Bonding and Isomerism in SF n −1 Cl ( n = 1−6): A Quantum Chemical Study

The Journal of Chemical Physics

Takeshita

2015

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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 a3B1 states of CF2 and SF2, which are formed by the addition of a fluorine atom to the a4Σ− 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(a4Σ−) 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(a4Σ−) 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 a3B1 states of CF2 and SF2 illustrate the fundamental differences between recoupled pair bond dyads formed from 2s and 3p lone pairs.

Section: Discussionsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 82%

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

The Journal of Chemical Physics

Takeshita

2015

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“…As has been shown by Dunning and coworkers, the structures and energetics of all of the hypervalent molecules that they studied-SFn, 137 SFn-1Cl, 143 SCln, 144 PFn, 145 PFn-1Cl, 146 H(SO), 147 S2F4, 148,149 SO2, 150,151 ClnSO, 152 ClFn, 153 ClFn + , 154 and ClFn -155 -follow the pattern observed above for SFn (see also Refs. 156, 157, and 158).…”

supporting

confidence: 76%

Spin-Coupled Generalized Valence Bond Theory: New Perspectives on the Electronic Structure of Molecules and Chemical Bonds

Cooper

et al. 2021

J. Phys. Chem. A

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Spin-Coupled Generalized Valence Bond (SCGVB) theory provides the foundation for a comprehensive theory of the electronic structure of molecules. SCGVB theory offers a compelling orbital description of the electronic structure of molecules as well as an efficient and effective zero-order wave function for calculations striving for quantitative predictions of molecular structures, energetics and other properties. The orbitals in the SCGVB wave function are usually semi-localized and, for most molecules, can be interpreted using concepts familiar to all chemists (hybrid orbitals, localized bond pair, lone pairs, etc.). SCGVB theory also provides new perspectives on the nature of the bonds in molecules such as C2, Be2 and SF4/SF6. SCGVB theory contributes unparalleled insights into the underlying cause of the first-row anomaly in inorganic chemistry as well as the electronic structure of organic molecules and the electronic mechanisms of organic reactions. The SCGVB wave function accounts for nondynamical correlation effects and, thus, corrects the most serious deficiency in molecular orbital (RHF) wave functions. Dynamical correlation effects, which are critical for quantitative predictions, can be taken into account using the SCGVB wave function as the zero-order wave function for multireference configuration interaction or coupled cluster calculations.* In prior articles in the literature the Spin-Coupled Generalized Valence Bond wave function has usually been referred to as the Spin-Coupled Valence Bond (SCVB) wave function or the full Generalized Valence Bond (full GVB) wave function. These wave functions are identical; to prevent confusion, the authors have adopted the allencompassing SCGVB name. Spin-Coupled Generalized Valence Bond Theory: New Perspectives on the Electronic Structure of Molecules and Chemical Bonds3 and recoupled pair bond dyad. 36 The SCGVB wave function describes molecules in terms of orbitals that are optimized at each geometry, allowing the atomic orbitals to change as a result of molecule formation. In addition to the changes in the orbitals, optimization of the general spin function used in the SCGVB wave function allows the spin function to change from that appropriate for the separated atoms or fragments to that appropriate for the molecule. SCGVB theory addresses a major limitation of RHF theory, providing an excellent zero-order description of molecules that require multiconfiguration descriptions, the making/breaking of bonds, and many chemical reactions and excited states. For most molecules, the SCGVB wave function captures the essential features of the FORS/CASSCF wave functions, with much reduced complexity compared to these latter wave functions.

“…5 Previous studies of the halides of the second row, late p-block elements, by our group have shown that hypervalent or hypercoordinated molecules possess a previously unrecognized type of bond, a recoupled pair bond. [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.…”

Section: Introductionmentioning

confidence: 99%

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

The Journal of Chemical Physics

Takeshita

2015

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The number of singly occupied orbitals in the ground-state atomic configuration of an element defines its nominal valence. For carbon and sulfur, with two singly occupied orbitals in their 3P ground states, the nominal valence is two. However, in both cases, it is possible to form more bonds than indicated by the nominal valence—up to four bonds for carbon and six bonds for sulfur. In carbon, the electrons in the 2s lone pair can participate in bonding, and in sulfur the electrons in both the 3p and 3s lone pairs can participate. Carbon 2s and sulfur 3p recoupled pair bonds are the basis for the tetravalence of carbon and sulfur, and 3s recoupled pair bonds enable sulfur to be hexavalent. In this paper, we report generalized valence bond as well as more accurate calculations on the a4Σ− states of CF and SF, which are archetypal examples of molecules that possess recoupled pair bonds. These calculations provide insights into the fundamental nature of recoupled pair bonds and illustrate the key differences between recoupled pair bonds formed with the 2s lone pair of carbon, as a representative of the early p-block elements, and recoupled pair bonds formed with the 3p lone pair of sulfur, as a representative of the late p-block elements.