Nitrogen rule

It is a fortunate coincidence that the most abundant isotope of all elements has either even mass and even valence, or both are odd, except for nitrogen, which has even mass but odd valence. This property leads to what is known as the 

Neutral Lost Formula of the Neutral Precursor Molecule 

The nitrogen rule has two important applications (1) it can be used to identify whether an ion is OE or EE (see Example 6.5), and (2) it can provide some idea of the presence or absence of nitrogen atoms in a compound (see Example 6.6). 

When we restrict our selection to common elements in organic mass spectrometry (H, B, C, N, O, Si, S, P, F, Cl, Br, I, etc.) all but one of these elements having an odd number of valences also possess an odd mass number and those having an even number of valences also have even mass numbers. Nitrogen represents this only exception, which gives rise to the so-called nitrogen rule (Table 6.7). 

Nitrogen rule If a compound contains an even number of nitrogen atoms (0, 2, 4,. ..), its monoisotopic molecular ion will be detected at an even-numbered nominal m/z value. Vice versa, an odd number of nitrogens (1, 3, 5.) is indicated by an odd-numbered nominal m/z. 

Rule Cleaving off a radical (that contains no nitrogen) from any ion changes the nominal m/z value from odd to even or vice versa. Loss of a molecule (that contains no nitrogen) from an ion produces even nominal mass fragments from even mass ions and odd mass fragments from odd mass ions. 

Example To rationalize the mass spectrum of methane, reactions 6.2-6.6 were proposed. They all obey the rule. Just check the mass spectra and fragmentation schemes throughout this chapter for additional examples of the nitrogen rule  

Unsaturated hydrocarbons and cyclic hydrocarbons have characteristic empirical formulae. The general formula of hydrocarbons is Cx 2x+2-2m while that of alcohols is C H2x-n-2m y (where m represents the number of rings and unsaturated bonds in the molecular structure). An important characteristic of molecules composed of C, H, O, and S is that their presence results in an even number molecular weight. This characteristic, however, disappears at the presence of a nitrogen atom, which relates to the so-called nitrogen rule . 

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A relatively simple example is the nitrogen rule A molecule with an odd number of nitrogens has an odd molecular weight a molecule with only C H and O or with an even number of nitrogens has an even molecular weight... 

Nitrogen rule (Section 13 23) The molecular weight of a sub stance that contains C H O and N is odd if the number of nitrogens is odd The molecular weight is even if the num her of nitrogens is even... 

B In light of the nitrogen rule mentioned in molecular formula of pyridine, M+ = 79 ... 

The nitrogen rule of mass spectrometry says that a compound with an odd number of nitrogen atoms has an odd-numbered molecular weight. Thus, the presence of nitrogen in a molecule is detected simply by observing its mass spectrum. An odd-numbered molecular ion usually means that the unknown... 

Nitrogen rule (Section 24.10) A compound with an odd number of nitrogen atoms has an odd-numbered molecular weight. 

The unknown gave a molecular ion at m/z 193 with fragment ions at m/zs 174, 148, and 42. From the abundance of the molecular ion, it is probably aromatic, and according to the Nitrogen Rule, contains at least one nitrogen atom. From accurate mass measurement data and an examination of the isotopic abundances in the molecular ion region, the molecular formula was found to be CnH15N02. 

In addition to the study of the isotopic cluster, useful information on analyte identity can be obtained by the nitrogen rule. It states that if a molecule contains an odd number of nitrogen atoms, its molecular weight must be an odd number. However, if an even number of nitrogen atoms or no nitrogen is present, its molecular weight must be an even number. 

Try to identify the molecular ion peak. Is it present in the spectrum Use four necessary conditions to consider a peak as representing M+. Do not forget about the nitrogen rule. 

Note The first decimal of the m/z values of carbenium ions continuously rises with increasing m/z as a result of mass sufficiency of hydrogen (Chap. 3.3). Theoretically from C32H6s onwards m/z 449.5081), rounding of the m/z value causes a shift to the next integer m/z, and thus result in confiision due to the nitrogen rule. Therefore, integer m/z values should not be used above m/z 400 instead use of the decimal is recommended for LR-MS. 

The derived molecular formula must obey the nitrogen rule (Chap. 6.2.5). An odd-numbered m/z value of the molecular ion requires 1, 3, 5,. .. nitrogens to be contained, whereas an even m/z value belongs to 0, 2, 4,... nitrogens. 

If the compound contains or may contain nitrogen, the nitrogen rule is applicable. According to this rule, any molecule with an odd number of nitrogen atoms has an odd mass. For example, in the compound N(CH3)3 the mass is 59 g/mole. 

Don t forget to consider the nitrogen rule when interpreting the mass spectra of amides. (See Chapter 5 to review the nitrogen rule.)... 

Many peaks can be ruled out as possible molecular ions simply on grounds of reasonable structure requirements. The nitrogen rule is often helpful. It states that a molecule of even-numbered molecular weight must contain either no nitrogen or an even number of nitrogen atoms an odd-numbered molecular weight requires... 

For the nitrogen rule to hold, only unit atomic unit masses (i.e., integers) are used in calculating the formula masses. 

In many laboratories, the El spectrum and the Cl spectrum (with methane or isobutane) are obtained routinely since they are complementary. The Cl spectrum will frequently provide the [M + H]+ peak when the El spectrum shows only a weak or undetectable M + peak. The [M - H]+ peak may also appear in the Cl spectrum by hydride abstraction. The Cl fragmentation pattern is usually difficult to predict or rationalize. Note that the nitrogen rule (see Section 2) does not apply to the [M + H]+ or the [M - H]+ peaks neither does it apply to the Cl fragmentation ions. 

A very simple and useful rule to aid in the interpretation of the MS data of organic compounds relates to the fact that nitrogen has an even mass but an odd valency, whereas the valency and mass for the most abundant isotope of other elements are either both odd or both even. This gives us a very simple rule that we can immediately apply to any mass spectral data the nitrogen rule which is given in Box 5.2. 

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