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1、Proton Nuclear Magnetic Resonance Spectrometry (1H NMR),What does a 13C NMR look like ?,Comparison Between 1H and 13C NMR,Similarity Both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons) Both give us information about the envi
2、ronment of the nuclei (hybridization state, attached atoms, etc.),Chemical Shift Equivalence,Chemical Shift Equivalence,75.5 MHz C/C coincident,150.9 MHz C/C are resolved,Comparison Between 1H and 13C NMR,Difference A signal for a 13C nucleus is only about 1% as intense as that for 1H because of the
3、 magnetic properties of the nuclei, and at the “natural abundance“ level only 1.1% of all the C atoms in a sample are 13C (most are 12C) 13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei (0200 ppm). Overlap of signals less frequen
4、tly encountered (it still happens though).,Chemical shift (, ppm),ClCH2,Proton Spectrum,CH3,ClCH2CH2CH2CH2CH3,1H,Chemical shift (, ppm),Carbon Spectrum,ClCH2CH2CH2CH2CH3,13C,CDCl3,a separate, distinct peak appears for each of the 5 carbons,Chemical Shift Scale and Range,For a given spectrometer, the
5、 resonance frequency for C13 is about that Of H1. To a first approximation, C13 chemical shifts parallel those of H1,13C NMR Chemical Shift,Chemical Shift () are measured in ppm from the carbons of TMS Influence Factors electronegativity of groups attached to carbon hybridization state of carbon,Ele
6、ctronegativity Effects,Greater effect on 13C chemical shifts than it does on 1H chemical shifts.,0.2,0.9,1.3,1.7,Replacing H by C (more electronegative) deshields C to which it is attached.,Types of Carbons,Electronegativity effects on CH3,CH3F,CH4,CH3NH2,CH3OH,Chemical shift, d,Chemical shift, d,45
7、,33,29,22,14,Deshielding effect of Cl decreases as number of bonds between Cl and C increases.,Electronegativity effects and chain length,Hybridization effects,sp3 hybridized carbon is more shielded than sp2,sp hybridized carbon is more shielded than sp2, but less shielded than sp3,Carbonyl carbons
8、are especially deshielded,CH2,C,O,127-134,41,14,61,171,Type of carbon,Chemical shift (), ppm,Type of carbon,Chemical shift (), ppm,RCH3,0-35,65-90,R2CH2,15-40,R3CH,25-50,R4C,30-40,100-150,110-175,Type of carbon,Chemical shift (), ppm,Type of carbon,Chemical shift (), ppm,RCH2Br,20-40,RCH2Cl,25-50,35
9、-50,RCH2NH2,50-65,RCH2OH,RCH2OR,50-65,RCOR,160-185,RCR,190-220,110-125,13CH Coupling,Peaks in a 13C NMR spectrum are typically singlets 13C-1H splitting is not seen because spectrum is measured under conditions that suppress this splitting (broadband decoupling).,1H Decoupling Techniques,J values fo
10、r C-H are typically 110-300 Hz (C-C-H and C-C-C-H are 0-60Hz). Thus a CH3 group would appear as a quartet, CH2-triplet CH-doublet etc. The 1H nuclei are irradiated with a broadband Rf (radio frequency) to remove coupling to Carbon.,Interpretation of a Simple 13C Spectrum:,decoupled,coupled,Expansion
11、s: Additional splitting Is due to J2 and J3 couplings,Chemical shift (, ppm),Proton Decoupled Spectrum,Peak Intensities,The 13C peak intensities do not correlate with the number of carbon atoms in a given peak in routine spectra, due to longer T1 values and NOE.,T1 Relaxation,T1 varies widely for di
12、fferent types of carbons. Generally, T1s decrease as the number of attached protons increases.,Attached Protons Affect T1 and Signal Intensity,7 carbons give 7 signals, but intensities are not equal.,NOE,NOE: Nulecar Overhauser Enhancement The net effect of NOE on 13C peak spectra is the enhancement
13、 of peaks whose carbon atoms have attached protons. 13C nuclei without an attached proton are characterized by peaks of distinctly low intensities.,Distortionless Enhancement of Polarization Transfer Using DEPT to count the hydrogens directly attached to 13C,DEPT,Std H1 decoupled spectrum,DEPT 135 deg. CH &CH3 up, CH2 down,DEPT 90 deg. CH only,DEPT is a helpful to determine proton inventory, but it does not Record Hs on heteroatoms; must correlate with H1 spectrum.,Sample Handling,
