Nuclear Magnetic Resonance ANIMATED ILLUSTRATIONS MS Powerpoint Presentation Files Uses Animation Schemes as available in MS XP or MS 2003 versions A class room educational material

Magnetic Resonance Phenomenon is a manifestation due to the presence of INTRINSIC SPIN angular momentum and the associated Magnetic Moment characteristically in electrons and Nuclei When the experimental conditions are set for the NUCLEI to resonate, then it is the Nuclear Magnetic Resonance. A definition: 0 ppm TMS 10ppm Single line NMR spectrum PMR spectrum NMR spectrum of a sample of spin ensemble……..

Single NMR line This NMR spectral line can be attributed with a characteristic shape and is describable by a mathematical equation. There are in general two distinct kind of shapes known for the spectral lines 1. Gaussian line shape 2. The Lorentzian shape. Line position δ ppm 0 ppm Reference line Both are typically symmetric line shapes:: symmetric about the line centre δ0δ0 Amplitude Full Width at Half Height Δ =FWHH Spectral features Besides these individual line features, it is the characteristic groups and patterns of lines in a spectrum which is useful for structure elucidation and the INTEGRATED INTENSTIEIS

NOTE that the previous slide contained data for ethyl alcohol spectrum in acidic medium. The above is in CDCl 3 The problem seems to be that the position of the -OH peak varies dramatically depending on the conditions - for example, what solvent is used, the concentration, and the purity of the alcohol - especially on whether or not it is totally dry. spectrum in acidic medium Deuterium exchange can cause the Proton signal to disappear

NMR spectrum for an alcohol like ethanol: When a few drops of deuterium oxide, D2O, is added to the solution, and allowed to settle, the -OH peak disappears!

2nI+1 and with I=1/2 2-spins cause a line to split into 3 lines 3-Spins cause a line to split into 4 lines

0 singlet 1 1 doublet triplet quartet pentet sextet septet octet nonet

1 ppm (δ) = 1 part per million ( 1 part in 10 6 ) of the Spectrometer frequency ν 0. T he spectrometer frequencies can be usually in MHz mostly above 200 MHz ( 1 MHz = 10 6 ) for protons 1 H; I=1/2 At 300 MHz 1ppm =300 Hz At 400MHz 1ppm =400 Hz 13 C I=1/2 At 75 MHz 1ppm = 75Hz At 100 MHz 1ppm= 100Hz 15 N I=1/2 At 30 MHz 1ppm = 30 Hz At 40 MHz 1ppm= 40 Hz Other nuclei commonly chosen for NMR study are 31 P (I=1/2), 19 F = (I=1/2) NMR of several of the other nuclei occurring in the chemical periodic table can be studied with a multi nuclear facility This phenomenon has been found to be capable of revealing the nature of nuclear environments in molecules (chemical compounds) because of the changes in the electronic structures due to the bonding criteria for the atoms forming the molecules. These are essentially the variations in the resonance frequencies due to electron circulations within molecules. And these variations called ‘Chemical Shifts’ are in the order of parts per million of the applied field/frequency.

Thus if proton nuclei has a characteristic resonance frequency of 300MHz corresponding to a applied magnetic field of 7.05 Tesla, then the total range for the variation of the proton resonance frequency due to differences in molecular electron circulations (the Chemical shift range) is 10ppm. This corresponds to a total variation of 3 KHz in 300 MHz (since 1ppm=300Hz). This implies a stringent stability criterion for the Magnetic field and RF frequency sources and the required ratio must be also maintained to the same accuracy to obtain reliable readout parameters from the spectrum obtained from spectrometers. This is the requirement of field-frequency lock in NMR spectrometers. With that good stability ensured, the magnetic field must be shimmed to get high homogeneity of the field in the sample region. By such techniques a reproducibility of the chemical shift to the accuracy of ppm are possible with the current generation of spectrometers.

All this is due to the advances in instrumentation on the rf detection side during the continuous wave mode of NMR detection and subsequently and the improvements in tuning of sample coils simultaneously used for the transmitter and receiver purposes with High Power [up to 3KW peak power for solid samples] pulsing and detecting the response, possibly in the range of 10μv, induced RF in the coil due to NMR induction in pulsed NMR detection. Up to 100MHz proton resonance frequency, Electromagnets (23 KG) can be used but for fields higher than this value Supercon Magnet Systems (with superconducting current carrying elements) are necessary. The possibility of realizing superconducting magnet systems has brought about a total revolution in what was possible by NMR Spectroscopic Technique. 1cc of water contains proton spins of the order of spins and the actual sample of water in the detectable region of nmr-probe would contain about spins corresponding to 100 micro liter of water sample. A typical spectrometer of the 300MHz frequency can detect conveniently a spin count of which amounts to volumes in a few ‘micro liter’ range. But the present generation of Spectrometers at as much high field as corresponding to 900MHz can be sensitive enough detect spins which in terms of sample volume in the ‘pico liter’ range.

Ethyl benzene