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.

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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.

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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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H2O

D2O

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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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THANK YOU

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