UNDERSTANDING THE MOLECULAR PROPERTIES OF ClF$_n$ ({$n=$1-7}) SPECIES: AN APPLICATION OF THE RECOUPLED PAIR BONDING MODEL FOR HYPERVALENT BONDS
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Date
2009
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Ohio State University
Abstract
Recently, new insight into the nature of hypervalent behavior led us to develop a model called recoupled pair bonding. In this model, two hypervalent bonds can be formed by decoupling a valence $p^2$ or $s^2$ electron pair. However, energy must be expended to decouple an electron pair, and the first bond is weakened as a consequence. The recoupled pair bonding model has been proven successful in our initial study of the SF$_n$ ({$n=$1-7}) species. To further examine the applicability of this new model, this study explored the molecular properties of the ClF$_n$ ({$n=$1-7}) series. Optimized ground state structures, bond energies, and spectral properties of these molecules were obtained by employing high level ab initio calculations [MRCI, CCSD(T)] with correlation consistent basis sets. Because of recoupled pair bonding, there are unanticipated low-lying excited states such as ClF (${^3}{\Pi}$) and ClF$_2$ (${^2}{\Pi}$,${^4}{\Sigma}$). We also systematically explored the bond formation processes, adding F atoms one at a time to the optimized ClF$_n$ ({1 $\leq n \leq$ 6 }) molecules. We find the bond energies for F addition to form ClF$_2$, ClF$_4$, and ClF$_6$ are much lower than those leading to ClF, ClF$_3$ and ClF$_5$. This oscillating trend is analogous to what is seen in the SF$_n$ species, though the bond energies of the SF$_n$ species are considerably greater than the ones for ClF$_n$. The lower bond energies of the even $n$ species in the ClF$_n$ series reflects the cost of decoupling paired electrons of the central atom, and the difference between ClF$_n$ and SF$_n$ reflects the fact that more energy is needed to decouple each of the $3p^2$ pairs of electrons of Cl than the single $3p^2$ pair of S. This behavior and other trends observed in ClF$_n$ species demonstrate the improved predictive ability of the recoupled pair bonding model over other models for describing hypervalent bonding.
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Author Institution: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801