tailieunhanh - Understanding NMR Spectroscopy phần 5

Một vô giá trị hơn nữa trong các tín hiệu đã được nhìn thấy ở 41,0 ms, để bạn làm những gì thuộc tính này? Đ 3-3 Sử dụng sơ đồ vector Như được hiển thị trong phần 6,9, một thuật ngữ đôi lượng tử phát triển dưới theo bù đắp cho | 180 rotation to each in succession cos J T Lr sin J T 2LJi 12 1x 12 1y 2 z where it has already been written in that O1tp n for a 180 pulse. The 180 rotation about x for spin 1 has no effect on the operator I1 x and I2 z and it simply reverses the sign of the operator I1 y cos J tL sin J t2 LJ- cos J tL - sin nJ t2 LJt 12 1x 12 1y 2 z 12 1x 12 1y 2 z The 180 rotation about x for spin 2 has no effect on the operators I1 x and I1 y but simply reverses the sign of the operator I2 z. The final result is thus cos nJ 2tI x sin nJ12T2I1yI2z cos J12tI1x - sin nJ . T 2I1yI2z cos nJ Tjr sin nJ t2 LL 12 1x 12 1y 2 z Nothing has happened the 180 pulse has left the operators unaffected So for the purposes of the calculation it is permissible to ignore the 180 pulse and simply allow the coupling to evolve for 2t. The final result can therefore just be written down I T-180 x -T C0S2J T I. sin2nJ12T 2I. I2 1x 12 1 x 12 1 y 2 z From this it is easy to see that complete conversion to anti-phase magnetization requires 2nJ12T n 2 . T 1 4 J12 . The calculation is not quite as simple if the initial state is chosen as I1 y but the final result is just the same - the coupling evolves for 2t. I -cos2nJ12TI. sin2nJ12T 1 y 12 1 y 12 1x 2 z In fact the general result is that the sequence - T - 180 x to spin 1 and spin 2 - T - is equivalent to the sequence - 2t- 180 x to spin 1 and spin 2 in which the offset is ignored and coupling is allowed to act for time 2t. Interconverting in-phase and anti-phase states So far spin echoes have been demonstrated as being useful for generating antiphase terms independent of offsets. For example the sequence 90 x - 1 4J12 - 180 x - 1 4J12 -generates pure anti-phase magnetization. Equally useful is the sequence - 1 4J12 - 180 x - 1 4J12 - which will convert pure anti-phase magnetization such as 2I1 xI2 z into in-phase magnetization I1 y . 6-10 Spin echoes in heteronuclear spin systems If spin 1 and spin 2 are different nuclear species .