Shielding effect,effect of chemical exchange,hydrogen bonding

1. SHIELDING EFFECT, EFFECT
OF CHEMICAL EXCHANGE &
HYDROGEN BONDING
Guided By Presented By
Dr.Ashish Asatkar Sumeet Kumar Jha
Mr. Nasiir Ahmed M.Sc.(Chemistry)
Smt. Chandra Kiran Patel 2nd Sem
Roll No.-30828
GOVT.GUNDADHUR
P.G.COLLEGE,KONDAGAON(C.G.)

2. CONTENTS
 SHIELDING EFFECT
 HYDROGEN BONDING
 EFFECT OF CHEMICAL EXCHANGE
 REFERENCE

3. SHIELDING EFFECT
 According to the equation ∆E = h/2πγb, single peak should be there for all
the hs. Fortunately, the situation is no so simple.
 In the vicinity of the nucleus, field genrated by the circulating electron
reduces the external magnetic field the nucleus “feels”.
 More the electron density around the nucleous, less the experienced
magnatic field by it. Such nuclei are called shielded and the effect is called
shielding effect.
 The need a higher-frequency to achieve resonance.

4.  At constant radio frequency, shielding shifts the absorption to lower
frequency in the molecules when there is a spherical distribution of electrons
around the protons. this is known as positive shielding.
 If the secondry field produced by the circulating electrons reinforce the
applied field, the position of resonance moves to higher frequency. this is
known as negative shielding.

5. HYDROGEN BONDING
 Hydrogen bonding involves transfer of electron cloud from hydrogen atom
to neighbouring electronigative atom.
 Protons that are involved in hydrogen bonding are typically change the
chemical shift values.
 The more hydrogen bonding, the more proton is deshielded and chemical
shift value is higher.
 Hydrogen bonding decrease the electron density around the proton, causing
deshielding.
 If temprature of the sample increases, weaker the hydrogen bonding cause up
shield shift.

6.  The chemical shieft of :-
(a) Inter molecular hydrogen bonded protons are dependent of temprature
and solute concentration.
(B) intra molecular hydrogen bonded protons are indipendent of temprature
and solute concentration.
 Protons that are involved in hydrogen bonding(this usually means –oh or
-nh) are typically observed over a large range of chemical shift values.
 The more hydrogen bonding there is the more the proton is deshielded and
the higher its chemical shift will be.
 The amount of hydrogen bonding is suscepitible to factors such as solvalation,
acidity, concentration and temprature, it can often be difficult to predict.

7.  Alcohols vary in chemical shift from 0.5 ppm(free -OH) to about 5ppm (H
bonding).

8. CHEMICAL EXCHANGE :-
Chemical exchange refers to any
process in which a nucleus exchange between two or more
enviorments in which its NMR parameters (e.g. chemical shift, scalar
coupling or relaxation) differ.
EFFECT OF CHEMICAL EXCHANGE :-
Rate of exchange
and sepration can lead to three orientation:-
1) When the exchange frequency is greater than the sepration
between in the interacting compound, spin decoupling always takes
place and peaks are sharp.
2) When the rate of exchange is significantaly greater than or equal
to the sepration frequency than spin decoupling and peaks are
broad.
EFFECT OF
CHEMICAL EXCHANGE

9. 3)When the exchange frequency is less than the sepration between in the
interacting compound, spin coupling always takes place.
 When acid, base or water is present in a compound, the rate of exchange is
faster than sepration.
 to reduce the rate of exchange with a solution or solvent, it is treated with
anhydrus sodium carbonate, alumina or molecular sieves.
Example:-
In pure ethyl alcohol, the triplet of -CH3 group, the quartet and
the doublet of the -CH2 group and the triplet of the –OH group signal
should be found thus:-

10.  If water is present in the alcohol solution, a rapid interchange occurs,
resulting in a single peak the actual shift positions of the HOH and ROH
peaks.The presence of water is reflected in the integration for the single
peak and we get singlet peaks for –OH, quartet peak for -CH2 and triplet
peaks for -CH3 ,which is as folloews:-

11. REFERENCE
SPECTROSCOPy(h.kaur)