion ABCD + .
Schematic of "tight" and "loose" transition state complexes for a hypothetical ion ABCD + .