When examining the formation energetics of a hydrogen-bonded complex R-X-H...Y-R', focus has been almost always on the atoms directly involved, namely the atoms X, Y, and H. Little attention has been paid to the effects of the secondary alkyl groups R and R'. Taking dimethyl sulfoxide (DMSO)-methanol binary system as an example, we have studied the roles of the alkyl groups in stabilizing the hydrogen bonds by employing FTIR and NMR techniques and quantum chemical calculations. We found that methyl groups play different roles in response to the hydrogen-bonding interactions. The methyl groups of DMSO are electron-donating, whereas that of methanol is electron-withdrawing, both making positive contributions. The findings reveal non-negligible effects of secondary alkyl groups in hydrogen bonding interaction and may shed light on the understanding of other more complicated hydrogen-bonded systems in chemical and biological systems.

The pathways for the hydrogenation of adsorbed acetylene and vinylidene on Pd(111) are investigated using temperature-programmed desorption and infrared reflection-absorption spectroscopies. The chemistry of the vinyl intermediate formed by the hydrogenation of both species is investigated by adsorbing vinyl iodide on Pd(111) where it is found that vinyl species hydrogenate more rapidly than adsorbed acetylene, indicating that the rate-limiting step in acetylene hydrogenation is the addition of the first hydrogen to acetylene to form a vinyl species. Infrared spectroscopy also reveals that vinyl species convert to ethylidynes as Pd( 111) is heated above ∼160 K. The hydrogenation of vinylidene by up to ∼0.2 Torr of hydrogen involves intermediate ethylidyne species in accord with this observation. Surprisingly, the rate constant for the conversion of vinylidene into ethylidyne is identical to that for the titration of ethylidyne from the surface by hydrogen, an effect that may be explained in terms of the different saturation coverages of the two species.

L a states for a series of linear acenes showed that a TDDFT scheme incorporating the Tamm-Dancoff approximation (TDDFT/ TDA) could decrease the estimation errors by a factor of about 50%, but keep the levels of 1 L b states almost unchanged. Thus, the TDDFT/TDA scheme gives an overall description for the low-lying excited states of linear acenes significantly better than the full TDDFT does. Furthermore, 16 nonlinear polycyclic aromatic hydrocarbons (PAHs) with various structures were examined to confirm the superiority of the TDDFT/TDA over the full TDDFT in its ability to describe the 1 L a states for conjugated -systems of large size.

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