This document provides an overview of C13 NMR spectroscopy. It discusses the principles and theory of NMR spectroscopy, the history of C13 NMR, and the information that can be obtained from C13 NMR spectra. Specifically, it explains that C13 NMR spectroscopy allows identification of carbon atoms in organic molecules similarly to how proton NMR identifies hydrogen atoms. It also discusses factors that influence chemical shifts in C13 NMR such as substitution effects, hybridization, and electronegativity. In summary, the document serves as an introduction to C13 NMR spectroscopy, its applications and principles.

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2. C13 NMR SPECTROSCOPY
 Presenter: Farah Naz
 Advance structure elucidation techniques
of Natural products

3. Contents;
Introductions of NMR spectroscopy
Principle and theory
History
C13 –NMR Spectroscopy
NMR –active Nuclei
Information from C13- NMR
Chemical shift of C13-NMR
Short coming of C13-NMR
Spin- Spin splitting of C13-NMR
 Factor effecting C13-NMR
Advantages and dis- advantages
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4. NMR SPECTROSCOPY
Nuclear magnetic resonance spectroscopy ,most
commonly known as NMR spectroscopy or magnetic
resonance spectroscopy ,is a spectroscopic technique to
observe local magnetic fields around atomic nuclei.
It results to given a spectrum with frequency on x- axis and
intensity of absorption on y-axis.
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5. PRINCIPLE AND THEORY
 The nuclear magnetic resonance occur when nuclei aligned with
an applied field are induced to absorb energy and change their spin
orientation with respect to the applied field.
 The energy absorption is quantized process, and energy absorbed
must equal to energy difference between the two states involved .
 E absobed = (E-1/2state –E+1/2state)=hv
 The stronger the applied magnetic field , greater the energy
difference between the possible spin states.
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6. PRINCIPLE OF NMR
 when energy in the form of radiofrequency is applied
 When applied frequency is equal to processional
frequency absorption of energy occures.
 Nucleus in resonance
 NMR signal is recorded
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7.  HISTORY OF C13 NMR
 The first NMR signal for 1H was observed in 1945 but the first 13C
NMR signal was detected in 1957 by Lauterbur.
 The first 13C NMR of some organic compound was recorded in
early 1960. This discovery made structural determination very
simple and interesting.
 This is more helpful to those molecules which have very few C-H
bonds. This is because, this type of molecules are not identified
completely by 1H NMR. Also, the complex molecules mainly
biomolecules found this method of identification more efficient.
 The study of biosynthetic pathways of natural products and brain
metabolism can easily be monitored with the help of 13C NMR
spectroscopy.
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8.  C13 – NMR SPECTROSCOPY
Carbon-13 nuclear magnetic resonance is the application of nuclear magnetic
resonance spectroscopy to carbon. It is analogous to proton NMR and allows the
identification of carbon atoms in an organic molecule just as proton NMR identifies
hydrogen atoms. ¹³C NMR detects only the ¹³ C isotope.
NUCLEI AND RELATIVE ABUNDANCE OF
CARBON ISOTOPES
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9.  NMR ACTIVE NUCLEI:
 nuclear spin quantum number (I) atomic mass and atomic number
 Even mass nuclei that have even number of neutron have I = 0 (NMR
inactive)
 Even mass nuclei that have odd number of neutrons have an integer spin
quantum number (I = 1, 2, 3, ) Odd mass nuclei have half-integer spin
quantum number (I = 1/2, 3/2, 5/2, )
 I= 1/2: 1H, 13C, 19F, 31P , I= 1: 2H, 14N
 I= 3/2: 15N ,I= 0: 12C, 16O
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 The most abundant isotope of carbon is C-12
(nuclear spin, I = 0) and this is NMR inactive.
 , the natural abundance of C-13 or 13C is 1.1% and
has I = 1/2 like a proton.
C-13 nuclei are NMR active.

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 The Spin Quantum Number
(ms) describes the angular momentum
of an electron. An electron spins around
an axis and has both angular momentum
and orbital angular momentum. Because
angular momentum is a vector, the Spin
Quantum Number (s) has both a
magnitude (1/2) and direction (+ or -)
 The gyromagnetic ratio, often denoted
by the symbol γ (gamma) is the ratio of
the magnetic momentum in a particle to
its angular momentum.

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INFORMATION FROM C13NMR SPECTRUM
Analysis of an NMR spectrum may involve analyzing: a)
The number of signals a molecule emits b) The frequencies
at which signals occur c) The intensity of signals d) The
splitting of signals

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 Information from 13C NMR
1.Number of signals: Tells about different types of carbon
atoms present (non-equivalent carbons)
2. Position of Signals: Gives information about either carbons
are aliphatic, alkene, aromatic, aldehydic, carbene or
carbonyl etc.
3.Splitting of signals: In proton-coupled 13C NMR, splitting
of signals gives information of number of hydrogen atoms
attached to a particular carbon.

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Ethyl acetate

15.  CHEMICAL SHIFT OF C13 NMR;
 The frequency at which a nucleus will resonate is
dependent on the magnetic field strength.
 Because this can vary from instrument to instrument,
frequency is expressed relative to magnetic field strength,
“chemical shift”
 Chemical Shift = frequency of resonance (Hz) /frequency
of instrument(MHz)
 units = parts per million = ppm
 The Chemical shift range for 1H is usually in δ 0 – 12
ppm.
 The Chemical shift range for 13C is quite large and usually
in δ0 – 220 ppm.
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This results is that almost all non-equivalent 13C peaks
appear at different Chemical shift values and rarely
overlap.

17. SOLVENTS FOR NMR SPECTROSCOPY
 NMR spectra are usually measured using solutions of the
substance being investigated. A commonly used solvent is CDCl3.
This is a trichloromethane (chloroform) molecule in which the
hydrogen has been replaced by its isotope, deuterium.
 CDCl3 is also commonly used as the solvent in proton-NMR
because it doesn't have any ordinary hydrogen nuclei (protons)
which would give a line in a proton-NMR spectrum. It does, of
course, have a carbon atom - so why doesn't it give a potentially
confusing line in a C-13 NMR spectrum?
 In fact it does give a line, but the line has an easily re-cognisable
chemical shift and so can be removed from the final spectrum.
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19.  SHORT COMINGS OF C13-NMR SPECTRA
 Only 1%of the carbon in the molecules is carbon -13.
 Sensitivity is consequently low.
 Recording the NMR – spectra is a tedious and time consuming
process .
 Magnetogyric ratio of carbon (g=68)is less than that of proton
(g=268).
 13C nuclei resonance 6000 times weaker than 1H-nuclei.
 13C-NMR resonance (25 MHz) frequency ¼less than1H
resonance (100 MHz)frequency.
The overall sensitivity c13 compound with 1H stands at 1/5700.
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H 3C –CO- CH3
1.1 % 1.1 % ACETONE

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The advent of recent developments in NMR-spectroscopy it is
quite possible to eliminate some of these short comings
adequately. They are :
 Development of powerful magnets (‘super con’ magnets)
has ultimately resulted in relatively stronger NMR-signals
from the same number of atoms
 Improved hardware in NMR-spectroscopy has gainfully
accomplished higher sensitivity
 Development of more sensitive strategies has made it
possible to record these C—H correlation spectra in a
much easier manner.
Eliminate short comings

21.  Spin-Spin Splitting in 13C NMR;
1-Homonuclear(13C-13C)Coupling;
low natural abundance of 13C nuclei,
the probability of finding two 13C
atoms adjacent to each other in the same
molecule is very low. Hence, no spin-
spin splitting between 13C–13C nuclei.

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2- proton coupled Spectra:
Spin-spin splitting occurs between hydrogen attached to the
particular 13C atom. Such spectrum called proton coupled 13C
NMR spectrum. The splitting of 13C signal occurs according to
n+1 rule. The proton coupled 13C NMR spectrum are very
difficult to interpret due to large coupling constant (J = 100-320
Hz). 1H–13C 1J = ~100-320 Hz; 1H–C–13C 2J = 0-60 Hz; 1H–
C–C–13C 3J = 0-60 Hz

23. 3- proton decoupled spectra;
When there is no coupling between 1H and 13C nuclei, then such 13C
spectrum called as proton decoupled 13C NMR spectrum i.e. 13C{1H}
NMR spectrum. This gives only singlets and thus simplifies the spectrum.
However, this lost the information about attached hydrogens. Majority of
13C NMR spectra are obtained as proton decoupled spectra.
13C{1H} NMR spectrum of [Au(CH3)(carbene)] in CDCl3. The solvent
signals are marked with ‘*’.
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 FACTOR EFFECTING C13- NMR

25.  FACTOR EFFECTING C-13NMR
 (a) α,b-and γ-Substituent Effects
 The α-substituents results from the replacement of a directly bonded H by an X
group:
C-H → C-X
 The b-substituents results from the replacement of a H on an adjacent atom by
an X group:
C-C-H → C-C-X
 The gamma -substituents results from the replacement of a H on an adjacent to
adjacent atom by an X group:
C-C-C-H → C-C-C-X
 The chemical shift increases on going from primary to secondary to tertiary to
quaternary carbon atoms.
 The average increase per carbon atom lies in the range of 7-10 ppm. The
substituents at alpha and beta -position increases the chemical shift value. In
other words, the 13C absorption position is shifted downfield.
 The case with gamma substitution is opposite. In this case, the 13C absorption
position is shifted up field i.e. the chemical shift value is decreased.
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The other important factor influencing the
substituent effects is the electronegativity of the
attached atom. The attachment of electronegative
atom shifts the δ value towards higher side.
For example, in the 13C NMR spectrum of ethanol
the signal for CH3 is observed at higherδ value in
comparison to the CH2. This is due to the attachment
of CH2 group with electron withdrawing group –
OH.
(b) Electronegative effect ;

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(c) Hybridization Effect;
The hybrid state of carbon also affects the chemical shift of carbon.
1-sp3 hybridised carbons
• The sp3hybridised carbons are either un-substituted or
substituted with electron withdrawing groups (EWG).
• Overall range of resonance observed in between 0 to 50 ppm
for un-substituted and between 60-90 ppm for EWG
substituted carbons. The influence of strong electronic and
steric effect cannot be ruled out. For an extreme case like
tetraiodomethane (CI4) where, carbon has been observed at
300 ppm lower than the TMS (δ= -300).

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2- sp2 hybridized carbons;
The sp2hybridised carbons include alkene and aromatic carbons. These carbons
give signals in overlapped regions. The range alkene and aromatic carbon signals
are between 90 to 150 and 100 to 170 respectively.
For example,
>C=O, >C=N etc. The shift in ppm differs for these cases depending upon the
electronic and steric factors. The >C=N present in the Schiffs bases are generally
observed in the range 130-150 ppm. The 13C NMR signal ranges of important
>C=O groups are:
 ester C=O ;170-175 ppm
 acid C=O ; 177-185 ppm
 aldehyde C=O ; 190-200 ppm
 ketone C=O ; 200-220 ppm
3-sp hybridized carbons The sp hybridized carbons include alkynes,
nitriles and isonitriles. The signals for these groups are generally present in
very narrow range. The unsubstituted alkynes have been observed in the
range of 40-80 ppm.

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(d)Three-Membered Rings:
The three-membered ring has significant effect on the
chemical shifts in 13C NMR spectra. The compounds
such as cyclopropanes, cyclopropenes, epoxides,
aziridines and other 3-membered rings tend to show
pronounced upfield shifts i.e. shift towards lower δvalues.
For example,
The C-2 carbon of propane is observed at 16 ppm in 13C
NMR spectra whereas the carbon in cyclopropane is
observed at -3 ppm . Similar trend has also been observed
for propene and dimethyl ether with respect to their cyclic
counterpart carbon

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(e) Conjugation to carbonyl group;
The conjugation to carbonyl carbon also affects the chemical
shift. The conjugation may be due to a double bond or
aromatic ring. This causes upfield shifts i.e. towards lower
δ value about 6-10 ppm for all types of carbonyl compounds.
The effect is smaller for nitriles also.
For example,
The carbonyl carbon of 2-butanone appears at 206 ppm
whereas, corresponding conjugated carbonyl carbon for but-
3-ene-2-one appears at 197 ppm and acetophenone at 195
ppm

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2- butanone
acetaphenone

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(f)Hydrogen Bonding Effects;
The intra-molecular hydrogen bonding causes substantial
downfield shifts in 13C NMR spectra i.e. towards higher δ value.
It has been observed that most carbon signals are resistant to
solvent effects but the carbonyl groups are an exception. The
chemical shifts for carbonyl carbon move to downfield in protic
solvents. This may be attributed to hydrogen bonding.

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 ADVANTAGES OF C13 NMR

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 C-13 NMR offers considerably better resolution, largely
because the C-13 absorptions for most ordinary organic
molecules are spread over 200 instead of 10 ppm.
 Carbons bearing no protons are directly visible.
 A count of the number of protons attached to each carbon
results from comparison of the broad-band decoupled C-13
NMR spectrum with the off-resonance C-13 spectrum. Thus
the number of methyl, methylene, methinyl, and quaternary
carbons in a fairly complex molecule is far more readily
determined by C-13 than by H-I NMR.

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 DIS-ADVANTAGES OF C13 NMR

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l. Larger sample size (up to 100 mg) or longer sampling time (up to
several days) is sometimes necessary; however, if 100 mg of a
sample is available and that sample has high solubility, the time
requirement may be only a few minutes; a good spectrum may even
be obtained on as little as a I-mg sample when several days are
available for scanning the sample.
2. Owing to variations in relaxation times and nuclear Overhauser
effects (NOE), the areas of absorption for individual carbons vary
considerably (up to a factor of about 10). Thus it is not as easy to tell
relative numbers of carbons from C-13 as it is protons from H-l
NMR; for this reason only the H-l spectra are integrated in the
problems below.
3. Protons attached to heteroatoms are not directly visible.

41. REFERENCE ;
 MCH 401;Application of Spectroscopy (Organic) UNIT- 4th:
Carbon-13 NMR Spectroscopy
 Introduction to spectroscopy book
 Pharmaceutical drug analysis book
 CARBON-13 NMR SPECTRAL PROBLEMS Robert B. Bates
and William A. Beavers Tems Eastman Company,
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