Ch.13 Flashcards
describe 1H NMR
lec 13, slide 3
- the number of samples tells us how many unique hydrogens there are in a sample
- each signal corresponds to each chemical environment (most of the time)
- must use “deuterated” solvents deuterium (i.e. hydrogen-2) does not show up in proton NMR
- proton NMR signals can undergo splitting
what is a chemical shift
When a nucleus experiences different local electron densities due to nearby atoms or functional groups, its resonant frequency is altered relative to a reference standard, usually tetramethylsilane in organic solvents. This alteration is expressed in parts per million (ppm) and is known as the chemical shift.
complete the practice problem on lec. 13, slide 6
demonstrate where EWGs fall vs. EDGs fall on a shift chart
complete lec. 13, slide 7
EWGs downfield (higher ppm)
EDGs upfield (lower ppm)
where do pi electrons fall on a ppm shift
lec. 13, slide 8
a triple bond, for example, is an EWG so why would it appear so far upfield (to the right)?
- when you have a pi system, you generate another electromagnetic field which decreases ppm
what is an integration and why is it important to draw? what symbol/s are used to depict them?
integrations tell you the area under a peak, which translates to how many hydrogens are there.
- denoted by the S looking antiderivative symbol (the bigger the symbol, the greater the area under the peak). also denoted under the peak.
what does it mean if the integration is 3.1 or 2.9?
the area under the peak is still 3, but it suggests that other impurities are present
is O EWG or EDG
EWG via induction
what happens if there is a plain of symmetry?
lec. 13, slide 11
if two groups are symmetrical and are connected to the same atom and are in the same space, the hydrogens on both groups are the same
what is splitting
the magnetic interactions between neighboring, non-equivalent NMR-active nuclei
what is spin-spin coupling
describes the magnetic interactions between non-equivalent hydrogen atoms that are separated by 2 or 3 sigma bonds. The nearby protons have a magnetic moment that can be either against or with the external magnetic field; therefore, the energy levels of the protons whose signal is being observed are split, and this results in the splitting of the signal into multiple peaks (the terms ‘splitting’ and ‘coupling’ are often used interchangeably when discussing NMR).
what is the splitting rule?
lec 13. slide 14
n + 1 rule –> this is only for very simple systems –> the more complex, the less it works
how do you determine the splitting value
lec 13. slide 14
lec 13. slide 20
add all of adjacent carbons and add one (n+1 rule)
what is the effect of the spin of two split protons
lec 13. slide 15
when spin is with the overall current of the system –> H feels a stronger field (downfield)
when spin is against the overall system –> H feels a weaker field (upfield)
explain a splitting tree
come back to
do equivalent protons couple?
lec 13. slide 17
no!
how do you name splitting peaks 0-6
vicinal proton = name
0 = singlet (s)
1 = doublet (d)
2 = triplet (t)
3 = quartet (q)
4 = pentet
5 = sextet
6 = septet
what is a coupling constant
J (usually in frequency units, Hz) is the difference between two neighboring protons that split each other –> these protons have the same J value
what does lec. 13 slide 19 mean
where does the reference signal for CDCl3 occur and how does the intensity of the peak vary
at 7.27
- varies in height as the more concentration, then the larger the signal is
(Deuterated chloroform)
homotopic vs. heterotopic
lec 13, slide 20
Homotopic molecules are often described as twin structures because the orientation of the hydrogen molecule is the same no matter how it is located about the central carbon atom, as with the hydrogens of a methane molecule. Heterotopic hydrogens occur when the two molecules under comparison form constitutional isomers.
what happens to ppm when shielding is increased
lowers ppm; upfield
go over ALWS for benzene nmr
how does electron density effect shielding
electron rich = lower ppm = EDG = shielded
what is the housing effect
lec 13 slide 21
tall peak/s in the middle and it gets smaller from there –> indicates they are coupling together –> they are pointing to each other
what is the chemical exchange for acidic hydrogens
lec 13 slide 23
groups with a pka lower than 20 can exchange (happens with hydrogens on EN elements, amines, carboxylic acids, alcohols)
what is the most acidic hydrogen that can undergo exchange on CH3OH
lec 13 slide 23
the OH hydrogen is the most acidic hydrogen
for CH3OH which hydrogen will be upfield vs. downfield
lec 13 slide 23
CH3 hydrogens are more upfield than the OH hydrogen
describe the chemical exchange with D2O
lec 13 slide 24
CH3OH – D2O –> CH3OD
how does D2O exchange effect the D2O peak present on the nmr?
with D2O exchange the peak completely goes away
what is conformational equilibrium
the dynamic process where a molecule interconverts between different conformers, or spatial arrangements of its atoms, that are related by rotations around single bonds.
do the practice problem on lec 13 slide 26
do practice problem on lec 13 slide 27
describe the product of a deuteron exchange of homotopic protons
lec 13 slide 29
D is added at the ends on both sides (same molecules) –> creates an enantiomer
describe the product of a deuteron exchange of heterotopic protons
lec 13 slide 29
D is added at the end of one side and in the center (different molecules) –> creates a diastereomers
draw enantiotopic protons for the deuteron exchange of CH3CH2CH2CH3. would you get two different signals between the enantiomers?
lec 13 slide 30
the nmr would not be able to tell the difference between the enantiomers (they would still have the same signal)
draw diasterotopic protons for the deuteron exchange of CH3CH2CH(OH)CH3. how many signals would you expect from the hydrogens?
lec 13 slide 30
there is a chirality center
- you will get two different signals
for CH3CH2CH(OH)CH3, how do the CH2 protons differ
lec 13 slide 31
the H closer to the oxygen is in a different environment from the H closer to the CH3, but the difference may be very small
is benzene EWG or EDG
EDG
for the compound on lec 13 slide 32, why is Hb in a different enviornment from Ha
the bond is more restricted and doesnt rotate
when does splitting not occur
lec 13 slide 34
when…
1. inequivalent protons split differently
2. ortho, meta, and para coupling is a thing
3. resonance based shifts
for a benzene ring with an O, where are the protons signals
lec 13 slide 34
the proton closest to the oxygen has the lowest signal because the ortho position makes it close to a resonance donor
does splitting occur between Ha and Hd
lec 13 slide 35
Ha is split by Hb and Hc
Ha and Hd could split, but most instruments can’t see this
what does a doublet vs. a doublet of doubles vs. a doublet of doublet of doublets look like
lec 13 slide 35
describe carbon-13 nmr
carbon-13 is an active nucleus
- the spectrum shows the total number pf unique carbons in a sample
what is an active nucleus
NMR active nuclei are those possessing a property called ‘spin’, whereby a charged nucleus spins about an axis and generates its own magnetic dipole moment.
how many carbon signals would be shown from CH3CH2CH(OH)CH3
lec 13 slide 37
4 signals –> 4 unique carbons
how does symmetry effect signals of a carbon nmr
lec 13 slide 39
fewer signals than carbons –> see example
how many carbon signals would a benzene have
lec 13 slide 40
1 signal –> due to symmetry
do the practice problem on lec 13 slide 41
why does shielding occur
- when the number of electrons increases (or the nucleus is in a more electron rich environment), the shielding effect increases.
- when the shielding increases, the effective magnetic force felt by the nucleus decreases.
- when the effective magnetic force felt decreases, the frequency necessary to achieve resonance decreases (hence, the peak appears at lower ppms on the right).
what is the general rule for NMR
the number of signals = the number of unique chemical environments
what is homotopic
same protons; symmetry –> 1 signal
what is heterotopic
different protons; asymmetry –> 2 signals
draw the difference between heterotopic and homotopic
see image
enantiotopic after exchange with solvent
1 chirality center after exchange with solvent (D2O) –> 1 signal
diastereotopic after exchange with solvent
2 chirality centers (1 already present) after exchange with solvent –> 2 signals
draw the difference between enantiotopic and diasterotopic
use image
what are delta scale / shifts
has to do with shielding
shielding
greater electron density around nucleus
EDG = lower ppm = electron rich
deshielding
EWG = higher ppm = electron poor
draw deshielding vs. shielding
see image
can the integral value be scaled
no - 1.5 can go to 3 or 6 depending on the H structure
draw an example of protons that split each other
see image (ex. cis vs. trans)
what are the ortho coupling rules
n+1 generally
- homotopic / enantiotopic (same protons) –> do not split each other
no adjacent hydrogens = signlet
1 adjacent = doublet
draw the difference between ortho, meta, and para coupling
what are the meta coupling rules
n+1 generally
only occur on benzene / aromatic systems
1 meta adjacent = ____(et) doublets
2 meta adjacent = _____(et) triplets
what are the solvent effects on NMR
TMS peak –> 0 ppm
COCl3 peak –> 7.3 ppm (singlets)
D2O peak –>
explain why deuterium signals do not show up on HNMR
- the hydrogen on an alcohol is replaced by D (ex HOCH2CH3 –> DOCH2CH3), so the D is not detected by the NMR (one less peak overall)
explain the use of deuterium to substitute hydrogens
used to figure out what peak is the alcohol (whichever one disappears is the alcohol)
does carbon NMR have coupling / splitting
no
what is DEPT
DEPT (Distortionless Enhancement by Polarization Transfer) ¹³C NMR Spectroscopy is a powerful technique used in organic chemistry to explain the structure of organic molecules.
what are the DEPT peaks
DEPT-0 = regular NMR
DEPT-45 = CH, CH2, CH3 peaks shown (no CH4)
DEPT-90 = CH onlu
DEPT-135 = CH, CH3 (positive –> peaks point up), CH2 (negative –> peaks point down)
1D vs. 2D NMR
-1D NMR can be used for simple organic compounds while two dimensional NMR can beused for complex organic compounds. -2D NMR gives information about the chemical shift, coupling constant and signals of compounds, while 1D doesn;t provide these informations.
what is 2D/3D NMR
they plot C13 NMR with H1 NMR for the determination of the structure of the molecule
how do cis and trans J values differ
cis –> 6-11 Hz
trans –> 11-18 Hz
