核磁共振中自旋裂分或j偶合核磁共振中自旋裂分或j偶合.docVIP

核磁共振中自旋裂分或J偶合 Spin-spin splitting or J coupling Coupling in 1H NMR spectra We have discussed how the chemical shift of an NMR absorption is affected by the magnetic field Be produced by the circulation of neighboring electrons. Now we wish to examine how the magnetic field produced by neighboring nuclei Bn affects the appearance of the 1H NMR absorption. The effect occurs through the interaction of nuclear spins with bonding electron spins rather than through space. Lets first consider the absorption of a hydrogen nucleus labeled A with only one neighboring hydrogen nucleus in a vicinal position labeled X. Lets also assume that HA and HX have significantly different chemical shifts. HX will have approximately equal probability of existing in either the low energy alpha state or high energy beta state. Again because of the small energy difference between the low and high energy states, the high energy state is easily populated from thermal energy. For those molecules in which HX exists in the low energy state, about half the molecules in the sample, its magnetic field Bn will subtract from the magnetic field Bo-Be and for those molecules in which HX exists in the higher energy state, again about half the molecules, its magnetic field Bn will add to Bo-Be. Note: whether Bn for a particular spin state adds to or subtracts from Bo is a function of the number of intervening bonds; this phenomenon doesnt usually affect the appearance of the signal and will not be explained here but results from the mechanism of coupling involving interaction of nuclear spins with electron spins. For the example of vicinal coupling (3 intervening bonds), the Bn field is negative for HX in the alpha spin state; for geminal coupling Bn is positive for HX in the alpha spin state. Geminal coupling occurs between protons of different chemical shift bonded to the same carbon (2 intervening bonds); it will be discussed later. As a consequence of the Bn field in a vicinal system, at fixed exter