Intro to NMR

Question 1

Electron (as well a protons and neutrons) posses a kind of angular momentum called?

Answer 1

Spin
If a particle has spin angular momentum the it shows magnetic properties

Question 2

What is included in the overall spin angular momentum for the nucleus (characteried by the quantum number i)

Answer 2

The angular momentum fron the individual protons and neutrons comes together to give an overall spin momentum

Question 3

What values can the quantum number l take

Answer 3

can be an interger, a half interger, or zero

Question 4

Will isotopes have the same of different value of angular momentum

Answer 4

Different isotopes of the same element have different values of i

Question 5

Nuclei with odd mass number will have what values for the quantum numberi

Answer 5

Have a half intergral spin
e.g: ¹H, ⁷Li, ¹¹B

Question 6

Nuclei with odd number of protons and odd numbers of neutrons have what values for the quantum number i

Answer 6

Intergal number
²H, ⁶Li, ¹⁰B

Question 7

Nuclei with even numbers of protons and even numbers of neutrons have what value for quantum number i

Answer 7

Have zero spin
⁴He, ¹²C, ¹⁶O

Question 8

A nucleus where i doesn’t equal 0, aligns in what orientation with respect to …

Answer 8

2i + 1 oritentation
With respect to the direction of the field, each distinguished by the quantum number mi

Question 9

What values does mi take?

Answer 9

from -i through all the way to +i

Question 10

Each of the discrete values for quantum number mi have a different …
They are known as …

Answer 10

energy
Spin states

Question 11

For a spin where i=1/2
The two orientation are…

Answer 11

Parallel and Antiparallel with the magnetic field

Question 12

NMR measured

Answer 12

The energy from transition from different spin states

Question 13

What is the basic method for NMR

Answer 13

The sample is dissolved within a suitable solvent and placed in a powerful magnetic (up to 20 T)
A short pulse of radio-wave radiation (pulse) causes a transition between spin states
NMR signal is detected as the spin return to equilibrium

Question 14

In the spectrum the NMR signal appears as a peak
They are known as

Answer 14

Resonance
The frequency of the peak is releated to the energy seperation between the spin states

Question 15

The resonance frequency depends on:

Answer 15

• the stregth of the applied magnetic field
• The isotope observed

Question 16

What value of i would not be shown on an NMR spectrum

Answer 16

i= 0

Question 17

The power of NMR spectroscopy arises from
The precise difference in energy between spin states depend on what

Answer 17

The fact that not all nuclei of the same isotope give a peak at the same frequency
The precise difference in energy between spin states depends on the local magnetic field experied by a given nucleus - which in turn depends on its chemical environment

Question 18

How does a chemical shift on a NMR comes about

Answer 18

• In response to the large magnetic field, the electrons in the surrounding bonds (that determine the chemical environment) flow so as to give a current
• In turn this current generates a local magnetic field, which opposes the applied magnetic field at the nucleus
• As a result, the nucleus experiences a reduced magnetic field, which results in a lower energy separation and a shift to lower resonance frequency

Question 19

Values of the chemical shift are measures in Hz. What do these values depend on?
Hence how do we standardise the measurements of NMR?

Answer 19

The magnetic field in the NMR spectrometer
Resonance position are reported as shift from the resonance of an agreed reference compound
Shift (ppm) is expressed as a proportion of the reference frequency so that the result is independent of the size of the magnetic field

Question 20

What is the reference compound in ¹H and ¹³C NMR

Answer 20

Si(CH₃)₄

Question 21

Each nucleus in a different environment in a molecule result in…

Answer 21

… a seperate resonance in the NMR spectrum

Question 22

Why does the chemical shift roughly correlated with the electronegativity of the substituents

Answer 22

The exact chemical shift depends on the electronic environment of the nucleus
Electrons are said to shield the nucleus but neighbouring functional groups can remove electron density from the nucleus deshields it
This results in a higher frequency and more positive value for chemical shift

Question 23

Why is the ¹³C signal much weaker than the ¹H signal

Answer 23

The natural abundance of ¹³C is only 1.1% that of naturally occuring carbon
And the most abundant isotope ¹²C doesnt have a magnetic moment