NMR SPECTROSCOPY

NMR SPECTROSCOPY

1. NMR SPECTROSCOPY By: V.Vidya dhari 170120885007 M pharm analysis-Sem II Advanced instrumental analysis.

2. INTRODUCTION – NMR SPECTROSCOPY. INSTRUMENTATION SOLVENT REQUIREMENT IN NMR. RELAXATION PROCESS IN NMR. CHEMICAL SHIFT. FACTORS AFFECTING CHEMICAL SHIFT. NMR SIGNALS IN VARIOUS COMPOUNDS. Contents

3. NMR SPECTROSCOPY - Introduction This spectroscopic technique gives us information about the number and types of atoms in a molecule. Nuclear magnetic spectroscopy is a powerful analytical technique used to organize organic molecules by identifying carbon hydrogen frameworks within molecules. Principle : The principle behind NMR is that  many nuclei have spin and all nuclei are electrically charged . If an external magnetic field is applied, an energy transfer is possible between the base energy to a higher energy level.

4. A NMR spectrophotometer consists of following components : A Magnet Sample and Sample holder Radio frequency generator Detector Reader WORKING : In NMR spectrometer, the sample is dissolved in CDCL3 and placed in magnetic field. Then radio frequency generator irradiates the sample with short pulse of radiation causing resonance. When nuclei fall back to their lower energy state, detector measures the energy released and the spectrum is recorded. The super conducting magnet in modern NMR spectrometers have coils that are cooled in liquid Helium and conduct electricity. Instrumentation

5. Solvent Requirement in NMR The solvents used in NMR spectroscopy should be chemically inert, magnetically isotropic, devoid of hydrogen atom and should dissolve the sample to a reasonable extent. • Eg : CCl4, CS2, CDCl3, DMSO etc. When selecting a solvent for running Nuclear Magnetic Resonance Spectroscopy (NMR) analysis, typically a deuterated solvent is used in order to minimize background signals and provide a lock signal to compensate for drifts in the magnetic field. NMR solvents are distinctly different from other spectroscopic solvents as the majority of hydrogen nuclei are replaced with deuterium so as to minimize the interference due to protons. Though deuterium also has a nuclear spin, it does not operate on the same frequency as protons in the given magnetic field. Therefore, it serves the purpose of NMR spectroscopy.

6. It is crucial to remember that the price increases with the degree of deuteration. Deuterated chloroform, is most commonly used because of its low price. Plus, the NMR peak is observed with the minimal chemical shift. A reference standard such as tetramethylsilane is commonly added (around 0.03%) to most commercially available solvent grades to serve as zero ppm chemical shift reference for NMR studies. However, there are several other factors to consider for NMR solvent selection: 1. Solubility 2. Interfering peaks 3. Price 4. Isotopic purity 5. Ease of NMR sample recovery. Common spectroscopic solvents are available commercially in different degrees of deuteration. Apart from CDCl3 other deuterated solvents in common use are:

7. Deuterated water-\(D_2O\) DMSO (There is no ???? NMR peak, making this one of the best solvents) Methanol Methylene chloride Pyridine Acetic acid Acetone For Example : CDCL3 : This solvent is the deuterated form of chloroform ,since it is relatively in expensive with isotopic purity, dissolves many compounds, and is easy to evaporate after analysis for NMR sample recovery. Since no solvent is 100% deuterated there will always be an observed ¹H peak for the solvent, which may interfere with the compound of interest. 2. DMSO : Compounds like DMSO have benefits to use as an NMR solvent as they are relatively inexpensive and can solubilize many compounds that are difficult to solubilize.

8. Relaxation Process In NMR Relaxation is the process by which the spins in the sample come to equilibrium with the surroundings. The rate of relaxation determines how fast an experiment can be repeated. The rate of relaxation is influenced by the physical properties of the molecule and the sample.

9. An understanding of relaxation processes is important for the proper measurement and interpretation of NMR spectra. There are three important considerations. 1. The very small energy difference between α and β states of a nuclear spin orientation in a magnetic field results in a very small excess population of nuclei in the ground vs the excited states. For many nuclei, relaxation is a very slow process. It is thus very easy to saturate an NMR transitions, with the resultant loss in signal quality, and failure to obtain correct peak areas. 2. NMR lines are extraordinarily sharp, and close compared to higher energy spectroscopic methods. When relaxation is very fast, NMR lines are broad, J-coupling may not be resolved or the signal may even be difficult or impossible to detect. 3. The success of many multipulse experiments, especially 2D and 3D spectra, depends crucially on proper consideration of relaxation times.

10. NMR Relaxation is of two types : 1. Spin-Lattice or Longitudinal Relaxation. 2. Spin-spin or Transverse Relaxation.

11. Relaxation process occurs along z-axis. Transfer of the energy to the lattice or the solvent material. Coupling of the nuclei magnetic field with the magnetic field of the ensemble of the vibrational and rotational motion of the lattice or the solvent. Results in a minimal temperature increase in sample. Relaxation time (T1) → Exponential decay. Spin-Lattice or Longitudinal Relaxation

12. Relaxation process in the X-Y plane. Exchange of energy between excited nucleus and low energy state nucleus. Randomization of spins or magnetic moment in X-Y plane. Related to NMR peak line-width. Relaxation time T2. T2 may be equal to T1. No energy change Mx = My = M0 [1- e(- t/T2] Spin-spin or Transverse Relaxation

13. 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. 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. CHEMICAL SHIFT

14. Under the influence of the magnetic field electrons bonding the protons tends to process around the nucleus in a plain perpendicular to magnetic field The position of the peaks in an NMR spectrum relative to the reference peak is expressed in terms of the chemical shift ᵟ = H0(reference) - H0(sample) X 106PPM H0(reference)

15. The value of H0 for the reference is usually greater than H0 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 106ppm ѵ(reference ) Chemical shift is dimension less and expressed in parts per million (ppm ). Alternative system used is tau scale. τ=10-ᵟ scale the position of TMS signal is taken as 0.0 ppm Most chemical shifts value ranges from 0-10. A small value in tau represents low field absorption. High value represents high field absorption. Greater the deshielding of protons larger the value of delta.

16. Reasons for Chemical Shift Positive shielding : Resonance position moves upfield. Negative shielding : Resonance position moves downfield.

17. In order to measure the magnitude of chemical shifts of different kinds of protons, There must be some standard signal . 0.5%Tetra methylsilane (TMS)(ch3)4si is used as reference or standard compound. Chemical shift is represented by δ. Δ scale: 0 to 10 scale. TMS is taken as zero markers. Dimensionless expression; negative for most protons Measurement of Chemical Shift

18. 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 . 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. P rotons in the methyl group of DSS gives a strong line. TMS – As Internal Standard

19. Factors Affecting Chemical Shift 1. Electronegativity and Inductive effect 2. Anisotropic effect 3. Vander waals Deshielding 4. Hydrogen Bonding

20. 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. There for the absorption occurs downfield. The deshielding is directly proportional to the halogens oxygen or nitrogen. +I effect : An electron releasing group increase the electron density around the proton and give rise to its shielding. -I effect : An electron withdrawing group is able to reduce electron density around the proton and Deshields the proton.

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

22. Anisotropic effect in Alkanes Alkanes do not possess the same degree of electron circulation as alkynes but do not exert nonlocal fields on adjacent nuclei. The c-c  

23. Anisotropy in Alkynes In a magnetic field, the pi-electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B॰). Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield .

24. Anisotropy In Benzene In a magnetic field, the six electrons in a benzene ring circulate around the ring creating a ring current. The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons. The protons feel a stronger magnetic field and thus are deshielded. A higher frequency is needed for resonance.

25. 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. The presence of bulky groups in a molecule cause deshielding due to weak VanderWaals force and give higher δ value.

26. 4. Hydrogen Bonding Intra-molecular hydrogen bonding does not show any change in absorption due to change in concentration. While hydrogen atom involved in the intermolecular H-bonding shares its electrons with two electronegative elements and as a result it itself deshielded and get higher δ value. • E.g. Carboxylic acid dimer and β-diketones.

27. NMR Signal in Various Compounds The number of NMR signals represent the number of protons in a molecule.

28. For example, let’s start with the simplest hydrocarbon; how many signals would you expect to see on the NMR spectrum of methane? Even though methane has four protons, they are all connected to the same atoms and have the same neighbors on all sides – in other words, they are equivalent because they are in the same environment. Remember, equivalent protons give one NMR signal:

29. It is the same with ethane (CH3-CH3); six protons – all equivalent, therefore one NMR signal:

30. For example: Bromoethane gives two NMR signals because the protons of the CH2 groups, being closer to the bromine, are different from those in the CH3 group:

31. Propane and butane give two signals. One because the protons of the CH2 group are different from those in the CH3 group, and the other, because despite having four carbon atoms, the molecule is a combination of two identical CH2 and CH3 groups:

32. Butane also gives 2 NMR signals. Because protons a are different from protons b. Each type gives one NMR signal.

33. Introduction to spectroscopy-5 th edition- NMR Spectroscopy by Donald Pavia, Pg no: 215 – 262. Spectrometric Identification of Organic Compounds-6 th Edition-- Robert Silverstein, Pg No: 222 - 245. www.slideshare.net/chemicalshift https://www.sciencedirect.com/science/nuclear-magnetic-resonance http://chem.ch.huji.ac.il/nmr/whatisnmr/whatisnmr.html Images – Internet search REFERENCES

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