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nmrextra - 13-13-11313C-NMR SpectroscopyC-NMR...

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Unformatted text preview: 13-13-11313C-NMR SpectroscopyC-NMR SpectroscopyEach nonequivalent 13C gives a different signala 13C signal is split by the 1H bonded to it according to the (n+ 1) rule coupling constants of 100-250 Hz are common, which means that there is often significant overlap between signals, and splitting patterns can be very difficult to determineThe most common mode of operation of a 13C-NMR spectrometer is a hydrogen-decoupled mode13-13-21313C-NMR SpectroscopyC-NMR SpectroscopyIn a hydrogen-decoupled mode, a sample is irradiated with two different radio frequenciesone to excite all 13C nucleia second broad spectrum of frequencies to cause all hydrogens in the molecule to undergo rapid transitions between their nuclear spin statesOn 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 decoupled13-13-31313C-NMR SpectroscopyC-NMR Spectroscopyhydrogen-decoupled 13C-NMR spectrum of 1-bromobutane13-13-4Chemical Shift - Chemical Shift - 1313C-NMRC-NMRRCH3RCH2RR3CHR2C=CR2RCCRR3CORRCH2ClRCH2BrRCH2IR3COHRCRCNR2ORCH, RCROORCCOHORCORO0-40110-160165 - 180160 - 180165 - 185180 - 21540-8040-8035-8025-6565-85100-15020-6015-5510-40Type of CarbonChemicalShift (29 (29 13-13-5Chemical Shift - Chemical Shift - 1313C-NMRC-NMR13-13-6The DEPT MethodThe DEPT MethodIn the hydrogen-decoupled mode, information on spin-spin coupling between 13C and hydrogens bonded to it is lostThe DEPT method is an instrumental mode that provides a way to acquire this informationDistortionless Enhancement by Polarization TransferDistortionless Enhancement by Polarization Transfer(DEPT):DEPT):an NMR technique for distinguishing among 13C signals for CH3, CH2, CH, and quaternary carbons13-13-7The DEPT MethodThe DEPT MethodThe DEPT methods uses a complex series of pulses in both the 1H and 13C ranges, with the result that CH3, CH2, and CH signals exhibit different phases;signals for CH3and CH carbons are recorded as positive signalssignals for CH2carbons are recorded as negative signalsquaternary carbons give no signal in the DEPT method13-13-8Isopentyl acetateIsopentyl acetate13C-NMR: (a) proton decoupled and (b) DEPT13-13-9Interpreting NMR SpectraInterpreting NMR SpectraAlkanesAlkanes1H-NMR signals appear in the range of 0.8-1.7 13C-NMR signals appear in the considerably wider range of 10-60AlkenesAlkenes1H-NMR signals appear in the range 4.6-5.71H-NMR coupling constants are generally larger for...
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This note was uploaded on 03/21/2008 for the course CHEM 261 taught by Professor Austell during the Spring '08 term at UNC.

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nmrextra - 13-13-11313C-NMR SpectroscopyC-NMR...

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