Experiment 3- NMR Spectroscopy

Question 1

What technique is used to visualize the molecular structure of chemical compounds?

Answer 1

NMR

Question 2

How does NMR work? (PRELAB Question)

Answer 2

It uses radio waves to excite the nuclear spun, which provides information on the chemical structure.

Question 3

In which common setting is NMR used in?

Answer 3

MRI scanning in hospitals.

Question 4

NMR spectroscopy relies not he use of a strong magnet. Which of the following statements are true? (You may select more than 1 option).

Answer 4

A) the strong magnet is only needed for 1H NMR experiments and not 13C NMR.
B) the strong magnet is generally safe for chemists and patients to use unless they wear metallic medical devices or jewelry.
C) the strong magnet removes magnetic metals from the sample/subject that could interfere with the analysis.
D) the strong magnet differentials the energies of nuclear spin states.
E) the strong magnet absorbs IR radiation.

ANSWER: B and D

Question 5

The _____ signal results from exiting the hydrogen nuclei. The signal decays with time and is said to be in the ______. This data is ______ into a spectrum in the frequency domain whereby the x-axis in units of relative frequency (ppm) called the chemical shift.

Answer 5

Free Induction Decay (FID). Time domain (x-axis being the time). Fourier Transformed.

Question 6

What are the units of ppm?

Answer 6

Hz/MHz

Question 7

Thing with a higher chemical shift are said to be ______ and _____ while things with a lower chemical shift are said to be _____ and _____.

Answer 7

Downfield/Deshielded; Upfield/Shielded.

Question 8

Is a 1.2ppm signal higher in frequency than a 4.7ppm signal?

Answer 8

No

Question 9

What does chemical equivalence refer to?

Answer 9

It means that hydrogens and carbons are respectively identical due to symmetry, whether it be due to rotation, axis of symmetry, or reflection.

Question 10

What is something to note about the chemically identical hydrogens/carbons?

Answer 10

Have the same chemical shift and do not couple to each other.

Question 11

What does it mean for hydrogens to be enantiotopic?

Answer 11

They have one element of symmetry, making them chemically identical.

Question 12

What’s the reason for hydrogens in a methyl group being chemically equivalent?

Answer 12

Free rotation around the C-C bond.

Question 13

What does it mean for hydrogens to be diastereotopic?

Answer 13

It means that there’s a stereo center in the molecule, making all the hydrogens in a CH2 chemically INEQUIVALENT.

Question 14

What information does integration tell us?

Answer 14

The number of hydrogens responsible for giving rise to the peak.

Question 15

How do we determine the number of hydrogens that correspond to a signal based on provided step curves?

Answer 15

The height of the step curve corresponds to the # of present hydrogens. The greater the height, the more hydrogens present.

Question 16

What is the equation for determining multiplicity?

Answer 16

Multiplicity=2nI+1, where n is the number of neighboring molecules that are spin active, I is the number of spin states.

Question 17

Define Coupling Constant

Answer 17

The magnitude of splitting is called the coupling constant.

Question 18

What does 3J AB signify?

Answer 18

The number 3 indicates that the hydrogens are separated by 3 sigma bonds, AB signifies HA and HB being coupled to one another.

Question 19

What happens to the magnitude of the coupling constant with increasing distance?

Answer 19

It goes down; there is an inverse relationship between the two.

Question 20

How do you determine the coupling constant from the chemical shifts provided?

Answer 20

Subtract the chemical shifts, giving you value in ppm. PPM is in Hz/MHz. So, from there, you multiply the value in ppm by the MHz value of the instrument/magnet, giving yo the value of the coupling constant.