Hydrogen-Bonded Complexes of Phenylacetylene-Acetylene: Who is the Proton Donor?

Dominance of unique Pπ phosphorus bonding with π donors: evidence using matrix isolation infrared spectroscopy and computational methodology

Albeit the first account of hypervalentπ interactions has been reported with halogenπ interactions, the feasibility of their extension to other hypervalent atoms as possible Lewis acids is still open. In this work, the role of phosphorus as an acceptor from the π electron cloud (Pπ pnicogen or phosphorus bonding) in PCl3-C2H2 and PCl3-C2H4 heterodimers is explored, by combining matrix isolation infrared spectroscopy with ab initio and DFT computational methodologies. The respective potential energy surfaces of the PCl3-C2H2 and PCl3-C2H4 heterodimers reveal unique minima stabilized by a concert of reasonably strong to weak interactions, of which Pπ phosphorus bonding was energetically dominant. Heterodimers, trimers and tetramers bound primarily by this unique phosphorus bond were generated at low temperatures. The dominance of phosphorus bonding in the PCl3-C2H2 and PCl3-C2H4 heterodimers over other interactions (such as Hπ, HCl, HP, Clπ and lone pair-π interactions) was confirmed and substantiated using extended quantum theory of atoms in molecules, natural bond orbital, electrostatic potential mapping and energy decomposition analyses. The following inferences in correlation with results from non-covalent-interaction analysis offer a complete understanding of the nature of the Pπ phosphorus bonding interactions. The significance of electrostatic forces kinetically favoring the formation of phosphorus bonded heterodimers, in addition to thermodynamic stabilization, is demonstrated experimentally.

From the propargyl alcohol–water complex to the propargyl alcohol dimer: where does the propargyl alcohol–methanol complex fit in?

A correlation was recognized between the structures of PA–H₂O, PA–MeOH and PA dimer complexes that could help predict the structures of larger systems in a systematic way.

An aromatic noble-gas hydride: C6H5CCXeH

We report on the aromatic noble-gas hydride, C6H5CCXeH, identified in a xenon matrix using infrared spectroscopy and extensive quantum chemical calculations. This molecule is prepared by 250-nm photolysis of phenylacetylene (C6H5CCH) isolated in a xenon matrix and subsequent thermal mobilization of hydrogen atoms at about 40 K. The characteristic H-Xe stretching mode of C6H5CCXeH is observed at about 1500 cm-1, and a number of other fundamentals also appear in the experimental spectra. The assignment is supported by deuteration experiments providing predictable shifts of the vibrational frequencies. The experimental and calculated spectra are in a good agreement. C6H5CCXeH is computationally lower in energy than the C6H5CC + Xe + H fragments by about 0.60 eV at the M06-2X/aug-cc-pVTZ-PP level of theory, which allows its formation at low temperatures. C6H5CCXeH is the first aromatic noble-gas hydride and the first halogen-free aromatic noble-gas compound.

The interaction strengths and spectroscopy parameters of the C2H2∙∙∙HX and HCN∙∙∙HX complexes (X = F, Cl, CN, and CCH) and related ternary systems valued by fluxes of charge densities: QTAIM, CCFO, and NBO calculations

This theoretical work exhibits a new systematic study of structural parameters, electronic properties, infrared vibration modes, and molecular topography of hydrogen complexes, namely linear-type HCN⋯HX and T-type C₂H₂⋯HX (X = F, Cl, CN, and CCH). Ideally, the knowledge of the ternary systems of C₂H₂⋯HCN⋯HF and HCN⋯HCN⋯HF whose subparts integrate the linear and T-shaped complexes were used to give support in this current research. By means of computational calculations carried out in both levels B3LYP and MP2, the variations of the HX bond lengths are clearly overestimated in the HCN⋯HX linear complexes. In agreement with the analyses of the electrostatic potentials, the higher intermolecular energies of these complexes agree with the larger red-shifts in the stretch frequencies in HX. Also, the QTAIM descriptors and NBO calculations were used to inspect the interaction strength as well as to confirm the π cloud as a proton accepting center. By taking into account the absorption intensity ratio as a standard parameter to predict the interaction strength and intermolecular characterization, the formalism of the charge-charge flux-overlap modified (CCFO) was applied.

Assessment of the Presence and Strength of H-Bonds by Means of Corrected NMR

The downfield shift of the NMR signal of the bridging proton in a H-bond (HB) is composed of two elements. The formation of the HB causes charge transfer and polarization that lead to a deshielding. A second factor is the mere presence of the proton-accepting group, whose electron density and response to an external magnetic field induce effects at the position of the bridging proton, exclusive of any H-bonding phenomenon. This second positional shielding must be subtracted from the full observed shift in order to assess the deshielding of the proton caused purely by HB formation. This concept is applied to a number of H-bonded systems, both intramolecular and intermolecular. When the positional shielding is removed, the remaining chemical shift is in much better coincidence with other measures of HB strength.