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Membranes derived from self-assembled block polymers have shown promise as highly selective and highly permeable filters, but the complex synthetic routes and limited pore functionalities of existing systems need to be improved if these materials are to serve as a platform for the next generation of nanostructured membranes. Here, the facile synthesis of a polyisoprene- b -polystyrene- b -poly( N,N -dimethylacrylamide) (PI–PS–PDMA) triblock polymer using a controlled reversible addition-fragmentation chain transfer (RAFT) polymerization mechanism is reported. This material is then processed into a membrane using a self-assembly and non-solvent induced phase separation (SNIPS) technique, which creates an asymmetric, porous structure consisting of a selective layer that contains a high density of PDMA-lined pores (9.4×10 13 pores m −2 ) with an average diameter of 8.1 nm, as determined using solute rejection tests. Solvent flow experiments demonstrate that the PI–PS–PDMA membrane has a pH-independent permeability of 6 L m −2 h −1 bar −1 . The PDMA moiety lining the pore walls is converted, through simple hydrolysis in the solid state, to yield a poly(acrylic acid)-lined (PAA-lined) structure. The permeability of the PI–PS–PAA membrane is pH-dependent, and ranges from 0.6 L m −2 h −1 bar −1 for solutions with a pH greater than 4 to 16 L m −2 h −1 bar −1 for a solution at pH 1. Solute rejection tests demonstrated a pore size of 3.4 nm for the PI–PS–PAA membrane, which is the smallest pore size reported to date for membranes fabricated from self-assembled block polymers. The facile synthesis of the PI–PS–PDMA material, the scalable SNIPS membrane fabrication protocol, and the simple conversion chemistry of the pore functionality demonstrate that these nanostructured membranes are a strong platform for applications within the range of water purification, pharmaceutical separations, sensors, and drug delivery.

The application of single-configuration molecular orbital theory to the conformations of small organic molecules is reviewed. Emphasis is laid on systematic ab initio studies using simple gaussion-type basis sets for expansion of the molecular orbitals. Topics dealt with include the prediction of bond angles, single-rotor potential functions, effects of single and double (1,2) substitutions on such rotors and double-rotor potentials involving two internal rotation coordinates.

~~~ ~ ~~ ~~ ~ ~ Abstract: The silicon hydrides, SiH, (n = 1-4), and the entire set of H3SiX (X = Li, BeH, BH,, CH3, NH,, OH, F and Na, MgH, AIH,, SiH3, PH2, SH, C1) molecules have been investigated by using ab initio methods. All structural parameters were optimized by use of the 3-21G and 3-21G(*) basis sets. The silyl derivatives are compared with the corresponding methyl compounds. In most cases, the equilibrium geometries of the methyl and silyl molecules are similar. The most notable exception is in silylamine, where a planar geometry is found about the nitrogen. Addition of &functions to the second-row atoms results in a decrease in the bond lengths. The relative H3Si-X and H3C-X bond energies depend principally on the electronegativity of the group X. Since SiH, has a higher electron affinity and a lower ionization potential than CH,, groups which are very electronegative or very electropositive have stronger bonds to silicon than carbon.

As models for polymeric dilithiomethane, a number of head-to-tail andhead-to-head CH2Li2 dimer structures were optimized by means of ab initio molecular orbital calculations using the RHF/STO-3G minimal basis method. Split valence basis RHF/4-31G single point calculations indicated VIII, with four lithium atoms bridging two perpendicular CH2 units, to be the most stable geometry for (CH2Li2)2. The dimerization energy at this level of theory, 37 kcal/mole, is already considerable; it should be even larger with higher association. Possible trimer and polymer structures are suggested.

Isotopic hydronium ions D/sub 2/H/sup 17/O/sup +/ and DH/sub 2//sup 17/O/sup +/ in 1:1 (molar) HF:SbF/sub 5//SO/sub 2/ and DF:SbF/sub 2/SO/sub 2/ solutions, respectively, at -15/sup 0/C undergo slow hydrogen-deuterium exchange as monitored by /sup 17/O NMR spectroscopy. The rate of such exchange increases with the increase in the acidity of the fluoroantimonic acid medium (1:2 molar composition). The previously observed lack of exchange of isotopic hydronium ions in the somewhat weaker Magic Acid, FSO/sub 3/H:SbF/sub 5/, medium (H/sub 0/ approx. -21.5) suggests that in the stronger HF:SbF/sub 5/ medium (H/sub 0/ approx. -25 to -28), the exchange occurs through the intermediacy of protonated hydronium dication. Consequently, the structure and stability of protonated hydronium dication has been probed by ab initio theory. The T/sub d/ symmetry structure was found to be the minimum energy structure at the HF/6.31G* level. At the HF/6.31G* level this structure is thermodynamically unstable (dissociation preferred by 59.2 kcal/mol), it seems to have significant kinetic stability (deprotonation barrier 39.4 kcal/mol). 20 references, 1 table.

Multiply bonded Si molecules, H2SiX (X = Be, BH, CH2, SiH2, NH, 0), HSiY (Y = B, CH, SiH, N), and SiZ (Z = 0, 02), along with their carbon analogues, have been investigated by ab initio methods. All structural parameters were optimized by use of the 3-21G basis set and the higher 3-21G(*) basis set for silicon. In addition, the isomeric substituted silylenes which result from 1,2-hydrogen shifts were also considered. In many cases, the substituted silylene structures were energetically favored. The effect of all first-row substituents on the singlet-triplet energy separation of divalent silicon molecules was assessed and compared with the corresponding carbenes.

Calculs MO ab initio des surfaces d'energie potentielle pour des systemes a nombre pair d'electrons: isomeres singulets les moins stables, barrieres pour les reactions de deplacement 1,2 de l'hydrogene, structures triplets a l'equilibre, etc.

The geometrical structure and vibrational frequencies of the ground triplet electronic state of HCCN have been examined at a wide range of levels of ab initio electronic structure theory. The potential energy surface of HCC bending is very flat for HCCN owing to a competition between linear allene OHC=C=N and bent carbene HCC=N valence structures. Evidence is presented that both the restricted Hartree-Fock (RHF) and unrestricted Hartree-Fock (UHF) methods treat this potential surface in a somewhat uneven manner. When the effects of electron correlation are included, however, RHF- and UHF-based methods converge to a similar set of structural and energetic predictions. The most reliable levels of theory suggest the HCCN is a quasi-linear molecule, with THETA/sub e/(HCC) approx. = 138/sup 0/ and a barrier to linearity of only about 2 kcal/mol.

ab initio MO calculations have been performed on the cis and trans isomers of the crotyl cation, free radical and anion in each of two orientations of the Me rotor about the allylic framework. In agreement with available experimental data, both the crotyl cation and free radical prefer trans skeletal geometries. On the other hand, the cis isomer of the crotyl anion is found to be more stable than the t rans , the same preference as has been noted for alkali metal allyl organometallics in solution, but opposite to that recently reported for the free (gas phase) anion. The Me groups are predicted to eclipse the partial double bond for the trans isomers of all three systems and for the cis cation. These results are rationalized with the aid of perturbation MO theory.

Complete optimization of classical and nonclassical allyllithium structures at the RHF/STO-3G level with subsequent RHF/4-31G and RHF/6-31G calculations confirms the C s , bridged species to have the lowest energy. The allyl fragment of this structure is significantly distorted from a planar arrangement in order to enhance bonding between the allyl anion HOMO (on C(1), C(3)) and the lithium p -orbital with axis parallel to C(1)–C(3).

The recently introduced 3-21G split-valence basis sets for second-row elements have been supplemented with functionsof d-type symmetry. The resulting basis sets, termed 3-21G(*), are for use in conjuction with unsupplemented 3-21G representationsfor first-row elements. Equilibrium structures calculated by using 3-21G(*) are generally in good accord with available experimentaldata and are markedly improved over the corresponding 3-2 1 G level geometries, especially for hypervalent compounds andfor molecules incorporating bonds between two second-row elements. 3-21G(*) level normal-mode vibration frequencies,hydrogenation energies, and electric dipole moments are also generally but not always in better agreement with their respectiveexperimental quantities than are those obtained by using the unsupplemented 3-21G basis set. Overall, the 3-21G(*) basisset yields molecular properties that are uniformly close to those obtained with the much larger 6-31G* representation. The3-21G(*) basis set is still relatively compact and as such is generally applicable to molecules of moderate size.

Ab initio molecular orbital calculations were made on methoxymethanol, a model for the hemiacetal and acetal moieties in aldopyranoses and methyl aldopyranosides, thereby improving on the previous calculations using methanediol. The new calculations confirmed the favored conformations already deduced, and gave, for the conformational-energy differences and CO bond-length variations, values more appropriate to the carbohydrate systems, as confirmed by a re-examination of the experimental data from crystal-structure determinations. From the results, it was predicted that the OCH3 bond in methyl aldopyranosides is lengthened; this is supported by the experimental data. An examination of the angles and bond-lengths in the pyranoid ring and of the linkage bonds of oligosaccharides indicated that similar electronic effects involving the oxygen lone-pair electrons apply to oligo- and poly-saccharides.

The energies of planar vs. tetrahedral geometries of tetracoordinate organic molecules have been surveyed by ab ini- tio molecular orbital calculations. Because of their 7~ acceptor and u donor character, electropositive substituents, especially lithium, are particularly effective in stabilizing the planar arrangements selectively. Multiple substitution by such metals and by three-membered rings provides further stabilization, and 1,l-dilithiocyclopropane (XVII) and 3,3-dilithiocyclopropene (XVIII), inter alia, are actually calculated (RHF/STO-3G theory) to be more stable planar than tetrahedral.