Quiz 1 - NMR Flashcards
Proton NMR is a technique that uses ____ to study the ____ in a sample
Proton NMR is a technique that uses NUCLEAR MAGNETIC RESONANCE to study the HYDROGEN ATOMS in a sample
In an NMR experiment, shielding refers to the reduced impact of the ____ on a nucleus due to the presence of _____ around the nucleus.
In an NMR experiment, shielding refers to the reduced impact of the APPLIED MAGNETIC FIELD on a nucleus due to the presence of ELECTRONS around the nucleus
Shielding can be _____ by the presence of more electronegative groups around the nucleus.
Shielding can be DECREASED by the presence of more electronegative groups around the nucleus.
A shielded nucleus appears further ____ whereas a deshielded nucleus appears further ____ in an NMR spectrum.
A shield nucleus appears further UPFIELD whereas a deshielded nucleus appears further DOWNFIELD in an NMR spectrum.
splitting
The appearance of multiple peaks for a single nucleus due to neighboring nuclei is called this
integration
The area of a signal that corresponds to the number of nuclei at that frequency is called this
The end of the NMR spectrum far from the reference signal is called this
downfield
The end of the NMR spectrum close to the reference signal is called this
upfield
chemical shift
The frequency at which a nucleus absorbs and resonates with a magnetic field is called this
- in ppm
what type of id test is NMR?
qualitative and quantitative
How does NMR work?
- Nuclei have spin property from intrinsic angular momentum
- Moving charges generate magnetic fields based on number of protons
- Must have net +/- charge to be active (not even N and p)
alpha VS beta spin states
Alpha = low energy and parallel H’s
Beta = high energy and anti-parallel H’s
NMR procedure
- Expose sample to external magnetic field.
- Separate protons into alpha and beta spin states.
3.Measure the difference between spin states - Energy difference associated with different frequencies of magnetic radiation
- Electromagnetic radiation excites protons from A to B spin states
- Protons relax down to A state and emit energy in the process
7.Energy at a specific resonant frequency which is detected and plotted for NMR spectrum
***Energy depends upon chemical environment of nuclei.
number of signals
Number of chemical environments
Unique groups of chemical equivalent H-atoms
deshielded
Deshielded (less ED) ⇒ more EN atoms close by (> +)
Downfield with larger ppm
shielded
Shielded (more ED) ⇒ less EN atoms close by (> -)
Upfield with smaller ppm
standard 0 ppm value
TMS (tetramethylsilane)
Chemical shift in rings
remember ortho, para, and meta positions being controlled by EWG (more EN) vs EDG (less EN)
Factors that affect chemical shift
- ED ⇒ inductive withdrawing from EN atoms/nearby (+/-)s OR resonance
- H-bonding
- Solvent effects
- Rings
splitting patterns
- Signals are split into multiple peaks based on the number of adjacent H-atoms (3 bonds away)
coupled
- Occurs when H’s in one environment interact with H’s in a different chemical environment
- spilt into multiple peaks
- *** H’s cannot couple with H’s in same chemical environment
n + 1 rule
- The number of peaks is equal to the number of H’s 3 bonds away plus one
*can only occur through hydrocarbon bonding - used to determine the number of peaks per signal (signal pattern)
Degrees of Unsaturation (DU)
-How many rings/pi bonds are present in a molecule
Equation = 2(carbons) + 2 - nitrogens - hydrogens - halogens2
J (coupling constant)
the distanced between coupled peaks
- larger distance = larger J in Hz
