Chapter 13 - Spectroscopy

Chapter 13 - SpectroscopyChapter 13 - Spectroscopy

YSU 400 MHz Nuclear Magnetic Resonance Spectrometer(s)

Techniques used to find structures of organic moleculesTechniques used to find structures of organic molecules

NMR spectroscopy: Based on the response of magnetic nuclei to an external magnetic field and an energy source (Radio frequency)

IR spectroscopy: Response of bonds within organic molecules to externally applied Infra Red light

UV/Vis spectroscopy: Response of electrons within bonds to externally applied UV or Visible light

Mass spectrometry: Response of molecules to being bombarded with high energy particles such as electrons

13.1 The Electromagnetic spectrum13.1 The Electromagnetic spectrum Figure 13.1

13.2 Two (quantized) energy states 13.2 Two (quantized) energy states Figure 13.2

13.2 Physics Concepts13.2 Physics Concepts

E = h i.e. Energy of the radiation is directly proportional to its frequency ( = Planck’s constant)

= c/i.e. Frequency of the radiation is inversely proportional to its wavelength (c = speed of light)

E = hc/ i.e. Energy of the radiation is inversely proportional to its wavelength

Take home : Longer wavelength, lower energy

Higher frequency, higher energy

Nuclear spins of protons (1H nucleus) Figure 13.3

13.3 Introduction to 13.3 Introduction to 11H NMR – Nuclear SpinH NMR – Nuclear Spin

Energy difference between states increases with field strength Energy difference between states increases with field strength (Fig. (Fig. 13.4)13.4)

Schematic diagram of a Schematic diagram of a nnuclear uclear mmagnetic agnetic rresonance spectrometeresonance spectrometer

Basic operation of a Fourier Transform (FT) NMR Instrument (Basic operation of a Fourier Transform (FT) NMR Instrument (Fig. 13.5)Fig. 13.5)

downfield upfield

upfielddownfield

13.4 NMR Spectrum Characteristics – 13.4 NMR Spectrum Characteristics – Chemical ShiftChemical Shift

Position of signal is the chemical shift

Chemical shift () = position of signal – position of TMS peak x 106

spectrometer frequency

Enables us to use same scale for different size spectrometers (60 MHz, 400 MHz, 850 MHz, etc.)

TMS = (CH3)4Si, signal appears at 0 Hz on spectrum, therefore used as reference

Chemical shifts are reported as ppm (parts per million) relative to TMS and usually occur in the 0-12 ppm range for 1H spectra

13.4 NMR Spectrum Characteristics – 13.4 NMR Spectrum Characteristics – Chemical ShiftChemical Shift

13.5 Effect of molecular structure on 13.5 Effect of molecular structure on 11H Chemical ShiftH Chemical Shift

CH3F CH3OCH3 (CH3)3N CH3CH3

4.3 3.2 2.2 0.9

i.e. electronegativity of other atoms plays a role in shift

CH3CH3

012PPM

~0.9 ppm

N

CH3

H3C CH3

012PPM

~2.2 ppm

H3CO

CH3

0123PPM

~3.2 ppm

CH3F

~4.3 ppm

H

H

H

H

H

HH H

HHCH3CH3

7.3 5.3 0.9Pi electrons reinforce external field and signals show downfield

13.5 Effect of structure on 13.5 Effect of structure on 11H Chemical ShiftH Chemical Shift

CH3CH3

012PPM

~0.9 ppm “R3C-H – alkyl”

012345PPM

H

H H

H ~5.3 ppm “C=C-H alkene”

01234567PPM

H

H

H

H

H

H

~7.3 ppm “Ar-H benzene”

01234567PPM

H3CCH3

CH3

O

CH3NH3C

CH3

13.5 Effect of structure on 13.5 Effect of structure on 11H Chemical ShiftH Chemical Shift

Spectra typically have multiple signals the number depending on the number of unique types of protons

012PPM

13.5 Typical 13.5 Typical 11H NMR SpectraH NMR Spectra

Simple alkane protons – R2CH2

From spectroscopy sheet – chemical shift ~ 0.9-1.8 ppm

0123PPM

H3CO

CH3

Ether protons -O-C-H

From spectroscopy sheet – chemical shift ~ 3.3-3.7 ppm



012345PPM

H3CO

CH2

OCH3

Two types of ether protons -O-C-HFrom spectroscopy sheet – chemical shift ~ 3.3-3.7 ppm

CH2 further downfield (two neighbouring O atoms)

0246810PPM


Aldehyde proton -CHOFrom spectroscopy sheet – chemical shift ~ 9-10 ppm3 types of Ar-H proton – chemical shift ~ 6.5-8.5 ppm

H

O

0246810PPM


OH

O

Carboxylic acid proton -CO2HFrom spectroscopy sheet – chemical shift ~ 10-13 ppm3 types of Ar-H proton – chemical shift ~ 6.5-8.5 ppm

13.6 Integration – Ratio of different types of H13.6 Integration – Ratio of different types of H

0246810PPM

15 OH

O

Lines on spectra are curves

Areas underneath each curve give a reliable ratio of the different numbers of each type of proton

0123PPM

CH3CH2OCH3

23

3


Areas are given as a ratio, not an absolute number

01234567PPM

2 22

3

3OCH2CH3

O CH3


13.7 Spin-Spin Splitting – Effect of neighbouring H on shape13.7 Spin-Spin Splitting – Effect of neighbouring H on shape

H C C

H

H

H

Cl

Cl

H C C

H

H

H

Br

H


H C C

H

H

H

Br

C

H

H

H



General rule for splitting patterns

For simple cases, multiplicity for H = n + 1

Where n = number of neighbouring protons

i.e 1 neighbour, signal appears as a doublet

2 neighbours, signal appears as a triplet

3 neighbours, signal appears as a quartet

4 neigbours, signal appears as a quintet, etc.

Complex splitting patterns are referred to as multiplets

13.7-13.10 Basis of Splitting Patterns13.7-13.10 Basis of Splitting Patterns

Cl C C

H

Cl

H

Br

Br

Ho

For red H : neighbouring H (blue) has two possible alignments, either with, or against, the external field (Ho). This effects the local magnetic environment around the red H and thus there are two slightly different frequencies (and thus chemical shifts) at which the red H resonates. Same applies to the blue H.

Cl C C

H

Cl

H

Br

H

Ho

Red H will be a triplet

Cl C C

H

Cl

H

Br

H

Ho

Blue H’s will be a doublet


Cl C C

H

Cl

H

H

H

Ho

Red H will be split into a quartet, blue H’s will be split into a doublet


Gaps between lines (in Hz) will be the same for adjacent protons (here ~7.4 Hz). This is the coupling constant.

13.7-13.10 Basis of Splitting Patterns - 13.7-13.10 Basis of Splitting Patterns - Coupling Constants

01234567PPM

O O

HH

HH

H

H H

H H

H

H

HH

H

CH3CH2 but which one?

CH3CH2O

Find J and match signals

Using Coupling Constants

If nonequivalent neighbours have same J value then n+1 applies for signal

0123PPM

Cl

H H

HH

H HH

HH

CH3CH2

CH3CH2

CH3CH2CH2

CH2CH2Cl

Coupling Constants – Nonequivalent Neighbours

13.11 Complex Splitting Patterns13.11 Complex Splitting Patterns

When nonequivalent neighbours have different J values then n+1 does not apply for signal

Generally for alkene protons:J trans > J cis

Figure 13.20

13.11 Complex Splitting Patterns13.11 Complex Splitting Patterns

O

H

AcOAcO

AcO

N3

OAc

H

H

HH H

H

13.12 13.12 11H NMR Spectra of AlcoholsH NMR Spectra of Alcohols

Acidic protons exchange with any H2O in sample

Figure 13.21

Glycosyl amide structure from NMR - NOESYGlycosyl amide structure from NMR - NOESY

N-H

N-H

ON

HH

H

H H

O

H

YSUYSU

Glycosyl amide structure from 2-D NMR - COSYGlycosyl amide structure from 2-D NMR - COSY

YSUYSU

H-5H-4H-3

H-2

N-H

H1, H2, H3, and H4 hard to distinguish just from coupling constants (all t, J~9 Hz)

David TemelkoffDavid Temelkoff

ON

H1H5

H3

H4 H2

O

H

13.14 13.14 1313C NMR Spectroscopy C NMR Spectroscopy

Figure 13.23

• Carbon 13 isotope and not 12C is observed in NMR

• 13C very low abundance (<1%), integration not useful

• Spectra usually “decoupled” and signals are singlets

• Number of distinct signals indicates distinct carbons

• Same ideas about shielding/deshielding apply

• Spectra often measured in CDCl3 and referenced to

either the C in TMS (0 ppm) or the C in CDCl3, which shows

as a triplet at 77.0 ppm

13.14 13.14 1313C NMR SpectroscopyC NMR Spectroscopy

13C NMR (ppm) 21, 52, 121, 122, 120, 126, 132, 134, 148, 168, 169

020406080100120140160180PPM

O

O

O

CH3

O

13.15 13.15 1313C NMR Chemical Shifts (see Sheet)C NMR Chemical Shifts (see Sheet)

13C NMR (ppm) 23, 28, 32, 128, 151, 197

020406080100120140160180200PPM

H3C

O

CH3

CH3

H

H


020406080100120140160180200

PPM

0123456PPM

H3C

O

CH3

CH3

H

H


020406080100120140160180200PPM

O

13.15 13.15 1313C NMR – Information on SymmetryC NMR – Information on Symmetry

020406080100120140160180200PPM

O

Not covering 13.17-13.19

Information on the types of bonds within moleculesH3C CH3

OH

13.20 Infrared Spectroscopy13.20 Infrared Spectroscopy

13.20 Infrared Spectrometer13.20 Infrared Spectrometer

13.20 Stretching and bending vibrations of a methylene unit13.20 Stretching and bending vibrations of a methylene unit

Figure 13.25

13.20 Stretching and bending vibrations from Spec Sheet13.20 Stretching and bending vibrations from Spec Sheet

Don’t memorize, learn to use as you practice problems

13.20 Interpreting IR Spectra – 13.20 Interpreting IR Spectra – nn-Hexane-Hexane

Figure 13.31

CH3CH2CH2CH2CH2CH3

13.20 Interpreting IR Spectra – 1-Hexene13.20 Interpreting IR Spectra – 1-Hexene

H2C=CHCH2CH2CH2CH3

Figure 13.32

13.20 Interpreting IR Spectra – 13.20 Interpreting IR Spectra – tt-Butylbenzene-Butylbenzene

Figure 13.33

C

CH3

CH3

CH3

13.20 Interpreting IR Spectra – 2-Hexanol13.20 Interpreting IR Spectra – 2-Hexanol

Figure 13.34

CH3CHCH2CH2CH2CH3

OH

13.20 Interpreting IR Spectra – 2-Hexanone13.20 Interpreting IR Spectra – 2-Hexanone

Figure 13.35

CH3CCH2CH2CH2CH3

O

13.21 Ultraviolet-Visible (UV-Vis) Spectroscopy13.21 Ultraviolet-Visible (UV-Vis) Spectroscopy

Figure 13.37 Figure 13.38

Useful for identifying chromophores in molecules (benzene rings, conjugated alkene systems) - More useful in Biochemistry

13.22 Mass Spectrometry13.22 Mass Spectrometry

Gives information on molecular mass and structure

13.22 Mass Spectrometry13.22 Mass Spectrometry

Cl

Exam ProblemsExam Problems

Chapter 13 - Spectroscopy

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