Chemical shift

CHEMICAL SHIFT
ISOTOPIC NUCLEI
REFERENCE STANDARDS

1. CHEMICAL SHIFT ISOTOPIC NUCLEI REFERENCE STANDARDS 1 PRESENTED BY ARANTHA J JOSEPH Dep of pharmaceutics St josephs college of pharmacy cherthala .

2. CHEMICAL SHIFT The shift in the position of the NMR region resulting from the shielding and deshielding by electrons is called chemical shift. When a proton is present inside the magnetic field more close to an electro positive atom more applied magnetic field is required to cause excitation. This effect is called shielding effect. When a proton is present outside the magnetic field close to a electronegative atom less applied magnetic field is required to cause excitation . This effect is called deshielding effect 2

3. Greater the electron density around the proton greater will be the induced secondary magnetic field. [ local diamagnetic effect] currents induced by fixed magnetic field result in secondary fields which can either enhance or decrease the field to given a proton responds. 3

4. under the influence of the magnetic field electrons bonding the pr o tons tends to process around the nucleus in a plain perpendicular to magnetic field 4

5. 5     The position of the peaks in an NMR spectrum relative to the reference peak is expressed in terms of the chemical shift ᵟ = H (reference)- H (sample) X 10 6PPM H (reference) The value of H0 for the reference is usually greater than H for the sample,so subtraction in the direction indicated gives a positive ᵟ.In terms of frequency unit ᵟ takes the form ᵟ = ѵ(sample)- ѵ(reference) X 10 6 ppm ѵ(reference)      

6. chemical shift is dimension less expressed in parts per million (ppm ) 10 Alternative system used us tau sc ale τ =10-ᵟ scale the position of TMS signal is taken as 0.0 ppm most chemical shifts value ranges from 0-10 6

7. a small value if tau represents low field absorptio n. high value represents high field absorption . 7 greater the deshielding of protons larger the value of delta

8. Reason for chemical shift 8 Positive shielding :resonance position moves upfield. Negative shielding: resonance position moves downfield.

9. Measurement of Chemical Shift In order to measure the magnitude of chemical shifts of different kinds of protons, T here must be some standard signal . 0.5% Tetra methylsilane (TMS) (ch 3 ) 4 si is used as reference or standard compound. Chemical shift is represented by δ Δ scale: 0 to -10 scale TMS as zero markers. Dimensionless expression; negative for most protons . 9

10. Chemical shift= shift in hz Machine frequency in mhz 10

11. Reason for TMS as reference std TETRA METHYL SILANE(TMS) Accepted internal standard. TMS has 12 equivalent protons and gives an intense single signal. Electro negativity of silicon is very low so the shielding of equivalent protons in TMS is more than other compound so all the signal arrives in a down field direction. 11

12. chemically inert low boiling point so it can be easily removed by evaporation after the spectrum has been recorded so the sample can easily recovered. TMS is not suitable in aqueous solution so DSS ( 2,2-dimethyl-2silapentane-5 sulphate) used as reference. protons in the methyl group of DSS gives a strong line. 12

13. FACTORS AFFECTING CHEMICAL SHIFT 1. ELECTRONEGATIVITY AND INDUCTIVE EFFECT The proton is said to be deshielded if its attached with an electronegative atom/group. Greater the electro negativity of atom greater is the deshielding caused to proton. If the deshielding is more, then δ value also more . Electronegative atoms like Halogens Oxygen and Nitrogen deshield the protons T here for the absorption occurs downfield . The deshielding is directly proportional to the halogens oxygen or nitrogen 13

14. + i effect - i effect An electron withdrawing group is able to reduce electron density around the proton and Deshields the proton. An electron releasing group increase the electron density around the proton and give rise to its shielding. 14

15. 15 Electronegativity Deshieldin g Delta value Electrons around proton induce magnetic field Induce magnetic field oppose applied magnetic field Shielding Neighboring electronegative atom decrease's electron density Deshielding Smaller force is required

16. 2. Anisotropic effect(space effect) Shielding and deshielding can be determined By location of proton in space Space effect Downfield or paramagnetic shift of protons attached to C = C, aldehydic , aromatic proton is experienced by the molecular magnetic field induced by an action of applied field H o on pi electrons, this magnetic field induced by pi electrons are directional or unsymmetrical and this directional measurement is called ANISOTROPY 16

17. ALKANE 17

18. ALKYNE 18

19. Benzene 19

20. 3. VANDERWAALS DESHIELDING The electron cloud of a bulkier group will tend to repel the electron cloud surrounding the proton. Thus such a proton will be deshielded will resonate at slightly higher value of δ than expected in the absence of this effect. 20

21. 4. HYDROGEN BONDING if an atom exhibits hydrogen boding in a comp ound . it will deshielded due to the strongly electronegative atom attached to it so absorption in shifted downfield. Intra molecular hydrogen bonding Inter molecular hydrogen bonding 21

22. ISOTOPIC NUCLEI Many isotopes of chemical elements can be used for NMR analysis 1h Most commonly used spin 1/2 nucleus in NMR Most sensitive nucleus. Produce narrow chemical shift with sharp signal Fast result is possible due to short relaxation time 22

23. 2H A spin 1nucleus commonly utilize as signal free medium Deuterium is used Commonly used in high resolution NMR. To monitor drifts in the magnetic field strength To improve the homogenicity of the external magnetic field 3h Very sensitive to nmr 23

24. 11B - More sensitive than 10 B -Yields sharper signals - Quarts tubes are used(borosilicate glass interfere with measurements) 13C - Spin 1/2 -Widely used, stable to nuclear decay - low sensitivity - Wide chemical shift -Yields sharp signals 24

25. 14N Spin 1 Medium sensitivity Wide chemical shift Limited use to small molecules and functional group. 17 O Spin 5/2 Low sensitivity Low natural abundance Wide chemical shift 25

26. 19f Spin 1/2 Yields sharp signal - Wide chemical shifts 31P Spin 1/2 Medium chemical shift Yield sharp line 26

27. 43ca Used in biochemistry to study Calcium binding to DNA protein Moderately sensitive Very low natural abundance 195 P t eg:-6Li,7 Li 9 Be , 19F 59Co , 61Ni , 77 Se etc ......... 27

28. REFERENCE STANDARDS USED IN NMR 1. Tetra Methylsilane ( tms ) 2. DSS( 4,4-dimethyl-4-silapentane-1-sulfonic Acid) Used In Proton And Carbon Related Nmr High Water Solubility High Intensity Signals Almost All Peaks Found In Naturally Occurring Org Molecule. Show Minor Peaks At 3.1ppm - Triplet 28

29. 3. TRIMETHYLSILYL PROPIONIC ACID Used for aqueous solvents (3-(trimethylsilyl)-2,2',3,3'-tetradeutero propionic acid ) Or tmsp-d4 4. DEUTERATED DMSO Show no peaks in 1+1 H1 NMR signal is observed at 2.20 ppm 13C chemical shift of DMSO - 39.52ppm 29

30. REFERENCE Instrumental Methods Of Chemical Analysis By Gurdeep R. Chatwal ,Sham K Anand, Pg No: 2.192-2.196 Www. Wikipedia.Org 30 Spectroscopy of organic compounds by P.S. Kalsi, page no.453-490. Spectroscopy of organic compound by Y.R. Sharma.

31. THANKUUUUUUUUU........... 31 PRESENTED BY: ARANTHA J JOSEPH