Spin spin splitting in esr

SPIN-SPIN SPLITTING IN
ELECTRON SPIN RESONANCE
NAME:-SARAF PRIYA MADHAV
ROLL NO 53
FINAL YEAR SEM VIII
COLLEGE NAME-PES’MODERN COLLEGE OF PHARMACY
(ONLY FOR LADIES),MOSHI
SUBJECT:-PHARMACEUTICAL ANALYSIS VI
GUIDED BY: MRS VRUSHALI TAMBE MAA’M

Twinkle twinkle little Spin ,Are you single or are you twin?
Are you real or are you false? How I crave your resonant
pulse

Let’s have look of electron spin resonance
 Applied to species having one or more unpaired Electrons. Free radicals, biradicals ,other triplet state
transitional metal compounds
Species having one unpaired electron has two
Electron Spin energy level:
Selection rule
g: proportionality constant,
2.00232 For free electron
1.99-2.01 For radicals
1.4-3.0 For transitional metal
compounds

or the microwave frequency of 9388.2 MHz, the predicted resonance occurs at a magnetic field of about = 0.3350 T = 3350 G
 In isotropic system {gas, liquid or solution ,cubic environment, g is independent of field direction
μB = Bohr
magneton = 9.274 × 10−24 (J/T) for electron
ms = +1/2 for spin up and ms = -1/2 for spin down.
B0 = external field Commonly 0.34-1.24 T
 The electron interact with neighboring nuclear magnetic dipole, energy level becomes:
M1 = nuclear spin quantum no.for the neighboring nucleus
a = hyperfine coupling constant
= 0.3350 T = 3350 G
Corresponding frequency
9.59(X-band)-35(Q-band) GHz

 Energy level and transition for signal unpaired electron in an electron in
an external magnetic field
with no coupling coupling to one nucleus with spin 1/2

Esr spectrum
A] absorption curve
B]first derivative spectrum
standard: DPPH 1, 1-diphenyl-2-picrylhydrazyl radical
g = 2.0036
Pitch g = 2.028

 Hyperfine coupling in isotropic system interaction between electron and nuclear spin magnetic
moments
fine structure in ESR spectrum
 Coupling constant arise in 2 ways:-
A]Direct dipole –dipole interactions
B]Fermi contact interaction
 Coupling pattern in ESR are determine by the same rules that apply to NMR
 Coupling to nuclei with spin > ½ are move frequently observed
Hyperfine coupling constant – g μB MHz
Hyperfine splitting constant – A gauss or millitesla

 It depends on the magnitude of magnetic moments of nuclear and electron
spins
 The electron spin density in the immediate vicinity of the nucleus
 It is independent of applied magnetic field
 It obeys (n + 1) rule for I = 1/2 i.e., (2nI + 1)
 The first two terms correspond to the Zeeman energy of the electron and
the nucleus of the system,
 The third term is the hyperfine coupling between the electron and nucleus where ai is the
hyperfine coupling constant

 splitting between energy levels and their dependence on magnetic field strength.
 In this figure, there are two resonances where frequency equals energy level splitting at
magnetic field strengths of B1B1 and B2B2.
a=gμeΔB hyperfine coupling constant

 Hyperfine Interactions
 Relative intensities determined by the number of interacting nuclei
 If only one nucleus interacting
 Distributions derived based upon spin
 For spin ½ (most common), intensities follow binomial distribution
 Example: Radical anion of benzene [C6H6 ]• -
• Electron is delocalized over all six carbon atoms Exhibits coupling to
six equivalent hydrogen atoms 2NI + 1 = 2(6)(1/2) + 1 = 7 7 lines
• relative intensities 1:6:15:20:15:6:1

Relative Intensities for I = 1
N Relative Intensities
1 : 6 : 21 : 40 : 80 : 116 : 141 : 116 : 80 : 40 : 21 : 6 : 1
1 : 5 : 15 : 20 : 45 : 51 : 45 : 20 : 15 : 5 : 1
1 : 4 : 10 : 16 : 19 : 16 : 10 : 4 : 1
1 : 3 : 6 : 7 : 6 : 3 : 1
1 : 2 : 3 : 2 : 1
1 : 1 : 1
10
2
3
4
5
6
1

 Relative Intensities for I = 1

 Hyperfine Interactions
Example:
VO(acac) 2
with vanadium nucleus
V: I = 7/2
2nI + 1 = 2(1)(7/2) + 1 = 8 line expected

 Hyperfine Splitting to provide information about a molecule, most often radicals.
 The number and identity of nuclei can be determined, as well as the distance of a nucleus from
the unpaired electron in the molecule.
 Hyperfine coupling is caused by the interaction between the magnetic moments arising from
the spins of both the nucleus and electrons in atoms.
B is magnetic field, μ is dipole moment, ‘N’ refers to the nucleus, ‘e’ refers to the
electron
 This spin interaction in turn causes splitting of the fine structure of spectral lines into
smaller components called hyperfine structure

Example :-
 Methyl Radical, CH3⋅
 It contains an unpaired electron with S = 1/2 and three equivalent protons.
 Each proton has a spin I = 1/2. Therefore, the total nuclear spin is I = 3/2.
 For each ms = ±1/2, the mI values are +3/2, +1/2, −1/2, −3/2 The ESR spectrum shows
four lines(n + 1), i.e., a quartet in the intensity ratio 1:3:3:1.
 The spacing between any two successive lines represents the isotropic
coupling constant
1. Proton—splits into 2 lines 1:1
2 .Proton—splits into 3 lines 1:2:1
3. Proton—splits into 4 lines 1:3:3:1

 SUPERHYPERFINE SPLITTING
 Further splitting may occur by the unpaired electron if the electron is subject to the influence of
multiple sets of equivalent nuclei. This splitting is on the order of 2nI+1 and is known
as superhyperfine splitting
 For example, in a CH2OH radical, show a triplet of doublets. The triplet would arise from the
three protons, but superhyperfine splitting would cause these to split further into doublets.

 EXAMPLE
2)Pyrazine radical anions
a)Coupling to 2 14N nuclei (1:2:3:2:1 quintet ),and split by
4H
b)Na+ salt further split into 1: 1 :1 :1 quartet

 Zero–field splitting :-
 The splitting of spin levels even in the absence of magnetic field ,2S+1 is
called zero–field splitting.
 Removes the degeneracy of transitions and more transitions are observed than expected , in
the presence of external magnetic field.
 Fine structure in ESR spectrum is obtained
 zero -field splitting and Kramer ’ s degeneracy ESR
spectra of second and third row transition metal
complexes are often hard to observed, however, rare
-earth metal complexes give clear, useful spectra
short spin -lattice relaxation times ==> broad
spectral lines low temperature experiments will be
needed to observe spectra

 EXAMPLE
In a d2 system with two unpaired electrons, S = +½ + ½ = 1. Therefore, ms= -1, 0, +1. In the
absence of zero – field splitting,
Two transitionsare possible as shown below: The first transition is ms= 0 to +1 and the second
transition is ms= -1 to 0.
The 1st transition is ms=0 to 1 & 2nd is ms = -1 to 0.
This transition have equal energy i.e degenerate so only one single is observed.
Kramer’s degeneracy is not operative.

Anisotropic systems
• Solids, frozen solutions, radicals prepared by irradiation of crystalline materials, radical trapped in host
matrices, paramagnetic point defect in single crystals
• for systems with spherical or cubic symmetry g factors
• for systems with lower symmetry, g ==> g‖ and g┴ ==> gxx, gyy, gzz
• ESR absorption line shapes show distinctive envelope
system with an axis of symmetry no symmetry

REFERENCES:-
 www.sciencedirect.com
 www.slideplayer.com
 https://chem.libretexts.org/
 www.spectroscopy in inorganic chemistry and instrumental analysis
 http://r.takjoo.profcms.um.ac.ir/imagesm/1006/stories/Spectroscopy/93/section%2023.pdf
 JOHN A. WEIL and JAMES R. BOLTON
 https://fen.nsu.ru/posob/organic/physmethods/lit/NMR/add/Rieger_Electron%20spin%20resonan
ce.pdf
 www.Wikipedia.com

Spin spin splitting in esr

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Spin spin splitting in esr