Structure, energetics, and infrared spectra of weakly bound HC(2n+1)N···HCl complexes. A theoretical study

Hydrogen-Bonded π Complexes between Phosphaethyne and Hydrogen Chloride

The highly labile complexes between phosphaethyne (HCP) and hydrogen chloride (HCl) with 1:1 and 1:2 stoichiometries have been generated in Ar and N₂ matrices at 10 K through laser photolysis of the molecular precursors 1-chlorophosphaethene (CH₂PCl) and dichloromethylphosphine (CH₃PCl₂), respectively. The IR spectrum of the 1:1 complex suggests the preference of a single "T-shaped" structure in which HCl acts as the hydrogen donor that interacts with the electron-rich C≡P triple bond. In contrast, three isomeric structures for the 1:2 complex bearing a core structure of the "T-shaped" 1:1 complex are present in the matrix. The spectroscopic identification of these rare HCP π-electron complexes is supported by D-isotope labeling and the quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level of theory.

Characterization of Competing Halogen- and Hydrogen-Bonding Motifs in Simple Mixed Dimers of HCN and HX (X = F, Cl, Br, and I)

This work performs the first systematic comparison of hydrogen- and halogen-bonded configurations of the HCN/HX mixed dimer, where X = F, Cl, Br, and I. Eleven different minima have been characterized for these four heterogeneous dimers near the CCSD(T) complete basis set (CBS) limit. For each complex, two different hydrogen-bonded minima were identified: the global minimum where HX acts as the hydrogen bond donor and a local minimum where HX acts as the hydrogen bond acceptor. A halogen-bonded local minimum was also identified for all but the fluorine mixed dimer. To the best of our knowledge, three of the minima are identified here for the first time. The hydrogen- and halogen-bonded local minima of each complex become more energetically competitive with the global minimum as the atomic radius of the halogen atom increases. CCSD(T) relative energies of the hydrogen-bonded local minima computed near the CBS limit decrease from 4.5 kcal mol^-1 for HCN/HF to 2.9, 2.4, and 1.2 kcal mol^-1 for X = Cl, Br, and I, respectively. Corresponding relative energies for the halogen-bonded local minima range from 4.0 kcal mol^-1 for X = Cl to 2.7 kcal mol^-1 for X = Br and to as little as 0.5 kcal mol^-1 X = I. Harmonic vibrational frequency shifts reported here suggest that it may be feasible to differentiate between the various minima for X = Cl, Br, and I via spectroscopic analysis, as was the case for the HCN/HF dimer.

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).