CH15: NMR Spectrometry Flashcards
(28 cards)
What is NMR?
Nuclear Magnetic Resonance is the most powerful form of spectroscopy that is the study of interactions between electromagnetic radiation and nuclei of atoms
What is a magnetic moment?
A magnetic field associated with a nucleus that exhibits a “spin”
What does it mean if a proton experiences nuclear spin?
A proton that experiences spin does not refer to the rotation of the particle but has a rotating sphere of charge called a magnetic moment. A nucleus must have an odd number of protons/neutrons for this
What are alpha and beta spin states?
When a proton of an atom is subjected to an external magnetic field..
Alpha: proton aligns with field
Beta: proton aligned against
- Quantifiable ΔE
Is NMR the same as resonance?
NO. NMR is when a proton in alpha experiences elecmag. radiation, absorption happens if energy of photon is = energy gap, causing a flip to beta
Define diamagnetism
Circulation of electron density occurs near external magnetic field. Produces induced magnetic field which opposes the other
Do all materials have diamagnetic properties?
YEAh. Without it, all protons would have same amount of rf radiation and NMR useless
What is a shielded proton or carbon.
Protons or carbons whose electron density is rich, net magnetic field strength approximately to external mf
What does it mean if a proton/carbon is unshielded?
Poor election density. Gap between alpha and beta spins and absorb different frequencies of rf radiation
1 tesla = ?
10,000 gauss
Three characteristics of 1HNMR spectrum?
- Location of each signal shows electronic environment
- area under signal shows number of protons
- shape of signal shows number of neighboring protons
When a proton/carbon atom are chemically equivalent….
occupy identical e.e. And produce one signal (rotation or reflection)
Define homotopic
Protons are interchangeable by rotational symmetry
Define enantiotopic protons
- If the two compounds drawn are enantiomers (cannot be placed on-top each other)
- no rotational, yes reflectional
Define diastereotopic protons
-Compound drawn are diastereomers (nonsuperimposable, not mirror)
- not chemically equivalent
Are enantiotopic protons always chemically equivalent?
-only when solvent is achiral (superimposed, mirrors)
- if solvent is chiral (no superimposed, mirror) will produce signal
Chemical shift
Location of signal (relative to frequency of absorption of reference compound tetramethylsilane (TMS))
What is frequency formula for NMR spec?
-NMR spec contains small amounts TMS in its deuerated solvents, produces signal at low freq
(Observed shift TMS)/(freq of instrument) = [ppm]
Where do most organic compounds observe a chemical shift?
0ppm - 12ppm
Integration (NMR)
Area under signal determines number of protons giving rise to signal
Multiplicity(NMR)
- Number of peaks in signal due to the magnetic effects of the number of neighboring protons
- singlet, doublet, triplet, quartet, quintet
Define coupling
Nonequivalent protons connected to adjacent carbons which multiplicity of each signal affects each other
The affect of a doublet for an Ha proton
-Hb proton aligns
-Hb proton against
The affect of a triplet for an Ha proton
- 2 Hb proton align
- 2 Hb proton against
- 1 aligns, 1 against
