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

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.

9. 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.

10. 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).

11. 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.

13. 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.

14.  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.

15. 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).

16. 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

17. 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

19. 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 amineto 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).

20. 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
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