Dynamics of Si-H vibrations in an amorphous environment

We present results of the first vibrational photon-echo, transient-grating, and temperature dependent transient-bleaching experiments on a-Si:H. Using these techniques, and the infrared light of a free electron laser, the vibrational population decay and phase relaxation of the Si-H stretching mode were investigated. Careful analysis of the data indicates that the vibrational energy relaxes directly into Si-H bending modes and Si phonons, with a distribution of rates determined by the amorphous host. Conversely, the pure dephasing appears to be single exponential, and can be modeled by dephasing via two-phonon interactions.     

Measurement error in laser interferometry caused by free convective boundary layers on optical windows

When convective heat transfer measurements are made using a laser interferometer, optical windows are often used to separate the test fluid inside the experimental model from the ambient air. In the present study, the measurement error caused by the thermal boundary layers that form on the exterior surfaces of the optical windows has been analyzed. A convenient analytical expression for the error in the fringe gradient has been derived, using the two-dimensional similarity solution for laminar free convective flow. For typical experimental conditions this source of bias error was found to be generally small, except near the leading edge of the thermal boundary layer. Based on the results, it is recommended that the measurement region be at least five centimeters away from the leading edge of these external thermal boundary layers.     

Computational Evaluation of Me2TCCP as Lewis Acid

Supramolecular adducts between dimethyl-2,2,3,3-tetracyanocyclopropane (Me₂TCCP) with 21 small (polar) molecules and 10 anions were computed with DFT (B3LYP-D3/def2-TZVP). Their optimized geometries were used to obtain interaction energies, and perform energy decomposition and ‘atoms-in-molecules’ analyses. A set of 38 other adducts were also evaluated for comparison purposes. Selected examples were further scrutinized by inspection of the molecular electrostatic potential maps, Noncovalent Interaction index plots, the Laplacian, the orbital interactions, and by estimating the Gibbs free energy of complexation in hexane solution. These calculations divulge the thermodynamic feasibility of Me₂TCCP adducts and show that complexation is typically driven by dispersion with less polarized partners, but by orbital interactions when more polarized or anionic guests are deployed. Most Me₂TCCP adducts are more stable than simple hydrogen bonding with water, but less stable than traditional Lewis adducts involving Me₃B, or a strong halogen bond such as with Br₂. Several bonding analyses showed that the locus of interaction is found near the electron poor sp³-hydridized (NC)₂C−C(CN)₂ carbon atoms. An empty hybrid σ*/π* orbital on Me₂TCCP was identified that can be held responsible for the stability of the most stable adducts due to donor-acceptor interactions.     

Pulse polarography: Part X. Formaldehyde hydration in aqueous acetate and phosphate buffer solutions

Formaldehyde hydration has been studied with pulse polarography in acetate and phosphate buffers, at 25°C. It could be shown that in both buffer systems extrapolation to zero buffer concentration produces the equilibrium constant K_d=[CH₂O]/[CH₂(OH)₂]=(6.31±0.05) 10^?4 in pure water, a value of as yet unmatched precision. In the buffers mentioned two novel features appear in the hydration mechanism: (a) buffer acids form addition compounds with CH₂O where the, corresponding equilibria are rapidly established. The equilibrium constants have been derived. (b) H₂PO₄^?/HPO₄^2? buffers are exceptional in that the polarographic specific rate of hydration as a function of the concentration of this buffer is less than expected on the basis of a normal linear dependence. This is explained, as in the case of free-aldehyde glucose [23], on the basis of a solvent (water)-shared associate of (presumably) H₂PO₄^? with CH₂O which decreases the rate of diffusion of the latter, the more so as the buffer concentration increases. It is pointed out, that the occurrence of such associates can explain the excess activity of H₂PO₄^? as an acid catalyst in aldehyde hydration [16].     

Engineering Crystals Using sp3-C Centred Tetrel Bonding Interactions

1,1,2,2-Tetracyanocyclopropane derivatives 1 and 2 were designed and synthesized to probe the utility of sp³-C centred tetrel bonding interactions in crystal engineering. The crystal packing of 1 and 2 and their 1,4-dioxane cocrystals is dominated by sp³-C(CN)₂⋅⋅⋅O interactions, has significant C⋅⋅⋅O van der Waals overlap (≤0.266 Å) and DFT calculations indicate interaction energies of up to −11.0 kcal mol⁻¹. A cocrystal of 2 with 1,4-thioxane reveals that the cyclopropane synthon prefers interacting with O over S. Computational analyses revealed that the electropositive C₂(CN)₄ pocket in 1 and 2 can be seen as a strongly directional ‘tetrel-bond donor’, similar to halogen bond or hydrogen bond donors. This disclosure is expected to have implications for the utility of such ‘tetrel bond donors’ in molecular disciplines such as crystal engineering, supramolecular chemistry, molecular recognition and medicinal chemistry.     

Observations of tetrel bonding between sp3-carbon and THF

We report the direct observation of tetrel bonding interactions between sp³-carbons of the supramolecular synthon 3,3-dimethyl-tetracyanocyclopropane (1) and tetrahydrofuran in the gas and crystalline phase. The intermolecular contact is established via σ-holes and is driven mainly by electrostatic forces. The complex manifests distinct binding geometries when captured in the crystalline phase and in the gas phase. We elucidate these binding trends using complementary gas phase quantum chemical calculations and find a total binding energy of −11.2 kcal mol⁻¹ for the adduct. Our observations pave the way for novel strategies to engineer sp³-C centred non-covalent bonding schemes for supramolecular chemistry.

sp³-C⋯THF tetrel bonding was observed in the crystalline state and in the gas phase. Density functional calculations revealed interaction energies up to −11.2 kcal mol⁻¹ and showed that these adducts are held together mainly by electrostatics.