 Sigma and pi bonds in IUPAC Gold Book

Primary terminology entry S05434, DOI10.1351/goldbook.S05434; localized two-centre sigma/pi symmetry definitions and distinction from whole-molecule orbital symmetry. Official indexed entry and official PDF goldbook.iupac.org/terms/view/S05434/pdf verified; direct HTML fetch returned403. Definition authority, not an experimental bond-energy measurement.