Download Spin-spin splitting in NMR spectrum

Spin-spin splitting in NMR spectrum
Spin-spin splitting in NMR spectrum is caused by the interaction of nuclear spins of one
set of protons with another set. The splitting occurs only between nuclei with different
chemical shifts, i.e. between chemically and magnetically non-equivalent protons.
Explanation of spin orientation of different set of protons in a molecule
e.g.
Ethanol, acetaldehyde etc. .....
General conclusions with regard to spin-spin interaction
i)
It is independent of the strength of the applied field B0
ii)
Equivalent nuclei do not interact with each other.
iii)
Multiplicity is determined by the no of neighboring groups of
nuclei. For protons, 'n' is the no-of equivalent nuclei
equivalent
in the neighboring group,
the expected multiplicity is n+1
iv)
The splitting of resonance into individual lines is called fine structure of the
spectrum.
v)
The intensities of multiplets are symmetric with relative intensities given by
the coefficients of the terms in the expansion of (a+b)n
vi)
The spacing of adjacent peaks in the multiplet is a direct measurement of the
spin coupling of the proton and is known as the spin-spin coupling constant.
Chemical shift
Chemical shift - shift of the resonance line of a given nucleus from that of a standard
reference sample. Absolute magnitude is extremely small.
Br = B0 (1-  r), Bs = Bo ((1-  s)
Br and Bs are the magnetic fields at which resonance occurs for the reference and
the given group respectively  r &  s - screenings constants depending upon the
electron density around proton for reference and sample respectively -
=
Br  Bs
=  s -  r, extremely small,
Bo
So different unit
=
Br  Bs
x 106 ppm
Bo
Another unit.
‫ = ﬢ‬10 - 
EPR Spectroscop
It is a spectroscopic technique confined to the study of those species having one or more unpaired e
orbital motion associated with the angular momentum there will be a magnetic moment. The magnetic
spectrum is very higher tan in NMR. ESR spectra can be recorded for radical concentration down t
for investigating species like free radical, Transition metal ions (Mn2+), co-ordination complexes [F
electrons. Free radicals are produced in these sample chemically or photo chemically by the use of e
E=  e  BMs.B
 e is the dimension less constant called electronic
 factor or spectroscopic splitting factor. For free electron
 = 2.002322.
 B  Bohr Magneton
Ms = quantum number for the L component of electronic spin.
B = Strength of the external magnetic field.
For a molecule with single unpaired electron
Ms  1 / 2. The selection rule for allowed transition are
∆Ms =  1 as in the case NMR.
Because of the -ve charge of the electron the magnetic moment e of the electron is in the direction op
the lower level in contrast to the situation in NMR. For a transition ms= -1/2 to Ms=+ 1/2 then ∆E =
frequency is required for transition between energy level falls in the microwave region.
The magnetic field seen by the unpaired electron is not the applied field but one screened by th
   e (1  
The  factor for many organic molecule is 2.0026 close to e indicating that the unpaired
localised near a metal ion site varies from 0 to 4. Since  is approximately equal to  e for many s
indispensable in identifying the parametric factor.
Conventional NMR signals are a plot of absorption against field while ESR signals are a plot
large and the first derivative modes of spectrum enable one to fix up the frequency position and
gives a well defined line width ∆V even there are overlap signals it id still possible to do a good appr
Mossbauer Spectros
Mossbauer effect:- The phenomena of recoilless emission and resonance absorption of r-ray
Mossbauer Spectroscopy:- The technique based on this effect which involves the study of n
absorption of r-rays by a nucleus depends upon the electron density about the nucleus and th
Nuclear resonance absorption.
It is difficult to achieve due to
(a)
Record of the nucleus
(b)
Doppler broadening
Recoil of the nucleus:-
𝐸𝑟𝑒 =
𝑃2
2𝑀
P - recoil momentum
M - mass of the recoil nucleus
The recoil of the nucleus introduces a difference of (Elr-Er) between the energy of the emitted
Elr - Er = 2E re=
If 2Ere <<
Eo 2
MC 2
the FWHM, of the line, the two curves would overlap and one would observ
Two methods to achieve it
1)
Put the source and absorber nuclei in a crystal lattice - spreading out the recoil energy.
2)
Cool the source and the sample to the liquid He temp. So that thermal motion of the lattice
Doppler broadening.
A movement of the source and absorber relative to each other with velocity V produces
resonance energies for source and absorber exactly match at a particular velocity V, the re
transmission of r-ray from a source through an absorber as a function of velocity V between th
The absorbers are stable Isotopes of elements with which the Mossbauer effect can be
an executed life time in the range 101−6 S<T>1010 S .Longer life time leads to narrow resonan
Some of the Mossbauer nuclei :57 119
Fe, Sn
125
Te,
129
I,
157
Eu
Isomer Shift :- If the environments of the emitting and absorbing nuclei are different, the
difference is known as the Isomer shift and this will shift the resonance line from the zero po
eg: [Fe(CN)6]4-
Determination of Isomer shift allows estimation of relative S-electron density which in t
Nuclei in the executed states usually have different radius from those in the ground state  Re x  Rgd
Rgd

Chemical Shift = Const  p 2 ex  p 2 gd  



p2 - S electron density at the excited and ground state nuclei.
R - Radius of the nucleus
Quadrupole interaction :- The spin % either or both of the Mossbauer nucleus levels may hav
moment eQ with the electric field gradient eq at the nucleus may lead to a splitting of the
by the surroundings, one would be able to get useful data on crystal fields from an investiga
Magnetic hyperfine interaction
Most of the Mossbauer nuclei have non zero spin in the ground and excited states.
equally spaced levels.
Applications
1)
In solid state - magnetic hyperfine interaction studies.
2)
Isomer shift :- bond character of atoms chemically bonded to Mossbauer nucleus.
3)
Molecular geometry of I2Cl2 Br2
4)
Surface studies.