Carbon-13 NMR Spectroscopy & 2D NMR Methods

13C & 2D NMR SPECTROSCOPY PRESENTED BY Madhavanand Ingalageri MSc IV-Semester Reg No: 1471092GUIDED BYProf. A R SaundaneM.Sc., PhD

Department of Chemistry, Gulbarga University Kalaburagi.

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IntroductionProton (1H) and carbon (13C) are the most important nuclear spins to organic chemists.12C nucleus is not magnetically active the spin number I being zero. But 13C Have I =

13C account for only 1.1% of naturally occurring carbon 13C- 13C coupling is negligible and not observed.

Each nonequivalent 13C gives a different signal.

The most common mode of operation of a 13C-NMR spectrometer is a hydrogen-decoupled mode.

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Nuclear Magnetic Resonance SpectroscopyMany nuclei exhibit NMR phenomenonAll nuclei with odd number of protons All nuclei with odd number of neutronsNuclei with even numbers of both protons and neutrons do not exhibit NMR phenomenon

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Nuclear Magnetic Resonance SpectroscopySome nuclear spins are aligned parallel to the external field Lower energy orientation

Some nuclear spins are aligned antiparallel to the external field Higher energy orientation(a)Nuclear spin oriented randomly in the absence of B0(b)Specific orientation in presence of external field B0

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Principle of NMRWhen energy in the form of radiofrequency is applied

When applied frequency is equal to precessional frequency.

Absorption of energy occurs

Nucleus is in resonance

NMR signal is recorded.

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InstrumentationThe basic features of the instrumentation needed to record NMR spectra are,

MagnetA radiofrequency sourceA detection System

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Why 13C NMR Required ??

Proton NMR used for study of number of nonequivalent proton present in unknown compound.

Carbon NMR can used to determine the number of non-equivalent carbons and to identify the types of carbon atoms(methyl, methylene, aromatic, carbonyl.) which may present in compound.

13C signals are spread over a much wider range than 1H signals making it easier to identify & count individual nuclei.6

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Chemical Shift

Carbon-13 chemical shifts are most affected by,Hybridiasation state of carbon & Electronegative group attached to carbon.

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1H and 13C NMR compared13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei.Figure #1 shows the 1H NMR spectrum of 1-chloropentane; Figure #2 shows the 13C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13C spectrum than by its 1H spectrum.

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1-Chloropentane

01.02.03.04.05.06.07.08.09.010.0Chemical shift (d, ppm)ClCH2CH3ClCH2CH2CH2CH2CH31H9

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1-Chloropentane

Chemical shift (d, ppm)ClCH2CH2CH2CH2CH3

02040608010012014016018020013CCDCl3a separate, distinct peak appears for each of the 5 carbons10

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Problems of 13CNatural abundance-13C natural abundance is very low (1.1%) .

Gyromagnetic Ratio-

13C nucleus gyro magnetic ratio is much lesser than proton nucleus. 13C-1.404; 1H-5.585

Coupling phenomenon-

13C & 1H have I=1/2 so that coupling between them probably occur.

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PROBLEMS IN NMR CAN BE OVERCOME BYFourier Transform Technique-

Decoupling Technique- 1) Broad Band Decoupling 2) Off Resonance Decoupling 3) DEPT (Pulse) Decoupling

Nuclear Overhauser Phenomenon-

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Hydrogen Decoupled mode(Broad Band Decoupled)A sample is irradiated with two different radio frequencies.One to excite all 13C nuclei.A second broad spectrum of frequencies to cause all hydrogen's in the molecule to undergo rapid transitions between their nuclear spin states.On the time scale of a 13C-NMR spectrum, each hydrogen is in an average or effectively constant nuclear spin state, with the result that 1H-13C spin-spin interactions are not observed; they are decoupled.Thus, each different kind of carbon gives a single, unsplit peak.

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ETHYL PHENYLACETATE

13C coupledto the hydrogens

13C decoupledfrom the hydrogens

in some casesthe peaks of the multiplets willoverlap

this is aneasier spectrumto interpret14

Off-Resonance DecouplingOff-Resonance decoupling simplifies the spectrum by allowing some of the splitting information to be retained.

In this technique only the 13C nuclei are split by the protons directly bounded to them and not by any other protons i.e., one observes only one bond coupling 13C -1HThe coupling between each carbon atom and each hydrogen attached directly to it, s observed according to n+1 rule. Q T D S

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Example: Propanol

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13C Off-resonance decoupled spectrum

1,2,2-trichloropropane17

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DEPT 13C NMR SpectroscopyDistortionless Enhancement by Polarization Transfer (DEPT-NMR) experiment.This technique used is used to distinguish between among CH3, CH2, CH groups.Run in 3 stages.

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Example: 6methylhept-5-en-2-olOrdinary broadband-decoupled spectrum showing signals for all eight of 6-methylhept-5-en-2-ol

DEPT-90 spectrum showing signals only for the two C-H carbons.

DEPT-135 spectrum showing positive signals for the two CH carbons and the three CH3 carbons and negative signals for the two CH2 carbons.

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At what approximate positions would you expect ethyl acrylate, H2C=CHCO2CH2CH3, to show 13C NMR absorptions?

StrategyIdentify the distinct carbons in the molecule, and note whether each is alkyl, vinylic, aromatic, or in a carbonyl group.then predict the value with the help of correlation chart.

Predicting Chemical Shifts in 13C NMR Spectra20

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Solution:

Ethyl acrylate has five distinct carbons: two different C=C, one C=O, one C(O)-C, and one alkyl C. From Correlation chart the likely absorptions are

Predicting Chemical Shifts in 13C NMR Spectra

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2D NMR Spectroscopy2D NMR Spectra have two frequency axis and one intensity axis.Example: COSY and HECTOR Correlation.

The Spectrum gives a look of mountain range when viewed from air. These mountain like spectra (Known as stack plots)

The compound is identified using a contour plot, where slice of each mountain represented by contour circles.

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2D NMR

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A MountainStack plotContour plot

Preparationspin system relaxes and then excited by r.f.Evolution (t1)chemical shifts & spin-spin couplings evolve. This is the time domain which is incremented during a 2D experiment.Mixingr.f. pulses are applied and create observable transverse magnetization.Detection (t2)observable transverse magnetization is recorded. It is usually labeled with t2.Four periods in 2D experiment24

COSY Spectrum

The common 2D spectra are 1 H -1H shift correlations known as COSY Spectrum.COSY identifies pair of protons which are coupled to each other.The compound is identified using a contour plotOne dimensional counterpart of a given peak on the diagonal lies directly below that peak on each axis.The presence of cross peak normally indicates protons giving the connected resonance on the diagonal are geminaly or vicinally coupled.

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COSY Spectrum of m-dinitrobenzene

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HECTOR Spectrum 2D-NMR spectra that displays 13C 1H-NMR shift correlations are called HETCOR spectra. It shows coupling between protons and the carbon to which they are attached.

The 2D spectrum is composed only of cross-peaks, each one relating carbon to its directly bonded proton(s).

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The HETCOR spectrum of 2-methyl-3-pentanone

HETCOR: Heteronuclear Chemical Shift CorrelationHECTOR Spectrum of 2 Methyl -3-pentanone28

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Applications of 13C NMRCMR is a noninvasive and nondestructive method,i.e,especially used in repetitive In-vivo analysis of the sample without harming the tissues.CMR, chemical shift range(0-240 ppm) is wider compared to H-NMR(0-14 ppm), which permits easy separation and identification of chemically closely related metabolites.C-13 enrichment, which the signal intensities and helps in tracing the cellular metabolism.CMR technique is used for quantification of drugs purity to determination of the composition of high molecular weight synthetic polymers.29

Applications of 2D-NMRTwo-dimensional NMR leads to the development of chemical shifts (and coupling constant) into two dimensional and to resolve overlap of resonance which enables the correlation of interacting nuclei to be determined.Two-dimensional NMR can be applied to complex spectra which are difficult to analyzed by conventional methods.COSY identifies pair of protons which are coupled to each other.

The HETCOR spectra used to detect directly bounded 13C 1H atoms.

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THANK YOU

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