MASS SPECTROSCOPY ( Molecular ion, Base peak, Isotopic abundance, Metastable ion & Nitrogen rule )

CONTENT:

Molecular Ion Peak
Significance of Molecular ion & Graphically Method
Base Peak
Isotopic Abundance
Metastable Ion
Significance of Metastable ion
Nitrogen Rule & graphs
Formulation of Rule

1. NAME: SACHIN ROHIDAS KALE SEM : II STD : MSC-I ROLL NO : 8044 PPT TOPIC : mass spectroscopy ORGANIC CHEMISTRY

2. CONTENT: Molecular Ion Peak Significance of Molecular ion & Graphically Method Base Peak Isotopic Abundance Metastable Ion Significance of Metastable ion Nitrogen Rule & graphs Formulation of Rule

3. Molecular ion or Parent ion: When a molecule is bombarded with electrons in high vacuum in Mass spectrometer, it is converted into positive ions by loss of an electron. These ions are called as Molecular or Parent ions. M + e → M+° + 2e— Where, M – represents the Molecule; M+°– represents the Molecular or Parent ion The order of energy required to remove electron is as follows— σ electrons > non-conjugated π > conjugated π > non bonding or lone pair of electrons.

4. Many of these molecular ions (M+°) disintegrate at 10-10 to 10-13 seconds to give a positively charged fragment and a radical in the simplest case. If some Molecular ions remain intact long enough (about 10-6 seconds) to reach the detector. Most molecules show a peak for the molecular ion, the stability of which is usually in the order— Aromatic > Conjugated acyclic polyenes > Alicyclics > nhydrocarbons > ketones > ethers> Branched chain hydrocarbons > Alcohols.

5. THE MOLECULAR ION (M + ) PEAK :

6. Significance of Molecular ion: Molecular ion peak gives the molecular weight of the compound. i.e. m/z of molecular ion = molecular weight of the compound. Ex: C2H5 + (m/e=29) gives the molecular weight of Ethane.

7. BASE PEAK : The most intense (tallest) peak in a mass spectrum, due to the ion with the greatest relative abundance (relative intensity; height of peak along the spectrum's y-axis ). Not to be confused with molecular ion: base peaks are not always molecular ions, and molecular ions are not always base peaks. The graphic representaion of mass spectrum of a compound is constructed by plotting mass/charge ratio(m/z) versus relative aboundance or percentage of base peak where the base peak is te most intense peak in the spectrum.

8. The common practice is to represent spectra of organic compounds, e.g., of methanol in above diagram in the form of bar graph and also in the tabulated form ( Diagram ).

9. ISOTOPIC ABUNDANCE : The  composition  of any object can be given as a set of elemental and isotopic abundances.  Isotopic abundances refer to the relative proportions of the stable isotopes of each element Since the late 1930s, geochemists, astrophysicists, and nuclear physicists have joined together to try to explain the observed pattern of elemental and isotopic abundances. 

10. METASTABLE ION : Metastable ion in mass spectrometry, An  ion  which is formed with sufficient excitation to dissociate spontaneously during its flight from the  ion  source to the detector.

11. Fragment of a parent ion will give rise to a new ion (daughter) plus either a neutral molecule or a radical. M1 + M2 + + non charged particle An intermediate situation is possible; M1 + may decompose to M2 + while being accelerated. The resultant daughter ion M2 + will not be recorded at either M1 or M2, but at a position M* as a rather broad, poorly focused peak. Such an ion is called a metastable ion.

12. Nature Of Metastable Ions: Metastable ions have lower kinetic energy than normal ions and metastable peaks are smaller than the M1 and M2 peaks and also broader. These metastable ions arise from fragmentation that takes place during the flight down through ion rather than in the ionization chamber. Molecular ions formed in the ionization chamber do one of the following things: 1. Either they decompose completely and very rapidly in the ion source and never reach the collector (as in case of highly branched molecular ions with life times less than 10-5 seconds). 2. Or else they survive long enough to reach the collector and be recorded there (life times longer than 10-5).

13. Significance of Metastable Ions: Metastable ions are useful in helping to establish fragments routes. Metastable ion peak can also be used to distinguish between fragmentation Processes, which occur in few microseconds

14. NITROGEN RULE: The  nitrogen rule  states that  organic compounds  containing exclusively  hydrogen ,  carbon ,  nitrogen ,  oxygen ,  silicon ,  phosphorus ,  sulfur , and the  halogens  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 in the molecular formula of the  molecular ion . The nitrogen rule is not a rule,  per se , as much as a general principlely prove useful when attempting to solve  organic   mass spectrometry  structures. An important corollary of this rule states that the fragmentation at a single bond gives an odd ion fragment from an even numbered molecular ion. Let us consider nitro-benzene (C6H5NO2) the signal for molrcular ion appears at m/e 123;i.e,,at odd numbered molecular mass since the compound contain only one (Odd number) nitrogen atom. Two important ion fragment which are formed in the mass spectrum of this compound are ( i ) NO2 at m/e 46 and (ii) NO+ at m/e 30.

15. Formulation of the rule This rule is derived from the fact that, perhaps coincidentally, for the most common  chemical elements  in neutral  organic compounds  (hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens), elements with even numbered nominal masses form even numbers of  covalent bonds , while elements with  odd numbered  nominal masses form odd numbers of covalent bonds, with the exception of nitrogen, which has a nominal (or  integer )  mass  of 14, but has a  valency  of 3. The nitrogen rule is only true for  neutral  structures in which all of the  atoms  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  ions .  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).

16. Sharma Y.R. Elementary organic spectroscopy principles and chemical applications. 1st ed. S. Chand and Company ltd; New Delhi :2008 . Tureček , František ; McLafferty , Fred W. (1993).  Interpretation of mass spectra . Sausalito, Calif : University Science Books. Organic Spectroscopy – Principle and Application Jagmohan Narsoa Publiction . Introduction of Spectroscopy, Donald L Pavia, Gary M Lampman , George S Kriz , Thomson Brooks. REFERENCE :