3. Nuclear Magnetic Resonance Spectra课件.pptVIP

3. Nuclear Magnetic Resonance Spectra ; 1945 E. M. Purcell in Harvard University F. Bloch in Stanford University won 1952 Nobel Physical 1992 Prof. R.R. Ernst won Nobel chemistry Prize ( 2D-NMR Fourier Transform NMR) Now HPLC-NMR High-sensitivity superconducting probes (HNMR CNMR will be discuss in this section );1. Basic Concept for Nuclear Magnetic Resonance ;For each nucleus with spin, the number of allowed spin states it may adopt is quantized and is determined by its nuclear spin quantum number, I. There are 2I+1 allowed spin states with integral differences ranging from +I to –I. The individual spin states fit into the sequence +I, (I-1), (-I+1),-I For instance, a proton (hydrogen nucleus) has the spin quantum number I=1/2 and has two allowed spin states [2(1/2)+1=2] for its nucleus: -1/2 and +1/2 ;Spin quantum Numbers of Some Common Nuclei;Problem: No Applied Magnetic field,How about the energy of spin states?; When the Applied is employed, the nucleus has a magnetic moment, ? , generated. ; As an external magnetic field is applied, the degenerate spin states split into two states of unequal energy, as shown below: ;The nuclear magnetic resonance phenomenon occurs when the nuclei aligned with an applied field are induced to absorb energy and change their spin orientation with respect to the applied field. The energy absorption is a quantized process, and the energy must equal the energy difference between two states involved. ??E=?(h/2?)B0=h? or ? =(? /2 ? ) B0 ， Where ? =frequency, in Hz B0 = Strength of the magnetic field, in gauss ?= a nuclear constant, the gyromagnetic ratio ;Place a substance in a magnetic field of constant strength, and then obtain a spectrum : pass radiation of steadily changing frequency through the substance, and observe the frequency at which radiation is absorbed. Keep the radiation frequency constant, and to vary the strength of the magnetic field;(Field-sweep) at