1

344 Organic Chemistry LaboratoryIntroduction to 1H-NMR Spectroscopy Part 1

Main topics

- Equivalent/non-equivalent protons

- Interpreting simple 1H-NMR spectra

- Spin-spin coupling, n+1 rule

How do we know the structure of these molecules?

Molecular Modeling

E = hν E = energy (kJ/mol) ν = frequency (Hz), h = Planck constant

Background & theory: pp. 536-538 “Organic Chemistry” 5th. Ed. Loudon.

Spin ½ nuclei 1H, 13C, 19F, 29Si, 31P

NMR active, easy to do, widely used

Spin 1 nuclei 2H (D), 14N

NMR active but not widely used

Spin 0 nuclei 12C, 16O, 32S

NMR silent

Background & theory: pp. 578-636 “Organic Chemistry” 5th. Ed. Loudon.

Nuclear Magnetic Resonance (NMR)

Nuclei behave like a bar magnet

4

6

7.04

2.29

parts per million (ppm)

1,4-Dimethylbenzene

(para-Xylene)

The 1H-NMR spectrum

C8H10

Analyzing para-Xylene

Hb

Hb

Hb

Hb

HbHb

HbHb

Ha

Ha

Ha

Ha

Ha

Ha

Ha

Ha

Ha

Ha

HaHa

C8H10

para-Xylene

4

6

7.04

2.29

parts per million (ppm)

1,4-Dimethylbenzene

(para-Xylene)

The 1H-NMR spectrum

C8H10

Ha

Hb

4

6

6.82

3.75

1,4-Dimethoxybenzene

Ha

Ha

Ha

HbHb

Electronegativity O = 3.44, C = 2.55, H = 2.20

O

O

Oxygen is a highly electronegative atom

Oxygen exerts a strong electron-withdrawing effect

in the sigma-framework (i.e. along the C-O bond)

This electron-withdrawing effect reduces the degree

of electron density around the Me-group C-atom

1,4-Dimethoxybenzene1,4-Dimethylbenzene

Electronegativity O = 3.44, C = 2.55, H = 2.20

3.75 ppm2.29 ppm

6.82 ppm7.04 ppm

Shielding/deshielding are relative terms

The Me-group of 1,4-dimethoxybenzene is deshielded relative to the Me-

group of 1,4-dimethylbenzene

1

9

8.00

1.50

tert-Butylformate

Hb

Ha

Ha

Ha

Ha

Hb

SiMe4 = TMS = Tetramethylsilane

0.00 ppm reference compound

Why use TMS?

Si

Don’t assign the TMS signal as part of your molecule!

- chemically inert (won’t react with most molecules)

- signal is strong and unsplit

- volatile (easily removed)

- inexpensive

- protons are strongly shielded

Ranges +/- ~0.5 ppm

2.45

4.56

9

6

Deuterated chloroform (CDCl3) used as solvent

2,4,6-Tris(bromomethyl)mesitylene

Electronegativity Br = 2.96

C = 2.55

2

3

5.80

2.14Standard notation for reporting NMR data

[chemical shift, (multiplicity, integration)]

Bromomethyl acetate

1H-NMR δ: 2.14, (s, 3H); 5.80 (s, 2H).

Reporting 1H-NMR data

Spin-spin coupling

4.42

2

1.58

3

N

O

O

Electronegativity O = 3.44, N = 3.04, C = 2.55

Nitroethane

HaHa

Ha

HbHb

N

O

OHa

Ha

Ha

HbHb

How do neighboring non-equivalent protons interact?

Consider how Hb protons interact with Ha protons

Hb protons align with or against direction of Bo

3 different combinations of Hb spin states

Bo

N

O

OHa

Ha

Ha

HbHb

How do neighboring non-equivalent protons interact?

Consider how Hb protons interact with Ha protons

Hb protons align with or against direction of Bo

1 2 1

Signal from Ha split into a

1:2:1 triplet by Hb

3 different combinations of Hb spin states

Both parallel

ap p

Both anti-parallelp ap

21 1

HaHa

Ha

HbHb

How do neighboring non-equivalent protons interact?

Consider how Ha protons interact with Hb protons

4 different combinations of Ha spin statesN

O

O

Bo

HaHa

Ha

HbHb

How do neighboring non-equivalent protons interact?

Consider how Ha protons interact with Hb protons

4 different combinations of Ha spin states

1 33 1 1 3 3 1

Signal from Hb split into a

1:3:3:1 quartet by Ha

N

O

O

Spin-spin splitting

4.42

2

1.58

3

N

O

O

Nitroethane

HaHaHa

Ha

Hb

HbHb

n + 1 rule

n = # non-equivalent neighboring protons

3.64

12

2

1.12n + 1 rule

Ha

Ha

Ha

Ha

Ha

Ha

Hb

Diisopropylether

O

1

6

15

20

15

6

1

Coupling constant J (Hz) – indicates strength of coupling

J ~ 7 Hz for alkyl (sp3) systems

Ha

Ha

Ha

Ha

Ha

Ha

Hb

O

2

1

Doublet

Pascal’s Triangle

1 1

1

1

1

1

1

1

1

1

1

1

2

3 3

4 46

6 6

5 510 10

2015 15

Singlet

Triplet

Quartet

Quintet

Sextet

Septet

n = 1

n = 2

n = 3

n = 4

1.18

2.793.51

7.02 6.63

4-isopropylaniline

1.18

6

12

2.793.51

4-isopropylaniline

Hb Ha

Hc

Hd

He

1:6:15:20:15:6:1

1

15

20

6

Coupling constant J (Hz) – indicates strength of coupling

J ~ 7 Hz for alkyl (sp3) systems

15

6

1

Part 2

Ha

HbHc

Hd + He Hb Ha

Hc

Hd

He

4-isopropylaniline

1.18

2.793.51

6.637.02