13-1 Nuclear Magnetic Resonance Spectroscopy Part-1 Prepared By Dr. Khalid Ahmad Shadid Islamic...

13-1

Nuclear Magnetic Resonance SpectroscopyPart-1

Prepared By

Dr. Khalid Ahmad Shadid

Islamic University in MadinahDepartment of Chemistry

13-2

Molecular Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy:

A spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of hydrogen atoms using 1H-NMR spectroscopy. carbon atoms using 13C-NMR spectroscopy. phosphorus atoms using 31P-NMR spectroscopy.

13-4

NMR: Basic Experimental Principles

Imagine placing a molecule, for example, CH4, in a magnetic field. We can probe the energy difference of the - and - state of the protons by irradiating them with EM radiation of just the right energy.In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz (radio waves).So, if we bombard the molecule with 300 MHz radio waves, the protons will absorb that energy and we can measure that absorbance.In a magnet of 11.75 Tesla, it takes EM radiation of about 500 MHz (stronger magnet means greater energy difference between the - and - state of the protons)

E

Bo

E = h x 300 MHz E = h x 500 MHz

7.05 T 11.75 T

proton spin state (lower energy)

proton spin state (higher energy)

Graphical relationship between

magnetic field (Bo) and frequency (

for 1H NMR absorptions

at no magnetic field,there is no difference beteen- and - states.

0 T

13-5

The Chemical Shift of Different Protons

NMR would not be very valuable if all protons absorbed at the same frequency. You’d see a signal that indicates the presence of hydrogens in your sample, but any fool knows there’s hydrogen in organic molecules. What makes it useful is that different protons usually appear at different chemical shifts (d. So, we can distinguish one kind of proton from another. Why do different protons appear at different d? There are several reasons, one of which is shielding. The electrons in a bond shield the nuclei from the magnetic field. So, if there is more electron density around a proton, it sees a slightly lower magnetic field, less electron density means it sees a higher magnetic field:

How do the electrons shield the magnetic field? By moving. A moving charge creates a magnetic field, and the field created by the moving electrons opposes the magnetic field of our NMR machine. It’s not a huge effect, but it’s enough to enable us to distinguish between different protons in our sample.

C H

ZThis represents the electron density of a C-H bond. How much electron density is on the proton depends on what else is attached to the carbon. If Z is an elelctronegative atom, the carbon becomes electron deficient and pulls some of the electron density away from the H. if Z is an electron donating group, more electron density ends up on the H.

13-6

The Hard Part - Interpreting Spectra

1) Chemical shift data - tells us what kinds of protons we have.2) Integrals - tells us the ratio of each kind of proton in our sample.3) 1H - 1H coupling - tells us about protons that are near other protons.

Learning how an NMR machine works is straightforward. What is less straightforward is learning how to use the data we get from an NMR machine (the spectrum of ethyl acetate is shown below). That’s because each NMR spectrum is a puzzle, and there’s no single fact that you simply have to memorize to solve these spectra. You have to consider lots of pieces of data and come up with a structure that fits all the data. What kinds of data do we get from NMR spectra? For 1H NMR, there are three kinds each of which we will consider each of these separately:

13-7

O

HH

H

H

H

H 1

2

34

5

6

RIGHT HAND RULE

• Acetone in the Absence of a magnetic Field Axis of nuclear spin is random عشوائي الغزل محور

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O

HH

H

H

H

H 1

2

34

5

6

EXTERNALMAGNETIC FIELD/NMR MACHINE

Acetone in the presence of a magnetic Field

• The magnetic moment of The proton will line up with Or against the external magnetic field

• These are known as nuclear spins and each represents an energy level

13-9

EXTERNALMAGNETIC FIELD/NMR MACHINEHo

H nucleus

E

- 1/2

1/2 parallel spin

opposite spin

ground state

excited state

Nuclear spin in the presence of an external Magnetic field

الغزل • مستويات بين الطاقة فرق تقابل طاقتها التي االشعةالراديو اشعة هي النووية

بشعاع • المغناطيسي المجال في الموجود البروتون عرضنا اذاالمستوى في الموجود البروتون ، الطاقة فرق مع يتطابق راديو

المثار المستوى الى وينتقل االشعة سيمتصهذه االرضي

EXTERNALMAGNETIC FIELD/NMR MACHINEHo

H nucleus

- 1/2

1/2

parallel spin

opposite spin

ground state

excited state

radio frequency

- 1/2

1/2

parallel spin

opposite spin

ground state

excited state

400 MHzHo

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NMR Active Nuclei النشيطة أال نوية

The number of protons or neutrons or both should be odd او مفرد ذري عدد على تحتوي ان االنوية هذه في يشترط

نترونات عدد معا كالهما او مفرد

Isotope # of Protons# of neutrons

# of Spin States

1H 1 0 22H 1 1 3

12C 6 6 013C 6 7 214N 7 7 317O 8 9 619F 9 10 231P 15 16 235Cl 17 18 4

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Nuclear Spin

Any atomic nucleus which possesses either odd mass, odd atomic number, or both has spin angular momentum and a magnetic moment

)2

1( H1

1)

2

5( O17

8)

2

1( N15

7

)1( H2

1)1( N14

7)3( B10

5

)0( C12

6 )0( O16

8)0( O34

16

I Atomic Mass Atomic No. Example (I)

Half integer Odd Odd or even

Integer Even Odd

Zero Even Even

13-1212

Bruker AV600 Bruker AV400Bruker DRX500 JEOL 270Bruker DRX300

NMR

Safety Precautions: Very VeryStrong Magnetic Field!

5G10G

13-13

Nuclear Spin States

An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2. This spinning charge has an associated magnetic field. In effect, an electron behaves as if it is a tiny bar

magnet and has what is called a magnetic moment. The same effect holds for certain atomic nuclei.

Any atomic nucleus that has an odd mass number, an odd atomic number, or both, also has a spin and a resulting nuclear magnetic moment.

The allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus.

13-14

Nuclear Spin States

A nucleus with spin quantum number I has 2I + 1 spin states. If I = 1/2, there are two allowed spin states.

Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds.

1H 2H 12C 13C 14N 16O 31P 32S15N 19FElement

Nuclear spinquantum number (I )

Number ofspin states

1/2 1 0 0 01/2 1

2 3 1 2 3 1

1/2

2 1

1/2

2

1/2

2

13-15

Nuclear Spins in Bo

Within a collection of 1H and 13C atoms, nuclear spins are completely random in orientation.

When placed in a strong external magnetic field of strength B0, however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed.

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Nuclear Spins in B0

Figure 13.1 for 1H and 13C, only two orientations are allowed.

13-17

Nuclear Spins in B0

In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for 1H is approximately 0.120 J (0.0286 cal)/mol, which

corresponds to a frequency of 300 MHz (300,000,000 Hz).

13C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to a frequency of 75MHz (75,000,000 Hz).

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Nuclear Spin in B0

Figure 13.2 The energy difference between allowed spin states increases linearly with applied field strength.

Values shown here are for 1H nuclei.

13-19

Nuclear Magnetic Resonance

When nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins align with the applied field in the lower energy state.

The nucleus begins to precess and traces out a cone-shaped surface, in much the same way a spinning top or gyroscope traces out a cone-shaped surface as it precesses in the earth’s gravitational field.

If the precessing nucleus is irradiated with electromagnetic radiation of the same frequency as the rate of precession, the two frequencies couple, energy is absorbed, and the nuclear spin is flipped from spin state +1/2 (with the applied field) to -1/2 (against the applied field).

13-20


Figure 13.3 The origin of nuclear magnetic “resonance”. (a) Precession and (b) after absorption of electromagnetic radiation.

13-21


Resonance: In NMR spectroscopy, resonance is the absorption of energy by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state.

The precessing spins induce an oscillating magnetic field that is recorded as a signal by the instrument. Signal: A recording in an NMR spectrum of a nuclear

magnetic resonance.

13-22


If we were dealing with 1H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H. The same is true of 13C nuclei.

Hydrogens in organic molecules, however, are not isolated from all other atoms. They are surrounded by electrons, which are caused to circulate by the presence of the applied field.

The circulation of electrons around a nucleus in an applied field is called diamagnetic current and the nuclear shielding resulting from it is called diamagnetic shielding.

13-23

NMR Spectrometer

Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio-frequency detector.

The sample is dissolved in a solvent, most commonly CDCl3 or D2O, and placed in a sample tube which is then suspended in the magnetic field and set spinning.

Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds.

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NMR Spectrometer

Figure 13.4 Schematic diagram of a nuclear magnetic resonance spectrometer.

13-25


The difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small.

The difference in resonance frequencies for hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency.

360 Hz300 x 106 Hz

1.2 = 1.2 ppm106=

13-26

Nuclear Magnetic Resonance NMR signals are measured relative to the signal of the reference

compound tetramethylsilane (TMS).

The signal from TMS becomes the reference point and is assigned a shift of zero ppm.

Chemical shift (): The shift in ppm of an NMR signal from the signal of TMS.

For a 1H-NMR spectrum, signals are reported by their shift from the signal of the 12 equivalent H atoms in TMS.

For a 13C-NMR spectrum, signals are reported by their shift from signal of the 4 equivalent carbons in TMS.

Tetramethylsilane (TMS)

CH3Si CH3CH3

CH3

13-27

Figure 13.5 1H-NMR spectrum of methyl acetate.

High frequency: The shift of an NMR signal to the left. Low frequency: The shift of an NMR signal to the right.

Terms used in an NMR spectrum.

Downfield Upfield

Less shielding More shielding

Low induced field High induced field

High frequency - Low frequency-

300 Hzppm = = -------------- = 5 or 5 ppm

60 MHz

13-28

H

O

H

H

H

H

H THE H'S ARE CHEMICALY AND MAGNETICALY NOTEQUIVALENT

على . 6يحتوي انوية كانت لو هيدروجين ذراتومغناطيسيا كيميائيا متكافئة الست الذرات

الخارجي المغناطيسي بالمجال تتاثر انها بمعنىعند الراديوا ستمتصاشعة فانها الدرجة بنفس

تقنية تكون وعندها التردد النووي نفس الرنين. المغناطيسي نجد الحظ لحسن فائدة بدون

البيئة الختالف متكافئة غير االيثانول بروتوناتفيها توجد التي التركيبية

Equivalent Hydrogens

Equivalent hydrogens: Hydrogens that have the same chemical environment.

13-29

How to determine Chemical and magnetic equivalenceالبروتونات وتصنيف والمغناطيسي الكيميائي التكافؤ

H

O

H

H

H

H

H ARE THE H'S CHEMICALY EQUIVALENT OR NOT??

ما جزيئ في هيدروجين ذرتا كانت اذا ما لمعرفةبمجموعة منهما كل نستبدل ال ام متكافئتين

. نتج اذا االستبدال نواتج ونقارن برومين ولتكنوالعكس متكافئتين الذرتان كانت المركب نفس

.صحيح

: متكافئة: المثل مجموعة ذرات انوية هل اوال مثالال؟؟ ام

13-30


H

O

H

H

H

Br

H

ARE THE H'S CHEMICALY EQUIVALENT OR NOT??

H

O

H

H

H

H

Br

Br

O

H

H

H

H

H

2-bromoethanol

2-bromoethanol

2-bromoethanol

SAME COMPOUND


13-31


H

O

H

H

H

Br

H


H

O

H

H

H

H

Br

Br

O

H

H

H

H

H

2-bromoethanol

2-bromoethanol

2-bromoethanol

SAME COMPOUND


مجموعة: ذرات انوية الخالصةمتكافئة المثل

ومغناطيسيا كيميائيا

A molecule with one set of equivalent hydrogens gives one NMR signal because each hydrogen is in the same environment.

13-32


ام: متكافئة المثلين مجموعة ذرات انوية هل ثانياال؟؟

H

O

H

H

H

H

H ARE THE H'S CHEMICALY EQUIVALENT OR NOT??

13-33


ام: متكافئة المثلين مجموعة ذرات انوية هل ثانياال؟؟

H

O

H

H

Br

H

H


H

O

H

Br

H

H

H

SAME COMPOUND

1-bromoethanol

1-bromoethanol مجموعة: ذرات انوية الخالصةمتكافئة المثلين


13-34


مع: متكافئة المثل مجموعة ذرات انوية هل ثالثاال؟؟ ام المثلين

H

O

H

H

H

H

H

ARE THE CH3 AND CH2 CHEMICALY EQUIVALENT OR NOT??

13-35


مع: متكافئة المثل مجموعة ذرات انوية هل ثالثاال؟؟ ام المثلين

H

O

H

H

H

Br

H


H

O

H

Br

H

H

H

DIFFERENT COMPOUND

2-bromoethanol

1-bromoethanol

مجموعة: ذرات انوية الخالصةمتكافئة غير المثلين

المثل مجموعة مع ومغناطيسيا كيميائيا

13-36


OH :ذرة رابعا

مجموعة: ذرات انوية الخالصةمتكافئة غير والكحول والمثل المثلين


H

O

Br

H

H

H

H

ethyl hypobromite

13-37


H

O

H

H

H

H

H

H

O

H

H

H

H

H

H

O

H

H

H

H

H

3 DIFFERENT TYPESOF PROTONS

مجموعة: ذرات انوية الخالصةغير والكحول والمثل المثلين

متكافئة مع ومغناطيسيا كيميائيا

هنالك – اصنافمن 3بعضهااشارات 3البروتونات –

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H3C C

CH3

Br

CH3 CH3CH2-Br

CH3CH2CH2-Br CH3CHCH3

Cl

onetwo

threetwo

CH3

CH2Cl

CH3CHCH2CH3

BrCl-CH2CH2CH2-Cl

four two

three

CCH3H3C

CH3C CH3

CH3

CH3

oneone

one

two

number of signals?

13-39


Equivalent hydrogens: Hydrogens that have the same chemical environment. A molecule with one set of equivalent hydrogens gives

one NMR signal because each hydrogen is in the same environment.

H3C

C C

CH3

H3C CH3

CH3CCH3 ClCH2CH2Cl

Propanone(Acetone)

1,2-Dichloro-ethane

Cyclopentane 2,3-Dimethyl-2-butene

O

13-40


A molecule with two or more sets of equivalent hydrogens gives a different NMR signal for each set. In the following structures, the sets of equivalent hydrogens are distinguished by the labels a, b, and c.

CH3CHCl

Cl ClC C

CH3

H H

O

Cyclopent-anone

(2 signals)

1,1-Dichloro-ethane

(2 signals)

(Z)-1-Chloro-propene

(3 signals)

Cyclohexene (3 signals)

a ca

a

a a

a

ab

b

bb b

b

b

c

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1H NMR Problems

How many unique proton environments are there in:

CH3CH2Br CH3CH2Br

CH3OCH2CH2CHCH3

CH3

CH3OCH2CH2CHCH3

CH3

CH3

C

CC

C

CC

CH3

H

H

H

H

H

CH3

2 environments

5 environments

4 environments

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1H NMR Problems

C CCH3

H

CH3

CH3

C CCH3

H

CH3

CH3

C CCH2CH3

H

CH3CH2

HC C

CH2CH3

H

CH3CH2

H

4 environments

3 environments

Symmetry Simplifies Spectra!!!

13-43

CH3CCH3

O

13-44

CH3COCH3

O

CH3OCH3

13-45

Signal Areas

Relative areas of signals are proportional to the number of H giving rise to each signal. Modern NMR spectrometers electronically integrate and record the relative area of each signal.

Figure 13.7 1H-NMR spectrum of tert-butyl acetate.

13-46

Good Luck

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