MC 13.1 Spectroscopy, Pt I 1 Spectrocopy Nuclear Magnetic Resonance (NMR)spectroscopy Infrared (IR) Spectroscopy Ultraviolet-Visible (UV-VIS) Spectroscopy

MC 13.1 Spectroscopy, Pt I1

Spectrocopy

Nuclear Magnetic Resonance (NMR)spectroscopy

Infrared (IR) Spectroscopy

Ultraviolet-Visible (UV-VIS) Spectroscopy

Mass Spectrometry (MS)

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Principles of Molecular Spectroscopy:Electromagnetic Radiation

Electromagnetic Radiation travels at the speed of light

Has both particle and wave properties

A single packet of energy is a photon

The energy of a photon is proportional to its frequency

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The Visible Electromagnetic Spectrum

400 nm 750 nm

Visible Light

Longer Wavelength ()Shorter Wavelength ()

Higher Frequency () Lower Frequency ()

Higher Energy (E) Lower Energy (E)

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The Electromagnetic Spectrum (cont)

Ultraviolet Infrared

Longer Wavelength ()Shorter Wavelength ()

Higher Frequency () Lower Frequency ()

Higher Energy (E) Lower Energy (E)

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Cosmic rays

Rays

X-rays

Ultraviolet light

Visible light

Infrared radiation

Microwaves

Radio waves

Energy

The Electromagnetic Spectrum (cont)

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Quantized Energy States

Electromagnetic radiation is absorbed when theenergy of the photon corresponds to difference in energy between two states

E = h

State #1

State #2

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Quantized Energy States (cont)

What Kind of States are of Interest?

Electronic

Nuclear spin

Vibrational

Rotational

UV-Vis

Radiofrequency

Infrared

Microwave

State Spectral Region Type of Spectroscopy

Ultraviolet-Visible

Nuclear Magnetic Resonance

Infrared

Raman

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Introduction to 1H NMR Spectroscopy

The nuclei that are most useful to organic chemists are:

1H and 13C

Both have spin = + or - 1/2

1H is 99% natural abundance

13C is 1.1% natural abundance

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MC 13.1 Spectroscopy, Pt I9continue…..

Introduction to 1H NMR Spectroscopy (cont)

Nuclear Spin:

A spinning charge, such as the nucleus of 1H or 13C, generates a magnetic field

The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2 + +

10

The distribution of nuclear spins is random in the absence of an external magnetic field

+

++

+

+

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An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field

++

+

+

+

H0

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There is a slight excess of nuclear magnetic moments aligned parallel to the applied field

++

+

+

+

H0

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Energy Differences Between Nuclear Spin States:

There is no difference in the absence of a magnetic field

The difference is proportional to the strength of the external magnetic field

+

+

E E '

Increasing Field Strength

No Magnetic

Field

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Some important relationships in NMR:

The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1H) depends on its molecular environment

This is why NMR is such a useful tool for structure determination

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Nuclear Shielding and 1H Chemical Shifts:

1) What do we mean by "shielding?“

2) What do we mean by "chemical shift?"

The induced field shields the nuclei (in this case, C and H) from the applied field

A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation

C H

H 0

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Chemical Shift:

What do we mean by "shielding?“

What do we mean by "chemical shift?"

Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded

Protons in different environments are shielded to greater or lesser degrees;

They have different chemical shifts

C H

H 0

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Chemical Shift (cont):

Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard

Si CH3

CH3

CH3

H3C

=Position of Signal - Position of TMS Signal

Spectrometer Frequencyx 106

Calculating Chemical Shift Numbers:

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The Proton NMR Chemical Shift Scale:

01.02.03.04.05.06.07.08.09.010.0

Chemical shift (, ppm) - measured relative to TMS

DownfieldDecreased shielding

UpfieldIncreased shielding

(CH3)4Si (TMS)

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01.02.03.04.05.06.07.08.09.010.0

Chemical shift (, ppm)

(CH3)4Si (TMS)


Effects of Molecular Structure on 1H Chemical Shifts:

Protons in different environments experience different degrees of shielding and have different chemical shifts

7.28 ppm

H C

Cl

Cl

Cl

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Effects of Molecular Structure on 1H Chemical Shifts (cont):

Protons in different environments experience different degrees of shielding and have different chemical shifts

Electronegative substituents decrease the shielding of hydrogen atoms on methyl groups

Decreased Shielding Increased shielding CH3F CH3OCH3 (CH3)3N CH3CH3 (CH3)4Si

4.3 3.2 2.2 0.9 0.0

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Electronegative substituents decrease shielding

H3C — CH2 — CH3

0.9 0.9 1.3

O2N — CH2 — CH2 — CH3

1.0 4.3 2.0

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Electronegative substituents decrease shielding

The electronegativity effect is cumulative

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CHCl3 7.3

CH2Cl2 5.3

CH3Cl 3.1


Methyl ( CH3 ), Methylene ( CH2 ), and Methine ( CH ), :



CH3 more shielded than CH2

CH2 more shielded than CH

H3C C

CH3

CH3

H

0.9

1.6

0.8

0.9

H3C C

CH3

CH3

CH2 CH3

1.2

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01.02.03.04.05.06.07.08.09.010.0

Chemical shift (, ppm)

(TMS)

Example:

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Interpreting Proton NMR Spectra (cont)

4 lines;quartet

2 lines;doublet

Cl2CH CH3

Cl2CH CH3 Cl2CH CH3

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Interpreting Proton NMR Spectra (cont)

CH3CH2X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet)

01.02.03.04.05.06.07.08.09.010.0Chemical shift (, ppm)

(TMS)

3 lines;tripletCH3

BrCH2 CH3

Br CH2 CH3

4 lines;quartetCH2

Splitting Patterns: The Ethyl Group BrCH2CH3

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Documents

MC 13.1 Spectroscopy, Pt I 1 Spectrocopy Nuclear Magnetic Resonance (NMR)spectroscopy Infrared (IR) Spectroscopy Ultraviolet-Visible (UV-VIS) Spectroscopy