NMR Assignment 

Instructions: For each unknown compound in this assignment you are given the IR and NMR spectra for the compound on the pages that follow. Propose a structure for each of these unknown compounds. You will need to draw the structures of the compounds using a chemistry drawing program such as ChemDoodle or ChemDraw and paste them onto this first page where indicated. Alternatively, you can draw the structures by hand, scan this page, and upload as a pdf file to Canvas (It is not necessary to include the pages containing the spectra).

Unknown Compound 1 Structure:

Unknown Compound 2 Structure:

Unknown Compound 3 Structure:

Unknown Compound 4 Structure:

Unknown Compound 5 Structure:

Unknown Compound 1

Formula: C₅H₁₀O

Unknown Compound 2

Formula: C₄H₁₀O

Unknown Compound 3

Formula: C₄H₈O₂

Unknown Compound 4

C₁₀H₁₄O

Unknown Compound 5

C₈H₁₄

Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance 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.

The Basis of NMR

NMR is based on the absorption of radiowaves by certain atomic nuclei when the molecule is in a strong magnetic field

The nuclei of certain elements act as though they are magnets spinning about an axis

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.

Spinning proton resembles a tiny magnet

Nuclear Spins in B₀

• 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. The result is that only certain orientations of nuclear magnetic moments are allowed.

Nuclear Spins in B₀

–

for

1

H and

13

C, only two orientations are allowed.

Absorption of energy causes nuclear “spin flip”

b spin state a spin stateefixetledrBna0l

D

E

An NMR spectrometer supplies this energy in

The form of radio frequency (RF) radiation

The NMR Event- What Causes “Spin Flip”?

The combination of radio frequency and appled magnetic field causes the nuclei to “come into resonance”

Nuclear Spin in B₀

• The energy difference between allowed spin states increases linearly with applied field strength.
• Values shown here are for ¹H nuclei.

FT NMR Spectrometer

NMR Spectrometer

• Schematic diagram of a nuclear magnetic resonance spectrometer.

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 the nuclei align themselves in the spin up or spin down orientation.
• The sample is then subjected to RF energy and all of the nuclei spin flip at once causing a complex signal that is then mathematically manipulated by “Fourier transforms (FT)” before it can be displayed as a peak on an NMR spectrum.
• Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds.

NMR Spectrum

• ¹H-NMR spectrum of methyl acetate.

–

High frequency:

The shift of an NMR signal to the left on the chart

paper.

–

Low frequency:

The shift of an NMR signal to the right on the

chart paper.

NMR Information

¹H NMR spectrum contains 4 important pieces of information:

1. Number of signals- how many different sets of non-equivalent protons are in the molecule
2. Chemical shift- position of signal relative to TMS standard (zero ppm). Chemical shift determined by magnetic environment surrounding the proton.
3. Integration- the relative ratio of non-equivalent protons in each signal.
4. Signal splitting- number of peaks in a signal is determined by the n + 1 rule.

Equivalent Hydrogens

• Equivalent hydrogens: Hydrogens that have the same chemical environment.

– A molecule with 1 set of equivalent hydrogens gives 1 NMR signal.

OH₃CC H3

C H3CCH3 ClCH2C H2C l

C

C

H₃CC H3

Propanone 1,2-Dichloro- Cyclopentane 2,3-Dimethyl-

(Acetone) ethane 2-butene

Equivalent Hydrogens

– A molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each

set.

Cl ClCH₃

Chemical Shift

– Signals are measured relative to the signal of the reference

compound tetramethylsilane (TMS).

CH3 CH₃Si CH3 CH3

Tetramethylsilane (TMS)

• For a ¹H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS.
• For a ¹³C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS.
• Chemical shift (d): The shift in ppm of an NMR signal from the signal of TMS.

Chemical Shift

H

a

H

e

H

a

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d

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d

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.

H

e

i

s

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d

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d

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d

.

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d

d

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w

n

f

i

e

l

d

C

H

3

O

C

H

2

O

C

H

2

H

b

C

H

2

C

H

3

H

a

H

c

H

d

H

e

p

p

m

Chemical Shift 6 regions of NMR spectrum

Table of Chemical Shifts

O

C

C

C

H

C

C

H

C

H

O

HHHH

C

C

C XC C H

OH

H RH allylic or saturated

H ^O vinyl Xor =h aOlo, gNe,n baceentzyylelincic adjacent to carbonyl

13 – 9.0 8.5 – 6.5 6.5 – 4.5 5.0 – 2.5 3.0 – 2.5 2.5 – 2.0 1.5 – 0

H 13 – 10 H 6.0 – 5.0 F >Cl >Br >I 3^o >2^o >1^o

H 10.5 – 9.5 H 5.5 – 4.5 O 4.0 – 3.0

N 3.0 – 2.0

(ppm)

Chemical Shift

• Chemical shift depends on the (1) electronegativity of nearby atoms, (2) hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds.
• Electronegativity Electroneg- Chemical

CH3-X ativity of X Shift (δ)

CH3F 4.0 4.26 CH₃OH 3.5 3.47

CH₃Cl 3.1 3.05

CH₃Br 2.8 2.68 CH3I 2.5 2.16

(CH3)4C 2.1 0.86

(CH3)4Si 1.8 0.00

Chemical Shift due to electron withdrawing effect

3

.

2

p

p

m

3

.

5

p

p

m

3

.

7

p

p

m

4

.

4

p

p

m

C

H

3

C

H

2

–

N

O

2

C

H

3

C

H

2

–

C

l

C

H

3

C

H

2

–

I

C

H

3

C

H

2

–

O

H

triplet and quartet = ethyl pattern

Integration of Peaks

The red curves represent the peak areas ( integration)

C

H

3

O

C

H

2

O

C

H

2

H

b

C

H

2

C

H

3

H

a

H

c

H

d

H

e

H

a

H

b

H

e

m

e

t

h

y

l

g

r

o

u

p

s

i

n

t

e

g

r

a

t

e

f

o

r

3

p

r

o

t

o

n

s

C

H

2

g

r

o

u

p

s

i

n

t

e

g

r

a

t

e

f

o

r

2

p

r

o

t

o

n

s

Splitting Patterns: n+1 Rule

• A signal is split into multiple peaks by adjacent protons.
• Splitting occurs only between nonequivalent protons.
• The signal is split into n+1 peaks, where n = the number of adjacent protons

Peak Splitting

The (n+1) rule indicates the number of protons attached to

_O neighboring carbon atoms

C

H

3

O

C

H

2

C

H

2

H

b

C

H

2

C

H

3

H

a

H

c

H

d

H

e

H

a

H

b

H

e

A triplet indicates there are 2

protons on adjacent carbon

atom

A singlet indicates there

are no protons on adjacent

carbon atom

S

h

i

f

t

S

i

g

n

a

l

S

p

l

i

t

t

i

n

g

3

I

n

t

e

g

r

a

t

i

o

n

Splitting Pattern for 1,1,2-tribromoethane

1,1,2-tribromoethane

B

r

C

C

B

r

B

r

H

a

H

b

H

b

H

a

H

b

H

b

i

s

s

p

l

i

t

b

y

a

d

j

a

c

e

n

t

H

a

p

r

o

t

o

n

.

H

b

s

i

g

n

a

l

i

s

s

p

l

i

t

i

n

t

o

a

d

o

u

b

l

e

t

.

H

a

i

s

s

p

l

i

t

b

y

a

d

j

a

c

e

n

t

H

b

p

r

o

t

o

n

s

.

H

a

s

i

g

n

a

l

i

s

s

p

l

i

t

i

n

t

o

a

t

r

i

p

l

e

t

b

y

a

p

p

l

y

i

n

g

t

h

e

(

n

+

1

r

u

l

e

)

.

Splitting Pattern for 1,1,2-trichloroethane

NMR spectrum of p-xylene (no signal splitting)

Interpreting Spectra

• Determine degrees of unsaturation (IHD) from molecular formula
• Count the # of signals to determine # of distinct proton environments
• Look for diagnostic chemical shifts
• Look for relative ratios (integration)
• Analyze splitting patterns

C3H7I

C2H4Cl2

C3H6Cl2