NMR SPECTROSCOPY

A complete Detail on nmr spectroscopy which gives a broad idea about the topic

1. Presented by : Arvind Singh Heer MSc-II ( Sem -III) Analytical Chemistry Paper-IV MITHIBAI COLLEGE NMR SPECTROSCOPY

2. CONTENT INTRODUCTION PRINCIPLE NUCLEAR RELAXATION

3. INTORDUCTION Nuclear Magnetic Resonance (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.

4. 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 (B O ) is present, the nuclei align themselves either with or against the field of the external magnet.

5. If an external 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.

6. 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.  An understanding of relaxation processes is important for the proper measurement and interpretation of NMR spectra. Nuclear Relaxation

7.   An Understanding of Relaxation Processes There are three important considerations .   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. 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 . The success of many multipulse experiments, especially 2D and 3D spectra, depends crucially on proper consideration of relaxation times.

8. NMR Relaxation - Spin-Lattice or Longitudinal 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 (T 1 ) → Exponential decay . M z = M [1- e (-t/T1) ]

9. NMR Relaxation - Spin-spin or Transverse 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 T 2 T 2 may be equal to T 1 , or differ by orders of magnitude No energy change M x = M y = M [1- e (-t/T2 ]

10. ( Sn ) Tin NMR Tin is unique in that it has no less than three NMR active spin ½ nuclei,  115 Sn, 117 Sn and 119 Sn. They all yield narrow signals over a very wide chemical shift range. 119 Sn is very slightly more sensitive than 117 Sn, so 119 Sn is therefore usually the preferred nucleus. 115 Sn is much less sensitive than either 117 Sn or 119 Sn. Tin NMR is mostly used for the study of organotin compounds, but is also applicable to inorganic tin compounds.

11. Comparison of the NMR spectra of the tin isotopes 115 Sn, 117 Sn and 119 Sn for SnCl 4 (neat)

12. (Sn) Tin NMR All the tin nuclei couple to other nuclei. 1 H, 13 C, 19 F, 31 P, etc couplings have been reported. One bond couplings to 13 C are between 1200 and 1500 Hz. 1 H one bond couplings are from 1750 to 3000 Hz, 19 F from 130 to 2000 Hz and for 31 P they range from 50 to 2400 Hz. Two bond Sn-H coupling constants are approximately 50 Hz. Homonuclear 119 Sn- 119 Sn and heteronuclear 119 Sn- 117 Sn have been reported from 200 to 4500 Hz. Three and four bond couplings have been reported.

13. Chemical shift ranges for tin NMR Each type of tin compound has its characteristic chemical shift range.

14. ( 195 Pt) Platinum NMR Platinum (Pt) has one medium sensitivity NMR spin -½ nucleus,  195 Pt that yields narrow signals over a very wide chemical shift range. Because platinum has such a wide chemical shift range and  195 Pt gives narrow signals, the slightest effect  can be resolved as in the spectrum in fig. 2 where replacing  35 Cl with  37 Cl gives extra signals . 195 Platinum 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.

15. Fig. 1.  195 Pt-NMR spectrum of K 2 PtCl 4  in D 2 O Fig. 2. Resolution enhanced  195 Pt-NMR spectrum of K 2 PtCl 4  in D 2 O showing isotopomers

16. Chemical shift ranges for platinum NMR Each type of platinum has its representative chemical shift range.

17. ( 195 Pt) Platinum NMR Platinum shows a wide variety of  couplings with other nuclei, 1 H, 13 C,   15 N, 31 P,   etc . Two-bond couplings to protons are between 25 and 90 Hz . One-bond  195 Pt- 15 N couplings are in the region of 160 to 390 Hz . Couplings to 31 P  are around 1300 to 4000 Hz for one-bond and 30 Hz for two-bond . The one-bond coupling to  77 Se is between 80 and 250 Hz. The platinum coupling to  119 Sn  is especially large and can be over 33000 Hz. Homonuclear platinum couplings can also be observed.

18. References Physical Methods in Inorganic Chemistry , R. S. Drago, John-Wiley Pub ., 1975 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. NMR Spectroscopy, Basic Principles, Concepts, and Applications in Chemistry , Günther, Harald, 3 rd edition, Wiley Publication. Introduction to Spectroscopy , Donald L. Pavia, Gary M. Lampman , S . Kriz , 5 th edition, Pearson Brook/Cole. -THANK YOU