Exam 2

Question 1

pulsed-Fourier Transform

Answer 1

• Standard method for recording NMR
• Sample placed in NMR probe in magnetic field and irradiated with short pulse of high-power radiofrequency
• pulse excites all nuclei of a given type in the sample-specific
• excited spins precess around the external magnetic field together creating current in receiver coil of NMR probe
• Resulting signal is recorded and digitized by computer

Question 2

Relaxation

Answer 2

-establishment or re-establishment of equilibrium state of nuclear spin magentization

Question 3

Longitudinal Spin Relaxation (T1)

Answer 3

• Allow z component of nuclear spin magnetization vectors to re-establish equilibrium according to Boltzmann distribution
• Determines how long he experimentalist must wait after an FID before repeating the process to acquire another FID
• Long T1 = takes relatively long time for magnetization along the z axis to be established

Question 4

Spin-spin relaxation (T2)

Answer 4

• enables net magnetization in the xy plane to decay to zero (equilibrium) after an RF pulse
• slower T2 = longer FIDs and sharper NMR peaks

Question 5

Requirements for Modern NMR instrumentation

Answer 5

• Field greater than 100 MHz
• Frequency field stability– come with superconducting magnets
• Field homogeneity– uniform as possible
• computer interface

Question 6

Nuclear Overhauser Effects (NOE)

Answer 6

• in everything with NMR

- Results from type of nuclear spin relaxation

Question 7

H NOE

Answer 7

• through-space effect which decreases with the sixth power of distance between the spins
• relies on dipolar interaction between spins
• utility is to determine which protons are close in space to which other protons

Question 8

C NOE

Answer 8

• net effect is enhancement of peaks whose carbon atoms have directly bonded protons
• large reduction in the time needed to obtain a decoupled spectrum as compared with coupled spectrum
• erratic contribution make peak intensities unrelated to number of 13-C nuclei

Question 9

DEPT

Answer 9

• proton detectant increased sensitivity

- Distortionless Enhancement by Polarization Transfer

Question 10

Improvements to S/N

Answer 10

• increase sample concentration
• number of scans (S/N proportional to square root of number of scans)
• increase magnetic field
• dynamic nuclear polarization
• cooling NMR probes

Question 11

Ideal Solvent

Answer 11

• contain no protons
• inert
• low boiling point
• inexpensive
• usually deuterated –> H wont show up

Question 12

Sample Handling conditions

Answer 12

• 5-10 mg of compound
• 0.5 mL of solvent in a 5 mm NMR tube
• no ferromagnetic impurities
• look out for common solvent impurities: greases, plasticizers

Question 13

Key Information to Extract form H NMR

Answer 13

• Number of signals in spectra
• chemical shift values
• integration of signal
• splitting patterns