TAILIEUCHUNG - Understanding NMR Spectroscopy phần 2

quang phổ kế hoạt động với một tần số Larmor là 600 MHz cho proton. Đối với một tập đặc biệt tăng cường độ trường RF, ω1 / (2π) đã được xác định là 25 kHz. Giả sử rằng máy phát được đặt ở 5 ppm, là những gì bù đắp (Hz) của một đỉnh cao ở mức 10 ppm? | Strong coupling 2-17 full spectrum J I . Ấ A JIM A V0 A V0 B 0 p sub-spectrum Á Ỷ . -V0 A 1 2JAX -V0 B 1 2JBX a sub-spectrum ư 10 -V0 A-1 2JAX 20 30 Ỳ V0 B 1 2JBX 40 50 frequency Hz Fig. AB parts of an ABX spectrum illustrating the decomposition into two sub-spectra with different effective Larmor frequencies indicated by the arrows . The parameters used in the simulation were v0 A -20 Hz V0 B -30 Hz JAB 5 Hz JAX 15 Hz and JBX 3 Hz. As spin X is weakly coupled to the others we can think of the AB part of the spectrum as two superimposed AB sub-spectra one is associated with the X spin being in the a state and the other with the spin being in the 3 state. If spins A and B have Larmor frequencies V0 A and V0 B respectively then one sub-spectrum has effective Larmor frequencies V0 A 2 JAX and V0 B 2 JBX. The other has effective Larmor frequencies V0 A 2 JAX and V0 B 2 JBX. The separation between the two effective Larmor frequencies in the two subspectra can easily be different and so the degree of strong coupling and hence the intensity patterns in the two subspectra will be different. All we can measure is the complete spectrum the sum of the two sub-spectra but once we know that it is in fact the sum of two AB-type spectra it is usually possible to disentangle these two contributions. Once we have done this the two sub-spectra can be analysed in exactly the way described above for an AB system. Figure illustrates this decomposition. The form of the X part of the ABX spectrum cannot be deduced from this simple analysis. In general it contains 6 lines rather than the four which would be expected in the weak coupling limit. The two extra lines are combination lines which become observable when strong coupling is present. 2-18 NMR and energy levels Of course in reality the Larmor frequencies out to be tens or hundreds of MHz not 100 Hz However it makes the numbers easier to handle if we use these unrealistic small values the principles remain the same .

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