NMR spectroscopy
Download as PPTX, PDF389 likes230,748 views
NMR spectroscopy is a technique that uses magnetic fields and radiofrequency pulses to analyze atomic nuclei and study the physical and chemical properties of molecules. It provides detailed information about molecular structure by detecting hydrogen and other nuclei. The document discusses the basic principles of NMR, instrumentation, factors affecting chemical shifts, and applications in medicine such as anatomical imaging and tumor detection.
NMR spectroscopy
- 1. NMR SPECTROSCOPY Afsath.B M Pharm 1st year Pharmacognosy and Phytochemistry Malik Deenar College of Pharmacy
- 2. • Spectroscpy is the study of interaction of EMR with matter, which may result in absorption, transmission, emission,reflection , rotation of EMR • Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy. Nuclear Magnetic Resonance (NMR) is a spectroscopy technique which is based on the absorption of electromagnetic radiation in the radio frequency region 4 to 900 MHz by nuclei of the atoms. • Proton Nuclear magnetic resonance spectroscopy is one of the most powerful tools for elucidating the number of hydrogen or proton in the compound. • Spectroscopy determines the physical and chemical properties of atoms or the molecules in which they are contained and provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. • It is used to study a wide variety of nuclei: •1H •15N •19F •13C •31P 2
- 3. Theory of NMR Spin quantum number (I) is related to the atomic and mass number of the nucleus Elements with either odd mass or odd atomic number have the property of nuclear “spin” I Z A Eg; Half integer Odd Odd 1H (1/2) Half integer Odd Even 13C (1/2) Integer Even Odd 2H(1) Zero Even Even 12C (0) 3
- 4. • If an external magnetic field is applied,the number of possible orientations calculated by (2I+1). Eg.:-Hydrogen has spin quantum number I=1/2 and possible orientation is (2*1/2+1=2) two , ie,+1/2 and -1/2. 4
- 5. PRINCIPLE OF NMR • The principle is based on the- spinning of nucleus and generating a magnetic field. • Without external magnetic(Bo) – field nuclear spin are random in direction. • With Bo ,nuclei align themselves either with or against field of external magnetic field 5
- 6. Eg, Liq.N2 Vs Liq. O2 6
- 7. • If an external magnetic field is applied, an energy transfer (ΔE) is possible between ground state to excited state. • When the spin returns to its ground state level, the absorbed radiofrequency energy is emitted at the same frequency level. • The emitted radiofrequency signal that give the NMR spectrum of the concerned nucleus and is directly proportional to the strength of the applied field • If an external magnetic field is applied, an energy transfer (ΔE) is possible between ground state to excited state. • When the spin returns to its ground state level, the absorbed radiofrequency energy is emitted at the same frequency level. • The emitted radiofrequency signal that give the NMR spectrum of the concerned nucleus. • If an external magnetic field is applied, an energy transfer (ΔE) is possible between ground state to excited state. • When the spin returns to its ground state level, the absorbed radiofrequency energy is emitted at the same frequency level. • The emitted radiofrequency signal that give the NMR spectrum of the concerned nucleus. 7
- 8. NMR SPECTRUM NMR spectrum is a plot of intensity of NMR signals VS magnetic field (frequency) in reference to TMS Reasons for taking TMS as reference Standard 1. Chemically inert, magnetically isotopic, volatile & soluble in most organic solvents. 2. TMS gives an intense signal. 3. TMS can be easily removed. 4. Electro negativity is low. 5. It doesn’t make any intermolecular association with sample 8
- 9. Chemical shift • Is the resonance frequency of a nucleus relative to a standard in a magnetic field. • TMS is the most common reference compound in NMR, it is set at 𝛿 = 0𝑝𝑝𝑚 𝛿=ν𝑠𝑎𝑚𝑝𝑙𝑒 − 𝜈𝑟𝑒𝑓 𝜈𝑟𝑒𝑓 Shielding of protons:- High electron density around a nucleus shields the nucleus from the external magnetic field and the signals are upfield in the NMR spectrum Deshielding of protons:- Lower electron density around a nucleus deshields the nucleus from the external magnetic field and the signals are downfield in the NMR spectrum 9
- 10. Factors affecting chemical shift • Electronegative groups/ Inductive effect • Magnetic anisotropy of π-systems / diamagnetic effect of pi bond • Hydrogen bonding • Vander Waal's deshielding • Effect of temperature &Effect of solvent Electronegative groups:- Electronegative groups attached to the C-H system decrease the electron density around the protons, and there is less shielding (i.e.deshielding) and chemical shift increases Compound Chemical shift CH3I 2.16 CH3Br 2.65 CH3Cl 3.10 CH3F 4.26 10
- 11. Magnetic anisotropy of π-systems-( space effect) So magnetic anisotropy means that there is a "non-uniform magnetic field". Electrons in π systems (e.g. aromatics, alkenes, alkynes, carbonyls etc.) interact with the applied field which induces a magnetic field that causes the anisotropy. It causes both shielding and de-shielding of protons. Example:-Benzene Vander Waal's deshielding The electron cloud of a bulkier group will tend to repel the electron cloud surrounding the proton. such a proton will be deshielded & will resonate at slightly higher value of δ than expected in the absence of this effect. Effect of temperature Resonance position of most signals is little affected by temperature. 11
- 12. Effect of solvent Chemical shift change when the solvent changed from CCl4 to CDCl3 is 0.1 ppm. But change to more polar solvents like methanol, the change is 0.3 ppm. Solvents used in NMR CCl4, CS2, CDCl3, C6D6, D2O Hydrogen bonding Protons that are involved in hydrogen bonding are typically change the chemical shift values. The more hydrogen bonding, the more proton is deshielded and chemical shift value is higher. 12
- 13. Proton NMR The most common for of NMR is based on the hydrogen-1 (1H), nucleus or proton. It can give information about the structure of any molecule containing hydrogen atoms. Eg, ethanol 3 types of ;-CH2,CH3,OH Interpretation of 1HNMR spectra:- Number of signals - Indicates how many "different kinds" of protons are present. Position of signals - Indicates something about (chemicalshift) magnetic (electronic) environment of protons Relative intensity of signals - Proportional to number of protons present signals Splitting of signals (spin spin coupling) - Indicates the number of nearby nuclei usually protons 13
- 14. n+1 rule • The multiplicity of signal is calculated by using n+1 rule. • This is one of the rule to predict the splitting of proton signals. This is considered by the nearby hydrogen nuclei. Therefore, n= Number of protons in nearby nuclei Zero H atom as neighbour n+1=0+1=1(singlet) One H atom as neighbour n+1=1+1 = 2(doublet) Two H atom as neighbour n+1=2+1 =3(triplet) Spin-spin coupling (splitting) The interaction between the spins of neighbouring nuclei in a molecule may cause the splitting of NMR spectrum. The splitting pattern is related to the number of equivalent H- atom at the nearby nuclei. Eg., Ethyl acetate 14
- 15. Spin-spin coupling (splitting) cont… 15
- 16. Rules for spin-spin coupling • Chemically equivalent protons do not show spin-spin coupling &Only nonequivalent protons couple. X X Hb Hc Hd H Hb couples with Hc Ha C C C C H Hb & Ha donot couple because they are equivalent H H H H Hc & Hd donot couple because they are equivalent • Protons on adjacent carbons normally will couple. • Protons separated by four or more bonds will not couple. 16
- 17. ORIGINS OF SIGNAL SPLITTING The origins of signal splitting patterns in which, each arrow represents an Hb nuclear spin orientation. COUPLING CONSTANT (J-Hz) • Measurement of splitting effect is based on the distance between the peak in a given multiplet. • Useful in 1H NMR of complex structure 17
- 18. INSTRUMENTATION 1. Sample tube/sample holder 2. Permanent magnet 3. Magnet coil 4. Sweep generator 5. Radio frequency transmitter 6. Radio frequency reviever 7. Read out system 18
- 19. • Sample tube / sample holder It should be chemically inert, durable & transparent to NMR radiation. Generally about 8.5 cm long & approximately 0.3 cm in diameter is employed. • Sample probe It’s the device that hold sample tube in position & is provided with an air driven turbine for rotating the sample tube almost 100 revolutions per min. • Permanent Magnet It provide homogenous magnetic field at 60-100MHz • Magnetic coil It induce magnetic field when current flow through them • Sweep generator To produce equal amount of magnetic field pass through the sample 19
- 20. • Radio frequency transmitter transmitter is fed on to a pair of coils mounted on right angles to the path of field. 60 MHz capacity is normally used. • Radio frequency receiver detect radio frequencies emitted as nuclei relax at lower energy level • Signal detector & recording system The electrical signal generated is amplified by means of amplifier & then recorded. 20
- 21. Application ……… • 1H NMR used for structural elucidation of organic and inorganic solids • determines the physical and chemical properties of atoms • Application in medicine • Anatomical imaging • Measuring physiological function • Flow measurement and angiography • Tissue perfusion studies • Tumours • MRI 21
- 22. Application in medicine… BRAIN • Distinguishing grey matter & white matter • Imaging posterior fossae , brain stem, spinal cord • Detect demyelinating lesions, tumour, haemorrhage, infarctions ABDOMEN • Metabolic liver disease • Focal areas of inflammation in chronic active hepatisis KIDNEY • Distinguishing renal corta & medulla • To evaluate transplanted kidney PELVIS • Differentiate between benign prostatic hyperplasia &prostatic carcinoma 22
- 23. REFERENCE • Organic spectroscopy, Willaim Kemp • Instrumental method of chemical analysis • Instrumental method of chemical analysis. Wilard • https://www.slideshare.net/solairajananant/nmr-spectroscopy-13887430 • http://sydney.edu.au/science/chemistry/facilities/nmr/nmr-applications.shtml 23