NMR Spectroscopy
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Transcript of NMR Spectroscopy
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NMR SPECTROSCOPYCHEM 212
Introduction to Spectroscopy
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Spectroscopy is the study of the interaction of matter with the electromagnetic spectrum
1. Electromagnetic radiation displays the properties of both particles and waves
2. This “packet” of wave and particle properties is called a photon
The term “photon” is implied to mean a small, massless particle that contains a small wave-packet of EM radiation/light
3. The energy E component of a photon is proportional to the frequency n
E = hn
The constant of proportionality is Plank’s constant, h
Introduction to Spectroscopy
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5. Because the speed of light (c ) is constant, the frequency (n) (number of cycles of the wave per second) can complete in the same time, must be inversely proportional to how long the oscillation is, or wavelength (l):
6. Amplitude describes the wave height, or strength of the oscillation
7. Because the atomic particles in matter also exhibit wave and particle properties (though opposite in how much) EM radiation can interact with matter in two ways:• Collision – particle-to-particle – energy is lost as heat and
movement
• Coupling – the wave property of the radiation matches the wave property of the particle and “couple” to the next higher quantum mechanical energy level
n = ___
l
c E = hn =
___
l
hc
Introduction to Spectroscopy
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8. Remember atoms and molecules are quantum mechanical particles
9. Where a photon is a wave with some particle character, matter is made of particles with some wave character – wave/particle duality
10. As a result of this, the energy of these particles can only exist at discrete energies – we say these energy levels are quantized
11. It is easy to understand if we visualize the “wave” property of matter as an oscillating string in a box—only certain “energy levels” can exist as the string is bound at both ends:
Ener
gy
The Spectroscopic Process5
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1. Irradiation: Molecule is bombarded with photons of various frequencies over the range desired
hn hn hn
5. Detection: Photons that are reemitted and detected by the spectrometer correspond to quantum mechanical energy levels of the molecule
Ener
gy
2. Absorption: Molecule takes on the quantum energy of a photon that matches the energy of a transition and becomes excited
hn4. Relaxation
rest state rest state
excited state
3. Ex
citati
on
Types of Spectroscopy6
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UVX-rays IRg-rays RadioMicrowave
Energy (kcal/mol) 300-30 300-30 ~10-4> 300 ~10-6
Visib
leFrequency, n (Hz) ~1015 ~1013 ~1010 ~105~1017
Wavelength, l 10 nm 1000 nm 0.01 cm 100 m~0.01 nm
nuclear excitation (PET)
core electron excitation (X-ray cryst.)
electronic excitation (p to p*)
molecular vibration
molecular rotation
Nuclear Magnetic Resonance NMR & MRI
Basis of NMR Spectroscopy7
Nuclear Spin States The sub-atomic particles within atomic nuclei possess a spin
quantum number just like electrons
Just as when using Hund’s rules to fill atomic orbitals with electrons, nucleons must each have a unique set of quantum numbers
The total spin quantum number of a nucleus is a physical constant, I
For each nucleus, the total number of spin states allowed is given by the equation:
2I + 1
Basis of NMR Spectroscopy8
6. Observe that for atoms with no net nuclear spin, there are zero allowed spin states
7. Nuclear Magnetic Resonance can only occur where there are allowed spin states
8. Note that two nuclei, prevalent in organic compounds have allowed nuclear spin states – 1H and 13C, while two others do not 12C and 16O
Spin Quantum Numbers of Common Nuclei
Element 1H 2H 12C 13C 14N 16O 17O 19F 31P 35ClNuclear Spin Quantum Number
½ 1 0 ½ 1 0 5/2 ½ ½ 3/2
# of spin states 2 3 0 2 3 0 6 2 2 4
Basis of NMR Spectroscopy9
Nuclear Magnetic Moments A nucleus contains protons, which each bear a +1 charge
If the nucleus has a net nuclear spin, and an odd number of protons, the rotation of the nucleus will generate a magnetic field along the axis of rotation
Thus, a nucleus has a magnetic moment, m, generated by its charge and spin
A hydrogen atom with its lone proton making up the nucleus, can have two possible spin states—degenerate in energy
mH H
mI = +½ I = -½
Creating Non-Degenerate Nuclei
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In the absence of stimulus all nuclear spin
sates are degenerate
When a large
magnetic field B0 is
applied the two spin states
become non-
degenerate
As B0 increases, the larger
DE becomes
Nuclear Magnetic Resonance
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When a nuclei of spin +½
encounters a photon where
n = E/h, the two
“couple”
The nuclei “flips” its spin state
from +½ to –½ and is
now opposed to
B0
The nuclei “relaxes”
with the re-emission of a photon
and returns to the + ½
spin state
Nuclear Magnetic Resonance
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For the 1H nucleus (proton) this resonance condition occurs at low energy (lots of noise) unless a very large magnetic field is applied
Early NMR spectrometers used a large permanent magnet with a field of 1.4 Tesla—protons undergo resonance at 60 MHz (1 MHz = 106 Hz)
Modern instruments use a large superconducting magnet—our NMR operates at 9.4 T where proton resonance occurs at 400 MHz
In short, higher field gives cleaner spectra and allows longer and more detailed experiments to be performed
Origin of the Chemical Shift13
Electrons surrounding the nucleus are opposite in charge to the proton, therefore
they generate an opposing b0
DeshiedingFactors which
lower e- density allow the nucleus to
“see” more of the B0 being
applied – resonance
occurs at higher energy
ShieldingFactors which
raise e- density reduce the
amount of B0 the nucleus “sees” –
resonance condition occurs at lower energy
The Proton (1H) NMR Spectrum
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The 1H NMR Spectrum15
A reference compound is needed—one that is inert and does not interfere with other resonances
Chemists chose a compound with a large number of highly shielded protons—tetramethylsilane (TMS)
No matter what spectrometer is used the resonance for the protons on this compound is set to d 0.00
Si
CH3
H3C CH3CH3
The 1H NMR Spectrum16
The chemical shift for a given proton is in frequency units (Hz)
This value will change depending on the B0 of the particular spectrometer
By reporting the NMR absorption as a fraction of the NMR operating frequency, we get units, ppm, that are independent of the spectrometer
The 1H NMR Spectrum17
We need to consider four aspects of a 1H spectrum:
a. Number of signalsb. Position of signalsc. Intensity of signals.d. Spin-spin splitting of signals
The Number of Signals18
The number of NMR signals equals the number of different types of protons in a compound
Protons in different environments give different NMR signals
Equivalent protons give the same NMR signal
The Number of Signals19
To determine if two protons are chemically equivalent, substitute “X” for that each respective hydrogen in the compound and compare the structures
If the two structures are fully superimposible (identical) the two hydrogens are chemically equivalent; if the two structures are different the two hydrogens were not equivalent
A simple example: p-xylene CH3
CH3
H
H
CH3
CH3
H
Z
CH3
CH3
Z
H
Same Compound
The Number of Signals20
Examples
Important: To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to show specific stereochemistry:
The Number of Signals21
In comparing two H atoms on a ring or double bond, two protons are equivalent only if they are cis or trans to the same groups.
The Number of Signals22
Proton equivalency in cycloalkanes can be determined similarly:
The Number of Signals23
Enantiotopic Protons – when substitution of two H atoms by Z forms enantiomers:a. The two H atoms are equivalent and give the same
NMR signalb. These two atoms are called enantiotopic
The Number of Signals24
Diastereotopic Protons - when substitution of two H atoms by Z forms diastereomersa. The two H atoms are not equivalent and give two NMR
signalsb. These two atoms are called diastereotopic
Chemical Shift – Position of Signals
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Remember:
Electrons surrounding the nucleus are opposite in charge to the proton, therefore
they generate an opposing b0
DeshiedingFactors which
lower e- density allow the nucleus to
“see” more of the B0 being
applied – resonance
occurs at higher energy
ShieldingFactors which
raise e- density reduce the
amount of B0 the nucleus “sees” –
resonance condition occurs at lower energy
Chemical Shift – Position of Signals
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• The less shielded the nucleus becomes, the more of the applied magnetic field (B0) it feels
• This deshielded nucleus experiences a higher magnetic field strength, to it needs a higher frequency to achieve resonance
• Higher frequency is to the left in an NMR spectrum, toward higher chemical shift—so deshielding shifts an absorption downfield
Downfield, deshielded Upfield, shielded
Chemical Shift – Position of Signals
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There are three principle effects that contribute to local diamagnetic shielding:
a. Electronegativity b. Hybridizationc. Proton acidity/exchange
Chemical Shift – Position of Signals
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Electronegative groups comprise most organic functionalities:
-F -Cl -Br -I -OH -OR -NH2
-NHR -NR2 -NH3+ -C=O -NO2 -NO -SO3H
-PO3H2 -SH -Ph -C=C and most others
In all cases, the inductive WD of electrons of these groups decreases the electron density in the C-H covalent bond – proton is deshielded – signal more downfield of TMS
Chemical Shift – Position of Signals
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Protons bound to carbons bearing electron withdrawing groups are deshielded based on the magnitude of the withdrawing effect – Pauling electronegativity:
CH3F CH3O- CH3Cl CH3Br CH3I CH4 (CH3)4Si
Pauling Electronegativity
4.0 3.5 3.1 2.8 2.5 2.1 1.8
d of H 4.26 3.40 3.05 2.68 2.16 0.23 0.0
Chemical Shift – Position of Signals
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3. The magnitude of the deshielding effect is cumulative:
As more chlorines are added d becomes larger
4. The magnitude of the deshielding effect is reduced by distance, as the inductive model suggests
CH3Cl CH2Cl2 CHCl3d of H 3.05 5.30 7.27
-CH2Br -CH2CH2Br -CH2CH2CH2Brd of H 3.30 1.69 1.25
Chemical Shift – Position of Signals
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Hybridization Increasing s-character (sp3 sp2 sp) pulls e- density
closer to nucleus effectively raising electronegativity of the carbon the H atoms are bound to – a deshielding effect
We would assume that H atoms on sp carbons should be well downfield (high d) and those on sp3 carbons should be upfield (low d)
Chemical Shift – Position of Signals
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What we observe is slightly different:Type of H Carbon
hybridizationName of H Chemical Shift, d
R-CH3, R2CH2, R3CH sp3 alkyl 0.8-1.7C=C-CH3 sp3 allyl 1.6-2.6CC-H sp acetylenic 2.0-3.0C=C-H sp2 vinylic 4.6-5.7Ar-H sp2 aromatic 6.5-8.5
O=C-H sp2 aldehydic 9.5-10.1
Chemists refer to this observation as magnetic anisotropy
Chemical Shift – Position of Signals
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Magnetic Anisotropy – Aromatic Protonsa. In a magnetic field, the six p electrons in benzene
circulate around the ring creating a ring current.b. The magnetic field induced by these moving electrons
reinforces the applied magnetic field in the vicinity of the protons.
c. The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield.
Chemical Shift – Position of Signals
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Similarly this effect operates in alkenes:
Chemical Shift – Position of Signals
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In alkynes there are two perpendicular sets of p-electrons—the molecule orients with the field lengthwise—opposing B0 shielding the terminal H atom
Chemical Shift – Position of Signals
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Chemical Shift – Position of Signals
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Intensity of Signals—Integration
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The area under an NMR signal is proportional to the number of absorbing protons
An NMR spectrometer automatically integrates the area under the peaks, and prints out a stepped curve (integral) on the spectrum
The height of each step is proportional to the area under the peak, which in turn is proportional to the number of absorbing protons
Modern NMR spectrometers automatically calculate and plot the value of each integral in arbitrary units
The ratio of integrals to one another gives the ratio of absorbing protons in a spectrum; note that this gives a ratio, and not the absolute number, of absorbing protons
Intensity of Signals—Integration
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Intensity of Signals—Integration
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Spin-Spin Splitting41
Consider the spectrum of ethyl alcohol: Why does each resonance “split” into
smaller peaks?
HOCH2
CH3
Spin-Spin Splitting42
The magnetic effects of nuclei in close proximity to those being observed have an effect on the local magnetic field, and therefore DE
Specifically, when proton is close enough to another proton, typically by being on an adjacent carbon (vicinal), it can “feel” the magnetic effects generated by that proton
On any one of the 108 of these molecules in a typical NMR sample, there is an equal statistical probability that the adjacent (vicinal) proton is either in the + ½ or – ½ spin state
If there is more than one proton on an adjacent carbon – all the statistical probabilities exist that each one is either + ½ or – ½ in spin
The summation of these effects over all of the observed nuclei in the sample is observed as the spin-spin splitting of resonances
Spin-Spin Splitting43
Recall, we are observing the frequency (E = hn) where a proton goes into resonance
Any change in B0 will cause a change in energy at which the resonance condition will occur for a proton of a given chemical shift
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In solution we are not looking at a single molecule but about 108
On some molecules the proton being observed may be next to another proton of spin + 1/2 :
Spin-Spin Splitting45
On some molecules the proton being observed may be next to another proton of spin – 1/2 :
Spin-Spin Splitting46
Observe what effect this has on an isolated ethyl group:
The two methylene Ha protons have three neighbors, Hb, on the adjacent methyl carbon
Each one of these hydrogens can be + ½ or – ½ , and since we are not looking at one molecule, but billions, we will observe all combinations
C C
HbHa
R
HbHb
Ha
Spin-Spin Splitting47
The first possibility is that all three Hb protons have a + ½ spin; in this case the three protons combine to generate three small magnetic fields that aid B0 and deshield the protons – pushing the resonance for Ha slightly downfield (the magnetic field of a proton is tiny compared to B0)
C C
HbHa
R
HbHb
Ha
All 3 Hb protons + ½
resonance for Ha in absence of spin-spin splitting
Spin-Spin Splitting48
The second possibility is that two Hb protons have a + ½ spin and the third a - ½ ; in this case the two protons combine to enhance B0 and the other against it, a net deshielding; there are 3 different combinations that generate this state
C C
HbHa
R
HbHb
Ha
2 Hb protons + ½
or or
resonance for Ha in absence of spin-spin splitting
Spin-Spin Splitting49
The third possibility is that two Hb protons have a –½ spin and the third +½; here, the two protons combine to reduce B0 and the other enforce it, a net shielding effect; there are 3 different combinations that generate this state
C C
HbHa
R
HbHb
Ha
2 Hb protons - ½
resonance for Ha in absence of spin-spin splitting
or or
Spin-Spin Splitting50
The last possibility is that all three Hb protons have a – ½ spin; in this case the three protons combine to oppose B0, a net shielding effect; there is one combination that generates this state
C C
HbHa
R
HbHb
Ha
All 3 Hb protons - ½
resonance for Ha in absence of spin-spin splitting
Spin-Spin Splitting51
The result is instead of one resonance (peak) for Ha, the peak is “split” into four, a quartet, with the constituent peaks having a ratio of 1:3:3:1 centered at the d (n) for the resonance
C C
HbHa
R
HbHb
Ha
resonance for Ha in absence of spin-spin splitting
Spin-Spin Splitting52
Similarly, the Hb protons having two protons, on the adjacent carbon each producing a magnetic field, cause the Hb resonance to be split into a triplet
resonance for Ha in absence of spin-spin splitting
C C
HbHa
R
HbHb
Ha
Spin-Spin Splitting53
Rather than having to do this exercise for every situation, it is quickly recognized that a given family of equivalent protons (in the absence of other spin-coupling) will have its resonance split into a multiplet containing n+1 peaks, where n is the number of hydrogens on carbons adjacent to the carbon bearing the proton giving the resonance – this is the n + 1 rule
# of Hs on adj. C’s
Multiplet # of peaks
The relative ratios of the peaks are a mathematical progression given by Pascal’s triangle:
0 singlet 1 11 doublet 2 1 12 triplet 3 1 2 13 quartet 4 1 3 3 1 4 quintet 5 1 4 6 4 15 sextet 6 1 5 10 10 5 16 septet 7 1 6 15 20 15 6 1
1H NMR—Spin-Spin Splitting54
Common patterns:X CH3
X
XX
X
tert-butyl - singletmethyl - singlet
iso-propyl – septet - doublet
ethyl – quartet - triplet
n-propyl – triplet - quintet - triplet
1H NMR—Spin-Spin Splitting55
1H NMR—Spin-Spin Splitting56
Another Example:
BrC
CBr
Br
HaHb
Hb
BrBr
Br=
1H NMR—Spin-Spin Splitting57
Another Example:
1H NMR—Spin-Spin Splitting58
Three general rules describe the splitting patterns commonly seen in the 1H NMR spectra of organic compounds:1.Equivalent protons do not split each other’s signals2.A set of n nonequivalent protons splits the signal of a nearby proton into n + 1 peaks3.Splitting is observed for nonequivalent protons on the same carbon or adjacent carbonsIf Ha and Hb are not equivalent, splitting is observed when:
1H NMR—Spin-Spin Splitting59
Magnetic influence falls off dramatically with distance
The n + 1 rule only works in the following situations:
H H
H HAliphatic compounds that have free rotation about each bond
GHa
HbHc
H
H
Aromatic compounds where each proton is held in position relative to one another
1H NMR—Spin-Spin Splitting60
The amount of influence exerted by a proton on an adjacent carbon is observed as the difference (in Hz) between component peaks within the multiplet it generates. This influence is quantified as the coupling constant, J
Two sets of protons that split one another are said to be “coupled”
J for two sets of protons that are coupled are equivalent—therefore on complex spectra we can tell what is next to what
This J Is equal to this J
-CH2- -CH3
1H NMR—Spin-Spin Splitting61
The next level of complexity (which at this level, is only introduced) is when protons on adjacent carbons exert different J’s than one another.
Consider the ethylene fragment:
C
CX
H
H
HThe magnetic influence of the trans- relationship is over the longest distance
The cis-relationship, is over an intermediate distance
The influence of the geminal-relationship is over the shortest distance
1H NMR—Spin-Spin Splitting62
For this substituted ethylene we see the following spectrum:
2Jgem = 0 – 1 Hz
3Jtrans = 11- 18 Hz
3Jcis = 6 - 15 Hz
The observed multiplet for Ha is a “doublet of doublets”
3JAB3JAB
3JACC
CX
Ha
Hc
Hb
1H NMR—Spin-Spin Splitting63
In general, when two sets of adjacent protons are different from each other (n protons on one adjacent carbon and m protons on the other), the number of peaks in an NMR signal = (n + 1)(M + 1)
In general the value of J falls off with distance; J values have been tabulated for virtually all alkene, aromatic and aliphatic ring systems
1H NMR—Spin-Spin Splitting64
Some common J-values
3J = 6-8
3Ja,a = 8-143Ja,e = 0-73Je,e = 0-5
3Jtrans = 4-83Jcis = 6-12
3Jtrans = 4-83Jcis = 6-12
3Jtrans = 11-18
3Jcis = 6-15
3Jallyl = 4-10
3J = 8-11
3J = 5-7
3Jortho = 7-10 Hz4Jmeta = 1-3 Hz5J para = 0-1 Hz
HH
Ha
He
Ha
He
H
H
HH
CH3
H
H
H
H
H
H
H
HH
O
HH
H
H
HHo
Hm
Hp
1H NMR—Spin-Spin Splitting65
We can now tell stereoisomers apart through 1H NMR:
1H NMR—Spin-Spin Splitting66
A combined example:
1H NMR—Spin-Spin Splitting67
• Under usual conditions, an OH proton does not split the NMR signal of adjacent protons
• Protons on electronegative atoms rapidly exchange between molecules in the presence of trace amounts of acid or base (usually with NH and OH protons)
Structure Determination68
Structure Determination69
Structure Determination70
Structure Determination71
13C NMR72
• The lack of splitting in a 13C spectrum is a consequence of the low natural abundance of 13C
• Recall that splitting occurs when two NMR active nuclei—like two protons—are close to each other. Because of the low natural abundance of 13C nuclei (1.1%), the chance of two 13C nuclei being bonded to each other is very small (0.01%), and so no carbon-carbon splitting is observed
• A 13C NMR signal can also be split by nearby protons. This 1H-13C splitting is usually eliminated from the spectrum by using an instrumental technique that decouples the proton-carbon interactions, so that every peak in a 13C NMR spectrum appears as a singlet
• The two features of a 13C NMR spectrum that provide the most structural information are the number of signals observed and the chemical shifts of those signals
13C NMR73
13C NMR74
• The number of signals in a 13C spectrum gives the number of different types of carbon atoms in a molecule.
• Because 13C NMR signals are not split, the number of signals equals the number of lines in the 13C spectrum.
• In contrast to the 1H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13C NMR signals are not integrated.
13C NMR75
• In contrast to the small range of chemical shifts in 1H NMR (1-10 ppm usually), 13C NMR absorptions occur over a much broader range (0-220 ppm).
• The chemical shifts of carbon atoms in 13C NMR depend on the same effects as the chemical shifts of protons in 1H NMR.
13C NMR76
13C NMR77
Running an NMR Experiment Sample sizes for a typical high-field NMR (300-600 MHz):
1-10 mg for 1H NMR 10-50 mg for 13C NMR
Solution phase NMR experiments are much simpler to run; solid-phase NMR requires considerable effort
Sample is dissolved in ~1 mL of a solvent that has no 1H hydrogens
Otherwise the spectrum would be 99.5% of solvent, 0.5% sample!
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Running an NMR Experiment Deuterated solvents are employed—all 1H atoms
replaced with 2H which resonates at a different frequency
Most common: CDCl3 and D2O
Employed if necessary: CD2Cl2, DMSO-d6, toluene-d8, benzene-d6, CD3OD, acetone-d6
Sample is contained in a high-tolerance thin glass tube (5 mm)
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Running an NMR Experiment IMPORTANT—no deuterated solvent is 100% deuterated,
there is always residual 1H material, and this will show up on the spectrum
CHCl3 in CDCl3 is a singlet at d 7.27
HOD in D2O is a broad singlet at d 4.8
No attempt is made to make solvents for 13C NMR free of 13C, as the resonances are so weak to begin with
13C NMR using CDCl3 shows a unique 1:1:1 triplet at d 77.00 (+1, 0, 1 spin states of deuterium coupled with 13C)
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