Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

We describe a simple method to automate the geometric optimization of molecular orbital calculations of supermolecules on potential surfaces that are corrected for basis set superposition error using the counterpoise (CP) method This method is applied to the H‐bonding complexes HF/HCN, HF/H2O, and HCCH/H2O using the 6‐31G(d,p) and D95++(d,p) basis sets at both the Hartree–Fock and second‐order Mo/ller–Plesset levels We report the interaction energies, geometries, and vibrational frequencies of these complexes on the CP‐optimized surfaces; and compare them with similar values calculated using traditional methods, including the (more traditional) single point CP correction Upon optimization on the CP‐corrected surface, the interaction energies become more negative (before vibrational corrections) and the H‐bonding stretching vibrations decrease in all cases The extent of the effects vary from extremely small to quite large depending on the complex and the calculational method The relative magnitudes of the vibrational corrections cannot be predicted from the H‐bond stretching frequencies alone

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How does basis set superposition error change the potential surfaces for hydrogen-bonded dimers?

The contrast-variation technique is employed in multiple-contrast neutron/X-ray reflectometry experiments to highlight scattering from different structural components that are present at a surface or interface. The advantage of this technique is that the structural model used to describe the interfacial scattering length density profile must apply to all the contrasts measured. A new reflectivity analysis package, MOTOFIT, which runs in the IGOR Pro environment (http://www.wavemetrics.com), has been created to aid the simultaneous fitting (with the same structural model) of these multiple-contrast data, using an intuitive graphical user interface, which most co-refinement packages do not possess. MOTOFIT uses a slab-model approach with the Abeles matrix method, and extensions for surface roughness to perform non-linear least-squares regression on the experimental reflectivity curves. Other features, such as the ability to create complicated interparameter constraints or analyse reflectivity from multilayers, simulated annealing, etc., make MOTOFIT a powerful reflectometry analysis package

Co‐refinement of multiple‐contrast neutron/X‐ray reflectivity data using MOTOFIT

Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

Hofmeister phenomena: an update on ion specificity in biology.

Surfactant layers at the air/water interface: structure and composition.

Polymer/surfactant interactions at the air/water interface

Microcalorimetric measurements have been made on the series of gemini surfactants [CMH2M+1(CH3)2N(CH2)SN(CH3)2CMH2M+1] Br2, designated CMCSCMBr2, where M and S indicate the numbers of carbons in the side chains and spacer respectively, for M = 12 and S = 3, 4, 6, 8, 10, 12. For comparison, parallel measurements have been made on the series of double-chain, singly charged surfactants dodecyldimethylalkylammonium bromides, designated as C12CNBr with N = 1, 2, 4, 6, 8, 10, 12, where N is the number of carbons in the secondary alkyl chain. The choice of this second series was to compare each CMCSCMBr2 with its nearest equivalent monomer C12CS/2Br. The values of the critical micelle concentrations (cmc) were found to be in good agreement with other measurements, showing a maximum at S = 4−6 for the geminis and a nonlinear variation with chain length for the C12CNBr series. For both series of surfactants, the enthalpies of micellization, ΔHmic, are all exothermic and show a marked minimum in magnitude at S = 4−...

Thermodynamics of Molecular Self-Assembly of Cationic Gemini and Related Double Chain Surfactants in Aqueous Solution

The formation of complex monolayers between gemini surfactants (CsH2s-α,ω-(CmH2m+1N+(CH3)2Br-)2, abbreviated as C12-Cs-C12, s = 3, 4, 6, 8, 10, 12) and DNA at the air−water interface was systematically investigated. The polyion-complex monolayers formed in situ through the electrostatic attraction between the ammonium groups of gemini surfactants and the phosphate groups of DNA. The effect of surfactant spacer length on the surface properties was investigated. A turning point of the surface properties (extrapolated molecular area and collapse pressure) of the gemini surfactant/DNA complex monolayers appears when the surfactant spacer is above a certain length (s = 6). The gemini surfactant spacer taking a reverse U-shape conformation at the air−water interface is proposed to interpret the turning point. A quantitative kinetic analysis of the decay curves further confirms that the turning point appears at the surfactant spacer above its critical length, s = 6. Moreover, the surface topographies of the gemi...

Gemini Surfactant/DNA Complex Monolayers at the Air−Water Interface: Effect of Surfactant Structure on the Assembly, Stability, and Topography of Monolayers

Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C16TAX) with different counterions, F−, Cl−, Br−, NO−3, and ½SO2−4, have been studied. Critical micelle concentration (CMC), degree of micelle ionization ( α ) , and enthalpy of micellization ( Δ H mic ) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the α increase in the order SO2−4  Δ H mic becomes less negative through the sequence NO−3  ( Δ G mic ) and entropies of micellization ( Δ S mic ) have been calculated from the values of Δ H mic , CMC, and α and can be rationalized in terms of the Hofmeister series. The variations in Δ H mic and Δ S mic have been compared with those for the corresponding series of gemini surfactants. Electron spin resonance has been used to assess the micropolarity and the microviscosity of the micelles. The results show that the microenvironment of the spin probe in the C16TAX surfactant micelles depends strongly on the binding of the counterion.

Robert K. Thomas

Papers

Surfactant layers at the air/water interface: structure and composition.

Polymer/surfactant interactions at the air/water interface

Thermodynamics of Molecular Self-Assembly of Cationic Gemini and Related Double Chain Surfactants in Aqueous Solution

Gemini Surfactant/DNA Complex Monolayers at the Air−Water Interface: Effect of Surfactant Structure on the Assembly, Stability, and Topography of Monolayers

Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution.