NMR SPECTROSCOPY

NMR SPECTROSCOPY

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  Presented by : ArvindSingh Heer MSc-II (Sem-III) Analytical Chemistry Paper-IV MITHIBAI COLLEGE NMR SPECTROSCOPY
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  CONTENT  INTRODUCTION  PRINCIPLE NUCLEAR RELAXATION
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  INTORDUCTION  Nuclear MagneticResonance (NMR) is a spectroscopic technique which is based on the absorption of elelctromagnetic radiation in the radio frequency region 4 to 900 MHz by nuclei of the atoms.  NMR is used in quality control and research for determining the content and purity of a sample as well as its molecular structure.  For e.g. NMR can quantitatively analyse mixtures containing known compounds.  For unknown compounds, NMR can either be used to match against spectral libraries or to infer the basic structure directly.
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  Principles of NMR The theory behind NMR comes from the spin of a nucleus and it generates a magnetic field.  Without an external applied magnetic field, the nuclear spins are random in directions.  But when an external magnetic field (BO) is present, the nuclei align themselves either with or against the field of the external magnet.
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   If anexternal magnetic field is applied, an energy transfer (∆E) 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 gives the NMR spectrum of the concerned nucleus.
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   Relaxation isthe 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.  An understanding of relaxation processes is important for the proper measurement and interpretation of NMR spectra. Nuclear Relaxation
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  An Understanding ofRelaxation Processes  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, with half-lives on the order of 0.1 to 100 seconds for a spin ½. It is thus very easy to saturate an NMR transition (equalize populations of excited and ground state), 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.
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  NMR Relaxation -Spin-Lattice orLongitudinal Relaxation  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. Mz = M0 [1- e(-t/T1) ]
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  NMR Relaxation -Spin-spin orTransverse Relaxation  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, or differ by orders of magnitude  No energy change Mx = My = M0 [1- e(- t/T2]
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  (Sn) Tin NMR Tin is unique in that it has no less than three NMR active spin ½ nuclei, 115Sn, 117Sn and 119Sn.  They all yield narrow signals over a very wide chemical shift range.  119Sn is very slightly more sensitive than 117Sn, so 119Sn is therefore usually the preferred nucleus.  115Sn is much less sensitive than either 117Sn or 119Sn.  Tin NMR is mostly used for the study of organotin compounds, but is also applicable to inorganic tin compounds.
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  Comparison of theNMR spectra of the tin isotopes 115Sn, 117Sn and 119Sn for SnCl4 (neat)
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  (Sn) Tin NMR All the tin nuclei couple to other nuclei.  1H, 13C, 19F, 31P, etc couplings have been reported.  One bond couplings to 13C are between 1200 and 1500 Hz.  1H one bond couplings are from 1750 to 3000 Hz, 19F from 130 to 2000 Hz and for 31P they range from 50 to 2400 Hz.  Two bond Sn-H coupling constants are approximately 50 Hz.  Homonuclear 119Sn- 119Sn and heteronuclear 119Sn- 117Sn have been reported from 200 to 4500 Hz.  Three and four bond couplings have been reported.
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  Chemical shift rangesfor tin NMR Each type of tin compound has its characteristic chemical shift range.
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  (195Pt) Platinum NMR Platinum (Pt) has one medium sensitivity NMR spin -½ nucleus, 195Pt that yields narrow signals over a very wide chemical shift range.  Because platinum has such a wide chemical shift range and 195Pt gives narrow signals, the slightest effect can be resolved as in the spectrum in fig. 2 where replacing 35Cl with 37Cl gives extra signals.  195Platinum NMR is mostly used for studying platinum complexes, their structure, conformation and dynamics, and platinum binding in biological systems.  Because platinum is widely used as an industrial catalyst and in medicine, its chemistry and NMR has been widely studied.
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  Fig. 1. 195Pt-NMR spectrumof K2PtCl4 in D2O Fig. 2. Resolution enhanced 195Pt-NMR spectrum of K2PtCl4 in D2O showing isotopomers
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  Chemical shift rangesfor platinum NMR  Each type of platinum has its representative chemical shift range.
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  (195Pt) Platinum NMR Platinum shows a wide variety of couplings with other nuclei, 1H, 13C, 15N, 31P, etc.  Two-bond couplings to protons are between 25 and 90 Hz.  One-bond 195Pt-15N couplings are in the region of 160 to 390 Hz.  Couplings to 31P are around 1300 to 4000 Hz for one-bond and 30 Hz for two-bond.  The one-bond coupling to 77Se is between 80 and 250 Hz.  The platinum coupling to 119Sn is especially large and can be over 33000 Hz.  Homonuclear platinum couplings can also be observed.
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  References 1. Physical Methodsin Inorganic Chemistry, R. S. Drago, John- Wiley Pub.,1975 2. Instrumental Methods of Analysis, H.H. Willard, L.L. Merrit, J.A. Dean and F.A. Settle, C.B.S. Publishers and Distributors, New Delhi, 1986. 3. NMR Spectroscopy, Basic Principles, Concepts, and Applications in Chemistry, Günther, Harald, 3rd edition, Wiley Publication. 4. Introduction to Spectroscopy, Donald L. Pavia, Gary M. Lampman, S. Kriz, 5th edition, Pearson Brook/Cole. -