Structures and energetics of clusters surrounding diatomic anions stabilized by hydrogen, halogen, and other noncovalent bonds

O-H stretching frequency red shifts do not correlate with the dissociation energies in the dimethylether and dimethylsulfide complexes of phenol derivatives

In this perspective, we present a comprehensive report on the spectroscopic and computational investigations of the hydrogen bonded (H-bonded) complexes of Me2O and Me2S with seven para-substituted H-bond donor phenols. The salient finding was that although the dissociation energies, D0, of the Me2O complexes were consistently higher than those of the analogous Me2S complexes, the red-shifts in phenolic O-H frequencies, Δν(O-H), showed the exactly opposite trend. This is in contravention of the general perception that the red shift in the X-H stretching frequency in the X-HY hydrogen bonded complexes is a reliable indicator of H-bond strength (D0), a concept popularly known as the Badger-Bauer rule. This is also in contrast to the trend reported for the H-bonded complexes of H2S/H2O with several para substituted phenols of different pKa values wherein the oxygen centered hydrogen bonded (OCHB) complexes consistently showed higher Δν(O-H) and D0 compared to those of the analogous sulfur centered hydrogen bonded (SCHB) complexes. Our effort was to understand these intriguing observations based on the spectroscopic investigations of 1 : 1 complexes in combination with a variety of high level quantum chemical calculations. Ab initio calculations at the MP2 level and the DFT calculations using various dispersion corrected density functionals (including DFT-D3) were performed on counterpoise corrected surfaces to compute the dissociation energy, D0, of the H-bonded complexes. The importance of anharmonic frequency computations is underscored as they were able to correctly reproduce the observed trend in the relative OH frequency shifts unlike the harmonic frequency computations. We have attempted to find a unified correlation that would globally fit the observed red shifts in the O-H frequency with the H-bonding strength for the four bases, namely, H2S, H2O, Me2O, and Me2S, in this set of H-bond donors. It was found that the proton affinity normalized Δν(O-H) values scale very well with the H-bond strength.

Comparison of Bifurcated Halogen with Hydrogen Bonds

Bifurcated halogen bonds are constructed with FBr and FI as Lewis acids, paired with NH₃ and NCH bases. The first type considered places two bases together with a single acid, while the reverse case of two acids sharing a single base constitutes the second type. These bifurcated systems are compared with the analogous H-bonds wherein FH serves as the acid. In most cases, a bifurcated system is energetically inferior to a single linear bond. There is a larger energetic cost to forcing the single σ-hole of an acid to interact with a pair of bases, than the other way around where two acids engage with the lone pair of a single base. In comparison to FBr and FI, the H-bonding FH acid is better able to participate in a bifurcated sharing with two bases. This behavior is traced to the properties of the monomers, in particular the specific shape of the molecular electrostatic potential, the anisotropy of the orbitals of the acid and base that interact directly with one another, and the angular extent of the total electron density of the two molecules.

Origins and properties of the tetrel bond

The tetrel bond (TB) recruits an element drawn from the C, Si, Ge, Sn, Pb family as electron acceptor in an interaction with a partner Lewis base. The underlying principles that explain this attractive interaction are described in terms of occupied and vacant orbitals, total electron density, and electrostatic potential. These principles facilitate a delineation of the factors that feed into a strong TB. The geometric deformation that occurs within the tetrel-bearing Lewis acid monomer is a particularly important issue, with both primary and secondary effects. As a first-row atom of low polarizability, C is a reluctant participant in TBs, but its preponderance in organic and biochemistry make it extremely important that its potential in this regard be thoroughly understood. The IR and NMR manifestations of tetrel bonding are explored as spectroscopy offers a bridge to experimental examination of this phenomenon. In addition to the most common σ-hole type TBs, discussion is provided of π-hole interactions which are a result of a common alternate covalent bonding pattern of tetrel atoms.

F-Halogen Bond: Conditions for Its Existence

The question as to whether the F atom can engage in a halogen bond (XB) remains unsettled. This issue is addressed via density functional theory calculations which pair a wide range of organic and inorganic F-acids with various sorts of Lewis bases. From an energetic perspective, perfluorinated hydrocarbons with sp, sp², or sp³ C-hybridization are unable to form an XB with an N-base, but a very weak bond can be formed if electron-withdrawing C≡N substituents are added to the acid. There is little improvement for inorganic acids O₂NF, FOF, ClF, BrF, SiF₄, or GeF₄, but F₂ is capable of a stronger XB of up to 5 kcal/mol. These results are consistent with a geometric criterion, which compares the intermolecular equilibrium distance with the sum of atomic van der Waals radii. The intensity of the σ-hole on the F atom has predictive value in that a V_s,max of at least 10-15 kcal/mol is required for XB formation. Adding a positive charge to the Lewis acid enhances the strength of any XB and even more so if the base is anionic. The acid-base interaction induces a contraction of the r(AF) covalent bond in the acid in most cases and a deshielding of the NMR signal of the F nucleus.

Tweaking the Charge Transfer: Bonding Analysis of Bismuth(III) Complexes with a Flexidentate Phosphane Ligand

To account for the charge transfer and covalent character in bonding between P and Bi centers, the electronic structures of [P(C6H4-o-CH2SCH3)3BiCln](3-n)+ (n = 0-3) model species have been investigated computationally. On the basis of this survey a synthetic target compound with a dative P→Bi bond has been selected. Consecutively, the highly reactive bismuth cage [P(C6H4-o-CH2SCH3)3Bi]3+ has been accessed experimentally and characterized. Importantly, our experiments (single-crystal X-ray diffraction and solid-state NMR spectroscopy) and computations (NBO and AIM analysis) reveal that the P···Bi bonding in this trication can be described as a dative bond. Here we have shown that our accordion-like molecular framework allows for tuning of the interaction between P and Bi centers.

How Many Pnicogen Bonds can be Formed to a Central Atom Simultaneously?

A central ZF₃ molecule (Z = P, As, Sb, Bi) is allowed to interact with a number of nucleophiles exemplified by NCH, NH₃, and NC^– anion. The Z···N pnicogen bond (ZB) to a single base grows stronger for heavier Z atom: P < A < Sb < Bi and follows the NCH < NH₃ < NC^– order for the three bases. The maximum number of ZBs depends on both the nature of the base and pnicogen atom. PF₃ and AsF₃ can pnicogen bond with only a single CN^–; SbF₃ and BiF₃ can interact with two anions but only weakly. The weak NCH nucleophile can engage in a maximum of two ZBs, while three ZBs occur for NH₃. The latter NH₃ maximum can be extended to four ZBs but only for BiF₃. The fourth ZB is somewhat longer and weaker than the others, and the entire (H₃N)₄···BiF₃ complex relies partially on secondary interactions for its stability.

Microhydration of a Carbonyl Group: How does the Molecular Electrostatic Potential (MESP) Impact the Formation of (H2O)n:(R2C═O)Complexes?

The presence of a carbonyl group in a molecule usually leads to the identification of a π-hole on the molecular electrostatic potential (MESP) of the species. How does this electrophilic site influence the formation of microhydrated complexes? To address this point, a panel of R₂CO solutes with various MESPs was selected, and we identified the structures and properties of several complexes containing one, two, three and six water molecules. The following solutes were considered in the present study: H₂CO, F₂CO, Cl₂CO,(NC)₂CO and H₂C═CO. Geometry optimizations and frequency calculations were carried out at the LC-ωPBE/6-311++G(d,p) level, with the GD3BJ empirical correction for dispersion. For a number of n water molecules around the R₂CO solute, the structure and the features of the most stable (H₂O)_n:(R₂CO) complexes are highly dependent on the MESP of the isolated R₂CO solute. The formation of pi-hole bondings appears to play a decisive role in the initiation of a three-dimensional organization of water molecules around the solute.

Direct investigation of chalcogen bonds by multinuclear solid-state magnetic resonance and vibrational spectroscopy

We report a multifaceted experimental and computational study of three self-complementary chalcogen-bond donors as well as a series of seven chalcogen bonded cocrystals.