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Seminar – 2011-2012
M.Sc. (Chemistry) Final
Submitted to: -Submitted by: -
Dr. Abhi Abhishek vaishnav
NUCLEAR MAGNETIC RESONANCE SPECRTOSCOPY
The phenomenon of nuclear magnetic resonance (NMR) was first observed in 1946and it has been routinely applied in organic chemistry since about 1960.NMR spectroscopy is one of the most useful technique for structural elucidation available to organic chemist. In NMR we are going to study the interaction of the magnetic component with the certain nuclei hence the name NMR spectroscopy is given.
The NMR spectroscopy deals with nucleus of an atom that possesses magnetic moment. The nucleus of an atom is positively charged. Like electron the nuclei of certain atom also spin about their axis . The spinning of these charged particle generate a magnetic moment along the axis of the spin, so that these nuclei act like tiny bar magnet. Individual protons & neutrons have spin quantum no. +1/2 & -1/2 . Therefore depending on number of nucleons (protons+ neutrons) certain nuclei also possesses spin. The total spin quantum no. ‘I’ is a characteristic of nucleus. The nuclei that have spin no. greater than zero possesses spin.
Magnetic and Nonmagnetic Nuclei
The nuclei which have odd mass no. and odd or even atomic no. have half integral spin like ½, 3/2, 5/2, and possesses spin quantum no. I=1/2. These are magnetic nuclei.
The nuclei having even mass no. and odd atomic no. have integral spin quantum no. such as 1, 2, 3 etc. possesses spin quantum no. I=1. These are also magnetic nuclei
To find out nuclear spin quantum no. I following rules are generally applied.
Nucleus with I>0 when spins about its own axis generates a magnetic moment and are known as magnetic nuclei.
The Nucleus with I=0 does not produce magnetic moment and are known as nonmagnetic nuclei . The nuclei which have even mass number and even atomic number have zero spin quantum no. are nonmagnetic
The nuclei with I>0 possesses spin angular momentum and produce magnetic field are known as magnetic nuclei.
Processional motion of nucleus in applied magnetic field.
The number of orientations of the spin state is given by (2I+1) where I is the spin quantum no. We know that proton (H) has I=1/2 and it has only two orientations.
No. of orientations for H =(2I+1)
Transition from one possible orientation to another may be achieved by absorption or emission of electromagnetic radiation.
The energy required for flipping depends upon strength of applied magnetic field.
Processional motion of nucleus in applied magnetic field
Shielding and Deshielding
The induced magnetic field tries to protection the nucleus from applied magnetic field H0 . This type of protection of nuclei from H0 is called as diamagnetic shielding . When nuclei is not protected by induced magnetic field then Deshielding takes place .Thus the shielding and Deshielding depends upon the electron density around the proton.
a) More the electron density around the proton more is induced magnetic field and therefore more shielding of proton from applied magnetic field.
For example:-Acetylene molecule
Each nucleus (proton) in different environment requires a slightly different applied magnetic field H0 for resonance and peaks occur in different regions of the spectrum . For proton NMR spectra of organic compound the single resonance peak if the methyl groups in ‘Tetra methyl silane’ (TMS) is taken as a internal reference standard.
The distance in ‘d’ values from TMS to each signal (absorption or peak ) in the spectrum is called the chemical shift for the proton or proton giving that signal.
Advantages of TMS
It is chemically inert.
It gives unique line position.
It is symmetrical molecule and gives single and strong , sharp absorption peak as all 12 protons are equivalent.
Methyl protons of TMS are strongly shielded and hence absorption occurs at high field . It is taken as zero ppm.
It is soluble in most of organic solvents.
It is volatile(b.p.270c)and hence the recovery of sample is possible.
Measurement of Chemical shift.
chemical shift is measured in frequency unit ‘Hertz’. Most of routine instruments operate at 60, 90, 100 MHz.
More sophisticated instruments operate as high as 600MHz. The chemical shift recorded in Hz may vary with the spectrometer. To avoid this complication the chemical shift values are expressed in terms of delta or tau scale. Which are independent of field strength. Chemical shift in delta scale are expressed in parts per million (ppm).
The NMR spectrum at low resolution shows a no. of broad absorption peaks (signals) corresponding to the protons in different chemical environment. Area under each peak is proportional to the no. of protons. However at high resolution these NMR signal shows multiplicity. That is they get split into several signals. It must be noted that area under signals remain the same. This multiplicity of lines is due to spin-spin coupling, which arise from small magnetic interactions that occur between the nuclei of neighboring atoms. The nuclei are said to be coupled and resulting NMR spectral pattern is known as spin-spin splitting.
n n + 1 Pascal pattern:
0 singlet 1
1 doublet 2
2 triplet 3
3 quartet 4
4 quintet 5
The N+1 Rule
If the signal is spit by N equivalent protons, it is spit into N+1 peaks.
Range of magnetic coupling
Other Techniques for NMR
19Fluorine is a sensitive nucleus which yields sharp signals and has a wide chemical shift range.
A typical analysis of a 19F NMR spectrum may proceed similarly to that of Proton (1H). Our NMR service provides 19F NMR along with many other NMR techniques. The number of fluorines of each type in the spectrum of a pure sample can be obtained directly from the integrals of each multiplet provided that the multiplets are well separated which is very likely to the large chemical shift range. A routine NMR spectrum yields integrals with an accuracy of ±10%. Accuracies of ±1% can be achieved by increasing the relaxation delay to five times the longitudinal relaxation times (T1) of the signals of interest.
The 1D 13Carbon NMR experiment is much less sensitive than Proton (1H) but has a much larger chemical shift range. Its low natural abundance (1.108%) and proton decoupling means that spin-spin couplings are seldom observed. This greatly simplifies the spectrum and makes it less crowded. 13C is a low sensitivity nucleus that yields sharp signals and has a wide chemical shift range.
A typical analysis of a 13C NMR spectrum consists of matching expected chemical shifts to the expected moieties. Our NMR service provides 13C NMR along with many other NMR techniques Each type of signal has a characteristic chemical shift range that can be used for assignment
The 1D 31Phosphorus NMR experiment is much less sensitive than Proton (1H) but more sensitive than 13Carbon. 31Phosphorus is a medium sensitivity nucleus that yields sharp lines (fig. 1) and has a wide chemical shift range. It is usually acquired with 1Hdecoupling (fig. 2) means that spin-spin couplings are seldom observed. This greatly simplifies the spectrum and makes it less crowded. Where there are one-bond 31P-1H couplings present then the decoupling power needs to be at lest twice that needed for 13C because of the large coupling constant.