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nmr spectroscopy


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  • 1. Guided by: Prepared by:Dimal Shah Shweta Singh IICP NEW V.V. NAGAR.... 1
  • 2.  Beginning in 1953 with the first commercial NMR spectrometer, the early instruments used permanent magnets or electromagnets with field of 60, 80, 90 or 100 MHz, respectively for proton resonance. All instruments above 100 MHz are based on helium- cooled superconducting magnets (selenoids) and operate in pulsed FT mode. The other basic requirements besides high field are frequency-field stability, field homogenity, and a computer interface. IICP NEW V.V. NAGAR.... 2
  • 3. • Types of NMR high resolution spectra Continuous wave Fourier transform NMR NMR• Imp parts of the NMR spectrometer:- Permanent magnet/electromagnet RF generator RF detector Sample holder Magnetic coils IICP NEW V.V. NAGAR.... 3
  • 4. • Principle:- based on• frequency sweep field sweep frequency of RF source frequency is is varied constantBo is constant Bo is varied IICP NEW V.V. NAGAR.... 4
  • 5.  Magnets:- permanent:- constant Bo is generated that is 0.7;1.4;2.1o adv:- construction is simple cheaper electromagnet:-Bo can be varied which is done by winding the electromagnetic coil around the magnet most expensive components of the nuclear magnetic resonance spectrometer system Shim Coils The purpose of shim coils on a spectrometer is to correct minor spatial inhomogeneities in the Bo magnetic field. These inhomogeneities could be caused by the magnet design, materials in the probe, variations in the thickness of the sample tube, sample permeability. IICP NEW V.V. NAGAR.... 5
  • 6.  A shim coil is designed to create a small magnetic field which will oppose and cancel out an inhomogeneity in the Bo magnetic field. Superconducting solenoids:- prepared from superconducting niobium-titanium wire and niobium-tin wire operated at lower temp. kept in liquid He(mostly preferred) or liquid N2 at temp of 4 K liquid N2 should be changed at 10 days while liquid He shouldd be changed at 80-130 days higher Bo can be produced that is upto 21 T.o Advantage:- High stability Low operating cost High sensitivity Small size compared to electromagnets simple IICP NEW V.V. NAGAR.... 6
  • 7.  RADIOFREQUNCY TRANSMITTER It is a 60 MHz crystal controlled oscillator. RF signal is fed into a pair of coils mounted at right angles to the path of field. The coil that transmit RF field is made into 2 halves in order to allow insertion of sample holder . 2 halves are placed in magnetic gap For high resolution the transmitted frequency must be highly constant. The basic oscillator is crystal controlled followed by a buffer doubler, the frequency being doubled by tunning the variable It is further connected to another buffer doubler tuned to 60 MHz Then buffer amplifier is provided to avoid circuit loading. IICP NEW V.V. NAGAR.... 7
  • 8.  Signal amplifier and detector Radiofrequency signal is produced by the resonating nuclei is detected by means of a coil that surrounds the sample holder The signal results from the absorption of energy from the receiver coil, when nuclear transitions are induced and the voltage across receiver coil drops This voltage change is very small and it must be amplified before it can be displayed. The display system The detected signal is applied to vertical plates of an oscilloscope to produce NMR spectrum Spectrum can also be recorded on a chart recorder. IICP NEW V.V. NAGAR.... 8
  • 9.  Sample Probe The sample probe is the name given to that part of the spectrometer which accepts the sample, sends RF energy into the sample, and detects the signal emanating from the sample. It contains the RF coil, sample spinner, temperature controlling circuitry, and gradient coils. It is also provided with an air driven turbine for rotating the sample tube at several hundred rpm This rotation averages out the effects of in homogeneities in the field and provide better resolution. IICP NEW V.V. NAGAR.... 9
  • 10.  The spectrum obtained either by CW scan or pulse FT at constant magnetic field is shown in series of peaks whose areas are proportional to the number of protons they represent. Peak areas are measured by an electronic integrator that traces a series of steps with heights proportional to the peak areas. IICP NEW V.V. NAGAR.... 10
  • 11.  A proton count from the integration is useful to determination or confirm molecular formulas, detect hidden peaks, determine sample purity, and to quantitative analysis. Peak positions are measured in frequency units from a reference peak. A routine sample for proton NMR on a 300MHz instrument consist of about 2mg of the compound in about 0.4 mL of solvent in a 5-mm tube. Under favorable conditions, it is possible to obtain a spectrum of 1μg of a compound of a compound of modest molecular weight in a microtube in a 300-MHz pulsed instrument. Microprobe that accept a 2.5mm or 3 mm o.d. tube are convenient and provide high sensitivity. IICP NEW V.V. NAGAR.... 11
  • 12. Fig. illustrates basic elements of a classical 60-MHz NMR spectrometer. IICP NEW V.V. NAGAR.... 12
  • 13.  The sample is dissolved in a solvent containing no interfering proton (usually CCl4 and CDCl3), and a small amount of TMS is added to serve as an internal reference. The sample is a small cylindrical glass tube that is suspended in the gap between the faces of the pole pieces of the magnet. The sample is spun around the axis to ensure that all parts of the solution experience a relatively uniform magnetic field. Also in a magnetic gap is a coil attached to 60-MHz radiofrequency generator. This coil supplies the electromagnetic energy used to change the spin orientation of the protons. Perpendicular to the RF oscillator coil is a detector coil. When no absorption of energy is taking place, the detector coil picks up non of the energy given off by the RF oscillator coil. IICP NEW V.V. NAGAR.... 13
  • 14.  when sample absorbs energy, however, the reorientation of the nuclear spins induces a radiofrequency signal in the plane of the detector coil, and the instrument responds by recording this as a resonance signal, or peak. Rather than changing the frequency of the RF oscillator to allow each of the protons in a molecule to come into a resonance, the typical NMR spectrometer uses a constant frequency RF- signal and varies the magnetic field strength. As the magnetic field strength is increased, the precessional frequency of all the protons increase. When the precesional frequency of a given type proton reaches 60MHz, it has resonance. IICP NEW V.V. NAGAR.... 14
  • 15.  As the field strength is increased linearly, a pen travels across a recording chart. A typical spectrum is recorded as shown in fig below.The 60MHz 1H NMR spectrum of phenylacetone. IICP NEW V.V. NAGAR.... 15
  • 16.  As each chemically distinct type of proton comes into resonance, it is recorded as a peak on the chart. The peak at δ = 0 ppm is due to the internal reference compound TMS. IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT SCANS FROM “LOW FIELD” TO “HIGH FIELD” LOW HIGH FIELD FIELD NMR CHART DOWNFIELD UPFIELD scan IICP NEW V.V. NAGAR.... 16
  • 17.  Since highly shield proton precess more slowly than unshield proton, it is necessary to increase field to induce them to precess at 60MHz and hence highly shield proton appear to the right of this chart and deshield proton appear to the left. Instruments which vary the magnetic field in continuous fashion, scanning from downfield to upfield end of the spectrum, are called continuous-wave(CW) instrument. Because the chemical shift of the peak in this spectrum are calculated from frequency difference from TMS, this type of spectrum is said to be a frequency-domain spectrum. Peaks generated by a CW instrument have ringing. Ringing occurs because the excited nuclei do not have time to relax back to their equilibrium state before the field. And pen, of the instrument have advanced to a new position. Ringing is most noticeable when a peak is a sharp Singlet. IICP NEW V.V. NAGAR.... 17
  • 18. IICP NEW V.V. NAGAR.... 18
  • 19.  FTNMR uses a pulse of RF radiation which causes a nuclei in a magnetic field to flip into higher energy alignment Applying such a pulse to a set of nuclear spins simultaneously excites all the nuclei in all local environment. All the nuclei will re emit RF radiation at their resonance frequencies which induces a current in a nearby pickup coil, creating an electrical signal oscillating at the NMR frequency. This signal is known as the free induction decay (FID) and contains the sum of the NMR responses from all the excited spins. In order to obtain the frequency-domain NMR spectrum (intensity vs. frequency) this time-domain signal (intensity vs. time) must be Fourier transformed. Fortunately the development of FT-NMR coincided with the development of digital computers and Fast Fourier Transform algorithms. . This is the principle on which a pulse Fourier transform spectrometer operates. By exposing the sample to a very short (10 to 100 μsec), relatively strong (about 10,000 times that used for a CW spectrometer) burst of RF energy, all of the protons in the sample are excited simultaneously. IICP NEW V.V. NAGAR.... 19
  • 20. PULSED EXCITATION N 2 1 OBROADBANDRF PULSE CH2 C CH3contains a rangeof frequencies 3 1 ..... n S All types of hydrogen are excited simultaneously with the single RF pulse. IICP NEW V.V. NAGAR.... 20
  • 21. FREE INDUCTION DECAY ( relaxation ) 1 O CH2 C CH3 3 1, 2, 3 have different half lifes IICP NEW V.V. NAGAR.... 21
  • 22. COMPOSITE FID “time domain“ spectrum 1 + 2 + 3 + ...... time IICP NEW V.V. NAGAR.... 22
  • 23. FOURIER TRANSFORMA mathematical technique that resolves a complexFID signal into the individual frequencies that addtogether to make it. ( Details not given here. ) converted to DOMAINS ARETIME DOMAIN FREQUENCY DOMAIN MATHEMATICAL FID NMR SPECTRUM TERMS FT-NMR computer COMPLEX SIGNAL 1 + 2 + 3 + ...... Fourier Transform individual a mixture of frequencies frequencies decaying (with time) converted to a spectrum IICP NEW V.V. NAGAR.... 23
  • 24. The Composite FID is Transformed into aclassical NMR Spectrum : O CH2 C CH3 “frequency domain” spectrum IICP NEW V.V. NAGAR.... 24
  • 25.  Advantage over continuous wave NMR Sample of low conc. can be determined Magnetic nuclei with low natural isotopic abundance can be determined eg 13C Very rapid pulse repetition can be possible Entire spectrum can be recorded, computerized and transformed in a few seconds that is every 2 sec For e.g. in 13 min 400 spectra can be recorded. So thus 20 times signal enhancement is seen Analysis can be possible where magnetogyric ratio is low IICP NEW V.V. NAGAR.... 25
  • 26.  Properties:- Nonviscous. Should dissolve analyte. Should not absorb within spectral range of analysis. All solvents used in NMR must be aprotic that is they should not possess proton. Chloroform-d (CDCl3) is the most common solvent for NMR measurements other deuterium labeled compounds, such as deuterium oxide (D2O), benzene-d6 (C6D6), acetone-d6 (CD3COCD3) and DMSO-d6 (CD3SOCD3) are also available for use as NMRsolvents. DMF, DMSO, cyclopropane, dimethyl ether can also be used IICP NEW V.V. NAGAR.... 26
  • 27. IICP NEW V.V. NAGAR.... 27