After reading this article you will learn about:- 1. Definition of NMR 2. Principle of NMR 3. Theory 4. Application.
Definition of NMR:
(1) Nuclear magnetic resonance is defined as a condition when the frequency of the rotating magnetic field becomes equal to the frequency of the processing nucleus.
(2) If ratio frequency energy and a, magnetic field are simultaneously applied to the nucleus, a condition as given by the equation v = үH0/2π is met. The system at this condition is said to be in resonance [v — frequency of radiation associated with transition from one state to the other; ү = proportionality constant and H0 = magnetic field]’.
Principle of NMR:
The principle of nuclear magnetic resonance is based on the spins of atomic nuclei. The magnetic measurements depend upon the spin of unpaired electron whereas nuclear magnetic resonance measures magnetic effect caused by the spin of protons and neutrons. Both these nucleons have intrinsic angular momenta or spins and hence act as elementary magnet.
The existence of nuclear magnetism was revealed in the hyper fine structure of spectral lines. If the nucleus with a certain magnetic moment is placed in the magnetic field, we can observe the phenomenon of space quantization and for each allowed direction there will be a slightly different energy level.
Theory of NMR:
The hydrogen nucleus or protons can be regarded as a spinning positively charged unit and so it will generate a tiny magnetic field HO along its spinning axis (as shown in figure 1). Now if this nucleus is placed in an external magnetic field H0, it will naturally line up either parallel A or antiparallel B to the direction of external field. The A will be more stable, being of lower energy.
The energy difference AE between two states will be absorbed or emitted as the nucleus flips from one orientation to the other.
AE = hv
where v = a radiation frequency and h = Planck’s constant
If correct frequency is applied to the sample containing hydrogen nuclie and sample is placed in the external field HQ, then low energy nuclie A will absorb AE = hv, and flips to B. Thus on flipping back down, they remit hv as a radiation signal which is picked up by the instrument.
In other words, if both radio frequency and magnetic field are simultaneously applied to the nucleus, transition from lower to higher level will occur when equation (1) will be equal to (2).
∆E = ∂hH/2π … (1)
∆E = hv … (2)
or v = δH/2π … (3)
δ = Gyromagnctic ratio, a constant characteristic of a particular nucleus.
Where ∆E = energy difference between two spin states, h = Planck’s constt, v = frequency of resonance absorption, H = strength of applied magnetic field at nucleus. The system at this condition is said to be in resonance and hence the name nuclear magnetic resonance. The observed value of H is therefore a function of molecular environment of proton affording the signal.
(1) Relaxation Process:
Relaxation processes are defined as different types of radiation-less transitions by which a nucleus in an upper spin slate returns to a lower spin state.
Generally there are two types of relaxation processes:
(a) Spin-spin Relaxation:
It is affected by mutual exchange of spins by two processing nuclei in close proximity to each other.
(b) Spin Lattice Relaxation (lattice term refers to frame work of molecules containing the precessing nuclei):
This process maintains an excess of nuclei in a lower state, which is the essential basic condition for the observation of nuclear resonance phenomenon.
In a NMR spectroscopy the sharp resonance lines are observed for stales of extended excitation, and broad lines are observed for short-lived excited stales. Both the processes, spin-spin relaxation and spin lattice relaxation contribute to he width of a spectral line.
(2) Condition of Resonance Signals:
The atoms like O16 and C12 which have even number of protons and neutrons have no magnetic moment and hence refuse to give resonance signals. While atoms such as P21, F19, which have odd number of protons and even numbers of neutrons, if any, generate nuclear magnetic moments and “hence give resonance signals.
(3) Units of NMR:
The nuclear magnetic resonance values are expressed in any of three ways:
(a) δ-the reference compound be quoted (δ denotes that chemical shift is independent of oscillator frequency).
(b) Cps — the reference compound must be quoted and the oscillator frequency given.
(c) τ-TMS (tetra methylsilane) or DSS (2, 2 dimethyl-2 silapentane-5 sulphonate) is assumed independent of both oscillator frequency and reference compound.
Nuclear Magnetic Resonance Spectrometers:
The basic elements of a typical n.m.r. spectrometer consist of the main parts;
(1) A magnet with strong, stable homogeneous field. The field must be constant over the area of the sample.
(2) A radio frequency oscillator (transmitter) connected to coil which transmits energy to the sample in a direction perpendicular to the magnetic field.
(3) A sample container, usually a glass tube spun by an air driven turbine to average the magnetic field over the sample dimensions.
(4) A radio frequency receiver connected to a coil encircling the sample. The two coils are perpendicular to each other and to the magnetic field.
(5) A read out system. The other supporting parts are— consisting of an amplifier, recorder and additional components for increasing sensitivity, accuracy or convenience.
(6) A sweep generator which supplies a variable d-c current to a secondary magnet so that the total applied magnetic field can be varied (swept) over a limited range.
Always a dilute solution is analysed. The compound to be studied is generally mixed with a solvent like CC14 or etramclhyl silane and the dilute solution is filled in a tube.
Now when a sample under investigation is placed in the magnetic field and subjected to rf field of oscillator then at particular combinations of the oscillator frequency and field strength, the rf. energy is absorbed by certain nuclei and an rf. signal is picked up by the detector.
Two ways have been employed in NMR experiments for getting the desired particular combinations:
(i) In one way, the magnetic field remains constant and radio frequency is varied.
(ii) In second, the radio frequency remains unchanged and magnetic field is varied till resonance conditions are obtained and there is detectable absorption by the nucleus.
The block diagram for a sample NMR spectrometer is shown in Fig, 3.
In block diagram, the blocks labelled N and S represent the poles of the large HO magnet, which is generally an electromagnet operated through a stabilized power supply. A field of up-to 1400 gauss and a pole of about 1.75 — 1.8 inch is necessary for high resolution spectra. The frequency and field strength are related to each other by Larmor condition.
v = үH0/2π
[This equation represents the condition of resonance.]
where HO = magnetic field,
v = is the frequency of radiation associate with transition from one state to another. It is generally known as Larmor frequency,
ү = proportionality constant or gyromagnetic ratio.
Experimental Parameter (Chemical shift):
The most important molecular parameter determined by NMR is the chemical shift. The chemical shift is defined as a measure of the resonance frequency of the nuclei in a given chemical environment.
The magnitude of the chemical shift is proportional to the strength of applied field and is caused by the circulations of surrounding electrons about the protons.
The chemical shift parameter 6 is defined
δ = (Hr – Hs)/Hr × 106 ppm
where Hr and Hs are field strengths corresponding to resonance for a particular nucleus in the sample (Hs) and reference (Hr).
But as spectra are usually calibrated in cycles per second (cps), the equation can be written as:
δ = ∆v × 106/Oscillator frequency (cps)
where ∆v = Difference in absorption frequencies of the sample and the reference in cps;
oscillator frequency is the characteristic of the instrument: For a 60 MHz instrument, the oscillator frequency is 60 × 106 cps.
The factor 106 has been included for convenience.
The units of 5, is expressed as parts per million (ppm). The tetra methyl silane (TMS) is generally taken as acceptable standard (because of low boiling point 27°C).
If the compound has a symmetrical structure, each proton is identical to all others and is found in an identical electronic environment which gives a very high shielding. As a result, TMS gives a single sharp resonance line.
Chemical shift is also designated by τ where τ = 10 – δ.
The standard (CH3)4Si protons appear between 0 on 5 scale and 10 on τ scale.
Measurement of Chemical Shift:
In fact the measurement of chemical shift gives information about the various types of magnetic environments. The chemical shift in simple molecules is fairly characteristics and may be used for analysis and characterization.
Factors which influence δ:
Actually the chemical shift parameter 8 is a function of electron density around the nucleus as the electrons are directly involved in the diamagnetic shielding which acts to attenuate the applied magnetic field.
Hence following factors are responsible for influencing its value:
(a) Specific solvent,
(b) Bulk diamagnetic susceptibility effect,
(c) Temperature (only when change in temperature causes changes in some type of association equilibrium or changes in amplitude of torsional vibrations),
(d) Electron density,
(e) Inductive effect,
(f) Vander Waal deshielding, and
(g) Hydrogen bonding.
Interpretation of NMR Spectrum:
The number spectrum gives several kinds of information:
(1) The number of signals (peaks) tells us how many kinds of protons (protons with different chemical environments) are present in a molecule.
(2) The position (chemical shift) of the signal informs about the bonding environment of each proton.
(3) The area under each signal tells us how many protons of each kind are in the molecule.
(4) All hydrogens with identical environments in a molecule have same chemical shift, e.g., (a) all the three protons of a methyl CH3; (b) the protons of a methylene — CH2; (c) one identical.
(5) Protons on heteroatoms (H—S, H —N, H—O etc.) show highly variable chemical shifts and sometimes broad peaks.
(6) Hydrogen on different carbons yields the same absorptional signal if they are structurally indistinguishable.
(7) Sometimes a proton exhibits an absorption signal which is split into several peaks because of coupling with its neighbouring protons. In such cases a coupling constant J is calculated.
(8) The number of peaks (N) into which a proton signal is split equals one more than the number of vicinal protons (n) (number of equivalent neighbours causing splitting):
N = n + 1
N = 2 (one vicinal H) = doublet (d)
= 3 (two vicinal H’s) = triplet (t)
= 4 (three vicinal H’s) = quartet (q).
Spectrum of-isomers—dimethyl ether and ethanol.
The low resolution spectra of isomers dimethyl ether and ethanol are shown in figures 4 (a) and (b). The spectrum of dimethyl ether shows only one signal because all 6H atoms are equivalent. The 3H atoms in a given CH3 group are indistinguishable, as are the two CH3 groups.
The spectrum of ethanol has three signals-one each for CH3, CH2 and OH protons. The relative areas under the peak for the ethanol are 3:2:1 for CH3, CH2, and OH groups respectively. CH2 signal is far away than CH3 signal because of electron withdrawing effect of two adjacent atoms.
In a high resolution NMR spectrum of ethyl alcohol (CH3—CH2—OH), the methyl peak is associated with two peaks, white methylene and OH are associated with four and three peaks respectively.
The three equivalent methyl protons are split into a triplet (1 + 2) by two equivalent methyl protons. Similarly two equivalent methylene protons are split into a quartet (1 + 3) by three equivalent methyl protons.
Applications of N.M.R. Spectroscope:
(1) Quantitative Analysis:
The area of peak is directly proportional to the number of nuclei responsible for that peak. Thus the concentration of species can be determined directly by making use of signal area per proton. The signal area per proton can easily be calculated by use of a known concentration of an internal standard.
Similarly, (he concentration of new species formed during the reaction can also be calculated from the spectrum of parent compound.
(2) Qualitative Analysis:
The qualitative analysis of the compound can easily be made by knowing:
(i) Chemical shift 8 values of hydrogen containing groups,
(ii) The presence of particular functional group,
(iii) The relative position of these groups and
(iv) The relative number of nuclei in these groups.
Nuclear Magnetic Double Resonance (NIVIDR):
When two oscillating magnetic fields are simultaneously applied to the sample, the experiment is called double resonance, double irradiation, or spin decoupling. In the usual nuclear magnetic double resonance experiment, a strong rf. field H2 is used to irradicate the sample while a weak rf. field H1 induces the transitions to be observed. We can sweep the magnetic field holding HI and H2 constant.
Electron Paramagnetic resonance (EPR):
The electron paramagnetic resonance (EPR) differs from NMR principally because in that the frequencies of electron resonance occur in microwave region for magnetic fields of the order of several thousand gauss. Therefore EPR spectrometer uses such components as Klystrons, wave guides and resonance cavities for the sample.
EPR method is applicable whenever the compound displays at least one unpaired electron, i.e., in free radicals, crystalline and amorphous solids subjected to irradiation or containing transition element ions and rare earths had some chelates. The other different examples are metals, odd molecules, graphite’s and impurities in semiconductors.