We have determined the structure of a monolayer of hexadecyltrimethylamonium bromide adsorbed at the airlwater interface at its critical micelle concentration (9.2 x lop4 M) and at two lower concentrations using neutron specular reflection. We have used isotopic labeling to determine the relative positions of chains, heads, and solvent, and more detailed labeling to determine the distributions of (26 chain fragments at either end of the alkyl chains. The use of the more detailed labeling scheme has allowed us to make a quantitative estimate of the contribution of surface roughness to the structure of the layer at the three concentrations, to show that the alkyl chains are, on average, strongly tilted away from the surface normal but that the part of the alkyl chain next to the head group is less tilted. The different tilt angle distributions for the two ends of the hydrocarbon chain also indicate that there are gauche defects in the chain.

The structure of layers of triethylene glycol monododecyl ether (C12E3) adsorbed at the air/water interface at four concentrations from the critical micelle concentration (cmc = 5.5 X lCh8 M) down to 1 X "6 M have been determined using neutron specular reflection together with isotopic substitution. The distribution of any part of the layer is described in terms of the full width of the distribution at half height, *; i.e. * is a measure of the thickness of that part of the layer. The width, *, of the alkyl chain part of the layer varied from 14 ± 1A at an area per molecule (A) of 36 A* 12 down to 10 ± 1.5 A at an A of 89 A2. The corresponding widths of the ethylene glycol region varied from 13 ± 1 to 5 ± 2 A, and the separation of the two chain regions changed from 8 ± 0.5 to 4.5 ± 1 A. The separation of the alkyl chains from the midpoint of the water distribution was found to be 2 A greater than from the ethylene glycol chains at the cmc but exactly the same at 1 X 10-6 M. Isotopic labeling of the two halves of the alkyl chain was used to establish that the alkyl chain is substantially tilted away from the surface normal at all concentrations and that at least one gauche conformation occurs on average in the alkyl chain at the highest surface concentration. The shape of the overall chain distribution is better described by a Gaussian distribution than by a uniform layer. The static displacement of the surfactant molecules in the direction normal to the surface is shown to be strongly disordered at the cmc, the mean displacement being about 9 A, which is of the order of half the projected length of the molecule in the vertical direction. The ethylene glycol ______is oriented close to the normal direction when plane when A = 89 A2.

The effect of the double-layer on the reduction of [Co(NH 3 ) 6 ] 3+ cation has been studied at four different single-crystal gold substrates, namely, Au(111), Au(100), Au(110), and Au(210), and at various concentrations of perchloric acid. The values of the experimental transfer coefficient (R ex ) are much larger than 1.0 at all gold surfaces and decrease with increasing concentration of the supporting electrolyte. The value of R ex also depends on the crystallographic orientation of the gold substrate and increases in the following order: Au(111) < Au(100) < Au(110) < Au(210), consistent with a decreasing reduction rate from Au(111) to Au(210) at constant electrode potential. The experimental transfer coefficients are rationalized on the basis of the classical theory of the double-layer with consideration of reactants and products in estimation of the diffuse-layer potential. The charge on the reactant transported through the double-layer is 2+. Corrected Tafel plots yield an apparent transfer coefficient R a equal to 1.1 ( 0.1 for all analyzed substrates. Corrected rate constants for double-layer effects indicate that the reduction of [Co(NH 3 ) 6 ] 3+ ion depends on the crystallographic orientation of the gold substrate only through the differences in the potential of zero charge. This behavior confirms that the role of the metal is that expected for an adiabatic outer-sphere electron-transfer process.

No abstract

Contrary to 4,4'-dipyridinium (i.e., archetypal methyl viologen), which is reduced by two single-electron transfers (stepwise reduction), the 4,1'-dipyridinium isomer (so-called "head-to-tail" isomer) undergoes two electron transfers at apparently the same potential (single-step reduction). A combined theoretical and experimental study has been undertaken to establish that the latter electrochemical behavior, also observed for other polyarylpyridinium electrophores, is due to potential compression originating in a large structural rearrangement. Three series of branched expanded pyridiniums (EPs) were prepared: N-aryl-2,4,6-triphenylpyridiniums (Ar-TP), N-aryl-2,3,4,5,6-pentaphenylpyridiniums (Ar-XP), and N-aryl-3,5-dimethyl-2,4,6-triphenylpyridinium (Ar-DMTP). The intramolecular steric strain was tuned via N-pyridinio aryl group (Ar) phenyl (Ph), 4-pyridyl (Py), and 4-pyridylium (qPy) and their bulky 3,5-dimethyl counterparts, xylyl (Xy), lutidyl (Lu), and lutidylium (qLu), respectively. Ferrocenyl subunits as internal redox references were covalently appended to representative electrophores in order to count the electrons involved in EP-centered reduction processes. Depending on the steric constraint around the N-pyridinio site, the two-electron reduction is single-step (Ar = Ph, Py, qPy) or stepwise (Ar = Xy, Lu, qLu). This steric switching of the potential compression is accurately accounted for by ab initio modeling (Density Functional Theory, DFT) that proposes a mechanism for pyramidalization of the N(pyridinio) atom coupled with reduction. When the hybridization change of this atom is hindered (Ar = Xy, Lu, qLu), the first reduction is a one-electron process. Theory also reveals that the single-step two-electron reduction involves couples of redox isomers (electromers) displaying both the axial geometry of native EPs and the pyramidalized geometry of doubly reduced EPs. This picture is confirmed by a combined UV-vis-NIR spectroelectrochemical and time-dependent DFT study: comparison of in situ spectroelectrochemical data with the calculated electronic transitions makes it possible to both evidence the distortion and identify the predicted electromers, which play decisive roles in the electron-transfer mechanism. Last, this mechanism is further supported by in-depth analysis of the electronic structures of electrophores in their various reduction states (including electromeric forms).