NMR Flashcards
(12 cards)
how does NMR work
We do this by irradiating the sample by a short pulse of radiofrequency (Rf) radiation across the whole Rf spectrum
This pushes a proportion of the nuclei into their high energy excited state
After we’ve done this, we can measure the output of energy as they flip back to their ground state
what is relaxation
After we’ve done this, we can measure the output of energy as they flip back to their ground state
what is the solvent used and why
Deuterated solvents are used (such as CDCl3 or DMSO-d6) to minimize interference from proton signals in the solvent
what is the calibration chgecmial used
or 1H NMR spectroscopy, a reference (tetramethyl silane, TMS) is added to provide an internal calibration of 0 ppm (this is the frequency at which the methyl protons of TMS resonate
why do diff hydrogen give diff values
Different 1H nuclei resonate at very slightly different values of the applied external magnetic field (B0) depending on their precise chemical environment
what 3 pieces of info does it tell us
chemical shift, intergration, splitting
what is checmial shift
Where peaks come on the graph tell us about the functional groups
what is intergration
The area under the groups of peaks (rather than the peak height) tells us how many protons are in each chemical environment.
what is splitting
tells us the relationships of groups of protons to one another
what is spin-spin coupling
t occurs as the magnetic spin of one nuclei affects that of an adjacent nucleus
2 pros
– Provides more molecular structure information cf. UV or IR
– Information on functional groups, connectivity and stereochemistry
2 cons
Expensive spectrometers (£200K to >£1M)
– Highly trained staff needed to operate spectrometers
– Analysis technique is insensitive so a larger sample is needed more than 5mg for HNMR
