Nitrogen rule, mass

The nitrogen rule of mass spectrometry says that a compound containing an odd number of nitrogens has an odd-numbered molecular ion. Conversely, a compound containing an even number of nitrogens has an even-numbered M+ peak. Explain. 

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. 

Restricting to the more common elements in organic mass spectrometry (H, B, C, N, O, Si, S, P, F, Cl, Br, I, etc.), a simple rule holds valid With the exception of N, all of the above elements having an odd number of valences also possess an odd mass number and those having an even number of valences have even mass numbers. This adds up to molecular masses fulfilling the nitrogen rule (Tab. 6.7). 

Examples To rationalize the mass spectrum of methane, reactions 6.2-6.6 were proposed. They all obey the mle. You should check the mass spectra and fragmentation schemes throughout this chapter for additional examples of 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. 

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.)... 

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

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. 

The nitrogen rule helps us propose compositions for molecular ions If a compound has an odd number of nitrogen atoms—in addition to any number of C, H, halogens, O, S, Si, and P—then M+ has an odd nominal mass. For a compound with an even number of nitrogen atoms (0, 2, 4, and so on), M+ has an even nominal mass. A molecular ion at mlz 128 can have 0 or 2 N atoms, but it cannot have 1 N atom. 

In the chemical ionization mass spectrum of pentobarbital in Figure 22-4, the peak with the most significant intensity at the high end of the mass spectrum at mlz 227 is suspected to be MH+ If this is so. then the nominal mass of M is 226. The nitrogen rule tells us that a molecule with an even mass must have an even number of nitrogen atoms. If you know from elemental analysis that the compound contains only C, H. N. and O. how many atoms of carbon would you suspect are in the molecule ... 

The nitrogen rule requires that the molecular mass is always even when the number of nitrogen atoms is even or zero. This results from the fact that nitrogen has a different mass parity and valence electrons parity mass 14 u, five peripheral electrons. Both of these parities are identical in the case of any other atom. It should be noted that this holds only if we consider the mass of the predominant isotope. Thus, the chemical mass of bromine is 80 u, an even number, but its predominant isotope is that of mass 79 u, an odd mass. In the same way, isotopically labelled compounds do not always obey this rule. 

Tlie nitrogen rule of mass peCtrotnetry aya that a compound wiUi an odd number of mtn n atoms has an odd nunibered molecular wcish ... 

In the case of monoamines, if the molecular-ion peak is present, it appears at an odd mass number in the spectrum. In the case of other nitrogen compounds also, it appears at odd mass number when the compound contains an odd number of nitrogen atoms (the nitrogen rule). 

The effect of N atoms on the mass of the molecular ion in a mass spectrum is called the nitrogen rule A compound that contains an odd number of N atoms gives an odd molecular ion. A compound that contains an even number of N atoms (including zero) gives an even molecular ion. Two street drugs that mimic the effects of heroin illustrate this principle 3-methylfentanyl (two N atoms, even molecular weight) and MPPP (one N atom, odd molecular weight). 

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