Substituent Effects in the Noncovalent Bonding of SO2 to Molecules containing a Carbonyl Group. The Dominating Role of the Chalcogen Bond

The SO2 molecule is paired with a number of carbonyl-containing molecules, and the properties of the resulting complexes are calculated by high-level ab initio theory. The global minimum of each pair is held together primarily by a S···O chalcogen bond wherein the lone pairs of the carbonyl O transfer charge to the π* antibonding SO orbital, supplemented by smaller contributions from weak CH···O H-bonds. The binding energies vary between 4.2 and 8.6 kcal/mol, competitive with even some of the stronger noncovalent forces such as H-bonds and halogen bonds. The geometrical arrangement places the carbonyl O atom above the plane of the SO2 molecule, consistent with the disposition of the molecular electrostatic potentials of the two monomers. This S···O bond differs from the more commonly observed chalcogen bond in both geometry and origin. Substituents exert their influence via inductive effects that change the availability of the carbonyl O lone pairs as well as the intensity of the negative electrostatic potential surrounding this atom

Effects of Charge and Substituent on the S∙∙∙N Chalcogen Bond

Neutral complexes containing a S···N chalcogen bond are compared with similar systems in which a positive charge has been added to the S-containing electron acceptor, using high-level ab initio calculations. The effects on both XS···N and XS+···N bonds are evaluated for a range of different substituents X = CH3, CF3, NH2, NO2, OH, Cl, and F, using NH3 as the common electron donor. The binding energy of XMeS···NH3 varies between 2.3 and 4.3 kcal/mol, with the strongest interaction occurring for X = F. The binding is strengthened by a factor of 2–10 in charged XH2S+···NH3 complexes, reaching a maximum of 37 kcal/mol for X = F. The binding is weakened to some degree when the H atoms are replaced by methyl groups in XMe2S+···NH3. The source of the interaction in the charged systems, like their neutral counterparts, is derived from a charge transfer from the N lone pair into the σ*(SX) antibonding orbital, supplemented by a strong electrostatic and smaller dispersion component. The binding is also derived from small contributions from a CH···N H-bond involving the methyl groups, which is most notable in the weaker complexes

The Magnitude and Mechanism of Charge Enhancement of CH∙∙O H-bonds

Quantum calculations find that neutral methylamines and thioethers form complexes, with N-methylacetamide (NMA) as proton acceptor, with binding energies of 2–5 kcal/mol. This interaction is magnified by a factor of 4–9, bringing the binding energy up to as much as 20 kcal/mol, when a CH3+ group is added to the proton donor. Complexes prefer trifurcated arrangements, wherein three separate methyl groups donate a proton to the O acceptor. Binding energies lessen when the systems are immersed in solvents of increasing polarity, but the ionic complexes retain their favored status even in water. The binding energy is reduced when the methyl groups are replaced by longer alkyl chains. The proton acceptor prefers to associate with those CH groups that are as close as possible to the S/N center of the formal positive charge. A single linear CH··O hydrogen bond (H-bond) is less favorable than is trifurcation with three separate methyl groups. A trifurcated arrangement with three H atoms of the same methyl group is even less favorable. Various means of analysis, including NBO, SAPT, NMR, and electron density shifts, all identify the +CH··O interaction as a true H-bond

Theoretical Evidence for a NH···XC Blue-Shifting Hydrogen Bond: Complexes Pairing Monohalomethanes with HNO

Correlated ab initio calculations are used to analyze the interaction between nitrosyl hydride (HNO) and CH3X (X = F, Cl, Br). Three minima are located on the potential energy surface of each complex. The more strongly bound contains a NH···X bond, along with CH···O; CH···O and CH···N bonds occur in the less stable minimum. Binding energies of the global minimum lie in the range of 11−13 kJ/mol, and there is little sensitivity to the identity of the halogen atom. Unlike most other such hydrogen bonds, the NH covalent bond in this set of complexes becomes shorter, and its stretching frequency shifts to the blue, upon forming the NH···X hydrogen bond. The amount of this blue shift varies in the order F \u3e Cl \u3e Br

Complexes Pairing Hypohalous Acids with Nitrosyl Hydride. Blue Shift of a NH Bond that is Uninvolved in a H-bond

Correlated calculations are used to analyze the interaction between nitrosyl hydride (HNO) and hypohalous acids (HOF, HOCl, and HOBr). Two minima are located on the potential energy surface of each complex, in both of which HOX acts as proton donor. Donation to the N atom of HNO makes for a more strongly bound complex, as compared to the OH··O bond in the secondary minimum. Binding energies of the global minimum are about 22 kJ/mol, as compared to 18 kJ/mol for the secondary structure; there is little sensitivity to the identity of the halogen atom. Whereas the covalent OH bond of HOX stretches and shifts to the red upon complexation, the NH bond of HNO, whether involved in a H-bond or not, behaves in the opposite manner

Theoretical Investigation of the Weakly Dihydrogen Bonded Complexes FArCCH..HBeX (X=H, F, Cl, Br)

An ab initio computational study of the properties of four linear dihydrogen-bonded complexes formed between the first compound with an Ar−C chemical bond (FArCCH) and HBeX (X = H, F, Cl, and Br) molecules was undertaken at the MP2/6-311++G(2d,2p) level of theory. The calculated complexation energy at MP2 and G2(MP2) levels decreases in the order HBeH···HCCArF \u3e BrBeH···HCCArF \u3e ClBeH···HCCArF \u3e FBeH···HCCArF. The intermolecular stretching frequency, and shifts within the monomers, are compared with the energetic strength of complexation

Stabilities and Properties of Complexes Pairing Hydroperoxyl Radical with Monohalomethanes

UMP2/aug-cc-pvdz calculations are used to analyze the interaction between hydroperoxyl radical (HOO) and CH3X (X = F, Cl, Br). Two minima are located on the potential energy surface of each complex. The more strongly bound contains a OH···X bond, along with CH···O; only CH···O bonds occur in the less stable minimum. Binding energies of the dominant minimum lie in the range of 20−24 kJ/mol, with X = F the most strongly bound. Analysis of the perturbations of the covalent bond lengths within each subunit caused by complexation, coupled with vibrational frequencies and charge transfers, opens a window into the nature of the interactions