On the electronic origins of barriers to methyl rotation: CNDO/2 calculations on (CH3)2XHn (X = C,Si, N,P, O,S) molecules

CNDO calculations are performed for the homogeneous series of (CH₃)₂XH_n compounds (X = C, Si, N, P, O, S) in order to determine the molecular equilibrium configuration. In agreement with available experimental data, for all investigated molecules, the theoretical energy minimum is found for the (θ = 60°, ψ = 60°) conformation in which one hydrogen of each methyl group is in the X heavy atoms plane but pointing outside the CXC angle. A partitioning of total energy shows that the variations of this quantity are completely reflected by the variations of the interaction energy between non-bonded terms. A more detailed analysis reveals, only for the third-row compounds, the essential role of the interactions between central atom and methyl hydrogens.     

Polymerized rodlike micelle adsorption at the solid-liquid interface

The adsorption of rodlike polymer-micelle aggregates of cetyltrimethylammonium 4-vinylbenzoate (p-C16TVB) at the silica-water interface has been characterized using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) studies. Adsorption isotherm data, recorded by QCM-D, indicate a two-stage mechanism: an adsorbed film of free CTA+ ions is initially produced at low concentrations until the surface is charge reversed, whereupon the weakly anionic aggregates can adsorb and the adsorbed mass is seen to increase dramatically. The adsorbed rodlike micelle aggregates are seen to form a close-packed monolayer from AFM images with a high degree of order over micrometer length scales. AFM force-distance data indicate that the adsorbed aggregates retain their cylindrical structure and little or no flattening is seen. Rinsing of the film did not result in removal of the adsorbed layer, and the persistence of these nanoscale ordered films at the solid-liquid interface suggests many possible applications.     

A CNDO/2 approach to electron delocalization in N4S4 and N4S4F4

The electronic structure of N₄S₄ and N₄S₄F₄ molecules is investigated within the framework of the CNDO/2 approximation. A pure alternated system is obtained for the fluorinated compound with respect to the Wiberg bond populations. On the other hand, the N₄S₄ molecule appears to be composed of three highly delocalized “islands”, as defined by Dewar in the case of cyclophosphazenes. The “tub form” of N₄S₄ is due to a strong spatial interaction between non-bonded sulphur atoms and, for this reason, the N₄S₄ molecule may be called the “inorganic cyclooctatetraene”.