Final answer:
The two main coupling mechanisms in NMR spectroscopy are scalar coupling, which is through-bond and observable in liquid NMR, and dipolar coupling, which is through-space and significant in solid-state NMR.
Step-by-step explanation:
In NMR spectroscopy, the two main coupling mechanisms mentioned are scalar coupling (also known as J-coupling or through-bond coupling) and dipolar coupling (also referred to as through-space coupling).
Scalar coupling occurs through chemical bonds and involves the interaction between nuclear spins mediated by the bonding electrons, being predominantly observable in high-resolution NMR of liquids. On the other hand, dipolar coupling is the direct interaction between nuclear magnetic moments and is significant in solids where molecules do not tumble rapidly.