Nuclear Magnetic Resonance Spectroscopy

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NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY : 

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY M.Sudha, I Year, M.Pharm. 1

OUTLINE OF PRESENTATION : 

OUTLINE OF PRESENTATION Introduction Theory Principle Relaxation process NMR Spectrum Chemical shift Factors influencing chemical shift Spin-Spin coupling Coupling constant Factors influencing coupling constant Spin - Spin decoupling Proton exchange reaction 2

NMR Spectroscopy - Intro : 

NMR Spectroscopy - Intro The study of spin changes at the nuclear level when radio frequency energy is absorbed in the presence of magnetic field. Measures the absorption of EM radiation in the radiofrequency region 4 MHz to 750 MHz (wavelength 0.4 m to 75 m) Most commonly done on 1H and 13C. OBJECTIVES : Structural elucidation Drug design MRI 3

THEORY & PRINCIPLE : 

THEORY & PRINCIPLE Nuclei of atoms with an odd atomic number or an odd mass number have a nuclear spin or angular momentum. The total angular momentum depends on the spin quantum number (I). Because nuclei are positively charged, their spin induce a magnetic field. When a magnetic field is applied to atomic nuclei, the magnetic fields of the nuclei align themselves either parallel or anti-parallel to the applied magnetic field. 4

THEORY & PRINCIPLE contnd… 5

Magnetic moments and energy states for a nucleus with a spin quantum number of + 1/2. : 

Magnetic moments and energy states for a nucleus with a spin quantum number of + 1/2. 6 THEORY & PRINCIPLE contnd…

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THEORY & PRINCIPLE contnd… E =  h Bo 2p The energy difference E between and the two spin states depends on the strength of the applied magnetic field Bo. h Planck’s constant ( 6.6262 × 10-27 erg sec )  Nuclear constant or Gyro magnetic ratio,  is a constant for each nucleus. (26,753 s-1gauss-1 for H and 6,728 s-1 Tesla-1 for C) 7

PRINCIPLE : 

PRINCIPLE When energy in the form of Radiofrequency is applied and when, Applied frequency = Precessional frequency absorption of energy occurs and a NMR signal is recorded . The nuclei are said to be in resonance, and the energy they emit when flipping from the high to the low energy state can be measured. 8

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PRINCIPLE OF NMR 9

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DE = h n n = g Bo / 2p DE = g h Bo / 2p w = 2pn w = g Bo = g h I / 2p n -Electromagnetic frequency in radio frequency w - Precessional frequency  - Magnetic dipole moment I - Spin quantum number DE - Energy Difference h - Planck’s constant  - Gyro magnetic ratio Bo - Applied magnetic field 10

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THEORY & PRINCIPLE contnd… 1.41 T 60 MHz 2.35 T 100 MHz 4.7 T 200 MHz E = hv 7.0 5T 300 MHz RELATIONSHIP BETWEEN APPLIED MAGNETIC FIELD & RADIOFREQUENCY 11

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NMR Spectra of Acrylonitryle at 60 ,100 and 220 MHz 12 THEORY & PRINCIPLE contnd…

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SPIN – LATTICE RELAXATION (longitudinal): The components of the lattice field can interact with nuclei in the higher energy state, and cause them to lose energy (returning to the lower state). RELAXATION PROCESS SPIN - SPIN RELAXATION (transverse): The interaction between neighbouring nuclei with identical precessional frequencies but differing magnetic quantum states. a nucleus in the lower energy level will be excited, while the excited nucleus relaxes to the lower energy state. There is no net change in the populations of the energy states, but The relaxation time, T1 (the average lifetime of a nucleus in the excited state) will decrease. 13

NMR Spectrum : 

NMR Spectrum A Spectrum of Absorption of Radiation Vs. Applied Magnetic Strength is called as NMR Spectrum. The number of signals shows how many different kinds of protons are present. The intensity of the signal shows the number of protons of each kinds. The location of the signals shows how shielded or deshielded the proton is. Signal splitting shows the number of protons on adjacent atoms. 14

Combined 13C and 1H Spectra : 

Combined 13C and 1H Spectra 15

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Diamagnetic shielding Aromatic Protons, 7-8 Vinyl Protons, 5-6 Acetylenic Protons, 2.5 Aldehyde Proton, 9-10 16

CHEMICAL SHIFT () : 

CHEMICAL SHIFT () The variations of nuclear magnetic resonance frequencies of the same kind of nucleus, due to variations in the electron distribution .  ppm w – w ref w ref Hz MHz Chemical Shift = Absorption Frequency relative to TMS (Hz) Spectrometer Frequency (MHz) 17

Tetramethylsilane (TMS) : 

Tetramethylsilane (TMS) Only one peak on NMR spectrum High electronic density of H in TMS. Almost all the H peaks of organic compounds appear on the left of the TMS peak. 18

Slide 19: 

The effective magnetic field at the nucleus can be expressed in terms of the externally applied field B0 by the expression Where  is called the shielding factor or screening factor. The factor  is small - typically 10-5 for protons and <10-3 for other nuclei When a signal is found with a higher chemical shift the signal or shift is downfield or at low field or paramagnetic Conversely a lower chemical shift is called a diamagnetic shift, and is upfield . B=Bo(1 -  ) 19

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FACTORS INFLUENCING CHEMICAL SHIFT : 

FACTORS INFLUENCING CHEMICAL SHIFT Both 1H and 13C Chemical shifts are related to the following major factors: Depends on Hydrogen bonding Depends on adjacent group Depends on carbon group attached Depends on hybridization Depends on anisotropy 22

HYDROGEN BONDING : 

HYDROGEN BONDING Molecules having hydrogen bonding have higher chemical shift and absorb radiation at low field. That is due to the decrease of electronic density around the nucleus 23

ADJACENT GROUP : 

ADJACENT GROUP For protons on carbon attached to an electronegative atom or group X( Cl , F ,Br ,I), the chemical shift increases with the electro negativity of X. This is due to the inductive effect on the shielding of the protons and is apparent in the methyl halides. 24

CARBON GROUP ATTACHED : 

CARBON GROUP ATTACHED 25

HYBRIDIZATION : 

HYBRIDIZATION 26

ANISOTROPY : 

ANISOTROPY Protons on an aromatic ring appears at very low field (7.27), due to the aromatic ring current. 27

Slide 28: 

one spin two spins see each other few Hz Ha Ha Hb Ha Hb Magnetic field of Hb adds to the applied field . Ha signal appears at a lower applied field Magnetic field of Hb subtracts to the applied field . Ha signal appears at a higher applied field SPIN - SPIN COUPLING The interactions between the spins of neighbouring nuclei in a molecule may cause the splitting of the lines in the NMR spectrum. Example:

The N + 1 Rule : 

The N + 1 Rule If a signal is split by N equivalent protons , It is split into N + 1 peaks. Example: 29

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1,1,2-Tribromoethane : 

1,1,2-Tribromoethane Doublet: 1 Adjacent Proton Triplet: 2 Adjacent Protons 31

COUPLING CONSTANT(J) : 

COUPLING CONSTANT(J) Distance between the peaks of multiplet . Measured in Hz Two types, Geminal Coupling : Depends on bond angle H-C-H, two sigma bonds Vicinal coupling: Depends on dihedral angle H-C-C-H, three sigma bonds 32

VALUES FOR COUPLING CONSTANTS : 

VALUES FOR COUPLING CONSTANTS 33

FACTORS INFLUENCING COUPLING CONSTANT : 

FACTORS INFLUENCING COUPLING CONSTANT Geminal coupling constant: Increasing bond angle-more + ve J Electronegative substituent - more + ve J Neighbouring pi bonds-more –ve J Vicinal coupling constant: Increasing Dihedral angle- more + ve J Electronegative substituent- less + ve J J Decreases with bond angle 34

SPIN - SPIN DECOUPLING : 

SPIN - SPIN DECOUPLING Irradiation of protons or groups of equivalent protons with sufficiently intense radio frequency energy to eliminate completely the observed coupling to the neighbouring protons. Process of removing spin – spin splitting between the spins. 35

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Types of Spin – Spin Decoupling in 13C NMR Broad-band Decoupling Off-Resonance Decoupling

PROTON EXCHANGE REACTION(PROTON TRANSFER) : 

PROTON EXCHANGE REACTION(PROTON TRANSFER) Describes the fact that in a given period of time, a single -OH proton may attach to a number of different ethyl alcohol molecules. The rate in pure alcohol ethyl alcohol is slow, but increased in acidic or basic impurities. If the rate is very slow, the expected multiplicity of hydroxyl group is observed. If it is rapid, a single sharp signal is observed. It causes spin decoupling. 37

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