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'''Unimolecular ion decomposition''' is the fragmentation of a gas phase ion in a reaction with a [[molecularity]] of one.<ref>{{Cite journal | doi = 10.1146/annurev.pc.30.100179.000411 | title = Unimolecular Ion Decomposition | year = 1979 | last1 = Brenton | first1 = A G | last2 = Morgan | first2 = R P | last3 = Beynon | first3 = J H | journal = Annual Review of Physical Chemistry | volume = 30 | pages = 51|bibcode = 1979ARPC...30...51B }}</ref> Ions with sufficient [[internal energy]] may fragment in a [[mass spectrometer]], which in some cases may degrade the mass spectrometer performance, but in other cases, such as [[tandem mass spectrometry]], the fragmentation can reveal information about the structure of the ion. 

==Wahrhaftig diagram==
[[File:Wahrhaftig Diagram.gif|thumb|right|400 px|Wahrhaftig diagram showing the relationship between internal energy and unimolecular ion decomposition for a hypothetical ion ABCD<sup>+</sup>.]]
A Wahrhaftig diagram illustrates the relative contributions in unimolecular ion decomposition of direct fragmentation and fragmentation following rearrangement. The [[x-axis]] of the digram represents the internal energy of the ion. The lower part of the diagram shows the [[logarithm]] of the [[rate constant]] ''k'' for [[unimolecular]] dissociation] whereas the upper portion of the diagram indicates the probability of forming a particular product ion. The green trace in the lower part of the diagram indicates the rate of the rearrangement reacton given by

:<math>ABCD^+  \to AD^+  + BC\,</math>

and the blue trace indicates the direct cleavage reaction

:<math>ABCD^+  \to AB^+ + CD\,</math>

A rate constant of 10<sup>6</sup> s<sup>-1</sup> is sufficiently fast for ion decomposition within the [[ion source]] of a typical mass spectrometer. Ions with rate constants less than 10<sup>6</sup> s<sup>-1</sup> and greater than approximately 10<sup>5</sup> s<sup>-1</sup> (lifetimes between 10<sup>5</sup> and 10<sup>6</sup> s) have a high probability of decomposing in the mass spectrometer between the ion source and the detector. These rate constants are indicated in the Wahrhaftig diagram by the log ''k'' = 5 and log ''k'' = 6 dashed lines.

Indicated on the rate constant plot are the reaction critical energy (also called the [[activation energy]]) for the formation of AD<sup>+</sup>, E<sub>0</sub>(AD<sup>+</sup>) and AB<sup>+</sup>, E<sub>0</sub>(AB<sup>+</sup>). These represent the minimum internal energy of ABCD<sup>+</sup> required to form the respective product ions:  the difference in the [[zero point energy]] of ABCD<sup>+</sup> and that of the [[activated complex]].

When the internal energy of ABCD<sup>+</sup> is greater than E<sub>m</sub>(AD<sup>+</sup>), the ions are metastable (indicated by m<sup>*</sup>); this occurs near log ''k'' > 5. A metastable ion has sufficient [[internal energy]] to dissociate prior to detection.<ref>{{GoldBookRef|title=metastable ion (in mass spectrometry)|file=M03874}}</ref><ref>{{Cite journal | doi = 10.1103/PhysRev.68.54 | title = Detection of Metastable Ions with the Mass Spectrometer | year = 1945 | last1 = Hipple | first1 = J. | last2 = Condon | first2 = E. | journal = Physical Review | volume = 68 | pages = 54|bibcode = 1945PhRv...68...54H }}</ref> The energy E<sub>s</sub>(AD<sup>+</sup>) is defined as the internal energy of ABCD<sup>+</sup> that results in an equal probability that ABCD<sup>+</sup>and AD<sup>+</sup> leave the ion source, which occurs at near log ''k'' = 6. When the precursor ion has an internal energy equal to E<sub>s</sub>(AB<sup>+</sup>), the rates of formation of AD<sup>+</sup> and AB<sup>+</sup> are equal.

==Thermodynamic and kinetic effects==
[[File:ABCD transition states.gif|thumb|right|400 px|Schematic of "tight" and "loose" transition state complexes for a hypothetical ion ABCD<sup>+</sup>.]]

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.<ref>{{cite book |author=Tureček, František; McLafferty, Fred W. |title=Interpretation of mass spectra |publisher=University Science Books |location=Sausalito, Calif |year=1993 |pages=115 |isbn=0-935702-25-3 |oclc= |doi= |accessdate=}}</ref> In the decomposition of  ABCD<sup>+</sup>, the reaction to form  AD<sup>+</sup> is thermodynamically favored and the reaction to form  AB<sup>+</sup>is kinetically favored. This is because the AD<sup>+</sup> reaction has favorable [[enthalpy]] and the AB<sup>+</sup> has favorable [[entropy]].

In the reaction depicted schematically in the figure, the rearrangement reaction forms a double bond B=C and a new single bond A-D, which offsets the cleavage of the A-B and C-D bonds. The formation of AB<sup>+</sup> requires bond cleavage without the offsetting bond formation. However, the [[steric effect]] makes it more difficult for the molecule to achieve the rearrangement transition state and form AD<sup>+</sup>. The activated complex with strict steric requirements is referred to as a "tight complex" whereas the transition state without such requirements is called a "loose complex."

==See also==
*[[Metastability]]
*[[Transition state theory]]
*[[RRKM theory]]
*[[Tandem mass spectrometry]]

==References==
{{Reflist}}

[[Category:Mass spectrometry]]
[[Category:Physical chemistry]]