Structure determination

Question 1

Why do nuclei in different environments absorb different amounts of energy at different frequencies?

Answer 1

∵ in differentenvironments,nuclei willfeel different magnetic fields

Question 2

What does NMR spectroscopy essentially look for?

Answer 2

Differences in energy absorption between environments

Question 3

Whatsubstance is used as a standard inNMR spectroscopy?

Answer 3

Tetramethylsilane (TMS), Si(CH3)4

Question 4

What is chemical shift(δ)?

Answer 4

The differences (in energy absorbed) NMR spectroscopy measuresrelative to the standard substance (TMS)

Question 5

Name 5reasons whyTMS is used as a standard

Answer 5

• So produces single absorption peak
• Well away from most other absorption peaks
• Inert (doesn’t react with sample)
• Non-toxic
• Volatile (easy to remove from sample) aka low boiling point

Question 6

What is a chemical environment?

Answer 6

When atoms are joined to exactly the same things

Question 7

How is NMR done?

Answer 7

substance is dissolved in a solvent in a long thin tube and placed inside the machine where it is bombarded with radiowaves in a strong magnetic field (the stronger the magnetic field the better the resoluton of the signals)

solvent used must contain no 1H atoms. Solvents contain deuterium (2H) instead as they have no nuclear spin

commonly used solvents = CCl4 and deuterated solvents like CDCl3

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Question 8

What does chemical shift depend on?

Answer 8

• what other atoms/groups are near the H
• closer the H is to electronegative atoms, the greater the shift
• the more electronegative atoms near, the greater the shift

Question 9

What does the area under peaks represent in NMR?

Answer 9

ratio of protons and how many protons there are

Question 10

What does 13 C NMR look for

Answer 10

how many different carbon environments there are in a carbon being tested

Question 11

What are the differences between H NMR and C NMR

Answer 11

Question 12

What are the problems that can occur with proton NMR?

Answer 12

• prootons can interfere with each others magnetic field = spin-spin comupling
• this causes peaks in NMR to split (because there are hydrogens near by)

Question 13

What is the spin-spin coupling rule?

Answer 13

number of hydrogens adjacent + 1

dont’ couple :

H’s in the same chemical environement

H attached to O

Question 14

What is the purpose of CCl4 and CDCl3

Answer 14

• solvent
• have no H (atoms so give no signals in spectrum) tied to either CDCl3 or CCl4
• CCl4 non polar (- good solvent for non-polar organic molecules)
• CDCl3 polar covalent molecule (– good solvent for polar organic compounds)