Nationalacademy of Sciences

Sulfur is unique among the elements in consisting in its standard state of octatomic molecules. It is interesting that the stability of the S8 molecule can be understood by consideration of the forces determining the orientation around the sulfur-sulfur single bond. Let us consider a molecule such as H2S2, in which each sulfur atom forms a single bond with the other sulfur atom and a single bond with another atom (hydrogen), and has in addition two unshared electron pairs in its outer shell. The bonds formed by the sulfur atoms involve mainly 3p orbi-tals, and one of the two unshared outer electron pairs occupies the third 3p orbital, the other unshared pair occupying essentially the 4s orbital. (A rough theoretical treatment which will be communicated later indicates that the bond orbitals have only about 1% s character.) The dihedral angle between the planes H-S-S and S-S-H is determined principally by the repulsion of p7r electrons. The repulsion between p7r electrons is at a maximum when the p7r orbitals of the two sulfur atoms are in the same plane, and falls to zero when these orbitals are in orthogonal planes. The energy of interaction of an unshared pair of pfr electrons on one atom and the similar unshared pair on the other atom in a coplanar p7r orbital is equal to-2 a (the negative sign corresponding to repulsion, because a, the exchange integral occurring in the electron-pair bond theory, is itself negative), that between an unshared pair on one atom and a single bonding electron on another is-a, and that between two bonding electrons is-1/2a. Hence the total interaction energy of the pir electrons in H2S2 varies from-5/2a for a planar configuration of the molecule (dihedral angle 'y equal to 0° or 1800) to a minimum of-2a for y = 900. It is then to be predicted that the molecule H2S2 has a skew configuration, with the dihedral angle y equal to approximately 900. The configuration of this molecule has not been precisely de