NMR S PECTROSCOPY. Electron has a spin and that spinning creates and associated magnetic field....

NMR SPECTROSCOPY

Electron has a spin and that spinning creates and associated magnetic field. Electron can behave like a tiny bar magnet.

Atomic nucleus can spin and behave as a tiny bar magnet. These spinning nuclei generate tiny magnetic fields Any nucleus will spin (odd mass or odd atomic number) Will not spin (if even mass or even atomic number)

Tiny magnets interact with an external magnetic field, denoted B0

Proton (1H) and carbon (13C) are the most important nuclear spins to organic chemists

NUCLEAR MAGNETIC RESONANCE

SPECTROSCOPY

RECALL

Mass number (atomic mass) is number of protons plus neutrons and can vary from isotopes of the same element

Atomic number (number of protons or electrons) is the same for all isotopes of that element

Nuclear Magnetic Resonance (NMR)

- the nuclei of some atoms spin: 1H, 13C, 19F, …

- Any nucleus will spin (odd mass or odd atomic number)

- the nuclei of many atoms do not spin: 2H, 12C, 16O, …

- Will not spin (if even mass or even atomic number)

- moving charged particles generate a magnetic field ()

- when placed between the poles of a powerful magnet, spinning nuclei will align with or against the applied magnetic field creating an energy difference.

Mass of the element

Nuclear spins are oriented randomly in the absence (a) of an external magnetic field but have a specific orientation in the presence (b) of an external field, B0

Some nuclear spins are aligned parallel to the external field Lower energy orientation More likely

Some nuclear spins are aligned antiparallel to the external field Higher energy orientation Less likely

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

When nuclei that are aligned parallel with an external magnetic field are irradiated with the proper frequency of electromagnetic radiation the energy is absorbed and the nuclei “spin-flips” to the higher-energy antiparallel alignmentNuclei that undergo “spin-flips” in response to

applied radiation are said to be in resonance with the applied radiation - nuclear magnetic resonance

Frequency necessary for resonance depends on strength of external field and the identity of the nuclei


The energy difference DE between nuclear spin states depends on the strength of the applied magnetic fieldAbsorption of energy with frequency n converts a

nucleus from a lower to a higher spin state DE = 8.0 x 10-5 kJ/mol for magnetic field strength of 4.7 T a

For field strength of 4.7 T a radiofrequency (rf) of n = 200 MHz is required to bring 1H nuclei into resonance

For a field strength of 4.7 T a radiofrequency (rf) of n = 50 MHz is required to bring 13C nuclei into resonance


The absorption frequency is not the same for all 1H and 13C nucleiNuclei in molecules are surrounded by electronsElectrons set up tiny local magnetic fields that act

in opposition to the applied field, shielding the nucleus from the full effect of the external magnetic field

The effective field actually felt by the nucleus is the applied field reduced by the local shielding effects

Beffective = Bapplied – Blocal

THE NATURE OF NMR ABSORPTIONS

The absorption frequency is not the same for all 1H and 13C nucleiEach chemically distinct nucleus in a molecule has

a slightly different electronic environment and consequently a different effective field

Each chemically distinct 13C or 1H nucleus in a molecule experiences a different effective field and will exhibit a distinct 13C or 1H NMR signal

Hydrogens in most organic molecules are surrounded by electrons and by other atoms.

THE NATURE OF NMR ABSORPTIONS

The NMR ChartThe downfield, deshielded side is on the left, and requires a lower

field strength for resonanceThe upfield, shielded side is on the right, and requires a higher

field strength for resonance The tetramethylsilane (TMS) absorption is used as a reference point

CHEMICAL SHIFTS

SHIELDING AND DESHIELDED

Shielded (by the local environment) Greater electron density around each hydrogen Due to poor electron withdrawing group Due to poor inductive effect

Deshielded (by the local environment) Less electron density around each hydrogen Due to high electronegative element Due to larger inductive effect

1H NMR spectroscopy determines how many kinds of electronically nonequivalent hydrogens are present in a molecule

Equivalence or nonequivalence of two protons determined by replacing each H by an X (halogen group)

Equivalent H give the same H-NMR signal

Non equivalent H give different H-NMR signal

Symmetrical compounds tend to contain a higher amount of equivalent hydrogens and thus fewer resonance signals in their NMRspectra

1H NMR SPECTROSCOPY AND PROTON EQUIVALENCE

Four possibilities:1. Protons are chemically unrelated and thus

nonequivalent

2. Protons are chemically identical and thus electronically equivalent

3. Protons are electronically equivalent but not identical

4. Replacement of a hydrogen gives a unique diastereomer (not mirror images of each other) with a second chirality center

1H NMR spectroscopy determines how many kinds of electronically nonequivalent hydrogens are present in a molecule

Four possibilities:

1. Protons that are unrelated and thus nonequivalent


2. Protons are chemically identical and thus electronically equivalent Chemically identical protons are said to be

homotopic


3. Protons are electronically equivalent but not identical The two –CH2 – hydrogens on C2 of butane (as well as

the two C3 hydrogens) are not identical because replacing one or the other would lead to a new chirality center

Different enantiomers would result if pro-R or pro-S hydrogen were replaced

Prochiral hydrogens are electronically equivalent and thus have the same NMR absorption


4. Replacement of a hydrogen gives a unique diastereomer (not mirror images of each other) with a second chirality center These hydrogens are neither electronically or

chemically equivalent and will most likely show different 1H NMR absorptions


Information from 1H-nmr spectra:

1. Number of signals: How many different types of hydrogens in the molecule.

2. Position of signals (chemical shift): What types of hydrogens.

3. Relative areas under signals (integration): How many hydrogens of each type.

4. Splitting pattern: How many neighboring hydrogens.

CCH3H3C

CH3C CH3

CH3

CH3

oneone

one

two

1. Number of signals: How many different types of hydrogens in the molecule

H3C C

CH3

Br

CH3 CH3CH2-Br

CH3CH2CH2-Br CH3CHCH3

Cl

onetwo

threetwo

PRACTICE, HOW MANY SIGNALS

H3C C

CH3

Br

CH3 CH3CH2-Br

CH3CH2CH2-Br CH3CHCH3

Cl

onetwo

threetwo

CH3

CH2Cl

CH3CHCH2CH3

BrCl-CH2CH2CH2-Cl

four two

three

2. Position of signals (chemical shift): what types of hydrogens.

primary 0.9 ppmsecondary 1.3tertiary 1.5aromatic 6-8.5allyl 1.7benzyl 2.2-3chlorides3-4 H-C-Clbromides 2.5-4 H-C-Briodides 2-4 H-C-Ialcohols 3.4-4 H-C-Oalcohols 1-5.5 H-O- (variable)

Note: combinations may greatly influence chemical shifts. For example, the benzyl hydrogens in benzyl chloride are shifted to lower field by the chlorine and resonate at 4.5 ppm.

reference compound = tetramethylsilane (CH3)4Si @ 0.0 ppm

remember: magnetic field

chemical shift

convention: let most upfield signal = a, next most upfield = b, etc.

… c b a tms

toluene

ab

CH3

ab

CCH3H3C

CH3C CH3

CH3

CH3

a a

a b

a

a

chemical shifts

3. Integration (relative areas under each signal): how many hydrogens of each type.

a b cCH3CH2CH2Br a 3H a : b : c = 3 : 2 : 2

b 2Hc 2H

a b aCH3CHCH3 a 6H a : b = 6 : 1

Cl b 1H

CCH3H3C

CH3C CH3

CH3

CH3

a a

a b

a

a

a 12 H a 12 H

a 6 H

a 6 Hb 4 H

integration

The area under each 1H NMR peak is proportional to the number of protons causing that peak

Integrating (electronically measuring) the area under each peak makes it possible to determine the relative number of each kind of proton in a molecule

Integrating the peaks of 2,2-dimethylpropanoate in a “stair-step” manner shows that they have 1:3 ratio, corresponding to the ratio of the numbers of protons (3:9)

INTEGRATION OF 1H NMR ABSORPTIONS: PROTON COUNTING

4. Splitting pattern: how many neighboring hydrogens.

In general, n-equivalent neighboring hydrogens will split a 1H signal into an ( n + 1 ) Pascal pattern.

“neighboring” – no more than three bonds away

n n + 1 Pascal pattern:

0 1 1singlet

1 2 1 1doublet

2 3 1 2 1 triplet

3 4 1 3 3 1quartet

4 5 1 4 6 4 1quintet

Multiple absorptions, called spin-spin splitting, are caused by the interaction (coupling) of the spins of nearby nuclei

Tiny magnetic fields produced by one nucleus affects the magnetic field felt by neighboring nuclei If protons align with the applied field the effective

field felt by neighboring protons is slightly larger If protons align against the applied field the

effective field felt by neighboring protons is slightly smaller

SPIN-SPIN SPLITTING IN 1H NMR SPECTRA

note: the alcohol hydrogen –OH usually does not split neighboring hydrogen signals nor is it split. Normally a singlet of integration 1 between 1 – 5.5 ppm (variable).

CCH3H3C

CH3C CH3

CH3

CH3

a a

a b

a

a

a 12 H singlet a 12 H singlet

a 6 H singlet

a 6 H singletb 4 H singlet

splitting pattern?

The absorption of a proton can split into multiple peaks called a multiplet

Remember ( n + 1 ) for splitting signals

1H NMR spectrum of bromoethane shows four peaks (a quartet) at 3.42 d for –CH2Br protons and three peaks (a triplet) at 1.68 d for –CH3 protons


Each –CH2Br proton of CH3CH2Br has its own nuclear spin which can align either with or against the applied field, producing a small change in the effective field experienced by the –CH3 protons

Three possible spin states (combinations)1. Both protons spin in alignment with applied field

Effective field felt by neighboring –CH3 protons is larger Applied field necessary to cause resonance is reduced

2. One proton spin is aligned with and one proton spin is aligned against the applied field (two possible combinations) No effect on neighboring protons

3. Both proton spins align against applied field Effective field felt by neighboring –CH3 is smaller Applied field necessary to cause resonance is increased


The origin of spin-spin splitting in bromoethane. The nuclear spins of –CH2Br protons, indicated by horizontal arrows, align either with or against the applied field, causing the splitting of –CH3 absorptions into a triplet


n + 1 rule Protons that have n equivalent neighboring protons show

n + 1 peaks in their 1H NMR spectrum The septet is caused by splitting of the –CHBr- proton signal

at 4.28 d by six equivalent neighboring protons on the two methyl groups (n = 6 leads to 6+1 = 7 peaks)

The doublet at 1.71 d is due to signal splitting of the six equivalent methyl protons by the single –CHBr- proton (n = 1 leads to 2 peaks)


cyclohexane

a singlet 12H

isopropyl chloride

a b aCH3CHCH3 Cl

a doublet 6Hb septet 1H

THE ISOPROPYL GROUP: EQUIVALENT NEIGHBORS

PEAK INTENSITY VARIATION

ALCOHOLS

C13 NMR: DIFFERENCES FROM 1H NMR

Signal for a carbon atom is 10-4 times weaker than hydrogen

Carbon signals are spread over a much wider range

All signals appear as singlets. Signal intensity varies and can not be used to

determine how many carbon atoms are responsible for each signal

13C – nmr 13C ~ 1.1% of carbons

1) number of signals: how many different types of carbons

2) splitting: number of hydrogens on the carbon

3) chemical shift: hybridization of carbon sp, sp2, sp3

4) chemical shift: evironment

Factors that affect chemical shifts:1. Chemical shift affected by nearby electronegative atoms

• Carbons bonded to electronegative atoms absorb downfield from typical alkane carbons

2. Hybridization of carbon atoms• sp3-hybridized carbons generally absorb from 0 to 90 d• sp2-hybridized carbons generally absorb from 110 to 220 d• C=O carbons absorb from 160 to 220 d

CHARACTERISTICS OF 13C NMR SPECTROSCOPY

2-bromobutane

a c d b CH3CH2CHCH3 Br

13C-nmr

13C spectrum for butan-2-one Butan-2-one contains 4 chemically nonequivalent

carbon atoms Carbonyl carbons (C=O) are always found at the low-

field end of the spectrum from 160 to 220 d

CHARACTERISTICS OF 13C NMR SPECTROSCOPY

At what approximate positions would you expect ethyl acrylate, H2C=CHCO2CH2CH3, to show 13C NMR absorptions?

PRACTICE

PREDICTING CHEMICAL SHIFTS IN 13C NMR SPECTRA

Solution Ethyl acrylate has five distinct carbons: two different

C=C, one C=O, one C(O)-C, and one alkyl C. From Figure 11.7, the likely absorptions are

The actual absorptions are at 14.1, 60.5, 128.5, 130.3, and 166.0 d

SOLUTION

PREDICTING CHEMICAL SHIFTS IN 13C NMR SPECTRA

NMR S PECTROSCOPY. Electron has a spin and that spinning creates and associated magnetic field....

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