I recently noticed that the 3JHH coupling constants from the C-6 to C-5 hydrogen atoms in 3-substituted pyridines are often in the range of 4-5 Hz. In pyridine itself, I found 4.8 Hz at the CHM605 site at the University of Wisconsin. The same site gives 6.0 to 9.5 as the range for aromatic coupling constants, but heteroaromatic compounds can sometimes have very small coupling constants. At the section of the site called, "5-HMR-5 Vicinal Proton-Proton Coupling 3JHH" there is mention of the effect of pi bond order. I am not sure whether this is the reason in pyridines. Just curious.