Relative magnitudes of 2- and 3-bond coupling constants

Why is geminal coupling weaker than 3-bond coupling in complex coupling? It would make more sense that atoms that are closer to each other influence each other more. Take methyl acrylate for example. The $\mathrm{H_a}$ is causing the bigger split, and the $\mathrm{H_b}$ the smaller ones. Why?

• chem.wisc.edu/areas/reich/nmr/05-hmr-04-2j.htm – DavePhD Apr 26 '16 at 20:03
• J coupling is mediated by the interaction of spin-polarised molecular orbitals. 120° bond angle geminal orbitals just don't interact well, as opposed to the situation at an sp3-hybridised CH2 group. – Karl Apr 26 '16 at 20:21
• What do you mean by weaker (and stronger)? You can’t weakly couple with pregnancy (i.e. can’t be a little bit pregnant). Unless you are talking about the magnitude of coupling? – Jan Apr 26 '16 at 20:43
• @DavePhD - Hooray! Someone else is onto Hans Reich's page as a great resource. – long Apr 26 '16 at 21:38
• @Jan - Take care when using the terms weaker and stronger when referring to coupling in NMR. A weakly coupled system is one where the distance between resonances is much greater than the coupling constant. Strongly coupled nuclei are closer together, and give rise to roofing/tenting in spectra. An AB spin system is a strongly coupled system. – long Apr 26 '16 at 21:45