Odd molecular mass

An odd number of nitrogen atoms brings about an odd molecular mass in daltons such as is defined in mass spectrometry NH3 17, CH3NH2 31, and so on. Thus, in the case of an odd number of nitrogens, the earlier rule must be inverted for the ion, the mass parity is the same as the electron parity. 

From an analytical point of view, we see that an odd molecular mass, based on the predominant isotopes, indicates the presence of an odd number of nitrogen atoms. 

This compound has one nitrogen atom, and therefore the parent ion has an odd molecular mass of 87. Loss of a methyl group as a radical gives the base peak, whose mass is 72. 

In summary, if the unknown mass spectrum has an intense peak at m/z 59 and an abundant m/z 72 with an odd molecular ion, this suggests a primary amide. 

The mass spectra of 80-91 show weak odd molecular ions and strong fragments owing to a cleavage of the side chains (129). The secondary nature of the nitrogen atom is proved by acetylation, which gives less polar materials 42 amu... 

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). 

Homolytic cleavages cause odd-numbered mass differences between fragment and molecular ion (Chap. 6.2.5). Rearrangement fragmentations cause even-numbered mass differences. This rule toggles if odd numbers of nitrogen are contained in the neutral loss. 

A neutral organic compound, composed of the most abundant isotopes of the el erne tils present, with an even number of nitrogen atoms 111 List have an even relative molecular mass, but a compound with an odd number of nitrogen atoms must have an odd relative molecular mass. 

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. 

Fogliano et al.72 have prepared melanoidin [molecular mass (MM) >12 kDa] from glucose-glycine, lactose-lysine, and lactose-iV-acetyllysine lactose-lysine proved to be the odd one out, exhibiting less than one-tenth the absorbance of the other two at 460 and 520 nm. Their CAVs (colour activity value,73 defined as the ratio of a colorant s concentration to its visual detection threshold, both in pg kg-1) were 8000, 200, and 4000, respectively. Examination by MALDI-TOF gave results only for the lactose-A-acetyllysine melanoidin, with peaks around 6,12, and 24 kDa (the spectrum reproduced by Borrelli et al.74 shows peaks around 6, 7.5, and 12 kDa). The antioxidant effects of the melanoidins were also examined (see p. 129). 

Mass spectrometry is used to measure the molecular mass of a compound and provides a method to obtain the molecular formula. It differs from the other instrumental techniques presented thus far because it does not involve the interaction of electromagnetic radiation with the compound. Instead, molecules of the compound being studied are bombarded with a high-energy beam of electrons in the vapor phase. When an electron from the beam impacts on a molecule of the sample, it knocks an electron out of the molecule. The product, called the molecular ion (represented as A/f), has the same mass as the original molecule but has one less electron. It has both an odd number of... 

The combination of IR and an odd molecular ion in the mass spectrum should confirm amines, amides, and nitriles. A strong alcohol —OH absorption in the IR might suggest that the apparent molecular ion in the mass spectrum could be low by 18 units from loss of water. 

Mass spectrum The MS shows an odd molecular weight at 121 and a large even-numbered fragment at 106. These features may indicate the presence of a nitrogen atom. 

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. 

The IOU of a molecular formula [Eq. (6.7)] is the sum of rings plus n bonds in the structure. The nitrogen rale says that the parity (odd or even) of the number of nitrogens in the molecular formula must match the parity of the nominal molecular mass and the number of hydrogens plus halogens. 

Nitrogen is the only element that does not have both odd or even valence and nominal mass. Valence (+3) is odd while nominal mass (14Da) is even. Any molecule that contains an odd number of nitrogen atoms will have an odd nominal mass. This can be used to Unlit the number of potential molecular formulae. 

How to use the mass of the molecular ion to propose molecular formulas for an unknown is shown in Sample Problem 13.2. In this process, keep in mind the following useful fact. Hydrocarbons like methane (CH4) and hexane (CgHiJ, as well as compounds that contain only C, H, and O atoms, always have a molecular ion with an even mass. An odd molecular ion indicates that a compound has an odd number of nitrogen atoms. 

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). 

Explain why compounds containing an odd number of nitrogen atoms have an odd molecular ion in their mass spectra. 

In this paper it has been shown that the polyethers based on the non-rigid rodlike mesogen bis(4-hydroxyphenoxy)-p-xylene and dibromoalkanes exhibit smectic meso-morphism. Furthermore, the molecular mass of the polyethers follows a pronounced odd-even dependence due to differences in solubility of the different polymers during the polymerization. DSC and polarized light microscopy also revealed an odd-even dependence of the temperatures and enthalpy and entropy changes associated with... 

By associating several other considerations, such as the nitrogen rule (if the compound contains an atom of nitrogen, its molecular mass is an odd number) or... 

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