3.3.15 Nuclear Magnetic Resonance Spectroscopy

Question 1

What does NMR analysis provide information on?

Answer 1

The positions of 13C and 1H atoms in a molecule.

Question 2

What is the reference compound all samples are measured against?

Answer 2

TMS, tetramethylsilane.

Question 3

Where does the reference sample show up on the spectra?

Answer 3

At 0 ppm.

Question 4

What is the x and y of an NMR spectra?

Answer 4

Y = absorption
X = chemical shift, ppm

Question 5

Why is TMS used as a reference?

Answer 5

Non toxic, doesn’t react with the sample, easily separated from the sample due to a low BP, produces one strong sharp absorption peak on the spectrum.

Question 6

What is the formula for TMS?

Answer 6

Si(CH3)4

Question 7

What is the area under the peak?

Answer 7

The number of protons in a particular environment.

Question 8

What causes a chemical shift of the absorption?

Answer 8

Different environments.

Question 9

What are chemically equivalent protons?

Answer 9

Protons in the same chemical environment.

Question 10

Each peak on an 1HNMR spectra relates to what?

Answer 10

To the protons in the same environment.

Question 11

What do peaks on a low resolution NMR spectrum refer to?

Answer 11

The molecular environments of an organic compound

Question 12

The strengths of the absorption lines are proportional to what?

Answer 12

The number of equivalent 1H atoms.

Question 13

What is the splitting pattern of each peak determined by?

Answer 13

The number of protons on neighbouring environments.

Question 14

What does the number of peaks a signal splits into =?

Answer 14

n + 1

Question 15

What information do splitting patterns give?

Answer 15

Relative intensities.

Question 16

The NMR spectronomer measures the area under each peak as what?

Answer 16

An integration spectra.

Question 17

What is spin-spin splitting caused by?

Answer 17

A protons spin interacting with the spin stated of nearby protons in different environments.

Question 18

What does spin spin splitting provide information on?

Answer 18

The number of protons bonded to adjacent carbon atoms.

Question 19

For a proton with n protons attached to a carbon atom, the number of sub peaks in a splitting pattern =?

Answer 19

n + 1

Question 20

Splitting patterns must occur in what? Why?

Answer 20

Pairs, because each proton splits the signal of the other.

Question 21

What reference compound is used for 13CNMR?

Answer 21

TMS

Question 22

On a 13CNMR spectra what are the peaks with different chemical shifts representative of?

Answer 22

The non-equivalent carbon atoms.

Question 23

Does 13CNMR produce splitting patterns?

Answer 23

No.

Question 24

In 13CNMR height is what?

Answer 24

Not proportional to the number of carbon atoms.

Question 25

What solvents are samples dissolved in? Why?

Answer 25

Deuterated solvents or CCl4. Have even mass numbers so they have no spin and don’t interfere with 1HNMR.