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Applications of NMR ( 1 H & 13 C) Spectroscopy P.Sindhuka Pharmaceutical Technology

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OUTLINE Features of a NMR spectrum II. Information could be deduced from an unknown NMR spectrum III. Structure Elucidation IV. General applications

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Features of a NMR spectrum: NMR spectrum consists of horizontal scale showing chemical shift and the externally applied magnetic field strength . Applied magnetic field strength increases from left to right. Signals to the left of the spectrum-Downfield. Signals on the right of the spectrum- U pfield . The horizontal scale is shown as δ ( ppm ). It is a spectra of chemical shift Vs intensity. 13 C resonances are 0 to 220 ppm downfield from TMS . 8 7 6 5 4 3 2 1 0 δ ( ppm ). Up field Down field

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The electrons surrounding a nucleus affect the effective magnetic field sensed by the nucleus

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The Larmor Equation!!! D E = f B o = f ( g ) B O = h n D E = g ( h / 2 p) B O = h n can be transformed into g n = 2 p B o gyromagnetic ratio g strength of the magnetic field g (Ratio of magnetic moment to angular momentum) is a constant which is different for each atomic nucleus (H, C, N, etc.) frequency of the incoming radiation that will cause a transition

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THE NMR SIGNAL h n = g ( h / 2 p) B O RF n = w R F 2. Irradiation of nuclei with RF ( n) signal (radiation) (Absorption) Ground state (Emission) Boltzmann equilibrium 3. Signal (Maximum): Resonance ∞ Bo υ i s kept constant, Bo is swiped/varied 4. Relaxation Bo Nuclei is kept in a homogenous magnetic field (Bo) 1> 2> 3> 4 δ E

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SHIELDED Lower frequency UPFIELD DESHIELDED Higher Frequency DOWNFIELD NMR Correlation Chart TMS Ranges can be defined for different general types of protons .

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NMR spectra: 1 H NMR spectrum of ethanol in CDCl 3

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1 H NMR spectrum of ethanol in CDCl 3

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13 C NMR spectra:

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II. Information could be deduced from an unknown NMR spectrum: 1.Each different type of hydrogen gives a peak or group of peaks( multiplet ). 2.The chemical shift ( δ ,in ppm )gives a clue as to the type of hydrogen generating the peak ( alkane , alkene , benzene , aldehyde ,etc.) 3.The integral gives the relative no. of each type of hydrogen. 4.Spin-Spin splitting gives the no. of hydrogens on adjacent carbons. 5.The coupling constant J also gives information about the arrangement of the atoms involved. 1 H NMR spectrum:

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13 C NMR— Number of Signals: The number of signals in a 13 C spectrum gives the number of different types of carbon atoms in a molecule. Because 13 C NMR signals are not split, the number of signals equals the number of lines in the 13 C spectrum. In contrast to the 1 H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13 C NMR signals are not integrated.

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In contrast to the small range of chemical shifts in 1 H NMR (10-20 ppm usually), 13 C NMR absorptions occur over a much broader range (0-220 ppm ). The chemical shifts of carbon atoms in 13 C NMR depend on the same effects as the chemical shifts of protons in 1 H NMR. Position of Signal:

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INVESTIGATION OF DYNAMIC PROPERTIES OF MOLECULES: 1.Conformational isomerism 2.Keto-enol tautomerism 3 .Optical purity 4.Structure determination III. General applications

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1.Cyclo hexane at room temperature exists as rapidly inter converting chair forms. 2.Spectrum consists of a single averaged peak. 1.Conformational isomerism:

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2.Keto-enol tautomerism (Integration & chemical shift): Ex:

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Ex: BARBITURIC ACID % Keto -Crystalline state % enol - Aqueous solution

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4.Optical purity: Diastereomers differ in their NMR characteristics. Enantiomeric ratios can be calculated from integration cruve . Ex : Optical mixture of Ephedra alkaloids.

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Enantiotopic Distinctions If H’s are in environments that are mirror images of each other, they are enantiotopic Replacement of each H with X produces a set of enantiomers The H’s have the same NMR signal (in the absence of chiral materials)

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Enantiotopic Distinctions

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Diastereotopic Distinctions In a chiral molecule, paired hydrogens can have different environments and different shifts Replacement of a hydrogen with X gives a different diastereomer . Diastereotopic hydrogens are distinct chemically and spectrocopically

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5.Structure determination: Structure of SOF 4: it shows only one signal . Structure of CIF 3: : it shows 2 types of flourine signals structure of polyethylene

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2. HYDROGEN BONDING: HYDROGEN BONDING DESHIELDS PROTONS The chemical shift depends on how much hydrogen bonding is taking place. Alcohols vary in chemical shift from 0.5 ppm (free OH) to about 5.0 ppm (lots of H bonding). Range: 0.5-5.0 Hydrogen bonding lengthens the O-H bond and reduces the valence electron density around the proton - it is deshielded and shifted downfield in the NMR spectrum. Concentrated solution ( Intramolecular ) R-O-H Free (dilute solution) The more H bonding that takes place, the more deshielded a proton becomes, depends on conc.

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Factors affecting H-bonding Solute concentration: Increased H bonding, greater deshielding Temperature: Raising temperature, weakening of H-bonds, up field shift (shielding) occurs Intramolecular H bonding are independent of temperature and solute concentration Note: NMR spectroscopy is thus used in studying H bonding .

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SOME MORE EXTREME EXAMPLE Carboxylic acids have strong hydrogen bonding - they form dimers . With carboxylic acids the O-H absorptions are found between 10 and 12 ppm very far downfield. Range: 10.5-12.0 Amines RNH 2 : 0.5-5.0 Phenols ArOH : 4.0-7.0

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3.DRUG-MACROMOLECULE INTERACTION : Interactions between molecules of low molecular weight and macromolecules 2 NMR parameters are observed. 1.Chemical shift 2.Line width Drug or Substrate Enzyme or macromolecule Complex Large T 2 Small T 2 Bound substrate has smaller T 2 than free substrate Narrow line Broad line Broader line than substrate alone

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Ex: Storage of catecholamines -Adrenaline form complexes with nucleotides. Acetyl choline interaction with Esteric site of Acetylcholinesterase .

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4.APPLICATIONS IN FORENSIC SCIENCE : 1 H-nuclear magnetic resonance (NMR) was applied to the study of metabolites in rat organs and tissues From 1 H-NMR spectra -extracts of various organs and tissues, lactate, pyruvate , and some other substances were simultaneously identified and quantitated .

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5. FUTURE APPLICATIONS: 1.Use of NMR to monitor rate processes 2. Materials science 3. Food chemistry 4. NMR as a microscope 5. Nuclear Magnetic Resonance Tomograph : Useful in the identification of different kinds of tissues as well as the visualization of organs.It is a very important field of medicare applications, knowing as magnetic tomography or Magnetic Resonance Imaging, MRI.

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IV.Quantitative analysis: Estimation : It has limited scope. A drug can be rapidly quantified by measuring suitable protans . t- butanol used as internal standard. The amount of drug present can be calculated by using the formula:

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Amount of drug= Area signal for int. std. protons Area signal for drug protons x Mass of int.std. X M d Mstd X n std . n d EX: NMR spectrum obtained from tablet containing Aspirin , Paracetamol and Codeine. t- butanol used as internal standard

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The researchers said MRI shows how the treatment targets a specific area of the brain — Brain 'pacemakers' promising for anorexia treatment Experimental deep brain stimulation could help treat eating disorder

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