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CHE 320: Organic Spectroscopy

NMRLecture Notes

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Introduction to NMR

NMR is a spectroscopic technique that gives information about number and type of atom in a molecule

The experiments on NMR involve the absorption of Radio frequencies on EMS

An NMR spectrum: is the plot of radio frequency applied against absorption frequencies usually in ppm on a chemical shift scale

A signal in an NMR spectrum is refered to as resonance 1H-NMR spectroscopic give the number and type of

hydrogen atoms in a molecule

CH3CH2OH 3 types of H atoms# of H = 6

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13C-NMR spectroscopy gives the number and types of carbon atoms

NB: The NMR spectra give information about the nature of the chemical environment of each magnetically active nucleus in the molecule

OC

H2C

CH3C

O

CH 5 types of C# of C = 5

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NMR spectrometer Essential parts of the spectrometer

A powerful magnet Radiofrequency generator Radiofrequency receiver Sample tube Readout

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NMR spectrometer cont…

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Features leading to the NMR – phenomenon

Nuclear spin state Nuclei with an odd mass/atomic number have nuclei

spin with quantum number of ½ with allowed value of +1/2 (alpha state) and -1/2 (beta state)

e.g 1H, 13C, 15N, 19F, 31P Spin change generates magnetic field These nuclei behave like a tiny magnetic bar and has

“magnetic moment” Magnetic quantum number is due to the fact that

nucleus has +ve and –ve spinning

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Recall; Proton in motion- current in motion- magnetic

field- electricityFlaming Rule “Right hand Rule”

In the absence of applied magnetic field all the spin states of a given nucleus are degenerate. Thus all the spin states should almost equally populated with the same numbers of atoms having each of the allowed spin

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Nuclear magnetic moment and magnetic resonance No equivalent energy in spin state in an applied magnetic field

(βo or Ho) Nucleus has a magnetic momentum, µ, generated by charge

and spin The magnetic momentum, µ, of lower energy +1/2 spin

aligned with βo while with high magnetic momentum, µ, -1/2 spin opposes the βo

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Nuclear magnetic moment…

A nucleus is in resonance when it absorbs radio frequency radiations and spin flip to a high energy state

Energy absorbed is quantized process Energy absorbed most equal the energy different

between two state involved Eabsorbed = (E -1/2 state – E +1/2 state) = hυ In practice ∆E = f (βo) The stronger the applied magnetic fields, the greater the

energy difference between possible spin. i.e the magnitude of the energy level separation depend on the particular nuclear involved

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Nuclear magnetic moment …

Magnetogyric ratio, γ is constant for each nucleus and determines the energy dependence on the MF

∆E = f (γβo) = hυ

β x

∆ Ε =l

where l = 1/2

µ β x

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Mechanism of absorption (Resonance) The coupling frequency (Larmor frequency, ω, also called natural frequency) a

frequency at which the nuclei precess For resonance to occur external energy must be applied: The nucleus starts to precess about its own axis of spin with ω (Larmor frequency)

Natural frequency, ω is proportion to the strength of βo Radio frequency is recorded as an absorption signal The βo managed to change the direction of the spinning nuclei The realigned MF induce a radio signal in the output circuit to generate the output

signal Magnetic field = 1.41 T (= 14,100 Gauss) ∴ ω ~ 60 MHz

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Magnetic resonance

When the frequency of the oscillating electric field component of the incoming radiation just matches the frequency of the electric field generated by the precessing nucleus the two fields can couple, and energy can be transformed from the incoming radiation to the nucleus thus causing spin change. This situation is called Resonance

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Magnetic resonance.... NB. Two variable that characterize NMR

1. An applied magnetic field, βo 2. the frequency of the radiation usedf for resonance (in Mhz)

The frequency needed for the resonance and the βo strength are proportional

If we keep a MF constant and vary radiofrequency, different nuclei will resonate at different frequencies

In practice it is easier to keep radio frequency and vary MF. Why?

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1H-NMR CHEMICAL SHIFTS

Chemical shift (δ in ppm) : is the frequency of signal on an NMR spectrum

OR is a position on the chemical shift scale (in ppm) where the peak

occursAtoms in different chemical environment will resonate at different

frequency and will appear at different chemical shifts

Observed shift from TMS in Hz Spectrometer frequency in Hz x 106Chemical shift, δ =

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Chemical shifts The scale is expressed in ppm and is independent of the spectrometer

frequency The scale is calibrated such that the frequency point is tetramethyl silane, (TMS) = 0 on the RIGHT to the LEFT (usully 10- 12 ppm).

H3C Si

CH3

CH3

CH3TMS:

0 1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

TMS

chemical shift, δ (ppm)

UpfieldShielded

DownfieldDeshielded

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Chemical shifts

Increasing chemical shift for 0 to 12 ppm the proton is said to be Downfield or Deshielded

Decreasing chemical shift from 10 to 0 ppm, proton is said to be Up field or Shielded

Qn. Why we measure the resonance of nuclei in ppm ? Because when we vary the MF, we only need to change its strength by a few millionth to observe all the different atoms in the compounds

Calculations Qn.What would be the chemical shift of a peak that occur 655.2 Hz

downfield of TMS on a spectrum recorded using a 90 MHz spectrometer?

Soln: 655.2 Hz/90x10^-6Hz = 7.28 ppm Qn.At what frequency would the chemical shift of CHCl3, δ = 7.28

ppm occur relative to TMS on a spectrum recorded on a 300MHz spectrometer?

Soln. 7.28 ppm x 300MHz = 2184 Hz

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How to interpret 1H-NMR spectra?

1st . How many type of Hs? This indicated by how many groups of signals are

there in a spectra 2nd. What type of Hs? Indicated by the chemical shift of each group 3rd. How many Hs of each type are there? indicated by integration (relative area of signal for

each group 4th. What is the connectivity? Look at the coupling patterns. This tell you what is

next to each group

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Chemical shifts

Different protons has different resonance because protons surrounded by different electronic environments from one another

Protons are shielded by electrons which surround them In applied magnetic field , the valence electrons of the proton

are cause to circulate The circulation, called Local Diamagnetic Current generates

a counter magnetic field which opposes the applied magnetic field called , Diamagnetic shielding or diamagnetic anisotropy

Chemical environment: different types of proton will occur at different chemical shifts

Spinning proton has electron which also do spin and both generate magnetic field.

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Examples

Ethyl acetone

0123PPM

CH2

O

CH3

H2C

H3C(c)

(b)

(a) (d)

(a)(b)(c)(d)

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Example 2

012345678PPM

(a)

(b)

(c)

(a)

Benzyl acetate Phenyl acetone

012345678PPM

(b) (c)

(a)

(d) (e)

H2C CH3

O

(d)

(e)

H(a)

H(a)

H(b)

(c)H

(b)H

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Benzyl acetate & Phenyl acetone

Both phenyl acetone and benzylacetate have δ = 7.3 ppm. Methyl groups attached directly to a carbonyl have resonance at δ = 2.1 ppm.

Aromatic protons basically characteristically have resonance near 7-8 ppm

Acetyl groups (methyl group of this type) resonance ~2 ppm Resonance of the benzyl (-CH2-) protons comes at higher value

of δ = 5.1 ppm in benzyl acetate than phenyl acetone δ = 3.6 ppm. Being attached to an electronegative element (oxygen atom) these electrons are more deshielding than those in phenyl acetone

Note : the higher the electron density the higher the shielding , hence the slow the resonance

The higher the shielding the more the external energy is required.

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Integrations

The intensity of a 1H-NMR signal is proportional to the number of proton of each type in the molecule

The integral measures the area of the peak and gives the relative ratio of the # of Hs for each peak

An FT NMR instrument digitally integrates each signal area and provides ratios of hydrogens for each signal.

There are two ways to determine the number of hydrogens under each signal from the measured heights

for known molecular formula

For unknown molecular formula integrate by determine the ratio between the signals round off to a nearest whole number or multiply by a factor to produce a whole

number

# of Hs in signal =total heights of all integrals

total # of Hsheight of integral x

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Integrations

(1)

(2)

(3)

(4)

NB: The reason for multiple peaks will be discussed later SOLUTION Signal (1) 2 units (2) 3 units (3) 2 units (4) 3 units Total units = 10

Signal (1) = (2 /10) x 10 = 2H Signal (2)---------= 3H Signal (3)---------- = 2H Signal(4) ---------- = 3H Calculate DBE = [(2C+2)-H]/2 =1 May be 1 double bond or a ring

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Integrations cont..

H3C

H2C

CH2

O

CH3(2)

(1)(4)

(3)

(1)

(2)

(3)

(4)

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Questions

Qn. The line of integration of the two signals in the 1H-NMr spectrum of a ketone of molecular formula C7H14O rises 62 and 10 chart divisions, respectively. Calculate the number of Hydrogens giving rise to each signal and propose a structural formula for this ketone

Qn. Following are structural formula for two constitutional isomers of molecular formula C6H12O2

(a) Predict the number of signals in the 1H-NMR spectrum of each isomer(b) Predict the ratio of areas of the signals in each spectrum(c) Show how you can distinguish these isomers in the basis of chemical shifts(d) Draw and label the proposed 1H-NMR for each.(e) Predict IR absorption frequencies of major FGs and describe their spectrum

O

O O

O

I II

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Factors that influence shielding

Electronegativity Electronegative groups cause inductive effects Electrons around the proton create an MF that

opposes the applied magnetic field. These electrons are said to shield the proton from βo

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Electronegativity cont…

The presence of more electronegative groups produce more deshielding and therefore, large chemical shifts

Reason: Electronegative substituents (with EW effect) attached to a

carbon atom reduce the valence electron density around the protons attached to that carbon. It reduces the local diamagnetic shielding in the vicinity of the attached carbon because they reduce electron density around those protons

The greater the electronegativity of the substituent, the more it deshields protons and hence the greater is chemical shift of those protons

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Hybridization effect

The trend is due to difference in the hybridization of the atom to which hydrogen is attached

Sp3-Hydrogen: All hydrogen attached to the purely Sp3 –carbon atoms have resonance

in the limited range from 0-2 ppm provided that no electronegative elements or π bonded groups are nearby.

Hydrogens attached on a sp3 to heteroatom (-O-CH2-, ) or unsaturated carbon (-C=C-CH2-) have higher chemical shifts

Sp2- Hydrogen Sp2 –carbon atom hold its electron more tightly, this results in less

shielding for the H-nucleus than in an sp3-1s bond In an sp2-1s C-H bond the carbon atom has more s-character (more

electronegative)Sp- hydrogen

An sp-carbon should behave like as if it were more electronegative than an sp2 carbon. This is the opposite of what is actually observed

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Magnetic anisotropy

Mean “non-uniform magnetic field” produced by π system that interact with the applied which induced a magnetic field that causes the anisotropy.

The near by proton experience the three fields: - the applied field; -the shielding field and-the shielded or deshielded which implies energy required

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Magnetic anisotropy cont… Example

When benzene is placed in a magnetic field the π-electrons in the aromatic ring system are induced to circulate around the ring.

This circulation is called ring current The magnetic field covers a spatial volume large enough that it influences the shielding

of the benzene hydrogens The benzene hydrogen are said to be deshielded by the diamagnetic anisotropy of the

ring. Protons attached to the benzene ring is influenced by the three magnetic fields applied

magnetic field, weaker fields due to usual shielding by the valence electrons around the protons and weaker magnet due to the anisotropy

Qn. Provide thorough explanation of circulation of electrons, shielding and/ or deshielding effect when external magnetic field is applied to the benzene and acetylene

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H-bonding

Usually protons that are involve in H-bonding are –OH or –NH which typically observed at large chemical shift value.

The H-bonding, the more proton deshielded and the higher its chemical will be.

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Questions for assignment

Qn. Suggest a reason why the acidic protons in a carboxylic acid appear so downfield (about 12 ppm)

Qn. Discuss the phenomenon of exchangeable hydrogens in 1H-NMR experiments.

Qn. Deuterated solvent are commonly used in NMR experiments. Explain

Qn. Give a brief account on MRI (magnetic resonance imaging)