> as lemmas

1929 R. S. Mulliken in Chem. Rev. 6 532 The molecule contains two 1sσ electrons (1sσ²)..and two 3sσ electrons, and (in NO and O₂) one or two 3ρπ electrons... The second (Greek) letter gives the value of a quantum number λ which does not exist for the atom, (σ, π, δ,…mean λ = 0, 1, 2,…).

1939 J. W. T. Spinks tr. G. Herzberg Molecular Spectra v. 260 For multiplet Σ states the character positive-negative depends on whether K..is even or odd.

1952 L. N. Ferguson Electron Struct. Org. Molecules ii. 21 In butadiene, each carbon atom forms three σ bonds.

1963 W. J. Moore Physical Chem. (ed. 4) xiv. 600 In the electronic excitation an electron is removed from a π orbital and placed in a σ orbital.

1964 R. G. Parr Quantum Theory Molec. Electronic Struct. iii. 41 It is supposed that somehow the effect of the others, the sigma electrons, can be lumped into the Hamiltonian for the pi electrons.

1966 C. S. G. Phillips & R. J. P. Williams Inorg. Chem. II. xxviii. 341 The vinyl anion, C₂H₃⁻, binds directly to cobalt (III) of vitamin B₁₂ as a simple σ-bonded ligand.

1972 C. H. DePuy & O. L. Chapman Molecular Reactions & Photochem. vi. 103 The numbers refer to the atoms at either end of the sigma bond which is thought of as moving.

1978 P. W. Atkins Physical Chem. xv. 471 Thus an oxygen molecule, which..has two π*-electrons, could be in either a Σ state (the electrons orbiting in opposite directions) or in a Δ state (the electrons orbiting in the same sense around the bond).