C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR...

12
1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has a nuclear spin I = 0, rendering it unobservable by NMR. Limited to the observation of the 13 C nucleus which constitutes only 1.1% of naturally occurring carbon. 40.0 1.00 251.7 19 F 75.0 7.05 50.0 4.70 10.7 1.00 67.28 13 C 6.5 1.00 41.1 2 H 300. 7.05 200. 4.70 42.6 1.00 267.53 1 H Frequency ν (MHz) Field strength B 0 (Tesla) γ (10 6 rad/Tesla × sec) Nucleus 13 C Transition Energy The magnetogyric ratio, γ, for the 13 C is 67.3 compared to 267.5 for 1 H. Remember the resonance condition for a nucleus is given by: ν = (γ/2π)B 0 If the gyromagnetic ratio is lowered, the ΔE is also lowered. Where a 1 H spectrum using a 1.41 T magnet is observed at 60 MHz, a 13 C spectrum is observed at 15 MHz – roughly 4 times less energetic . Boltzmann: N upper /N lower = e -ΔE/kT = e -hν/kT @ 298 K the ratio is 1,000,000 / 1,000,002

Transcript of C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR...

Page 1: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

1

13C NMR Spectroscopy

13C NMR

12C is the most abundant natural isotope of carbon,

but has a nuclear spin I = 0, rendering it

unobservable by NMR.

Limited to the observation of the 13C nucleus which

constitutes only 1.1% of naturally occurring carbon.

40.01.00251.719F

75.07.05

50.04.70

10.71.0067.2813C

6.51.0041.12H

300.7.05

200.4.70

42.61.00267.531H

Frequency νννν

(MHz)

Field

strength B0

(Tesla)

γγγγ

(106 rad/Tesla ×××× sec)

Nucleus

13C Transition Energy

The magnetogyric ratio, γγγγ, for the 13C is 67.3 compared to

267.5 for 1H.

Remember the resonance condition for a nucleus is given by:

νννν = (γγγγ/2ππππ)B0

If the gyromagnetic ratio is lowered, the ∆∆∆∆E is also lowered.

Where a 1H spectrum using a 1.41 T magnet is observed at 60

MHz, a 13C spectrum is observed at 15 MHz – roughly 4 times

less energetic.

Boltzmann: Nupper/Nlower = e-∆∆∆∆E/kT = e-hνννν/kT

@ 298 K the ratio is 1,000,000 / 1,000,002

Page 2: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

2

13C NMR

The combined effects of smaller excess populations in the

lower energy state, low natural abundance, and slow

relaxation rates result in a 13C signal that is typically 6000

times weaker than that observed for 1H.

With FT instruments, this is not a problem – simply take

more scans! (recall S/N increases as the square root of the

number of scans).

16 scans on a 5-10 mg sample will give a good 1H spectrum,

512 scans on a 50 mg sample will give a good 13C spectrum.

Fourier Transform NMR

•Radio-frequency pulse given.

•Nuclei absorb energy and precess (spin) like little tops.

•A complex signal is produced, then decays as the nuclei lose energy.

•Free induction decay is converted to spectrum.

13C NMR

• low 13C abundance

• a single molecule will have at most only one 13C

atom

• however, we are sampling a very large number of

molecules, even in a 50 mg sample!

• thus our sampling will ‘see’ a 13C at every C

position in the molecule!

Chem 325

TUTORIAL

TONIGHT @ 7PM

Page 3: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

3

13C Shielding

13C spectra are typically recorded from 0 – 220 ppm;

with the zero being the methyl carbon in TMS

(much wider range than 1H spectra!)

13C nuclei are shielded or deshielded (CHEMICAL

SHIFT) due to the same factors as for 1H NMR.

1. Electron withdrawing ability (by inductance or

resonance) of nearby groups.

2. Hybridization.

3. Electron current effects.

13C NMR Chemical Shifts

Several functionalities appear directly on 13C NMR which are

not ‘visible’ in 1H NMR:

- Quaternary carbons

- ipso carbons

- Carbonyl carbonsSi

CH3

H3C CH3CH3

downfield δ (ppm) upfielddeshielded shieldedhigher ∆E lower ∆E

0.020406080100120140160180200220

carbonyl carbons

aromatic carbons

alkene carbons

alkyne carbons

13C-EWG

sp3 carbon

Carbonyl Carbon Chemical Shifts

110120130140150160170180190200210220

ketones

conj. ketones

aldehydes

carboxylic acids

anhydrides nitriles

esters

acid chlorides

amides

Spin-Spin Coupling in 13C NMR

Homonuclear coupling of 13C-13C is possible in theory.

However, due to the low natural abundance of 13C, it is rare to

find two 13C’s in the same molecule, let alone adjacent to one

another.

No need to consider 13C-13C coupling except for enrichment

studies!

Heteronuclear coupling between 13C and the 1H atoms attached

to them is observed (1H abundance ~99%).

Because the 1H atoms are directly attached, the coupling

constants (1J)are large, typically 100-250 Hz.

When such spectra are observed, they are referred to as proton

coupled spectra (or non-decoupled spectra).

Page 4: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

4

1H – 13C Splitting

The splitting follows the simple N+1 rule:

The multiplet analysis gives useful information, but there are

two major limitations:

1) If the 13C signal is weak (common) the outer peaks of the

multiplet may be lost in the noise of the spectrum.

2) Due to the large J-constants, the multiplets quickly begin to

overlap and become congested.

C13

C

H

13C

H

H13

C

H

H

H13

quaternary

singlet

methine

doublet

methylene

triplet

methyl

quartet

13C NMR Spectrum

Proton-Coupled

Effect of Coupling

Coupling can cause 13C NMR spectra to become very complicated (convoluted) quite easily.

1H Coupled

Three equal intensity lines

at 77 ppm

CDCl3 solvent

13C- 2D coupling

1H Decoupling

To simplify the 13C spectrum, and to increase the intensity of

the observed signals, a decoupler is used to remove the spin

effects of the 1H nucleus.

A second RF generator irradiates at the 1H resonance

frequency causing the saturation – effectively averaging all

their spin states to zero.

1H channel-

13C channel

13C νννν pulse

13C FID

Page 5: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

5

13C Proton Decoupled Spectrum

13C{1H}

Effect of Decoupling

1H Coupled

1H Decoupled

13C NMR Spectra

Due to signal enhancement and spectral simplification, 13C

spectra are usually reported as 1H decoupled.

Each chemically unique carbon in the molecule gives rise to a

single peak.

Of course chemically equivalent carbons contribute to the

same peak!

The number of different signals (peaks) indicates the number

of different kinds of carbon.

The location (chemical shift) indicates the type of functional

group.

13C NMR Intensities

Peak areas (~heights) are NOT proportional to number of carbons.

Carbon atoms with more hydrogens give strongersignals, due to more efficient relaxation (transfer of spin to the hydrogens).

However, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls)

Page 6: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

6

Example: Ethanol

CH2 CH3

OH

Example: 1-bromohexane

CH2

CH2

CH2

CH2

CH2

CH3Br

Example: cyclohexane Example: cyclohexene

Page 7: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

7

Example: 1,3-cyclohexadiene Example: 1,4-cyclohexadiene

Example: m-nitrotoluene

CH3O2N1

23

46

7

5

3

1

2

65

7

4

28

13C Chemical Shift Predictions

Examining a large set of chemical shift data has allowed the

development of ‘empirical’ rules or substituent parameters to

allow chemical shift predictions for most commonly

encountered situations.

Example: the carbon atoms of a substituted benzene ring.

Benzene itself → single peak at 128.7 ppm

Add to this value substituent increments which depend on the

chemical nature of the substituent and where it is on the ring

relative to the carbon whose shift is being predicted.

Page 8: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

8

29

13C Aromatic Substituent Parameters

CH3O2N 12

3

46

7

5

C1 = 128.7 + (CH3)ipso + (NO2)meta = 128.7 + 8.9 + 0.8 = 138.9 ppm

C2 = 128.7 + (CH3)ortho + (NO2)ortho = 128.7 + 0.7 + (-5.3) = 124.1 ppm

31

Example: m-nitrotolueneCH3O2N

12

3

46

7

5

3

1

2

65

7

4

135.4135.46

129.2129.45

120.6120.54

148.4148.23

123.8124.12

139.9138.41

Obs’dCalc’dC

32

Example: p-Hydroxyacetophenone

CO CH3

OH

1

6

5

4

3

2

131.4130.26

115.8115.85

162.1158.54

115.8115.83

131.4130.22

129.2128.71

Obs’dCalc’dC

4

3

5

2

61

Page 9: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

9

33

13C Shift Predictions – Alkyls

Can also make predictions for alkyl groups

Base value: use

unsubstituted

hydrocarbon 34

Example: bromocyclopentane

Br

23.3233

37.9362

53.5511

Obs’dCalc’dC 1

2

3

1

2

3

35

13C NMR Intensities

Peak areas (~heights) are NOT proportional to number of carbons.

Carbon atoms with more hydrogens give strongersignals, due to more efficient relaxation (transfer of spin to the hydrogens).

However, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls)

Nuclear Overhauser Enhancement (NOE)

A phenomenon observed with proton-decoupled 13C-NMR is

that the intensity of the signal for a given 13C increases versus

the proton-coupled spectrum roughly proportional to the

number of protons attached.

The degree of this signal enhancement is called the Nuclear

Overhauser Enhancement (NOE).

This effect is general, and appears anytime when one of two

types of atoms is irradiated, while the spectrum of the other is

observed. In this case, while the 1H population is irradiated to

saturation, the 13C is observed. Here: a heteronuclear effect.

Page 10: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

10

NOE

The effect can be a positive or negative one, but for the case

of 1H-13C, the effect is positive

The maximum enhancement is given by:

NOEmax = 1 (γγγγ irradiated)

2 (γγγγ observed)

This value is what is added to the observed intensity in the

coupled spectrum to give the intensity observed in the

decoupled spectrum:

total predicted intensity = 1 + NOEmax

NOE

For 1H – 13C, NOE = ½ (267.5/67.28) = 1.988

A maximum enhancement of almost 200% is possible.

NOE operates in both directions – 13C nuclei (if decoupled)

would enhance the signal of 1H – however, this signal would

be weak due to the low abundance of 13C.

Because NOE for 13C – 1H operates in the opposite direction

(a rare nuclei always bound to an abundant one) it is a useful

probe into structural assignments.

The NOE effect is very short-range, falling off as 1/r3 the

distance between the nuclei.

Origin of NOE

An isolated two spin system between

a single carbon and single hydrogen

atom

The effects of coupling are left out for

simplicty

Shown are the four combinations of

spin states of these two nuclei, N1-4

The two energy states where both are

spin up or spin down are the lowest

and highest energy states

The “mixed” states are roughly

degenerate in energy

N1

N3

N2

N4

C H

C H

C HC H

Origins of NOE

Quantum mechanics dictates that

allowed transitions involve only

one change of spin at a time –

single quantum transitions

The allowed transitions are

shown in red

N1

N3

N2

N4

C H

C H

C HC H

Page 11: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

11

Origins of NOE

Let the equilibrium population

of the two degenerate states be B

The N1 level would be higher

than B by a small amount, δδδδ

The N4 level would be lower

than B by a the same amount, δδδδ

The signal for a 13C in this case

would be proportional to δδδδ at

equilibrium

The two 13C transitions are N1 –

N2 and N3 – N4

N1

N3

N2

N4

C H

C H

C HC H

Origins of NOE

When a decoupler is used, the 1H

populations are disturbed from

their equilibrium values

Relaxation processes restore these

disturbed populations to their

equilibrium values

One such process is a double-

quantum transition, where both

the C and H nuclei relax

simultaneously (blue line)

This “leak” in the upper state

enhances the population of the

lower energy state for carbon – the

excess population is larger – and

the signal intensifies

N1

N3

N2

N4

C H

C H

C HC H

double

quantum transition

NOE

NOE: an example of cross-polarization, polarization of spin

states of one type of nucleus causes a polarization of the spin

states of another nucleus.

A heteronuclear NOE effect is always observed in ‘normal’ 1H

decoupled 13C spectra.

Total NOE for a given C increases with number of nearby H’s.

Thus intensities of C signals are generally:

CH3 > CH2 > CH > C

NOE effect is quite general. Can also be applied in a

homonuclear sense, i.e. 1H{1H}

NOE

Difference

Difference

Page 12: C NMR Spectroscopy - StFXalonso.stfx.ca/dklapste/Chem325/c325notes/c325_C13_NMR.pdf · 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of carbon, but has

12

NOE

Depends on cross-polarization of spin states.

Can tell us what nuclei are close together.

In contrast to J-coupling (spin-spin) which operates through

the bonding electrons, NOE is a through-space effect.

Thus NOE can tell us about the proximity of atoms which are

separated by many bonds, e.g. proteins, RNA, DNA

46

Example: m-nitrotoluene

CH3O2N1

23

46

7

5

3

1

2

65

7

4

47

Example: benzonitrile

C

N

Very weak: no attached H’s

No NOE effect!