Chapter 1

1.1 INTRODUCTION

Nuclear magnetic resonance spectroscopy is a technique which gives us information about the number and types of atoms of a particular element in the molecule, for example, about the number and types of : (a) hydrogen atoms using 1H-NMR spectroscopy: also called proton magnetic resonance (PMR)

spectroscopy. (b) carbon atoms using 13C-NMR spectroscopy.

The technique was developed by Edward M. Purcell and Felix Bloch during late 1940's for which they shared the 1952 Nobel Prize in physics. Their discovery paved the way for development of NMR spectroscopy as a useful tool for structure elucidation of organic compounds. Since then this branch of knowledge has developed by leaps and bounds as evident by another Nobel Prize in Chemistry awarded to Professor Richard R. Ernest in 1991 for his pioneer work on applications of this technique.

We have learnt the application of UV and IR spectroscopic techniques for elucidation of structures of organic compounds. But they have limited use, e.g, UV helps us in identifying chromophore and conjugation and IR helps us in identifying the functional group(s). Contrary to these techniques, NMR gives us a deeper insight into the structure giving very useful information about the carbon-hydrogen framework of organic molecules. In a number of cases this technique can be used to establish the complete structure of an unknown compound.

1.2 THE PRINCIPLE

Like electrons, the nuclei (positively charged particles) of certain atoms or their isotopes possess intrinsic property of spinning along their axis. These spinning nuclei, being charged, generate a magnetic moment along the axis of spin. Thus nuclei behave as tiny bar magnets. Here it may be worthwhile to mention that the nuclei of all atoms do not possess such magnetic property. Only those nuclei which have either (a) odd atomic numbers and odd mass numbers or (b) odd atomic numbers and even mass numbers or (c) even atomic numbers and odd mass numbers, possess a quantized spin angular momentum, i.e., a magnetic moment.

Further, all these nuclei possess either half integral (n/2) or full integral (n) spin quantum numbers. It means that atomic nuclei having I = n or n/2 will have magnetic properties. In contrast, the nuclei of atoms

Nuclear Magnetic Resonance (NMR)Spectroscopy

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possessing even mass number and even atomic number do not possess a nuclear spin and hence do not have magnetic properties.

For a nucleus having a spin quantum number I, there are 2I + 1 allowed spin states. The atomic numbers, mass numbers, spin quantum numbers and number of allowed spin states of a few typical nuclei are listed in table 1.1.

table 1.1 Mass numbers, atomic numbers, spin quantum numbers and allowed spin states of some typical nuclei

In the absence of applied field all the allowed spin states of a nucleus are randomly oriented, degenerate and are also equally populated. (Fig. 1.1(a))

115 B

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1

(b)

(Applied

magnetic

field)

In presence of applied magnetic field

(two split spin states)No applied magnetic field

(degenerated spin states)

Energy Lower

energy state

Higher

energy state

+1/2 (aligned

with the

applied field)

–1/2 (aligned

against the

applied field)

(c)

(a)

No magnetic field

(all spin states are

degenerate)

14100

gauss

23500

gauss

70460

gauss

Strength of applied magnetic field (gauss)

+1/2 (aligned with

the applied field)

–1/2 (aligned against

the applied field)

0.0239 J mol–1

0.120 J mol–1

H0

Energy

Fig. 1.1 Energy levels of protons in the absence and presence of applied magnetic field

However, when such nuclei are placed under the influence of strong external magnetic field of strength H0, their different spin states no more remain random and degenerate. The interaction between nuclear spins and the applied field is quantized. For example, in the case of hydrogen nucleus (the proton having two spin

states + 12

and −

12

the magnetic moments will either be aligned with +

12

or aligned against −

12

the

applied field, i.e., two energy states will be generated. In the lower energy state the nuclear magnetic moment

will be aligned with the applied field +

12 and in the higher energy state it will be aligned against the

applied field −

12

(Fig. 1.1(b)).

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The difference in energy between the two energy states is very small being 0.0239 J mole–1 when the strength of applied magnetic field is 14100 gauss (or 1.41 Tesla). Because of such a small difference , the two energy states are nearly equally populated. However, calculations have shown that there is always slight excess of nuclei in the lower energy state (0.001%; Boltzmann distribution). It is this excess which is responsible for absorption of energy in the radio frequency (rf) region of the spectrum. It may further be noted that the difference in energy of two states increases with the increase in the strength of applied magnetic field (Fig. 1.1 (c)).

The frequency of absorption (n) is given by the expression —

0 MHz

µν =

HhI

where, m = magnetic moment; H0 = strength of the applied field; h = Planck’s constant; I = spin quantum number.

Now for a proton H0 = 14100 gauss; I = ; 12

; h = 6.624×10–27; m = 1.41×10–23

23

27

1.41 10 14100 MHz1 6.624 102

ν−

−

× ×∴ =

× ×

= 60 MHz

So for we have seen that the effect of applied magnetic field is to segregate the protons into two energy states—aligned with (lower energy) and aligned against (higher energy). A second effect of the applied magnetic field on the nucleus is that it begins to ‘Wobble’ or ‘precess’ and traces a conical surface in the same manner as a spinning top or a gyroscope traces under the influence of earth’s gravitational field (Fig. 1.2).

(a)

(b)

Precessionalorbit

Axis of rotation

Spinning nucleus

Direction ofmagnetic fieldH0

Fig. 1.2 (a) Spinning top precessing in the earth’s gravitational field; (b) Precession of a spinning nucleus under the influence of the applied magnetic field.

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11.3 thE ProCESS oF AbSorPtIon: rESonAnCE

The precession of the nucleus, which is a charged particle, generates an oscillating electric field of the same frequency (w) as the frequency of precessing nucleus (w). Now when such a nucleus is irradiated with electromagnetic radiation also of the same frequency (w), the two frequencies couple and the nucleus attains

the state of resonance and absorbs energy. The nuclear spin is flipped from spin state +12

to –�12

(Fig 1.3).

The instrument detects and records this as a signal to yield a spectrum called nuclear magnetic resonance

spectrum.

1.4 thE ChEmICAl ShIFt : ShIEldIng And dEShIEldIng

So far our discussion on NMR spectroscopy had been based on the presumption that this technique is related to the study of ‘free’ protons. If this were the case, then any one combination of applied field and electromagnetic radiation which produces a signal for one particular proton would also produce the same signal for all the protons in the molecule. In other words, the spectrum would consist of one single signal which would be of no use in structural elucidation.

However, in a molecule, the protons are not at all ‘free’. They are rather surrounded by electron cloud which starts circulating under the influence of applied magnetic field. This circulation of electrons, which is called diamagnetic current, induces a local magnetic field (also called counterfield) which opposes the applied field near the nucleus (Fig. 1.4). Consequently, the field strength actually felt by the nucleus (H

eff) is always less than the strength of applied field (H0). Such a nucleus (Heff < H0) is said to be shielded. As this shielding is due to diamagnetic current, it is also called diamagnetic shielding.

Fig. 1.3 resonance process

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Induced magnetic fieldopposes the applied fieldnear the nucleus

Nucleus

Induced magnetic field

Circulating electron producesdiamagnetic current

H0

Applied field

Fig. 1.4 diamagnetic shielding of nucleus

For example, we know that under the influence of applied magnetic field of 14100 gauss (or 1.41 Tesla) an isolated proton resonates at 60 MHz. But when we examine the 1H-NMR spectrum of phenylacetone we find that it exhibits three types of signals as given below:

CH2

59.999995 MHz

59.99982 MHz

59.999700 MHz

60 MHz CO CH3

It is quite clear from the above example that each type of proton is shielded as compared to an isolated proton. Therefore, to cause absorption of radiowaves of 60 MHz, by each type of proton the strength of the applied magnetic field should be increased (or decreased) to a different extent. Obviously, the degree of shielding or dishielding of a given proton attached to a carbon atom depends upon the inductive effect of the other atoms or groups attached to this carbon. The electron - releasing groups (+ I effect) will increase the electron density around the proton under investigation and would cause its shielding. On the contrary, electron - withdrawing groups (– I effect) will decrease the electron density around the proton under examination and would cause its deshielding. In the former case the strength of the applied magnetic field should be increased while in the latter case it should be decreased.

As different types of protons absorb differently, there is a need to interpret these results by comparing their absorption with a standard proton. This difference between absorption position of a particular proton and that of a reference (or standard) proton is called chemical shift of that proton.

1.5 SElECtIon oF A rEFErEnCE Proton

It has been observed that the difference in resonance frequencies of different types of protons due to shielding and deshielding is generally very small. For instance, in the case of CH3F and CH3Cl, this difference is just

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172 Hz under the influence of applied magnetic field of 14,000 gauss which is 1.2 ppm of the irradiating frequency (60 MHz) and as such it is extremely difficult to measure such a small difference accurately.

To overcome this difficulty, a suitable reference compound is chosen and the resonance frequency of the given proton is measured relative to a resonance frequency of proton(s) of this standard compound. The most commonly used reference compound is tetramethylsilane(CH3)4Si (abbreviated as TMS).

SiH3C

CH3

CH3

CH3

TMS

The choice of TMS as a reference compound is associated with the following advantages: (i) It is relatively inert as it has no polar group. (ii) It is a highly volatile liquid (b.p. 299.5K) and , therefore, can be easily removed after use thereby

making the recovery of the sample (in pure condition) easy. (iii) It is miscible with most of the organic solvents and does not form any type of complexes with

them. (iv) It has only one type of protons, so it exhibits one sharp single peak (a singlet). (v) Due to, the presence of silicon atom, which is more electropositive than carbon, the chemical

environment of all the protons of TMS is quite different than that of most of the protons. Hence the signal due to protons of TMS appears outside the normal range of protons of majority of organic compounds. In fact the protons of TMS are highly shielded so chances of overlapping of the signal due to these protons with that of other protons are very rare.

1.6 unItS And SCAlE oF ChEmICAl ShIFt

The chemical shift can be measured in Hz which is directly proportional to the strength of applied magnetic field. Since a number of instruments operating at different field strengths (60 MHz, 100 MHz, 200 MHz, 300 MHz, etc.) are available, the chemical shift, measured in Hz, will be different with different instruments which would create problem in interpreting the results. To overcome this problem a field independent scale, was developed. The unit used for this is d (delta) which is the ratio of observed chemical shift (in Hz) to the radio frequency used (in MHz). Since the resulting number is too small, it is multiplied by 106 so that we obtain a value which can be conveniently handled. Thus

6Observed chemicalshift (Hz) from TMS 10Spectrometer frequency (MHz)

δ = ×

For example, for an instrument operating at 60 MHz the resonance frequency of a particular proton is 120 Hz and for the same proton it is 200 Hz for an instrument operating at 100 MHz. Though the frequencies in Hz are different, their d value is the same (i.e. 2.0) as shown below.

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First instrument Second instrument6120Hz × 10 2.0

60MHzδ = =

6200Hz × 10 2.0100MHz

δ = =

The d value assigned to TMS is 0(zero) and the chemical shifts of majority of protons fall between d 0 to d 10 on a ten point scale. A small d value indicates upfield shift (higher field strength) and the proton under examination is said to be shielded. On the contrary a large d value indicates downfield shift (lower field strength) and the proton is said to be deshielded. Here it may be noted that another scale called t (tau) scale has been developed and on this scale the chemical shift of TMS is assigned the value t as 10.

Thus both d and t values are related to each other as follows:

d = 10–t or t = 10 – d

The relationship between the two scales, position of TMS signal and the correlation of shielding and deshielding effects with magnetic field effect are shown in Fig. 1.5.

Fig. 1.5 scales of nMr spectra

In the above figure, a shift of NMR signal to the left on the chart paper is called downfield shift and is attributed to deshielding. A shift to the right on the chart paper is called upfield shift and is attributed to shielding.

1.7 SolVEntS uSEd In Pmr SPECtroSCoPY

Any solvent which does not contain protons can be used in PMR spectroscopy. The most widely used solvent is deuterated chloroform (CDCl3). Other solvents which are commonly used in PMR spectroscopy are: CCl4; deuterated acetone, i.e., (CD3)2CO (acetone-d6); deuterated dimethyl sulphoxide, i.e., (CD3)2SO (DMSO-d6) and deuterated benzene, i.e., C6D6 (benzene-d6), etc.

Trifluoroacetic acid (F3CCOOH) is also used sometimes even though it contains a proton. The reason being that this acidic proton absorbs at a very low field (11.2d) and hence does not interfere with the absorptions of most of the compounds which usually occurs in the range 0–10d.

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11.8 nmr SPECtromEtEr

Generally the following two types of spectrometers are used: (i) Continuous wave (CW) NMR spectrometer (ii) Fourier transform (FT) NMR spectrometer

(i) Continuous Wave nmr SpectrometerIn this type of instrument frequency of the radiation is kept constant, while the magnetic field strength is continuously varied. The essential components of a CW NMR spectrometer (Fig. 1.6) are as follows: (a) A powerful magnet (b) A radiofrequency generator (c) A radiofrequency detector (d) A radiofrequency recorder (e) A probe unit (a) Magnet: It is large, strong magnet and is homogeneous over the area of the sample. The strength

of the magnetic field can be varied by fixing electromagnetic coils on the poles of the main magnet.

(b) Radiofrequency generator: It generates the electromagnetic waves in the radiofrequency region and transmits these waves to the sample through a coil fixed in the probe.

(c) Radiofrequency detector: It detects the absorption of radiofrequency when a particular proton attains the state of resonance through a coil which is also fixed in the probe.

(d) Radiofrequency recorder: The radiofrequency detector transmits the signal received directly to the oscilloscope which records the spectrum (on a graph paper) as a plot of resonance signal v/s strength of the applied field.

(e) Probe: It is the sensing device which is inserted between poles of the magnet. It actually houses the sample tube, (a thin walled glass tube about 15-20 cm long and 5 mm diameter), radiofrequency generator coil and radiofrequency detector coil.

Radiofrequencygenerator

NMR spectrum

Detector andamplifier

Recorder

Probe

Variablemagneticfield

Sample tube

S

Fig 1.6 schematic diagram of a typical nMr spectrometer

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Working: The sample is dissolved in a suitable solvent which is usually NMR inactive and devoid of protons, eg., CCl4, CDCl3 (chloroform-d1) DMSO-d6 (Dimethyl sulphoxide - d6) CD3OD (Methanol-d4) etc. A small amount of TMS is added to this solution which is now transferred to the sample tube. The tube is then capped, inserted in the probe, and spun to ensure that all parts of the solution experience uniform magnetic field. The coil coming from the radiofrequency generator transmits the electromagnetic energy for flipping to take place. The other coil connected to radiofrequency detector picks up a signal the moment a proton attains the state of resonance. This signal is transmitted to the recorder which generates the NMR spectrum. As a typical example, the NMR spectrum of phenyl acetone is shown in Fig. 1.7.

Fig. 1.7 1hnMr spectrum of phenylacetone

(ii) Fourier transform nmr (Ft nmr) Spectrometer

Nowadays more advanced instruments known as FT NMR spectrometers are available. In these instruments, the magnetic field is kept constant whereas, the radiofrequency is varied. The greatest advantage of FT NMR over CW NMR is that the former records the spectrum within 5 seconds whereas the latter takes 2-5 minutes to scan it.

1.9 FACtorS AFFECtIng ChEmICAl ShIFtSThe chemical shifts of different types of protons are effected by local environments. Sometimes these effects are so strong that the values of chemical shifts can be used as diagnostic tools for characterisation of different types of protons thereby playing a major role in elucidation of the structure of unknown.

The values of the chemical shifts of different types of protons, listed in table 1.2, are illustrative.

table 1.2 Chemical shifts of different protons

Proton d Proton d

CH3—R 0.9 —C—CH2—OR 3.4

CH3—C=C 1.6 —C—CH2—I 3.2

CH3—COOR 2.0 —C—CH2—Br 3.5

CH3—COR 2.1 —C—CH2—Cl 3.6

CH3—S— 2.1 —C—CH2—OH 3.6Contd....