J耦合、谱分析以及偶极耦合.ppt

J coupling Spin-spin Coupling dipole-dipole interaction： magnetic dipole interaction between nuclei directly Spin-spin Coupling： It will occur between two magnetically active nuclei that are connected through chemical bonds. We can see it for two atoms directly connected, or for atoms that ‘see’ one another across several bonds. Couplings are perhaps the most important parameter in NMR, as they allow us to elucidate chemical structure. Example J Coupling J Coupling J Coupling Example J Coupling J Coupling J Coupling J Coupling J Coupling J Coupling Analysis of 1st Order Systems Analysis of 1st Order Systems In a spin system in which we have a certain nuclei coupled to more than one nuclei, all first order, the splitting will be basically an extension of what we saw before. Say that we have a CH (A) coupled to a CH3 (M) with a JAM of 7 Hz, and to a CH2 (X) with a JAX of 3 Hz. We basically go in steps. First the big coupling, which will give a quartet: Then the small coupling, which will split each line in the quartet into a triplet: This is called a triplet of quartets (big effect is the last…). Analysis of 1st Order Systems Analysis of 2nd Order Systems Magnetic Chemical Equivalence As we saw last time, we can do a very simple first order analysis of this spin system, because we assumed that all CH2 protons were ‘equal’, and all CH3 protons were ‘equal’. We can easily see that they are chemically equivalent. Additionally, we have free rotation around the bond, which makes their chemical shifts and couplings equal. So in effect, the coupling of any proton in CH2 to any proton in the CH3 will be the same. Magnetic Chemical Equivalence Spectral Analysis Dipolar coupling Scalar and Dipolar Coupling Thank You ! * * I S J ? 0 I S J = 0 Ethanol: CH3CH2OH Three singlets The spectrum is a result of spin-spin coupling (scale coupling) Each nucleus act as a small magnet and will affec