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SEMINAR ON
INTERNAL STANDARDS USED IN NMR
Facilitator:
Dr. B.M.Gurupadayya
Professor
Mr. Chandan .R.S
Asst. professor
J.S.S.C.P, Mysore.
Presented by:
Rajendra Prasad.P.C
I.M.Pharma
Industrial Pharmacy
J.S.S.C.P, Mysore.
1
INTERNAL STANDARDS:
Definition:
An internal standard in analytical chemistry is a chemical
substance that is added in a constant amount to samples, the blank
and calibration standards in a chemical analysis. This substance
can then be used for calibration by plotting the ratio of the analyte
signal to the internal standard signal as a function of the analyte
concentration of the standard.
2
The commonly used internal standards are:
Tetramethyl Silane (TMS): It is generally employed as internal
standard for measuring the position of 1H, 13C and 29Si in NMR
spectroscopy.
It is due to the following facts:
It is chemically inert and miscible with a large range of solvents.
Its twelve protons are all magnetically equivalent.
It is highly volatile and can be easily removed to get back the sample
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It does not take part in intermolecular associations with the sample.
Its resonance position is far away from absorptions due to protons in the
molecule. Thus, assigning TMS = 0 (δ), a scale can be devised in which
most of the proton resonances are of the same sign.
3-(trimethyl silyl) propane sulphonate(sodium salts):
It is a water soluble compound. It is used as internal standard for running
PMR spectra of water soluble substances in deuterium oxide (D2O) solvent.
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SOVENTS USED IN NMR:
A substances free of proton should be used as a solvent, i.e., which
does not give absorption of its own in NMR spectrum. Moreover, the
solvent should be capable of dissolving at least 10% of the substance
under investigation
To satisfy the condition that nonviscous samples give the sharpest
NMR spectra, it is usually necessary to record the spectra of organic
compounds in solution.
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Following solvents are commonly used in NMR spectroscopy:
CCl4 carbon tetrachloride
CS2 carbon disulfide
CDCl3 deuteriochloroform (chloroform-d)
C6D6 hexadeuteriobenzene (benzene-d6)
D2O deuterium oxide (heavy water)
(CD3)2SO hexadeuteriodimethylsulfoxide
(CD3)2CO hexadeuterioacetone (acetone-d6)
(CCl3)2CO hexachloroacetone
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These solvents differ considerably as regards their polarity.
Clearly, NMR spectrum of a compound measured in one solvent
may be slightly different from that measured in another solvent of
different polarity
Hydrogen bonding also affects the value of in this technique.
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As hydrogen bonding involves electron cloud transfer from hydrogen
atom to a neighbouring electronegative atom (O, N, S etc), the
hydrogen atom experience a net deshielding effects.
At higher concentration of -OH, -NH compounds (strong
intermolecular hydrogen bonding), deshielding of proton is greater
and signal appears at higher value than that at low concentration.
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With increase in temperature, the extent of hydrogen bonding falls.
Thus, signal in NMR appears at smaller value at high
temperature.
Carboxylic acids exist as dimmers (by hydrogen bonding) which
are resonance stabilised.
For a compound showing intramolecular hydrogen bonding, the
concerned value remains unchanged even if the solution is
diluted.2 March 2013 11
Trifluro acetic acid is a very useful solvents as its proton absorbs
at a very low field ( = 11.2) and hence, does not interfere with most
of the spectra. It is especially useful for the NMR spectrum of
amines.
The proton of the substituted aliphatic and aromatic ammonium ion
absorbs in the range 0.5-2.9 (aromatic at low field).
Dimethyl sulphoxide (DMSO) high polar solvents can be used
in NMR spectroscopy.2 March 2013 12
Some important characteristics of solvents used in this technique
are :
It should be chemically inert and magnetically isotropic,
It should be devoid of hydrogen atom, and
It should dissolve the sample to a reasonable extent.
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References:
1.Instumental and bio medical analysis by BK Sharma, page
no:500-516.
2. Organic spectroscopy by William kemp, 3rd edt, page no:109-
112, 131-132.
3. Elementary organic spectroscopy Y.R.Sharma, page no: 191,192.
198-200.
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