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Flashcards in Exam 2 Deck (13):
1

pulsed-Fourier Transform

-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

2

Relaxation

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

3

Longitudinal Spin Relaxation (T1)

-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

4

Spin-spin relaxation (T2)

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

5

Requirements for Modern NMR instrumentation

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

6

Nuclear Overhauser Effects (NOE)

-in everything with NMR
-Results from type of nuclear spin relaxation

7

H NOE

-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

8

C NOE

-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

9

DEPT

-proton detectant increased sensitivity
-Distortionless Enhancement by Polarization Transfer

10

Improvements to S/N

-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

11

Ideal Solvent

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

12

Sample Handling conditions

-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

13

Key Information to Extract form H NMR

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