13-1 Nuclear Magnetic Resonance Spectroscopy Part-3 Prepared By Dr. Khalid Ahmad Shadid Islamic...

13-1

Nuclear Magnetic Resonance SpectroscopyPart-3

Prepared By

Dr. Khalid Ahmad Shadid

Islamic University in MadinahDepartment of Chemistry

13-2

Spin-Spin Splitting in 1H NMR Spectra

• Peaks are often split into multiple peaks due to interactions between nonequivalent protons on adjacent carbons, called spin-spin splitting (multiplicity or coupling pattern)

Remember that a proton will exist in two spin states andSimple coupling patterns (obey the n+1 rule, see later)

• protons are only coupled to one other set of neighboring protons or • protons are coupled to multiple sets of neighboring protons by identical coupling constants

If the neighbours are not all equivalent, more complex patterns

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• In the absence of any other hydrogen atoms, Ha (ABOVE) has a conventional resonance frequency/chemical shift that is determined by its local magnetic field as usual


• The PRESENCE OF Hb, however, alters the local magnetic field at Ha, the TWO possible spin states at Hb results in both a small INCREASE in the local field (alpha spin) and also a small DECREASE in the local field (beta spin), this results in TWO NEW resonance frequencies/chemical shifts, the peak is split into TWO, it becomes a DOUBLET

13-4

The presence of the TWO hydrogens HB (BELOW) result in THREE local magnetic fields at proton Ha, the peak for HA is similarly split into THREE, it appears as a triplet.


13-5

We will discuss only splitting occurs with protons on ADJACENT carbon atoms.The area ratio's of the peaks in a splitting pattern is given by Pascal's triangle


13-6

Signal Splitting; the (n + 1) Rule

Peak: The units into which an NMR signal is split; singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and so forth.

Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens.

(n + 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks.

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Signal Splitting (n + 1)

Figure 13.12 1H-NMR spectrum of 1,1-dichloroethane.

CH3-CH-ClCl

For these hydrogens, n = 1;their signal is split into(1 + 1) = 2 peaks; a doublet

For this hydrogen, n = 3;its signal is split into(3 + 1) = 4 peaks; a quartet

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Origins of Signal Splitting

Signal coupling: An interaction in which the nuclear spins of adjacent atoms influence each other and lead to the splitting of NMR signals.

Coupling constant (J) : The separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet. A quantitative measure of the spin-spin coupling with

adjacent nuclei.

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Figure 13.13 Vicinal H atoms and the spin-spin coupling that occurs between them. H atoms with more than three bonds between them

generally do not exhibit noticeable coupling. For H atoms three bonds apart, the coupling is referred

to as vicinal coupling.

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Figure 13.15 The quartet-triplet 1H-NMR signals of 3-pentanone showing the original trace and a scale expansion to show the signal splitting more clearly.

13-11

Spin-Spin Splitting: Simple Example: 1,1-dichloroethane

13-12

Coupling in1H NMR Spectra

Coupling arises because the magnetic field of vicinal protons influences the field that the proton experiences

Consider: effect of -CH group on the adjacent -CH3

The methine -CH can adopt two alignments and as a result, the signal for the adjacent methyl -CH3 is split in two lines, of equal intensity, a doublet

13-13

Consider: effect of -CH3 group on the adjacent -CH The methyl -CH3 protons give rise to 8 possible

combinations with respect to the applied field Some combinations are equivalent and there are four

magnetically different effects As a result, the signal for the adjacent methine -CH is split

into four lines, of intensity ratio 1:3:3:1, a quartet

Coupling in 1H NMR Spectra

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Simple Spin-Spin Splitting

An adjacent CH3 group can have four different spin alignments as 1:3:3:1

This gives peaks in ratio of the adjacent H signal

An adjacent CH2 gives a ratio of 1:2:1 The separation of peaks in a multiplet

is measured is a constant, in Hz J (coupling constant)

13-15

Spin-Spin Splitting Patterns

The relative intensities, given by Pascal's triangle, are due to interactions between nuclear spins that can have two possible alignments with respect to the magnetic field

To derive Pascal's triangle, start at the apex, and generate each lower row by creating each number by adding the two numbers above and to either side in the row above together

13-16

Rules for Spin-Spin Splitting

Equivalent protons do not split each other 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

13-1717

Molecules With Two Type of Protons How many signals?

13-18

Two groups of protons coupled to each other have the same coupling constant, J J coupling constant is the distance between peaks in a

multiplet (in Hz) Peaks for protons that split each other will always have

the same coupling constant J useful in determining which peaks are adjacent to

each other Multiplets will often be skewed in the direction of the

peak to which they are coupled

Rules for Spin-Spin Splitting

13-19

For the peaks of bromoethane, the coupling constant J is 7 Hz J calculation: take the distance (in ppm) between any two

adjacent split peaks and multiply it by the frequency of the NMR machine

Coupling Constant

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How to distinguish between A and B?

Example

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Example

13-22

Coupling between two nuclei can be: Homonuclear: between same nuclei (e.g. 1H-C-C-1H) Heteronuclear: between different nuclei (e.g. 1H-13C)

Factors affecting spin-spin couplings : the nuclei involved, the distance between the two nuclei, the angle of interaction between the two nuclei, and the nuclear spin of the nuclei

Distance Dependence

Basics of Nuclear Coupling

one-bond (1J) > two-bond > (2J) three-bond (3J) > long-range (4J - nJ)

110 - 270 Hz 9 - 15 Hz 6 - 8 Hz 1 - 7 Hz

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Geminal coupling or two-bond coupling (2J) Dependent upon the bond angle between the

nuclei Generally, the smaller the angle the bigger the coupling constant


13-24

Vicinal coupling or three-bond coupling (3J) Dependent upon the dihedral angle between the nuclei.

Generally, the more eclipsed or antiperiplanar the nuclei the greater the coupling constant


13-2525

Complex Patterns of Proton Coupling

The n+1 rule fails when protons in a group are magnetically nonequivalent and may have different chemical shifts and coupling constants

Spectra can be more complex due to overlapping signals, multiple nonequivalence

Example: trans-cinnamaldehyde

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Complex Patterns of Proton Coupling

13-27

Complex Patterns of Proton Coupling Doublet-of-doublets! Hb is

split by Ha with a J of around 15 Hz. It is further split by proton Hc with a J of around 10 Hz. One way that you know it is not a quartet is that the peaks are all (roughly) the same height

Because of the small coupling between Ha and Hc, one of the splittings is very small, and consequently is difficult to see in the real spectrum

13-28

Coupling of Protons on Oxygen

Normally, no coupling is observed between the O-H and H’s on the carbon atom to which the hydroxyl group is attached

The average time of residence of a proton on oxygen is 10-5 seconds. About 10-2 to 10-3 second is required for an NMR transition event to be recorded. Thus, the hydroxyl proton is unattached more frequently than it is attached to oxygen, and the spin interaction between the hydroxyl proton and any other protons is effectively decoupled

Likewise, protons on nitrogen also exchange quickly enough to be uncoupled from other protons in the molecule

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Uses of 1H NMR Spectroscopy

The technique is used to identify likely products in the laboratory quickly and easily

Example: regiochemistry of hydroboration/oxidation of methylenecyclohexane

Only that for cyclohexylmethanol is observed

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13C NMR Spectroscopy

13-31

13C NMR Spectroscopy: Background

Like 1H, 13C has I = ½. For 12C, I = 0, no NMR signal observed 13C has only about 1.1% natural abundance. As a result,

C is about 400 times less sensitive than H nucleus to the NMR phenomena

13C resonances are 0 to 220 ppm downfield from TMS Similar factors affect the chemical shifts in 13C as seen

for 1H NMR Long relaxation times (excited state to ground state)

mean no integrations

13-32

Characteristics of 13C NMR Spectroscopy

Provides a count of the different types of environments of carbon atoms in a molecule

Number of peaks indicates the number of types of C Spectrum of 2-butanone is illustrative: signal for C=O

carbons on left edge

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Proton-Coupled 13C Spectra

Due to low natural abundance of 13C (1.1%), any 13C is unlikely to have another 13C nucleus as a neighbour in natural material. Therefore: we do not usually see 13C-13C coupling in “normal” 13C

spectra 13C–1H couplings are observed in “normal” 13C spectra

unless the interactions are “decoupled” As a result H’s split the carbon peaks into multiplets

13-3434

Proton-Broad Band Decoupled 13C Spectra

13C Broad Band Decoupled (BBD) spectra are normally obtained as proton-decoupled spectra to remove all interaction between 13C nuclei and protons

In the proton-decoupled spectra, each carbon atom gives a singlet

Proton-decoupled spectra are much easier to interpret but it dose not indicate the number of hydrogen atoms attached to each carbon atom

From BBD spectrum you know: number of chemically nonequivalent carbons, and chemical shift (environment of

each type of carbon)

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13C-NMR Spectroscopy

Figure 13.29 Proton-decoupled 13C-NMR spectrum of 1-bromobutane.

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How would you determine the nature of the carbon? The good news is (as you know): both 13C and 1H have

nuclear spin quantum numbers I = 1/2 Therefore: coupling (and J) can be observe between

13C and 1H Spin-spin splitting between 13C and 1H follows the same

rules as 1H–1H splitting The N+1 Rule can be applied:

singlet doublet triplet quartet

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Proton-Coupled Viz Proton-Decoupled 13C Spectra

Example: ethyl phenylacetate

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13C NMR Chemical Shifts

Similar factors affect (electronegativity, hybridization, anisotropy) the chemical shifts in 13C as seen for 1H NMR

Since C is more electronegative than H, A 13C atom linked to more C atoms and therefore fewer 1H atoms will be deshielded & is at a lower field (i.e. higher value)

a) Saturated C 8 - 60 ppm

b) C attached to electronegative atoms 40 - 70 ppm

c) Alkene/ aromatic C 100-150 ppm

d) Carbonyl carbons 155-220 ppm

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Simplified 13C Correlation Charts

13-40

Chemical Shift - 13C-NMR

Figure 13.30 13C-NMR chemical shifts of representative groups.

13-41

Chemical Shift - 13C-NMR

RCH3RCH2R

R3CH

R2C=CR2

RC CR

R3COR

RCH2Cl

RCH2BrRCH2I

R3COH

RC

RCNR2

O

RCH, RCRO O

RCCOHO

RCORO

0-40

110-160

165 - 180

160 - 180

165 - 185

180 - 215

40-80

40-80

35-80

25-65

65-85

100-150

20-6015-5510-40

Type of Carbon

ChemicalShift ()

ChemicalShift ()

Type of Carbon

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Interpreting NMR Spectra

Alkanes 1H-NMR signals appear in the range of 0.8-1.7. 13C-NMR signals appear in the considerably wider range

of 10-60. Alkenes

1H-NMR signals appear in the range 4.6-5.7. 1H-NMR coupling constants are generally larger for

trans-vinylic hydrogens (J= 11-18 Hz) compared with cis-vinylic hydrogens (J= 5-10 Hz).

13C-NMR signals for sp2 hybridized carbons appear in the range 100-160, which is to higher frequency from the signals of sp3 hybridized carbons.

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Figure 13.31 1H-NMR spectrum of vinyl acetate.

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Alcohols 1H-NMR O-H chemical shift often appears in the

range 3.0-4.0, but may be as low as 0.5. 1H-NMR chemical shifts of hydrogens on the carbon

bearing the -OH group are deshielded by the electron-withdrawing inductive effect of the oxygen and appear in the range 3.0-4.0.

Ethers A distinctive feature in the 1H-NMR spectra of ethers is

the chemical shift, 3.3-4.0, of hydrogens on the carbons bonded to the ether oxygen.

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Figure 13.32 1H-NMR spectrum of 1-propanol.

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Aldehydes and ketones 1H-NMR: aldehyde hydrogens appear at 9.5-10.1. 1H-NMR: a-hydrogens of aldehydes and ketones appear

at 2.2-2.6. 13C-NMR: carbonyl carbons appear at 180-215.

Amines 1H-NMR: amine hydrogens appear at 0.5-5.0

depending on conditions.

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Carboxylic acids 1H-NMR: carboxyl hydrogens appear at 10-13, to

higher frequency of most other types of hydrogens. 13C-NMR: carboxyl carbons in acids and esters appear

at 160-180.

Figure 13.33 1H-NMR spectrum if 2-methylpropanoic acid.

13-48


Spectral Problem 1; molecular formula C5H10O.

CH3-CH2-C-CH2-CH3

O

3-Pentanone

2,42 (q)

1.07 (t)

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Spectral Problem 2; molecular formula C7H14O.

CH3-C-CH2-C-CH3

OCH3

CH3

4,4-Dimethyl-2-pentanone

2,32 (s)

2.11 (s)

1.01 (s)

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Problem 13.21

Following is the 1H-NMR spectrum of compound O, molecular formula C7H12. Compound O reacts with bromine in carbon tetrachloride to give a compound with the molecular formula C7H12Br2. The 13C-NMR spectrum of compound O shows signals at 150.12,106.43, 35.44, 28.36, and 26.36. Deduce the structural formula of O.

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Problem 13.21

The molecular formula, C7H12, indicates a hydrogen deficiency of 2, so there are two rings and/or pi bonds. Compound O reacts with only one mole of bromine, so there is only one double bond and there then must be one ring. Furthermore, the 13C-NMR shows only two sp2 resonances,another evidence that there is only one double bond.

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Good Luck

With thanks to Dr. Ziad Ali, Taibah University for providing NMR resources

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