仪器——课件CNMR幻灯片.pptVIP

6 6 6 25 25 1 3 3 3 3 Proton Nuclear Magnetic Resonance Spectrometry (1H NMR) What does a 13C NMR look like ? Chemical shift (?, ppm) 0 20 40 60 80 100 120 140 160 180 200 O C C CH CH CH CH2 CH2 CH2 CH3 CCH2CH2CH2CH3 O 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 environment 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 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 (0–200 ppm). Overlap of signals less frequently encountered (it still happens though). 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (?, ppm) ClCH2 Proton Spectrum CH3 ClCH2CH2CH2CH2CH3 1H Chemical shift (?, ppm) Carbon Spectrum ClCH2CH2CH2CH2CH3 0 20 40 60 80 100 120 140 160 180 200 13C CDCl3 a separate, distinct peak appears for each of the 5 carbons Chemical Shift Scale and Range For a given spectrometer, the 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 (δ) are measured in ppm from the carbons of TMS Influence Factors electronegativity of groups attached to carbon hybridization state of carbon Electronegativity Effects Greater effect on 13C chemical shifts than it does on 1H chemical shifts. (CH3)3CH CH4 CH3CH3 CH3CH2CH3 (CH3)4C primary secondary tertiary quaternary Classification Chemical shift, d 1H 13C 0.2 0.9 1.3 1.7 -2 8 16 25 28 Replacing H by C (more el