Theoretical Studies of the Excited Doublet States of SF and SCl and Singlet States of SF₂, SFCl, and SCl₂

Abstract

In previous work, we reported that the lowest-lying excited states of SF, SCl, SF(2), SFCl, and SCl(2) have recoupled pair bonds. In this study, we examine the analogous low-spin states--the (2)Σ(-) and (2)Δ states of SF and SCl and the excited singlet states of SF(2), SCl(2), and SFCl--which also possess recoupled pair bonds. In contrast to the excited states treated previously, the states studied in the present work have the same spin multiplicities as their respective ground states and are thus potentially observable via electronic excitation. Of particular interest are the minima on the (1)A″ potential energy surface of SFCl corresponding to bond-stretch isomers analogous to those found on the (3)A″ surface. In addition, we discovered that the first two excited states ((1)A″) accessible via vertical excitations from the ground state of SFCl have the electronic structure of the bond-stretch isomers. Thus, electronic excitation spectroscopic studies of SFCl could reveal a signature of the bond-stretch isomers. We will also present limited data on the lowest singlet Rydberg states of the triatomic species. Calculations were performed at the MRCI+Q/aug-cc-pV(Q+d,5+d)Z levels of theory.

Fundamental aspects of recoupled pair bonds. I. Recoupled pair bonds in carbon and sulfur monofluoride

“…At long R, S 13 also increases with decreasing R, reaching a maximum of 0.12 at ∆R = 0.77 Å, after which it decreases to −0.16 at ∆R = 0.43 Å; there is then a very small maximum after which S 13 continues to decrease to −0.19 at R e . S 13 crosses zero at ∆R = 0.56 Å. The large value of S 23 at R e , the magnitude of which is significantly larger in SF than in CF (0.60 versus 0.43) is consistent with resulting much weaker and longer bond in the SF(a 4 Σ − ) state.…”

“…The shape of the S 13 curve in CF is quite different, rising to a maximum of 0.15 at ∆R = 0.68 Å and then decreasing to −0.16 at R e . In both cases, the change in sign of S 13 and S 23 at short R is a manifestation of the phase change in the ϕ a3 orbital.…”

“…The other overlaps in CF, S 13 , and S 23 are energetically unfavorable overlaps since they represent overlaps between an unpaired orbital and essentially a singlet-coupled pair. These overlaps give rise to Pauli repulsion, 39 a consequence of the Pauli principle and is similar to the repulsive interaction in the HeH( 2 Σ + ) state.…”

“…[6][7][8][9][10][11][12][13][14] Recoupled pair bonds and recoupled pair bond dyads 14 the stability of SF 4 and SF 6 , 7 as well as other hypervalent species such as PF 5 , 9 ClF 3 , and ClF 5 , 8 providing a simple, straightforward explanation of the low-lying states, structures, and energetics of these species as well as those of the intermediate SF n , PF n , and ClF n radicals. In fact, it has been found that the ability of elements beyond the first row to form recoupled pair bonds accounts for a number of the differences between the structures, energetics, and spectra of compounds of the first and second row elements, including differences in reactivity 13 -the so-called first-row anomaly. 14 We have also found that recoupled pair bonds and recoupled pair bond dyads are important in the oxides and hydroxides of the second row, late p-block elements.…”

The First Row Anomaly and Recoupled Pair Bonding in the Halides of the Late p-Block Elements

“…Our recent theoretical and computational studies of the second row halides 4−11 have shed further light on these anomalies: all of the above differences can be explained by the ability of the second row, late p-block elements to form a new type of chemical bond—the recoupled pair bond . A recoupled pair bond is formed when an electron in a singly occupied ligand orbital recouples the pair of electrons in a formally doubly occupied lone pair orbital on a central atom, forming a central atom-ligand bond.…”