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NMR SPECTROSCOPY. Introduction. NMR srectroscopy is the study of spin changes at the nuclear level when radio frequency energy is absorbed in the presence of magnetic field. Nuclei with odd mass number only Nuclei with odd mass number only give NMR spectra.
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Introduction • NMR srectroscopy is the study of spin changes at the nuclear level when radio frequency energy is absorbed in the presence of magnetic field. • Nuclei with odd mass number only Nuclei with odd mass number only give NMR spectra. eg:1H,13C,19F,35Cl • NMR Spectroscopy is an important method for molecular structural determination
This spectroscopy gives information about the number and types of protons in a molecule. Different types of NMR spectroscopy are available 1H-NMR spectroscopy 13C-NMR spectroscopy 31P-NMR spectroscopy 29Si-NMR spectroscopy Fourier-transform NMR Solid state NMR 19F, 119Sn, 195Pt, 123Te,129Xe etc.
PRINCIPLE: • Any nucleus with odd mass number spins on its own axis. • By the application of an external magnetic field the nucleus spins on its own axis and magnetic moment created.This state is called ground state. • The magnetic field caused by the spin of nuclei aligned with externally applied magnetic field.When energy applied in the form radiofrequency,absorption of energy occur and NMR signal is recorded.
INSTRUMENTATION • Magnet poles • Sample cell(tube ) • Sweep generator • RF transmitter • RF receiver • Recorder
Magnet poles: • Generation of magnetic field. • Sweep generator: • To vary the strength of applied magnetic field. Eg:14,092 gauss,21,140gauss. • RF receiver: • To Measure the intensity of unabsorbed radio frequency energy.
RF transmitter: • Used to apply radiofrequency radiation. Eg:60MHz,90MHz,100MHz,220MHz. • Sample cell: • sample test tube which is about 25mm long and 5mm diameter. • This is kept inside the sample cavity and is spun at 30rps. • Recorder: • Record NMR signal obtained from detector.
Parameters involved in NMR • Relaxation process • Shielding and desheilding • Chemical shift • Spin-Spin coupling
Relaxation process: It is the process of transmission from excited state to ground state. Where the absorbed enegy can be lost by Two ways • Spin-lattice(longitudinal) • Spin-Spin(Transverse) Spin-Spin(Transverse): It involves the transfer of energy from one nucleus to the other. There is net loss of energy.
Spin-lattice(longitudinal): • It involves the transfer of energy from the nucleus in its higher energy state to the molecular lattice. • The energy is transferred to the components of the lattice as the additional transitional, vibrational and rotational energy. • Thus the total energy of the system remains the same.
Shielding and desheilding: Nucleus is surrounded by the electrons.Protons can be present within or outside the magnetic field caused by the electrons in double bond. • When a proton is present inside magnetic field more applied magnetic field is required to cause excitation.such protons are called Shielded protons. • When a proton is present outside the magnetic field less applied magnetic field is required to cause excitation.such protons are called deShielded protons.
Chemical shift: chemical shift is the difference between the absorption position of a sample and the absorption position of the reference compound.TMS (tetra methyl silane) used as reference compound. Tetra methyl silane (TMS)
For a 1H-NMR spectrum, signals are reported by their shift from the 12-H signal in TMS. • For a 13C-NMR spectrum, signals are reported by their shift from the 4-C signal in TMS. • Chemical shift (δ): the shift of an NMR signal from the signal of TMS. Normally given in parts per million (ppm). • TMS is not useful for water soluble compounds,for this DSS used.(DSS-2,2-dimethyl-2 silapentane-5-sulphonate)
Spin - spin coupling: Consider the structure of ethanol: • The 1H NMR spectrum of ethanol (below) shows the methyl peak has been split into three peaks (a triplet) and the methylene peak has been split into four peaks (a quartet). This occurs because there is a small interaction (coupling) between the two groups of protons.
The effect of the methyl protons on the methylene protons will be present. • .The splitting pattern of a given nucleus can be predicted by the n+1 rule, where n is the number of neighboring spin-coupled nuclei. • Coupling constant: The distance between the two peaks. It is represented by J
Solvent requirements • Solvent used in NMR soectroscopy should not contain Hydrogen atoms. • Hence we use solvents like carbon tetra chloride(ccl4),deuteratedachloroform(CDCl3),Deuterated water, Deuterated dimethyl sulphoxide.
Properties of solvent • Chemical inertness • Magnetic isotropy • Volatility • Absence of hydrogen atoms • Easily available • Inexpensive
Applications • Structure elucidation of organic compounds. • Types of protons • Environment of protons • No.of protons of each type • No.of adjacent protons • Investigation of conformational isomerism, molecular symmetry hydrogen bonding keto-enol tautomerism
Determination of optical purity. • Study of molecular interaction micelle formation Drug receptor interaction • Quantitative analysis Assay of components Surfactant chain length determination Hydrogen analysis Iodine value Moisture analysis
Used to determine the molar ratio of the components in a mixure. • Analysis of mixure of isomeric compounds like o-xylene and p-xylene. • Elimental analysis. • Rates of reaction.
limitations • The minimum sample size is 0.1ml of 1%con • Lack of sensitivity. • It does n’t give information about molecular weight. • Only liquids can be studied.
REFERENCES • Spin Dynamics: Basics of Nuclear Magnetic Resonance, 2nd Edition, Wiley 2008. • Fundamentals of Nuclear Magnetic Resonance Spectroscopy, Durham Eagle Press, Durham, North Carolina, 2007.
