L39 - NMR And MRI - Advance Interpretation

Question 1

How can you measure coupling?

Answer 1

• measure a, measure b
• measure spliiting for a, how far a ppm is for b
• divide a by b, multiply by the Hz per ppm

Question 2

What is coupling constant?

Answer 2

Protons couple to each other, with certain intensity (vary from 0 to 18 Hz, usually ~ 7)

Question 3

What happens when a molecule contains a proton which is coupled to 2 different protons with different coupling constants?

Answer 3

Different splitting pattern

Question 4

What happens to the coupling constant when there’s a freely rotating alkane?

Answer 4

There’s am observed average across the different coupling constants

Question 5

What are coupling constants like in alkenes?

Answer 5

• differ depending on cist or trans
• trans - quite large (12-18Hz)
• cis - smaller (8-11Hz)

Question 6

How does symmetry in an aromatic systems affect NMR?

Answer 6

• 2 Ha protons = 2 Hb protons
• chemically equivalent, same chem shift, same coupling
• different couping pathway, JHH not the same

Question 7

What is coupling like in simple aromatic rings?

Answer 7

• size of coupling depends on orientation of protons to each other
• coupled by 2C (ortho) = JHH 8-10Hz
• coupled by 3C (meta) = JHH 1-2.5Hz
• coupled by 4C (para)= JHH 0.25-0.50Hz (not observed)
• meta coupling (3C) aka W coupling

Question 8

What is 13C-NMR like compared to H-NMR?

Answer 8

• had 1.1%natural abundance
• 1/2 nucleus
• 400 times less sensitive than H nucleus
• dont usually see 13C-13C coupling
• chem shift 0-220ppm
• chem shift measured with respect to tetramethylsilane
• simalar factors affect chem shift
• long relaxation times = no integrations
• broadband, proton decoupled = single lines
• number of peaks = number o types of C atom

Question 9

What is broadband decoupled like?

Answer 9

• one bond C-H coupling retained
• signal for particular carbon determined bt number of attached protons in accord with n+1 rule

Question 10

What is 13C chemical shift like?

Answer 10

• electronegativity of groups attached to C atom
• Hybridisation of C

Question 11

What is fully proton decoupled 13C NMR like?

Answer 11

• rapidly collected
• appear as single peaks irrespective to num of protons connected to C
• same height unless originate from 2 chemically and structurally equivalent protons

Question 12

What is distortionless enhancement of polarization tranfer (DEPT)?

Answer 12

• used to give similar info to off-resonance decoupled spectra
• sequence of pulses with various delays, used to create DEPT
• dif peaks respond in different fashions, based on num of protons attaches

Question 13

What are the 2 DEPT experiments used in Bath?

Answer 13

• DEPT-135
• DEPT-90

Question 14

What is DEPT-135 like?

Answer 14

• CH and CH3 = positive peaks
• CH2 = negative peaks
• Cq = invisible

Question 15

What is DEPT-90 like?

Answer 15

• CH = yields peaks
• Cq, CH2 and CH3 = invisible peaks