Description Of Hydrogen Research Articles

What Is the Nature of Supramolecular Bonding? Comprehensive NBO/NRT Picture of Halogen and Pnicogen Bonding in RPH2···IF/FI Complexes (R = CH3, OH, CF3, CN, NO2).

We employ a variety of natural bond orbital (NBO) and natural resonance theory (NRT) tools to comprehensively investigate the nature of halogen and pnicogen bonding interactions in RPH2···IF/FI binary complexes (R = CH3, OH, CF3, CN, and NO2) and the tuning effects of R-substituents. Though such interactions are commonly attributed to “sigma-hole”-type electrostatic effects, we show that they exhibit profound similarities and analogies to the resonance-type 3-center, 4-electron (3c/4e) donor-acceptor interactions of hydrogen bonding, where classical-type “electrostatics” are known to play only a secondary modulating role. The general 3c/4e resonance perspective corresponds to a continuous range of interatomic A···B bond orders (bAB), spanning both the stronger “covalent” interactions of the molecular domain (say, bAB ≥ ½) and the weaker interactions (bAB ˂ ½, often misleadingly termed “noncovalent”) that underlie supramolecular complexation phenomena. We show how a unified NBO/NRT-based description of hydrogen, halogen, pnicogen, and related bonding yields an improved predictive utility and intuitive understanding of empirical trends in binding energies, structural geometry, and other measurable properties that are expected to be manifested in all such supramolecular interaction phenomena.

Identification of the best DFT functionals for a reliable prediction of lignin vibrational properties

Lignin is the most abundant aromatic plant polymer on earth. Useful information on its structure and interactions is gained by vibrational spectroscopy and relies on the quality of band assignments. B3LYP predictions were recently shown to support band assignments. Further progress calls for a comprehensive study of the quality of available theoretical methods in relation to the task of predicting lignin vibrational properties. The present study examined more than 50 functionals for prediction of IR vibrations of an appropriate lignin model. Based on a basis set incompleteness study, the pc-2 basis set was used. B98, X3LYP and B97-1 were the overall best-performing functionals, and “fingerprint” band positions were predicted by single-factor scaling of harmonic frequencies to an average error of ±3 cm−1 by optimized scaling factors of 1.017, 1.021 and 1.016, respectively. Their performance using instead explicit anharmonic correction was slightly worse giving an error of ca. ±5 cm−1. The X3LYP and B97-1 functionals offer also good description of hydrogen bonding. Error compensation from, e.g., insufficient treatment of solvation is likely to affect these results, and thus at this stage no single functional stands out. These results provide a needed basis for further theoretical developments in relation to vibrational assignments of Infrared and Raman spectra of lignin.

Theoretical study of the hydrogen atom transfer in the heterodimer indole-ammonia and comparison with experimental results.

Ab initio calculations on the heterodimer C8H6NH...NH3 are carried out for its ground, the excited pisigma*, and the ground cationic electronic states, enabling the description of hydrogen or proton transfer, respectively. Two-dimensional quantum-dynamical computations on the pisigma* potential surface help one to understand the mechanism and the time scale of the hydrogen transfer. Subsequent decay processes are discussed depending on the vibrational excitation of the ammonium constituent. Finally, the theoretical results obtained are used for the interpretation of the time-dependent signals observed in femtosecond pump-probe experiments.

Theory of dense hydrogen plasma

A partially ionized plasma model is applied for the description of hydrogen from the molecular fluid domain at low temperatures up to the plasma state at high temperatures. Pressure dissociation of H2 molecules is taken into account via the dissociation equilihrium H2 ⇌ H + H. The proton-proton pair distribution function gP&) is determined and compared with available results from computer simulations. The treatment of the ionization equilibrium H ⇌ e + p is needed for the description of the plasma state at higher temperatures. The Hugoniot curves and the electrical conductivity are calculated up to Mbar pressures. The transition from semiconducting to metallic-like behavior as found experimentally is verified.