核磁共振实验课.ppt

静磁场B0中大π键环电流的作用 (4n+2)个π电子形成芳香性的抗磁环流 4n个π电子在静磁场中会形成顺磁性的环流，屏蔽和非屏蔽区域与(4n+2)个π电子的抗磁环流恰好相反。 单键、双键的磁各向异性 空间距离、Van Der Waals半径和化学位移 Hb：3.55 Ha：0.88 CH2(环己烷)：1.40 极性键会产生电场影响附近的氢原子，这个电场破坏了氢核外电子结构，是去屏蔽作用，而且是通过空间进行作用的。极性键需要靠近氢核，但不需要近到Van Der Waals半径之和以内 I1x, I2x is in-phase I1y, I2y is in-phase, but with a dispersion shape 2I1xI2z, 2I1yI2z, 2I2xI1z, 2I2yI1z: anti-phase 2I1xI2x, 2I1yI2y, 1I1yI2x, 2I1xI2y: multi-quantum For 1H at 14.1 Tesla (600 MHz) the ratio of the two populations is only: N+1/2 / N-1/2 = 1.0002. Because ω0 is proportional to γ and B → 0 , nuclei with high γ are more sensitive than those with low γ, and higher magnetic fields increase the sensitivity dramatically. In practice, the sensitivity of NMR spectrometers increases approximately according to B3/2. The bandwidth of excitation ( the frequency region in which spins are more or less equally excited) is directly dependent on the intensity of the pulse (peak to peak voltage, B1 amplitude). The first zero crossing of the excitation function occurs at ω0/2π± γB1/2π Hz. * * * * A simple solenoid coil best illustrates how RF energy, in the form of alternating current (AC), is transmitted into the sample (placed inside the coil). Current passing through the coil at a given instant induces a magnetic field (direction is determined by right-hand rule). By alternating the current (switching the direction in a sinusoidal manner), a llinearly polarized, alternating magnetic field is generated along the center axis of the coil. The coil is placed in the static magnetic field (Bo) such that the axis of the coil is perpendicular to Bo as shown. This same coil is used to receive the signal from the sample. The nuclei act like tiny bar magnets, and the net magnetization vector also acts like a tiny bar magnet. We have just seen how, in a non-equilibrium situation, M precesses about Bo. This oscillating bar magnet induces a current in the surrounding coil, and it is this tiny current that we measure as the NM