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.

Bonding and spectroscopic analyses of N2O-CS2 and N2O-OCS heterodimer complexes and their atmospheric consequences

Different isomers of the valence isoelectronic pairs of the heterodimers N₂O-SCS and N₂O-OCS were investigated using MP2 and CCSD(T) methods with the aug-cc-pVXZ (X = D, T) basis set with anharmonic frequency calculations. Interaction energies of both the heterodimers were estimated at the CBS limit and for CCSD(T)/aug-cc-pVTZ. One isomer was located for the N₂O-SCS heterodimer. In the case of the N₂O-OCS heterodimer, three planar slipped-parallel isomers were characterized. There was no significant change in electron density at the bond critical points (BCPs) of various intra-bonds but the presence of inter-bonds with a weak electron density at the BCPs in the complexes was found as per the results from the quantum theory of atoms in molecules (QTAIM) analysis. The interorbital interactions of the monomer in the heterodimers are discussed herein in terms of the natural bond orbital (NBO) charge transfer. Symmetry-adapted perturbation analysis was performed for all the isomers. In the case of the stable complex of N₂O-SCS, induction dominated over the dispersion energy. Among the N₂O-OCS isomers, the dispersion effect predominates for the most stable isomer, while for the other two isomers, the induction effect is dominant. Frequency calculations for these complexes showed a number of interactions induced by the low frequency modes in the far IR region. The atmospheric implications are also discussed for these complexes.

A theoretical investigation of the energetics and spectroscopic properties of the gas-phase linear proton-bound cation-molecule complexes, XCH(+)-N2 (X = O, S)

The structural features, spectroscopic properties, and interaction energies of the linear proton-bound complexes of OCH(+) and its sulfur analog SCH(+) with N2 were investigated using the high-level ab initio methods MP2 and CCSD(T) as well as density functional theory with the aug-cc-pVXZ (X = D, T) basis sets. The rotational constants along with the vibrational frequencies of the cation-molecule complexes are reported here. A comparison of the interaction energies of the OCH(+)-N2 and SCH(+)-N2 complexes with those of the OCH(+)-CO and OCH(+)-OC complexes was also performed. The energies of all the complexes were determined at the complete basis set (CBS) limit. CS shows higher proton affinity at the C site than CO does, so the complex OCH(+)-N2 is relatively strongly bound and has a higher interaction energy than the SCH(+)-N2 complex. Symmetry-adapted perturbation theory (SAPT) was used to decompose the total interaction energies of the complexes into the attractive electrostatic interaction energy (E elst), induction energy (E ind), dispersion energy (E disp), and repulsive exchange energy (E exch). We found that the ratio of E ind to E disp is large for these linear proton-bound complexes, meaning that inductive effects are favored in these complexes. The bonding characteristics of the linear complexes were elucidated using natural bond orbital (NBO) theory. NBO analysis showed that the attractive interaction is caused by NBO charge transfer from the lone pair on N to the σ*(C-H) antibonding orbital in XCH(+)-N2 (X = O, S). The quantum theory of atoms in molecules (QTAIM) was used to analyze the strengths of the various bonds within and between the cation and molecule in each of these proton-bound complexes in terms of the electron density at bond critical points (BCP). Graphical Abstract Linear proton-bound complexes of OCH(+)-N2 and SCH(+)-N2. In these complexes, inductive effect is favored over dispersive effect. The attractive interaction is the NBO charge transfer from N-lone pair of N2 to CH σ* antibonding orbital of XCH(+) (X = O, S).