Consider a spin system consisting of a l3C, spin X, and two protons, A and B. The carbon has a one-bond coupling of 150 Hz to proton A, and a long-range coupling of 15 Hz to proton B. Protons A and B are also coupled.
(a) Describe the circumstances in which degree of strong coupling in the two AB subspectra will be substantially different.
(b) Assuming that the proton Larmor frequency is 500 MHz, compute the difference in chemical shift (in ppm) between protons A and B which will result in infinite strong coupling in first one subspectrum and then in the other. [Hint: compute the frequency separation in Hz and then convert this to ppm.]
(c) Explain why your answers will be the same if the sign of the A-B coupling is reversed, but will change if the sign of the B-X coupling is reversed.
(d) Explain why it is that in a homonuclear AB spin system the degree of strong coupling always decreases as the static field is increased, whereas in an ABX system of the type described at the start of the question increasing the static field can, under some circumstances, increase the degree of strong coupling.