The structural properties and phase stability of the four common polytypes of boron nitride, cubic zinc blende (c-BN͒, hexagonal (h-BN͒, wurtzite (w-BN͒ and rhombohedral (r-BN͒, are studied by ab initio calculations. Electronic energies are calculated using an ultra soft pseudopotential method under the densityfunctional theory, and phonon dispersions are calculated using the first-principles force-constant method. The p-T phase diagrams of these four boron nitride phases are constructed with the quasiharmonic approximation. Direct compression simulations are then performed to find probable phase transformation paths among these polytypes, with additional energy calculations of plausible transition structures. The c-BN phase is the most thermodynamically stable in ambient conditions among these four polytypes, and the transformation between r-BN and c-BN has the smallest energy barrier. Direct transformation between h-BN and c-BN is far less favorable than indirect transformation, with w-BN or r-BN as an intermediate. The presence of structural defects is a key attribute in reducing the energy barrier of phase transformation. The results in this work offer theoretical clues to experimental data on c-BN film growth, particularly the absence of w-BN.

The fluxional behavior of the protonated ethane ion was examined using both static and dynamic modeling. Static ab initio calculations, including perturbation theory (MP2), coupled cluster (CCSD(T)), and density functional theory, were used to locate various minima, saddle points, and G2-quality relative energies on the potential energy surface for atomic motions. In tandem, Car−Parrinello molecular dynamics simulations were performed to aid the stationary-point search and to examine the stabilities of various isomers at different temperatures. Predicted infrared spectra were also obtained from both techniques. Unlike most previous experimental and theoretical investigations which have focused upon the relative energies and stabilities of σ C - C-protonated (bridged) structures and σC - H-protonated (open or “classical”) structures, this work establishes the existence of a third isomer, the ion−molecule or solvated-ion complex C2H5 +···H2, which is the more likely candidate for the second isomer of experiments by Hiraoka and Kebarle and by Yeh, Price, and Lee. The open isomer may still be experimentally unknown. Peculiar discrepancies remain, however, and further experimental work is needed to resolve them.

The chemisorption of pyrazine on Si(100) is a unique and experimentally demonstrated example for the growth of an ordered 1D line of organic molecules on a very reactive silicon surface. Two key factors are identified, using first principles calculations, as being responsible for this remarkable process. First, van der Waals interaction between pyrazine and Si(100) varies considerably depending on the orientation of a pyrazine molecule, which opens up a significant gap in the activation barriers and makes the adsorption selective. Second, the presence of multiple reaction channels at elevated temperature could actually facilitate the self-assembly of adsorbed molecules into ordered structure, due to the presence of a cooperative effect. Consideration of these two factors should be useful in the search for other organic molecules to grow 1D lines on clean silicon surfaces.

It has long been understood that a precursor mediated chemisorption is a significant part of the dynamics for the adsorption of O(2) on Si(100), which is a much studied model system of surface reaction with considerable technological relevance. However, theoretical studies on the interaction between O(2) and Si(100) have been focused on the excited singlet state of O2 and unable to explain the observations in surface scattering experiments. We demonstrate by first principles calculations that such a focus is misplaced. In reality, triplet O(2) can also react with Si(100), after overcoming small barriers, and its reaction paths provide a full account for experiments. Our results highlight the important role played by triplet O(2) in surface oxidation.