PRINCIPLES of FT-NMR & 13C NMR

PRINCIPLES of FT-NMR & 13C NMR
Fourier Transform
FOURIER TRANSFORM NMR SPECTROSCOPY
THEORY OF FT-NMR 
13C NMR SPECTROSCOPY
Principle
Why C13-NMR is required though we have H1-NMR?
CHARACTERISTIC FEATURES OF 13 C NMR 
Chemical Shifts
NUCLEAR OVERHAUSER ENHANCEMENT
Short-Comings of 13C-NMR Spectra

1. Presented by: Aditya Sharma M.S. (Pharm) Pharmaceutical Analysis NIPER Guwahati 1 PRINCIPLES of FT-NMR & 13 C NMR

2. It is the mathematical operation in which the complex waveform can be broken-down into simple mathematical operations. It is the mathematical operation required to convert a time domain spectrum to frequency domain spectrum (or vice versa). Following an adequate S/N ratio, digital data must be transformed into the frequency data . A computer is essential to solve the complex equations. 2

3. In FT-NMR instrument, small energy change takes place in the magnitude, present in NMR and hence the sensitivity of this instrument is very less. The sensitivity in FT-NMR can be increased by adding the square root of recorded spectra's together. Simultaneous irradiation of frequency occurs in a spectrum having a Radio frequency pulse and then the nuclei returns back to thermal equilibrium on its normal state. 3

4. The Fourier Transformation is the basic mathematical calculation necessary to convert the data in time domain ( interferogram ) to frequency domain(NMR Spectrum ) i.e., time domain- Intensity v/s Time. Frequency domain- Intensity v/s Frequency. It was developed by JEAN BAPTISE JOSEPH FOURIER. 4

5. When magnetic nuclei are placed in a magnetic field and irradiated with a pulse of radio frequency close to their resonant frequency, the nuclei absorb some of the energy and precis's like little tops at their resonant frequencies . This precession of many nuclei at slightly different frequencies produces a complex signal that decays as the nuclei loses the energy they had gained from the pulse. This signal is called as free induction decay(FID) or transient, it contains all the information needed to calculate a spectrum. The free induction decay can be recorded by a radio receiver and a computer in 1-2 seconds and many FIDs can be averaged in few minutes. A computer converts the averaged transients into a spectrum. 5

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7. A Fourier transform is the mathematical technique used to compute the spectrum from the free induction decay. This technique of using pulses and collecting transients is called Fourier transform spectroscopy. A Fourier transform spectrometer is usually more expensive than a continuous wave spectrometer, since it must have fairly sophisticated electronics capable of generating precise pulses and accurately receiving the complicated transients. 7

8. 13 C NMR SPECTROSCOPY 8

9.  In all carbon atoms about 1.1% of all carbon atoms are the C-13 isotope; the rest (apart from tiny amounts of the radioactive C-14) is C-12.Obtaining C-13 spectra is more complex than for proton NMR. This is primarily because of the low isotopic abundance of 13C in nature. C-13 NMR relies on the magnetic properties of the C-13 nuclei. Carbon-13 nuclei fall into a class known as "spin ½" nuclei for reasons. The effect of this is that a C-13 nucleus can behave as a little magnet. C-12 nuclei don't have this property.  Proton Decoupled Techniques: Broad Band hydrogen (or proton) Decoupling Off – resonance Decoupling DEPT – NMR 9

10. 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. If an external magnetic field is applied, an energy transfer (ΔE) 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 radio frequency signal that give the NMR spectrum of the concerned nucleus . The emitted radio frequency is directly proportional to the strength of the applied field. v  = γB /2П 10

11. Why C13-NMR is required though we have H1-NMR? 1H nmr spectroscopy - The powerful and useful tool a tool for structural analysis. Useless when portions of a molecule lack C-H bonds, no information is forthcoming. Ex : polychlorinated compounds such as chlordane, polycarbonyl compounds such as croconic acid, and compounds incorporating triple bonds. Even when numerous C-H groups are present, an unambiguous interpretation of a proton nmr spectrum may not be possible. 11

12. CHARACTERISTIC FEATURES OF 13 C NMR  The chemical shift of the CMR is wider ( δ is 0-220ppm relative to TMS) in comparison to PMR( δ is 0-12ppm relative to TMS ). 13 C - 13 C coupling is negligible because of low natural abundance of 13 C in the compound. Thus in one type of CMR. S pectrum(proton de coupled) each magnetically non equivalent carbon gives a single sharp peak that does undergo further splitting. The area under the peak in CMR spectrum is not necessary to be proportional to the number of carbon responsible for the signal. Therefore not necessary to consider the area under ratio. Proton coupled spectra the signal for each carbon or a group of magnetically equivalent carbon is split by proton bonded directly to that carbon & the n+1 rule is follwed . 13 C nucleus is about one-fourth the frequency required to observe proton resonance. The chemical shift is greater for 13 C atom than for proton due to direct attachment of the electronegative atom to 13 C 12

13. 13 C chemical shifts follow the same principles as those of  1 H, although the typical range of chemical shifts is much larger than for  1 H (by a factor of about 20). The chemical shift reference standard for  13 C is the carbons in tetramethylsilane (TMS ), chemical shift is considered to be 0.0 ppm. 13

14. NOE effect for heteronuclear nuclei: when one of two different types of atoms irradiated & NMR spectrum of other type is determined they show change in absorption intensities of the observed atom, enhancement occurred, is called nuclear overhauser effect & the degree of increase in the signal is called nuclear overhauser enhancement. Effect can be either positive or negative. I n 13 C interacting with 1 H effect is positive. NOE is enhancement of signals , it add in to original signal strength. 14

15. Short-Comings of 13 C-NMR Spectra Only 1% of the carbon in the molecule is carbon-13, Sensitivity is consequently low, and Recording the NMR-spectra is a tedious and time consuming process. However, with the advent of recent developments in NMR-spectroscopy it is quite possible to eliminate some of these short comings adequately. They are: Development of powerful magnets (‘supercon’ magnets) has ultimately resulted in relatively stronger NMR-signals from the same number of atoms, Improved hardware in NMR-spectroscopy has gainfully accomplished higher sensitivity, and Development of more sensitive strategy 15

16. References R. M. Silverstein, G. C. Bassler and T. C. Morrill (1991). Spectrometric Identification of Organic Compounds . Wiley. Keeler, James (2010). Understanding NMR Spectroscopy (2nd ed.). John Wiley & Sons. p. 457. ISBN 978-0-470- 74608-0. C.O.Wilson and O. Givold , "Text book of Organic Medicinal and pharmaceutical Chemistry", 5 Medical Publishing Co. LTD, London copy right. by. J. B. Lippin Cott Company (1966).  Ashutosh Kar ,”Pharmaceutical Drug Analysis’,2’nd edition , New Age International Pvt.Ltd.Publishers , Page No:-348 Eberhard Breitmaier & Wolfgang Voelter Carbon 13 nmr Spectroscopy”,3’rd revised Edition, VCH Publication ,Page N:-12, 107 16