Chemical shift reagents and solvents used in NMR.18-03-11

Chemical shift reagents and solvents used in nuclear magnetic resonance spectroscopy A.Haripriya M.Pharm(analysis&qa) :

Chemical shift reagents and solvents used in nuclear magnetic resonance spectroscopy A.Haripriya M.Pharm(analysis&qa) 1

Introduction:

Introduction As is implied in the name, nuclear magnetic resonance is concerned with the magnetic properties of certain atomic nuclei. Analysis of a NMR spectrum provides information on the number and type of chemical entities in a molecule. However, NMR provides much more information than IR. 2

Basic concepts::

Basic concepts: Basically the nuclei of all elements carry a charge. When the spins of the protons and neutrons present in the nuclei are not paired, the overall spin of the charged nucleus generates a magnetic dipole along the spin axis. A nucleus of odd mass number A will have a half-integer spin and a nucleus of even A will have integer spin. 3

principle:

principle radio Freq waves 4

Chemical shift: :

Chemical shift: NMR has great utility because not all protons in a molecule have resonance at the same frequency. This variability is due to the fact that the protons in a molecule are surrounded by electrons and exist in slightly different environment from one another. The valence shell electron densities vary from one proton to another. so they precise at different resonance frequencies. 5

Chemical shift::

Chemical shift: These differences in resonance frequency are very small. Hence it is very difficult to measure exact frequencies to that precision Hence shift is measured relative to the resonance frequency of the protons of the reference substance. The shift from TMS for a given proton depends on the strength of the applied magnetic field. 6

Chemical shift::

Chemical shift: To avoid the confusion in working with instruments of different frequencies can be solved by introducing a field independent measure called “chemical shift”. The observed chemical shifts depend not only on the structure of molecule being studied, but also on the interaction between the molecule & the surrounding solvent molecules. Hence selection of solvents also plays a vital role in NMR. 7

solvents used in NMR:

solvents used in NMR NMR spectra are usually obtained by dissolving the sample in an appropriate solvent. Because the solvent is present in much higher concentration than the sample, protons in the solvent would overwhelm the resonance peaks for the sample. For this reason, deuterated solvents are used in NMR spectroscopy 8

Solvents::

Solvents: Substance must be dissolved in a solvent, and the solvent that is selected should have certain desirable properties. It should be cheap, It should dissolve a wide range of substances and It should contain deuterium. E.g. 9

Solvent shifts::

Solvent shifts: Solvent Non polar e.g. hydrocarbons Weak interactions between Solute and solvent Minimum effect on chemical shift Polar e.g. acetone, chloroform Strong interactions between solute and solvent Shift is more than that in non polar solvent. 10

Solvent shifts::

Solvent shifts: Strong diamagnetic anisotropic solvent Eg benzene, pyridine. Solvent interacts more strongly with Solute than TMS More shift of solute molecules with respect to shift of TMS 11

Solvent shifts::

Solvent shifts: At low concentrations, intermolecular hydrogen bonding is diminished in simple OH, NH, and SH compounds At high concentrations (strong hydrogen bonding, strong desheilding) OH, NH and SH protons appear at higher δ than in dilute solutions. Since hydrogen bonding involves electron cloud transfer from the hydrogen atom to neighboring electronegative atom(O,N ,S), the hydrogen experiences a net desheilding effect when hydrogen bonding is strong, and is less deshielded when hydrogen bonding is diminished. 12

Solvent shifts::

Solvent shifts: Intra molecular hydrogen bonding is unchanged by dilution and the NMR spectrum from such systems is virtually unaltered by varying concentrations. Increased temperature also reduces intermolecular hydrogen bonding, so the resonance positions for these protons are temperature dependent . These solvent induced shifts mainly useful in clarifying the complex spectra which has a overlapping multiplets. 13

Chemical shift reagents:

Chemical shift reagents 14

Why Chemical shift reagents? :

Why Chemical shift reagents? Because the chemical shifts of several groups of protons are all very similar , which shows their proton resonances in the same area of the spectrum and often peak overlap so extensively that individual peaks and splitting cannot be extracted. 15

Chemical shift reagents:

Chemical shift reagents are organic complexes of paramagnetic rare earth metals from the lanthanide series. Of the lanthanides, europium is probably the most commonly used metal. Two of its widely used complexes are Tris (dipavalomethanato) europium and tris-(6,6,7,7, 8,8,8- heptafluoro-2,2-dimethyl-3,5-octanedionato) europium, frequently abbreviated Eu(dpm)3 and Eu(fod)3, respectively 16

Chemical shift reagents:

Chemical shift reagents 17

Chemical shift reagents:

Chemical shift reagents When such metal complexes are added to the compound whose spectrum is being determined, there is a profound shifts in the various groups of protons. The direction of the shift (up field or downfield) depends primarily on which metal is being used. Complexes of europium, erbium, thulium and ytterbium shift resonances to lower field, while complexes of cerium, praseodymium, neodymium, samarium, terbium, and holmium generally shift resonances to higher field. 18

Interaction of chemical shift reagents::

Interaction of chemical shift reagents: These lanthanide complexes interact with a relatively basic pair of electrons ( an unshared pair ) which can coordinate with Eu+3. Typically, aldehydes, ketones, alcohols, thiols, ethers and amines all interact: 19

Chemical shift reagents:

Chemical shift reagents The amount of shift a given group of protons experiences depends on the distance separating the metal (Eu3+)and that group of protons and the concentration of shift reagent in the solution. Hence it is necessary to include the number of mole equivalents of shift reagents used or its molar concentration when reporting a lanthanide shifted spectrum. 20

Chemical shift reagents::

Chemical shift reagents: E.g:The spectra of 1- hexanol: In the absence of shift reagent, the spectrum shown Only the triplet of the terminal methyl group the triplet of the methylene group next to the hydroxyl are resolved in the spectrum. The protons (aside from O-H) are found together in a broad, unresolved group. With the shift reagent added each of the methylene groups is clearly separated and is resolved into proper multiplet structure. 21

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Advantages of using chemical shift reagents:

Advantages of using chemical shift reagents gives spectra which are much easier to interpret. No chemical manipulation of the sample is required with the use of shift reagents. more easily obtained. 24

disadvantage:

disadvantage shift reagents cause a small amount of line broadening At high shift reagent concentrations this problem becomes serious, but at most useful concentrations the amount of broadening is tolerable . 25

Conclusion::

Conclusion: Thus the chemical shift reagents and solvent induced shifts have their application in resolving the NMR spectra of complex structures by inducing shifts with respect to reference compound. Thus useful in interpretation of structures of complex organic compounds. 26

References: :

References: Donald L.pavia, Gary M.Lampman, Georges S.Kriz , Introduction to spectroscopy , 3 rd edition, printed in the United States Of America, P.322-323, 109-110. William Kemp, organic spectroscopy , 3 rd edition, printed by replica press ltd., India P.102-106, 131-133, 169-170. http://jr.stryker.tripod.com/pchem/nmr.html http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nspin.html http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm. 27

Any questions??:

Any questions?? 28

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