Mechanisms and Energetics of the Reaction of Si+ with CH3-SiH3

An ab initio quantum chemical study of the reactions of Si+ with methylsilane has been carried out: SCF I 6-31 G(d) wave functions were used to predict structures of the possible products and transition states; relative energies were obtained by means of single point electron correlation corrections with fourth-order perturbation theory using the larger 6-31 G( d,p) basis set. Three different mechanisms involving initial complex formation, followed by insertion of Si+ into Si-C, Si-H, and C-H bonds leading to the eliminations of H2 and other products have been investigated in detail. This involves the detailed mapping of ShCH6+, Si2CH5+, and ShCH4+ potential energy surfaces. Results of the calculations are compared with the experimental observations of Mandich et al., Lim et al., and Kickel et al. Good agreement with experiments is obtained

Tautomerization of H⁺KPGG: Entropic Consequences of Strong Hydrogen-Bond Networks in Peptides

Ion mobility spectrometry-mass spectrometry and quantum chemical calculations are used to determine the structures and stabilities of the singly protonated peptide H+KPGG. The two peaks making up the IMS distribution are shown to be tautomers differing by the location of the extra proton on either the lysine side chain or the N-terminus. The lysine-protonated tautomer is strongly preferred entropically while being disfavored in terms of the electronic energy and enthalpy. This relationship is shown, through comparison of all low-lying conformers of both tautomers, to be related to the strong hydrogen-bond network of the N-terminally protonated tautomer. A general relationship is demonstrated wherein stronger cross-peptide hydrogen-bond networks result in entropically disfavored conformers. Further effects of the H+KPGG hydrogen-bond network are probed by computationally examining singly and doubly methylated analogues. These results demonstrate the importance of the entropic consequences of hydrogen bonds to peptide stability as well as techniques for perturbing the hydrogen-bond network and folding preferences of peptides via minimal chemical modification

Phosphine Adsorption on the In-Rich InP(001) Surface: Evidence of Surface Dative Bonds at Room Temperature

Adsorption of phosphine on indium phosphide compound semiconductor surfaces is a key process during the chemical vapor deposition of this material. Recent experimental infrared studies of the In-rich InP surfaces exposed to phosphine show a complex vibrational pattern in the P-H stretch region, presumably due to overlapping contributions from several structural species. We have performed density functional calculations using finite-sized cluster models to investigate the dissociative adsorption of PH 3 on the In-rich InP surface. We find that initially PH 3 forms a dative bond with one of the surface In atoms with a binding energy of approximately 11 kcal mol -1 at 298 K. The In-PH 3 bond length is 2.9 Å, 0.3 Å greater than the In-P covalent bond length computed for In-PH 2 species produced by hydrogen migration to a neighboring atom. However, the dissociation process, though exothermic, involves a significant activation barrier of ∼23 kcal mol -1 , suggesting the possibility of metastable trapping of the dative bonded PH 3 molecules. Indeed, a careful vibrational analysis of different P-H stretching modes of the surface-bound PH 3 and PH 2 units gives excellent agreement with the observed infrared frequencies and their relative intensities. Moreover, at higher temperatures the frequency modes associated with PH 3 disappear either due to desorption or dissociation of this molecule, an observation also well supported from the computed thermochemical parameters at different temperatures. The computed energy parameters and infrared analysis provide direct evidence that PH 3 is present as a dative bonded complex on the InP surface at room temperature

Quantum mechanical ab-initio simulation of the electron screening effect in metal deuteride crystals

In antecedent experiments the electron screening energies of the d+d reactions in metallic environments have been determined to be enhanced by an order of magnitude in comparison to the case of gaseous deuterium targets. The analytical models describing averaged material properties have not been able to explain the experimental results so far. Therefore, a first effort has been undertaken to simulate the dynamics of reacting deuterons in a metallic lattice by means of an ab-initio Hartree-Fock calculation of the total electrostatic force between the lattice and the successively approaching deuterons via path integration. The calculations have been performed for Li and Ta, clearly showing a migration of electrons from host metallic to the deuterium atoms. However, in order to avoid more of the necessary simplifications in the model the utilization of a massive parallel supercomputer would be required.Comment: 11 pages, 12 figures, svjour class. To be published in Eur. Phys. J.

Chlorination of hydrogen-terminated silicon (111) surfaces

Infrared absorption spectroscopy was used to investigate the chlorination of hydrogen-terminated Si(111) surfaces by three different methods: (a) exposure to a saturated solution of phosphorus pentachloride (PCl5) in chlorobenzene; (b) exposure to chlorine gas, Cl2(g), and (c) exposure to Cl2(g) under UV illumination. X-ray photoelectron spectroscopy and first principles model (clusters) calculations were used to explore the structure and dynamics of these surfaces. The infrared spectra exhibited sharp chlorine-related vibrations at 586 and 527 cm^–1. The narrow full width at half maximum of these vibrations for all three preparation methods indicated that all functionalization schemes produced a nearly complete monolayer of Cl with little surface roughening or introduction of step edges. The 527 cm^–1 mode was at a much higher frequency than might be expected for the bending vibration of Si monochloride. Theoretical calculations show, however, that this vibration involves the displacement of the top Si atom parallel to the surface, subject to a relatively stiff potential, shifting its frequency to a value fairly close to that of the Si–Cl stretching mode on a Si(111) surface

Boron centres allow design, control and systematic tuning of neutral homoaromatics for functionalization purposes

Homoaromatic compounds are currently viewed more as an interesting novelty with little to no practical application. Based on calculations within density functional theory, we show that the unique charge redirection properties of tricoordinate boron, along with it being isolobal to a carbocation allow for a larger range of two‐electron donors to be utilized, leading to the rational design of homoaromatic compounds better suited to functionalization. Among others, these compounds show a strong dependency on the relative positioning of the hetero‐atoms within the ring system, a modulation control rendered possible by the insertion of the boron centres