1H-NMR

(Nuclear Magnetic Resonance) in Chem242

- In Chem242, you will submit your sample to obtain 1H-NMR spectra. - You will analyze each 1H-NMR data in a computer lab.

Nuclear Spin and Magnetic Moment

NMR looks at magnetically active nuclei that is nuclei with non-zero “nuclear spin”.

Nuclear Spin and Spin States 1H

has a spin quantum number of 1/2.

They can have two different spin states - ± 1/2 or α/β The different spin states have the same energy unless they are put in an external magnetic field

or

Small Magnet in an External Magnetic Field

or

ΔE = B0

γh 2π

h : Plank’s constant γ: gyromagnetic ratio B0 : external magnetic field

spin state β

Population Difference and NMR Experiment • The energy gap between the two spin states 100,000

ΔE α E

100,001 in 200 MHz instrument

creates a small population difference. • When the system is irradiated with electromagnetic radiation, energy is absorbed at the frequency that matches to the energy gap.

E = hν hν = B0

γh 2π

ν

γB0

∴ ν = 2π

frequency

Chemical Shift Scale • Different NMR instruments have different magnets, and operate at different frequencies. The same proton would resonate at different frequencies.

Chemical shift (δ) =

νobs - νref νinstrument

x 106 ppm

NMR Reference Compounds CH3

• Chemically stable • Can be removed easily (b.p. = 27°C) • Most protons appear to the left of TMS

H3C Si CH3 CH3 Tetramethylsilane (TMS)

Positive chemical shifts

CH3 H3C Si CH3

SO3H

For D2O

4,4-dimethyl-4-silapentane-1-sulfonic acid(DSS)

Common NMR Solvents

Do not use them as references

What Do We Look for in 1H-NMR? • # of unique protons • Peak positions (chemical shift) – Shielding by bonding electrons – Magnetic anisotropy (or ring current effect in aromatic compounds • Peak splitting (spin-spin coupling) • Integrals – Peak area are proportional to # of protons

Chemical Shift Range for 1HNMR TMS (δ=0)

Typical 1H Chemical Shift Ranges

Ethyl p-Hydroxybenzoate, 1H-NMR 3H E (triplet) 1H, singlet 2H 2H A C B

2H D(quartet)

Chemical shift scale

Chemical Shifts and Chemical Equivalence • Chemically equivalent protons have the same chemical shift. How do we find chemically equivalent(or non-equivalent) protons? H

Z

H

H

H

H

Z substitution

H

H

H

H

H

H

Z

H

H

H

H

H

H

H

H

Z

H

H

H

H

H H Z H

H

H H Z

H H

Z

H

H

H

H H

Same compound Chemically equivalent (homotopic)

Toluene and p-Xylene a

a H

H

b H

H

H H

H

H H

c H

H

b c

b b

H

H

b

H

H

b

H

H H

d a

Halogenated Methanes Ha C Ha

Ha Ha

C Ha

Ha

Enantiotopic protons

Ha Cl

C Ha

Ha

Ha

Cl

Z

Ha C

Cl

Cl Br

C H

H Cl Br

C Z

Cl Br

A pair of enatiomers • Enantiotopic protons have the same chemical shift although they are not chemically equivalent.

Halogenated Ethanes Ha

Ha

Ha

C

C

Ha

Ha

Ha

Cl

Ha

Hb

C

C

Ha

Hb

Enantiotopic protons

Diastereotopic protons

Hb Cl

C Hc

Cl

Cl

Ha

C

C

Ha

Ha

Cl

Homotopic protons

Ha C

Hb

Ha

Cl

Z

H

C

C

H

Br

Cl

Cl

H

H

C

C

Z

Br

Br

A pair of diastereomers

• Diastereotopic protons have different chemical shift.

Cl

Spin-spin Coupling 2H Cl

Ha

Cl

C

C

Ha

Hb

Cl

a

1H b

TMS

Spin-spin coupling

Ha : doublet (d) Hb : triplet (t)

Diethyl Ether H3C

O CH2

CH3

b

CH2

a

Less shielded

More shielded

N+1 Rule and Pascal’s Triangle singlet doublet triplet quartet quintet sextet septet etc

• A proton with N adjacent neighbors will split into N+1 lines. • The intensity of the lines may be determined using Pascal’s triangle. • Spin-spin couplings are commonly observed between protons that are separated by three bonds or less.

Commonly Observed Splitting Patterns

Ethyl-trans-Crotonate H3C

H

O

H O

CH2

CH3

Alkenic Protons b a

c

H3C

b

H

O

H

CH2

CH3

O

c Jab = 7 Hz Jac = 1 Hz Jbc = 16Hz “a doublet of quartets”

Allylic Protons a

c

H3C

b

H

O

H

CH2

CH3

O

Jab = 7 Hz Jac = 1 Hz Jbc = 16Hz “a doublet of doublets (dd)”

Typical Coupling Constants

Cis and Trans Alkenes

Vinyl Acetate dd

singlet dd

dd

Important to recognize splitting patterns

Trans-3-methoxyacrylonitrile “A 1st order spectrum” HC

HA

HB

Bromostilbene “A 2nd order spectrum” Ha Br Hb

Ha = 7.024 ppm Hb = 7.072 ppm

A2 to AB to AX

Ha

A2 Ha

HA

AB

Br HB

AX

HA

OCH3

NC

HX

O(C=O)R

Jab Ha

Jab

O(C=O)R

Hb

Ha, Hb: diastereotopic protons

Jac

Jac

Jbc

Jbc

4.126 4.096

Hc

4.340 4.319

Hb

O(C=O)R

4.185 4.156

Ha

4.281 4.259

1H-NMR of Vegetable Oil

4.185 4.156 4.126 4.096

4.340 4.319

4.281 4.259

Calculating J Values and Chemical Shifts 4.340 - 4.319 = 0.021 4.281 - 4.259 = 0.022

Average = 0.022 ppm

0.022 x 10-6 x 200 x 106 = 4.4 Hz = Jac 4.185 - 4.156 = 0.029 4.126 - 4.096 = 0.030

Average = 0.030 ppm

0.030 x 10-6 x 200 x 106 = 6.0 Hz = Jbc 4.340 - 4.281 =0.059 4.319 - 4.259 = 0.060 4.185 - 4.126 = 0.059 4.156 - 4.096 = 0.060

Average = 0.060 ppm

0.060 x 10-6 x 200 x 106 = 12.0 Hz = Jab

Hydrogen Bonded Protons • • •

Usually appear as a broad peak Their chemical shifts depend on concentration, temperature, solvent etc. Generally do not show spin-spin coupling.

•

They are exchangeable, and can be identified by “D2O” shake.

* R-OH + D2O

R-OD + HDO

+ D2O (excess) HDO

Ethanol

N-Methyl Aniline NH CH3

NH

Problem 1

Problem 1

H H H

H H

H

Br

Br

quintet

H

sextet

H

triplet 3H

triplet 2H Br

2H 2H

Br

• Don’t be afraid of writing wrong structures!

H

Problem 2

Problem 2 H

H

X

Y H

2H

2H

H

2H

2H

Z CH2 CH3

NH2

3H

Problem 2 H

H

X

NH2

Y H

C9H12NO2

Z CH2 CH3

H

CO2 missing

O

O CH2 CH3 H2N O

CH2 H2N

O

CH3