[PDF] - 1 YSU YSU YSU YSU 2 E = h i.e. Energy of the radiation is PDF Document

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YSU 400 MHz Nuclear Magnetic Resonance Spectrometer(s)

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

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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

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3 E = h

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

 = c/

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

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E = hc/

i.e. Energy of the radiation is inversely proportional to its wavelength Take home : Longer wavelength, lower energy Higher frequency, higher energy

YSU YSU Nuclear spins of protons (1H nucleus) ‐ Figure 13.3

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upfield downfield

Position of signal is the chemical shift

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downfield upfield

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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

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MHz, 850 MHz, etc.) throughout the world 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

CH3F CH3OCH3 (CH3)3N CH3CH3 4.3 3.2 2.2 0.9

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

CH CH

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1 2 PPM

~0.9 ppm

CH3CH3

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~2.2 ppm YSU YSU

1 2 PPM

H3C O CH3

~3.2 ppm YSU YSU

1 2 3 PPM

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CH3F ~4.3 ppm YSU YSU

H H H H H H H H H H CH3CH3

7 3 5 3 0 9

Fig 13.8 YSU YSU

7.3 5.3 0.9

Pi electrons reinforce external field and signals show downfield

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1 2 PPM

~0.9 ppm “R3C‐H – alkyl” ~5.3 ppm “C=C‐H alkene”

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1 2 3 4 5 PPM

pp

1 2 3 4 5 6 7 PPM

~7.3 ppm “Ar‐H benzene”

H3C CH3 CH3 CH3 N H3C

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1 2 3 4 5 6 7 PPM

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

O CH3

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1 2 PPM

Simple alkane protons – R2CH2 From spectroscopy sheet – chemical shift ~ 0.9‐1.8 ppm

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H3C O CH3

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1 2 3 PPM

Ether protons ‐O‐C‐H From spectroscopy sheet – chemical shift ~ 3.3‐3.7 ppm CH3OCH2OCH3

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1 2 3 4 5 PPM

Two types of ether protons ‐O‐C‐H From spectroscopy sheet – chemical shift ~ 3.3‐3.7 ppm CH2 further downfield (two neighbouring O atoms)

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H O

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2 4 6 8 10 PPM

Aldehyde proton ‐CHO From spectroscopy sheet – chemical shift ~ 9‐10 ppm 3 types of Ar‐H proton – chemical shift ~ 6.5‐8.5 ppm

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2 4 6 8 10 PPM

Carboxylic acid proton ‐CO2H From spectroscopy sheet – chemical shift ~ 10‐13 ppm 3 types of Ar‐H proton – chemical shift ~ 6.5‐8.5 ppm

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14 1 5

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2 4 6 8 10 PPM

Lines on spectra are curves Areas underneath each curve give a reliable ratio of the different numbers of each type of proton CH3CH2OCH3

2 3 3

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1 2 3 PPM

Areas are given as a ratio, not an absolute number

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OCH2CH3

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1 2 3 4 5 6 7 PPM

2

O CH3

H C C H H H Cl Cl

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H Cl

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H C C H H H Br H

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H Br H C C H H H Br C H H H

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H Br H

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17 General rule for splitting patterns

1 i hb i l d bl t

For simple cases, multiplicity for H = n + 1 Where n = number of neighbouring protons

YSU YSU 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

Cl C C H Cl H Br Br

YSU YSU 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. The same applies to the blue H.

Ho

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Red H will be a triplet

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Blue H’s will be a doublet

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

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Gaps between lines (in Hz) will be the same for adjacent protons (here

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~7.4 Hz). This is known as the coupling constant. CH3CH2 but which one? CH3CH2O

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1 2 3 4 5 6 7 PPM

Find J and match signals

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

CH3CH2 CH3CH2 CH CH CH CH2CH2Cl YSU YSU

1 2 3 PPM

CH3CH2CH2

When nonequivalent neighbours have different J values then n+1 does not apply for signal Generally for alkene protons: J trans > J cis

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Figure 13.21

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Acidic protons exchange with any H2O in sample

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N-H

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N-H H-5 H-4 H-3 H-2 N-H

O N H1 H5 H3 H4 H2 O H

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

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Figure 13.23

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Carbon 13 isotope and not 12C is observed in NMR spectroscopy
13C very low abundance (<1%), consequently integration not useful
Spectra usually “decoupled” and signals are observed as singlets

N b f di i i l i di di i f b

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Number of distinct signals indicates distinct types of carbon
Same ideas about shielding/deshielding apply in 13C spectroscopy
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

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O O O CH3 O

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

20 40 60 80 100 120 140 160 180 PPM

O H3C O CH3 CH3 H H

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13C NMR (ppm) 23, 28, 32, 128, 151, 197 20 40 60 80 100 120 140 160 180 200 PPM

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1 2 3 4 5 6 PPM

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20 40 60 80 100 120 140 160 180 200 PPM 20 40 60 80 100 120 140 160 180 200

O

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20 40 60 80 100 120 140 160 180 200 PPM 20 40 60 80 100 120 140 160 180 200 PPM

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26 Not covering 13.17‐13.19

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Information on the types of bonds within molecules

H3C CH3 O H YSU YSU

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Figure 13.25

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

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CH3CH2CH2CH2CH2CH3

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Figure 13.31

H2C=CHCH2CH2CH2CH3 YSU YSU

Figure 13.32

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Figure 13.33

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Figure 13.34

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Figure 13.35

YSU YSU Useful for identifying chromophores in molecules (benzene rings, conjugated alkene systems) ‐ More useful in Biochemistry Figure 13.37 Figure 13.38

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Gives information on molecular mass and structure

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Gives information on molecular mass and structure

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