This document provides an overview of electron spin resonance (ESR) spectroscopy. It discusses key concepts like spin-spin splitting, hyperfine coupling and splitting patterns. Examples are provided to illustrate hyperfine splitting from interactions with nuclei like protons and isotopes. Superhyperfine splitting from interactions between the unpaired electron and multiple equivalent nuclei is also described. The summary concludes with a brief discussion of anisotropic systems and non-spherical symmetry leading to orientation-dependent g-factors.

1. 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

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

3. 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

4. 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

5.  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

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

7.  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

8.  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

9.  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

10.  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

11. 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

12.  Relative Intensities for I = 1

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

14.  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

15. 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

16.  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.

17.  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

18.  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

19.  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.

20. 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

21. 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