Fundamental aspects of recoupled pair bonds. II. Recoupled pair bond dyads in carbon and sulfur difluoride.

Formation of a bond between a second ligand and a molecule with a recoupled pair bond results in a recoupled pair bond dyad. We examine the recoupled pair bond dyads in the a(3)B1 states of CF2 and SF2, which are formed by the addition of a fluorine atom to the a(4)Σ(-) states of CF and SF, both of which possess recoupled pair bonds. The two dyads are very different. In SF2, the second FS-F bond is very strong (De = 106.3 kcal/mol), the bond length is much shorter than that in the SF(a(4)Σ(-)) state (1.666 Å versus 1.882 Å), and the three atoms are nearly collinear (θe = 162.7°) with only a small barrier to linearity (0.4 kcal/mol). In CF2, the second FC-F bond is also very strong (De = 149.5 kcal/mol), but the bond is only slightly shorter than that in the CF(a(4)Σ(-)) state (1.314 Å versus 1.327 Å), and the molecule is strongly bent (θe = 119.0°) with an 80.5 kcal/mol barrier to linearity. The a(3)B1 states of CF2 and SF2 illustrate the fundamental differences between recoupled pair bond dyads formed from 2s and 3p lone pairs.