NMR - Spectropy I Flashcards
isotopes
different numbers of protons
have spins
no magnetic field=
no spin
nuclei with magnetic field
spins = aligned same direction/ with or against the field
the energy difference between two spin states is
small
resonance = when nucleus is subjected to the right combination of magnetic filed and electromagnetic radiation the spin can flip
detected by what?
back and forth
detected by NMR
after alignment what happens
the nuclei gets excited
hv = AE
^ energy ^ magnetic force
then radiation is introduced =
energy back and forth
Simple H1 NMR
difference between where absorbance happens - different shielding depends on absorbance
What is shielding?
magnetic forcefield exerted by electrons: e- circulating and generating small induced magnetic field that opposes the external field (B0)
What happen if shielding is decreased?
felt by the nucleus - results in a stronger filed being NEEDED for resonance to occur
Electronegative atoms (O2) ____ e- away from near atoms =
pull
deshielding them
Shilded atoms need higher field energy to cause
resonance
What is the chemical shift?
difference in magnetic filed strength between the resonance field of the proton being observed and that tetramethylsilane measured in parts per million (ppm)
chemical shift definition
the resonant frequency of an atomic nucleus relative to a standard in a magnetic field.
does the chemical shift stay in a given proton?
yes, regardless of operating field and frequency of the spectrometer
chemical shift =
shift downfield from TMS (Hz) / spec frequency (MHz)
1) electronegativity
can cause deshieling of neighbouring atoms as the - are drawn towards the MORE electronegative of that atom
2) distance from e- withdrawing effect
further away
less deshielding
3) addition of e- withdrawing groups
can decrease the electron density at the nucleus, deshielding the nucleus and result in a larger chemical shift
4) other groups with deshielding effects
- aromatic rings
- aldehydes - carbonyl groups (deshielding)
- OH / NH - hydrogen bonding = broad peaks absorbed
- COOH - C=O and O are involved
number of signals depends:
can figure out how many protons there are in each peak = proportional
e.g. peak - 3.2 = 11.5 mm ( if other peak is 1/3 of original peak =
share with 3 = it is in proportion
measure peak to make sure to get ratio
spin-spin splitting
protons from different environments can influence the magnetic filed of each other they are close to one and other
results in splitting of their peaks as seen in the NMR spectrum - chemically equivalent protons do not split each other
1 protons next to 2 protons what will happen to splitting
n+1
1 will be 2 (1+1=2)
2 will be 3 (2+1=3)
splitting will NOT be observed if more
that 3 bonds exist between protons
why does signal appear thicker?
extra protons - opening and lengthening
coupling constants
distance between the peaks of multiplets measured in Hz
do not vary with the field strength of spectrometer
coupling constant are used to distinguish between the isomers of compounds incl.
stereoisomers
are 2 signals equivalent or not?
homotropic
ditropic
heteroprotic
READ
organic chemistry, Wade and Simek, 9th Ed - chapter 13
Can academy
