Proton NMR

Question 1

Why is proton NMR used over 13C NMR?

Answer 1

• nearly all hydrogen atoms are ^`1H
• So it is easier to get a spectrum

Question 2

What causes the hydrogen atoms to resonate at different frequencies?

Answer 2

• different functional groups
• nuclei with more electrons around them are more shielded

Question 3

What is significant about the size of the peak>

Answer 3

• areas under the peak are proportional to the number of hydrogen atoms in that environment

Question 4

WHat effects the size of the chemical shift value?

Answer 4

• further away a hydrogen atom is from an electronegative atom, the smaller its chemical shift

Question 5

What is the intergration trace?

Answer 5

• area under curve proportional to number of hydrogen atoms producing
• can be difficult to evaluate by eye, so the instrument produces a line called an intergration trace
• the relative heights of the steps of this trace give the relative number of each type of hydrogen

Question 6

What does the chemical shift value show?

Answer 6

• tells you about its environment - the type of functional group it is part of

Question 7

What is the structure of TMS? What is TMS?

Answer 7

• tetramethylsilane, Si(CH₄)₄
• chemical shift value for these hydrogen atoms is zero by definition

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Question 8

How is the spectrum calibrated>

Answer 8

• liquid TMS added to samples before NMR run
• give a peak at a δ value of zero ppm
• modern techniques do not require this

Question 9

What are the other reasons for using TMS?

Answer 9

• inert
• non-toxic
• easy to remove from the sample