Chapter 13 NMR Spectroscopy

Chapter 13 NMR Spectroscopy

NMR - Nuclear Magnetic Resonance

NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic compounds.

Invented by physicists (1950’s), then used by chemists (1960’s).

MRI – Magnetic Resonance Imaging (1980’s) A special form of NMR used in medicine.

What is NMR?NMR – a tool to determine the structure of an

organic compound.

magnet

NMRSpectrometer

computer

1H NMRSpectrum

This instrument gives you

information about an organic

compound’s structure.

What is NMR?NMR Instruments

Small, 60 mHz

instrument for

undergraduate

student use.

magnetmagnet

computercomputer

studentstudent


Research grade

instrument,

300 mHz magnet,

that we use at

Western.

magnetmagnet


Research grade

instrument,

300 mHz magnet,

that we use at

Western.

computercomputer

What is NMR?

NMR Instruments

State-of-the-art

instrument,

950 mHz magnet.

Rather large and

expensive!

magnetmagnet

researcherresearcher

Why is it called NMR?

Nuclear Magnetic ResonanceNuclear – because it looks at the nucleus of an atom,

most commonly a hydrogen atom. A hydrogen atom nucleus consists of one proton with a

+1 charge and “spin” of ½. It acts like a tiny bar magnet.

protonspinning proton generatesmagnetic

field

bar magnet

No External Magnetic FieldNuclear spins are pointedin random directions

NMR – Effect of Magnetic FieldSample in Magnetic Field

Spins align with or against the external magnetic field

No external magnetic field applied to sample

Random orientation of nuclear spins

Sample placed in an external magnetic field H0

NMR – Effect of Magnetic Field

aligned with field(lower energy)

alignedagainst field

(higher energy)

hydrogennuclei

Spins align with or against field (most align with field)

NMR: Absorption of Energy

Initial State – nucleus at low energy level

Scan with RF field – nucleus absorbs energy, giving a signal in the NMR spectrum

radio waves

nucleus absorbs energy

NMR: Information Obtained from a Spectrum

An NMR Spectrum will generally provide three types of information:

Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded)

Integration – gives the relative number of nuclei producing a given signal

Spin-Spin Coupling – describes the connectivity

1H NMR Spectrum – H2O

A sample of water is placed in an NMR instrument, and a proton spectrum is recorded (scanned from left to right).

signal fromprotons in H20

An NMR signal appears. This proves thatwater contains hydrogen atoms!

scanning

When does nucleus absorb energy?

Absorption depends on shielding by electron cloud around the nucleus. More electron density = more shielding = signal shifted to the right.

Magnetic Fields: 1. from spinning proton 2. from magnet 3. from electrons

2, External Field (Ho)from magnet

Not allprotons arethe same!

3.

NMR: Simple 1H NMR Spectrum Showing Chemical Shift

Two types of protons (a CH2 and a CH3) give two separate signals at two different chemical shifts.

Chemical Shift:location of the signal

on the spectrum.

Right Side:high electron

densityLeft Side:

low electrondensity

NMR: Chemical Shift Practice

Assign the four groups shown to the four NMR singals, based on each element’s electronegativity.

Group

-O-CH3

-Si-CH3

-C-CH3

Cl3C-H

-SiCH3

3.5

1.8

2.5

3.0

EN

Left Side:low electron

density(high EN)

-CCH3 -OCH3 Cl3C-H

3 electronegative atoms

NMR: Chemical Shift Reference

Chemical shift measured in ppm. For 1H: roughly 0 to 10 ppm.

Chemical shift zero is set to TMS (tetramethylsilane).

TMS =

(silicon – low electroneg.)

Si CH3

CH3

CH3

CH3

NMR: Chemical Shift Regions

Alkane region (high electron density) is from about .8 – 2.5 ppm.

-CH2-CH3


Heteroatom region (low electron density) is from about 2.5 to 5.

-O-CH3


Double bond region is on the left, from about 5 – 10 ppm.

C=C H H

NMR: Chemical Equivalence and Number of Signals

How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry)

OMe

Br

O

Cl

Cl

NH2Cl

H

Cl

Cl

H

H

HNH2

Cl

HH

2 4 5

2 4 7

NMR: Chemical Equivalence and Number of Signals

How many signals should appear in the proton NMR spectrum for these compounds?

In theory: 9 4

O

octane

Signals actually resolved: 3-4 2

1

2

3

4

NMR: Overlapping Proton Signals

Protons b, c, and d are in roughly the same environment, and their chemical shifts are also about the same.

The -CH2- groups allappear in the same spot

(not resolved)

octane

Review: How Many NMR Signals?

How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry)

ClH

H HHH

H

H CH3

H

H

H

H H

CH3CH2Cl

CC

1 2 5

Fast chairflips at RT

Fast rotation aboutC-C single bond

No rotation aboutdouble bonds

These H’s are different

NMR: Chloroethane

CC

ClH

H

HH

H

Fast rotation around single bonds gives an “averaged” spectrum for the three methyl hydrogens.

An NMR spectrometer is like a camera with a slow shutter speed.

NMR: Chair Cyclohexane

Rapid chair flipping makes all H’s equivalent. Cylcohexane gives one peak in the 1H NMR spectrum.

An NMR spectrometer is like a camera with a slow shutter speed.

NMR: A Second Proton Spectrum

Note: the signal for the nine methyl H’s (red) is larger than the signal for the CH2 group (blue)

bigger (9 H’s)

smaller (2 H’s)




Integration – gives the relative number of nuclei that produces a given signal.

The integral (area under the curve) is drawnon the spectrum by the instrument.

Spin-Spin Coupling – describes the connectivity

NMR: Integration Indicates Relative Number of Nuclei

The height of the integration line (“integral”) gives you the relative number of nuclei producing each signal.

Integral has relative height 9

Relative height 2




Integration – gives the relative number of nuclei producing a given signal

Spin-Spin Coupling: - describes the carbon connectivity - follows the “n+1”rule”

NMR: Splitting into a Doublet

Note that the red signal at 1.6 ppm for the methyl group is split into two peaks.

Remember that this is one signal, composed of two separate peaks.

doublet

NMR: Signal Splitting, n+1 Rule

• A signal is often split into multiple peaks due to interactions with protons on carbons next door. Called spin-spin splitting

• The splitting is into one more peak than the number of H’s on adjacent carbons (“n+1 rule”)

• Splitting of a signal can give doublets (two peaks), triplets (three peaks), quartets (4 peaks), ect.

• The relative intensities given by Pascal’s Triangle:doublet 1 : 1triplet 1 : 2 : 1quartet 1 : 3 : 3 : 1pentet: 1 : 4 : 6 : 4 : 1


n+1 Rule: A signal in the proton NMR spectrum will be split into n+1 peaks, where n is the number of protons on adjacent carbons.

For the Methyl Group: There are two protons ‘next door’ (n=2), so the methyl signal will be split into three peaks (2+1), which is called a triplet. Chemical shift will be about 1.5 (alkane region), integration = 3.

For the -CH2- Group: Three protons next door means the CH2 signal will be split into 4 (3+1) peaks, called a quartet. Chemical shift = 3.3 (heteroatom region), integration = 2.

Example: CH3-CH2-Br

1H NMR Spectrum for Bromoethane

Four peaks, a quartet (1:3:3:1)

Three peaks, a triplet (1:2:1)

integration:2 H 3 H

Note the expansionsprinted above the spectrum


Peak Heights - Pascal’s Triangle singlet 1doublet 1 : 1triplet 1 : 2 : 1quartet 1 : 3 : 3 : 1pentet 1 : 4 : 6 : 4 : 1


many lines = “mulitplet”


seven peaksHow many neighbors?

H

n + 1 = 7

n = 6

NMR: Origin of Spin-Spin Splitting

NMR: Doublets and Triplets

Doublet: the one proton next doorcan be either up or down (α or β)

Triplet: for the two protons next door,there are four combinations possible: α α α β β β β α

NMR: Using the n+1 Rule

Using the n+1 rule, predict the 1H NMR spectrum of 2-iodopropane.Give splitting pattern, integration, and approximate chemical shift.

IHI

CH3H3CC

six neighbors

one neighborNote that the methyl groups are equivalent, so they will give one signal in the NMR spectrum.

NMR: Spectrum of 2-iodopropane

Seven line pattern

doublet

NMR: Rules for Spin-Spin Splitting• The signal of a proton with n equivalent neighboring H’s

is split into n + 1 peaks

• Protons that are farther than two carbon atoms apart do not split each other

• Equivalent protons do not split each other

Common 1H NMR Patterns 1. triplet (3H) + quartet (2H) -CH2CH3

2. doublet (1H) + doublet (1H) -CH-CH-

3. large singlet (9H) t-butyl group

4. singlet 3.5 ppm (3H) -OCH3 group

5. large double (6H) + muliplet (1H) isopropyl

6. singlet 2.1 ppm (3H) methyl ketone

O

CH3

Common 1H NMR Patterns

7. multiplet ~7.2 ppm (5H) aromatic ring, monosubstituted

8. multiplet ~7.2 ppm (4H) aromatic ring, disubstituted

9. broad singlet, variable -OH or –NH chemical shift (H on heteratom)

Solving NMR Problems 1. Check the molecular formula and degree of unsaturation. How many rings/double bonds?

2. Make sure that the integration adds up to the total number of H’s in the formula.

3. Are there signals in the double bond region?

4. Check each signal and write down a possible sub-structure for each one.

5. Try to put the sub-structures together to find the structure of the compound.

Proton NMR Spectrum: C9H12

Degree of Unsat = 4aromatic,disubst.

H

HH

H

CH3

CH3-CH2-

CH3-CH2-

1H NMR Spectrum: C4H7O2Br

Br O

Ot2H

t2H

s3H

O CH3

5.0 4.0 3.0 2.0 1.0 0

H H

HH

CC

O

CBr

Degree of Unsat = 1

Electronegative Substituents: Shift Left

HH33C—CHC—CH22—CH—CH33 OO22N—CHN—CH22—CH—CH22—CH—CH33

0.90.9 0.90.9 1.31.3 1.01.0 4.3 2.02.0

– CHCH33ClCl 3.1 3.1 (one Cl) (one Cl)

– CHCH22ClCl22 5.3 5.3 (two (two

Cl’s)Cl’s)

– CHClCHCl33 7.3 7.3 (three Cl’s) (three Cl’s)

Effect is cumulativeEffect is cumulative

Propane:Propane: heteroatomregion

smalleffect

~noeffect

Hydrogens on Heteroatoms

Type of protonType of protonChemical shift (ppm)Chemical shift (ppm)

1-31-3 HH NRNR

0.5-50.5-5 HH OROR

6-86-8 HH OArOAr

10-1310-13 CC

OO

HOHO

Chemical shifts for protons on heteroatoms are variable, Chemical shifts for protons on heteroatoms are variable, and signals are often and signals are often broadbroad (not generally useful). (not generally useful).

may beuseful

farleft

13C NMR Spectroscopy

• Carbon-13: only carbon isotope with a nuclear spin

natural abundance of 13C is only 1.1%

(99% of carbon atoms are 12C, with no NMR signal)

• All signals are obtained simultaneously using a broad pulse of energy. The resulting “mass signal” changed into an NMR spectrum mathematically using the operation of Fourier transform (FT-NMR)

• Frequent repeated pulses give many data sets that are averaged to eliminate noise

13C signals go from 0 to 240 ppm. 13C signals: always sharp singlets.

(wider range than in 1H NMR) (1H signals: broad multiplets)

These two facts mean that in carbon-13 NMR, each separate signal is usually visible, and you can accurately count the number of different carbons in the molecule.

Chemical shift affected by electronegativity of nearby atoms:

alkane-like range: 0 – 40 ppm (R-CH2-R)

heteroatom range: 50 – 100 ppm (O-CH2-R)

double bond range: 100 – 220 ppm (sp2 carbons)

No signal overlap!No signal overlap!

13C NMR Spectroscopy

NMR: Scanning for All Nuclei

An instrument can only examine one area at a time.

To see both proton and C-13 nuclei, a very wide region would have to be scanned.

1H area is small

13C area is much wider

Why does 13C NMR give singlets?13C is only 1.1% natural abundant, so most carbons are 12C,

and give no NMR signal.

No splitting seen with carbon, because carbons next to the 13C are likely to be carbon-12:

Sample of 1-Propanol:

12CH3-12CH2-12CH2-OH 12CH3-12CH2-12CH2-OH




NMR: Number of Signals for 13C NMR

How many signals should appear in the carbon-13 NMR spectrum for these compounds?

In theory: 10 4

Signals actually resolved: 10 4

O

octane

13C NMR ExampleNote the wide spectral width and the sharp singlets in the

spectrum below.

Also note that there is no integration with 13C NMR.

13C NMR: smaller signal to noise ratio

Noise

13C NMR: smaller signal to noise ratio

Noise

morescans(noise

smaller)

Signal

13C NMR Spectrum: C5H11Cl

Cl

Cl

Cl

5 signals

5 signals

3 signals

D. of Unsat = 0

five 13Csignals

O

C

13C NMR Spectrum: C4H7O2Br

200 150 100 50 0

CDCl3

double bond region

D. of Unsat = 1

Chapter 13 NMR Spectroscopy

Documents

Transcript of Chapter 13 NMR Spectroscopy