The production of a molecular ion is frequently followed via its dissociation or fragmentation into ions and neutral species of the lower mass, which in turn might dissociate further. Fragmentation patterns are characteristic of specific molecular structures and can point out the presence of specific functional groups, therefore providing helpful information on the structure and identity of the original molecule. The points of cleavage in a molecule are found out by individual bond strengths all through the structure and additionally, molecular rearrangements and recombination can take place.
Fragmentation patterns are an invaluable help in the interpretation of mass spectra and in the recognition or confirmation of the structural characteristics.
Interpretation of the Mass Spectrum:
Mass spectra can be hard to interpret, due to the complexity of the fragmentation. Usually, one identified the peak because of the molecular ion (M+) or MH+ ion and uses this as the reference point. The mass differences between this peak and other peaks are found out and the likely nature of the fragments lost from the reference peak deduced from the reference tables. One can work the other manner round and consult tables that point out the most likely composition of the fragment. The peaks on either side of the main peak might be made because of the presence of isotopes in a fragment. These peaks should be taken into account whenever making any deductions, and initially it is best to consider just the most abundant peaks. Unfortunately, the great diversity of fragments generated in a mass spectrum signifies that these deductions are of limited value on their own. However taken in conjunction by other experimental proof can be of considerable use either in recognizing or finding out the structure of a compound.
Some Rules employed in the Interpretation of Mass Spectra:
1) The nitrogen rule defines that compounds having an even numbered RMM should contain zero or an even number of nitrogen atoms and those having an odd numbered RMM should contain an odd number of nitrogen atoms.
2) The unsaturated sites rule gives a means of computation of the number of double-bond equivalent in the molecule from the formula:
No of C atoms + 1/2(no of N atoms) - 1/2(no of H atoms + halogen atoms) + 1
For illustration: C7H7ON, the formula provides 7 + 0.5 - 3.5 - 1 = 5 double bond equivalents. This corresponds to benzamide, C6H5CONH2, the aromatic ring being counted as the three double bonds plus one for the ring.
3) The intensity of molecular ion peak reduces with increasing chain in length in the spectra of a homologous series of compounds and by increased branching of the chain.
4) Double bonds are cyclic structures tend to stabilize the molecular ion, saturated rings losing side chains at the α-position.
5) Alkyl-substituted aromatic rings (that is, benzyl group) undergo rearrangement to form a tropylium cation C7H7+ (see below) providing a prominent peak at m/z = 91.
6) Small neutral molecules like CO, C2H4, C2H2, H2O and NH3 are often lost throughout fragmentation.
7) The C-C bond adjacent to a heteroatom (N, O and S) is often cleaved leaving the charge on the fragment having the heteroatom (Y), whose nonbonding electrons give resonance stabilization example:
CH3-CH2-Y+•-R → (-CH3•) → CH2=Y+-R ↔ •CH2-Y-R
8) McLafferty rearrangements:
McLafferty rearrangement takes place in carbonyl compounds example:
HCH2-CH2-CH2-CO+•-OR ↔ C2H2 + CH2=C(O+•)-
A neutral molecule of ethene is lost in the procedure.
The mass spectrum of toluene (that is, methyl benzene) is illustrated below (figure shown below). The spectrum displays a strong molecular ion at m/z = 92, small m + 1 and m +2 peaks, a base peak at m/z = 91 and an assortment of the minor peaks m/z =65 and below.
Fig: Mass Spectrum of Toluene
The molecular ion again symbolizes loss of an electron and the peaks above the molecular ion are due to isotopic abundance. The base peak in toluene is because of loss of a hydrogen atom to form the relatively stable benzyl cation. This is thought to experience rearrangement to form the extremely stable tropylium cation, and this strong peak at m/z = 91 is a hallmark of compounds having a benzyl unit. The minor peak at m/z = 65 symbolizes loss of a neutral acetylene from the tropylium ion and the minor peak beneath this occur from more complex fragmentation.
Simple alkanes tend to experience fragmentation via the initial loss of a methyl group to form a (m-15) species. This carbocation can then experience stepwise cleavage down the alkyl chain, expelling neutral two-carbon units (that is, ethane). Branched hydrocarbons form more stable secondary and tertiary carbocation, and these peaks will tend to control the mass spectrum.
Fig: Alkanes tend to undergo fragmentation
b) Aromatic Hydrocarbons:
The fragmentation of the aromatic nucleus is fairly complex, producing series of peaks having m/z = 77, 65, 63 and so on. While these peaks are difficult to explain in simple terms, they do form a pattern (the aromatic cluster) that becomes recognizable with experience. If the molecule includes a benzyl unit, the main cleavage will be to produce the benzyl carbocation that rearranges to form the tropylium ion. Expulsion of acetylene (that is, ethyne) from this produces a characteristic m/z = 65 peak.
c) Aldehydes and Ketones:
The predominate cleavage in aldehydes and ketones is loss of one of the side chains to produce the substituted oxonium ion. This is an extremely favorable cleavage and this ion frequently represents the base peak in the spectrum. The methyl derivative (CH3C ≡ O+) is generally referred to as the acylium ion.
Fig: Predominate cleavage in aldehydes and ketones
The other common fragmentation noticed in the carbonyl compounds (and in nitriles and so on) comprises the expulsion of neutral ethane through a process termed as the McLafferty rearrangement.
d) Esters, acids and amides:
As with aldehydes and ketones, the main cleavage observed for these compounds comprises expulsion of the 'X' group, as illustrated below to form the substituted oxonium ion. For carboxylic acids and unsubstituted amides, characteristic peaks at m/z = 45 and 44 are as well often noticed.
Fig: Cleavage observed for Esters, acids and amides
Moreover to losing a proton and hydroxyl radical, alcohols tend to loose on the α-alkyl groups (or hydrogen) to form the oxonium ions illustrated below. For primary alcohols, this produces a peak at m/z = 31; secondary alcohols produced peaks with m/z = 45, 59, 73 and according to substitution.
Fig: Alcohols tend to lose α-alkyl groups
Following the trend of alcohols, ethers will fragment often via loss of an alkyl radical, to form a substituted oxonium ion.
Organic halides fragment having simple expulsion of the halogen, as illustrated below. The molecular ions of chlorine and bromine having compounds will illustrate multiple peaks because of the fact that each of these exists as two isotopes in relatively high abundance. Thus for chlorine,
the 35Cl-37Cl ratio is roughly 3.08:1 and for bromine, the 79Br-81
Br ratio is 1.02:1. The molecular ion of a chlorine-containing compound will have two peaks, separated by two mass units, in the ratio ~ 3:1, and a bromine-containing compound will have two peaks, again separated by two mass units, having approximately equivalent intensities.
The lists given above are by no means exhaustive and represent just the simplest and most common fragments seen in the mass spectrum.
Examples of Mass Spectra Interpretation:
a) The figure below includes the mass spectra data of octane and 2,2,4-trimethylpentane. Octane is a saturated straight chain therefore the spectra is characterized via clusters of peaks 14 mass units (CH2 groups) apart, as successive C-C bonds all along the chain are cleaved in various molecules. Octane consists of a base peak at m/z = 43 due to the CH3CH2CH2+ fragment ion and a small molecular ion peak at m/z (rule 3).
Table: Commonly lost Fragments from a Molecular Ion
Fig: Mass Spectrum of 2,2,4-Trimethylpentane
Branching of the chain modify the relative intensities of the clusters, as illustrated by the spectrum of the isomeric 2,2,4-trimethylpentane that consists of a base peak at m/z = 57 due to the (CH3)3C+ fragment ion, and no significant m/z = 71, 85 or molecular ion peak (rule 3).
b) The spectrum of methylbenzene (figure below) typifies alkyl-substituted aromatic compounds, having a base peak corresponding to the tropylium ion, C7H7+, at m/z = 91 and a large molecular ion peak at m/z = 92 (rules 4 and 5).
Fig: Mass Spectrum of Methylbenzene
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