Nitrogen rule
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2008-06-05T18:33:21Z
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The '''nitrogen rule''' states that [[organic compound]]s containing exclusively [[hydrogen]], [[carbon]], [[nitrogen]], [[oxygen]], [[silicon]], [[phosphorus]], [[sulfur]], and the [[halogen]]s either have 1) an ''odd [[nominal mass]]'' that indicates an ''[[odd number]]'' of nitrogen atoms are present or 2) an ''even nominal mass'' that indicates an ''[[even number]]'' of nitrogen atoms are present in the [[molecular ion]].<ref name="isbn0-9660813-9-0">{{cite book |author=David O. Sparkman |title=Mass Spectrometry Desk Reference |publisher=Global View Pub |location=Pittsburgh |year= |pages=p64 |isbn=0-9660813-9-0 |oclc= |doi=}}</ref> The nitrogen rule is not a rule, ''per se'', as much as a general principle which may prove useful when attempting to solve [[organic chemistry|organic]] [[mass spectrometry]] structures.
==Formulation of the rule==
This rule is derived from the fact that, perhaps coincidentally, for the most common [[chemical elements]] in neutral [[organic compound]]s (hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens), elements with even numbered nominal masses form even numbers of [[covalent bond]]s, while elements with [[odd number]]ed nominal masses form odd numbers of covalent bonds, with the exception of nitrogen, which has a nominal (or [[integer]]) [[molecular mass|mass]] of 14, but has a [[valence (chemistry)|valency]] of 3. <!-- It would be useful to mathematically prove this here, if anyone is so inclined. -->
It should be noted that the nitrogen rule is only true for [[Neutral particle|neutral]] structures in which all of the [[atom]]s in the molecule have a number of covalent bonds equal to their standard valency (counting each [[sigma bond]] and [[pi bond]] as a separate covalent bond for the purposes of the calculation). Therefore, the rule is typically only applied to the molecular ion signal in the [[mass spectrum]].
Mass spectrometry generally operates by measuring the mass of [[ion]]s. If the measured ion is generated by creating or breaking a single covalent bond (such as protonating an [[amine]] to form an [[ammonium]] center or removing a [[hydride]] from a [[molecule]] to leave a positively charged ion) then the nitrogen rule becomes reversed (odd numbered masses indicate even numbers of nitrogens and vice versa). However, for each consecutive covalent bond that is broken or formed, the nitrogen rule again reverses.
Therefore, a more rigorous definition of the nitrogen rule for [[organic compound]]s containing exclusively hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens would be as follows:
An even [[nominal mass]] indicates that a net even number of covalent bonds have been broken or formed and an even number of nitrogen atoms are present, or that a net odd number of covalent bonds have been broken or formed and an odd number of nitrogen atoms are present. An odd nominal mass indicates that a net even number of covalent bonds have been broken or formed and an odd number of nitrogen atoms are present, or that a net odd number of covalent bonds have been broken or formed and an even number of nitrogen atoms are present.
Inorganic molecules do not necessarily follow the rule. For example the nitrogen oxides NO and NO<sub>2</sub> have an odd number of nitrogens but even masses of 30 and 46.
==See also==
* [[Mass (mass spectrometry)]]
==References==
{{Reflist}}
[[Category:Mass spectrometry]]
[[Category:Parity]]