The original Bürgi-Dunitz measurements were of a series of intramolecular amine-ketone carbonyl interactions, in crystals of compounds bearing both functionalities—e.g., methadone and protopine. These gave a narrow range of BD angle values (105 ± 5°); corresponding computations—molecular orbital calculations of the SCF-LCAO-type—describing the approach of the s-orbital of a hydride anion (H) to the pi-system of the simplest aldehyde, formaldehyde (HC=O), gave a BD angle value of 107°.
Hence, Bürgi, Dunitz, and thereafter many others noted that the crystallographic measurements of the aminoketones and the computational estimate for the simplest nucleophile-electrophile system were quite close to a theoretical ideal, the tetrahedral angle (internal angles of a tetrahedron, 109.5°), and so consistent with a geometry understood to be important to developing transition states in nucleophilic attacks at trigonal centers.
In the structure of -methadone (above, left), note the tertiary amine projecting to the lower right, and the carbonyl (CO) group at the center, which engage in an intramolecular interaction in the crystal structure (after rotation around the single bonds connecting them, during the crystallization process).
Similarly, in the structure of protopine (above, center), note the tertiary amine at the center of the molecule, part of a ten-membered ring, and the CO group opposite it on the ring; these engage in an intramolecular interaction allowed by changes in the torsion angles of the atoms of the ring.