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Presented by : Arvind Singh Heer MSc-II(Sem-III)Analytical ChemistryPaper-IV MITHIBAI COLLEGE

NMR SPECTROSCOPY

CONTENTINTRODUCTIONPRINCIPLENUCLEAR RELAXATION

INTORDUCTIONNuclear 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.

Principles of NMRThe 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.

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.

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

An Understanding of Relaxation ProcessesThere are three important considerations.The very small energy difference between and 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 . It is thus very easy to saturatean 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.

NMR Relaxation-Spin-Lattice or Longitudinal RelaxationRelaxation process occurs along z-axisTransfer of the energy to the lattice or the solvent materialCoupling 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) ]

NMR Relaxation-Spin-spin or Transverse RelaxationRelaxation process in the X-Y planeExchange of energy between excited nucleus and low energy state nucleus.Randomization of spins or magnetic moment in X-Y planeRelated to NMR peak line-widthRelaxation time T2T2 may be equal to T1, or differ by orders of magnitudeNo energy change

Mx = My = M0 [1- e(-t/T2]

(Sn) Tin NMRTin is unique in that it has no less than three NMR active spin 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.

Comparison of the NMR spectra of the tin isotopes 115Sn, 117Sn and 119Sn for SnCl4 (neat)

(Sn) Tin NMRAll 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.

Chemical shift ranges for tin NMR

Each type of tin compound has its characteristic chemical shiftrange.

(195Pt) Platinum NMR

Platinum (Pt) has one medium sensitivity NMR spin - nucleus,195Pt that yields narrow signals over a very wide chemical shift range.Because platinum has such a wide chemical shift range and195Pt gives narrow signals, the slightest effectcan be resolved as in the spectrum in fig. 2 where replacing35Cl with37Cl 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.

Fig. 1.195Pt-NMR spectrum of K2PtCl4in D2O

Fig. 2. Resolution enhanced195Pt-NMR spectrum of K2PtCl4in D2O showing isotopomers

Chemical shift ranges for platinum NMREach type of platinum has its representative chemical shift range.

(195Pt) Platinum NMRPlatinum shows a wide variety ofcouplings with other nuclei, 1H, 13C,15N, 31P,etc.Two-bond couplings to protons are between 25 and 90 Hz.One-bond195Pt-15N couplings are in the region of 160 to 390 Hz.Couplings to 31Pare around 1300 to 4000 Hz for one-bond and 30 Hz for two-bond.The one-bond coupling to77Seis between 80 and 250 Hz. The platinum coupling to119Snis especially large and can be over 33000 Hz. Homonuclear platinum couplings can also be observed.

ReferencesPhysical Methods in Inorganic Chemistry, R. S. Drago, John-Wiley Pub.,1975Instrumental 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, Gnther, Harald, 3rd edition, Wiley Publication.Introduction to Spectroscopy, Donald L. Pavia, Gary M. Lampman, S. Kriz, 5th edition, Pearson Brook/Cole.

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