2 Nuclear Magnetic Resonance

Prof. K. M. Muraleedharan

Engineering Chemistry III

NMR - Basic principles Subatomic particles like electrons, protons and neutrons are associated with ‘spin’- a fundamental property like charge or mass. In the case of nuclei with even number of protons and neutrons, individual spins are paired and the overall spin becomes zero. However, there are many cases such as 1H and 13C, where the nuclei possess a net spin, which is important in Nuclear Magnetic Resonance (NMR) Spectroscopy. A set of rules to determine the overall spin a nucleus is given below. •

When there are even number of protons and even number of neutrons in the nucleus, the net spin is equal to zero.

•

When there are odd number of neutrons and odd number of protons in the nucleus, it will have an integer spin (i.e. 1, 2, 3)

•

If the sum of the number of neutrons and the number of protons is odd number, the nucleus will have a half-integer spin (i.e. 1/2, 3/2, 5/2).

These rules can be summarized in terms of atomic mass and atomic number as shown below.

I Halfinteger Halfinteger Integer Zero

Atomic Mass Odd

Atomic Number Odd

Examples 1H

(1/2) NMR active

Odd

Even

13C

Even

Odd

2H

Even

Even

12C

(1/2)

(1) (0)

Not NMR active

Nuclei are charged and those with a net spin would generate a magnetic dipole along the spin axis. The magnitude of this dipole is given by the nuclear magnetic moment μ, which is give by

μ = γ I h / 2π ..................................(1) where I is the spin quantum number (with values ½, 1, 3/2 etc.) and

Indian Institute of Technology Madras

Prof. K. M. Muraleedharan

Engineering Chemistry III

γ = Gyromagnetic ratio, which is a characteristic constant for individual nuclei. In Nuclear Magnetic Resonance Spectroscopy, we study the behavior of magnetic nuclei in presence of an external magnetic field. Quantum mechanics tells us that a nucleus of spin = I can have 2I+1 orientations with respect to an external magnetic field. Since the main aim of this chapter is to illustrate the use of NMR in the study of aromaticity, we will focus on the magnetic properties of 1H and see what special information does 1H NMR spectroscopy provide when this nuclei (1H) is part of an aromatic system. A nuclei such as 1H with spin = ½, can orient in two ways (2 x ½ + 1 = 2) with respect to the external field. Of these, the spin state represented as +1/2 (or α state) is of lower energy where as the one represented as -1/2 (or β state) is of higher energy. The former reinforces the applied field and the latter opposes it. The energy difference between the two spin states is proportional to the applied field and can be written as ΔE = hν = hγB0/2π ..................................(2) B

where, h = Plank’s constant, γ = Gyromagnetic ratio which is a characteristic constant for individual nuclei, B0 = strength of the magnetic field at the nucleus B

From equation (2), it is clear that a radiation of frequency ν = γB0/2π possess the B

right amount of energy to effect a transition from lower energy α state to higher energy β state. Absorption of energy takes place only with a certain combinations of field strengths and radio frequencies during which the system is said to be in a state of resonance. For proton, γ = 2.675x108 T-1s-1 If the NMR spectrometer is equipped with a magnet with a field of 7.046T, ν = γ B0/2π =

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2.675 x 108 T-1S-1 x 7.046 T 2 x 3.1416

= 300 x 106 Hz = 300 MHz

Prof. K. M. Muraleedharan

Engineering Chemistry III

This can be pictorially represented as shown below: Energy Spin state I = -1/2 (β ) I = 1/2 Spin state I = +1/2 (α ) Field strength B0

NMR experiments can be performed in one of the following ways, i)

keep the external magnetic field strength constant and vary the frequency of radiation to see an absorption

ii)

keep the radio frequency constant and slowly vary the field strength until the splitting of spin states corresponds to the energy of radio wave.

In new versions of instruments, instead of Continuous Wave (RF) sweeping, an intense radiofrequency pulse is used to excite all nuclei simultaneously and their individual absorptions determined using Fourier transform methods

Shielding and Deshielding So far we were considering the magnetic moments generated by the spinning nucleus and its interaction with an external field.

However, in realty, nuclei are

surrounded by electrons which also generate small local magnetic fields (Bloc) as they circulate. These local magnetic fields can either oppose or augment the external magnetic field. If the field created by the electron oppose the external field, nuclei ‘experience’ an effective field which is smaller than the external field and it is said to be SHIELDED. If the field created by the electron augments the external field, nuclei ‘experience’ an effective field which is larger than the external field. It is said to be DE-SHIELDED.

i.e.

Beff = Bo - Bloc B

This can be represented as Beff = Bo( 1 - σ )

Indian Institute of Technology Madras

Prof. K. M. Muraleedharan

Engineering Chemistry III

σ – magnetic shielding or screening constant; depends on electron density. Equation (2) can now be written as ΔE = hν = hγ Bo( 1 - σ )/2π ..................................(3) Equation (3) shows that magnetic field ‘felt’ by individual nuclei varies depending upon their chemical environment; ΔE and hence the energy of the radiation required to excite them differ consequently.

Diamagnetic and paramagnetic anisotropy Two effects that originate from electronic delocalization in aromatic systems are 1) External field induces a flow (current) of electrons in π system – ring current effect 2) Ring current induces a local magnetic field with shielding (decreased chemical shift) and deshielding (increased chemical shifts) zones. This is the basis of diamagnetic anisotropy in aromatic systems which can be diagrammatically represented as shown below.

In the case of compounds with unpaired electrons, paramagnetism associated with the net spin overrides the diamagnetic effects and lead to a different type of magnetic effect (vide infra)

Chemical shift and δ scale As mentioned, effective magnetic field at individual nuclei vary depending upon their chemical environment.

Radiofrequency required to excite them also will be

different under different external field strengths. This means that if we take NMR spectra of a compound using instruments of different field strengths, peaks appear at different positions and it would be difficult to compare them without applying corrections for

Indian Institute of Technology Madras

Prof. K. M. Muraleedharan

Engineering Chemistry III

differences in absorptions. To avoid this problem, a different system, based on ‘chemical shift’ is often used. In this, we use a reference compound in our experiments. It is generally tetramethylsilane, which is unreactive with other organic compounds, volatile, and gives an absorption which is relatively shielded in comparison with most of the protons important to organic chemists. Solvent used in NMR experiments should not contain 1H due to interference. Deuterated solvents such as CDCl3, DMSO-D6 etc. are generally used in modern instruments. Now let us see what are chemical shift and δ scale. At first, the absorbance frequencies of the standard and our sample are measured. The latter is then subtracted from the former and then divided by the frequency of the standard. This gives a number called the chemical shift (δ). Let us assume that in a given magnetic field (external field), the ‘standard’ absorbs at 300,000,000 Hz (300 megahertz), and our sample absorbs at 300,000,300 Hz. The difference between our sample and the standard is 300 Hz, and we take 300/300,000,000 = 1/1,000,000 and call it 1 part per million (or 1 ppm). If we examine the same sample in a stronger magnetic field where the reference comes at 500,000,000 Hz (or 500 megahertz), the frequency of our sample will increase proportionally, and will come at 500,000,500 Hz.

The

difference in this case would be 500 Hz. But if we divide this difference by 500,000,000 ie., 500/500,000,000, we will get 1/1,000,000, = 1 ppm! So, chemical shift values remain the same irrespective of the instruments (field strengths).

Although we did these

calculations manually, all these are done automatically in the computer associated with the NMR spectrometer. A correlation of radiofrequencies and δ values and their relative positions with respect to TMS (standard) is presented below.

Indian Institute of Technology Madras

Prof. K. M. Muraleedharan

Engineering Chemistry III

A representative NMR spectra is given below

H H

H H H C

C

H C

Br

C

Br

H H

H H

Shielding and deshielding zones in Aromatic systems Due to the magnetic field generated by the circulating electrons, hydrogens which lie in the plane of the ring experiences a deshielding effect as shown below (A). At the same time those situated above and below the plane experience shielding, as the magnetic lines of force are opposite in direction with respect to the applied field. Similar effect, but to a lesser extent, can be seen in simple olefins (B), aldehydes etc.

Applied magnetic field

H

B0

H

δ 7-8 ppm

A

δ 5-7 ppm

B

Illustrative examples: Chemical shift positions of hydrogen atoms which are placed in the shielding and deshielding zones of aromatic systems are given below.

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Prof. K. M. Muraleedharan

Engineering Chemistry III

Bridge head protons at -0.5 δ Inner protons 0.00 δ

Outer protons at 6.9-7.3 δ

H

CH3 protons at -4.25 δ

Outer protons 7.6 δ H

H

H

H

H H

Outer ring protons at 8.14-8.67 δ

H3C

H

CH3

H [14]-annulene

H

Outer protons at ~ 9 δ

H

H H

H H H H H H H

H

2

Inner protons at ~ -3 δ

Ha

H

H1 H7

4

H

H

Hb

3

5

6

Ha at -0.3 δ Hb at 5.1 δ H1 & H7 at 6.4 δ H2-H6 at 8.5 δ

H H

H

[18]-annulene

Antiaromatic systems Antiaromatic systems are paratropic. That is, they are able to sustain a paramagnetic ring current, which lead to shielding of outer ring protons and deshielding of inner protons (opposite to that of aromatic compounds which show diamagnetic effect). Examples presented below demonstrate the behavior of magnetic nuclei situated in an antiaromatic environment. 1) At -170oC, inner protons of [12]-annulene comes at ~8 ppm and outer protons comes at ~6 ppm which is characteristic of antiaromaticity. Above -150oC, all protons are magnetically equivalent showing conformational flexibility. Above -50oC, it rearranges to a bicyclic system as shown below.

above -50oC

[12]-annulene

Indian Institute of Technology Madras

Prof. K. M. Muraleedharan

Engineering Chemistry III

At -130oC [16]-annulene is paratropic with four central protons at 10.56 δ, and

2)

twelve outer protons at 5.35 δ. Above -50oC, all protons are magnetically equivalent showing conformational flexibility

H

H H H

in solution

Nonaromatic [16]-annulene

3)

As discussed previously, the locked form of [14]-annulene show significant

aromatic character, with outer protons resonating at 8.14-8.67 δ and CH3 protons coming at -4.25 δ. However, the dianion of this compound is antiaromatic which is evident from the paramagnetic anisotropic effect seen in its NMR.

H

Outer ring protons at 8.14-8.67 δ

H

CH3 protons at -4.25 δ H3C

CH3

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CH3 protons at 21 δ H3C

reduction

Outer ring protons at -3 δ

CH3