я п к ь
advertisement
Chemial Shift The referene point of an NMR spetrum is set at the point where TMS resonates CH3 H3C Si CH3 CH3 tetramethylsilane (TMS) Chemial shift is a measure of how far a signal relative to the referene signal The ommon sale for hemial shift is dened as: distane downeld from TMS (Hz ) δ = operating frequeny of the spetrometer (MHz ) Dr. Yuming Zhao Leture Notes for Chem 2401 Chemial Shift Dr. Yuming Zhao Leture Notes for Chem 2401 Chemial Shift Chemial shift is independent of the spetrometer's operating frequeny Dr. Yuming Zhao Leture Notes for Chem 2401 1H NMR Spetra Dr. Yuming Zhao Leture Notes for Chem 2401 Chemial Shifts Of Funtional Groups Eletron-withdrawal auses NMR signal to appear at higher frequeny (i.e. larger δ value) Charateristi hemial shift values for ommon organi funtional groups Dr. Yuming Zhao Leture Notes for Chem 2401 Table Of Chemial Shifts Dr. Yuming Zhao Leture Notes for Chem 2401 Diamagneti Anisotropy π -Eletrons are less tightly held by the nuleus than σ -eletrons; therefore, they are more free to move in response to an applied magneti eld Hydrogens bonded to π -bonds (e.g. benzene, alkenes) usually appear at the downeld region of the spetrum Dr. Yuming Zhao Leture Notes for Chem 2401 Integration Line The area under eah signal is proportional to the number of protons that give rise to that signal The height of eah integration step is proportional to the area under a spei signal The integration tells the relative number of protons, not the absolute number Dr. Yuming Zhao Leture Notes for Chem 2401 Integration Line Dr. Yuming Zhao Leture Notes for Chem 2401 Splitting of NMR Signals An 1 H NMR signal is split into N + 1 peaks, where N is the number of equivalent protons bonded to adjaent arbons Coupled protons split eah other's signal The number of peaks in a signal is alled the multipliity of the signal The splitting of signals, aused by spin-spin oupling, when dierent kinds of protons are lose to one another Dr. Yuming Zhao Leture Notes for Chem 2401 N + 1 Rule Dr. Yuming Zhao Leture Notes for Chem 2401 Multipliity Of NMR Signals Dr. Yuming Zhao Leture Notes for Chem 2401 Spin-Spin Coupling Splitting is observed if the protons oupled to one another are separated by no more than three σ bonds Long-range oupling may our between protons that are separated by more than three bonds and one of the bonds is a double or triple bond Dr. Yuming Zhao Leture Notes for Chem 2401 An Example Of Spin-Spin Coupling Dr. Yuming Zhao Leture Notes for Chem 2401
