One of the most prevalent classes of sigmatropic shifts is classified as where "j" is odd. That means one terminus of the σ-bond migrates ("j" − 1) bonds away across a π-system while the other terminus does not migrate. It is a reaction involving "j" + 1 electrons: j − 1 from the π-system and 2 from σ-bond. Using FMO analysis, [1,"j"-sigmatropic rearrangements are allowed if the transition state has constructive overlap between the migrating group and the accepting p orbital of the HOMO. In [1,"j"]-sigmatropic rearrangements if "j" + 1 = 4"n", then supra/antara is thermally allowed, and if "j" + 1 = 4"n" + 2, then supra/supra or antara/antara is thermally allowed.