Thom H. Dunning

A comparison between polar covalent bonding and hypervalent recoupled pair bonding in diatomic chalcogen halide species {O,S,Se} {F,Cl,Br}

The 2Π ground states and low-lying 4Σ– excited states of the nine diatomic species formed by combining O, S, or Se with F, Cl, or Br were characterized with multireference configuration interaction calculations in order to compare and contrast the behaviour of the bonding in these two states of the chalcogen halides. For each of these species, the 2Π ground state is polar covalently bound via simple singlet coupling of unpaired electrons on each atom. But for each compound there is also a bound 4Σ– state where bond formation requires recoupling the valence p 2 pair of electrons on the chalcogen atom. This mode of bonding makes more electrons available for additional bond formation and is the basis of hypercoordination. The behaviour of S and Se differs significantly from that of O. Although the hypervalent 4Σ– states of the diatomic chalcogen halides are bound for all three elements, the O species are only weakly bound and exhibit minimal recoupling compared with the corresponding S and Se compounds.

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

The dramatic differences between the properties of molecules formed from the late p-block elements of the first row of the periodic table (N–F) and those of the corresponding elements in subsequent rows is well recognized as the first row anomaly. Certain properties of the atoms, such as the relative energies and spatial extents of the ns and np orbitals, can explain some of these differences, but not others. In this Account, we summarize the results of our recent computational studies of the halides of the late p-block elements. Our studies point to a single underlying cause for many of these differences: the ability of the late p-block elements in the second and subsequent rows of the periodic table to form recoupled pair bonds and recoupled pair bond dyads with very electronegative ligands. Recoupled pair bonds form when an electron in a singly occupied ligand orbital recouples the pair of electrons in a doubly occupied lone pair orbital on the central atom, leading to a central atom-ligand bond. Recoupled pair bond dyads occur when a second ligand forms a bond with the orbital left over from the initial recoupled pair bond. Recoupled pair bonds and recoupled pair bond dyads enable the late p-block elements to form remarkably stable hypervalent compounds such as PF5 and SF6 and lead to unexpected excited states in smaller halides of the late p-block elements such as SF and SF2. Recoupled pair bonding also causes the Fn–1X–F bond energies to oscillate dramatically once the normal valences of the central atoms have been satisfied. In addition, recoupled pair bonding provides a lower-energy pathway for inversion in heavily fluorinated compounds (PF3 and PF2H, but not PH2F and PH3) and leads to unusual intermediates and products in reactions involving halogens and late p-block element compounds, such as (CH3)2S + F2. Although this Account focuses on the halides of the second row, late p-block elements, recoupled pair bonds and recoupled pair bond dyads are important in the chemistry of p-block elements beyond the second row (As, Se, and Br) and for compounds of these elements with other very electronegative ligands, such as OH and O. Knowledge of recoupled pair bonding is thus critical to understanding the properties and reactivity of molecules containing the late p-block elements beyond the first row.

Recoupled pair bonding in PFn (n=1-5)

Following our previous studies of hypervalency in SF(n) (n = 1-6) and ClF(n) (n = 1-7), we have characterized the structures and energetics of PF(n) (n = 1-5) species with RCCSD(T) coupled cluster calculations and triple- and quadruple-zeta quality correlation consistent basis sets. The prior studies demonstrated that hypervalent bonding occurs when it is energetically favorable to uncouple a pair of electrons to form new bonds, a process we describe as recoupled pair bonding. In contrast to S and Cl, ground state P((4)S) has no 3p(2) pairs that can be recoupled, but the 3s(2) pair of all three elements is susceptible to recoupled pair bonding when more energetically accessible bonding pathways have been exhausted. We found that this can first occur when F is added to PF(2)(X(2)B(1)), which yields PF(3)(X(1)A(1)) via normal covalent bonding but yields PF(3)(a(3)B(1)) via recoupled pair bonding. PF(3)(a(3)B(1)) lies 92.1 kcal/mol above PF(3)(X(1)A(1)) but is still bound by 42.0 kcal/mol with respect to PF(2)(X(2)B(1)) + F at the RCCSD(T)/aug-cc-pVQZ level. We characterized both of the isomers of PF(4): the more stable and familiar one that has two covalent equatorial bonds and two axial hypervalent bonds (that use both electrons of the recoupled 3s(2) pair) and the less-studied one that has three covalent bonds and only one hypervalent bond. The transition state between these two minima was also located. In addition to the states that can be formed from P((4)S), there is another group of low-lying excited state species that can be formed from P((2)D) via various combinations of covalent and recoupled pair bonding. Additions of the latter type include PF(B(3)Pi) formed from P((1)D) + F and PF(2)(B(2)B(2)) formed from either PF(a(1)Delta) + F or PF(B(3)Pi) + F.

Insights into the Perplexing Nature of the Bonding in C2 from Generalized Valence Bond Calculations

Diatomic carbon, C2, has been variously described as having a double, triple, or quadruple bond. In this article, we report full generalized valence bond (GVB) calculations on C2. The GVB wave function-more accurate than the Hartree-Fock wave function and easier to interpret than traditional multiconfiguration wave functions-is well-suited for characterizing the bonding in C2. The GVB calculations show that the electronic wave function of C2 is not well described by a product of singlet-coupled, shared electron pairs (perfect pairing), which is the theoretical basis for covalent chemical bonds. Rather, C2 is best described as having a traditional covalent σ bond with the electrons in the remaining orbitals of the two carbon atoms antiferromagnetically coupled. However, even this description is incomplete as the perfect pairing spin function also makes a significant contribution to the full GVB wave function. The complicated structure of the wave function of C2 is the source of the uncertainty about the nature of the bonding in this molecule.

A DFT and ab Initio Benchmarking Study of Metal-Alkane Interactions and the Activation of Carbon-Hydrogen Bonds

Density functional theory and ab initio methods have been used to calculate the structures and energies of minima and transition states for the reactions of methane coordinated to a transition metal. The reactions studied are reversible C-H bond activation of the coordinated methane ligand to form a transition metal methyl hydride complex and dissociation of the coordinated methane ligand. The reaction sequence can be summarized as L(x)M(CH(3))H <==> L(x)M(CH(4)) <==> L(x)M + CH(4), where L(x)M is the osmium-containing fragment (C(5)H(5))Os(R(2)PCH(2)PR(2))(+) and R is H or CH(3). Three-center metal-carbon-hydrogen interactions play an important role in this system. Both basis sets and functionals have been benchmarked in this work, including new correlation consistent basis sets for a third transition series element, osmium. Double zeta quality correlation consistent basis sets yield energies close to those from calculations with quadruple-zeta basis sets, with variations that are smaller than the differences between functionals. The energies of important species on the potential energy surface, calculated by using 10 DFT functionals, are compared both to experimental values and to CCSD(T) single point calculations. Kohn-Sham natural bond orbital descriptions are used to understand the differences between functionals. Older functionals favor electrostatic interactions over weak donor-acceptor interactions and, therefore, are not particularly well suited for describing systems--such as sigma-complexes--in which the latter are dominant. Newer kinetic and dispersion-corrected functionals such as MPW1K and M05-2X provide significantly better descriptions of the bonding interactions, as judged by their ability to predict energies closer to CCSD(T) values. Kohn-Sham and natural bond orbitals are used to differentiate between bonding descriptions. Our evaluations of these basis sets and DFT functionals lead us to recommend the use of dispersion corrected functionals in conjunction with double-zeta or larger basis sets with polarization functions for calculations involving weak interactions, such as those found in sigma-complexes with transition metals.

Bonding in Sulfur–Oxygen Compounds—HSO/SOH and SOO/OSO: An Example of Recoupled Pair π Bonding

The ground states (X(2)A″) of HSO and SOH are extremely close in energy, yet their molecular structures differ dramatically, e.g., re(SO) is 1.485 Å in HSO and 1.632 Å in SOH. The SO bond is also much stronger in HSO than in SOH: 100.3 kcal/mol versus 78.8 kcal/mol [RCCSD(T)-F12/AVTZ]. Similar differences are found in the SO2 isomers, SOO and OSO, depending on whether the second oxygen atom binds to oxygen or sulfur. We report generalized valence bond and RCCSD(T)-F12 calculations on HSO/SOH and OSO/SOO and analyze the bonding in all four species. We find that HSO has a shorter and stronger SO bond than SOH due to the presence of a recoupled pair bond in the π(a″) system of HSO. Similarly, the bonding in SOO and OSO differs greatly. SOO is like ozone and has substantial diradical character, while OSO has two recoupled pair π bonds and negligible diradical character. The ability of the sulfur atom to form recoupled pair bonds provides a natural explanation for the dramatic variation in the bonding in these and many other sulfur-oxygen compounds.

Insights into the Electronic Structure of Molecules from Generalized Valence Bond Theory

In this article we describe the unique insights into the electronic structure of molecules provided by generalized valence bond (GVB) theory. We consider selected prototypical hydrocarbons as well as a number of hypervalent molecules and a set of first- and second-row valence isoelectronic species. The GVB wave function is obtained by variationally optimizing the orbitals and spin coupling in the valence bond wave function. The GVB wave function is a generalization of the Hartree-Fock (HF) wave function, lifting the double occupancy restriction on a subset of the HF orbitals as well as the associated orthogonality and spin coupling constraints. The GVB wave function includes a major fraction (if not all) of the nondynamical correlation energy of a molecule. Because of this, GVB theory properly describes bond formation and can answer one of the most compelling questions in chemistry: How are atoms changed by molecular formation? We show that GVB theory provides a unified description of the nature of the bonding in all of the above molecular species as well as contributing new insights into the well-known, but poorly understood, first-row anomaly.

Theoretical studies of the excited doublet states of SF and SCl and singlet states of SF2, SFCl, and SCl2.

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.

Insights into the unusual barrierless reaction between two closed shell molecules, (CH3)2S + F2, and its H2S + F2 analogue: role of recoupled pair bonding.

Early flowtube studies showed that (CH(3))(2)S (DMS) reacted very rapidly with F(2); hydrogen sulfide (H(2)S), however, did not. Recent crossed molecular beam studies found no barrier to the reaction between DMS and F(2) to form CH(2)S(F)CH(3) + HF. At higher collision energies, a second product channel yielding (CH(3))(2)S-F + F was identified. Both reaction channels proceed through an intermediate with an unusual (CH(3))(2)S-F-F bond structure. Curiously, these experimental studies have found no evidence of direct F(2) addition to DMS, resulting in (CH(3))(2)SF(2), despite the fact that the isomer in which both fluorines occupy axial positions is the lowest energy product. We have characterized both reactions, H(2)S + F(2) and DMS + F(2), with high-level ab initio and generalized valence bond calculations. We found that recoupled pair bonding accounts for the structure and stability of the intermediates present in both reactions. Further, all sulfur products possess recoupled pair bonds with CH(2)S(F)CH(3) having an unusual recoupled pair bond dyad involving π bonding. In addition to explaining why DMS reacts readily with F(2) while H(2)S does not, we have studied the pathways for direct F(2) addition to both sulfide species and found that (for (CH(3))(2)S + F(2)) the CH(2)S(F)CH(3) + HF channel dominates the potential energy surface, effectively blocking access to F(2) addition. In the H(2)S + F(2) system, the energy of the transition state for formation of H(2)SF(2) lies very close to the H(2)SF + F asymptote, making the potential pathway a roaming atom mechanism.

Insights into the electronic structure of disulfur tetrafluoride isomers from generalized valence bond theory.

Sulfur and fluorine can participate in a variety of bonding motifs, lending significant diversity to their chemistry. Prior work has identified three distinct minima for disulfur tetrafluoride (S2F4) compounds: two FSSF3 isomers and one SSF4 species. We used a combination of sophisticated explicitly correlated coupled cluster calculations and generalized valence bond (GVB) theory to characterize the electronic structure of these species as well as additional stationary points on the potential energy surface with F2SSF2 connectivity. On the singlet surface, the two stationary points considered in this work with an F2SSF2 structure are first- or second-order saddle points and not minima. However, on the triplet surface, we discovered a novel C2 symmetric F2SSF2 minimum that was anticipated from the structure of an excited state ((3)B1) of SF2. Analysis using the GVB wave function in conjunction with the recoupled pair bonding model developed by our group provides a straightforward explanation of the bonding in all of the S2F4 structures considered here. In addition, the model predicted the existence of the F2SSF2((3)B) minimum.