NMR stuff

Question 1

science behind NMR

Answer 1

• nuclei of atoms usually spin in a random direction, unless in a strong magnetic field
• radio waves will flip spins, and difference between spin up and spin down gives info on functional groups nearby

Question 2

elements of NMR

Answer 2

• powerful superconducting magnet (surrounded by layers of liquid helium and liquid nitrogen)
• powerful radiofrequency transmitter
• detector for emitted radio waves

Question 3

how is emission measured in NMR?

Answer 3

1. pulses of radio waves flip spins
2. nuclei emit radio signals as they flip back
3. emissions are measured

Question 4

3 main pieces of info from NMR

Answer 4

1. integration: area under peaks (how many of each type of proton)
2. splitting: how many peaklets are there (how many neighboring protons there are)
3. chemical shift: where peaks show up (depends on functional groups)

Question 5

signal integration

Answer 5

• area under peak indicates relative number of protons
• in symmetrical molecules, same kinds of protons attached to different carbons can integrate!

Question 6

splitting

Answer 6

• multiplicity: peaks are split into peaklets depending on how many nearby H spins there are
• multiplicity = n + 1, where n is the number of equivalent neighboring protons

Question 7

chemical shift

Answer 7

• different functional groups show up in different regions
• upfield (lower frequencies on right): more shielding and harder to flip
• downfield (higher frequencies on left): less shielding and easier to flip

Question 8

NMR shifts that are common

Answer 8

• electronegative atoms pull electrons away from nearby atoms, causing a downfield shift
• sp2 and sp carbons cause a downfield shift (more significant if proton directly attached to them)

Question 9

OH and NH groups in NMR

Answer 9

• OH and NH groups can H-bond, leading to broad peaks or no peaks at all
• OH may not split neighboring protons, and vice versa
• adding D2O will make OH peak disappear completely!

Question 10

nonequivalent protons with aklenes

Answer 10

• “equivalent” protons on an alkene are nonequivalent if there’s no symmetry

Question 11

what causes splitting into peaklets?

Answer 11

• vicinial protons may be spinning in same or opposite direction, which causes slight shielding or deshielding
• when there’s >1 vicinial proton, there’s more possibilities but some are more likely (larger center peak)

Question 12

spacing of protons that split each other

Answer 12

• spacing between peaklets for each peak set should be the same
• peaks may lean towards eachother because the up/down spin is preferred

Question 13

spliting by nonequivalent protons

Answer 13

• causes complex patterns
• “doublet of doublets”: proton split 2 separate times by nonequivalent alkene protons

Question 14

13C spectroscopy

Answer 14

• way more 12C in nature, so many repeated scans must be done
• splitting is not observed
• integration doesn’t correspond to # identical carbons
• chemical shifts are similar (electronegative substituents
  and alkenes/alkynes deshield)

Question 15

chemical shifts in 13C NMR

Answer 15

0-50: sp3
50-80: sp3 near fxl group
80-100: sp
100-160: sp2
160-200: C=O

Question 16

integration in 13C NMR

Answer 16

• symmetrical molecules often have taller integrated peaks for identical carbons
• taller peaks often represent carbon atoms with more H bonds