S⋯N chalcogen bonded complexes of carbon disulfide with diazines. Theoretical study

Ability of the Spectroscopic Properties of the P═Se Bond of a Base to Assess Noncovalent Bond Strength

The viability of the P═Se bond to serve as a monitor of the strength of a noncovalent bond was tested in the context of the (CH3)3PSe molecule. Density functional theory (DFT) computations paired this base with a collection of Lewis acids that spanned hydrogen, halogen, chalcogen, pnicogen, and tetrel bonding interactions and covered a wide range of bond strengths. A very strong linear correlation was observed between the interaction energy and the nuclear magnetic resonance (NMR) 1J(PSe) coupling constant, which could serve as an accurate indicator of bond strength. Also correlating very well with the interaction energy is the stretch of the P═Se bond caused by complexation and the red shift of its stretching frequency. Moderate correlations arise in the chemical shifts of the P and Se nuclei. The σ-hole depth on the Lewis acid is poorly correlated with the energetics, and the same is true for the full electrostatic contribution to the bond energy. Of the various components, it is the polarization energy that correlates most closely with the interaction energy.

Correlation between Noncovalent Bond Strength and Spectroscopic Perturbations within the Lewis Base

Me2CO was allowed to interact with 20 different Lewis acids so as to engage in various sorts of noncovalent interactions, encompassing hydrogen, halogen, chalcogen, pnictogen, and tetrel bonds. Density functional theory computations evaluated the interaction energy of each dyad, which was compared with spectroscopic, geometric, AIM, and energy decomposition elements so as to elucidate any correlations. The red shift of the C═O stretching frequency, and the changes in the nuclear magnetic resonance shielding of the O and C atoms of acetone, are closely correlated with the interaction energy so can be used to estimate the latter from experimental measurements. The standard AIM measures at the bond critical point, ρ, ∇2ρ, and V also correlate with the energy, albeit not as well as the spectroscopic parameters. The σ-hole depth on the Lewis acid is not well correlated with the energetics, due in part to the fact that electrostatics in general are not an accurate metric of bond strength.

Participation of S and Se in hydrogen and chalcogen bonds

The heavier chalcogen atoms S, Se, and Te can each participate in a range of different noncovalent interactions. They can serve as both proton donor and acceptor in H-bonds. Each atom can also act as electron acceptor in a chalcogen bond.

The role of sulfur interactions in crystal architecture: experimental and quantum theoretical studies on hydrogen, halogen, and chalcogen bonds in trithiocyanuric acid–pyridine N-oxide co-crystals

Hydrogen, halogen, chalcogen bonds and π interactions of the trithiocyanuric acid ring are responsible for crystal structure architecture and have been classified according to the QTAIM approach as closed-shell interactions.

Versatility of the Cyano Group in Intermolecular Interactions

Several cyano groups are added to an alkane, alkene, and alkyne group so as to construct a Lewis acid molecule with a positive region of electrostatic potential in the area adjoining these substituents. Although each individual cyano group produces only a weak π-hole, when two or more such groups are properly situated, they can pool their π-holes into one much more intense positive region that is located midway between them. A NH3 base is attracted to this site, where it forms a strong noncovalent bond to the Lewis acid, amounting to as much as 13.6 kcal/mol. The precise nature of the bonding varies a bit from one complex to the next but typically contains a tetrel bond to the C atoms of the cyano groups or the C atoms of the linkage connecting the C≡N substituents. The placement of the cyano groups on a cyclic system like cyclopropane or cyclobutane has a mild weakening effect upon the binding. Although F is comparable to C≡N in terms of electron-withdrawing power, the replacement of cyano by F substituents substantially weakens the binding with NH3.

Coordination of a Central Atom by Multiple Intramolecular Pnicogen Bonds

A central pnicogen Z atom (Z = Sb, As) is covalently attached to the O atom of three -O(CH₂)_nX chains where X represents either an aldehyde or amine group. The chain can fold around so that the basic X group can engage in a noncovalent pnicogen bond with the central Z. The formation of up to three pnicogen bonds is energetically favored. The amine appears to engage in stronger pnicogen bonds than does the aldehyde, and bonds to Sb are favored over As, but there is little dependence on the length of the chain. The formation of each successive pnicogen bond reduces the magnitude of the σ-holes surrounding the Z atom, which tends to weaken the attraction for the basic end of the chain.

Nature of the Interaction of Pyridines with OCS. A Theoretical Investigation

Ab initio calculations were carried out to investigate the interaction between para-substituted pyridines (X-C₅H₄N, X=NH₂, CH₃, H, CN, NO₂) and OCS. Three stable structures of pyridine.OCS complexes were detected at the MP2=full/aug-cc-pVDZ level. The A structure is characterized by N…S chalcogen bonds and has binding energies between −9.58 and −12.24 kJ/mol. The B structure is bonded by N…C tetrel bond and has binding energies between −10.78 and −11.81 kJ/mol. The C structure is characterized by π-interaction and has binding energies between −10.76 and −13.33 kJ/mol. The properties of the systems were analyzed by AIM, NBO, and SAPT calculations. The role of the electrostatic potential of the pyridines on the properties of the systems is outlined. The frequency shift of relevant vibrational modes is analyzed.

Chalcogen bonding of two ligands to hypervalent YF4 (Y = S, Se, Te, Po)

The ability of two NH₃ ligands to engage in simultaneous chalcogen bonds to a hypervalent YF₄ molecule, with Y = S, Se, Te, Po, is assessed via quantum calculations. The complex can take on one of two different geometries. The cis structure places the two ligands adjacent to one another in a pseudo-octahedral geometry, held there by a pair of σ-hole chalcogen bonds. The bases can also lie nearly opposite one another, in a distorted octahedron containing one π-hole and one strained σ-hole bond. The cis geometry is favored for Y = S, while Te, and Po tend toward the trans structure; they are nearly equally stable for Se. In either case, the binding energy rises rapidly with the size of the Y atom, exceeding 30 kcal mol^-1 for PoF₄.

Theoretical Studies of IR and NMR Spectral Changes Induced by Sigma-Hole Hydrogen, Halogen, Chalcogen, Pnicogen, and Tetrel Bonds in a Model Protein Environment

Various types of σ-hole bond complexes were formed with FX, HFY, H2FZ, and H3FT (X = Cl, Br, I; Y = S, Se, Te; Z = P, As, Sb; T = Si, Ge, Sn) as Lewis acid. In order to examine their interactions with a protein, N-methylacetamide (NMA), a model of the peptide linkage was used as the base. These noncovalent bonds were compared by computational means with H-bonds formed by NMA with XH molecules (X = F, Cl, Br, I). In all cases, the A-F bond, which lies opposite the base and is responsible for the σ-hole on the A atom (A refers to the bridging atom), elongates and its stretching frequency undergoes a shift to the red with a band intensification, much as what occurs for the X-H bond in a H-bond (HB). Unlike the NMR shielding decrease seen in the bridging proton of a H-bond, the shielding of the bridging A atom is increased. The spectroscopic changes within NMA are similar for H-bonds and the other noncovalent bonds. The C=O bond of the amide is lengthened and its stretching frequency red-shifted and intensified. The amide II band shifts to higher frequency and undergoes a small band weakening. The NMR shielding of the O atom directly involved in the bond rises, whereas the C and N atoms both undergo a shielding decrease. The frequency shifts of the amide I and II bands of the base as well as the shielding changes of the three pertinent NMA atoms correlate well with the strength of the noncovalent bond.

Effects of Halogen, Chalcogen, Pnicogen, and Tetrel Bonds on IR and NMR Spectra

Complexes were formed pairing FX, FHY, FH2Z, and FH3T (X = Cl, Br, I; Y = S, Se, Te; Z = P, As, Sb; T = Si, Ge, Sn) with NH3 in order to form an A⋯N noncovalent bond, where A refers to the central atom. Geometries, energetics, atomic charges, and spectroscopic characteristics of these complexes were evaluated via DFT calculations. In all cases, the A-F bond, which is located opposite the base and is responsible for the σ-hole on the A atom, elongates and its stretching frequency undergoes a shift to the red. This shift varies from 42 to 175 cm-1 and is largest for the halogen bonds, followed by chalcogen, tetrel, and then pnicogen. The shift also decreases as the central A atom is enlarged. The NMR chemical shielding of the A atom is increased while that of the F and electron donor N atom are lowered. Unlike the IR frequency shifts, it is the third-row A atoms that undergo the largest change in NMR shielding. The change in shielding of A is highly variable, ranging from negligible for FSnH3 all the way up to 1675 ppm for FBr, while those of the F atom lie in the 55-422 ppm range. Although smaller in magnitude, the changes in the N shielding are still easily detectable, between 7 and 27 ppm.