Dynamic Effects in NMR

The timescale in nmr is fairly long; processes occurring at frequencies of the order of chemical shift differences will tend to average out.

For a simple exchange process coalescence /21/2

This suggests that if proton spins can be made to change of the order of 20 to 40 Hz, coupling could be averaged out and its effects eliminated (recall the decoupling observed in the alcohol OH)

Effects of a Resonance frequency on a Nuclear Spin State

2

Irradiate 2 but observe 1

1

1

Processes occurring during double resonance1. Spins change2. Ratio of populations of ground and excited states 1 3. System reacts by redistributing other populations of

spin states

decoupling

Nuclear Overhauser Effect

We will return to other aspects of dynamic NMR later but first lets apply double resonance to 13C spectra.

13 C NMR Spectra

Unlike 1H nuclei, 13C are rare nuclei. The probability of finding a 13 C nucleu is approximately 1/100. The probability of finding 2 13 C next to each other is 2*.01*.01 = 2*10-4

In a molecule like n-butyl vinyl ether, the probability of finding a 13 C nucleus at any of the carbon positions is equal. The problem is that 1H will couple with 13C rendering a weak signal even weaker.

Advantage: signal to noise goes up

Disadvantage: spin coupling lost

Summary: Irradiation of the all the protons using a second broadband series of frequencies simultaneously while acquiring 13C spectrum as well causes?

Double resonance:

1. Multiplicity is lost and some structural information is lost (JCH)

2. When the protons are irradiated, the Boltzman distribution of spin states is perturbed, resulting in more H in the excited state than usual; if we apply Le Chatelier’s principle, the system responds to minimize the perturbation; if a 13C is next to one of the protons being irradiated, this perturbation results in more 13C nuclei returning to their ground state. This is a T1 process, meaning it will take a few seconds or longer (5 T1)to achieve this new equilibrium state. Once equilibrium is achieved, this leads to an enhancement of the 13C signal and is called the Nuclear Overhauser effect

NOE observed when the decoupler is left on

Gated Decoupling: using the decoupler to effect characteristic changes

in the spectrum by turning the decoupler frequency on and off at specific intervals

1. Broadband decoupling at protons; observe 13C

Effect: decoupling, NOE effect;

Gated Decoupling: using the decoupler to effect characteristic changes in the spectrum2. Gated decoupling to collape coupling without any NOE

NOE builds up with a time constant associated with 13C T1 values. If the rf frequency that irradiates the protons is left on, NOE is observed in a minute or so.

Why would you want gated decoupling without NOE?Interested in area under the curves (quantitative analysis)

3. Gated decoupling with NOE without loss of coupling; retains NOE enhancement and coupling

4. Off resonance decoupling: some coupling is retained so that the multiplicity is retained providing information regarding

neighbors; the NOE effect is partially retained; information regarding the magnitude of the JCH coupling is lost. The closer a nucleus is to the irradiating field, the more the coupling constant is reduced.

Carbon chemical shifts

The use of ACD to predict 13C NMR spectra

1. Estimation of : CH3CH2CH2CH2OCH=CH2

2. Estimation of : CHO

OC2H5

Coupling constants

in 13C NMR

Typical coupling

constants

Coupling constants in 13C NMR

2. Long range CH coupling

Coupling constants in 13C NMR

3. The relationship between hybridization and coupling constant

Coupling constants in 13C NMR

3. The relationship between hybridization and coupling constant

4. 1JCH CHCl3: 209 Hz; CH2CH2: 178; CH3Cl 150; CH2=CH2 156 Hz

cyclopropane

Measurement of T1’s

In a pulse experiment, if the rf field is left on long enough, the magnetization can be tipped 90°. What happens if the strong rf field is left on longer?

N

S

Before the rf pulse

rf generator

signal coil

N

S

Just after a 90 ° rf pulse

pulse width = τ

Just after a 180 ° pulse; no signal generated in detector coil

pulse width 2 τ

NS

The result of applying a second short rf pulse shortly after the 180° pulse

S N

Weak rf pulse turned off

1. Measure the signal immediately after the 180 ° pulse by using a second weak pulse to tip the nuclei and generate a signal in the xy plane. Wait 5 T1

2. Repeat the experiment, now waiting seconds after the 180 ° pulse.

3. Vary

= 0 after 180 ° pulse and weak second pulse

= 5T1

repeat but wait sec before second pulse

wait 5 T1

repeat varying

population of ground and excited states are equal

Inversion recovery method is a way of measuring T1

The decrease in intensity and then buildup again is a first order rate process. The change in ln(magnetization) plotted against time results in a straight line. The slope of the line is the rate constant and 1/slope = T1

Any other uses ?

Solvent suppression: T1’s for small molecules such as solvents are usually longer than for other nuclei for both 13C and 1H

3-fluoroalanine

Measurement of T2 Spin Echo Technique

Suppose we give a 90 rf pulse

to a set of identical uncoupled

nuclei. Magnetization is developed

in the xy plane. After a period τ

a 180 ° pulse is given. An echo is

observed at 2 τ

rf generator

signal coil

signal coil, rf generator

N

S

1. apply 90 Hrf pulse

rf generator

signal coil

signal coil, rf generator

N

S

2. apply 2nd 180° pulse

red: faster rotating

blue: slower rotating

rf generator

signal coil

signal coil, rf generator

N

S

1. apply 90 Hrf pulse

2. apply 2nd 180° pulse

blue: faster rotating

red: slower rotating

rf generator

signal coil

signal coil, rf generator

N

S

1. apply 90 Hrf pulse

2. apply 2nd 180° pulse

blue: faster rotating

red: slower rotating

Suppose that we repeat this experiment varying the length of of time between the original pulse and the second 180 ° pulse.

The intensity of the spin echo will decrease as a result of magnetic inhomogeneity and this decrease will follow first order kinetics. The reciprocal of the rate constant is equal to T2

Now consider a 13 CH fragment. The 13 C will signal will be a doublet due to the fact that half of the H’s will be and the others will be . Suppose our rotating frame of reference is at the chemical shift of the 13 C. Some of the magnetization of the 13 C signal will be moving J/2 faster than our rotating frame and half will be moving J/2 slower.

Chemical shift of 13 C

= 0 = Ta

= 3Ta

Observing a CH

= 0 = Ta

=6Ta

180° pulse = 0

= 2Ta

A spin echo 180 °out of phase will be observes at Ta later

Following an initial 90 ° pulse

180 ° pulse

The phase of the spin echo of a 13 CH can be both positive and negative.

The spin echo of a 13 C is always has the same phase (quaternary carbon)

Lets now consider a 13 CH2 and use for our rotating frame the chemical shift of the

13 C

= 0 = Ta = 3Ta

= 0

180 ° pulse

= 0

Net magnetization never out of phase

This forms the basis of the DEPT experiment also called APT and other similar experiments. It recovers the information lost when using broadband decoupling (ie. The number of attached protons)

Summary

Quaternary carbons and CH2 behave differently from CH and CH3 groups.