BT631-17-NMR_2

Although TMS can be used to determine the zero point, this practice is generally no longer

necessary as modern NMR instruments are designed to use the deuterium signal from the

solvent as a standard reference point.

In an applied field of 7.1T, the deuterium atom in CDCl3 resonates at 32 MHz, compared to

300 MHz for the protons in TMS.

Do we always need to use TMS to determine zero point?

Most organic molecules have several sets of protons in different chemical environments and

each set, in theory, will have a different resonance frequency in 1H-NMR spectroscopy.

The basis for differences in chemical shift

The chemical shift of a given proton is determined primarily by its immediate electronic

environment.

For example, the valence electrons around the methyl carbon, when subjected to B0, are

induced to circulate and thus generate their own very small magnetic field that opposes B0.

This induced field shields the nearby protons from experiencing the full force of B0, an effect

known as local diamagnetic shielding. The methane protons therefore do not experience the

full force of B0. What they experience is called effective field (Beff), which is slightly weaker

than B0.

Why nonequivalent protons have different chemical shifts?

Beff= B0 – Blocal

Therefore, their resonance frequency is slightly lower than what it would be if they did not

have electrons nearby to shield them.

For the protons, lower electron density means less diamagnetic shielding, which in turn

means a greater overall exposure to B0, a stronger Beff and a higher resonance frequency.

For example, in the methane molecule (CH4), all protons have a chemical shift of 0.23 ppm.

In methyl fluoride, CH3F, protons have a chemical shift of 4.26 ppm, significantly higher than

that of methane.

This is caused by something called the deshielding effect. Because fluorine is more

electronegative than carbon, it pulls valence electrons away from the carbon, effectively

decreasing the electron density around each of the protons.

To a large extent, then, we can predict trends in chemical shift by considering how much

deshielding is taking place near a proton. The chemical shift of trichloromethane is, as

expected, higher than that of dichloromethane, which is in turn higher than that of

chloromethane.

As the electronegativity of the substituent increases, so does the extent of deshielding and so

does the chemical shift. This is evident when we look at the chemical shifts of methane and

three halomethane compounds.

The deshielding effect of an electronegative substituent diminishes sharply with increasing

distance:

The presence of an electronegative oxygen, nitrogen, sulfur or sp2-hybridized carbon also

tends to shift the NMR signals of nearby protons slightly downfield:

Signal integration

In 1H-NMR spectroscopy, the area under a signal is proportional to the number of hydrogens

to which the peak corresponds.

The spectrum of methyl acetate The spectrum of para-xylene

For example, the two signals in the methyl acetate spectrum integrate to approximately the

same area, because they both correspond to a set of three equivalent protons.

The integration function can also be used to determine the relative amounts of two or more

compounds in a mixed sample.

If we have a sample that is a 50:50 (mole/mole) mixture

of benzene and acetone, the acetone signal should

integrate to the same value as the benzene sample,

because both signals represent six equivalent protons.

If we have a 50:50 mixture of acetone and

cyclopentane, the ratio of the acetone peak area

to the cylopentane peak area will be 1:2 (or

6:12), because the cyclohexane signal

represents twelve protons.

Hydrogen type Chemical shift (ppm) Hydrogen type Chemical shift (ppm)

Typical values for 1H-NMR chemical shifts

The chemical shifts of aromatic and vinylic protons

Some protons resonate much further downfield than

can be accounted for simply by the deshielding effect

of nearby electronegative atoms. Vinylic protons

(those directly bonded to an alkene carbon) and

aromatic (benzylic) protons are dramatic examples.

Hydrogen-bonded protons

Protons that are directly bonded to oxygen and nitrogen have chemical shifts that can vary

widely depending on solvent and concentration. This is because these protons can participate

to varying degrees in hydrogen-bonding interactions and hydrogen bonding greatly influences

the electron density around the proton. Signals for hydrogen-bonding protons also tend to be

broader than those of hydrogens bonded to carbon, a phenomenon that is also due to hydrogen

bonding.

Predicting chemical shifts in 1H NMR spectra

Methyl 2,2-dimethylpropanoate, (CH3)3CCO2CH3, has two peaks in its 1H NMR spectrum.

Strategy

• Identify the types of hydrogen in the molecule and note whether each is alkyl, vinylic or

next to an electronegative atom. Then predict where each absorbs.

Solution

• The –OCH3 protons absorb around 3.5 to 4.0δ because they are on carbon bonded to

oxygen. The (CH3)3C-protons absorb near 1.0δ because they are typical alkane-like

protons.

What are their approximate chemical shifts?

Spin-spin coupling

The 1H-NMR spectra of most organic molecules contain proton signals that are 'split' into two

or more sub-peaks. This splitting behavior actually provides us with more information about

our sample molecule.

Consider the spectrum for 1,1,2-trichloroethane.

The signal at 3.96 ppm, corresponding to

the two Ha protons, is split into two

subpeaks of equal height and area

(doublet).

The Hb signal at 5.76 ppm is split into three

sub-peaks (triplet) with the middle peak

higher than the two outside peaks. If we

were to integrate each subpeak, one would

see that the area under the middle peak is

twice that of each of the outside peaks.

What is the source of spin-spin coupling?

The source of signal splitting is a term that describes the magnetic interactions between

neighboring, non-equivalent NMR-active nuclei.

In 1,1,2-trichloromethane example, the Ha and Hb protons are spin-coupled to each other.

Let us look at the Ha signal first: in addition to being shielded by nearby valence electrons,

each of the Ha protons is also influenced by the small magnetic field generated by Hb next

door.

The magnetic moment of Hb will be aligned with B0 in half of the molecules in the sample,

while in the remaining half of the molecules it will be opposed to B0.

The Beff ‘felt’ by Ha is a slightly weaker if Hb is aligned against B0, or slightly stronger if Hb is

aligned with B0.

In other words, in half of the molecules Ha is shielded by Hb (thus the NMR signal is shifted

slightly upfield) and in the other half Ha is deshielded by Hb (and the NMR signal shifted

slightly downfield).

Let us see the Hb signal:

The magnetic environment experienced by Hb is influenced by the fields of both neighboring

Ha protons, which we will call Ha1 and Ha2.

There are four possibilities here, each of which is equally probable.

First, the magnetic fields of both Ha1 and Ha2 could be aligned with B0, which would deshield

Hb, shifting its NMR signal slightly downfield.

Second, both the Ha1 and Ha2 magnetic fields could be aligned opposed to B0, which would

shield Hb, shifting its resonance signal slightly upfield.

Third and fourth, Ha1 could be with B0 and Ha2 opposed, or Ha1 opposed to B0 and Ha2 with B0.

In each of the last two cases, the shielding effect of one Ha proton would cancel the

deshielding effect of the other and the chemical shift of Hb would be unchanged.

Now, consider the spectrum for ethyl acetate:

How many peaks and sub-peaks do you expect from this molecule?

We see an unsplit 'singlet' peak at 1.833 ppm that corresponds to the acetyl (Ha) hydrogens.

This signal is unsplit because there are no adjacent hydrogens on the molecule.

The signal at 1.055 ppm for the Hc hydrogens is split into a triplet by the two Hb hydrogens

next door.

The Hb hydrogens give rise to a quartet signal at 3.915 ppm – notice that the two middle

peaks are taller than the two outside peaks. This splitting pattern results from the spin-

coupling effect of the three Hc hydrogens next door.

n+1 rule

If a set of hydrogens has n neighboring, non-equivalent hydrogens, it will be split into n + 1

subpeaks. This is very useful information if we are trying to determine the structure of an

unknown molecule.

(1) Signal splitting only occurs between

non-equivalent hydrogens.

(2) Splitting occurs primarily between

hydrogens that are separated by three

bonds.

(3) Splitting is most noticeable with hydrogens bonded to carbon.

Three important points need to be emphasized here.

Coupling constants

For example, for a doublet in the 1,1,2-

trichloroethane spectrum, the two subpeaks are

separated by 6.1 Hz and thus it is written as 3Ja-b =

6.1 Hz. The superscript 3 tells us that this is a

three-bond coupling interaction and the a-b

subscript tells us that we are talking about

coupling between Ha and Hb.

The spin-spin coupling effect is quantified using the coupling constant (J), expressed in Hz.

The coupling constant is simply the difference between two adjacent sub-peaks in a split

signal.

Note that the coupling constant (the gap between subpeaks) is 6.1 Hz for both doublet as well

triplet).

Coupling constants between proton sets on neighboring sp3-hybridized carbons is typically in

the region of 6-8 Hz. With protons bound to sp2-hybridized carbons, coupling constants can

range from 0 Hz (no coupling at all) to 18 Hz, depending on the bonding arrangement.

The coupling constant 3Ja-b quantifies the magnetic interaction between the Ha and Hb

hydrogen sets and this interaction is of the same magnitude in either direction i.e. as Ha

influences Hb to the same extent that Hb influences Ha. Thus, it is a reciprocal coupling

constants.

Ortho hydrogens on a benzene ring couple at 6-10 Hz, while 4-bond coupling of up to 4 Hz is

sometimes seen between meta hydrogens.

Unlike the chemical shift value, the coupling constant is the same regardless of the applied

field strength of the NMR magnet. This is because the strength of the magnetic moment of a

neighboring proton, which is the source of the spin-spin coupling phenomenon, does not

depend on the applied field strength.

H-H coupling C-H coupling

Phosphate coupling

Complex coupling

When a set of hydrogens is coupled to two or more sets of nonequivalent neighbors, the result

is a phenomenon called complex coupling. For example, the 1H-NMR spectrum of methyl

acrylate:

Let's first consider the Hc signal (centered at 6.21 ppm):

The Hc signal is actually composed of four sub-

peaks.

This is referred to as a doublet of doublets.

Hc is coupled to both Ha and Hb, but with two

different coupling constants. Ha is trans to Hc across

the double bond and splits the Hc signal into a

doublet with a coupling constant of 3Jac = 17.4 Hz. In

addition, each of these Hc doublet sub-peaks is split

again by Hb (geminal coupling) into two more

doublets, each with a much smaller coupling constant

of 2Jbc = 1.5 Hz.

Why is this?

The signal for Ha at 5.95 ppm is also a doublet of

doublets with coupling constants 3Jac = 17.4 Hz and3Jab = 10.5 Hz.

The signal for Hb at 5.64 ppm is split into a doublet by

Ha, a cis coupling with 3Jab = 10.4 Hz. Each of the

resulting sub-peaks is split again by Hc, with the same

geminal coupling constant 2Jbc = 1.5 Hz. The overall

result is again a doublet of doublets.

When a proton is coupled to two different neighboring proton sets with identical coupling

constants, the splitting pattern that emerges often appears to follow the simple `n + 1 rule` of

non-complex splitting.

Ha and Hc are not equivalent (their chemical

shifts are different), but it turns out that 3Jab is

very close to 3Jbc. If we perform a splitting

diagram analysis for Hb, due to the overlap of

sub-peaks, the signal appears to be a quartet

and for all intents and purposes follows the n

+ 1 rule.

In the spectrum of 1,1,3-trichloropropane, for example, we

would expect the signal for Hb to be split into a triplet by

Ha and again into doublets by Hc, resulting in a 'triplet of

doublets'.

For similar reasons, the Hc peak in the spectrum of 2-pentanone appears as a sextet, split by

the five combined Hb and Hd protons. Technically, this 'sextet' could be considered to be a

'triplet of quartets' with overlapping sub-peaks.

Assigning a chemical structure from a 1H NMR spectrum

Exercise: Propose a structure for a compound, C5H12O, that fits the following 1H NMR data:

• 0.92 δ (3 H, triplet, J= 7 Hz)

• 1.20 δ (6 H, singlet)

• 1.50 δ (2 H, quartet, J= 7 Hz)

• 1.64 δ (1H, broad singlet)