THE LOW-LYING STATES OF SF$_n$ SPECIES ($n$=1--6): INSIGHTS INTO HYPERVALENCY FROM THE RECOUPLED PAIR BONDING MODEL

Abstract

High level MRCI and RCCSD(T) calculations with correlation consistent basis sets were used to characterize SF$_n$ species. By examining both the stable structures and the bonding processes that occur during SF$_n$ + F $\rightarrow$ SF$_{n+1}$ additions, we have derived a new model for describing hypervalent behavior that we call recoupled pair bonding, in which a pair of electrons on S can be decoupled to allow formation of a bond with F. The new model accounts for the origin of hypervalency, the presence of low-lying excited states, and the structures and spectral properties of neutral and ionic SF$_n$ species; it has more predictive capability than other models. For example, while SF and SF$_2$ both have covalently bonded ground states, they each have low-lying excited states with at least one recoupled pair bond. To the best of our knowledge, the SF($^4\Sigma^-$), SF$_2$($^3B_1$), and SF$_2$($^3A_2$) states have not yet been observed experimentally.